Gforth
1 Goals of Gforth
2 Gforth Environment
  2.1 Invoking Gforth
  2.2 Leaving Gforth
  2.3 Command-line editing
  2.4 Environment variables
  2.5 Gforth files
  2.6 Gforth in pipes
  2.7 Startup speed
3 Forth Tutorial
  3.1 Starting Gforth
  3.2 Syntax
  3.3 Crash Course
  3.4 Stack
  3.5 Arithmetics
  3.6 Stack Manipulation
  3.7 Using files for Forth code
  3.8 Comments
  3.9 Colon Definitions
  3.10 Decompilation
  3.11 Stack-Effect Comments
  3.12 Types
  3.13 Factoring
  3.14 Designing the stack effect
  3.15 Local Variables
  3.16 Conditional execution
  3.17 Flags and Comparisons
  3.18 General Loops
  3.19 Counted loops
  3.20 Recursion
  3.21 Leaving definitions or loops
  3.22 Return Stack
  3.23 Memory
  3.24 Characters and Strings
  3.25 Alignment
  3.26 Floating Point
  3.27 Files
    3.27.1 Open file for input
    3.27.2 Create file for output
    3.27.3 Scan file for a particular line
    3.27.4 Copy input to output
    3.27.5 Close files
  3.28 Interpretation and Compilation Semantics and Immediacy
  3.29 Execution Tokens
  3.30 Exceptions
  3.31 Defining Words
  3.32 Arrays and Records
  3.33 'POSTPONE'
  3.34 'Literal'
  3.35 Advanced macros
  3.36 Compilation Tokens
  3.37 Wordlists and Search Order
4 An Introduction to ANS Forth
  4.1 Introducing the Text Interpreter
  4.2 Stacks, postfix notation and parameter passing
  4.3 Your first Forth definition
  4.4 How does that work?
  4.5 Forth is written in Forth
  4.6 Review - elements of a Forth system
  4.7 Where To Go Next
  4.8 Exercises
5 Forth Words
  5.1 Notation
  5.2 Case insensitivity
  5.3 Comments
  5.4 Boolean Flags
  5.5 Arithmetic
    5.5.1 Single precision
    5.5.2 Double precision
    5.5.3 Bitwise operations
    5.5.4 Numeric comparison
    5.5.5 Mixed precision
    5.5.6 Floating Point
  5.6 Stack Manipulation
    5.6.1 Data stack
    5.6.2 Floating point stack
    5.6.3 Return stack
    5.6.4 Locals stack
    5.6.5 Stack pointer manipulation
  5.7 Memory
    5.7.1 ANS Forth and Gforth memory models
    5.7.2 Dictionary allocation
    5.7.3 Heap allocation
    5.7.4 Memory Access
    5.7.5 Address arithmetic
    5.7.6 Memory Blocks
  5.8 Control Structures
    5.8.1 Selection
    5.8.2 Simple Loops
    5.8.3 Counted Loops
    5.8.4 Arbitrary control structures
      5.8.4.1 Programming Style
    5.8.5 Calls and returns
    5.8.6 Exception Handling
  5.9 Defining Words
    5.9.1 'CREATE'
    5.9.2 Variables
    5.9.3 Constants
    5.9.4 Values
    5.9.5 Colon Definitions
    5.9.6 Anonymous Definitions
    5.9.7 Supplying the name of a defined word
    5.9.8 User-defined Defining Words
      5.9.8.1 Applications of 'CREATE..DOES>'
      5.9.8.2 The gory details of 'CREATE..DOES>'
      5.9.8.3 Advanced does> usage example
      5.9.8.4 'Const-does>'
    5.9.9 Deferred Words
    5.9.10 Aliases
  5.10 Interpretation and Compilation Semantics
    5.10.1 Combined Words
  5.11 Tokens for Words
    5.11.1 Execution token
    5.11.2 Compilation token
    5.11.3 Name token
  5.12 Compiling words
    5.12.1 Literals
    5.12.2 Macros
  5.13 The Text Interpreter
    5.13.1 Input Sources
    5.13.2 Number Conversion
    5.13.3 Interpret/Compile states
    5.13.4 Interpreter Directives
  5.14 The Input Stream
  5.15 Word Lists
    5.15.1 Vocabularies
    5.15.2 Why use word lists?
    5.15.3 Word list example
  5.16 Environmental Queries
  5.17 Files
    5.17.1 Forth source files
    5.17.2 General files
    5.17.3 Redirection
    5.17.4 Search Paths
      5.17.4.1 Source Search Paths
      5.17.4.2 General Search Paths
  5.18 Blocks
  5.19 Other I/O
    5.19.1 Simple numeric output
    5.19.2 Formatted numeric output
    5.19.3 String Formats
    5.19.4 Displaying characters and strings
    5.19.5 Terminal output
    5.19.6 Single-key input
    5.19.7 Line input and conversion
    5.19.8 Pipes
    5.19.9 Xchars and Unicode
  5.20 OS command line arguments
  5.21 Locals
    5.21.1 Gforth locals
      5.21.1.1 Where are locals visible by name?
      5.21.1.2 How long do locals live?
      5.21.1.3 Locals programming style
      5.21.1.4 Locals implementation
    5.21.2 ANS Forth locals
  5.22 Structures
    5.22.1 Why explicit structure support?
    5.22.2 Structure Usage
    5.22.3 Structure Naming Convention
    5.22.4 Structure Implementation
    5.22.5 Structure Glossary
    5.22.6 Forth200x Structures
  5.23 Object-oriented Forth
    5.23.1 Why object-oriented programming?
    5.23.2 Object-Oriented Terminology
    5.23.3 The 'objects.fs' model
      5.23.3.1 Properties of the 'objects.fs' model
      5.23.3.2 Basic 'objects.fs' Usage
      5.23.3.3 The 'object.fs' base class
      5.23.3.4 Creating objects
      5.23.3.5 Object-Oriented Programming Style
      5.23.3.6 Class Binding
      5.23.3.7 Method conveniences
      5.23.3.8 Classes and Scoping
      5.23.3.9 Dividing classes
      5.23.3.10 Object Interfaces
      5.23.3.11 'objects.fs' Implementation
      5.23.3.12 'objects.fs' Glossary
    5.23.4 The 'oof.fs' model
      5.23.4.1 Properties of the 'oof.fs' model
      5.23.4.2 Basic 'oof.fs' Usage
      5.23.4.3 The 'oof.fs' base class
      5.23.4.4 Class Declaration
      5.23.4.5 Class Implementation
    5.23.5 The 'mini-oof.fs' model
      5.23.5.1 Basic 'mini-oof.fs' Usage
      5.23.5.2 Mini-OOF Example
      5.23.5.3 'mini-oof.fs' Implementation
    5.23.6 Comparison with other object models
  5.24 Programming Tools
    5.24.1 Examining data and code
    5.24.2 Forgetting words
    5.24.3 Debugging
    5.24.4 Assertions
    5.24.5 Singlestep Debugger
  5.25 C Interface
    5.25.1 Calling C functions
    5.25.2 Declaring C Functions
    5.25.3 Calling C function pointers from Forth
    5.25.4 Defining library interfaces
    5.25.5 Declaring OS-level libraries
    5.25.6 Callbacks
    5.25.7 How the C interface works
    5.25.8 Low-Level C Interface Words
  5.26 Assembler and Code Words
    5.26.1 'Code' and ';code'
    5.26.2 Common Assembler
    5.26.3 Common Disassembler
    5.26.4 386 Assembler
    5.26.5 Alpha Assembler
    5.26.6 MIPS assembler
    5.26.7 PowerPC assembler
    5.26.8 ARM Assembler
    5.26.9 Other assemblers
  5.27 Threading Words
  5.28 Passing Commands to the Operating System
  5.29 Keeping track of Time
  5.30 Miscellaneous Words
6 Error messages
7 Tools
  7.1 'ans-report.fs': Report the words used, sorted by wordset
    7.1.1 Caveats
  7.2 Stack depth changes during interpretation
8 ANS conformance
  8.1 The Core Words
    8.1.1 Implementation Defined Options
    8.1.2 Ambiguous conditions
    8.1.3 Other system documentation
  8.2 The optional Block word set
    8.2.1 Implementation Defined Options
    8.2.2 Ambiguous conditions
    8.2.3 Other system documentation
  8.3 The optional Double Number word set
    8.3.1 Ambiguous conditions
  8.4 The optional Exception word set
    8.4.1 Implementation Defined Options
  8.5 The optional Facility word set
    8.5.1 Implementation Defined Options
    8.5.2 Ambiguous conditions
  8.6 The optional File-Access word set
    8.6.1 Implementation Defined Options
    8.6.2 Ambiguous conditions
  8.7 The optional Floating-Point word set
    8.7.1 Implementation Defined Options
    8.7.2 Ambiguous conditions
  8.8 The optional Locals word set
    8.8.1 Implementation Defined Options
    8.8.2 Ambiguous conditions
  8.9 The optional Memory-Allocation word set
    8.9.1 Implementation Defined Options
  8.10 The optional Programming-Tools word set
    8.10.1 Implementation Defined Options
    8.10.2 Ambiguous conditions
  8.11 The optional Search-Order word set
    8.11.1 Implementation Defined Options
    8.11.2 Ambiguous conditions
9 Should I use Gforth extensions?
10 Model
11 Integrating Gforth into C programs
12 Emacs and Gforth
  12.1 Installing gforth.el
  12.2 Emacs Tags
  12.3 Hilighting
  12.4 Auto-Indentation
  12.5 Blocks Files
13 Image Files
  13.1 Image Licensing Issues
  13.2 Image File Background
  13.3 Non-Relocatable Image Files
  13.4 Data-Relocatable Image Files
  13.5 Fully Relocatable Image Files
    13.5.1 'gforthmi'
    13.5.2 'cross.fs'
  13.6 Stack and Dictionary Sizes
  13.7 Running Image Files
  13.8 Modifying the Startup Sequence
14 Engine
  14.1 Portability
  14.2 Threading
    14.2.1 Scheduling
    14.2.2 Direct or Indirect Threaded?
    14.2.3 Dynamic Superinstructions
    14.2.4 DOES>
  14.3 Primitives
    14.3.1 Automatic Generation
    14.3.2 TOS Optimization
    14.3.3 Produced code
  14.4 Performance
15 Cross Compiler
  15.1 Using the Cross Compiler
  15.2 How the Cross Compiler Works
Appendix A Bugs
Appendix B Authors and Ancestors of Gforth
  B.1 Authors and Contributors
  B.2 Pedigree
Appendix C Other Forth-related information
Appendix D Licenses
  D.1 GNU Free Documentation License
    D.1.1 ADDENDUM: How to use this License for your documents
  D.2 GNU GENERAL PUBLIC LICENSE
Word Index
Concept and Word Index
Gforth
******

This manual is for Gforth (version 0.7.3, June 14, 2014), a fast and
portable implementation of the ANS Forth language.  It serves as
reference manual, but it also contains an introduction to Forth and a
Forth tutorial.

   Copyright (C) 1995, 1996, 1997, 1998, 2000, 2003,
2004,2005,2006,2007,2008 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.1 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: "You have freedom to copy and
     modify this GNU Manual, like GNU software.  Copies published by the
     Free Software Foundation raise funds for GNU development."

1 Goals of Gforth
*****************

The goal of the Gforth Project is to develop a standard model for ANS
Forth.  This can be split into several subgoals:

   * Gforth should conform to the ANS Forth Standard.
   * It should be a model, i.e.  it should define all the
     implementation-dependent things.
   * It should become standard, i.e.  widely accepted and used.  This
     goal is the most difficult one.

   To achieve these goals Gforth should be
   * Similar to previous models (fig-Forth, F83)
   * Powerful.  It should provide for all the things that are considered
     necessary today and even some that are not yet considered
     necessary.
   * Efficient.  It should not get the reputation of being exceptionally
     slow.
   * Free.
   * Available on many machines/easy to port.

   Have we achieved these goals?  Gforth conforms to the ANS Forth
standard.  It may be considered a model, but we have not yet documented
which parts of the model are stable and which parts we are likely to
change.  It certainly has not yet become a de facto standard, but it
appears to be quite popular.  It has some similarities to and some
differences from previous models.  It has some powerful features, but
not yet everything that we envisioned.  We certainly have achieved our
execution speed goals (*note Performance::)(1).  It is free and
available on many machines.

   ---------- Footnotes ----------

   (1) However, in 1998 the bar was raised when the major commercial
Forth vendors switched to native code compilers.

2 Gforth Environment
********************

Note: ultimately, the Gforth man page will be auto-generated from the
material in this chapter.

   For related information about the creation of images see *note Image
Files::.

2.1 Invoking Gforth
===================

Gforth is made up of two parts; an executable "engine" (named 'gforth'
or 'gforth-fast') and an image file.  To start it, you will usually just
say 'gforth' - this automatically loads the default image file
'gforth.fi'.  In many other cases the default Gforth image will be
invoked like this:
     gforth [file | -e forth-code] ...
This interprets the contents of the files and the Forth code in the
order they are given.

   In addition to the 'gforth' engine, there is also an engine called
'gforth-fast', which is faster, but gives less informative error
messages (*note Error messages::) and may catch some errors (in
particular, stack underflows and integer division errors) later or not
at all.  You should use it for debugged, performance-critical programs.

   Moreover, there is an engine called 'gforth-itc', which is useful in
some backwards-compatibility situations (*note Direct or Indirect
Threaded?::).

   In general, the command line looks like this:

     gforth[-fast] [engine options] [image options]

   The engine options must come before the rest of the command line.
They are:

'--image-file file'
'-i file'
     Loads the Forth image file instead of the default 'gforth.fi'
     (*note Image Files::).

'--appl-image file'
     Loads the image file and leaves all further command-line arguments
     to the image (instead of processing them as engine options).  This
     is useful for building executable application images on Unix, built
     with 'gforthmi --application ...'.

'--path path'
'-p path'
     Uses path for searching the image file and Forth source code files
     instead of the default in the environment variable 'GFORTHPATH' or
     the path specified at installation time (e.g.,
     '/usr/local/share/gforth/0.2.0:.').  A path is given as a list of
     directories, separated by ':' (on Unix) or ';' (on other OSs).

'--dictionary-size size'
'-m size'
     Allocate size space for the Forth dictionary space instead of using
     the default specified in the image (typically 256K). The size
     specification for this and subsequent options consists of an
     integer and a unit (e.g., '4M').  The unit can be one of 'b'
     (bytes), 'e' (element size, in this case Cells), 'k' (kilobytes),
     'M' (Megabytes), 'G' (Gigabytes), and 'T' (Terabytes).  If no unit
     is specified, 'e' is used.

'--data-stack-size size'
'-d size'
     Allocate size space for the data stack instead of using the default
     specified in the image (typically 16K).

'--return-stack-size size'
'-r size'
     Allocate size space for the return stack instead of using the
     default specified in the image (typically 15K).

'--fp-stack-size size'
'-f size'
     Allocate size space for the floating point stack instead of using
     the default specified in the image (typically 15.5K). In this case
     the unit specifier 'e' refers to floating point numbers.

'--locals-stack-size size'
'-l size'
     Allocate size space for the locals stack instead of using the
     default specified in the image (typically 14.5K).

'--vm-commit'
     Normally, Gforth tries to start up even if there is not enough
     virtual memory for the dictionary and the stacks (using
     'MAP_NORESERVE' on OSs that support it); so you can ask for a
     really big dictionary and/or stacks, and as long as you don't use
     more virtual memory than is available, everything will be fine (but
     if you use more, processes get killed).  With this option you just
     use the default allocation policy of the OS; for OSs that don't
     overcommit (e.g., Solaris), this means that you cannot and should
     not ask for as big dictionary and stacks, but once Gforth
     successfully starts up, out-of-memory won't kill it.

'--help'
'-h'
     Print a message about the command-line options

'--version'
'-v'
     Print version and exit

'--debug'
     Print some information useful for debugging on startup.

'--offset-image'
     Start the dictionary at a slightly different position than would be
     used otherwise (useful for creating data-relocatable images, *note
     Data-Relocatable Image Files::).

'--no-offset-im'
     Start the dictionary at the normal position.

'--clear-dictionary'
     Initialize all bytes in the dictionary to 0 before loading the
     image (*note Data-Relocatable Image Files::).

'--die-on-signal'
     Normally Gforth handles most signals (e.g., the user interrupt
     SIGINT, or the segmentation violation SIGSEGV) by translating it
     into a Forth 'THROW'.  With this option, Gforth exits if it
     receives such a signal.  This option is useful when the engine
     and/or the image might be severely broken (such that it causes
     another signal before recovering from the first); this option
     avoids endless loops in such cases.

'--no-dynamic'
'--dynamic'
     Disable or enable dynamic superinstructions with replication (*note
     Dynamic Superinstructions::).

'--no-super'
     Disable dynamic superinstructions, use just dynamic replication;
     this is useful if you want to patch threaded code (*note Dynamic
     Superinstructions::).

'--ss-number=N'
     Use only the first N static superinstructions compiled into the
     engine (default: use them all; note that only 'gforth-fast' has
     any).  This option is useful for measuring the performance impact
     of static superinstructions.

'--ss-min-codesize'
'--ss-min-ls'
'--ss-min-lsu'
'--ss-min-nexts'
     Use specified metric for determining the cost of a primitive or
     static superinstruction for static superinstruction selection.
     'Codesize' is the native code size of the primive or static
     superinstruction, 'ls' is the number of loads and stores, 'lsu' is
     the number of loads, stores, and updates, and 'nexts' is the number
     of dispatches (not taking dynamic superinstructions into account),
     i.e.  every primitive or static superinstruction has cost 1.
     Default: 'codesize' if you use dynamic code generation, otherwise
     'nexts'.

'--ss-greedy'
     This option is useful for measuring the performance impact of
     static superinstructions.  By default, an optimal shortest-path
     algorithm is used for selecting static superinstructions.  With
     '--ss-greedy' this algorithm is modified to assume that anything
     after the static superinstruction currently under consideration is
     not combined into static superinstructions.  With '--ss-min-nexts'
     this produces the same result as a greedy algorithm that always
     selects the longest superinstruction available at the moment.
     E.g., if there are superinstructions AB and BCD, then for the
     sequence A B C D the optimal algorithm will select A BCD and the
     greedy algorithm will select AB C D.

'--print-metrics'
     Prints some metrics used during static superinstruction selection:
     'code size' is the actual size of the dynamically generated code.
     'Metric codesize' is the sum of the codesize metrics as seen by
     static superinstruction selection; there is a difference from 'code
     size', because not all primitives and static superinstructions are
     compiled into dynamically generated code, and because of markers.
     The other metrics correspond to the 'ss-min-...' options.  This
     option is useful for evaluating the effects of the '--ss-...'
     options.

   As explained above, the image-specific command-line arguments for the
default image 'gforth.fi' consist of a sequence of filenames and '-e
FORTH-CODE' options that are interpreted in the sequence in which they
are given.  The '-e FORTH-CODE' or '--evaluate FORTH-CODE' option
evaluates the Forth code.  This option takes only one argument; if you
want to evaluate more Forth words, you have to quote them or use '-e'
several times.  To exit after processing the command line (instead of
entering interactive mode) append '-e bye' to the command line.  You can
also process the command-line arguments with a Forth program (*note OS
command line arguments::).

   If you have several versions of Gforth installed, 'gforth' will
invoke the version that was installed last.  'gforth-version' invokes a
specific version.  If your environment contains the variable
'GFORTHPATH', you may want to override it by using the '--path' option.

   Not yet implemented: On startup the system first executes the system
initialization file (unless the option '--no-init-file' is given; note
that the system resulting from using this option may not be ANS Forth
conformant).  Then the user initialization file '.gforth.fs' is
executed, unless the option '--no-rc' is given; this file is searched
for in '.', then in '~', then in the normal path (see above).

2.2 Leaving Gforth
==================

You can leave Gforth by typing 'bye' or 'Ctrl-d' (at the start of a
line) or (if you invoked Gforth with the '--die-on-signal' option)
'Ctrl-c'.  When you leave Gforth, all of your definitions and data are
discarded.  For ways of saving the state of the system before leaving
Gforth see *note Image Files::.

'bye'       -         tools-ext       "bye"
   Return control to the host operating system (if any).

2.3 Command-line editing
========================

Gforth maintains a history file that records every line that you type to
the text interpreter.  This file is preserved between sessions, and is
used to provide a command-line recall facility; if you type 'Ctrl-P'
repeatedly you can recall successively older commands from this (or
previous) session(s).  The full list of command-line editing facilities
is:

   * 'Ctrl-p' ("previous") (or up-arrow) to recall successively older
     commands from the history buffer.
   * 'Ctrl-n' ("next") (or down-arrow) to recall successively newer
     commands from the history buffer.
   * 'Ctrl-f' (or right-arrow) to move the cursor right,
     non-destructively.
   * 'Ctrl-b' (or left-arrow) to move the cursor left,
     non-destructively.
   * 'Ctrl-h' (backspace) to delete the character to the left of the
     cursor, closing up the line.
   * 'Ctrl-k' to delete ("kill") from the cursor to the end of the line.
   * 'Ctrl-a' to move the cursor to the start of the line.
   * 'Ctrl-e' to move the cursor to the end of the line.
   * <RET> ('Ctrl-m') or <LFD> ('Ctrl-j') to submit the current line.
   * <TAB> to step through all possible full-word completions of the
     word currently being typed.
   * 'Ctrl-d' on an empty line line to terminate Gforth (gracefully,
     using 'bye').
   * 'Ctrl-x' (or 'Ctrl-d' on a non-empty line) to delete the character
     under the cursor.

   When editing, displayable characters are inserted to the left of the
cursor position; the line is always in "insert" (as opposed to
"overstrike") mode.

   On Unix systems, the history file is '~/.gforth-history' by
default(1).  You can find out the name and location of your history file
using:

     history-file type \ Unix-class systems

     history-file type \ Other systems
     history-dir  type

   If you enter long definitions by hand, you can use a text editor to
paste them out of the history file into a Forth source file for reuse at
a later time.

   Gforth never trims the size of the history file, so you should do
this periodically, if necessary.

   ---------- Footnotes ----------

   (1) i.e.  it is stored in the user's home directory.

2.4 Environment variables
=========================

Gforth uses these environment variables:

   * 'GFORTHHIST' - (Unix systems only) specifies the directory in which
     to open/create the history file, '.gforth-history'.  Default:
     '$HOME'.

   * 'GFORTHPATH' - specifies the path used when searching for the
     gforth image file and for Forth source-code files.

   * 'LANG' - see 'LC_CTYPE'

   * 'LC_ALL' - see 'LC_CTYPE'

   * 'LC_CTYPE' - If this variable contains "UTF-8" on Gforth startup,
     Gforth uses the UTF-8 encoding for strings internally and expects
     its input and produces its output in UTF-8 encoding, otherwise the
     encoding is 8bit (see *note Xchars and Unicode::).  If this
     environment variable is unset, Gforth looks in 'LC_ALL', and if
     that is unset, in 'LANG'.

   * 
     'GFORTHSYSTEMPREFIX' - specifies what to prepend to the argument of
     'system' before passing it to C's 'system()'.  Default:
     '"./$COMSPEC /c "' on Windows, '""' on other OSs.  The prefix and
     the command are directly concatenated, so if a space between them
     is necessary, append it to the prefix.

   * 'GFORTH' - used by 'gforthmi', *Note gforthmi::.

   * 'GFORTHD' - used by 'gforthmi', *Note gforthmi::.

   * 'TMP', 'TEMP' - (non-Unix systems only) used as a potential
     location for the history file.

   All the Gforth environment variables default to sensible values if
they are not set.

2.5 Gforth files
================

When you install Gforth on a Unix system, it installs files in these
locations by default:

   * '/usr/local/bin/gforth'
   * '/usr/local/bin/gforthmi'
   * '/usr/local/man/man1/gforth.1' - man page.
   * '/usr/local/info' - the Info version of this manual.
   * '/usr/local/lib/gforth/<version>/...' - Gforth '.fi' files.
   * '/usr/local/share/gforth/<version>/TAGS' - Emacs TAGS file.
   * '/usr/local/share/gforth/<version>/...' - Gforth source files.
   * '.../emacs/site-lisp/gforth.el' - Emacs gforth mode.

   You can select different places for installation by using 'configure'
options (listed with 'configure --help').

2.6 Gforth in pipes
===================

Gforth can be used in pipes created elsewhere (described here).  It can
also create pipes on its own (*note Pipes::).

   If you pipe into Gforth, your program should read with 'read-file' or
'read-line' from 'stdin' (*note General files::).  'Key' does not
recognize the end of input.  Words like 'accept' echo the input and are
therefore usually not useful for reading from a pipe.  You have to
invoke the Forth program with an OS command-line option, as you have no
chance to use the Forth command line (the text interpreter would try to
interpret the pipe input).

   You can output to a pipe with 'type', 'emit', 'cr' etc.

   When you write to a pipe that has been closed at the other end,
Gforth receives a SIGPIPE signal ("pipe broken").  Gforth translates
this into the exception 'broken-pipe-error'.  If your application does
not catch that exception, the system catches it and exits, usually
silently (unless you were working on the Forth command line; then it
prints an error message and exits).  This is usually the desired
behaviour.

   If you do not like this behaviour, you have to catch the exception
yourself, and react to it.

   Here's an example of an invocation of Gforth that is usable in a
pipe:

     gforth -e ": foo begin pad dup 10 stdin read-file throw dup while \
      type repeat ; foo bye"

   This example just copies the input verbatim to the output.  A very
simple pipe containing this example looks like this:

     cat startup.fs |
     gforth -e ": foo begin pad dup 80 stdin read-file throw dup while \
      type repeat ; foo bye"|
     head

   Pipes involving Gforth's 'stderr' output do not work.

2.7 Startup speed
=================

If Gforth is used for CGI scripts or in shell scripts, its startup speed
may become a problem.  On a 300MHz 21064a under Linux-2.2.13 with
glibc-2.0.7, 'gforth -e bye' takes about 24.6ms user and 11.3ms system
time.

   If startup speed is a problem, you may consider the following ways to
improve it; or you may consider ways to reduce the number of startups
(for example, by using Fast-CGI).

   An easy step that influences Gforth startup speed is the use of the
'--no-dynamic' option; this decreases image loading speed, but increases
compile-time and run-time.

   Another step to improve startup speed is to statically link Gforth,
by building it with 'XLDFLAGS=-static'.  This requires more memory for
the code and will therefore slow down the first invocation, but
subsequent invocations avoid the dynamic linking overhead.  Another
disadvantage is that Gforth won't profit from library upgrades.  As a
result, 'gforth-static -e bye' takes about 17.1ms user and 8.2ms system
time.

   The next step to improve startup speed is to use a non-relocatable
image (*note Non-Relocatable Image Files::).  You can create this image
with 'gforth -e "savesystem gforthnr.fi bye"' and later use it with
'gforth -i gforthnr.fi ...'.  This avoids the relocation overhead and a
part of the copy-on-write overhead.  The disadvantage is that the
non-relocatable image does not work if the OS gives Gforth a different
address for the dictionary, for whatever reason; so you better provide a
fallback on a relocatable image.  'gforth-static -i gforthnr.fi -e bye'
takes about 15.3ms user and 7.5ms system time.

   The final step is to disable dictionary hashing in Gforth.  Gforth
builds the hash table on startup, which takes much of the startup
overhead.  You can do this by commenting out the 'include hash.fs' in
'startup.fs' and everything that requires 'hash.fs' (at the moment
'table.fs' and 'ekey.fs') and then doing 'make'.  The disadvantages are
that functionality like 'table' and 'ekey' is missing and that text
interpretation (e.g., compiling) now takes much longer.  So, you should
only use this method if there is no significant text interpretation to
perform (the script should be compiled into the image, amongst other
things).  'gforth-static -i gforthnrnh.fi -e bye' takes about 2.1ms user
and 6.1ms system time.

3 Forth Tutorial
****************

The difference of this chapter from the Introduction (*note
Introduction::) is that this tutorial is more fast-paced, should be used
while sitting in front of a computer, and covers much more material, but
does not explain how the Forth system works.

   This tutorial can be used with any ANS-compliant Forth; any
Gforth-specific features are marked as such and you can skip them if you
work with another Forth.  This tutorial does not explain all features of
Forth, just enough to get you started and give you some ideas about the
facilities available in Forth.  Read the rest of the manual and the
standard when you are through this.

   The intended way to use this tutorial is that you work through it
while sitting in front of the console, take a look at the examples and
predict what they will do, then try them out; if the outcome is not as
expected, find out why (e.g., by trying out variations of the example),
so you understand what's going on.  There are also some assignments that
you should solve.

   This tutorial assumes that you have programmed before and know what,
e.g., a loop is.

3.1 Starting Gforth
===================

You can start Gforth by typing its name:

     gforth

   That puts you into interactive mode; you can leave Gforth by typing
'bye'.  While in Gforth, you can edit the command line and access the
command line history with cursor keys, similar to bash.

3.2 Syntax
==========

A "word" is a sequence of arbitrary characters (except white space).
Words are separated by white space.  E.g., each of the following lines
contains exactly one word:

     word
     !@#$%^&*()
     1234567890
     5!a

   A frequent beginner's error is to leave away necessary white space,
resulting in an error like 'Undefined word'; so if you see such an
error, check if you have put spaces wherever necessary.

     ." hello, world" \ correct
     ."hello, world"  \ gives an "Undefined word" error

   Gforth and most other Forth systems ignore differences in case (they
are case-insensitive), i.e., 'word' is the same as 'Word'.  If your
system is case-sensitive, you may have to type all the examples given
here in upper case.

3.3 Crash Course
================

Type

     0 0 !
     here execute
     ' catch >body 20 erase abort
     ' (quit) >body 20 erase

   The last two examples are guaranteed to destroy parts of Gforth (and
most other systems), so you better leave Gforth afterwards (if it has
not finished by itself).  On some systems you may have to kill gforth
from outside (e.g., in Unix with 'kill').

   Now that you know how to produce crashes (and that there's not much
to them), let's learn how to produce meaningful programs.

3.4 Stack
=========

The most obvious feature of Forth is the stack.  When you type in a
number, it is pushed on the stack.  You can display the content of the
stack with '.s'.

     1 2 .s
     3 .s

   '.s' displays the top-of-stack to the right, i.e., the numbers appear
in '.s' output as they appeared in the input.

   You can print the top of stack element with '.'.

     1 2 3 . . .

   In general, words consume their stack arguments ('.s' is an
exception).

     Assignment: What does the stack contain after '5 6 7 .'?

3.5 Arithmetics
===============

The words '+', '-', '*', '/', and 'mod' always operate on the top two
stack items:

     2 2 .s
     + .s
     .
     2 1 - .
     7 3 mod .

   The operands of '-', '/', and 'mod' are in the same order as in the
corresponding infix expression (this is generally the case in Forth).

   Parentheses are superfluous (and not available), because the order of
the words unambiguously determines the order of evaluation and the
operands:

     3 4 + 5 * .
     3 4 5 * + .

     Assignment: What are the infix expressions corresponding to the
     Forth code above?  Write '6-7*8+9' in Forth notation(1).

   To change the sign, use 'negate':

     2 negate .

     Assignment: Convert -(-3)*4-5 to Forth.

   '/mod' performs both '/' and 'mod'.

     7 3 /mod . .

   Reference: *note Arithmetic::.

   ---------- Footnotes ----------

   (1) This notation is also known as Postfix or RPN (Reverse Polish
Notation).

3.6 Stack Manipulation
======================

Stack manipulation words rearrange the data on the stack.

     1 .s drop .s
     1 .s dup .s drop drop .s
     1 2 .s over .s drop drop drop
     1 2 .s swap .s drop drop
     1 2 3 .s rot .s drop drop drop

   These are the most important stack manipulation words.  There are
also variants that manipulate twice as many stack items:

     1 2 3 4 .s 2swap .s 2drop 2drop

   Two more stack manipulation words are:

     1 2 .s nip .s drop
     1 2 .s tuck .s 2drop drop

     Assignment: Replace 'nip' and 'tuck' with combinations of other
     stack manipulation words.

          Given:          How do you get:
          1 2 3           3 2 1
          1 2 3           1 2 3 2
          1 2 3           1 2 3 3
          1 2 3           1 3 3
          1 2 3           2 1 3
          1 2 3 4         4 3 2 1
          1 2 3           1 2 3 1 2 3
          1 2 3 4         1 2 3 4 1 2
          1 2 3
          1 2 3           1 2 3 4
          1 2 3           1 3

     5 dup * .

     Assignment: Write 17^3 and 17^4 in Forth, without writing '17' more
     than once.  Write a piece of Forth code that expects two numbers on
     the stack (A and B, with B on top) and computes '(a-b)(a+1)'.

   Reference: *note Stack Manipulation::.

3.7 Using files for Forth code
==============================

While working at the Forth command line is convenient for one-line
examples and short one-off code, you probably want to store your source
code in files for convenient editing and persistence.  You can use your
favourite editor (Gforth includes Emacs support, *note Emacs and
Gforth::) to create FILE.FS and use

     s" FILE.FS" included

   to load it into your Forth system.  The file name extension I use for
Forth files is '.fs'.

   You can easily start Gforth with some files loaded like this:

     gforth FILE1.FS FILE2.FS

   If an error occurs during loading these files, Gforth terminates,
whereas an error during 'INCLUDED' within Gforth usually gives you a
Gforth command line.  Starting the Forth system every time gives you a
clean start every time, without interference from the results of earlier
tries.

   I often put all the tests in a file, then load the code and run the
tests with

     gforth CODE.FS TESTS.FS -e bye

   (often by performing this command with 'C-x C-e' in Emacs).  The '-e
bye' ensures that Gforth terminates afterwards so that I can restart
this command without ado.

   The advantage of this approach is that the tests can be repeated
easily every time the program ist changed, making it easy to catch bugs
introduced by the change.

   Reference: *note Forth source files::.

3.8 Comments
============

     \ That's a comment; it ends at the end of the line
     ( Another comment; it ends here: )  .s

   '\' and '(' are ordinary Forth words and therefore have to be
separated with white space from the following text.

     \This gives an "Undefined word" error

   The first ')' ends a comment started with '(', so you cannot nest
'('-comments; and you cannot comment out text containing a ')' with '(
... )'(1).

   I use '\'-comments for descriptive text and for commenting out code
of one or more line; I use '('-comments for describing the stack effect,
the stack contents, or for commenting out sub-line pieces of code.

   The Emacs mode 'gforth.el' (*note Emacs and Gforth::) supports these
uses by commenting out a region with 'C-x \', uncommenting a region with
'C-u C-x \', and filling a '\'-commented region with 'M-q'.

   Reference: *note Comments::.

   ---------- Footnotes ----------

   (1) therefore it's a good idea to avoid ')' in word names.

3.9 Colon Definitions
=====================

are similar to procedures and functions in other programming languages.

     : squared ( n -- n^2 )
        dup * ;
     5 squared .
     7 squared .

   ':' starts the colon definition; its name is 'squared'.  The
following comment describes its stack effect.  The words 'dup *' are not
executed, but compiled into the definition.  ';' ends the colon
definition.

   The newly-defined word can be used like any other word, including
using it in other definitions:

     : cubed ( n -- n^3 )
        dup squared * ;
     -5 cubed .
     : fourth-power ( n -- n^4 )
        squared squared ;
     3 fourth-power .

     Assignment: Write colon definitions for 'nip', 'tuck', 'negate',
     and '/mod' in terms of other Forth words, and check if they work
     (hint: test your tests on the originals first).  Don't let the
     'redefined'-Messages spook you, they are just warnings.

   Reference: *note Colon Definitions::.

3.10 Decompilation
==================

You can decompile colon definitions with 'see':

     see squared
     see cubed

   In Gforth 'see' shows you a reconstruction of the source code from
the executable code.  Informations that were present in the source, but
not in the executable code, are lost (e.g., comments).

   You can also decompile the predefined words:

     see .
     see +

3.11 Stack-Effect Comments
==========================

By convention the comment after the name of a definition describes the
stack effect: The part in front of the '--' describes the state of the
stack before the execution of the definition, i.e., the parameters that
are passed into the colon definition; the part behind the '--' is the
state of the stack after the execution of the definition, i.e., the
results of the definition.  The stack comment only shows the top stack
items that the definition accesses and/or changes.

   You should put a correct stack effect on every definition, even if it
is just '( -- )'.  You should also add some descriptive comment to more
complicated words (I usually do this in the lines following ':').  If
you don't do this, your code becomes unreadable (because you have to
work through every definition before you can understand any).

     Assignment: The stack effect of 'swap' can be written like this:
     'x1 x2 -- x2 x1'.  Describe the stack effect of '-', 'drop', 'dup',
     'over', 'rot', 'nip', and 'tuck'.  Hint: When you are done, you can
     compare your stack effects to those in this manual (*note Word
     Index::).

   Sometimes programmers put comments at various places in colon
definitions that describe the contents of the stack at that place (stack
comments); i.e., they are like the first part of a stack-effect comment.
E.g.,

     : cubed ( n -- n^3 )
        dup squared  ( n n^2 ) * ;

   In this case the stack comment is pretty superfluous, because the
word is simple enough.  If you think it would be a good idea to add such
a comment to increase readability, you should also consider factoring
the word into several simpler words (*note Factoring: Factoring
Tutorial.), which typically eliminates the need for the stack comment;
however, if you decide not to refactor it, then having such a comment is
better than not having it.

   The names of the stack items in stack-effect and stack comments in
the standard, in this manual, and in many programs specify the type
through a type prefix, similar to Fortran and Hungarian notation.  The
most frequent prefixes are:

'n'
     signed integer
'u'
     unsigned integer
'c'
     character
'f'
     Boolean flags, i.e.  'false' or 'true'.
'a-addr,a-'
     Cell-aligned address
'c-addr,c-'
     Char-aligned address (note that a Char may have two bytes in
     Windows NT)
'xt'
     Execution token, same size as Cell
'w,x'
     Cell, can contain an integer or an address.  It usually takes 32,
     64 or 16 bits (depending on your platform and Forth system).  A
     cell is more commonly known as machine word, but the term _word_
     already means something different in Forth.
'd'
     signed double-cell integer
'ud'
     unsigned double-cell integer
'r'
     Float (on the FP stack)

   You can find a more complete list in *note Notation::.

     Assignment: Write stack-effect comments for all definitions you
     have written up to now.

3.12 Types
==========

In Forth the names of the operations are not overloaded; so similar
operations on different types need different names; e.g., '+' adds
integers, and you have to use 'f+' to add floating-point numbers.  The
following prefixes are often used for related operations on different
types:

'(none)'
     signed integer
'u'
     unsigned integer
'c'
     character
'd'
     signed double-cell integer
'ud, du'
     unsigned double-cell integer
'2'
     two cells (not-necessarily double-cell numbers)
'm, um'
     mixed single-cell and double-cell operations
'f'
     floating-point (note that in stack comments 'f' represents flags,
     and 'r' represents FP numbers).

   If there are no differences between the signed and the unsigned
variant (e.g., for '+'), there is only the prefix-less variant.

   Forth does not perform type checking, neither at compile time, nor at
run time.  If you use the wrong oeration, the data are interpreted
incorrectly:

     -1 u.

   If you have only experience with type-checked languages until now,
and have heard how important type-checking is, don't panic!  In my
experience (and that of other Forthers), type errors in Forth code are
usually easy to find (once you get used to it), the increased vigilance
of the programmer tends to catch some harder errors in addition to most
type errors, and you never have to work around the type system, so in
most situations the lack of type-checking seems to be a win (projects to
add type checking to Forth have not caught on).

3.13 Factoring
==============

If you try to write longer definitions, you will soon find it hard to
keep track of the stack contents.  Therefore, good Forth programmers
tend to write only short definitions (e.g., three lines).  The art of
finding meaningful short definitions is known as factoring (as in
factoring polynomials).

   Well-factored programs offer additional advantages: smaller, more
general words, are easier to test and debug and can be reused more and
better than larger, specialized words.

   So, if you run into difficulties with stack management, when writing
code, try to define meaningful factors for the word, and define the word
in terms of those.  Even if a factor contains only two words, it is
often helpful.

   Good factoring is not easy, and it takes some practice to get the
knack for it; but even experienced Forth programmers often don't find
the right solution right away, but only when rewriting the program.  So,
if you don't come up with a good solution immediately, keep trying,
don't despair.

3.14 Designing the stack effect
===============================

In other languages you can use an arbitrary order of parameters for a
function; and since there is only one result, you don't have to deal
with the order of results, either.

   In Forth (and other stack-based languages, e.g., PostScript) the
parameter and result order of a definition is important and should be
designed well.  The general guideline is to design the stack effect such
that the word is simple to use in most cases, even if that complicates
the implementation of the word.  Some concrete rules are:

   * Words consume all of their parameters (e.g., '.').

   * If there is a convention on the order of parameters (e.g., from
     mathematics or another programming language), stick with it (e.g.,
     '-').

   * If one parameter usually requires only a short computation (e.g.,
     it is a constant), pass it on the top of the stack.  Conversely,
     parameters that usually require a long sequence of code to compute
     should be passed as the bottom (i.e., first) parameter.  This makes
     the code easier to read, because the reader does not need to keep
     track of the bottom item through a long sequence of code (or,
     alternatively, through stack manipulations).  E.g., '!' (store,
     *note Memory::) expects the address on top of the stack because it
     is usually simpler to compute than the stored value (often the
     address is just a variable).

   * Similarly, results that are usually consumed quickly should be
     returned on the top of stack, whereas a result that is often used
     in long computations should be passed as bottom result.  E.g., the
     file words like 'open-file' return the error code on the top of
     stack, because it is usually consumed quickly by 'throw'; moreover,
     the error code has to be checked before doing anything with the
     other results.

   These rules are just general guidelines, don't lose sight of the
overall goal to make the words easy to use.  E.g., if the convention
rule conflicts with the computation-length rule, you might decide in
favour of the convention if the word will be used rarely, and in favour
of the computation-length rule if the word will be used frequently
(because with frequent use the cost of breaking the computation-length
rule would be quite high, and frequent use makes it easier to remember
an unconventional order).

3.15 Local Variables
====================

You can define local variables (_locals_) in a colon definition:

     : swap { a b -- b a }
       b a ;
     1 2 swap .s 2drop

   (If your Forth system does not support this syntax, include
'compat/anslocal.fs' first).

   In this example '{ a b -- b a }' is the locals definition; it takes
two cells from the stack, puts the top of stack in 'b' and the next
stack element in 'a'.  '--' starts a comment ending with '}'.  After the
locals definition, using the name of the local will push its value on
the stack.  You can leave the comment part ('-- b a') away:

     : swap ( x1 x2 -- x2 x1 )
       { a b } b a ;

   In Gforth you can have several locals definitions, anywhere in a
colon definition; in contrast, in a standard program you can have only
one locals definition per colon definition, and that locals definition
must be outside any control structure.

   With locals you can write slightly longer definitions without running
into stack trouble.  However, I recommend trying to write colon
definitions without locals for exercise purposes to help you gain the
essential factoring skills.

     Assignment: Rewrite your definitions until now with locals

   Reference: *note Locals::.

3.16 Conditional execution
==========================

In Forth you can use control structures only inside colon definitions.
An 'if'-structure looks like this:

     : abs ( n1 -- +n2 )
         dup 0 < if
             negate
         endif ;
     5 abs .
     -5 abs .

   'if' takes a flag from the stack.  If the flag is non-zero (true),
the following code is performed, otherwise execution continues after the
'endif' (or 'else').  '<' compares the top two stack elements and
produces a flag:

     1 2 < .
     2 1 < .
     1 1 < .

   Actually the standard name for 'endif' is 'then'.  This tutorial
presents the examples using 'endif', because this is often less
confusing for people familiar with other programming languages where
'then' has a different meaning.  If your system does not have 'endif',
define it with

     : endif postpone then ; immediate

   You can optionally use an 'else'-part:

     : min ( n1 n2 -- n )
       2dup < if
         drop
       else
         nip
       endif ;
     2 3 min .
     3 2 min .

     Assignment: Write 'min' without 'else'-part (hint: what's the
     definition of 'nip'?).

   Reference: *note Selection::.

3.17 Flags and Comparisons
==========================

In a false-flag all bits are clear (0 when interpreted as integer).  In
a canonical true-flag all bits are set (-1 as a twos-complement signed
integer); in many contexts (e.g., 'if') any non-zero value is treated as
true flag.

     false .
     true .
     true hex u. decimal

   Comparison words produce canonical flags:

     1 1 = .
     1 0= .
     0 1 < .
     0 0 < .
     -1 1 u< . \ type error, u< interprets -1 as large unsigned number
     -1 1 < .

   Gforth supports all combinations of the prefixes '0 u d d0 du f f0'
(or none) and the comparisons '= <> < > <= >='.  Only a part of these
combinations are standard (for details see the standard, *note Numeric
comparison::, *note Floating Point:: or *note Word Index::).

   You can use 'and or xor invert' as operations on canonical flags.
Actually they are bitwise operations:

     1 2 and .
     1 2 or .
     1 3 xor .
     1 invert .

   You can convert a zero/non-zero flag into a canonical flag with '0<>'
(and complement it on the way with '0=').

     1 0= .
     1 0<> .

   You can use the all-bits-set feature of canonical flags and the
bitwise operation of the Boolean operations to avoid 'if's:

     : foo ( n1 -- n2 )
       0= if
         14
       else
         0
       endif ;
     0 foo .
     1 foo .

     : foo ( n1 -- n2 )
       0= 14 and ;
     0 foo .
     1 foo .

     Assignment: Write 'min' without 'if'.

   For reference, see *note Boolean Flags::, *note Numeric comparison::,
and *note Bitwise operations::.

3.18 General Loops
==================

The endless loop is the most simple one:

     : endless ( -- )
       0 begin
         dup . 1+
       again ;
     endless

   Terminate this loop by pressing 'Ctrl-C' (in Gforth).  'begin' does
nothing at run-time, 'again' jumps back to 'begin'.

   A loop with one exit at any place looks like this:

     : log2 ( +n1 -- n2 )
     \ logarithmus dualis of n1>0, rounded down to the next integer
       assert( dup 0> )
       2/ 0 begin
         over 0> while
           1+ swap 2/ swap
       repeat
       nip ;
     7 log2 .
     8 log2 .

   At run-time 'while' consumes a flag; if it is 0, execution continues
behind the 'repeat'; if the flag is non-zero, execution continues behind
the 'while'.  'Repeat' jumps back to 'begin', just like 'again'.

   In Forth there are many combinations/abbreviations, like '1+'.
However, '2/' is not one of them; it shifts its argument right by one
bit (arithmetic shift right):

     -5 2 / .
     -5 2/ .

   'assert(' is no standard word, but you can get it on systems other
than Gforth by including 'compat/assert.fs'.  You can see what it does
by trying

     0 log2 .

   Here's a loop with an exit at the end:

     : log2 ( +n1 -- n2 )
     \ logarithmus dualis of n1>0, rounded down to the next integer
       assert( dup 0 > )
       -1 begin
         1+ swap 2/ swap
         over 0 <=
       until
       nip ;

   'Until' consumes a flag; if it is non-zero, execution continues at
the 'begin', otherwise after the 'until'.

     Assignment: Write a definition for computing the greatest common
     divisor.

   Reference: *note Simple Loops::.

3.19 Counted loops
==================

     : ^ ( n1 u -- n )
     \ n = the uth power of n1
       1 swap 0 u+do
         over *
       loop
       nip ;
     3 2 ^ .
     4 3 ^ .

   'U+do' (from 'compat/loops.fs', if your Forth system doesn't have it)
takes two numbers of the stack '( u3 u4 -- )', and then performs the
code between 'u+do' and 'loop' for 'u3-u4' times (or not at all, if
'u3-u4<0').

   You can see the stack effect design rules at work in the stack effect
of the loop start words: Since the start value of the loop is more
frequently constant than the end value, the start value is passed on the
top-of-stack.

   You can access the counter of a counted loop with 'i':

     : fac ( u -- u! )
       1 swap 1+ 1 u+do
         i *
       loop ;
     5 fac .
     7 fac .

   There is also '+do', which expects signed numbers (important for
deciding whether to enter the loop).

     Assignment: Write a definition for computing the nth Fibonacci
     number.

   You can also use increments other than 1:

     : up2 ( n1 n2 -- )
       +do
         i .
       2 +loop ;
     10 0 up2

     : down2 ( n1 n2 -- )
       -do
         i .
       2 -loop ;
     0 10 down2

   Reference: *note Counted Loops::.

3.20 Recursion
==============

Usually the name of a definition is not visible in the definition; but
earlier definitions are usually visible:

     1 0 / . \ "Floating-point unidentified fault" in Gforth on some platforms
     : / ( n1 n2 -- n )
       dup 0= if
         -10 throw \ report division by zero
       endif
       /           \ old version
     ;
     1 0 /

   For recursive definitions you can use 'recursive' (non-standard) or
'recurse':

     : fac1 ( n -- n! ) recursive
      dup 0> if
        dup 1- fac1 *
      else
        drop 1
      endif ;
     7 fac1 .

     : fac2 ( n -- n! )
      dup 0> if
        dup 1- recurse *
      else
        drop 1
      endif ;
     8 fac2 .

     Assignment: Write a recursive definition for computing the nth
     Fibonacci number.

   Reference (including indirect recursion): *Note Calls and returns::.

3.21 Leaving definitions or loops
=================================

'EXIT' exits the current definition right away.  For every counted loop
that is left in this way, an 'UNLOOP' has to be performed before the
'EXIT':

     : ...
      ... u+do
        ... if
          ... unloop exit
        endif
        ...
      loop
      ... ;

   'LEAVE' leaves the innermost counted loop right away:

     : ...
      ... u+do
        ... if
          ... leave
        endif
        ...
      loop
      ... ;

   Reference: *note Calls and returns::, *note Counted Loops::.

3.22 Return Stack
=================

In addition to the data stack Forth also has a second stack, the return
stack; most Forth systems store the return addresses of procedure calls
there (thus its name).  Programmers can also use this stack:

     : foo ( n1 n2 -- )
      .s
      >r .s
      r@ .
      >r .s
      r@ .
      r> .
      r@ .
      r> . ;
     1 2 foo

   '>r' takes an element from the data stack and pushes it onto the
return stack; conversely, 'r>' moves an elementm from the return to the
data stack; 'r@' pushes a copy of the top of the return stack on the
data stack.

   Forth programmers usually use the return stack for storing data
temporarily, if using the data stack alone would be too complex, and
factoring and locals are not an option:

     : 2swap ( x1 x2 x3 x4 -- x3 x4 x1 x2 )
      rot >r rot r> ;

   The return address of the definition and the loop control parameters
of counted loops usually reside on the return stack, so you have to take
all items, that you have pushed on the return stack in a colon
definition or counted loop, from the return stack before the definition
or loop ends.  You cannot access items that you pushed on the return
stack outside some definition or loop within the definition of loop.

   If you miscount the return stack items, this usually ends in a crash:

     : crash ( n -- )
       >r ;
     5 crash

   You cannot mix using locals and using the return stack (according to
the standard; Gforth has no problem).  However, they solve the same
problems, so this shouldn't be an issue.

     Assignment: Can you rewrite any of the definitions you wrote until
     now in a better way using the return stack?

   Reference: *note Return stack::.

3.23 Memory
===========

You can create a global variable 'v' with

     variable v ( -- addr )

   'v' pushes the address of a cell in memory on the stack.  This cell
was reserved by 'variable'.  You can use '!' (store) to store values
into this cell and '@' (fetch) to load the value from the stack into
memory:

     v .
     5 v ! .s
     v @ .

   You can see a raw dump of memory with 'dump':

     v 1 cells .s dump

   'Cells ( n1 -- n2 )' gives you the number of bytes (or, more
generally, address units (aus)) that 'n1 cells' occupy.  You can also
reserve more memory:

     create v2 20 cells allot
     v2 20 cells dump

   creates a word 'v2' and reserves 20 uninitialized cells; the address
pushed by 'v2' points to the start of these 20 cells.  You can use
address arithmetic to access these cells:

     3 v2 5 cells + !
     v2 20 cells dump

   You can reserve and initialize memory with ',':

     create v3
       5 , 4 , 3 , 2 , 1 ,
     v3 @ .
     v3 cell+ @ .
     v3 2 cells + @ .
     v3 5 cells dump

     Assignment: Write a definition 'vsum ( addr u -- n )' that computes
     the sum of 'u' cells, with the first of these cells at 'addr', the
     next one at 'addr cell+' etc.

   You can also reserve memory without creating a new word:

     here 10 cells allot .
     here .

   'Here' pushes the start address of the memory area.  You should store
it somewhere, or you will have a hard time finding the memory area
again.

   'Allot' manages dictionary memory.  The dictionary memory contains
the system's data structures for words etc.  on Gforth and most other
Forth systems.  It is managed like a stack: You can free the memory that
you have just 'allot'ed with

     -10 cells allot
     here .

   Note that you cannot do this if you have created a new word in the
meantime (because then your 'allot'ed memory is no longer on the top of
the dictionary "stack").

   Alternatively, you can use 'allocate' and 'free' which allow freeing
memory in any order:

     10 cells allocate throw .s
     20 cells allocate throw .s
     swap
     free throw
     free throw

   The 'throw's deal with errors (e.g., out of memory).

   And there is also a garbage collector
(http://www.complang.tuwien.ac.at/forth/garbage-collection.zip), which
eliminates the need to 'free' memory explicitly.

   Reference: *note Memory::.

3.24 Characters and Strings
===========================

On the stack characters take up a cell, like numbers.  In memory they
have their own size (one 8-bit byte on most systems), and therefore
require their own words for memory access:

     create v4
       104 c, 97 c, 108 c, 108 c, 111 c,
     v4 4 chars + c@ .
     v4 5 chars dump

   The preferred representation of strings on the stack is 'addr
u-count', where 'addr' is the address of the first character and
'u-count' is the number of characters in the string.

     v4 5 type

   You get a string constant with

     s" hello, world" .s
     type

   Make sure you have a space between 's"' and the string; 's"' is a
normal Forth word and must be delimited with white space (try what
happens when you remove the space).

   However, this interpretive use of 's"' is quite restricted: the
string exists only until the next call of 's"' (some Forth systems keep
more than one of these strings, but usually they still have a limited
lifetime).

     s" hello," s" world" .s
     type
     type

   You can also use 's"' in a definition, and the resulting strings then
live forever (well, for as long as the definition):

     : foo s" hello," s" world" ;
     foo .s
     type
     type

     Assignment: 'Emit ( c -- )' types 'c' as character (not a number).
     Implement 'type ( addr u -- )'.

   Reference: *note Memory Blocks::.

3.25 Alignment
==============

On many processors cells have to be aligned in memory, if you want to
access them with '@' and '!' (and even if the processor does not require
alignment, access to aligned cells is faster).

   'Create' aligns 'here' (i.e., the place where the next allocation
will occur, and that the 'create'd word points to).  Likewise, the
memory produced by 'allocate' starts at an aligned address.  Adding a
number of 'cells' to an aligned address produces another aligned
address.

   However, address arithmetic involving 'char+' and 'chars' can create
an address that is not cell-aligned.  'Aligned ( addr -- a-addr )'
produces the next aligned address:

     v3 char+ aligned .s @ .
     v3 char+ .s @ .

   Similarly, 'align' advances 'here' to the next aligned address:

     create v5 97 c,
     here .
     align here .
     1000 ,

   Note that you should use aligned addresses even if your processor
does not require them, if you want your program to be portable.

   Reference: *note Address arithmetic::.

3.26 Floating Point
===================

Floating-point (FP) numbers and arithmetic in Forth works mostly as one
might expect, but there are a few things worth noting:

   The first point is not specific to Forth, but so important and yet
not universally known that I mention it here: FP numbers are not reals.
Many properties (e.g., arithmetic laws) that reals have and that one
expects of all kinds of numbers do not hold for FP numbers.  If you want
to use FP computations, you should learn about their problems and how to
avoid them; a good starting point is 'David Goldberg, What Every
Computer Scientist Should Know About Floating-Point Arithmetic
(http://docs.sun.com/source/806-3568/ncg_goldberg.html), ACM Computing
Surveys 23(1):5-48, March 1991'.

   In Forth source code literal FP numbers need an exponent, e.g.,
'1e0'; this can also be written shorter as '1e', '+1.0e+0', and many
variations in between.  The reason for this is that, for historical
reasons, Forth interprets a decimal point alone (e.g., '1.') as
indicating a double-cell integer.  Another requirement for literal FP
numbers is that the current base is decimal; with a hex base '1e' is
interpreted as an integer.

   Forth has a separate stack for FP numbers.(1)  One advantage of this
model is that cells are not in the way when accessing FP values, and
vice versa.  Forth has a set of words for manipulating the FP stack:
'fdup fswap fdrop fover frot' and (non-standard) 'fnip ftuck fpick'.

   FP arithmetic words are prefixed with 'F'.  There is the usual set
'f+ f- f* f/ f** fnegate' as well as a number of words for other
functions, e.g., 'fsqrt fsin fln fmin'.  One word that you might expect
is 'f='; but 'f=' is non-standard, because FP computation results are
usually inaccurate, so exact comparison is usually a mistake, and one
should use approximate comparison.  Unfortunately, 'f~', the standard
word for that purpose, is not well designed, so Gforth provides 'f~abs'
and 'f~rel' as well.

   And of course there are words for accessing FP numbers in memory ('f@
f!'), and for address arithmetic ('floats float+ faligned').  There are
also variants of these words with an 'sf' and 'df' prefix for accessing
IEEE format single-precision and double-precision numbers in memory;
their main purpose is for accessing external FP data (e.g., that has
been read from or will be written to a file).

   Here is an example of a dot-product word and its use:

     : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
       >r swap 2swap swap 0e r> 0 ?DO
         dup f@ over + 2swap dup f@ f* f+ over + 2swap
       LOOP
       2drop 2drop ;

     create v 1.23e f, 4.56e f, 7.89e f,

     v 1 floats  v 1 floats  3  v* f.

     Assignment: Write a program to solve a quadratic equation.  Then
     read 'Henry G. Baker, You Could Learn a Lot from a Quadratic
     (http://home.pipeline.com/~hbaker1/sigplannotices/sigcol05.ps.gz),
     ACM SIGPLAN Notices, 33(1):30-39, January 1998', and see if you can
     improve your program.  Finally, find a test case where the original
     and the improved version produce different results.

   Reference: *note Floating Point::; *note Floating point stack::;
*note Number Conversion::; *note Memory Access::; *note Address
arithmetic::.

   ---------- Footnotes ----------

   (1) Theoretically, an ANS Forth system may implement the FP stack on
the data stack, but virtually all systems implement a separate FP stack;
and programming in a way that accommodates all models is so cumbersome
that nobody does it.

3.27 Files
==========

This section gives a short introduction into how to use files inside
Forth.  It's broken up into five easy steps:

  1. Opened an ASCII text file for input
  2. Opened a file for output
  3. Read input file until string matched (or some other condition
     matched)
  4. Wrote some lines from input ( modified or not) to output
  5. Closed the files.

   Reference: *note General files::.

3.27.1 Open file for input
--------------------------

     s" foo.in"  r/o open-file throw Value fd-in

3.27.2 Create file for output
-----------------------------

     s" foo.out" w/o create-file throw Value fd-out

   The available file modes are r/o for read-only access, r/w for
read-write access, and w/o for write-only access.  You could open both
files with r/w, too, if you like.  All file words return error codes;
for most applications, it's best to pass there error codes with 'throw'
to the outer error handler.

   If you want words for opening and assigning, define them as follows:

     0 Value fd-in
     0 Value fd-out
     : open-input ( addr u -- )  r/o open-file throw to fd-in ;
     : open-output ( addr u -- )  w/o create-file throw to fd-out ;

   Usage example:

     s" foo.in" open-input
     s" foo.out" open-output

3.27.3 Scan file for a particular line
--------------------------------------

     256 Constant max-line
     Create line-buffer  max-line 2 + allot

     : scan-file ( addr u -- )
       begin
           line-buffer max-line fd-in read-line throw
       while
              >r 2dup line-buffer r> compare 0=
          until
       else
          drop
       then
       2drop ;

   'read-line ( addr u1 fd -- u2 flag ior )' reads up to u1 bytes into
the buffer at addr, and returns the number of bytes read, a flag that is
false when the end of file is reached, and an error code.

   'compare ( addr1 u1 addr2 u2 -- n )' compares two strings and returns
zero if both strings are equal.  It returns a positive number if the
first string is lexically greater, a negative if the second string is
lexically greater.

   We haven't seen this loop here; it has two exits.  Since the 'while'
exits with the number of bytes read on the stack, we have to clean up
that separately; that's after the 'else'.

   Usage example:

     s" The text I search is here" scan-file

3.27.4 Copy input to output
---------------------------

     : copy-file ( -- )
       begin
           line-buffer max-line fd-in read-line throw
       while
           line-buffer swap fd-out write-line throw
       repeat ;

3.27.5 Close files
------------------

     fd-in close-file throw
     fd-out close-file throw

   Likewise, you can put that into definitions, too:

     : close-input ( -- )  fd-in close-file throw ;
     : close-output ( -- )  fd-out close-file throw ;

     Assignment: How could you modify 'copy-file' so that it copies
     until a second line is matched?  Can you write a program that
     extracts a section of a text file, given the line that starts and
     the line that terminates that section?

3.28 Interpretation and Compilation Semantics and Immediacy
===========================================================

When a word is compiled, it behaves differently from being interpreted.
E.g., consider '+':

     1 2 + .
     : foo + ;

   These two behaviours are known as compilation and interpretation
semantics.  For normal words (e.g., '+'), the compilation semantics is
to append the interpretation semantics to the currently defined word
('foo' in the example above).  I.e., when 'foo' is executed later, the
interpretation semantics of '+' (i.e., adding two numbers) will be
performed.

   However, there are words with non-default compilation semantics,
e.g., the control-flow words like 'if'.  You can use 'immediate' to
change the compilation semantics of the last defined word to be equal to
the interpretation semantics:

     : [FOO] ( -- )
      5 . ; immediate

     [FOO]
     : bar ( -- )
       [FOO] ;
     bar
     see bar

   Two conventions to mark words with non-default compilation semantics
are names with brackets (more frequently used) and to write them all in
upper case (less frequently used).

   In Gforth (and many other systems) you can also remove the
interpretation semantics with 'compile-only' (the compilation semantics
is derived from the original interpretation semantics):

     : flip ( -- )
      6 . ; compile-only \ but not immediate
     flip

     : flop ( -- )
      flip ;
     flop

   In this example the interpretation semantics of 'flop' is equal to
the original interpretation semantics of 'flip'.

   The text interpreter has two states: in interpret state, it performs
the interpretation semantics of words it encounters; in compile state,
it performs the compilation semantics of these words.

   Among other things, ':' switches into compile state, and ';' switches
back to interpret state.  They contain the factors ']' (switch to
compile state) and '[' (switch to interpret state), that do nothing but
switch the state.

     : xxx ( -- )
       [ 5 . ]
     ;

     xxx
     see xxx

   These brackets are also the source of the naming convention mentioned
above.

   Reference: *note Interpretation and Compilation Semantics::.

3.29 Execution Tokens
=====================

'' word' gives you the execution token (XT) of a word.  The XT is a cell
representing the interpretation semantics of a word.  You can execute
this semantics with 'execute':

     ' + .s
     1 2 rot execute .

   The XT is similar to a function pointer in C. However, parameter
passing through the stack makes it a little more flexible:

     : map-array ( ... addr u xt -- ... )
     \ executes xt ( ... x -- ... ) for every element of the array starting
     \ at addr and containing u elements
       { xt }
       cells over + swap ?do
         i @ xt execute
       1 cells +loop ;

     create a 3 , 4 , 2 , -1 , 4 ,
     a 5 ' . map-array .s
     0 a 5 ' + map-array .
     s" max-n" environment? drop .s
     a 5 ' min map-array .

   You can use map-array with the XTs of words that consume one element
more than they produce.  In theory you can also use it with other XTs,
but the stack effect then depends on the size of the array, which is
hard to understand.

   Since XTs are cell-sized, you can store them in memory and manipulate
them on the stack like other cells.  You can also compile the XT into a
word with 'compile,':

     : foo1 ( n1 n2 -- n )
        [ ' + compile, ] ;
     see foo

   This is non-standard, because 'compile,' has no compilation semantics
in the standard, but it works in good Forth systems.  For the broken
ones, use

     : [compile,] compile, ; immediate

     : foo1 ( n1 n2 -- n )
        [ ' + ] [compile,] ;
     see foo

   ''' is a word with default compilation semantics; it parses the next
word when its interpretation semantics are executed, not during
compilation:

     : foo ( -- xt )
       ' ;
     see foo
     : bar ( ... "word" -- ... )
       ' execute ;
     see bar
     1 2 bar + .

   You often want to parse a word during compilation and compile its XT
so it will be pushed on the stack at run-time.  '[']' does this:

     : xt-+ ( -- xt )
       ['] + ;
     see xt-+
     1 2 xt-+ execute .

   Many programmers tend to see ''' and the word it parses as one unit,
and expect it to behave like '[']' when compiled, and are confused by
the actual behaviour.  If you are, just remember that the Forth system
just takes ''' as one unit and has no idea that it is a parsing word
(attempts to convenience programmers in this issue have usually resulted
in even worse pitfalls, see 'State'-smartness--Why it is evil and How to
Exorcise it (http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz)).

   Note that the state of the interpreter does not come into play when
creating and executing XTs.  I.e., even when you execute ''' in compile
state, it still gives you the interpretation semantics.  And whatever
that state is, 'execute' performs the semantics represented by the XT
(i.e., for XTs produced with ''' the interpretation semantics).

   Reference: *note Tokens for Words::.

3.30 Exceptions
===============

'throw ( n -- )' causes an exception unless n is zero.

     100 throw .s
     0 throw .s

   'catch ( ... xt -- ... n )' behaves similar to 'execute', but it
catches exceptions and pushes the number of the exception on the stack
(or 0, if the xt executed without exception).  If there was an
exception, the stacks have the same depth as when entering 'catch':

     .s
     3 0 ' / catch .s
     3 2 ' / catch .s

     Assignment: Try the same with 'execute' instead of 'catch'.

   'Throw' always jumps to the dynamically next enclosing 'catch', even
if it has to leave several call levels to achieve this:

     : foo 100 throw ;
     : foo1 foo ." after foo" ;
     : bar ['] foo1 catch ;
     bar .

   It is often important to restore a value upon leaving a definition,
even if the definition is left through an exception.  You can ensure
this like this:

     : ...
        save-x
        ['] word-changing-x catch ( ... n )
        restore-x
        ( ... n ) throw ;

   However, this is still not safe against, e.g., the user pressing
'Ctrl-C' when execution is between the 'catch' and 'restore-x'.

   Gforth provides an alternative exception handling syntax that is safe
against such cases: 'try ... restore ... endtry'.  If the code between
'try' and 'endtry' has an exception, the stack depths are restored, the
exception number is pushed on the stack, and the execution continues
right after 'restore'.

   The safer equivalent to the restoration code above is

     : ...
       save-x
       try
         word-changing-x 0
       restore
         restore-x
       endtry
       throw ;

   Reference: *note Exception Handling::.

3.31 Defining Words
===================

':', 'create', and 'variable' are definition words: They define other
words.  'Constant' is another definition word:

     5 constant foo
     foo .

   You can also use the prefixes '2' (double-cell) and 'f' (floating
point) with 'variable' and 'constant'.

   You can also define your own defining words.  E.g.:

     : variable ( "name" -- )
       create 0 , ;

   You can also define defining words that create words that do
something other than just producing their address:

     : constant ( n "name" -- )
       create ,
     does> ( -- n )
       ( addr ) @ ;

     5 constant foo
     foo .

   The definition of 'constant' above ends at the 'does>'; i.e., 'does>'
replaces ';', but it also does something else: It changes the last
defined word such that it pushes the address of the body of the word and
then performs the code after the 'does>' whenever it is called.

   In the example above, 'constant' uses ',' to store 5 into the body of
'foo'.  When 'foo' executes, it pushes the address of the body onto the
stack, then (in the code after the 'does>') fetches the 5 from there.

   The stack comment near the 'does>' reflects the stack effect of the
defined word, not the stack effect of the code after the 'does>' (the
difference is that the code expects the address of the body that the
stack comment does not show).

   You can use these definition words to do factoring in cases that
involve (other) definition words.  E.g., a field offset is always added
to an address.  Instead of defining

     2 cells constant offset-field1

   and using this like

     ( addr ) offset-field1 +

   you can define a definition word

     : simple-field ( n "name" -- )
       create ,
     does> ( n1 -- n1+n )
       ( addr ) @ + ;

   Definition and use of field offsets now look like this:

     2 cells simple-field field1
     create mystruct 4 cells allot
     mystruct .s field1 .s drop

   If you want to do something with the word without performing the code
after the 'does>', you can access the body of a 'create'd word with
'>body ( xt -- addr )':

     : value ( n "name" -- )
       create ,
     does> ( -- n1 )
       @ ;
     : to ( n "name" -- )
       ' >body ! ;

     5 value foo
     foo .
     7 to foo
     foo .

     Assignment: Define 'defer ( "name" -- )', which creates a word that
     stores an XT (at the start the XT of 'abort'), and upon execution
     'execute's the XT. Define 'is ( xt "name" -- )' that stores 'xt'
     into 'name', a word defined with 'defer'.  Indirect recursion is
     one application of 'defer'.

   Reference: *note User-defined Defining Words::.

3.32 Arrays and Records
=======================

Forth has no standard words for defining data structures such as arrays
and records (structs in C terminology), but you can build them yourself
based on address arithmetic.  You can also define words for defining
arrays and records (*note Defining Words: Defining Words Tutorial.).

   One of the first projects a Forth newcomer sets out upon when
learning about defining words is an array defining word (possibly for
n-dimensional arrays).  Go ahead and do it, I did it, too; you will
learn something from it.  However, don't be disappointed when you later
learn that you have little use for these words (inappropriate use would
be even worse).  I have not found a set of useful array words yet; the
needs are just too diverse, and named, global arrays (the result of
naive use of defining words) are often not flexible enough (e.g.,
consider how to pass them as parameters).  Another such project is a set
of words to help dealing with strings.

   On the other hand, there is a useful set of record words, and it has
been defined in 'compat/struct.fs'; these words are predefined in
Gforth.  They are explained in depth elsewhere in this manual (see *note
Structures::).  The 'simple-field' example above is simplified variant
of fields in this package.

3.33 'POSTPONE'
===============

You can compile the compilation semantics (instead of compiling the
interpretation semantics) of a word with 'POSTPONE':

     : MY-+ ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
      POSTPONE + ; immediate
     : foo ( n1 n2 -- n )
      MY-+ ;
     1 2 foo .
     see foo

   During the definition of 'foo' the text interpreter performs the
compilation semantics of 'MY-+', which performs the compilation
semantics of '+', i.e., it compiles '+' into 'foo'.

   This example also displays separate stack comments for the
compilation semantics and for the stack effect of the compiled code.
For words with default compilation semantics these stack effects are
usually not displayed; the stack effect of the compilation semantics is
always '( -- )' for these words, the stack effect for the compiled code
is the stack effect of the interpretation semantics.

   Note that the state of the interpreter does not come into play when
performing the compilation semantics in this way.  You can also perform
it interpretively, e.g.:

     : foo2 ( n1 n2 -- n )
      [ MY-+ ] ;
     1 2 foo .
     see foo

   However, there are some broken Forth systems where this does not
always work, and therefore this practice was been declared non-standard
in 1999.

   Here is another example for using 'POSTPONE':

     : MY-- ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
      POSTPONE negate POSTPONE + ; immediate compile-only
     : bar ( n1 n2 -- n )
       MY-- ;
     2 1 bar .
     see bar

   You can define 'ENDIF' in this way:

     : ENDIF ( Compilation: orig -- )
       POSTPONE then ; immediate

     Assignment: Write 'MY-2DUP' that has compilation semantics
     equivalent to '2dup', but compiles 'over over'.

3.34 'Literal'
==============

You cannot 'POSTPONE' numbers:

     : [FOO] POSTPONE 500 ; immediate

   Instead, you can use 'LITERAL (compilation: n --; run-time: -- n )':

     : [FOO] ( compilation: --; run-time: -- n )
       500 POSTPONE literal ; immediate

     : flip [FOO] ;
     flip .
     see flip

   'LITERAL' consumes a number at compile-time (when it's compilation
semantics are executed) and pushes it at run-time (when the code it
compiled is executed).  A frequent use of 'LITERAL' is to compile a
number computed at compile time into the current word:

     : bar ( -- n )
       [ 2 2 + ] literal ;
     see bar

     Assignment: Write ']L' which allows writing the example above as ':
     bar ( -- n ) [ 2 2 + ]L ;'

3.35 Advanced macros
====================

Reconsider 'map-array' from *note Execution Tokens: Execution Tokens
Tutorial.  It frequently performs 'execute', a relatively expensive
operation in some Forth implementations.  You can use 'compile,' and
'POSTPONE' to eliminate these 'execute's and produce a word that
contains the word to be performed directly:

     : compile-map-array ( compilation: xt -- ; run-time: ... addr u -- ... )
     \ at run-time, execute xt ( ... x -- ... ) for each element of the
     \ array beginning at addr and containing u elements
       { xt }
       POSTPONE cells POSTPONE over POSTPONE + POSTPONE swap POSTPONE ?do
         POSTPONE i POSTPONE @ xt compile,
       1 cells POSTPONE literal POSTPONE +loop ;

     : sum-array ( addr u -- n )
      0 rot rot [ ' + compile-map-array ] ;
     see sum-array
     a 5 sum-array .

   You can use the full power of Forth for generating the code; here's
an example where the code is generated in a loop:

     : compile-vmul-step ( compilation: n --; run-time: n1 addr1 -- n2 addr2 )
     \ n2=n1+(addr1)*n, addr2=addr1+cell
       POSTPONE tuck POSTPONE @
       POSTPONE literal POSTPONE * POSTPONE +
       POSTPONE swap POSTPONE cell+ ;

     : compile-vmul ( compilation: addr1 u -- ; run-time: addr2 -- n )
     \ n=v1*v2 (inner product), where the v_i are represented as addr_i u
       0 postpone literal postpone swap
       [ ' compile-vmul-step compile-map-array ]
       postpone drop ;
     see compile-vmul

     : a-vmul ( addr -- n )
     \ n=a*v, where v is a vector that's as long as a and starts at addr
      [ a 5 compile-vmul ] ;
     see a-vmul
     a a-vmul .

   This example uses 'compile-map-array' to show off, but you could also
use 'map-array' instead (try it now!).

   You can use this technique for efficient multiplication of large
matrices.  In matrix multiplication, you multiply every line of one
matrix with every column of the other matrix.  You can generate the code
for one line once, and use it for every column.  The only downside of
this technique is that it is cumbersome to recover the memory consumed
by the generated code when you are done (and in more complicated cases
it is not possible portably).

3.36 Compilation Tokens
=======================

This section is Gforth-specific.  You can skip it.

   '' word compile,' compiles the interpretation semantics.  For words
with default compilation semantics this is the same as performing the
compilation semantics.  To represent the compilation semantics of other
words (e.g., words like 'if' that have no interpretation semantics),
Gforth has the concept of a compilation token (CT, consisting of two
cells), and words 'comp'' and '[comp']'.  You can perform the
compilation semantics represented by a CT with 'execute':

     : foo2 ( n1 n2 -- n )
        [ comp' + execute ] ;
     see foo

   You can compile the compilation semantics represented by a CT with
'postpone,':

     : foo3 ( -- )
       [ comp' + postpone, ] ;
     see foo3

   '[ comp' word postpone, ]' is equivalent to 'POSTPONE word'.  'comp''
is particularly useful for words that have no interpretation semantics:

     ' if
     comp' if .s 2drop

   Reference: *note Tokens for Words::.

3.37 Wordlists and Search Order
===============================

The dictionary is not just a memory area that allows you to allocate
memory with 'allot', it also contains the Forth words, arranged in
several wordlists.  When searching for a word in a wordlist,
conceptually you start searching at the youngest and proceed towards
older words (in reality most systems nowadays use hash-tables); i.e., if
you define a word with the same name as an older word, the new word
shadows the older word.

   Which wordlists are searched in which order is determined by the
search order.  You can display the search order with 'order'.  It
displays first the search order, starting with the wordlist searched
first, then it displays the wordlist that will contain newly defined
words.

   You can create a new, empty wordlist with 'wordlist ( -- wid )':

     wordlist constant mywords

   'Set-current ( wid -- )' sets the wordlist that will contain newly
defined words (the _current_ wordlist):

     mywords set-current
     order

   Gforth does not display a name for the wordlist in 'mywords' because
this wordlist was created anonymously with 'wordlist'.

   You can get the current wordlist with 'get-current ( -- wid)'.  If
you want to put something into a specific wordlist without overall
effect on the current wordlist, this typically looks like this:

     get-current mywords set-current ( wid )
     create someword
     ( wid ) set-current

   You can write the search order with 'set-order ( wid1 .. widn n -- )'
and read it with 'get-order ( -- wid1 .. widn n )'.  The first searched
wordlist is topmost.

     get-order mywords swap 1+ set-order
     order

   Yes, the order of wordlists in the output of 'order' is reversed from
stack comments and the output of '.s' and thus unintuitive.

     Assignment: Define '>order ( wid -- )' with adds 'wid' as first
     searched wordlist to the search order.  Define 'previous ( -- )',
     which removes the first searched wordlist from the search order.
     Experiment with boundary conditions (you will see some crashes or
     situations that are hard or impossible to leave).

   The search order is a powerful foundation for providing features
similar to Modula-2 modules and C++ namespaces.  However, trying to
modularize programs in this way has disadvantages for debugging and
reuse/factoring that overcome the advantages in my experience (I don't
do huge projects, though).  These disadvantages are not so clear in
other languages/programming environments, because these languages are
not so strong in debugging and reuse.

   Reference: *note Word Lists::.

4 An Introduction to ANS Forth
******************************

The difference of this chapter from the Tutorial (*note Tutorial::) is
that it is slower-paced in its examples, but uses them to dive deep into
explaining Forth internals (not covered by the Tutorial).  Apart from
that, this chapter covers far less material.  It is suitable for reading
without using a computer.

   The primary purpose of this manual is to document Gforth.  However,
since Forth is not a widely-known language and there is a lack of
up-to-date teaching material, it seems worthwhile to provide some
introductory material.  For other sources of Forth-related information,
see *note Forth-related information::.

   The examples in this section should work on any ANS Forth; the output
shown was produced using Gforth.  Each example attempts to reproduce the
exact output that Gforth produces.  If you try out the examples (and you
should), what you should type is shown 'like this' and Gforth's response
is shown 'like this'.  The single exception is that, where the example
shows <RET> it means that you should press the "carriage return" key.
Unfortunately, some output formats for this manual cannot show the
difference between 'this' and 'this' which will make trying out the
examples harder (but not impossible).

   Forth is an unusual language.  It provides an interactive development
environment which includes both an interpreter and compiler.  Forth
programming style encourages you to break a problem down into many small
fragments ("factoring"), and then to develop and test each fragment
interactively.  Forth advocates assert that breaking the
edit-compile-test cycle used by conventional programming languages can
lead to great productivity improvements.

4.1 Introducing the Text Interpreter
====================================

When you invoke the Forth image, you will see a startup banner printed
and nothing else (if you have Gforth installed on your system, try
invoking it now, by typing 'gforth<RET>').  Forth is now running its
command line interpreter, which is called the "Text Interpreter" (also
known as the "Outer Interpreter").  (You will learn a lot about the text
interpreter as you read through this chapter, for more detail *note The
Text Interpreter::).

   Although it's not obvious, Forth is actually waiting for your input.
Type a number and press the <RET> key:

     45<RET>  ok

   Rather than give you a prompt to invite you to input something, the
text interpreter prints a status message after it has processed a line
of input.  The status message in this case ("' ok'" followed by
carriage-return) indicates that the text interpreter was able to process
all of your input successfully.  Now type something illegal:

     qwer341<RET>
     *the terminal*:2: Undefined word
     >>>qwer341<<<
     Backtrace:
     $2A95B42A20 throw
     $2A95B57FB8 no.extensions

   The exact text, other than the "Undefined word" may differ slightly
on your system, but the effect is the same; when the text interpreter
detects an error, it discards any remaining text on a line, resets
certain internal state and prints an error message.  For a detailed
description of error messages see *note Error messages::.

   The text interpreter waits for you to press carriage-return, and then
processes your input line.  Starting at the beginning of the line, it
breaks the line into groups of characters separated by spaces.  For each
group of characters in turn, it makes two attempts to do something:

   * It tries to treat it as a command.  It does this by searching a
     "name dictionary".  If the group of characters matches an entry in
     the name dictionary, the name dictionary provides the text
     interpreter with information that allows the text interpreter
     perform some actions.  In Forth jargon, we say that the group of
     characters names a "word", that the dictionary search returns an
     "execution token (xt)" corresponding to the "definition" of the
     word, and that the text interpreter executes the xt.  Often, the
     terms "word" and "definition" are used interchangeably.
   * If the text interpreter fails to find a match in the name
     dictionary, it tries to treat the group of characters as a number
     in the current number base (when you start up Forth, the current
     number base is base 10).  If the group of characters legitimately
     represents a number, the text interpreter pushes the number onto a
     stack (we'll learn more about that in the next section).

   If the text interpreter is unable to do either of these things with
any group of characters, it discards the group of characters and the
rest of the line, then prints an error message.  If the text interpreter
reaches the end of the line without error, it prints the status message
"' ok'" followed by carriage-return.

   This is the simplest command we can give to the text interpreter:

     <RET>  ok

   The text interpreter did everything we asked it to do (nothing)
without an error, so it said that everything is "' ok'".  Try a slightly
longer command:

     12 dup fred dup<RET>
     *the terminal*:3: Undefined word
     12 dup >>>fred<<< dup
     Backtrace:
     $2A95B42A20 throw
     $2A95B57FB8 no.extensions

   When you press the carriage-return key, the text interpreter starts
to work its way along the line:

   * When it gets to the space after the '2', it takes the group of
     characters '12' and looks them up in the name dictionary(1).  There
     is no match for this group of characters in the name dictionary, so
     it tries to treat them as a number.  It is able to do this
     successfully, so it puts the number, 12, "on the stack" (whatever
     that means).
   * The text interpreter resumes scanning the line and gets the next
     group of characters, 'dup'.  It looks it up in the name dictionary
     and (you'll have to take my word for this) finds it, and executes
     the word 'dup' (whatever that means).
   * Once again, the text interpreter resumes scanning the line and gets
     the group of characters 'fred'.  It looks them up in the name
     dictionary, but can't find them.  It tries to treat them as a
     number, but they don't represent any legal number.

   At this point, the text interpreter gives up and prints an error
message.  The error message shows exactly how far the text interpreter
got in processing the line.  In particular, it shows that the text
interpreter made no attempt to do anything with the final character
group, 'dup', even though we have good reason to believe that the text
interpreter would have no problem looking that word up and executing it
a second time.

   ---------- Footnotes ----------

   (1) We can't tell if it found them or not, but assume for now that it
did not

4.2 Stacks, postfix notation and parameter passing
==================================================

In procedural programming languages (like C and Pascal), the
building-block of programs is the "function" or "procedure".  These
functions or procedures are called with "explicit parameters".  For
example, in C we might write:

     total = total + new_volume(length,height,depth);

where new_volume is a function-call to another piece of code, and total,
length, height and depth are all variables.  length, height and depth
are parameters to the function-call.

   In Forth, the equivalent of the function or procedure is the
"definition" and parameters are implicitly passed between definitions
using a shared stack that is visible to the programmer.  Although Forth
does support variables, the existence of the stack means that they are
used far less often than in most other programming languages.  When the
text interpreter encounters a number, it will place ("push") it on the
stack.  There are several stacks (the actual number is
implementation-dependent ...)  and the particular stack used for any
operation is implied unambiguously by the operation being performed.
The stack used for all integer operations is called the "data stack"
and, since this is the stack used most commonly, references to "the data
stack" are often abbreviated to "the stack".

   The stacks have a last-in, first-out (LIFO) organisation.  If you
type:

     1 2 3<RET>  ok

   Then this instructs the text interpreter to placed three numbers on
the (data) stack.  An analogy for the behaviour of the stack is to take
a pack of playing cards and deal out the ace (1), 2 and 3 into a pile on
the table.  The 3 was the last card onto the pile ("last-in") and if you
take a card off the pile then, unless you're prepared to fiddle a bit,
the card that you take off will be the 3 ("first-out").  The number that
will be first-out of the stack is called the "top of stack", which is
often abbreviated to "TOS".

   To understand how parameters are passed in Forth, consider the
behaviour of the definition '+' (pronounced "plus").  You will not be
surprised to learn that this definition performs addition.  More
precisely, it adds two number together and produces a result.  Where
does it get the two numbers from?  It takes the top two numbers off the
stack.  Where does it place the result?  On the stack.  You can act-out
the behaviour of '+' with your playing cards like this:

   * Pick up two cards from the stack on the table
   * Stare at them intently and ask yourself "what is the sum of these
     two numbers"
   * Decide that the answer is 5
   * Shuffle the two cards back into the pack and find a 5
   * Put a 5 on the remaining ace that's on the table.

   If you don't have a pack of cards handy but you do have Forth
running, you can use the definition '.s' to show the current state of
the stack, without affecting the stack.  Type:

     clearstacks 1 2 3<RET> ok
     .s<RET> <3> 1 2 3  ok

   The text interpreter looks up the word 'clearstacks' and executes it;
it tidies up the stacks and removes any entries that may have been left
on it by earlier examples.  The text interpreter pushes each of the
three numbers in turn onto the stack.  Finally, the text interpreter
looks up the word '.s' and executes it.  The effect of executing '.s' is
to print the "<3>" (the total number of items on the stack) followed by
a list of all the items on the stack; the item on the far right-hand
side is the TOS.

   You can now type:

     + .s<RET> <2> 1 5  ok

which is correct; there are now 2 items on the stack and the result of
the addition is 5.

   If you're playing with cards, try doing a second addition: pick up
the two cards, work out that their sum is 6, shuffle them into the pack,
look for a 6 and place that on the table.  You now have just one item on
the stack.  What happens if you try to do a third addition?  Pick up the
first card, pick up the second card - ah!  There is no second card.
This is called a "stack underflow" and consitutes an error.  If you try
to do the same thing with Forth it often reports an error (probably a
Stack Underflow or an Invalid Memory Address error).

   The opposite situation to a stack underflow is a "stack overflow",
which simply accepts that there is a finite amount of storage space
reserved for the stack.  To stretch the playing card analogy, if you had
enough packs of cards and you piled the cards up on the table, you would
eventually be unable to add another card; you'd hit the ceiling.  Gforth
allows you to set the maximum size of the stacks.  In general, the only
time that you will get a stack overflow is because a definition has a
bug in it and is generating data on the stack uncontrollably.

   There's one final use for the playing card analogy.  If you model
your stack using a pack of playing cards, the maximum number of items on
your stack will be 52 (I assume you didn't use the Joker).  The maximum
value of any item on the stack is 13 (the King).  In fact, the only
possible numbers are positive integer numbers 1 through 13; you can't
have (for example) 0 or 27 or 3.52 or -2.  If you change the way you
think about some of the cards, you can accommodate different numbers.
For example, you could think of the Jack as representing 0, the Queen as
representing -1 and the King as representing -2.  Your range remains
unchanged (you can still only represent a total of 13 numbers) but the
numbers that you can represent are -2 through 10.

   In that analogy, the limit was the amount of information that a
single stack entry could hold, and Forth has a similar limit.  In Forth,
the size of a stack entry is called a "cell".  The actual size of a cell
is implementation dependent and affects the maximum value that a stack
entry can hold.  A Standard Forth provides a cell size of at least
16-bits, and most desktop systems use a cell size of 32-bits.

   Forth does not do any type checking for you, so you are free to
manipulate and combine stack items in any way you wish.  A convenient
way of treating stack items is as 2's complement signed integers, and
that is what Standard words like '+' do.  Therefore you can type:

     -5 12 + .s<RET> <1> 7  ok

   If you use numbers and definitions like '+' in order to turn Forth
into a great big pocket calculator, you will realise that it's rather
different from a normal calculator.  Rather than typing 2 + 3 = you had
to type 2 3 + (ignore the fact that you had to use '.s' to see the
result).  The terminology used to describe this difference is to say
that your calculator uses "Infix Notation" (parameters and operators are
mixed) whilst Forth uses "Postfix Notation" (parameters and operators
are separate), also called "Reverse Polish Notation".

   Whilst postfix notation might look confusing to begin with, it has
several important advantages:

   * it is unambiguous
   * it is more concise
   * it fits naturally with a stack-based system

   To examine these claims in more detail, consider these sums:

     6 + 5 * 4 =
     4 * 5 + 6 =

   If you're just learning maths or your maths is very rusty, you will
probably come up with the answer 44 for the first and 26 for the second.
If you are a bit of a whizz at maths you will remember the convention
that multiplication takes precendence over addition, and you'd come up
with the answer 26 both times.  To explain the answer 26 to someone who
got the answer 44, you'd probably rewrite the first sum like this:

     6 + (5 * 4) =

   If what you really wanted was to perform the addition before the
multiplication, you would have to use parentheses to force it.

   If you did the first two sums on a pocket calculator you would
probably get the right answers, unless you were very cautious and
entered them using these keystroke sequences:

   6 + 5 = * 4 = 4 * 5 = + 6 =

   Postfix notation is unambiguous because the order that the operators
are applied is always explicit; that also means that parentheses are
never required.  The operators are active (the act of quoting the
operator makes the operation occur) which removes the need for "=".

   The sum 6 + 5 * 4 can be written (in postfix notation) in two
equivalent ways:

     6 5 4 * +      or:
     5 4 * 6 +

   An important thing that you should notice about this notation is that
the order of the numbers does not change; if you want to subtract 2 from
10 you type '10 2 -'.

   The reason that Forth uses postfix notation is very simple to
explain: it makes the implementation extremely simple, and it follows
naturally from using the stack as a mechanism for passing parameters.
Another way of thinking about this is to realise that all Forth
definitions are active; they execute as they are encountered by the text
interpreter.  The result of this is that the syntax of Forth is
trivially simple.

4.3 Your first Forth definition
===============================

Until now, the examples we've seen have been trivial; we've just been
using Forth as a bigger-than-pocket calculator.  Also, each calculation
we've shown has been a "one-off" - to repeat it we'd need to type it in
again(1) In this section we'll see how to add new words to Forth's
vocabulary.

   The easiest way to create a new word is to use a "colon definition".
We'll define a few and try them out before worrying too much about how
they work.  Try typing in these examples; be careful to copy the spaces
accurately:

     : add-two 2 + . ;
     : greet ." Hello and welcome" ;
     : demo 5 add-two ;

Now try them out:

     greet<RET> Hello and welcome  ok
     greet greet<RET> Hello and welcomeHello and welcome  ok
     4 add-two<RET> 6  ok
     demo<RET> 7  ok
     9 greet demo add-two<RET> Hello and welcome7 11  ok

   The first new thing that we've introduced here is the pair of words
':' and ';'.  These are used to start and terminate a new definition,
respectively.  The first word after the ':' is the name for the new
definition.

   As you can see from the examples, a definition is built up of words
that have already been defined; Forth makes no distinction between
definitions that existed when you started the system up, and those that
you define yourself.

   The examples also introduce the words '.' (dot), '."' (dot-quote) and
'dup' (dewp).  Dot takes the value from the top of the stack and
displays it.  It's like '.s' except that it only displays the top item
of the stack and it is destructive; after it has executed, the number is
no longer on the stack.  There is always one space printed after the
number, and no spaces before it.  Dot-quote defines a string (a sequence
of characters) that will be printed when the word is executed.  The
string can contain any printable characters except '"'.  A '"' has a
special function; it is not a Forth word but it acts as a delimiter (the
way that delimiters work is described in the next section).  Finally,
'dup' duplicates the value at the top of the stack.  Try typing '5 dup
.s' to see what it does.

   We already know that the text interpreter searches through the
dictionary to locate names.  If you've followed the examples earlier,
you will already have a definition called 'add-two'.  Lets try modifying
it by typing in a new definition:

     : add-two dup . ." + 2 =" 2 + . ;<RET> redefined add-two  ok

   Forth recognised that we were defining a word that already exists,
and printed a message to warn us of that fact.  Let's try out the new
definition:

     9 add-two<RET> 9 + 2 =11  ok

All that we've actually done here, though, is to create a new
definition, with a particular name.  The fact that there was already a
definition with the same name did not make any difference to the way
that the new definition was created (except that Forth printed a warning
message).  The old definition of add-two still exists (try 'demo' again
to see that this is true).  Any new definition will use the new
definition of 'add-two', but old definitions continue to use the version
that already existed at the time that they were 'compiled'.

   Before you go on to the next section, try defining and redefining
some words of your own.

   ---------- Footnotes ----------

   (1) That's not quite true.  If you press the up-arrow key on your
keyboard you should be able to scroll back to any earlier command, edit
it and re-enter it.

4.4 How does that work?
=======================

Now we're going to take another look at the definition of 'add-two' from
the previous section.  From our knowledge of the way that the text
interpreter works, we would have expected this result when we tried to
define 'add-two':

     : add-two 2 + . ;<RET>
     *the terminal*:4: Undefined word
     : >>>add-two<<< 2 + . ;

   The reason that this didn't happen is bound up in the way that ':'
works.  The word ':' does two special things.  The first special thing
that it does prevents the text interpreter from ever seeing the
characters 'add-two'.  The text interpreter uses a variable called '>IN'
(pronounced "to-in") to keep track of where it is in the input line.
When it encounters the word ':' it behaves in exactly the same way as it
does for any other word; it looks it up in the name dictionary, finds
its xt and executes it.  When ':' executes, it looks at the input
buffer, finds the word 'add-two' and advances the value of '>IN' to
point past it.  It then does some other stuff associated with creating
the new definition (including creating an entry for 'add-two' in the
name dictionary).  When the execution of ':' completes, control returns
to the text interpreter, which is oblivious to the fact that it has been
tricked into ignoring part of the input line.

   Words like ':' - words that advance the value of '>IN' and so prevent
the text interpreter from acting on the whole of the input line - are
called "parsing words".

   The second special thing that ':' does is change the value of a
variable called 'state', which affects the way that the text interpreter
behaves.  When Gforth starts up, 'state' has the value 0, and the text
interpreter is said to be "interpreting".  During a colon definition
(started with ':'), 'state' is set to -1 and the text interpreter is
said to be "compiling".

   In this example, the text interpreter is compiling when it processes
the string "'2 + . ;'".  It still breaks the string down into character
sequences in the same way.  However, instead of pushing the number '2'
onto the stack, it lays down ("compiles") some magic into the definition
of 'add-two' that will make the number '2' get pushed onto the stack
when 'add-two' is "executed".  Similarly, the behaviours of '+' and '.'
are also compiled into the definition.

   One category of words don't get compiled.  These so-called "immediate
words" get executed (performed now) regardless of whether the text
interpreter is interpreting or compiling.  The word ';' is an immediate
word.  Rather than being compiled into the definition, it executes.  Its
effect is to terminate the current definition, which includes changing
the value of 'state' back to 0.

   When you execute 'add-two', it has a "run-time effect" that is
exactly the same as if you had typed '2 + . <RET>' outside of a
definition.

   In Forth, every word or number can be described in terms of two
properties:

   * Its "interpretation semantics" describe how it will behave when the
     text interpreter encounters it in "interpret" state.  The
     interpretation semantics of a word are represented by an "execution
     token".
   * Its "compilation semantics" describe how it will behave when the
     text interpreter encounters it in "compile" state.  The compilation
     semantics of a word are represented in an implementation-dependent
     way; Gforth uses a "compilation token".

Numbers are always treated in a fixed way:

   * When the number is "interpreted", its behaviour is to push the
     number onto the stack.
   * When the number is "compiled", a piece of code is appended to the
     current definition that pushes the number when it runs.  (In other
     words, the compilation semantics of a number are to postpone its
     interpretation semantics until the run-time of the definition that
     it is being compiled into.)

   Words don't behave in such a regular way, but most have default
semantics which means that they behave like this:

   * The "interpretation semantics" of the word are to do something
     useful.
   * The "compilation semantics" of the word are to append its
     "interpretation semantics" to the current definition (so that its
     run-time behaviour is to do something useful).

   The actual behaviour of any particular word can be controlled by
using the words 'immediate' and 'compile-only' when the word is defined.
These words set flags in the name dictionary entry of the most recently
defined word, and these flags are retrieved by the text interpreter when
it finds the word in the name dictionary.

   A word that is marked as "immediate" has compilation semantics that
are identical to its interpretation semantics.  In other words, it
behaves like this:

   * The "interpretation semantics" of the word are to do something
     useful.
   * The "compilation semantics" of the word are to do something useful
     (and actually the same thing); i.e., it is executed during
     compilation.

   Marking a word as "compile-only" prohibits the text interpreter from
performing the interpretation semantics of the word directly; an attempt
to do so will generate an error.  It is never necessary to use
'compile-only' (and it is not even part of ANS Forth, though it is
provided by many implementations) but it is good etiquette to apply it
to a word that will not behave correctly (and might have unexpected
side-effects) in interpret state.  For example, it is only legal to use
the conditional word 'IF' within a definition.  If you forget this and
try to use it elsewhere, the fact that (in Gforth) it is marked as
'compile-only' allows the text interpreter to generate a helpful error
message rather than subjecting you to the consequences of your folly.

   This example shows the difference between an immediate and a
non-immediate word:

     : show-state state @ . ;
     : show-state-now show-state ; immediate
     : word1 show-state ;
     : word2 show-state-now ;

   The word 'immediate' after the definition of 'show-state-now' makes
that word an immediate word.  These definitions introduce a new word:
'@' (pronounced "fetch").  This word fetches the value of a variable,
and leaves it on the stack.  Therefore, the behaviour of 'show-state' is
to print a number that represents the current value of 'state'.

   When you execute 'word1', it prints the number 0, indicating that the
system is interpreting.  When the text interpreter compiled the
definition of 'word1', it encountered 'show-state' whose compilation
semantics are to append its interpretation semantics to the current
definition.  When you execute 'word1', it performs the interpretation
semantics of 'show-state'.  At the time that 'word1' (and therefore
'show-state') are executed, the system is interpreting.

   When you pressed <RET> after entering the definition of 'word2', you
should have seen the number -1 printed, followed by "' ok'".  When the
text interpreter compiled the definition of 'word2', it encountered
'show-state-now', an immediate word, whose compilation semantics are
therefore to perform its interpretation semantics.  It is executed
straight away (even before the text interpreter has moved on to process
another group of characters; the ';' in this example).  The effect of
executing it are to display the value of 'state' at the time that the
definition of 'word2' is being defined.  Printing -1 demonstrates that
the system is compiling at this time.  If you execute 'word2' it does
nothing at all.

   Before leaving the subject of immediate words, consider the behaviour
of '."' in the definition of 'greet', in the previous section.  This
word is both a parsing word and an immediate word.  Notice that there is
a space between '."' and the start of the text 'Hello and welcome', but
that there is no space between the last letter of 'welcome' and the '"'
character.  The reason for this is that '."' is a Forth word; it must
have a space after it so that the text interpreter can identify it.  The
'"' is not a Forth word; it is a "delimiter".  The examples earlier show
that, when the string is displayed, there is neither a space before the
'H' nor after the 'e'.  Since '."' is an immediate word, it executes at
the time that 'greet' is defined.  When it executes, its behaviour is to
search forward in the input line looking for the delimiter.  When it
finds the delimiter, it updates '>IN' to point past the delimiter.  It
also compiles some magic code into the definition of 'greet'; the xt of
a run-time routine that prints a text string.  It compiles the string
'Hello and welcome' into memory so that it is available to be printed
later.  When the text interpreter gains control, the next word it finds
in the input stream is ';' and so it terminates the definition of
'greet'.

4.5 Forth is written in Forth
=============================

When you start up a Forth compiler, a large number of definitions
already exist.  In Forth, you develop a new application using bottom-up
programming techniques to create new definitions that are defined in
terms of existing definitions.  As you create each definition you can
test and debug it interactively.

   If you have tried out the examples in this section, you will probably
have typed them in by hand; when you leave Gforth, your definitions will
be lost.  You can avoid this by using a text editor to enter Forth
source code into a file, and then loading code from the file using
'include' (*note Forth source files::).  A Forth source file is
processed by the text interpreter, just as though you had typed it in by
hand(1).

   Gforth also supports the traditional Forth alternative to using text
files for program entry (*note Blocks::).

   In common with many, if not most, Forth compilers, most of Gforth is
actually written in Forth.  All of the '.fs' files in the installation
directory(2) are Forth source files, which you can study to see examples
of Forth programming.

   Gforth maintains a history file that records every line that you type
to the text interpreter.  This file is preserved between sessions, and
is used to provide a command-line recall facility.  If you enter long
definitions by hand, you can use a text editor to paste them out of the
history file into a Forth source file for reuse at a later time (for
more information *note Command-line editing::).

   ---------- Footnotes ----------

   (1) Actually, there are some subtle differences - see *note The Text
Interpreter::.

   (2) For example, '/usr/local/share/gforth...'

4.6 Review - elements of a Forth system
=======================================

To summarise this chapter:

   * Forth programs use "factoring" to break a problem down into small
     fragments called "words" or "definitions".
   * Forth program development is an interactive process.
   * The main command loop that accepts input, and controls both
     interpretation and compilation, is called the "text interpreter"
     (also known as the "outer interpreter").
   * Forth has a very simple syntax, consisting of words and numbers
     separated by spaces or carriage-return characters.  Any additional
     syntax is imposed by "parsing words".
   * Forth uses a stack to pass parameters between words.  As a result,
     it uses postfix notation.
   * To use a word that has previously been defined, the text
     interpreter searches for the word in the "name dictionary".
   * Words have "interpretation semantics" and "compilation semantics".
   * The text interpreter uses the value of 'state' to select between
     the use of the "interpretation semantics" and the "compilation
     semantics" of a word that it encounters.
   * The relationship between the "interpretation semantics" and
     "compilation semantics" for a word depend upon the way in which the
     word was defined (for example, whether it is an "immediate" word).
   * Forth definitions can be implemented in Forth (called "high-level
     definitions") or in some other way (usually a lower-level language
     and as a result often called "low-level definitions", "code
     definitions" or "primitives").
   * Many Forth systems are implemented mainly in Forth.

4.7 Where To Go Next
====================

Amazing as it may seem, if you have read (and understood) this far, you
know almost all the fundamentals about the inner workings of a Forth
system.  You certainly know enough to be able to read and understand the
rest of this manual and the ANS Forth document, to learn more about the
facilities that Forth in general and Gforth in particular provide.  Even
scarier, you know almost enough to implement your own Forth system.
However, that's not a good idea just yet...  better to try writing some
programs in Gforth.

   Forth has such a rich vocabulary that it can be hard to know where to
start in learning it.  This section suggests a few sets of words that
are enough to write small but useful programs.  Use the word index in
this document to learn more about each word, then try it out and try to
write small definitions using it.  Start by experimenting with these
words:

   * Arithmetic: '+ - * / /MOD */ ABS INVERT'
   * Comparison: 'MIN MAX ='
   * Logic: 'AND OR XOR NOT'
   * Stack manipulation: 'DUP DROP SWAP OVER'
   * Loops and decisions: 'IF ELSE ENDIF ?DO I LOOP'
   * Input/Output: '. ." EMIT CR KEY'
   * Defining words: ': ; CREATE'
   * Memory allocation words: 'ALLOT ,'
   * Tools: 'SEE WORDS .S MARKER'

   When you have mastered those, go on to:

   * More defining words: 'VARIABLE CONSTANT VALUE TO CREATE DOES>'
   * Memory access: '@ !'

   When you have mastered these, there's nothing for it but to read
through the whole of this manual and find out what you've missed.

4.8 Exercises
=============

TODO: provide a set of programming excercises linked into the stuff done
already and into other sections of the manual.  Provide solutions to all
the exercises in a .fs file in the distribution.

5 Forth Words
*************

5.1 Notation
============

The Forth words are described in this section in the glossary notation
that has become a de-facto standard for Forth texts:

word     Stack effect   wordset   pronunciation
   Description

WORD
     The name of the word.

STACK EFFECT
     The stack effect is written in the notation 'before -- after',
     where before and after describe the top of stack entries before and
     after the execution of the word.  The rest of the stack is not
     touched by the word.  The top of stack is rightmost, i.e., a stack
     sequence is written as it is typed in.  Note that Gforth uses a
     separate floating point stack, but a unified stack notation.  Also,
     return stack effects are not shown in stack effect, but in
     Description.  The name of a stack item describes the type and/or
     the function of the item.  See below for a discussion of the types.

     All words have two stack effects: A compile-time stack effect and a
     run-time stack effect.  The compile-time stack-effect of most words
     is - .  If the compile-time stack-effect of a word deviates from
     this standard behaviour, or the word does other unusual things at
     compile time, both stack effects are shown; otherwise only the
     run-time stack effect is shown.

PRONUNCIATION
     How the word is pronounced.

WORDSET
     The ANS Forth standard is divided into several word sets.  A
     standard system need not support all of them.  Therefore, in
     theory, the fewer word sets your program uses the more portable it
     will be.  However, we suspect that most ANS Forth systems on
     personal machines will feature all word sets.  Words that are not
     defined in ANS Forth have 'gforth' or 'gforth-internal' as word
     set.  'gforth' describes words that will work in future releases of
     Gforth; 'gforth-internal' words are more volatile.  Environmental
     query strings are also displayed like words; you can recognize them
     by the 'environment' in the word set field.

DESCRIPTION
     A description of the behaviour of the word.

   The type of a stack item is specified by the character(s) the name
starts with:

'f'
     Boolean flags, i.e.  'false' or 'true'.
'c'
     Char
'w'
     Cell, can contain an integer or an address
'n'
     signed integer
'u'
     unsigned integer
'd'
     double sized signed integer
'ud'
     double sized unsigned integer
'r'
     Float (on the FP stack)
'a-'
     Cell-aligned address
'c-'
     Char-aligned address (note that a Char may have two bytes in
     Windows NT)
'f-'
     Float-aligned address
'df-'
     Address aligned for IEEE double precision float
'sf-'
     Address aligned for IEEE single precision float
'xt'
     Execution token, same size as Cell
'wid'
     Word list ID, same size as Cell
'ior, wior'
     I/O result code, cell-sized.  In Gforth, you can 'throw' iors.
'f83name'
     Pointer to a name structure
'"'
     string in the input stream (not on the stack).  The terminating
     character is a blank by default.  If it is not a blank, it is shown
     in '<>' quotes.

5.2 Case insensitivity
======================

Gforth is case-insensitive; you can enter definitions and invoke
Standard words using upper, lower or mixed case (however, *note
Implementation-defined options: core-idef.).

   ANS Forth only requires implementations to recognise Standard words
when they are typed entirely in upper case.  Therefore, a Standard
program must use upper case for all Standard words.  You can use
whatever case you like for words that you define, but in a Standard
program you have to use the words in the same case that you defined
them.

   Gforth supports case sensitivity through 'table's (case-sensitive
wordlists, *note Word Lists::).

   Two people have asked how to convert Gforth to be case-sensitive;
while we think this is a bad idea, you can change all wordlists into
tables like this:

     ' table-find forth-wordlist wordlist-map  !

   Note that you now have to type the predefined words in the same case
that we defined them, which are varying.  You may want to convert them
to your favourite case before doing this operation (I won't explain how,
because if you are even contemplating doing this, you'd better have
enough knowledge of Forth systems to know this already).

5.3 Comments
============

Forth supports two styles of comment; the traditional in-line comment,
'(' and its modern cousin, the comment to end of line; '\'.

'('       compilation 'ccc<close-paren>' - ; run-time -         core,file       "paren"
   Comment, usually till the next ')': parse and discard all subsequent
characters in the parse area until ")" is encountered.  During
interactive input, an end-of-line also acts as a comment terminator.
For file input, it does not; if the end-of-file is encountered whilst
parsing for the ")" delimiter, Gforth will generate a warning.

'\'       compilation 'ccc<newline>' - ; run-time -         core-ext,block-ext       "backslash"
   Comment till the end of the line if 'BLK' contains 0 (i.e., while not
loading a block), parse and discard the remainder of the parse area.
Otherwise, parse and discard all subsequent characters in the parse area
corresponding to the current line.

'\G'       compilation 'ccc<newline>' - ; run-time -         gforth       "backslash-gee"
   Equivalent to '\' but used as a tag to annotate definition comments
into documentation.

5.4 Boolean Flags
=================

A Boolean flag is cell-sized.  A cell with all bits clear represents the
flag 'false' and a flag with all bits set represents the flag 'true'.
Words that check a flag (for example, 'IF') will treat a cell that has
any bit set as 'true'.

'true'       - f         core-ext       "true"
   'Constant' - f is a cell with all bits set.

'false'       - f         core-ext       "false"
   'Constant' - f is a cell with all bits clear.

'on'       a-addr -         gforth       "on"
   Set the (value of the) variable at a-addr to 'true'.

'off'       a-addr -         gforth       "off"
   Set the (value of the) variable at a-addr to 'false'.

5.5 Arithmetic
==============

Forth arithmetic is not checked, i.e., you will not hear about integer
overflow on addition or multiplication, you may hear about division by
zero if you are lucky.  The operator is written after the operands, but
the operands are still in the original order.  I.e., the infix '2-1'
corresponds to '2 1 -'.  Forth offers a variety of division operators.
If you perform division with potentially negative operands, you do not
want to use '/' or '/mod' with its undefined behaviour, but rather
'fm/mod' or 'sm/mod' (probably the former, *note Mixed precision::).

5.5.1 Single precision
----------------------

By default, numbers in Forth are single-precision integers that are one
cell in size.  They can be signed or unsigned, depending upon how you
treat them.  For the rules used by the text interpreter for recognising
single-precision integers see *note Number Conversion::.

   These words are all defined for signed operands, but some of them
also work for unsigned numbers: '+', '1+', '-', '1-', '*'.

'+'       n1 n2 - n        core       "plus"

'1+'       n1 - n2        core       "one-plus"

'under+'       n1 n2 n3 - n n2        gforth       "under-plus"
   add n3 to n1 (giving n)

'-'       n1 n2 - n        core       "minus"

'1-'       n1 - n2        core       "one-minus"

'*'       n1 n2 - n        core       "star"

'/'       n1 n2 - n        core       "slash"

'mod'       n1 n2 - n        core       "mod"

'/mod'       n1 n2 - n3 n4        core       "slash-mod"

'negate'       n1 - n2        core       "negate"

'abs'       n - u        core       "abs"

'min'       n1 n2 - n        core       "min"

'max'       n1 n2 - n        core       "max"

'FLOORED'       - f         environment       "FLOORED"
   True if '/' etc.  perform floored division

5.5.2 Double precision
----------------------

For the rules used by the text interpreter for recognising
double-precision integers, see *note Number Conversion::.

   A double precision number is represented by a cell pair, with the
most significant cell at the TOS. It is trivial to convert an unsigned
single to a double: simply push a '0' onto the TOS. Since numbers are
represented by Gforth using 2's complement arithmetic, converting a
signed single to a (signed) double requires sign-extension across the
most significant cell.  This can be achieved using 's>d'.  The moral of
the story is that you cannot convert a number without knowing whether it
represents an unsigned or a signed number.

   These words are all defined for signed operands, but some of them
also work for unsigned numbers: 'd+', 'd-'.

's>d'       n - d         core       "s-to-d"

'd>s'       d - n         double       "d-to-s"

'd+'       d1 d2 - d        double       "d-plus"

'd-'       d1 d2 - d        double       "d-minus"

'dnegate'       d1 - d2        double       "d-negate"

'dabs'       d - ud         double       "d-abs"

'dmin'       d1 d2 - d         double       "d-min"

'dmax'       d1 d2 - d         double       "d-max"

5.5.3 Bitwise operations
------------------------

'and'       w1 w2 - w        core       "and"

'or'       w1 w2 - w        core       "or"

'xor'       w1 w2 - w        core       "x-or"

'invert'       w1 - w2        core       "invert"

'lshift'       u1 n - u2        core       "l-shift"

'rshift'       u1 n - u2        core       "r-shift"
   Logical shift right by n bits.

'2*'       n1 - n2        core       "two-star"
   Shift left by 1; also works on unsigned numbers

'd2*'       d1 - d2        double       "d-two-star"
   Shift left by 1; also works on unsigned numbers

'2/'       n1 - n2        core       "two-slash"
   Arithmetic shift right by 1.  For signed numbers this is a floored
division by 2 (note that '/' not necessarily floors).

'd2/'       d1 - d2        double       "d-two-slash"
   Arithmetic shift right by 1.  For signed numbers this is a floored
division by 2.

5.5.4 Numeric comparison
------------------------

Note that the words that compare for equality ('= <> 0= 0<> d= d<> d0=
d0<>') work for for both signed and unsigned numbers.

'<'       n1 n2 - f        core       "less-than"

'<='       n1 n2 - f        gforth       "less-or-equal"

'<>'       n1 n2 - f        core-ext       "not-equals"

'='       n1 n2 - f        core       "equals"

'>'       n1 n2 - f        core       "greater-than"

'>='       n1 n2 - f        gforth       "greater-or-equal"

'0<'       n - f        core       "zero-less-than"

'0<='       n - f        gforth       "zero-less-or-equal"

'0<>'       n - f        core-ext       "zero-not-equals"

'0='       n - f        core       "zero-equals"

'0>'       n - f        core-ext       "zero-greater-than"

'0>='       n - f        gforth       "zero-greater-or-equal"

'u<'       u1 u2 - f        core       "u-less-than"

'u<='       u1 u2 - f        gforth       "u-less-or-equal"

'u>'       u1 u2 - f        core-ext       "u-greater-than"

'u>='       u1 u2 - f        gforth       "u-greater-or-equal"

'within'       u1 u2 u3 - f        core-ext       "within"
   u2=<u1<u3 or: u3=<u2 and u1 is not in [u3,u2).  This works for
unsigned and signed numbers (but not a mixture).  Another way to think
about this word is to consider the numbers as a circle (wrapping around
from 'max-u' to 0 for unsigned, and from 'max-n' to min-n for signed
numbers); now consider the range from u2 towards increasing numbers up
to and excluding u3 (giving an empty range if u2=u3); if u1 is in this
range, 'within' returns true.

'd<'       d1 d2 - f        double       "d-less-than"

'd<='       d1 d2 - f        gforth       "d-less-or-equal"

'd<>'       d1 d2 - f        gforth       "d-not-equals"

'd='       d1 d2 - f        double       "d-equals"

'd>'       d1 d2 - f        gforth       "d-greater-than"

'd>='       d1 d2 - f        gforth       "d-greater-or-equal"

'd0<'       d - f        double       "d-zero-less-than"

'd0<='       d - f        gforth       "d-zero-less-or-equal"

'd0<>'       d - f        gforth       "d-zero-not-equals"

'd0='       d - f        double       "d-zero-equals"

'd0>'       d - f        gforth       "d-zero-greater-than"

'd0>='       d - f        gforth       "d-zero-greater-or-equal"

'du<'       ud1 ud2 - f        double-ext       "d-u-less-than"

'du<='       ud1 ud2 - f        gforth       "d-u-less-or-equal"

'du>'       ud1 ud2 - f        gforth       "d-u-greater-than"

'du>='       ud1 ud2 - f        gforth       "d-u-greater-or-equal"

5.5.5 Mixed precision
---------------------

'm+'       d1 n - d2        double       "m-plus"

'*/'       n1 n2 n3 - n4        core       "star-slash"
   n4=(n1*n2)/n3, with the intermediate result being double.

'*/mod'       n1 n2 n3 - n4 n5        core       "star-slash-mod"
   n1*n2=n3*n5+n4, with the intermediate result (n1*n2) being double.

'm*'       n1 n2 - d        core       "m-star"

'um*'       u1 u2 - ud        core       "u-m-star"

'm*/'       d1 n2 u3 - dquot         double       "m-star-slash"
   dquot=(d1*n2)/u3, with the intermediate result being
triple-precision.  In ANS Forth u3 can only be a positive signed number.

'um/mod'       ud u1 - u2 u3        core       "u-m-slash-mod"
   ud=u3*u1+u2, u1>u2>=0

'fm/mod'       d1 n1 - n2 n3        core       "f-m-slash-mod"
   Floored division: d1 = n3*n1+n2, n1>n2>=0 or 0>=n2>n1.

'sm/rem'       d1 n1 - n2 n3        core       "s-m-slash-rem"
   Symmetric division: d1 = n3*n1+n2, sign(n2)=sign(d1) or 0.

5.5.6 Floating Point
--------------------

For the rules used by the text interpreter for recognising
floating-point numbers see *note Number Conversion::.

   Gforth has a separate floating point stack, but the documentation
uses the unified notation.(1)

   Floating point numbers have a number of unpleasant surprises for the
unwary (e.g., floating point addition is not associative) and even a few
for the wary.  You should not use them unless you know what you are
doing or you don't care that the results you get are totally bogus.  If
you want to learn about the problems of floating point numbers (and how
to avoid them), you might start with 'David Goldberg, What Every
Computer Scientist Should Know About Floating-Point Arithmetic
(http://docs.sun.com/source/806-3568/ncg_goldberg.html), ACM Computing
Surveys 23(1):5-48, March 1991'.

'd>f'       d - r        float       "d-to-f"

'f>d'       r - d        float       "f-to-d"

'f+'       r1 r2 - r3        float       "f-plus"

'f-'       r1 r2 - r3        float       "f-minus"

'f*'       r1 r2 - r3        float       "f-star"

'f/'       r1 r2 - r3        float       "f-slash"

'fnegate'       r1 - r2        float       "f-negate"

'fabs'       r1 - r2        float-ext       "f-abs"

'fmax'       r1 r2 - r3        float       "f-max"

'fmin'       r1 r2 - r3        float       "f-min"

'floor'       r1 - r2        float       "floor"
   Round towards the next smaller integral value, i.e., round toward
negative infinity.

'fround'       r1 - r2        float       "f-round"
   Round to the nearest integral value.

'f**'       r1 r2 - r3        float-ext       "f-star-star"
   r3 is r1 raised to the r2th power.

'fsqrt'       r1 - r2        float-ext       "f-square-root"

'fexp'       r1 - r2        float-ext       "f-e-x-p"

'fexpm1'       r1 - r2        float-ext       "f-e-x-p-m-one"
   r2=e**r1-1

'fln'       r1 - r2        float-ext       "f-l-n"

'flnp1'       r1 - r2        float-ext       "f-l-n-p-one"
   r2=ln(r1+1)

'flog'       r1 - r2        float-ext       "f-log"
   The decimal logarithm.

'falog'       r1 - r2        float-ext       "f-a-log"
   r2=10**r1

'f2*'       r1 - r2         gforth       "f2*"
   Multiply r1 by 2.0e0

'f2/'       r1 - r2         gforth       "f2/"
   Multiply r1 by 0.5e0

'1/f'       r1 - r2         gforth       "1/f"
   Divide 1.0e0 by r1.

'precision'       - u         float-ext       "precision"
   u is the number of significant digits currently used by 'F.'  'FE.'
and 'FS.'

'set-precision'       u -         float-ext       "set-precision"
   Set the number of significant digits currently used by 'F.'  'FE.'
and 'FS.' to u.

   Angles in floating point operations are given in radians (a full
circle has 2 pi radians).

'fsin'       r1 - r2        float-ext       "f-sine"

'fcos'       r1 - r2        float-ext       "f-cos"

'fsincos'       r1 - r2 r3        float-ext       "f-sine-cos"
   r2=sin(r1), r3=cos(r1)

'ftan'       r1 - r2        float-ext       "f-tan"

'fasin'       r1 - r2        float-ext       "f-a-sine"

'facos'       r1 - r2        float-ext       "f-a-cos"

'fatan'       r1 - r2        float-ext       "f-a-tan"

'fatan2'       r1 r2 - r3        float-ext       "f-a-tan-two"
   r1/r2=tan(r3).  ANS Forth does not require, but probably intends this
to be the inverse of 'fsincos'.  In gforth it is.

'fsinh'       r1 - r2        float-ext       "f-cinch"

'fcosh'       r1 - r2        float-ext       "f-cosh"

'ftanh'       r1 - r2        float-ext       "f-tan-h"

'fasinh'       r1 - r2        float-ext       "f-a-cinch"

'facosh'       r1 - r2        float-ext       "f-a-cosh"

'fatanh'       r1 - r2        float-ext       "f-a-tan-h"

'pi'       - r         gforth       "pi"
   'Fconstant' - r is the value pi; the ratio of a circle's area to its
diameter.

   One particular problem with floating-point arithmetic is that
comparison for equality often fails when you would expect it to succeed.
For this reason approximate equality is often preferred (but you still
have to know what you are doing).  Also note that IEEE NaNs may compare
differently from what you might expect.  The comparison words are:

'f~rel'       r1 r2 r3 - flag         gforth       "f~rel"
   Approximate equality with relative error: |r1-r2|<r3*|r1+r2|.

'f~abs'       r1 r2 r3 - flag         gforth       "f~abs"
   Approximate equality with absolute error: |r1-r2|<r3.

'f~'       r1 r2 r3 - flag         float-ext       "f-proximate"
   ANS Forth medley for comparing r1 and r2 for equality: r3>0: 'f~abs';
r3=0: bitwise comparison; r3<0: 'fnegate f~rel'.

'f='       r1 r2 - f        gforth       "f-equals"

'f<>'       r1 r2 - f        gforth       "f-not-equals"

'f<'       r1 r2 - f        float       "f-less-than"

'f<='       r1 r2 - f        gforth       "f-less-or-equal"

'f>'       r1 r2 - f        gforth       "f-greater-than"

'f>='       r1 r2 - f        gforth       "f-greater-or-equal"

'f0<'       r - f        float       "f-zero-less-than"

'f0<='       r - f        gforth       "f-zero-less-or-equal"

'f0<>'       r - f        gforth       "f-zero-not-equals"

'f0='       r - f        float       "f-zero-equals"

'f0>'       r - f        gforth       "f-zero-greater-than"

'f0>='       r - f        gforth       "f-zero-greater-or-equal"

   ---------- Footnotes ----------

   (1) It's easy to generate the separate notation from that by just
separating the floating-point numbers out: e.g.  '( n r1 u r2 -- r3 )'
becomes '( n u -- ) ( F: r1 r2 -- r3 )'.

5.6 Stack Manipulation
======================

Gforth maintains a number of separate stacks:

   * A data stack (also known as the "parameter stack") - for
     characters, cells, addresses, and double cells.

   * A floating point stack - for holding floating point (FP) numbers.

   * A return stack - for holding the return addresses of colon
     definitions and other (non-FP) data.

   * A locals stack - for holding local variables.

5.6.1 Data stack
----------------

'drop'       w -        core       "drop"

'nip'       w1 w2 - w2        core-ext       "nip"

'dup'       w - w w        core       "dupe"

'over'       w1 w2 - w1 w2 w1        core       "over"

'tuck'       w1 w2 - w2 w1 w2        core-ext       "tuck"

'swap'       w1 w2 - w2 w1        core       "swap"

'pick'       S:... u - S:... w        core-ext       "pick"
   Actually the stack effect is ' x0 ... xu u -- x0 ... xu x0 '.

'rot'       w1 w2 w3 - w2 w3 w1        core       "rote"

'-rot'       w1 w2 w3 - w3 w1 w2        gforth       "not-rote"

'?dup'       w - S:... w        core       "question-dupe"
   Actually the stack effect is: '( w -- 0 | w w )'.  It performs a
'dup' if w is nonzero.

'roll'       x0 x1 .. xn n - x1 .. xn x0         core-ext       "roll"

'2drop'       w1 w2 -        core       "two-drop"

'2nip'       w1 w2 w3 w4 - w3 w4        gforth       "two-nip"

'2dup'       w1 w2 - w1 w2 w1 w2        core       "two-dupe"

'2over'       w1 w2 w3 w4 - w1 w2 w3 w4 w1 w2        core       "two-over"

'2tuck'       w1 w2 w3 w4 - w3 w4 w1 w2 w3 w4        gforth       "two-tuck"

'2swap'       w1 w2 w3 w4 - w3 w4 w1 w2        core       "two-swap"

'2rot'       w1 w2 w3 w4 w5 w6 - w3 w4 w5 w6 w1 w2        double-ext       "two-rote"

5.6.2 Floating point stack
--------------------------

Whilst every sane Forth has a separate floating-point stack, it is not
strictly required; an ANS Forth system could theoretically keep
floating-point numbers on the data stack.  As an additional difficulty,
you don't know how many cells a floating-point number takes.  It is
reportedly possible to write words in a way that they work also for a
unified stack model, but we do not recommend trying it.  Instead, just
say that your program has an environmental dependency on a separate
floating-point stack.

'floating-stack'       - n         environment       "floating-stack"
   N is non-zero, showing that Gforth maintains a separate
floating-point stack of depth N.

'fdrop'       r -        float       "f-drop"

'fnip'       r1 r2 - r2        gforth       "f-nip"

'fdup'       r - r r        float       "f-dupe"

'fover'       r1 r2 - r1 r2 r1        float       "f-over"

'ftuck'       r1 r2 - r2 r1 r2        gforth       "f-tuck"

'fswap'       r1 r2 - r2 r1        float       "f-swap"

'fpick'       f:... u - f:... r        gforth       "fpick"
   Actually the stack effect is ' r0 ... ru u -- r0 ... ru r0 '.

'frot'       r1 r2 r3 - r2 r3 r1        float       "f-rote"

5.6.3 Return stack
------------------

A Forth system is allowed to keep local variables on the return stack.
This is reasonable, as local variables usually eliminate the need to use
the return stack explicitly.  So, if you want to produce a standard
compliant program and you are using local variables in a word, forget
about return stack manipulations in that word (refer to the standard
document for the exact rules).

'>r'       w - R:w        core       "to-r"

'r>'       R:w - w        core       "r-from"

'r@'       - w ; R: w - w         core       "r-fetch"

'rdrop'       R:w -        gforth       "rdrop"

'2>r'       d - R:d        core-ext       "two-to-r"

'2r>'       R:d - d        core-ext       "two-r-from"

'2r@'       R:d - R:d d        core-ext       "two-r-fetch"

'2rdrop'       R:d -        gforth       "two-r-drop"

5.6.4 Locals stack
------------------

Gforth uses an extra locals stack.  It is described, along with the
reasons for its existence, in *note Locals implementation::.

5.6.5 Stack pointer manipulation
--------------------------------

'sp0'       - a-addr         gforth       "sp0"
   'User' variable - initial value of the data stack pointer.  OBSOLETE
alias of 'sp0'

'sp@'       S:... - a-addr        gforth       "sp-fetch"

'sp!'       a-addr - S:...        gforth       "sp-store"

'fp0'       - a-addr         gforth       "fp0"
   'User' variable - initial value of the floating-point stack pointer.

'fp@'       f:... - f-addr        gforth       "fp-fetch"

'fp!'       f-addr - f:...        gforth       "fp-store"

'rp0'       - a-addr         gforth       "rp0"
   'User' variable - initial value of the return stack pointer.
OBSOLETE alias of 'rp0'

'rp@'       - a-addr        gforth       "rp-fetch"

'rp!'       a-addr -        gforth       "rp-store"

'lp0'       - a-addr         gforth       "lp0"
   'User' variable - initial value of the locals stack pointer.
OBSOLETE alias of 'lp0'

'lp@'       - addr         gforth       "lp-fetch"

'lp!'       c-addr -        gforth       "lp-store"

5.7 Memory
==========

In addition to the standard Forth memory allocation words, there is also
a garbage collector
(http://www.complang.tuwien.ac.at/forth/garbage-collection.zip).

5.7.1 ANS Forth and Gforth memory models
----------------------------------------

ANS Forth considers a Forth system as consisting of several address
spaces, of which only "data space" is managed and accessible with the
memory words.  Memory not necessarily in data space includes the stacks,
the code (called code space) and the headers (called name space).  In
Gforth everything is in data space, but the code for the primitives is
usually read-only.

   Data space is divided into a number of areas: The (data space portion
of the) dictionary(1), the heap, and a number of system-allocated
buffers.

   In ANS Forth data space is also divided into contiguous regions.  You
can only use address arithmetic within a contiguous region, not between
them.  Usually each allocation gives you one contiguous region, but the
dictionary allocation words have additional rules (*note Dictionary
allocation::).

   Gforth provides one big address space, and address arithmetic can be
performed between any addresses.  However, in the dictionary headers or
code are interleaved with data, so almost the only contiguous data space
regions there are those described by ANS Forth as contiguous; but you
can be sure that the dictionary is allocated towards increasing
addresses even between contiguous regions.  The memory order of
allocations in the heap is platform-dependent (and possibly different
from one run to the next).

   ---------- Footnotes ----------

   (1) Sometimes, the term "dictionary" is used to refer to the search
data structure embodied in word lists and headers, because it is used
for looking up names, just as you would in a conventional dictionary.

5.7.2 Dictionary allocation
---------------------------

Dictionary allocation is a stack-oriented allocation scheme, i.e., if
you want to deallocate X, you also deallocate everything allocated after
X.

   The allocations using the words below are contiguous and grow the
region towards increasing addresses.  Other words that allocate
dictionary memory of any kind (i.e., defining words including ':noname')
end the contiguous region and start a new one.

   In ANS Forth only 'create'd words are guaranteed to produce an
address that is the start of the following contiguous region.  In
particular, the cell allocated by 'variable' is not guaranteed to be
contiguous with following 'allot'ed memory.

   You can deallocate memory by using 'allot' with a negative argument
(with some restrictions, see 'allot').  For larger deallocations use
'marker'.

'here'       - addr         core       "here"
   Return the address of the next free location in data space.

'unused'       - u         core-ext       "unused"
   Return the amount of free space remaining (in address units) in the
region addressed by 'here'.

'allot'       n -         core       "allot"
   Reserve n address units of data space without initialization.  n is a
signed number, passing a negative n releases memory.  In ANS Forth you
can only deallocate memory from the current contiguous region in this
way.  In Gforth you can deallocate anything in this way but named words.
The system does not check this restriction.

'c,'       c -         core       "c-comma"
   Reserve data space for one char and store c in the space.

'f,'       f -         gforth       "f,"
   Reserve data space for one floating-point number and store f in the
space.

','       w -         core       "comma"
   Reserve data space for one cell and store w in the space.

'2,'       w1 w2 -         gforth       "2,"
   Reserve data space for two cells and store the double w1 w2 there, w2
first (lower address).

   Memory accesses have to be aligned (*note Address arithmetic::).  So
of course you should allocate memory in an aligned way, too.  I.e.,
before allocating allocating a cell, 'here' must be cell-aligned, etc.
The words below align 'here' if it is not already.  Basically it is only
already aligned for a type, if the last allocation was a multiple of the
size of this type and if 'here' was aligned for this type before.

   After freshly 'create'ing a word, 'here' is 'align'ed in ANS Forth
('maxalign'ed in Gforth).

'align'       -         core       "align"
   If the data-space pointer is not aligned, reserve enough space to
align it.

'falign'       -         float       "f-align"
   If the data-space pointer is not float-aligned, reserve enough space
to align it.

'sfalign'       -         float-ext       "s-f-align"
   If the data-space pointer is not single-float-aligned, reserve enough
space to align it.

'dfalign'       -         float-ext       "d-f-align"
   If the data-space pointer is not double-float-aligned, reserve enough
space to align it.

'maxalign'       -         gforth       "maxalign"
   Align data-space pointer for all alignment requirements.

'cfalign'       -         gforth       "cfalign"
   Align data-space pointer for code field requirements (i.e., such that
the corresponding body is maxaligned).

5.7.3 Heap allocation
---------------------

Heap allocation supports deallocation of allocated memory in any order.
Dictionary allocation is not affected by it (i.e., it does not end a
contiguous region).  In Gforth, these words are implemented using the
standard C library calls malloc(), free() and resize().

   The memory region produced by one invocation of 'allocate' or
'resize' is internally contiguous.  There is no contiguity between such
a region and any other region (including others allocated from the
heap).

'allocate'       u - a-addr wior        memory       "allocate"
   Allocate u address units of contiguous data space.  The initial
contents of the data space is undefined.  If the allocation is
successful, a-addr is the start address of the allocated region and wior
is 0.  If the allocation fails, a-addr is undefined and wior is a
non-zero I/O result code.

'free'       a-addr - wior        memory       "free"
   Return the region of data space starting at a-addr to the system.
The region must originally have been obtained using 'allocate' or
'resize'.  If the operational is successful, wior is 0.  If the
operation fails, wior is a non-zero I/O result code.

'resize'       a-addr1 u - a-addr2 wior        memory       "resize"
   Change the size of the allocated area at a-addr1 to u address units,
possibly moving the contents to a different area.  a-addr2 is the
address of the resulting area.  If the operation is successful, wior is
0.  If the operation fails, wior is a non-zero I/O result code.  If
a-addr1 is 0, Gforth's (but not the Standard) 'resize' 'allocate's u
address units.

5.7.4 Memory Access
-------------------

'@'       a-addr - w        core       "fetch"
   w is the cell stored at a_addr.

'!'       w a-addr -        core       "store"
   Store w into the cell at a-addr.

'+!'       n a-addr -        core       "plus-store"
   Add n to the cell at a-addr.

'c@'       c-addr - c        core       "c-fetch"
   c is the char stored at c_addr.

'c!'       c c-addr -        core       "c-store"
   Store c into the char at c-addr.

'2@'       a-addr - w1 w2        core       "two-fetch"
   w2 is the content of the cell stored at a-addr, w1 is the content of
the next cell.

'2!'       w1 w2 a-addr -        core       "two-store"
   Store w2 into the cell at c-addr and w1 into the next cell.

'f@'       f-addr - r        float       "f-fetch"
   r is the float at address f-addr.

'f!'       r f-addr -        float       "f-store"
   Store r into the float at address f-addr.

'sf@'       sf-addr - r        float-ext       "s-f-fetch"
   Fetch the single-precision IEEE floating-point value r from the
address sf-addr.

'sf!'       r sf-addr -        float-ext       "s-f-store"
   Store r as single-precision IEEE floating-point value to the address
sf-addr.

'df@'       df-addr - r        float-ext       "d-f-fetch"
   Fetch the double-precision IEEE floating-point value r from the
address df-addr.

'df!'       r df-addr -        float-ext       "d-f-store"
   Store r as double-precision IEEE floating-point value to the address
df-addr.

'sw@'       c-addr - n        gforth       "s-w-fetch"
   n is the sign-extended 16-bit value stored at c_addr.

'uw@'       c-addr - u        gforth       "u-w-fetch"
   u is the zero-extended 16-bit value stored at c_addr.

'w!'       w c-addr -        gforth       "w-store"
   Store the bottom 16 bits of w at c_addr.

'sl@'       c-addr - n        gforth       "s-l-fetch"
   n is the sign-extended 32-bit value stored at c_addr.

'ul@'       c-addr - u        gforth       "u-l-fetch"
   u is the zero-extended 32-bit value stored at c_addr.

'l!'       w c-addr -        gforth       "l-store"
   Store the bottom 32 bits of w at c_addr.

5.7.5 Address arithmetic
------------------------

Address arithmetic is the foundation on which you can build data
structures like arrays, records (*note Structures::) and objects (*note
Object-oriented Forth::).

   ANS Forth does not specify the sizes of the data types.  Instead, it
offers a number of words for computing sizes and doing address
arithmetic.  Address arithmetic is performed in terms of address units
(aus); on most systems the address unit is one byte.  Note that a
character may have more than one au, so 'chars' is no noop (on platforms
where it is a noop, it compiles to nothing).

   The basic address arithmetic words are '+' and '-'.  E.g., if you
have the address of a cell, perform '1 cells +', and you will have the
address of the next cell.

   In ANS Forth you can perform address arithmetic only within a
contiguous region, i.e., if you have an address into one region, you can
only add and subtract such that the result is still within the region;
you can only subtract or compare addresses from within the same
contiguous region.  Reasons: several contiguous regions can be arranged
in memory in any way; on segmented systems addresses may have unusual
representations, such that address arithmetic only works within a
region.  Gforth provides a few more guarantees (linear address space,
dictionary grows upwards), but in general I have found it easy to stay
within contiguous regions (exception: computing and comparing to the
address just beyond the end of an array).

   ANS Forth also defines words for aligning addresses for specific
types.  Many computers require that accesses to specific data types must
only occur at specific addresses; e.g., that cells may only be accessed
at addresses divisible by 4.  Even if a machine allows unaligned
accesses, it can usually perform aligned accesses faster.

   For the performance-conscious: alignment operations are usually only
necessary during the definition of a data structure, not during the
(more frequent) accesses to it.

   ANS Forth defines no words for character-aligning addresses.  This is
not an oversight, but reflects the fact that addresses that are not
char-aligned have no use in the standard and therefore will not be
created.

   ANS Forth guarantees that addresses returned by 'CREATE'd words are
cell-aligned; in addition, Gforth guarantees that these addresses are
aligned for all purposes.

   Note that the ANS Forth word 'char' has nothing to do with address
arithmetic.

'chars'       n1 - n2         core       "chars"
   n2 is the number of address units of n1 chars.""

'char+'       c-addr1 - c-addr2        core       "char-plus"
   '1 chars +'.

'cells'       n1 - n2        core       "cells"
   n2 is the number of address units of n1 cells.

'cell+'       a-addr1 - a-addr2        core       "cell-plus"
   '1 cells +'

'cell'       - u         gforth       "cell"
   'Constant' - '1 cells'

'aligned'       c-addr - a-addr        core       "aligned"
   a-addr is the first aligned address greater than or equal to c-addr.

'floats'       n1 - n2        float       "floats"
   n2 is the number of address units of n1 floats.

'float+'       f-addr1 - f-addr2        float       "float-plus"
   '1 floats +'.

'float'       - u         gforth       "float"
   'Constant' - the number of address units corresponding to a
floating-point number.

'faligned'       c-addr - f-addr        float       "f-aligned"
   f-addr is the first float-aligned address greater than or equal to
c-addr.

'sfloats'       n1 - n2        float-ext       "s-floats"
   n2 is the number of address units of n1 single-precision IEEE
floating-point numbers.

'sfloat+'       sf-addr1 - sf-addr2         float-ext       "s-float-plus"
   '1 sfloats +'.

'sfaligned'       c-addr - sf-addr        float-ext       "s-f-aligned"
   sf-addr is the first single-float-aligned address greater than or
equal to c-addr.

'dfloats'       n1 - n2        float-ext       "d-floats"
   n2 is the number of address units of n1 double-precision IEEE
floating-point numbers.

'dfloat+'       df-addr1 - df-addr2         float-ext       "d-float-plus"
   '1 dfloats +'.

'dfaligned'       c-addr - df-addr        float-ext       "d-f-aligned"
   df-addr is the first double-float-aligned address greater than or
equal to c-addr.

'maxaligned'       addr1 - addr2         gforth       "maxaligned"
   addr2 is the first address after addr1 that satisfies all alignment
restrictions.  maxaligned"

'cfaligned'       addr1 - addr2         gforth       "cfaligned"
   addr2 is the first address after addr1 that is aligned for a code
field (i.e., such that the corresponding body is maxaligned).

'ADDRESS-UNIT-BITS'       - n         environment       "ADDRESS-UNIT-BITS"
   Size of one address unit, in bits.

'/w'       - u         gforth       "slash-w"
   address units for a 16-bit value

'/l'       - u         gforth       "slash-l"
   address units for a 32-bit value

5.7.6 Memory Blocks
-------------------

Memory blocks often represent character strings; For ways of storing
character strings in memory see *note String Formats::.  For other
string-processing words see *note Displaying characters and strings::.

   A few of these words work on address unit blocks.  In that case, you
usually have to insert 'CHARS' before the word when working on character
strings.  Most words work on character blocks, and expect a char-aligned
address.

   When copying characters between overlapping memory regions, use
'chars move' or choose carefully between 'cmove' and 'cmove>'.

'move'       c-from c-to ucount -        core       "move"
   Copy the contents of ucount aus at c-from to c-to.  'move' works
correctly even if the two areas overlap.

'erase'       addr u -         core-ext       "erase"
   Clear all bits in u aus starting at addr.

'cmove'       c-from c-to u -        string       "c-move"
   Copy the contents of ucount characters from data space at c-from to
c-to.  The copy proceeds 'char'-by-'char' from low address to high
address; i.e., for overlapping areas it is safe if c-to=<c-from.

'cmove>'       c-from c-to u -        string       "c-move-up"
   Copy the contents of ucount characters from data space at c-from to
c-to.  The copy proceeds 'char'-by-'char' from high address to low
address; i.e., for overlapping areas it is safe if c-to>=c-from.

'fill'       c-addr u c -        core       "fill"
   Store c in u chars starting at c-addr.

'blank'       c-addr u -         string       "blank"
   Store the space character into u chars starting at c-addr.

'compare'       c-addr1 u1 c-addr2 u2 - n        string       "compare"
   Compare two strings lexicographically.  If they are equal, n is 0; if
the first string is smaller, n is -1; if the first string is larger, n
is 1.  Currently this is based on the machine's character comparison.
In the future, this may change to consider the current locale and its
collation order.

'str='       c-addr1 u1 c-addr2 u2 - f         gforth       "str="

'str<'       c-addr1 u1 c-addr2 u2 - f         gforth       "str<"

'string-prefix?'       c-addr1 u1 c-addr2 u2 - f         gforth       "string-prefix?"
   Is C-ADDR2 U2 a prefix of C-ADDR1 U1?

'search'       c-addr1 u1 c-addr2 u2 - c-addr3 u3 flag         string       "search"
   Search the string specified by c-addr1, u1 for the string specified
by c-addr2, u2.  If flag is true: match was found at c-addr3 with u3
characters remaining.  If flag is false: no match was found; c-addr3, u3
are equal to c-addr1, u1.

'-trailing'       c_addr u1 - c_addr u2         string       "dash-trailing"
   Adjust the string specified by c-addr, u1 to remove all trailing
spaces.  u2 is the length of the modified string.

'/string'       c-addr1 u1 n - c-addr2 u2        string       "slash-string"
   Adjust the string specified by c-addr1, u1 to remove n characters
from the start of the string.

'bounds'       addr u - addr+u addr         gforth       "bounds"
   Given a memory block represented by starting address addr and length
u in aus, produce the end address addr+u and the start address in the
right order for 'u+do' or '?do'.

'pad'       - c-addr         core-ext       "pad"
   C-ADDR is the address of a transient region that can be used as
temporary data storage.  At least 84 characters of space is available.

5.8 Control Structures
======================

Control structures in Forth cannot be used interpretively, only in a
colon definition(1).  We do not like this limitation, but have not seen
a satisfying way around it yet, although many schemes have been
proposed.

   ---------- Footnotes ----------

   (1) To be precise, they have no interpretation semantics (*note
Interpretation and Compilation Semantics::).

5.8.1 Selection
---------------

     flag
     IF
       code
     ENDIF

   If flag is non-zero (as far as 'IF' etc.  are concerned, a cell with
any bit set represents truth) code is executed.

     flag
     IF
       code1
     ELSE
       code2
     ENDIF

   If FLAG is true, code1 is executed, otherwise code2 is executed.

   You can use 'THEN' instead of 'ENDIF'.  Indeed, 'THEN' is standard,
and 'ENDIF' is not, although it is quite popular.  We recommend using
'ENDIF', because it is less confusing for people who also know other
languages (and is not prone to reinforcing negative prejudices against
Forth in these people).  Adding 'ENDIF' to a system that only supplies
'THEN' is simple:
     : ENDIF   POSTPONE then ; immediate

   [According to 'Webster's New Encyclopedic Dictionary', "then (adv.)"
has the following meanings:
     ...  2b: following next after in order ...  3d: as a necessary
     consequence (if you were there, then you saw them).
   Forth's 'THEN' has the meaning 2b, whereas 'THEN' in Pascal and many
other programming languages has the meaning 3d.]

   Gforth also provides the words '?DUP-IF' and '?DUP-0=-IF', so you can
avoid using '?dup'.  Using these alternatives is also more efficient
than using '?dup'.  Definitions in ANS Forth for 'ENDIF', '?DUP-IF' and
'?DUP-0=-IF' are provided in 'compat/control.fs'.

     n
     CASE
       n1 OF code1 ENDOF
       n2 OF code2 ENDOF
       ...
       ( n ) default-code ( n )
     ENDCASE ( )

   Executes the first codei, where the ni is equal to n.  If no ni
matches, the optional default-code is executed.  The optional default
case can be added by simply writing the code after the last 'ENDOF'.  It
may use n, which is on top of the stack, but must not consume it.  The
value n is consumed by this construction (either by a OF that matches,
or by the ENDCASE, if no OF matches).

   Programming style note: To keep the code understandable, you should
ensure that you change the stack in the same way (wrt.  number and types
of stack items consumed and pushed) on all paths through a selection
construct.

5.8.2 Simple Loops
------------------

     BEGIN
       code1
       flag
     WHILE
       code2
     REPEAT

   code1 is executed and flag is computed.  If it is true, code2 is
executed and the loop is restarted; If flag is false, execution
continues after the 'REPEAT'.

     BEGIN
       code
       flag
     UNTIL

   code is executed.  The loop is restarted if 'flag' is false.

   Programming style note: To keep the code understandable, a complete
iteration of the loop should not change the number and types of the
items on the stacks.

     BEGIN
       code
     AGAIN

   This is an endless loop.

5.8.3 Counted Loops
-------------------

The basic counted loop is:
     limit start
     ?DO
       body
     LOOP

   This performs one iteration for every integer, starting from start
and up to, but excluding limit.  The counter, or index, can be accessed
with 'i'.  For example, the loop:
     10 0 ?DO
       i .
     LOOP
prints '0 1 2 3 4 5 6 7 8 9'

   The index of the innermost loop can be accessed with 'i', the index
of the next loop with 'j', and the index of the third loop with 'k'.

'i'       R:n - R:n n        core       "i"

'j'       R:w R:w1 R:w2 - w R:w R:w1 R:w2        core       "j"

'k'       R:w R:w1 R:w2 R:w3 R:w4 - w R:w R:w1 R:w2 R:w3 R:w4        gforth       "k"

   The loop control data are kept on the return stack, so there are some
restrictions on mixing return stack accesses and counted loop words.  In
particuler, if you put values on the return stack outside the loop, you
cannot read them inside the loop(1).  If you put values on the return
stack within a loop, you have to remove them before the end of the loop
and before accessing the index of the loop.

   There are several variations on the counted loop:

   * 'LEAVE' leaves the innermost counted loop immediately; execution
     continues after the associated 'LOOP' or 'NEXT'.  For example:

          10 0 ?DO  i DUP . 3 = IF LEAVE THEN LOOP
     prints '0 1 2 3'

   * 'UNLOOP' prepares for an abnormal loop exit, e.g., via 'EXIT'.
     'UNLOOP' removes the loop control parameters from the return stack
     so 'EXIT' can get to its return address.  For example:

          : demo 10 0 ?DO i DUP . 3 = IF UNLOOP EXIT THEN LOOP ." Done" ;
     prints '0 1 2 3'

   * If start is greater than limit, a '?DO' loop is entered (and 'LOOP'
     iterates until they become equal by wrap-around arithmetic).  This
     behaviour is usually not what you want.  Therefore, Gforth offers
     '+DO' and 'U+DO' (as replacements for '?DO'), which do not enter
     the loop if start is greater than limit; '+DO' is for signed loop
     parameters, 'U+DO' for unsigned loop parameters.

   * '?DO' can be replaced by 'DO'.  'DO' always enters the loop,
     independent of the loop parameters.  Do not use 'DO', even if you
     know that the loop is entered in any case.  Such knowledge tends to
     become invalid during maintenance of a program, and then the 'DO'
     will make trouble.

   * 'LOOP' can be replaced with 'n +LOOP'; this updates the index by n
     instead of by 1.  The loop is terminated when the border between
     limit-1 and limit is crossed.  E.g.:

          4 0 +DO  i .  2 +LOOP
     prints '0 2'

          4 1 +DO  i .  2 +LOOP
     prints '1 3'

   * The behaviour of 'n +LOOP' is peculiar when n is negative:

          -1 0 ?DO  i .  -1 +LOOP
     prints '0 -1'

          0 0 ?DO  i .  -1 +LOOP
     prints nothing.

     Therefore we recommend avoiding 'n +LOOP' with negative n.  One
     alternative is 'u -LOOP', which reduces the index by u each
     iteration.  The loop is terminated when the border between limit+1
     and limit is crossed.  Gforth also provides '-DO' and 'U-DO' for
     down-counting loops.  E.g.:

          -2 0 -DO  i .  1 -LOOP
     prints '0 -1'

          -1 0 -DO  i .  1 -LOOP
     prints '0'

          0 0 -DO  i .  1 -LOOP
     prints nothing.

   Unfortunately, '+DO', 'U+DO', '-DO', 'U-DO' and '-LOOP' are not
defined in ANS Forth.  However, an implementation for these words that
uses only standard words is provided in 'compat/loops.fs'.

   Another counted loop is:
     n
     FOR
       body
     NEXT
   This is the preferred loop of native code compiler writers who are
too lazy to optimize '?DO' loops properly.  This loop structure is not
defined in ANS Forth.  In Gforth, this loop iterates n+1 times; 'i'
produces values starting with n and ending with 0.  Other Forth systems
may behave differently, even if they support 'FOR' loops.  To avoid
problems, don't use 'FOR' loops.

   ---------- Footnotes ----------

   (1) well, not in a way that is portable.

5.8.4 Arbitrary control structures
----------------------------------

ANS Forth permits and supports using control structures in a non-nested
way.  Information about incomplete control structures is stored on the
control-flow stack.  This stack may be implemented on the Forth data
stack, and this is what we have done in Gforth.

   An orig entry represents an unresolved forward branch, a dest entry
represents a backward branch target.  A few words are the basis for
building any control structure possible (except control structures that
need storage, like calls, coroutines, and backtracking).

'IF'       compilation - orig ; run-time f -         core       "IF"

'AHEAD'       compilation - orig ; run-time -         tools-ext       "AHEAD"

'THEN'       compilation orig - ; run-time -         core       "THEN"

'BEGIN'       compilation - dest ; run-time -         core       "BEGIN"

'UNTIL'       compilation dest - ; run-time f -         core       "UNTIL"

'AGAIN'       compilation dest - ; run-time -         core-ext       "AGAIN"

'CS-PICK'       ... u - ... destu         tools-ext       "c-s-pick"

'CS-ROLL'       destu/origu .. dest0/orig0 u - .. dest0/orig0 destu/origu         tools-ext       "c-s-roll"

   The Standard words 'CS-PICK' and 'CS-ROLL' allow you to manipulate
the control-flow stack in a portable way.  Without them, you would need
to know how many stack items are occupied by a control-flow entry (many
systems use one cell.  In Gforth they currently take three, but this may
change in the future).

   Some standard control structure words are built from these words:

'ELSE'       compilation orig1 - orig2 ; run-time -         core       "ELSE"

'WHILE'       compilation dest - orig dest ; run-time f -         core       "WHILE"

'REPEAT'       compilation orig dest - ; run-time -         core       "REPEAT"

Gforth adds some more control-structure words:

'ENDIF'       compilation orig - ; run-time -         gforth       "ENDIF"

'?DUP-IF'       compilation - orig ; run-time n - n|         gforth       "question-dupe-if"
   This is the preferred alternative to the idiom "'?DUP IF'", since it
can be better handled by tools like stack checkers.  Besides, it's
faster.

'?DUP-0=-IF'       compilation - orig ; run-time n - n|         gforth       "question-dupe-zero-equals-if"

Counted loop words constitute a separate group of words:

'?DO'       compilation - do-sys ; run-time w1 w2 - | loop-sys         core-ext       "question-do"

'+DO'       compilation - do-sys ; run-time n1 n2 - | loop-sys         gforth       "plus-do"

'U+DO'       compilation - do-sys ; run-time u1 u2 - | loop-sys         gforth       "u-plus-do"

'-DO'       compilation - do-sys ; run-time n1 n2 - | loop-sys         gforth       "minus-do"

'U-DO'       compilation - do-sys ; run-time u1 u2 - | loop-sys         gforth       "u-minus-do"

'DO'       compilation - do-sys ; run-time w1 w2 - loop-sys         core       "DO"

'FOR'       compilation - do-sys ; run-time u - loop-sys         gforth       "FOR"

'LOOP'       compilation do-sys - ; run-time loop-sys1 - | loop-sys2         core       "LOOP"

'+LOOP'       compilation do-sys - ; run-time loop-sys1 n - | loop-sys2         core       "plus-loop"

'-LOOP'       compilation do-sys - ; run-time loop-sys1 u - | loop-sys2         gforth       "minus-loop"

'NEXT'       compilation do-sys - ; run-time loop-sys1 - | loop-sys2         gforth       "NEXT"

'LEAVE'       compilation - ; run-time loop-sys -         core       "LEAVE"

'?LEAVE'       compilation - ; run-time f | f loop-sys -         gforth       "question-leave"

'unloop'       R:w1 R:w2 -        core       "unloop"

'DONE'       compilation orig - ; run-time -         gforth       "DONE"

   The standard does not allow using 'CS-PICK' and 'CS-ROLL' on do-sys.
Gforth allows it, but it's your job to ensure that for every '?DO' etc.
there is exactly one 'UNLOOP' on any path through the definition ('LOOP'
etc.  compile an 'UNLOOP' on the fall-through path).  Also, you have to
ensure that all 'LEAVE's are resolved (by using one of the loop-ending
words or 'DONE').

Another group of control structure words are:

'case'       compilation  - case-sys ; run-time  -         core-ext       "case"

'endcase'       compilation case-sys - ; run-time x -         core-ext       "end-case"

'of'       compilation  - of-sys ; run-time x1 x2 - |x1         core-ext       "of"

'endof'       compilation case-sys1 of-sys - case-sys2 ; run-time  -         core-ext       "end-of"

   case-sys and of-sys cannot be processed using 'CS-PICK' and
'CS-ROLL'.

5.8.4.1 Programming Style
.........................

In order to ensure readability we recommend that you do not create
arbitrary control structures directly, but define new control structure
words for the control structure you want and use these words in your
program.  For example, instead of writing:

     BEGIN
       ...
     IF [ 1 CS-ROLL ]
       ...
     AGAIN THEN

we recommend defining control structure words, e.g.,

     : WHILE ( DEST -- ORIG DEST )
      POSTPONE IF
      1 CS-ROLL ; immediate

     : REPEAT ( orig dest -- )
      POSTPONE AGAIN
      POSTPONE THEN ; immediate

and then using these to create the control structure:

     BEGIN
       ...
     WHILE
       ...
     REPEAT

   That's much easier to read, isn't it?  Of course, 'REPEAT' and
'WHILE' are predefined, so in this example it would not be necessary to
define them.

5.8.5 Calls and returns
-----------------------

A definition can be called simply be writing the name of the definition
to be called.  Normally a definition is invisible during its own
definition.  If you want to write a directly recursive definition, you
can use 'recursive' to make the current definition visible, or 'recurse'
to call the current definition directly.

'recursive'       compilation - ; run-time -         gforth       "recursive"
   Make the current definition visible, enabling it to call itself
recursively.

'recurse'       compilation - ; run-time ?? - ??         core       "recurse"
   Call the current definition.

     Programming style note: I prefer using 'recursive' to 'recurse',
     because calling the definition by name is more descriptive (if the
     name is well-chosen) than the somewhat cryptic 'recurse'.  E.g., in
     a quicksort implementation, it is much better to read (and think)
     "now sort the partitions" than to read "now do a recursive call".

   For mutual recursion, use 'Defer'red words, like this:

     Defer foo

     : bar ( ... -- ... )
      ... foo ... ;

     :noname ( ... -- ... )
      ... bar ... ;
     IS foo

   Deferred words are discussed in more detail in *note Deferred
Words::.

   The current definition returns control to the calling definition when
the end of the definition is reached or 'EXIT' is encountered.

'EXIT'       compilation - ; run-time nest-sys -         core       "EXIT"
   Return to the calling definition; usually used as a way of forcing an
early return from a definition.  Before 'EXIT'ing you must clean up the
return stack and 'UNLOOP' any outstanding '?DO'...'LOOP's.

';s'       R:w -        gforth       "semis"
   The primitive compiled by 'EXIT'.

5.8.6 Exception Handling
------------------------

If a word detects an error condition that it cannot handle, it can
'throw' an exception.  In the simplest case, this will terminate your
program, and report an appropriate error.

'throw'       y1 .. ym nerror - y1 .. ym / z1 .. zn error         exception       "throw"
   If nerror is 0, drop it and continue.  Otherwise, transfer control to
the next dynamically enclosing exception handler, reset the stacks
accordingly, and push nerror.

   'Throw' consumes a cell-sized error number on the stack.  There are
some predefined error numbers in ANS Forth (see 'errors.fs').  In Gforth
(and most other systems) you can use the iors produced by various words
as error numbers (e.g., a typical use of 'allocate' is 'allocate
throw').  Gforth also provides the word 'exception' to define your own
error numbers (with decent error reporting); an ANS Forth version of
this word (but without the error messages) is available in
'compat/except.fs'.  And finally, you can use your own error numbers
(anything outside the range -4095..0), but won't get nice error
messages, only numbers.  For example, try:

     -10 throw                    \ ANS defined
     -267 throw                   \ system defined
     s" my error" exception throw \ user defined
     7 throw                      \ arbitrary number

'exception'       addr u - n         gforth       "exception"
   N is a previously unused 'throw' value in the range (-4095...-256).
Consecutive calls to 'exception' return consecutive decreasing numbers.
Gforth uses the string ADDR U as an error message.

   A common idiom to 'THROW' a specific error if a flag is true is this:

     ( flag ) 0<> errno and throw

   Your program can provide exception handlers to catch exceptions.  An
exception handler can be used to correct the problem, or to clean up
some data structures and just throw the exception to the next exception
handler.  Note that 'throw' jumps to the dynamically innermost exception
handler.  The system's exception handler is outermost, and just prints
an error and restarts command-line interpretation (or, in batch mode
(i.e., while processing the shell command line), leaves Gforth).

   The ANS Forth way to catch exceptions is 'catch':

'catch'       ... xt - ... n         exception       "catch"

'nothrow'       -         gforth       "nothrow"
   Use this (or the standard sequence '['] false catch drop') after a
'catch' or 'endtry' that does not rethrow; this ensures that the next
'throw' will record a backtrace.

   The most common use of exception handlers is to clean up the state
when an error happens.  E.g.,

     base  >r hex \ actually the hex should be inside foo, or we h
     ['] foo catch ( nerror|0 )
     r> base !
     ( nerror|0 ) throw \ pass it on

   A use of 'catch' for handling the error 'myerror' might look like
this:

     ['] foo catch
     CASE
       myerror OF ... ( do something about it ) nothrow ENDOF
       dup throw \ default: pass other errors on, do nothing on non-errors
     ENDCASE

   Having to wrap the code into a separate word is often cumbersome,
therefore Gforth provides an alternative syntax:

     TRY
       code1
       IFERROR
         code2
       THEN
       code3
     ENDTRY

   This performs code1.  If code1 completes normally, execution
continues with code3.  If code1 or there is an exception before
'endtry', the stacks are reset to the state during 'try', the throw
value is pushed on the data stack, and execution constinues at code2,
and finally falls through the code3.

'try'       compilation  - orig ; run-time  - R:sys1         gforth       "try"
   Start an exception-catching region.

'endtry'       compilation  - ; run-time  R:sys1 -         gforth       "endtry"
   End an exception-catching region.

'iferror'       compilation  orig1 - orig2 ; run-time  -         gforth       "iferror"
   Starts the exception handling code (executed if there is an exception
between 'try' and 'endtry').  This part has to be finished with 'then'.

   If you don't need code2, you can write 'restore' instead of 'iferror
then':

     TRY
       code1
     RESTORE
       code3
     ENDTRY

   The cleanup example from above in this syntax:

     base @ { oldbase }
     TRY
       hex foo \ now the hex is placed correctly
       0       \ value for throw
     RESTORE
       oldbase base !
     ENDTRY
     throw

   An additional advantage of this variant is that an exception between
'restore' and 'endtry' (e.g., from the user pressing 'Ctrl-C') restarts
the execution of the code after 'restore', so the base will be restored
under all circumstances.

   However, you have to ensure that this code does not cause an
exception itself, otherwise the 'iferror'/'restore' code will loop.
Moreover, you should also make sure that the stack contents needed by
the 'iferror'/'restore' code exist everywhere between 'try' and
'endtry'; in our example this is achived by putting the data in a local
before the 'try' (you cannot use the return stack because the exception
frame (sys1) is in the way there).

   This kind of usage corresponds to Lisp's 'unwind-protect'.

   If you do not want this exception-restarting behaviour, you achieve
this as follows:

     TRY
       code1
     ENDTRY-IFERROR
       code2
     THEN

   If there is an exception in code1, then code2 is executed, otherwise
execution continues behind the 'then' (or in a possible 'else' branch).
This corresponds to the construct

     TRY
       code1
     RECOVER
       code2
     ENDTRY

   in Gforth before version 0.7.  So you can directly replace
'recover'-using code; however, we recommend that you check if it would
not be better to use one of the other 'try' variants while you are at
it.

   To ease the transition, Gforth provides two compatibility files:
'endtry-iferror.fs' provides the 'try ... endtry-iferror ... then'
syntax (but not 'iferror' or 'restore') for old systems;
'recover-endtry.fs' provides the 'try ... recover ... endtry' syntax on
new systems, so you can use that file as a stopgap to run old programs.
Both files work on any system (they just do nothing if the system
already has the syntax it implements), so you can unconditionally
'require' one of these files, even if you use a mix old and new systems.

'restore'       compilation  orig1 - ; run-time  -         gforth       "restore"
   Starts restoring code, that is executed if there is an exception, and
if there is no exception.

'endtry-iferror'       compilation  orig1 - orig2 ; run-time  R:sys1 -         gforth       "endtry-iferror"
   End an exception-catching region while starting exception-handling
code outside that region (executed if there is an exception between
'try' and 'endtry-iferror').  This part has to be finished with 'then'
(or 'else'...'then').

   Here's the error handling example:

     TRY
       foo
     ENDTRY-IFERROR
       CASE
         myerror OF ... ( do something about it ) nothrow ENDOF
         throw \ pass other errors on
       ENDCASE
     THEN

   Programming style note: As usual, you should ensure that the stack
depth is statically known at the end: either after the 'throw' for
passing on errors, or after the 'ENDTRY' (or, if you use 'catch', after
the end of the selection construct for handling the error).

   There are two alternatives to 'throw': 'Abort"' is conditional and
you can provide an error message.  'Abort' just produces an "Aborted"
error.

   The problem with these words is that exception handlers cannot
differentiate between different 'abort"'s; they just look like '-2
throw' to them (the error message cannot be accessed by standard
programs).  Similar 'abort' looks like '-1 throw' to exception handlers.

'ABORT"'       compilation 'ccc"' - ; run-time f -         core,exception-ext       "abort-quote"
   If any bit of f is non-zero, perform the function of '-2 throw',
displaying the string ccc if there is no exception frame on the
exception stack.

'abort'       ?? - ??         core,exception-ext       "abort"
   '-1 throw'.

5.9 Defining Words
==================

Defining words are used to extend Forth by creating new entries in the
dictionary.

5.9.1 'CREATE'
--------------

Defining words are used to create new entries in the dictionary.  The
simplest defining word is 'CREATE'.  'CREATE' is used like this:

     CREATE new-word1

   'CREATE' is a parsing word, i.e., it takes an argument from the input
stream ('new-word1' in our example).  It generates a dictionary entry
for 'new-word1'.  When 'new-word1' is executed, all that it does is
leave an address on the stack.  The address represents the value of the
data space pointer ('HERE') at the time that 'new-word1' was defined.
Therefore, 'CREATE' is a way of associating a name with the address of a
region of memory.

'Create'       "name" -         core       "Create"

   Note that in ANS Forth guarantees only for 'create' that its body is
in dictionary data space (i.e., where 'here', 'allot' etc.  work, *note
Dictionary allocation::).  Also, in ANS Forth only 'create'd words can
be modified with 'does>' (*note User-defined Defining Words::).  And in
ANS Forth '>body' can only be applied to 'create'd words.

   By extending this example to reserve some memory in data space, we
end up with something like a variable.  Here are two different ways to
do it:

     CREATE new-word2 1 cells allot  \ reserve 1 cell - initial value undefined
     CREATE new-word3 4 ,            \ reserve 1 cell and initialise it (to 4)

   The variable can be examined and modified using '@' ("fetch") and '!'
("store") like this:

     new-word2 @ .      \ get address, fetch from it and display
     1234 new-word2 !   \ new value, get address, store to it

   A similar mechanism can be used to create arrays.  For example, an
80-character text input buffer:

     CREATE text-buf 80 chars allot

     text-buf 0 chars + c@ \ the 1st character (offset 0)
     text-buf 3 chars + c@ \ the 4th character (offset 3)

   You can build arbitrarily complex data structures by allocating
appropriate areas of memory.  For further discussions of this, and to
learn about some Gforth tools that make it easier, *Note Structures::.

5.9.2 Variables
---------------

The previous section showed how a sequence of commands could be used to
generate a variable.  As a final refinement, the whole code sequence can
be wrapped up in a defining word (pre-empting the subject of the next
section), making it easier to create new variables:

     : myvariableX ( "name" -- a-addr ) CREATE 1 cells allot ;
     : myvariable0 ( "name" -- a-addr ) CREATE 0 , ;

     myvariableX foo \ variable foo starts off with an unknown value
     myvariable0 joe \ whilst joe is initialised to 0

     45 3 * foo !   \ set foo to 135
     1234 joe !     \ set joe to 1234
     3 joe +!       \ increment joe by 3.. to 1237

   Not surprisingly, there is no need to define 'myvariable', since
Forth already has a definition 'Variable'.  ANS Forth does not guarantee
that a 'Variable' is initialised when it is created (i.e., it may behave
like 'myvariableX').  In contrast, Gforth's 'Variable' initialises the
variable to 0 (i.e., it behaves exactly like 'myvariable0').  Forth also
provides '2Variable' and 'fvariable' for double and floating-point
variables, respectively - they are initialised to 0.  and 0e in Gforth.
If you use a 'Variable' to store a boolean, you can use 'on' and 'off'
to toggle its state.

'Variable'       "name" -         core       "Variable"

'2Variable'       "name" -         double       "two-variable"

'fvariable'       "name" -         float       "f-variable"

   The defining word 'User' behaves in the same way as 'Variable'.  The
difference is that it reserves space in user (data) space rather than
normal data space.  In a Forth system that has a multi-tasker, each task
has its own set of user variables.

'User'       "name" -         gforth       "User"

5.9.3 Constants
---------------

'Constant' allows you to declare a fixed value and refer to it by name.
For example:

     12 Constant INCHES-PER-FOOT
     3E+08 fconstant SPEED-O-LIGHT

   A 'Variable' can be both read and written, so its run-time behaviour
is to supply an address through which its current value can be
manipulated.  In contrast, the value of a 'Constant' cannot be changed
once it has been declared(1) so it's not necessary to supply the address
- it is more efficient to return the value of the constant directly.
That's exactly what happens; the run-time effect of a constant is to put
its value on the top of the stack (You can find one way of implementing
'Constant' in *note User-defined Defining Words::).

   Forth also provides '2Constant' and 'fconstant' for defining double
and floating-point constants, respectively.

'Constant'       w "name" -         core       "Constant"
   Define a constant name with value w.

   name execution: - w

'2Constant'       w1 w2 "name" -         double       "two-constant"

'fconstant'       r "name" -         float       "f-constant"

   Constants in Forth behave differently from their equivalents in other
programming languages.  In other languages, a constant (such as an EQU
in assembler or a #define in C) only exists at compile-time; in the
executable program the constant has been translated into an absolute
number and, unless you are using a symbolic debugger, it's impossible to
know what abstract thing that number represents.  In Forth a constant
has an entry in the header space and remains there after the code that
uses it has been defined.  In fact, it must remain in the dictionary
since it has run-time duties to perform.  For example:

     12 Constant INCHES-PER-FOOT
     : FEET-TO-INCHES ( n1 -- n2 ) INCHES-PER-FOOT * ;

   When 'FEET-TO-INCHES' is executed, it will in turn execute the xt
associated with the constant 'INCHES-PER-FOOT'.  If you use 'see' to
decompile the definition of 'FEET-TO-INCHES', you can see that it makes
a call to 'INCHES-PER-FOOT'.  Some Forth compilers attempt to optimise
constants by in-lining them where they are used.  You can force Gforth
to in-line a constant like this:

     : FEET-TO-INCHES ( n1 -- n2 ) [ INCHES-PER-FOOT ] LITERAL * ;

   If you use 'see' to decompile this version of 'FEET-TO-INCHES', you
can see that 'INCHES-PER-FOOT' is no longer present.  To understand how
this works, read *note Interpret/Compile states::, and *note Literals::.

   In-lining constants in this way might improve execution time
fractionally, and can ensure that a constant is now only referenced at
compile-time.  However, the definition of the constant still remains in
the dictionary.  Some Forth compilers provide a mechanism for
controlling a second dictionary for holding transient words such that
this second dictionary can be deleted later in order to recover memory
space.  However, there is no standard way of doing this.

   ---------- Footnotes ----------

   (1) Well, often it can be - but not in a Standard, portable way.
It's safer to use a 'Value' (read on).

5.9.4 Values
------------

A 'Value' behaves like a 'Constant', but it can be changed.  'TO' is a
parsing word that changes a 'Values'.  In Gforth (not in ANS Forth) you
can access (and change) a 'value' also with '>body'.

   Here are some examples:

     12 Value APPLES     \ Define APPLES with an initial value of 12
     34 TO APPLES        \ Change the value of APPLES. TO is a parsing word
     1 ' APPLES >body +! \ Increment APPLES.  Non-standard usage.
     APPLES              \ puts 35 on the top of the stack.

'Value'       w "name" -         core-ext       "Value"

'TO'       c|w|d|r "name" -         core-ext,local       "TO"

5.9.5 Colon Definitions
-----------------------

     : name ( ... -- ... )
         word1 word2 word3 ;

Creates a word called 'name' that, upon execution, executes 'word1 word2
word3'.  'name' is a "(colon) definition".

   The explanation above is somewhat superficial.  For simple examples
of colon definitions see *note Your first definition::.  For an in-depth
discussion of some of the issues involved, *Note Interpretation and
Compilation Semantics::.

':'       "name" - colon-sys         core       "colon"

';'       compilation colon-sys - ; run-time nest-sys         core       "semicolon"

5.9.6 Anonymous Definitions
---------------------------

Sometimes you want to define an "anonymous word"; a word without a name.
You can do this with:

':noname'       - xt colon-sys         core-ext       "colon-no-name"

   This leaves the execution token for the word on the stack after the
closing ';'.  Here's an example in which a deferred word is initialised
with an 'xt' from an anonymous colon definition:

     Defer deferred
     :noname ( ... -- ... )
       ... ;
     IS deferred

Gforth provides an alternative way of doing this, using two separate
words:

'noname'       -         gforth       "noname"
   The next defined word will be anonymous.  The defining word will
leave the input stream alone.  The xt of the defined word will be given
by 'latestxt'.

'latestxt'       - xt         gforth       "latestxt"
   xt is the execution token of the last word defined.

The previous example can be rewritten using 'noname' and 'latestxt':

     Defer deferred
     noname : ( ... -- ... )
       ... ;
     latestxt IS deferred

'noname' works with any defining word, not just ':'.

   'latestxt' also works when the last word was not defined as 'noname'.
It does not work for combined words, though.  It also has the useful
property that is is valid as soon as the header for a definition has
been built.  Thus:

     latestxt . : foo [ latestxt . ] ; ' foo .

prints 3 numbers; the last two are the same.

5.9.7 Supplying the name of a defined word
------------------------------------------

By default, a defining word takes the name for the defined word from the
input stream.  Sometimes you want to supply the name from a string.  You
can do this with:

'nextname'       c-addr u -         gforth       "nextname"
   The next defined word will have the name C-ADDR U; the defining word
will leave the input stream alone.

   For example:

     s" foo" nextname create

is equivalent to:

     create foo

'nextname' works with any defining word.

5.9.8 User-defined Defining Words
---------------------------------

You can create a new defining word by wrapping defining-time code around
an existing defining word and putting the sequence in a colon
definition.

   For example, suppose that you have a word 'stats' that gathers
statistics about colon definitions given the xt of the definition, and
you want every colon definition in your application to make a call to
'stats'.  You can define and use a new version of ':' like this:

     : stats ( xt -- ) DUP ." (Gathering statistics for " . ." )"
       ... ;  \ other code

     : my: : latestxt postpone literal ['] stats compile, ;

     my: foo + - ;

   When 'foo' is defined using 'my:' these steps occur:

   * 'my:' is executed.
   * The ':' within the definition (the one between 'my:' and
     'latestxt') is executed, and does just what it always does; it
     parses the input stream for a name, builds a dictionary header for
     the name 'foo' and switches 'state' from interpret to compile.
   * The word 'latestxt' is executed.  It puts the xt for the word that
     is being defined - 'foo' - onto the stack.
   * The code that was produced by 'postpone literal' is executed; this
     causes the value on the stack to be compiled as a literal in the
     code area of 'foo'.
   * The code '['] stats' compiles a literal into the definition of
     'my:'.  When 'compile,' is executed, that literal - the execution
     token for 'stats' - is layed down in the code area of 'foo' ,
     following the literal(1).
   * At this point, the execution of 'my:' is complete, and control
     returns to the text interpreter.  The text interpreter is in
     compile state, so subsequent text '+ -' is compiled into the
     definition of 'foo' and the ';' terminates the definition as
     always.

   You can use 'see' to decompile a word that was defined using 'my:'
and see how it is different from a normal ':' definition.  For example:

     : bar + - ;  \ like foo but using : rather than my:
     see bar
     : bar
       + - ;
     see foo
     : foo
       107645672 stats + - ;

     \ use ' foo . to show that 107645672 is the xt for foo

   You can use techniques like this to make new defining words in terms
of any existing defining word.

   If you want the words defined with your defining words to behave
differently from words defined with standard defining words, you can
write your defining word like this:

     : def-word ( "name" -- )
         CREATE code1
     DOES> ( ... -- ... )
         code2 ;

     def-word name

   This fragment defines a "defining word" 'def-word' and then executes
it.  When 'def-word' executes, it 'CREATE's a new word, 'name', and
executes the code code1.  The code code2 is not executed at this time.
The word 'name' is sometimes called a "child" of 'def-word'.

   When you execute 'name', the address of the body of 'name' is put on
the data stack and code2 is executed (the address of the body of 'name'
is the address 'HERE' returns immediately after the 'CREATE', i.e., the
address a 'create'd word returns by default).

   You can use 'def-word' to define a set of child words that behave
similarly; they all have a common run-time behaviour determined by
code2.  Typically, the code1 sequence builds a data area in the body of
the child word.  The structure of the data is common to all children of
'def-word', but the data values are specific - and private - to each
child word.  When a child word is executed, the address of its private
data area is passed as a parameter on TOS to be used and manipulated(2)
by code2.

   The two fragments of code that make up the defining words act (are
executed) at two completely separate times:

   * At define time, the defining word executes code1 to generate a
     child word
   * At child execution time, when a child word is invoked, code2 is
     executed, using parameters (data) that are private and specific to
     the child word.

   Another way of understanding the behaviour of 'def-word' and 'name'
is to say that, if you make the following definitions:
     : def-word1 ( "name" -- )
         CREATE code1 ;

     : action1 ( ... -- ... )
         code2 ;

     def-word1 name1

Then using 'name1 action1' is equivalent to using 'name'.

   The classic example is that you can define 'CONSTANT' in this way:

     : CONSTANT ( w "name" -- )
         CREATE ,
     DOES> ( -- w )
         @ ;

   When you create a constant with '5 CONSTANT five', a set of
define-time actions take place; first a new word 'five' is created, then
the value 5 is laid down in the body of 'five' with ','.  When 'five' is
executed, the address of the body is put on the stack, and '@' retrieves
the value 5.  The word 'five' has no code of its own; it simply contains
a data field and a pointer to the code that follows 'DOES>' in its
defining word.  That makes words created in this way very compact.

   The final example in this section is intended to remind you that
space reserved in 'CREATE'd words is data space and therefore can be
both read and written by a Standard program(3):

     : foo ( "name" -- )
         CREATE -1 ,
     DOES> ( -- )
         @ . ;

     foo first-word
     foo second-word

     123 ' first-word >BODY !

   If 'first-word' had been a 'CREATE'd word, we could simply have
executed it to get the address of its data field.  However, since it was
defined to have 'DOES>' actions, its execution semantics are to perform
those 'DOES>' actions.  To get the address of its data field it's
necessary to use ''' to get its xt, then '>BODY' to translate the xt
into the address of the data field.  When you execute 'first-word', it
will display '123'.  When you execute 'second-word' it will display
'-1'.

   In the examples above the stack comment after the 'DOES>' specifies
the stack effect of the defined words, not the stack effect of the
following code (the following code expects the address of the body on
the top of stack, which is not reflected in the stack comment).  This is
the convention that I use and recommend (it clashes a bit with using
locals declarations for stack effect specification, though).

   ---------- Footnotes ----------

   (1) Strictly speaking, the mechanism that 'compile,' uses to convert
an xt into something in the code area is implementation-dependent.  A
threaded implementation might spit out the execution token directly
whilst another implementation might spit out a native code sequence.

   (2) It is legitimate both to read and write to this data area.

   (3) Exercise: use this example as a starting point for your own
implementation of 'Value' and 'TO' - if you get stuck, investigate the
behaviour of ''' and '[']'.

5.9.8.1 Applications of 'CREATE..DOES>'
.......................................

You may wonder how to use this feature.  Here are some usage patterns:

   When you see a sequence of code occurring several times, and you can
identify a meaning, you will factor it out as a colon definition.  When
you see similar colon definitions, you can factor them using
'CREATE..DOES>'.  E.g., an assembler usually defines several words that
look very similar:
     : ori, ( reg-target reg-source n -- )
         0 asm-reg-reg-imm ;
     : andi, ( reg-target reg-source n -- )
         1 asm-reg-reg-imm ;

This could be factored with:
     : reg-reg-imm ( op-code -- )
         CREATE ,
     DOES> ( reg-target reg-source n -- )
         @ asm-reg-reg-imm ;

     0 reg-reg-imm ori,
     1 reg-reg-imm andi,

   Another view of 'CREATE..DOES>' is to consider it as a crude way to
supply a part of the parameters for a word (known as "currying" in the
functional language community).  E.g., '+' needs two parameters.
Creating versions of '+' with one parameter fixed can be done like this:

     : curry+ ( n1 "name" -- )
         CREATE ,
     DOES> ( n2 -- n1+n2 )
         @ + ;

      3 curry+ 3+
     -2 curry+ 2-

5.9.8.2 The gory details of 'CREATE..DOES>'
...........................................

'DOES>'       compilation colon-sys1 - colon-sys2 ; run-time nest-sys -         core       "does"

   This means that you need not use 'CREATE' and 'DOES>' in the same
definition; you can put the 'DOES>'-part in a separate definition.  This
allows us to, e.g., select among different 'DOES>'-parts:
     : does1
     DOES> ( ... -- ... )
         ... ;

     : does2
     DOES> ( ... -- ... )
         ... ;

     : def-word ( ... -- ... )
         create ...
         IF
            does1
         ELSE
            does2
         ENDIF ;

   In this example, the selection of whether to use 'does1' or 'does2'
is made at definition-time; at the time that the child word is
'CREATE'd.

   In a standard program you can apply a 'DOES>'-part only if the last
word was defined with 'CREATE'.  In Gforth, the 'DOES>'-part will
override the behaviour of the last word defined in any case.  In a
standard program, you can use 'DOES>' only in a colon definition.  In
Gforth, you can also use it in interpretation state, in a kind of
one-shot mode; for example:
     CREATE name ( ... -- ... )
       initialization
     DOES>
       code ;

is equivalent to the standard:
     :noname
     DOES>
         code ;
     CREATE name EXECUTE ( ... -- ... )
         initialization

'>body'       xt - a_addr         core       "to-body"
   Get the address of the body of the word represented by xt (the
address of the word's data field).

5.9.8.3 Advanced does> usage example
....................................

The MIPS disassembler ('arch/mips/disasm.fs') contains many words for
disassembling instructions, that follow a very repetetive scheme:

     :noname DISASM-OPERANDS s" INST-NAME" type ;
     ENTRY-NUM cells TABLE + !

   Of course, this inspires the idea to factor out the commonalities to
allow a definition like

     DISASM-OPERANDS ENTRY-NUM TABLE define-inst INST-NAME

   The parameters DISASM-OPERANDS and TABLE are usually correlated.
Moreover, before I wrote the disassembler, there already existed code
that defines instructions like this:

     ENTRY-NUM INST-FORMAT INST-NAME

   This code comes from the assembler and resides in
'arch/mips/insts.fs'.

   So I had to define the INST-FORMAT words that performed the scheme
above when executed.  At first I chose to use run-time code-generation:

     : INST-FORMAT ( entry-num "name" -- ; compiled code: addr w -- )
       :noname Postpone DISASM-OPERANDS
       name Postpone sliteral Postpone type Postpone ;
       swap cells TABLE + ! ;

   Note that this supplies the other two parameters of the scheme above.

   An alternative would have been to write this using 'create'/'does>':

     : INST-FORMAT ( entry-num "name" -- )
       here name string, ( entry-num c-addr ) \ parse and save "name"
       noname create , ( entry-num )
       latestxt swap cells TABLE + !
     does> ( addr w -- )
       \ disassemble instruction w at addr
       @ >r
       DISASM-OPERANDS
       r> count type ;

   Somehow the first solution is simpler, mainly because it's simpler to
shift a string from definition-time to use-time with 'sliteral' than
with 'string,' and friends.

   I wrote a lot of words following this scheme and soon thought about
factoring out the commonalities among them.  Note that this uses a
two-level defining word, i.e., a word that defines ordinary defining
words.

   This time a solution involving 'postpone' and friends seemed more
difficult (try it as an exercise), so I decided to use a
'create'/'does>' word; since I was already at it, I also used
'create'/'does>' for the lower level (try using 'postpone' etc.  as an
exercise), resulting in the following definition:

     : define-format ( disasm-xt table-xt -- )
         \ define an instruction format that uses disasm-xt for
         \ disassembling and enters the defined instructions into table
         \ table-xt
         create 2,
     does> ( u "inst" -- )
         \ defines an anonymous word for disassembling instruction inst,
         \ and enters it as u-th entry into table-xt
         2@ swap here name string, ( u table-xt disasm-xt c-addr ) \ remember string
         noname create 2,      \ define anonymous word
         execute latestxt swap ! \ enter xt of defined word into table-xt
     does> ( addr w -- )
         \ disassemble instruction w at addr
         2@ >r ( addr w disasm-xt R: c-addr )
         execute ( R: c-addr ) \ disassemble operands
         r> count type ; \ print name

   Note that the tables here (in contrast to above) do the 'cells +' by
themselves (that's why you have to pass an xt).  This word is used in
the following way:

     ' DISASM-OPERANDS ' TABLE define-format INST-FORMAT

   As shown above, the defined instruction format is then used like
this:

     ENTRY-NUM INST-FORMAT INST-NAME

   In terms of currying, this kind of two-level defining word provides
the parameters in three stages: first DISASM-OPERANDS and TABLE, then
ENTRY-NUM and INST-NAME, finally 'addr w', i.e., the instruction to be
disassembled.

   Of course this did not quite fit all the instruction format names
used in 'insts.fs', so I had to define a few wrappers that conditioned
the parameters into the right form.

   If you have trouble following this section, don't worry.  First, this
is involved and takes time (and probably some playing around) to
understand; second, this is the first two-level 'create'/'does>' word I
have written in seventeen years of Forth; and if I did not have
'insts.fs' to start with, I may well have elected to use just a
one-level defining word (with some repeating of parameters when using
the defining word).  So it is not necessary to understand this, but it
may improve your understanding of Forth.

5.9.8.4 'Const-does>'
.....................

A frequent use of 'create'...'does>' is for transferring some values
from definition-time to run-time.  Gforth supports this use with

'const-does>'       run-time: w*uw r*ur uw ur "name" -         gforth       "const-does>"
   Defines NAME and returns.

   NAME execution: pushes W*UW R*UR, then performs the code following
the 'const-does>'.

   A typical use of this word is:

     : curry+ ( n1 "name" -- )
     1 0 CONST-DOES> ( n2 -- n1+n2 )
         + ;

     3 curry+ 3+

   Here the '1 0' means that 1 cell and 0 floats are transferred from
definition to run-time.

   The advantages of using 'const-does>' are:

   * You don't have to deal with storing and retrieving the values,
     i.e., your program becomes more writable and readable.

   * When using 'does>', you have to introduce a '@' that cannot be
     optimized away (because you could change the data using
     '>body'...'!'); 'const-does>' avoids this problem.

   An ANS Forth implementation of 'const-does>' is available in
'compat/const-does.fs'.

5.9.9 Deferred Words
--------------------

The defining word 'Defer' allows you to define a word by name without
defining its behaviour; the definition of its behaviour is deferred.
Here are two situation where this can be useful:

   * Where you want to allow the behaviour of a word to be altered
     later, and for all precompiled references to the word to change
     when its behaviour is changed.
   * For mutual recursion; *Note Calls and returns::.

   In the following example, 'foo' always invokes the version of 'greet'
that prints "'Good morning'" whilst 'bar' always invokes the version
that prints "'Hello'".  There is no way of getting 'foo' to use the
later version without re-ordering the source code and recompiling it.

     : greet ." Good morning" ;
     : foo ... greet ... ;
     : greet ." Hello" ;
     : bar ... greet ... ;

   This problem can be solved by defining 'greet' as a 'Defer'red word.
The behaviour of a 'Defer'red word can be defined and redefined at any
time by using 'IS' to associate the xt of a previously-defined word with
it.  The previous example becomes:

     Defer greet ( -- )
     : foo ... greet ... ;
     : bar ... greet ... ;
     : greet1 ( -- ) ." Good morning" ;
     : greet2 ( -- ) ." Hello" ;
     ' greet2 IS greet  \ make greet behave like greet2

   Programming style note: You should write a stack comment for every
deferred word, and put only XTs into deferred words that conform to this
stack effect.  Otherwise it's too difficult to use the deferred word.

   A deferred word can be used to improve the statistics-gathering
example from *note User-defined Defining Words::; rather than edit the
application's source code to change every ':' to a 'my:', do this:

     : real: : ;     \ retain access to the original
     defer :         \ redefine as a deferred word
     ' my: IS :      \ use special version of :
     \
     \ load application here
     \
     ' real: IS :    \ go back to the original

   One thing to note is that 'IS' has special compilation semantics,
such that it parses the name at compile time (like 'TO'):

     : set-greet ( xt -- )
       IS greet ;

     ' greet1 set-greet

   In situations where 'IS' does not fit, use 'defer!' instead.

   A deferred word can only inherit execution semantics from the xt
(because that is all that an xt can represent - for more discussion of
this *note Tokens for Words::); by default it will have default
interpretation and compilation semantics deriving from this execution
semantics.  However, you can change the interpretation and compilation
semantics of the deferred word in the usual ways:

     : bar .... ; immediate
     Defer fred immediate
     Defer jim

     ' bar IS jim  \ jim has default semantics
     ' bar IS fred \ fred is immediate

'Defer'       "name" -         gforth       "Defer"
   Define a deferred word name; its execution semantics can be set with
'defer!' or 'is' (and they have to, before first executing name.

'defer!'       xt xt-deferred -         gforth       "defer-store"
   Changes the 'defer'red word XT-DEFERRED to execute XT.

'IS'       compilation/interpretation "name-deferred" - ; run-time xt -         gforth       "IS"
   Changes the 'defer'red word NAME to execute XT.  Its compilation
semantics parses at compile time.

'defer@'       xt-deferred - xt         gforth       "defer-fetch"
   xt represents the word currently associated with the deferred word
xt-deferred.

'action-of'       interpretation "name" - xt; compilation "name" - ; run-time - xt         gforth       "action-of"
   Xt is the XT that is currently assigned to name.

'defers'       compilation "name" - ; run-time ... - ...         gforth       "defers"
   Compiles the present contents of the deferred word name into the
current definition.  I.e., this produces static binding as if name was
not deferred.

   Definitions of these words (except 'defers') in ANS Forth are
provided in 'compat/defer.fs'.

5.9.10 Aliases
--------------

The defining word 'Alias' allows you to define a word by name that has
the same behaviour as some other word.  Here are two situation where
this can be useful:

   * When you want access to a word's definition from a different word
     list (for an example of this, see the definition of the 'Root' word
     list in the Gforth source).
   * When you want to create a synonym; a definition that can be known
     by either of two names (for example, 'THEN' and 'ENDIF' are
     aliases).

   Like deferred words, an alias has default compilation and
interpretation semantics at the beginning (not the modifications of the
other word), but you can change them in the usual ways ('immediate',
'compile-only').  For example:

     : foo ... ; immediate

     ' foo Alias bar \ bar is not an immediate word
     ' foo Alias fooby immediate \ fooby is an immediate word

   Words that are aliases have the same xt, different headers in the
dictionary, and consequently different name tokens (*note Tokens for
Words::) and possibly different immediate flags.  An alias can only have
default or immediate compilation semantics; you can define aliases for
combined words with 'interpret/compile:' - see *note Combined words::.

'Alias'       xt "name" -         gforth       "Alias"

5.10 Interpretation and Compilation Semantics
=============================================

The "interpretation semantics" of a (named) word are what the text
interpreter does when it encounters the word in interpret state.  It
also appears in some other contexts, e.g., the execution token returned
by '' word' identifies the interpretation semantics of word (in other
words, '' word execute' is equivalent to interpret-state text
interpretation of 'word').

   The "compilation semantics" of a (named) word are what the text
interpreter does when it encounters the word in compile state.  It also
appears in other contexts, e.g, 'POSTPONE word' compiles(1) the
compilation semantics of word.

   The standard also talks about "execution semantics".  They are used
only for defining the interpretation and compilation semantics of many
words.  By default, the interpretation semantics of a word are to
'execute' its execution semantics, and the compilation semantics of a
word are to 'compile,' its execution semantics.(2)

   Unnamed words (*note Anonymous Definitions::) cannot be encountered
by the text interpreter, ticked, or 'postpone'd, so they have no
interpretation or compilation semantics.  Their behaviour is represented
by their XT (*note Tokens for Words::), and we call it execution
semantics, too.

   You can change the semantics of the most-recently defined word:

'immediate'       -         core       "immediate"
   Make the compilation semantics of a word be to 'execute' the
execution semantics.

'compile-only'       -         gforth       "compile-only"
   Remove the interpretation semantics of a word.

'restrict'       -         gforth       "restrict"
   A synonym for 'compile-only'

   By convention, words with non-default compilation semantics (e.g.,
immediate words) often have names surrounded with brackets (e.g., '[']',
*note Execution token::).

   Note that ticking (''') a compile-only word gives an error
("Interpreting a compile-only word").

   ---------- Footnotes ----------

   (1) In standard terminology, "appends to the current definition".

   (2) In standard terminology: The default interpretation semantics are
its execution semantics; the default compilation semantics are to append
its execution semantics to the execution semantics of the current
definition.

5.10.1 Combined Words
---------------------

Gforth allows you to define "combined words" - words that have an
arbitrary combination of interpretation and compilation semantics.

'interpret/compile:'       interp-xt comp-xt "name" -         gforth       "interpret/compile:"

   This feature was introduced for implementing 'TO' and 'S"'.  I
recommend that you do not define such words, as cute as they may be:
they make it hard to get at both parts of the word in some contexts.
E.g., assume you want to get an execution token for the compilation
part.  Instead, define two words, one that embodies the interpretation
part, and one that embodies the compilation part.  Once you have done
that, you can define a combined word with 'interpret/compile:' for the
convenience of your users.

   You might try to use this feature to provide an optimizing
implementation of the default compilation semantics of a word.  For
example, by defining:
     :noname
        foo bar ;
     :noname
        POSTPONE foo POSTPONE bar ;
     interpret/compile: opti-foobar

as an optimizing version of:

     : foobar
         foo bar ;

   Unfortunately, this does not work correctly with '[compile]', because
'[compile]' assumes that the compilation semantics of all
'interpret/compile:' words are non-default.  I.e., '[compile]
opti-foobar' would compile compilation semantics, whereas '[compile]
foobar' would compile interpretation semantics.

   Some people try to use "state-smart" words to emulate the feature
provided by 'interpret/compile:' (words are state-smart if they check
'STATE' during execution).  E.g., they would try to code 'foobar' like
this:

     : foobar
       STATE @
       IF ( compilation state )
         POSTPONE foo POSTPONE bar
       ELSE
         foo bar
       ENDIF ; immediate

   Although this works if 'foobar' is only processed by the text
interpreter, it does not work in other contexts (like ''' or
'POSTPONE').  E.g., '' foobar' will produce an execution token for a
state-smart word, not for the interpretation semantics of the original
'foobar'; when you execute this execution token (directly with 'EXECUTE'
or indirectly through 'COMPILE,') in compile state, the result will not
be what you expected (i.e., it will not perform 'foo bar').  State-smart
words are a bad idea.  Simply don't write them(1)!

   It is also possible to write defining words that define words with
arbitrary combinations of interpretation and compilation semantics.  In
general, they look like this:

     : def-word
         create-interpret/compile
         code1
     interpretation>
         code2
     <interpretation
     compilation>
         code3
     <compilation ;

   For a word defined with 'def-word', the interpretation semantics are
to push the address of the body of word and perform code2, and the
compilation semantics are to push the address of the body of word and
perform code3.  E.g., 'constant' can also be defined like this (except
that the defined constants don't behave correctly when '[compile]'d):

     : constant ( n "name" -- )
         create-interpret/compile
         ,
     interpretation> ( -- n )
         @
     <interpretation
     compilation> ( compilation. -- ; run-time. -- n )
         @ postpone literal
     <compilation ;

'create-interpret/compile'       "name" -         gforth       "create-interpret/compile"

'interpretation>'       compilation. - orig colon-sys         gforth       "interpretation>"

'<interpretation'       compilation. orig colon-sys -         gforth       "<interpretation"

'compilation>'       compilation. - orig colon-sys         gforth       "compilation>"

'<compilation'       compilation. orig colon-sys -         gforth       "<compilation"

   Words defined with 'interpret/compile:' and
'create-interpret/compile' have an extended header structure that
differs from other words; however, unless you try to access them with
plain address arithmetic, you should not notice this.  Words for
accessing the header structure usually know how to deal with this; e.g.,
''' word '>body' also gives you the body of a word created with
'create-interpret/compile'.

   ---------- Footnotes ----------

   (1) For a more detailed discussion of this topic, see M. Anton Ertl,
''State'-smartness--Why it is Evil and How to Exorcise it
(http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz)', EuroForth '98.

5.11 Tokens for Words
=====================

This section describes the creation and use of tokens that represent
words.

5.11.1 Execution token
----------------------

An "execution token" (XT) represents some behaviour of a word.  You can
use 'execute' to invoke this behaviour.

   You can use ''' to get an execution token that represents the
interpretation semantics of a named word:

     5 ' .   ( n xt )
     execute ( )      \ execute the xt (i.e., ".")

'''       "name" - xt         core       "tick"
   xt represents name's interpretation semantics.  Perform '-14 throw'
if the word has no interpretation semantics.

   ''' parses at run-time; there is also a word '[']' that parses when
it is compiled, and compiles the resulting XT:

     : foo ['] . execute ;
     5 foo
     : bar ' execute ; \ by contrast,
     5 bar .           \ ' parses "." when bar executes

'[']'       compilation. "name" - ; run-time. - xt         core       "bracket-tick"
   xt represents name's interpretation semantics.  Perform '-14 throw'
if the word has no interpretation semantics.

   If you want the execution token of word, write '['] word' in compiled
code and '' word' in interpreted code.  Gforth's ''' and '[']' behave
somewhat unusually by complaining about compile-only words (because
these words have no interpretation semantics).  You might get what you
want by using 'COMP' word DROP' or '[COMP'] word DROP' (for details
*note Compilation token::).

   Another way to get an XT is ':noname' or 'latestxt' (*note Anonymous
Definitions::).  For anonymous words this gives an xt for the only
behaviour the word has (the execution semantics).  For named words,
'latestxt' produces an XT for the same behaviour it would produce if the
word was defined anonymously.

     :noname ." hello" ;
     execute

   An XT occupies one cell and can be manipulated like any other cell.

   In ANS Forth the XT is just an abstract data type (i.e., defined by
the operations that produce or consume it).  For old hands: In Gforth,
the XT is implemented as a code field address (CFA).

'execute'       xt -        core       "execute"
   Perform the semantics represented by the execution token, xt.

'perform'       a-addr -        gforth       "perform"
   '@ execute'.

5.11.2 Compilation token
------------------------

Gforth represents the compilation semantics of a named word by a
"compilation token" consisting of two cells: w xt.  The top cell xt is
an execution token.  The compilation semantics represented by the
compilation token can be performed with 'execute', which consumes the
whole compilation token, with an additional stack effect determined by
the represented compilation semantics.

   At present, the w part of a compilation token is an execution token,
and the xt part represents either 'execute' or 'compile,'(1).  However,
don't rely on that knowledge, unless necessary; future versions of
Gforth may introduce unusual compilation tokens (e.g., a compilation
token that represents the compilation semantics of a literal).

   You can perform the compilation semantics represented by the
compilation token with 'execute'.  You can compile the compilation
semantics with 'postpone,'.  I.e., 'COMP' word postpone,' is equivalent
to 'postpone word'.

'[COMP']'       compilation "name" - ; run-time - w xt         gforth       "bracket-comp-tick"
   Compilation token w xt represents name's compilation semantics.

'COMP''       "name" - w xt         gforth       "comp-tick"
   Compilation token w xt represents name's compilation semantics.

'postpone,'       w xt -         gforth       "postpone-comma"
   Compile the compilation semantics represented by the compilation
token w xt.

   ---------- Footnotes ----------

   (1) Depending upon the compilation semantics of the word.  If the
word has default compilation semantics, the xt will represent
'compile,'.  Otherwise (e.g., for immediate words), the xt will
represent 'execute'.

5.11.3 Name token
-----------------

Gforth represents named words by the "name token", (nt).  Name token is
an abstract data type that occurs as argument or result of the words
below.

   The closest thing to the nt in older Forth systems is the name field
address (NFA), but there are significant differences: in older Forth
systems each word had a unique NFA, LFA, CFA and PFA (in this order, or
LFA, NFA, CFA, PFA) and there were words for getting from one to the
next.  In contrast, in Gforth 0...n nts correspond to one xt; there is a
link field in the structure identified by the name token, but searching
usually uses a hash table external to these structures; the name in
Gforth has a cell-wide count-and-flags field, and the nt is not
implemented as the address of that count field.

'find-name'       c-addr u - nt | 0         gforth       "find-name"
   Find the name c-addr u in the current search order.  Return its nt,
if found, otherwise 0.

'latest'       - nt         gforth       "latest"
   NT is the name token of the last word defined; it is 0 if the last
word has no name.

'>name'       xt - nt|0         gforth       "to-name"
   tries to find the name token NT of the word represented by XT;
returns 0 if it fails.  This word is not absolutely reliable, it may
give false positives and produce wrong nts.

'name>int'       nt - xt         gforth       "name-to-int"
   xt represents the interpretation semantics of the word nt.  If nt has
no interpretation semantics (i.e.  is 'compile-only'), xt is the
execution token for 'ticking-compile-only-error', which performs '-2048
throw'.

'name?int'       nt - xt         gforth       "name-question-int"
   Like 'name>int', but perform '-2048 throw' if nt has no
interpretation semantics.

'name>comp'       nt - w xt         gforth       "name-to-comp"
   w xt is the compilation token for the word nt.

'name>string'       nt - addr count         gforth       "name-to-string"
   addr count is the name of the word represented by nt.

'id.'       nt -         gforth       "i-d-dot"
   Print the name of the word represented by NT.

'.name'       nt -         gforth-obsolete       "dot-name"
   Gforth <=0.5.0 name for 'id.'.

'.id'       nt -         F83       "dot-i-d"
   F83 name for 'id.'.

5.12 Compiling words
====================

In contrast to most other languages, Forth has no strict boundary
between compilation and run-time.  E.g., you can run arbitrary code
between defining words (or for computing data used by defining words
like 'constant').  Moreover, 'Immediate' (*note Interpretation and
Compilation Semantics:: and '['...']'  (see below) allow running
arbitrary code while compiling a colon definition (exception: you must
not allot dictionary space).

5.12.1 Literals
---------------

The simplest and most frequent example is to compute a literal during
compilation.  E.g., the following definition prints an array of strings,
one string per line:

     : .strings ( addr u -- ) \ gforth
         2* cells bounds U+DO
     	cr i 2@ type
         2 cells +LOOP ;

   With a simple-minded compiler like Gforth's, this computes '2 cells'
on every loop iteration.  You can compute this value once and for all at
compile time and compile it into the definition like this:

     : .strings ( addr u -- ) \ gforth
         2* cells bounds U+DO
     	cr i 2@ type
         [ 2 cells ] literal +LOOP ;

   '[' switches the text interpreter to interpret state (you will get an
'ok' prompt if you type this example interactively and insert a newline
between '[' and ']'), so it performs the interpretation semantics of '2
cells'; this computes a number.  ']' switches the text interpreter back
into compile state.  It then performs 'Literal''s compilation semantics,
which are to compile this number into the current word.  You can
decompile the word with 'see .strings' to see the effect on the compiled
code.

   You can also optimize the '2* cells' into '[ 2 cells ] literal *' in
this way.

'['       -         core       "left-bracket"
   Enter interpretation state.  Immediate word.

']'       -         core       "right-bracket"
   Enter compilation state.

'Literal'       compilation n - ; run-time - n         core       "Literal"
   Compilation semantics: compile the run-time semantics.
Run-time Semantics: push n.
Interpretation semantics: undefined.

']L'       compilation: n - ; run-time: - n         gforth       "]L"
   equivalent to '] literal'

   There are also words for compiling other data types than single cells
as literals:

'2Literal'       compilation w1 w2 - ; run-time  - w1 w2         double       "two-literal"
   Compile appropriate code such that, at run-time, w1 w2 are placed on
the stack.  Interpretation semantics are undefined.

'FLiteral'       compilation r - ; run-time - r         float       "f-literal"
   Compile appropriate code such that, at run-time, r is placed on the
(floating-point) stack.  Interpretation semantics are undefined.

'SLiteral'       Compilation c-addr1 u ; run-time - c-addr2 u         string       "SLiteral"
   Compilation: compile the string specified by c-addr1, u into the
current definition.  Run-time: return c-addr2 u describing the address
and length of the string.

   You might be tempted to pass data from outside a colon definition to
the inside on the data stack.  This does not work, because ':' puhes a
colon-sys, making stuff below unaccessible.  E.g., this does not work:

     5 : foo literal ; \ error: "unstructured"

   Instead, you have to pass the value in some other way, e.g., through
a variable:

     variable temp
     5 temp !
     : foo [ temp @ ] literal ;

5.12.2 Macros
-------------

'Literal' and friends compile data values into the current definition.
You can also write words that compile other words into the current
definition.  E.g.,

     : compile-+ ( -- ) \ compiled code: ( n1 n2 -- n )
       POSTPONE + ;

     : foo ( n1 n2 -- n )
       [ compile-+ ] ;
     1 2 foo .

   This is equivalent to ': foo + ;' ('see foo' to check this).  What
happens in this example?  'Postpone' compiles the compilation semantics
of '+' into 'compile-+'; later the text interpreter executes 'compile-+'
and thus the compilation semantics of +, which compile (the execution
semantics of) '+' into 'foo'.(1)

'postpone'       "name" -         core       "postpone"
   Compiles the compilation semantics of name.

'[compile]'       compilation "name" - ; run-time ? - ?         core-ext       "bracket-compile"

   Compiling words like 'compile-+' are usually immediate (or similar)
so you do not have to switch to interpret state to execute them;
mopifying the last example accordingly produces:

     : [compile-+] ( compilation: --; interpretation: -- )
       \ compiled code: ( n1 n2 -- n )
       POSTPONE + ; immediate

     : foo ( n1 n2 -- n )
       [compile-+] ;
     1 2 foo .

   Immediate compiling words are similar to macros in other languages
(in particular, Lisp).  The important differences to macros in, e.g., C
are:

   * You use the same language for defining and processing macros, not a
     separate preprocessing language and processor.

   * Consequently, the full power of Forth is available in macro
     definitions.  E.g., you can perform arbitrarily complex
     computations, or generate different code conditionally or in a loop
     (e.g., *note Advanced macros Tutorial::).  This power is very
     useful when writing a parser generators or other code-generating
     software.

   * Macros defined using 'postpone' etc.  deal with the language at a
     higher level than strings; name binding happens at macro definition
     time, so you can avoid the pitfalls of name collisions that can
     happen in C macros.  Of course, Forth is a liberal language and
     also allows to shoot yourself in the foot with text-interpreted
     macros like

          : [compile-+] s" +" evaluate ; immediate

     Apart from binding the name at macro use time, using 'evaluate'
     also makes your definition 'state'-smart (*note state-smartness::).

   You may want the macro to compile a number into a word.  The word to
do it is 'literal', but you have to 'postpone' it, so its compilation
semantics take effect when the macro is executed, not when it is
compiled:

     : [compile-5] ( -- ) \ compiled code: ( -- n )
       5 POSTPONE literal ; immediate

     : foo [compile-5] ;
     foo .

   You may want to pass parameters to a macro, that the macro should
compile into the current definition.  If the parameter is a number, then
you can use 'postpone literal' (similar for other values).

   If you want to pass a word that is to be compiled, the usual way is
to pass an execution token and 'compile,' it:

     : twice1 ( xt -- ) \ compiled code: ... -- ...
       dup compile, compile, ;

     : 2+ ( n1 -- n2 )
       [ ' 1+ twice1 ] ;

'compile,'       xt -         core-ext       "compile-comma"
   Compile the word represented by the execution token xt into the
current definition.

   An alternative available in Gforth, that allows you to pass
compile-only words as parameters is to use the compilation token (*note
Compilation token::).  The same example in this technique:

     : twice ( ... ct -- ... ) \ compiled code: ... -- ...
       2dup 2>r execute 2r> execute ;

     : 2+ ( n1 -- n2 )
       [ comp' 1+ twice ] ;

   In the example above '2>r' and '2r>' ensure that 'twice' works even
if the executed compilation semantics has an effect on the data stack.

   You can also define complete definitions with these words; this
provides an alternative to using 'does>' (*note User-defined Defining
Words::).  E.g., instead of

     : curry+ ( n1 "name" -- )
         CREATE ,
     DOES> ( n2 -- n1+n2 )
         @ + ;

   you could define

     : curry+ ( n1 "name" -- )
       \ name execution: ( n2 -- n1+n2 )
       >r : r> POSTPONE literal POSTPONE + POSTPONE ; ;

     -3 curry+ 3-
     see 3-

   The sequence '>r : r>' is necessary, because ':' puts a colon-sys on
the data stack that makes everything below it unaccessible.

   This way of writing defining words is sometimes more, sometimes less
convenient than using 'does>' (*note Advanced does> usage example::).
One advantage of this method is that it can be optimized better, because
the compiler knows that the value compiled with 'literal' is fixed,
whereas the data associated with a 'create'd word can be changed.

   ---------- Footnotes ----------

   (1) A recent RFI answer requires that compiling words should only be
executed in compile state, so this example is not guaranteed to work on
all standard systems, but on any decent system it will work.

5.13 The Text Interpreter
=========================

The text interpreter(1) is an endless loop that processes input from the
current input device.  It is also called the outer interpreter, in
contrast to the inner interpreter (*note Engine::) which executes the
compiled Forth code on interpretive implementations.

   The text interpreter operates in one of two states: "interpret state"
and "compile state".  The current state is defined by the aptly-named
variable 'state'.

   This section starts by describing how the text interpreter behaves
when it is in interpret state, processing input from the user input
device - the keyboard.  This is the mode that a Forth system is in after
it starts up.

   The text interpreter works from an area of memory called the "input
buffer"(2), which stores your keyboard input when you press the <RET>
key.  Starting at the beginning of the input buffer, it skips leading
spaces (called "delimiters") then parses a string (a sequence of
non-space characters) until it reaches either a space character or the
end of the buffer.  Having parsed a string, it makes two attempts to
process it:

   * It looks for the string in a "dictionary" of definitions.  If the
     string is found, the string names a "definition" (also known as a
     "word") and the dictionary search returns information that allows
     the text interpreter to perform the word's "interpretation
     semantics".  In most cases, this simply means that the word will be
     executed.
   * If the string is not found in the dictionary, the text interpreter
     attempts to treat it as a number, using the rules described in
     *note Number Conversion::.  If the string represents a legal number
     in the current radix, the number is pushed onto a parameter stack
     (the data stack for integers, the floating-point stack for
     floating-point numbers).

   If both attempts fail, or if the word is found in the dictionary but
has no interpretation semantics(3) the text interpreter discards the
remainder of the input buffer, issues an error message and waits for
more input.  If one of the attempts succeeds, the text interpreter
repeats the parsing process until the whole of the input buffer has been
processed, at which point it prints the status message "' ok'" and waits
for more input.

   The text interpreter keeps track of its position in the input buffer
by updating a variable called '>IN' (pronounced "to-in").  The value of
'>IN' starts out as 0, indicating an offset of 0 from the start of the
input buffer.  The region from offset '>IN @' to the end of the input
buffer is called the "parse area"(4).  This example shows how '>IN'
changes as the text interpreter parses the input buffer:

     : remaining >IN @ SOURCE 2 PICK - -ROT + SWAP
       CR ." ->" TYPE ." <-" ; IMMEDIATE

     1 2 3 remaining + remaining .

     : foo 1 2 3 remaining SWAP remaining ;

The result is:

     ->+ remaining .<-
     ->.<-5  ok

     ->SWAP remaining ;-<
     ->;<-  ok

   The value of '>IN' can also be modified by a word in the input buffer
that is executed by the text interpreter.  This means that a word can
"trick" the text interpreter into either skipping a section of the input
buffer(5) or into parsing a section twice.  For example:

     : lat ." <<foo>>" ;
     : flat ." <<bar>>" >IN DUP @ 3 - SWAP ! ;

When 'flat' is executed, this output is produced(6):

     <<bar>><<foo>>

   This technique can be used to work around some of the
interoperability problems of parsing words.  Of course, it's better to
avoid parsing words where possible.

Two important notes about the behaviour of the text interpreter:

   * It processes each input string to completion before parsing
     additional characters from the input buffer.
   * It treats the input buffer as a read-only region (and so must your
     code).

When the text interpreter is in compile state, its behaviour changes in
these ways:

   * If a parsed string is found in the dictionary, the text interpreter
     will perform the word's "compilation semantics".  In most cases,
     this simply means that the execution semantics of the word will be
     appended to the current definition.
   * When a number is encountered, it is compiled into the current
     definition (as a literal) rather than being pushed onto a parameter
     stack.
   * If an error occurs, 'state' is modified to put the text interpreter
     back into interpret state.
   * Each time a line is entered from the keyboard, Gforth prints "'
     compiled'" rather than " 'ok'".

   When the text interpreter is using an input device other than the
keyboard, its behaviour changes in these ways:

   * When the parse area is empty, the text interpreter attempts to
     refill the input buffer from the input source.  When the input
     source is exhausted, the input source is set back to the previous
     input source.
   * It doesn't print out "' ok'" or "' compiled'" messages each time
     the parse area is emptied.
   * If an error occurs, the input source is set back to the user input
     device.

   You can read about this in more detail in *note Input Sources::.

'>in'       - addr         core       "to-in"
   'input-var' variable - a-addr is the address of a cell containing the
char offset from the start of the input buffer to the start of the parse
area.

'source'       - addr u         core       "source"
   Return address addr and length u of the current input buffer

'tib'       - addr         core-ext-obsolescent       "t-i-b"

'#tib'       - addr         core-ext-obsolescent       "number-t-i-b"
   'input-var' variable - a-addr is the address of a cell containing the
number of characters in the terminal input buffer.  OBSOLESCENT:
'source' superceeds the function of this word.

   ---------- Footnotes ----------

   (1) This is an expanded version of the material in *note Introducing
the Text Interpreter::.

   (2) When the text interpreter is processing input from the keyboard,
this area of memory is called the "terminal input buffer" (TIB) and is
addressed by the (obsolescent) words 'TIB' and '#TIB'.

   (3) This happens if the word was defined as 'COMPILE-ONLY'.

   (4) In other words, the text interpreter processes the contents of
the input buffer by parsing strings from the parse area until the parse
area is empty.

   (5) This is how parsing words work.

   (6) Exercise for the reader: what would happen if the '3' were
replaced with '4'?

5.13.1 Input Sources
--------------------

By default, the text interpreter processes input from the user input
device (the keyboard) when Forth starts up.  The text interpreter can
process input from any of these sources:

   * The user input device - the keyboard.
   * A file, using the words described in *note Forth source files::.
   * A block, using the words described in *note Blocks::.
   * A text string, using 'evaluate'.

   A program can identify the current input device from the values of
'source-id' and 'blk'.

'source-id'       - 0 | -1 | fileid         core-ext,file       "source-i-d"
   Return 0 (the input source is the user input device), -1 (the input
source is a string being processed by 'evaluate') or a fileid (the input
source is the file specified by fileid).

'blk'       - addr         block       "b-l-k"
   'input-var' variable - This cell contains the current block number

'save-input'       - x1 .. xn n         core-ext       "save-input"
   The n entries xn - x1 describe the current state of the input source
specification, in some platform-dependent way that can be used by
'restore-input'.

'restore-input'       x1 .. xn n - flag         core-ext       "restore-input"
   Attempt to restore the input source specification to the state
described by the n entries xn - x1.  flag is true if the restore fails.
In Gforth with the new input code, it fails only with a flag that can be
used to throw again; it is also possible to save and restore between
different active input streams.  Note that closing the input streams
must happen in the reverse order as they have been opened, but in
between everything is allowed.

'evaluate'       ... addr u - ...         core,block       "evaluate"
   Save the current input source specification.  Store '-1' in
'source-id' and '0' in 'blk'.  Set '>IN' to '0' and make the string
c-addr u the input source and input buffer.  Interpret.  When the parse
area is empty, restore the input source specification.

'query'       -         core-ext-obsolescent       "query"
   Make the user input device the input source.  Receive input into the
Terminal Input Buffer.  Set '>IN' to zero.  OBSOLESCENT: superceeded by
'accept'.

5.13.2 Number Conversion
------------------------

This section describes the rules that the text interpreter uses when it
tries to convert a string into a number.

   Let <digit> represent any character that is a legal digit in the
current number base(1).

   Let <decimal digit> represent any character in the range 0-9.

   Let {a b} represent the optional presence of any of the characters in
the braces (a or b or neither).

   Let * represent any number of instances of the previous character
(including none).

   Let any other character represent itself.

Now, the conversion rules are:

   * A string of the form <digit><digit>* is treated as a
     single-precision (cell-sized) positive integer.  Examples are 0 123
     6784532 32343212343456 42
   * A string of the form -<digit><digit>* is treated as a
     single-precision (cell-sized) negative integer, and is represented
     using 2's-complement arithmetic.  Examples are -45 -5681 -0
   * A string of the form <digit><digit>*.<digit>* is treated as a
     double-precision (double-cell-sized) positive integer.  Examples
     are 3465.  3.465 34.65 (all three of these represent the same
     number).
   * A string of the form -<digit><digit>*.<digit>* is treated as a
     double-precision (double-cell-sized) negative integer, and is
     represented using 2's-complement arithmetic.  Examples are -3465.
     -3.465 -34.65 (all three of these represent the same number).
   * A string of the form {+ -}<decimal digit>{.}<decimal digit>*{e E}{+
     -}<decimal digit><decimal digit>* is treated as a floating-point
     number.  Examples are 1e 1e0 1.e 1.e0 +1e+0 (which all represent
     the same number) +12.E-4

   By default, the number base used for integer number conversion is
given by the contents of the variable 'base'.  Note that a lot of
confusion can result from unexpected values of 'base'.  If you change
'base' anywhere, make sure to save the old value and restore it
afterwards; better yet, use 'base-execute', which does this for you.  In
general I recommend keeping 'base' decimal, and using the prefixes
described below for the popular non-decimal bases.

'dpl'       - a-addr         gforth       "dpl"
   'User' variable - a-addr is the address of a cell that stores the
position of the decimal point in the most recent numeric conversion.
Initialised to -1.  After the conversion of a number containing no
decimal point, 'dpl' is -1.  After the conversion of '2.' it holds 0.
After the conversion of 234123.9 it contains 1, and so forth.

'base-execute'       i*x xt u - j*x         gforth       "base-execute"
   execute xt with the content of 'BASE' being u, and restoring the
original 'BASE' afterwards.

'base'       - a-addr         core       "base"
   'User' variable - a-addr is the address of a cell that stores the
number base used by default for number conversion during input and
output.  Don't store to 'base', use 'base-execute' instead.

'hex'       -         core-ext       "hex"
   Set 'base' to &16 (hexadecimal).  Don't use 'hex', use 'base-execute'
instead.

'decimal'       -         core       "decimal"
   Set 'base' to &10 (decimal).  Don't use 'hex', use 'base-execute'
instead.

   Gforth allows you to override the value of 'base' by using a
prefix(2) before the first digit of an (integer) number.  The following
prefixes are supported:

   * '&' - decimal
   * '#' - decimal
   * '%' - binary
   * '$' - hexadecimal
   * '0x' - hexadecimal, if base<33.
   * ''' - numeric value (e.g., ASCII code) of next character; an
     optional ''' may be present after the character.

   Here are some examples, with the equivalent decimal number shown
after in braces:

   -$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision
number), 'A (65), -'a' (-97), &905 (905), $abc (2478), $ABC (2478).

Number conversion has a number of traps for the unwary:

   * You cannot determine the current number base using the code
     sequence 'base @ .' - the number base is always 10 in the current
     number base.  Instead, use something like 'base @ dec.'
   * If the number base is set to a value greater than 14 (for example,
     hexadecimal), the number 123E4 is ambiguous; the conversion rules
     allow it to be intepreted as either a single-precision integer or a
     floating-point number (Gforth treats it as an integer).  The
     ambiguity can be resolved by explicitly stating the sign of the
     mantissa and/or exponent: 123E+4 or +123E4 - if the number base is
     decimal, no ambiguity arises; either representation will be treated
     as a floating-point number.
   * There is a word 'bin' but it does not set the number base!  It is
     used to specify file types.
   * ANS Forth requires the '.' of a double-precision number to be the
     final character in the string.  Gforth allows the '.' to be
     anywhere after the first digit.
   * The number conversion process does not check for overflow.
   * In an ANS Forth program 'base' is required to be decimal when
     converting floating-point numbers.  In Gforth, number conversion to
     floating-point numbers always uses base &10, irrespective of the
     value of 'base'.

   You can read numbers into your programs with the words described in
*note Line input and conversion::.

   ---------- Footnotes ----------

   (1) For example, 0-9 when the number base is decimal or 0-9, A-F when
the number base is hexadecimal.

   (2) Some Forth implementations provide a similar scheme by
implementing '$' etc.  as parsing words that process the subsequent
number in the input stream and push it onto the stack.  For example, see
'Number Conversion and Literals', by Wil Baden; Forth Dimensions 20(3)
pages 26-27.  In such implementations, unlike in Gforth, a space is
required between the prefix and the number.

5.13.3 Interpret/Compile states
-------------------------------

A standard program is not permitted to change 'state' explicitly.
However, it can change 'state' implicitly, using the words '[' and ']'.
When '[' is executed it switches 'state' to interpret state, and
therefore the text interpreter starts interpreting.  When ']' is
executed it switches 'state' to compile state and therefore the text
interpreter starts compiling.  The most common usage for these words is
for switching into interpret state and back from within a colon
definition; this technique can be used to compile a literal (for an
example, *note Literals::) or for conditional compilation (for an
example, *note Interpreter Directives::).

5.13.4 Interpreter Directives
-----------------------------

These words are usually used in interpret state; typically to control
which parts of a source file are processed by the text interpreter.
There are only a few ANS Forth Standard words, but Gforth supplements
these with a rich set of immediate control structure words to compensate
for the fact that the non-immediate versions can only be used in compile
state (*note Control Structures::).  Typical usages:

     FALSE Constant HAVE-ASSEMBLER
     .
     .
     HAVE-ASSEMBLER [IF]
     : ASSEMBLER-FEATURE
       ...
     ;
     [ENDIF]
     .
     .
     : SEE
       ... \ general-purpose SEE code
       [ HAVE-ASSEMBLER [IF] ]
       ... \ assembler-specific SEE code
       [ [ENDIF] ]
     ;

'[IF]'       flag -         tools-ext       "bracket-if"
   If flag is 'TRUE' do nothing (and therefore execute subsequent words
as normal).  If flag is 'FALSE', parse and discard words from the parse
area (refilling it if necessary using 'REFILL') including nested
instances of '[IF]'..  '[ELSE]'..  '[THEN]' and '[IF]'..  '[THEN]' until
the balancing '[ELSE]' or '[THEN]' has been parsed and discarded.
Immediate word.

'[ELSE]'       -         tools-ext       "bracket-else"
   Parse and discard words from the parse area (refilling it if
necessary using 'REFILL') including nested instances of '[IF]'..
'[ELSE]'..  '[THEN]' and '[IF]'..  '[THEN]' until the balancing '[THEN]'
has been parsed and discarded.  '[ELSE]' only gets executed if the
balancing '[IF]' was 'TRUE'; if it was 'FALSE', '[IF]' would have parsed
and discarded the '[ELSE]', leaving the subsequent words to be executed
as normal.  Immediate word.

'[THEN]'       -         tools-ext       "bracket-then"
   Do nothing; used as a marker for other words to parse and discard up
to.  Immediate word.

'[ENDIF]'       -         gforth       "bracket-end-if"
   Do nothing; synonym for '[THEN]'

'[IFDEF]'       "<spaces>name" -         gforth       "bracket-if-def"
   If name is found in the current search-order, behave like '[IF]' with
a 'TRUE' flag, otherwise behave like '[IF]' with a 'FALSE' flag.
Immediate word.

'[IFUNDEF]'       "<spaces>name" -         gforth       "bracket-if-un-def"
   If name is not found in the current search-order, behave like '[IF]'
with a 'TRUE' flag, otherwise behave like '[IF]' with a 'FALSE' flag.
Immediate word.

'[?DO]'       n-limit n-index -         gforth       "bracket-question-do"

'[DO]'       n-limit n-index -         gforth       "bracket-do"

'[FOR]'       n -         gforth       "bracket-for"

'[LOOP]'       -         gforth       "bracket-loop"

'[+LOOP]'       n -         gforth       "bracket-question-plus-loop"

'[NEXT]'       n -         gforth       "bracket-next"

'[BEGIN]'       -         gforth       "bracket-begin"

'[UNTIL]'       flag -         gforth       "bracket-until"

'[AGAIN]'       -         gforth       "bracket-again"

'[WHILE]'       flag -         gforth       "bracket-while"

'[REPEAT]'       -         gforth       "bracket-repeat"

5.14 The Input Stream
=====================

The text interpreter reads from the input stream, which can come from
several sources (*note Input Sources::).  Some words, in particular
defining words, but also words like ''', read parameters from the input
stream instead of from the stack.

   Such words are called parsing words, because they parse the input
stream.  Parsing words are hard to use in other words, because it is
hard to pass program-generated parameters through the input stream.
They also usually have an unintuitive combination of interpretation and
compilation semantics when implemented naively, leading to various
approaches that try to produce a more intuitive behaviour (*note
Combined words::).

   It should be obvious by now that parsing words are a bad idea.  If
you want to implement a parsing word for convenience, also provide a
factor of the word that does not parse, but takes the parameters on the
stack.  To implement the parsing word on top if it, you can use the
following words:

'parse'       char "ccc<char>" - c-addr u         core-ext       "parse"
   Parse ccc, delimited by char, in the parse area.  c-addr u specifies
the parsed string within the parse area.  If the parse area was empty, u
is 0.

'parse-name'       "name" - c-addr u         gforth       "parse-name"
   Get the next word from the input buffer

'parse-word'       - c-addr u         gforth-obsolete       "parse-word"
   old name for 'parse-name'

'name'       - c-addr u         gforth-obsolete       "name"
   old name for 'parse-name'

'word'       char "<chars>ccc<char>- c-addr         core       "word"
   Skip leading delimiters.  Parse ccc, delimited by char, in the parse
area.  c-addr is the address of a transient region containing the parsed
string in counted-string format.  If the parse area was empty or
contained no characters other than delimiters, the resulting string has
zero length.  A program may replace characters within the counted
string.  OBSOLESCENT: the counted string has a trailing space that is
not included in its length.

'refill'       - flag         core-ext,block-ext,file-ext       "refill"
   Attempt to fill the input buffer from the input source.  When the
input source is the user input device, attempt to receive input into the
terminal input device.  If successful, make the result the input buffer,
set '>IN' to 0 and return true; otherwise return false.  When the input
source is a block, add 1 to the value of 'BLK' to make the next block
the input source and current input buffer, and set '>IN' to 0; return
true if the new value of 'BLK' is a valid block number, false otherwise.
When the input source is a text file, attempt to read the next line from
the file.  If successful, make the result the current input buffer, set
'>IN' to 0 and return true; otherwise, return false.  A successful
result includes receipt of a line containing 0 characters.

   Conversely, if you have the bad luck (or lack of foresight) to have
to deal with parsing words without having such factors, how do you pass
a string that is not in the input stream to it?

'execute-parsing'       ... addr u xt - ...         gforth       "execute-parsing"
   Make addr u the current input source, execute xt '( ... -- ... )',
then restore the previous input source.

   A definition of this word in ANS Forth is provided in
'compat/execute-parsing.fs'.

   If you want to run a parsing word on a file, the following word
should help:

'execute-parsing-file'       i*x fileid xt - j*x         gforth       "execute-parsing-file"
   Make fileid the current input source, execute xt '( i*x -- j*x )',
then restore the previous input source.

5.15 Word Lists
===============

A wordlist is a list of named words; you can add new words and look up
words by name (and you can remove words in a restricted way with
markers).  Every named (and 'reveal'ed) word is in one wordlist.

   The text interpreter searches the wordlists present in the search
order (a stack of wordlists), from the top to the bottom.  Within each
wordlist, the search starts conceptually at the newest word; i.e., if
two words in a wordlist have the same name, the newer word is found.

   New words are added to the "compilation wordlist" (aka current
wordlist).

   A word list is identified by a cell-sized word list identifier (wid)
in much the same way as a file is identified by a file handle.  The
numerical value of the wid has no (portable) meaning, and might change
from session to session.

   The ANS Forth "Search order" word set is intended to provide a set of
low-level tools that allow various different schemes to be implemented.
Gforth also provides 'vocabulary', a traditional Forth word.
'compat/vocabulary.fs' provides an implementation in ANS Forth.

'forth-wordlist'       - wid         search       "forth-wordlist"
   'Constant' - wid identifies the word list that includes all of the
standard words provided by Gforth.  When Gforth is invoked, this word
list is the compilation word list and is at the top of the search order.

'definitions'       -         search       "definitions"
   Set the compilation word list to be the same as the word list that is
currently at the top of the search order.

'get-current'       - wid         search       "get-current"
   wid is the identifier of the current compilation word list.

'set-current'       wid -         search       "set-current"
   Set the compilation word list to the word list identified by wid.

'get-order'       - widn .. wid1 n         search       "get-order"
   Copy the search order to the data stack.  The current search order
has n entries, of which wid1 represents the wordlist that is searched
first (the word list at the top of the search order) and widn represents
the wordlist that is searched last.

'set-order'       widn .. wid1 n -         search       "set-order"
   If N=0, empty the search order.  If N=-1, set the search order to the
implementation-defined minimum search order (for Gforth, this is the
word list 'Root').  Otherwise, replace the existing search order with
the N wid entries such that WID1 represents the word list that will be
searched first and WIDN represents the word list that will be searched
last.

'wordlist'       - wid         search       "wordlist"
   Create a new, empty word list represented by wid.

'table'       - wid         gforth       "table"
   Create a case-sensitive wordlist.

'>order'       wid -         gforth       "to-order"
   Push WID on the search order.

'previous'       -         search-ext       "previous"
   Drop the wordlist at the top of the search order.

'also'       -         search-ext       "also"
   Like 'DUP' for the search order.  Usually used before a vocabulary
(e.g., 'also Forth'); the combined effect is to push the wordlist
represented by the vocabulary on the search order.

'Forth'       -         search-ext       "Forth"
   Replace the wid at the top of the search order with the wid
associated with the word list 'forth-wordlist'.

'Only'       -         search-ext       "Only"
   Set the search order to the implementation-defined minimum search
order (for Gforth, this is the word list 'Root').

'order'       -         search-ext       "order"
   Print the search order and the compilation word list.  The word lists
are printed in the order in which they are searched (which is reversed
with respect to the conventional way of displaying stacks).  The
compilation word list is displayed last.

'find'       c-addr - xt +-1 | c-addr 0         core,search       "find"
   Search all word lists in the current search order for the definition
named by the counted string at c-addr.  If the definition is not found,
return 0.  If the definition is found return 1 (if the definition has
non-default compilation semantics) or -1 (if the definition has default
compilation semantics).  The xt returned in interpret state represents
the interpretation semantics.  The xt returned in compile state
represented either the compilation semantics (for non-default
compilation semantics) or the run-time semantics that the compilation
semantics would 'compile,' (for default compilation semantics).  The ANS
Forth standard does not specify clearly what the returned xt represents
(and also talks about immediacy instead of non-default compilation
semantics), so this word is questionable in portable programs.  If
non-portability is ok, 'find-name' and friends are better (*note Name
token::).

'search-wordlist'       c-addr count wid - 0 | xt +-1         search       "search-wordlist"
   Search the word list identified by wid for the definition named by
the string at c-addr count.  If the definition is not found, return 0.
If the definition is found return 1 (if the definition is immediate) or
-1 (if the definition is not immediate) together with the xt.  In
Gforth, the xt returned represents the interpretation semantics.  ANS
Forth does not specify clearly what xt represents.

'words'       -         tools       "words"
   Display a list of all of the definitions in the word list at the top
of the search order.

'vlist'       -         gforth       "vlist"
   Old (pre-Forth-83) name for 'WORDS'.

'Root'       -         gforth       "Root"
   Add the root wordlist to the search order stack.  This vocabulary
makes up the minimum search order and contains only a search-order
words.

'Vocabulary'       "name" -         gforth       "Vocabulary"
   Create a definition "name" and associate a new word list with it.
The run-time effect of "name" is to replace the wid at the top of the
search order with the wid associated with the new word list.

'seal'       -         gforth       "seal"
   Remove all word lists from the search order stack other than the word
list that is currently on the top of the search order stack.

'vocs'       -         gforth       "vocs"
   List vocabularies and wordlists defined in the system.

'current'       - addr         gforth       "current"
   'Variable' - holds the wid of the compilation word list.

'context'       - addr         gforth       "context"
   'context' '@' is the wid of the word list at the top of the search
order.

5.15.1 Vocabularies
-------------------

Here is an example of creating and using a new wordlist using ANS Forth
words:

     wordlist constant my-new-words-wordlist
     : my-new-words get-order nip my-new-words-wordlist swap set-order ;

     \ add it to the search order
     also my-new-words

     \ alternatively, add it to the search order and make it
     \ the compilation word list
     also my-new-words definitions
     \ type "order" to see the problem

   The problem with this example is that 'order' has no way to associate
the name 'my-new-words' with the wid of the word list (in Gforth,
'order' and 'vocs' will display '???' for a wid that has no associated
name).  There is no Standard way of associating a name with a wid.

   In Gforth, this example can be re-coded using 'vocabulary', which
associates a name with a wid:

     vocabulary my-new-words

     \ add it to the search order
     also my-new-words

     \ alternatively, add it to the search order and make it
     \ the compilation word list
     my-new-words definitions
     \ type "order" to see that the problem is solved

5.15.2 Why use word lists?
--------------------------

Here are some reasons why people use wordlists:

   * To prevent a set of words from being used outside the context in
     which they are valid.  Two classic examples of this are an
     integrated editor (all of the edit commands are defined in a
     separate word list; the search order is set to the editor word list
     when the editor is invoked; the old search order is restored when
     the editor is terminated) and an integrated assembler (the op-codes
     for the machine are defined in a separate word list which is used
     when a 'CODE' word is defined).

   * To organize the words of an application or library into a
     user-visible set (in 'forth-wordlist' or some other common
     wordlist) and a set of helper words used just for the
     implementation (hidden in a separate wordlist).  This keeps
     'words'' output smaller, separates implementation and interface,
     and reduces the chance of name conflicts within the common
     wordlist.

   * To prevent a name-space clash between multiple definitions with the
     same name.  For example, when building a cross-compiler you might
     have a word 'IF' that generates conditional code for your target
     system.  By placing this definition in a different word list you
     can control whether the host system's 'IF' or the target system's
     'IF' get used in any particular context by controlling the order of
     the word lists on the search order stack.

   The downsides of using wordlists are:

   * Debugging becomes more cumbersome.

   * Name conflicts worked around with wordlists are still there, and
     you have to arrange the search order carefully to get the desired
     results; if you forget to do that, you get hard-to-find errors (as
     in any case where you read the code differently from the compiler;
     'see' can help seeing which of several possible words the name
     resolves to in such cases).  'See' displays just the name of the
     words, not what wordlist they belong to, so it might be misleading.
     Using unique names is a better approach to avoid name conflicts.

   * You have to explicitly undo any changes to the search order.  In
     many cases it would be more convenient if this happened implicitly.
     Gforth currently does not provide such a feature, but it may do so
     in the future.

5.15.3 Word list example
------------------------

The following example is from the garbage collector
(http://www.complang.tuwien.ac.at/forth/garbage-collection.zip) and uses
wordlists to separate public words from helper words:

     get-current ( wid )
     vocabulary garbage-collector also garbage-collector definitions
     ... \ define helper words
     ( wid ) set-current \ restore original (i.e., public) compilation wordlist
     ... \ define the public (i.e., API) words
         \ they can refer to the helper words
     previous \ restore original search order (helper words become invisible)

5.16 Environmental Queries
==========================

ANS Forth introduced the idea of "environmental queries" as a way for a
program running on a system to determine certain characteristics of the
system.  The Standard specifies a number of strings that might be
recognised by a system.

   The Standard requires that the header space used for environmental
queries be distinct from the header space used for definitions.

   Typically, environmental queries are supported by creating a set of
definitions in a word list that is only used during environmental
queries; that is what Gforth does.  There is no Standard way of adding
definitions to the set of recognised environmental queries, but any
implementation that supports the loading of optional word sets must have
some mechanism for doing this (after loading the word set, the
associated environmental query string must return 'true').  In Gforth,
the word list used to honour environmental queries can be manipulated
just like any other word list.

'environment?'       c-addr u - false / ... true         core       "environment-query"
   c-addr, u specify a counted string.  If the string is not recognised,
return a 'false' flag.  Otherwise return a 'true' flag and some
(string-specific) information about the queried string.

'environment-wordlist'       - wid         gforth       "environment-wordlist"
   wid identifies the word list that is searched by environmental
queries.

'gforth'       - c-addr u         gforth-environment       "gforth"
   Counted string representing a version string for this version of
Gforth (for versions>0.3.0).  The version strings of the various
versions are guaranteed to be ordered lexicographically.

'os-class'       - c-addr u         gforth-environment       "os-class"
   Counted string representing a description of the host operating
system.

   Note that, whilst the documentation for (e.g.)  'gforth' shows it
returning two items on the stack, querying it using 'environment?' will
return an additional item; the 'true' flag that shows that the string
was recognised.

   Here are some examples of using environmental queries:

     s" address-unit-bits" environment? 0=
     [IF]
          cr .( environmental attribute address-units-bits unknown... ) cr
     [ELSE]
          drop \ ensure balanced stack effect
     [THEN]

     \ this might occur in the prelude of a standard program that uses THROW
     s" exception" environment? [IF]
        0= [IF]
           : throw abort" exception thrown" ;
        [THEN]
     [ELSE] \ we don't know, so make sure
        : throw abort" exception thrown" ;
     [THEN]

     s" gforth" environment? [IF] .( Gforth version ) TYPE
                             [ELSE] .( Not Gforth..) [THEN]

     \ a program using v*
     s" gforth" environment? [IF]
       s" 0.5.0" compare 0< [IF] \ v* is a primitive since 0.5.0
        : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
          >r swap 2swap swap 0e r> 0 ?DO
            dup f@ over + 2swap dup f@ f* f+ over + 2swap
          LOOP
          2drop 2drop ;
       [THEN]
     [ELSE] \
       : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
       ...
     [THEN]

   Here is an example of adding a definition to the environment word
list:

     get-current environment-wordlist set-current
     true constant block
     true constant block-ext
     set-current

   You can see what definitions are in the environment word list like
this:

     environment-wordlist >order words previous

5.17 Files
==========

Gforth provides facilities for accessing files that are stored in the
host operating system's file-system.  Files that are processed by Gforth
can be divided into two categories:

   * Files that are processed by the Text Interpreter ("Forth source
     files").
   * Files that are processed by some other program ("general files").

5.17.1 Forth source files
-------------------------

The simplest way to interpret the contents of a file is to use one of
these two formats:

     include mysource.fs
     s" mysource.fs" included

   You usually want to include a file only if it is not included already
(by, say, another source file).  In that case, you can use one of these
three formats:

     require mysource.fs
     needs mysource.fs
     s" mysource.fs" required

   It is good practice to write your source files such that interpreting
them does not change the stack.  Source files designed in this way can
be used with 'required' and friends without complications.  For example:

     1024 require foo.fs drop

   Here you want to pass the argument 1024 (e.g., a buffer size) to
'foo.fs'.  Interpreting 'foo.fs' has the stack effect ( n - n ), which
allows its use with 'require'.  Of course with such parameters to
required files, you have to ensure that the first 'require' fits for all
uses (i.e., 'require' it early in the master load file).

'include-file'       i*x wfileid - j*x         file       "include-file"
   Interpret (process using the text interpreter) the contents of the
file WFILEID.

'included'       i*x c-addr u - j*x         file       "included"
   'include-file' the file whose name is given by the string C-ADDR U.

'included?'       c-addr u - f         gforth       "included?"
   True only if the file C-ADDR U is in the list of earlier included
files.  If the file has been loaded, it may have been specified as, say,
'foo.fs' and found somewhere on the Forth search path.  To return 'true'
from 'included?', you must specify the exact path to the file, even if
that is './foo.fs'

'include'       ... "file" - ...         gforth       "include"
   'include-file' the file FILE.

'required'       i*x addr u - i*x         gforth       "required"
   'include-file' the file with the name given by ADDR U, if it is not
'included' (or 'required') already.  Currently this works by comparing
the name of the file (with path) against the names of earlier included
files.

'require'       ... "file" - ...         gforth       "require"
   'include-file' FILE only if it is not included already.

'needs'       ... "name" - ...         gforth       "needs"
   An alias for 'require'; exists on other systems (e.g., Win32Forth).

'sourcefilename'       - c-addr u         gforth       "sourcefilename"
   The name of the source file which is currently the input source.  The
result is valid only while the file is being loaded.  If the current
input source is no (stream) file, the result is undefined.  In Gforth,
the result is valid during the whole seesion (but not across
'savesystem' etc.).

'sourceline#'       - u         gforth       "sourceline-number"
   The line number of the line that is currently being interpreted from
a (stream) file.  The first line has the number 1.  If the current input
source is not a (stream) file, the result is undefined.

   A definition in ANS Forth for 'required' is provided in
'compat/required.fs'.

5.17.2 General files
--------------------

Files are opened/created by name and type.  The following file access
methods (FAMs) are recognised:

'r/o'       - fam         file       "r-o"

'r/w'       - fam         file       "r-w"

'w/o'       - fam         file       "w-o"

'bin'       fam1 - fam2         file       "bin"

   When a file is opened/created, it returns a file identifier, wfileid
that is used for all other file commands.  All file commands also return
a status value, wior, that is 0 for a successful operation and an
implementation-defined non-zero value in the case of an error.

'open-file'       c-addr u wfam - wfileid wior        file       "open-file"

'create-file'       c-addr u wfam - wfileid wior        file       "create-file"

'close-file'       wfileid - wior        file       "close-file"

'delete-file'       c-addr u - wior        file       "delete-file"

'rename-file'       c-addr1 u1 c-addr2 u2 - wior        file-ext       "rename-file"
   Rename file c_addr1 u1 to new name c_addr2 u2

'read-file'       c-addr u1 wfileid - u2 wior        file       "read-file"

'read-line'       c_addr u1 wfileid - u2 flag wior         file       "read-line"

'key-file'       wfileid - c        gforth       "paren-key-file"
   Read one character c from wfileid.  This word disables buffering for
wfileid.  If you want to read characters from a terminal in
non-canonical (raw) mode, you have to put the terminal in non-canonical
mode yourself (using the C interface); the exception is 'stdin': Gforth
automatically puts it into non-canonical mode.

'key?-file'       wfileid - f        gforth       "key-q-file"
   f is true if at least one character can be read from wfileid without
blocking.  If you also want to use 'read-file' or 'read-line' on the
file, you have to call 'key?-file' or 'key-file' first (these two words
disable buffering).

'write-file'       c-addr u1 wfileid - wior        file       "write-file"

'write-line'       c-addr u fileid - ior         file       "write-line"

'emit-file'       c wfileid - wior        gforth       "emit-file"

'flush-file'       wfileid - wior        file-ext       "flush-file"

'file-status'       c-addr u - wfam wior        file-ext       "file-status"

'file-position'       wfileid - ud wior        file       "file-position"

'reposition-file'       ud wfileid - wior        file       "reposition-file"

'file-size'       wfileid - ud wior        file       "file-size"

'resize-file'       ud wfileid - wior        file       "resize-file"

'slurp-file'       c-addr1 u1 - c-addr2 u2         gforth       "slurp-file"
   C-ADDR1 U1 is the filename, C-ADDR2 U2 is the file's contents

'slurp-fid'       fid - addr u         gforth       "slurp-fid"
   ADDR U is the content of the file FID

'stdin'       - wfileid        gforth       "stdin"
   The standard input file of the Gforth process.

'stdout'       - wfileid        gforth       "stdout"
   The standard output file of the Gforth process.

'stderr'       - wfileid        gforth       "stderr"
   The standard error output file of the Gforth process.

5.17.3 Redirection
------------------

You can redirect the output of 'type' and 'emit' and all the words that
use them (all output words that don't have an explicit target file) to
an arbitrary file with the 'outfile-execute', used like this:

     : some-warning ( n -- )
         cr ." warning# " . ;

     : print-some-warning ( n -- )
         ['] some-warning stderr outfile-execute ;

   After 'some-warning' is executed, the original output direction is
restored; this construct is safe against exceptions.  Similarly, there
is 'infile-execute' for redirecting the input of 'key' and its users
(any input word that does not take a file explicitly).

'outfile-execute'       ... xt file-id - ...         gforth       "outfile-execute"
   execute xt with the output of 'type' etc.  redirected to file-id.

'infile-execute'       ... xt file-id - ...         gforth       "infile-execute"
   execute xt with the input of 'key' etc.  redirected to file-id.

   If you do not want to redirect the input or output to a file, you can
also make use of the fact that 'key', 'emit' and 'type' are deferred
words (*note Deferred Words::).  However, in that case you have to worry
about the restoration and the protection against exceptions yourself;
also, note that for redirecting the output in this way, you have to
redirect both 'emit' and 'type'.

5.17.4 Search Paths
-------------------

If you specify an absolute filename (i.e., a filename starting with '/'
or '~', or with ':' in the second position (as in 'C:...')) for
'included' and friends, that file is included just as you would expect.

   If the filename starts with './', this refers to the directory that
the present file was 'included' from.  This allows files to include
other files relative to their own position (irrespective of the current
working directory or the absolute position).  This feature is essential
for libraries consisting of several files, where a file may include
other files from the library.  It corresponds to '#include "..."' in C.
If the current input source is not a file, '.' refers to the directory
of the innermost file being included, or, if there is no file being
included, to the current working directory.

   For relative filenames (not starting with './'), Gforth uses a search
path similar to Forth's search order (*note Word Lists::).  It tries to
find the given filename in the directories present in the path, and
includes the first one it finds.  There are separate search paths for
Forth source files and general files.  If the search path contains the
directory '.', this refers to the directory of the current file, or the
working directory, as if the file had been specified with './'.

   Use '~+' to refer to the current working directory (as in the
'bash').

5.17.4.1 Source Search Paths
............................

The search path is initialized when you start Gforth (*note Invoking
Gforth::).  You can display it and change it using 'fpath' in
combination with the general path handling words.

'fpath'       - path-addr         gforth       "fpath"

Here is an example of using 'fpath' and 'require':

     fpath path= /usr/lib/forth/|./
     require timer.fs

5.17.4.2 General Search Paths
.............................

Your application may need to search files in several directories, like
'included' does.  To facilitate this, Gforth allows you to define and
use your own search paths, by providing generic equivalents of the Forth
search path words:

'open-path-file'       addr1 u1 path-addr - wfileid addr2 u2 0 | ior         gforth       "open-path-file"
   Look in path PATH-ADDR for the file specified by ADDR1 U1.  If found,
the resulting path and and (read-only) open file descriptor are
returned.  If the file is not found, IOR is what came back from the last
attempt at opening the file (in the current implementation).

'path-allot'       umax -         gforth       "path-allot"
   'Allot' a path with umax characters capacity, initially empty.

'clear-path'       path-addr -         gforth       "clear-path"
   Set the path path-addr to empty.

'also-path'       c-addr len path-addr -         gforth       "also-path"
   add the directory c-addr len to path-addr.

'.path'       path-addr -         gforth       ".path"
   Display the contents of the search path PATH-ADDR.

'path+'       path-addr  "dir" -         gforth       "path+"
   Add the directory DIR to the search path PATH-ADDR.

'path='       path-addr "dir1|dir2|dir3"         gforth       "path="
   Make a complete new search path; the path separator is |.

   Here's an example of creating an empty search path:
     create mypath 500 path-allot \ maximum length 500 chars (is checked)

5.18 Blocks
===========

When you run Gforth on a modern desk-top computer, it runs under the
control of an operating system which provides certain services.  One of
these services is FILE SERVICES, which allows Forth source code and data
to be stored in files and read into Gforth (*note Files::).

   Traditionally, Forth has been an important programming language on
systems where it has interfaced directly to the underlying hardware with
no intervening operating system.  Forth provides a mechanism, called
"blocks", for accessing mass storage on such systems.

   A block is a 1024-byte data area, which can be used to hold data or
Forth source code.  No structure is imposed on the contents of the
block.  A block is identified by its number; blocks are numbered
contiguously from 1 to an implementation-defined maximum.

   A typical system that used blocks but no operating system might use a
single floppy-disk drive for mass storage, with the disks formatted to
provide 256-byte sectors.  Blocks would be implemented by assigning the
first four sectors of the disk to block 1, the second four sectors to
block 2 and so on, up to the limit of the capacity of the disk.  The
disk would not contain any file system information, just the set of
blocks.

   On systems that do provide file services, blocks are typically
implemented by storing a sequence of blocks within a single "blocks
file".  The size of the blocks file will be an exact multiple of 1024
bytes, corresponding to the number of blocks it contains.  This is the
mechanism that Gforth uses.

   Only one blocks file can be open at a time.  If you use block words
without having specified a blocks file, Gforth defaults to the blocks
file 'blocks.fb'.  Gforth uses the Forth search path when attempting to
locate a blocks file (*note Source Search Paths::).

   When you read and write blocks under program control, Gforth uses a
number of "block buffers" as intermediate storage.  These buffers are
not used when you use 'load' to interpret the contents of a block.

   The behaviour of the block buffers is analagous to that of a cache.
Each block buffer has three states:

   * Unassigned
   * Assigned-clean
   * Assigned-dirty

   Initially, all block buffers are unassigned.  In order to access a
block, the block (specified by its block number) must be assigned to a
block buffer.

   The assignment of a block to a block buffer is performed by 'block'
or 'buffer'.  Use 'block' when you wish to modify the existing contents
of a block.  Use 'buffer' when you don't care about the existing
contents of the block(1).

   Once a block has been assigned to a block buffer using 'block' or
'buffer', that block buffer becomes the current block buffer.  Data may
only be manipulated (read or written) within the current block buffer.

   When the contents of the current block buffer has been modified it is
necessary, _before calling 'block' or 'buffer' again_, to either abandon
the changes (by doing nothing) or mark the block as changed
(assigned-dirty), using 'update'.  Using 'update' does not change the
blocks file; it simply changes a block buffer's state to assigned-dirty.
The block will be written implicitly when it's buffer is needed for
another block, or explicitly by 'flush' or 'save-buffers'.

   word 'Flush' writes all assigned-dirty blocks back to the blocks file
on disk.  Leaving Gforth with 'bye' also performs a 'flush'.

   In Gforth, 'block' and 'buffer' use a direct-mapped algorithm to
assign a block buffer to a block.  That means that any particular block
can only be assigned to one specific block buffer, called (for the
particular operation) the victim buffer.  If the victim buffer is
unassigned or assigned-clean it is allocated to the new block
immediately.  If it is assigned-dirty its current contents are written
back to the blocks file on disk before it is allocated to the new block.

   Although no structure is imposed on the contents of a block, it is
traditional to display the contents as 16 lines each of 64 characters.
A block provides a single, continuous stream of input (for example, it
acts as a single parse area) - there are no end-of-line characters
within a block, and no end-of-file character at the end of a block.
There are two consequences of this:

   * The last character of one line wraps straight into the first
     character of the following line
   * The word '\' - comment to end of line - requires special treatment;
     in the context of a block it causes all characters until the end of
     the current 64-character "line" to be ignored.

   In Gforth, when you use 'block' with a non-existent block number, the
current blocks file will be extended to the appropriate size and the
block buffer will be initialised with spaces.

   Gforth includes a simple block editor (type 'use blocked.fb 0 list'
for details) but doesn't encourage the use of blocks; the mechanism is
only provided for backward compatibility - ANS Forth requires blocks to
be available when files are.

   Common techniques that are used when working with blocks include:

   * A screen editor that allows you to edit blocks without leaving the
     Forth environment.
   * Shadow screens; where every code block has an associated block
     containing comments (for example: code in odd block numbers,
     comments in even block numbers).  Typically, the block editor
     provides a convenient mechanism to toggle between code and
     comments.
   * Load blocks; a single block (typically block 1) contains a number
     of 'thru' commands which 'load' the whole of the application.

   See Frank Sergeant's Pygmy Forth to see just how well blocks can be
integrated into a Forth programming environment.

'open-blocks'       c-addr u -         gforth       "open-blocks"
   Use the file, whose name is given by c-addr u, as the blocks file.

'use'       "file" -         gforth       "use"
   Use file as the blocks file.

'block-offset'       - addr         gforth       "block-offset"
   User variable containing the number of the first block (default since
0.5.0: 0).  Block files created with Gforth versions before 0.5.0 have
the offset 1.  If you use these files you can: '1 offset !'; or add 1 to
every block number used; or prepend 1024 characters to the file.

'get-block-fid'       - wfileid         gforth       "get-block-fid"
   Return the file-id of the current blocks file.  If no blocks file has
been opened, use 'blocks.fb' as the default blocks file.

'block-position'       u -         block       "block-position"
   Position the block file to the start of block u.

'list'       u -         block-ext       "list"
   Display block u.  In Gforth, the block is displayed as 16 numbered
lines, each of 64 characters.

'scr'       - a-addr         block-ext       "s-c-r"
   'User' variable - a-addr is the address of a cell containing the
block number of the block most recently processed by 'list'.

'block'       u - a-addr         block       "block"
   If a block buffer is assigned for block u, return its start address,
a-addr.  Otherwise, assign a block buffer for block u (if the assigned
block buffer has been 'update'd, transfer the contents to mass storage),
read the block into the block buffer and return its start address,
a-addr.

'buffer'       u - a-addr         block       "buffer"
   If a block buffer is assigned for block u, return its start address,
a-addr.  Otherwise, assign a block buffer for block u (if the assigned
block buffer has been 'update'd, transfer the contents to mass storage)
and return its start address, a-addr.  The subtle difference between
'buffer' and 'block' mean that you should only use 'buffer' if you don't
care about the previous contents of block u.  In Gforth, this simply
calls 'block'.

'empty-buffers'       -         block-ext       "empty-buffers"
   Mark all block buffers as unassigned; if any had been marked as
assigned-dirty (by 'update'), the changes to those blocks will be lost.

'empty-buffer'       buffer -         gforth       "empty-buffer"

'update'       -         block       "update"
   Mark the state of the current block buffer as assigned-dirty.

'updated?'       n - f         gforth       "updated?"
   Return true if 'updated' has been used to mark block n as
assigned-dirty.

'save-buffers'       -         block       "save-buffers"
   Transfer the contents of each 'update'd block buffer to mass storage,
then mark all block buffers as assigned-clean.

'save-buffer'       buffer -         gforth       "save-buffer"

'flush'       -         block       "flush"
   Perform the functions of 'save-buffers' then 'empty-buffers'.

'load'       i*x n - j*x         block       "load"
   Save the current input source specification.  Store n in 'BLK', set
'>IN' to 0 and interpret.  When the parse area is exhausted, restore the
input source specification.

'thru'       i*x n1 n2 - j*x         block-ext       "thru"
   'load' the blocks n1 through n2 in sequence.

'+load'       i*x n - j*x         gforth       "+load"
   Used within a block to load the block specified as the current block
+ n.

'+thru'       i*x n1 n2 - j*x         gforth       "+thru"
   Used within a block to load the range of blocks specified as the
current block + n1 thru the current block + n2.

'-->'       -         gforth       "chain"
   If this symbol is encountered whilst loading block n, discard the
remainder of the block and load block n+1.  Used for chaining multiple
blocks together as a single loadable unit.  Not recommended, because it
destroys the independence of loading.  Use 'thru' (which is standard) or
'+thru' instead.

'block-included'       a-addr u -         gforth       "block-included"
   Use within a block that is to be processed by 'load'.  Save the
current blocks file specification, open the blocks file specified by
a-addr u and 'load' block 1 from that file (which may in turn chain or
load other blocks).  Finally, close the blocks file and restore the
original blocks file.

   ---------- Footnotes ----------

   (1) The ANS Forth definition of 'buffer' is intended not to cause
disk I/O; if the data associated with the particular block is already
stored in a block buffer due to an earlier 'block' command, 'buffer'
will return that block buffer and the existing contents of the block
will be available.  Otherwise, 'buffer' will simply assign a new, empty
block buffer for the block.

5.19 Other I/O
==============

5.19.1 Simple numeric output
----------------------------

The simplest output functions are those that display numbers from the
data or floating-point stacks.  Floating-point output is always
displayed using base 10.  Numbers displayed from the data stack use the
value stored in 'base'.

'.'       n -         core       "dot"
   Display (the signed single number) N in free-format, followed by a
space.

'dec.'       n -         gforth       "dec."
   Display n as a signed decimal number, followed by a space.

'hex.'       u -         gforth       "hex."
   Display u as an unsigned hex number, prefixed with a "$" and followed
by a space.

'u.'       u -         core       "u-dot"
   Display (the unsigned single number) U in free-format, followed by a
space.

'.r'       n1 n2 -         core-ext       "dot-r"
   Display N1 right-aligned in a field N2 characters wide.  If more than
N2 characters are needed to display the number, all digits are
displayed.  If appropriate, N2 must include a character for a leading
"-".

'u.r'       u n -         core-ext       "u-dot-r"
   Display U right-aligned in a field N characters wide.  If more than N
characters are needed to display the number, all digits are displayed.

'd.'       d -         double       "d-dot"
   Display (the signed double number) D in free-format.  followed by a
space.

'ud.'       ud -         gforth       "u-d-dot"
   Display (the signed double number) UD in free-format, followed by a
space.

'd.r'       d n -         double       "d-dot-r"
   Display D right-aligned in a field N characters wide.  If more than N
characters are needed to display the number, all digits are displayed.
If appropriate, N must include a character for a leading "-".

'ud.r'       ud n -         gforth       "u-d-dot-r"
   Display UD right-aligned in a field N characters wide.  If more than
N characters are needed to display the number, all digits are displayed.

'f.'       r -         float-ext       "f-dot"
   Display (the floating-point number) r without exponent, followed by a
space.

'fe.'       r -         float-ext       "f-e-dot"
   Display r using engineering notation (with exponent dividable by 3),
followed by a space.

'fs.'       r -         float-ext       "f-s-dot"
   Display r using scientific notation (with exponent), followed by a
space.

'f.rdp'       rf +nr +nd +np -         gforth       "f.rdp"
   Print float rf formatted.  The total width of the output is nr.  For
fixed-point notation, the number of digits after the decimal point is
+nd and the minimum number of significant digits is np.  'Set-precision'
has no effect on 'f.rdp'.  Fixed-point notation is used if the number of
siginicant digits would be at least np and if the number of digits
before the decimal point would fit.  If fixed-point notation is not
used, exponential notation is used, and if that does not fit, asterisks
are printed.  We recommend using nr>=7 to avoid the risk of numbers not
fitting at all.  We recommend nr>=np+5 to avoid cases where 'f.rdp'
switches to exponential notation because fixed-point notation would have
too few significant digits, yet exponential notation offers fewer
significant digits.  We recommend nr>=nd+2, if you want to have
fixed-point notation for some numbers.  We recommend np>nr, if you want
to have exponential notation for all numbers.

   Examples of printing the number 1234.5678E23 in the different
floating-point output formats are shown below:

     f. 123456779999999000000000000.
     fe. 123.456779999999E24
     fs. 1.23456779999999E26

5.19.2 Formatted numeric output
-------------------------------

Forth traditionally uses a technique called "pictured numeric output"
for formatted printing of integers.  In this technique, digits are
extracted from the number (using the current output radix defined by
'base'), converted to ASCII codes and appended to a string that is built
in a scratch-pad area of memory (*note Implementation-defined options:
core-idef.).  Arbitrary characters can be appended to the string during
the extraction process.  The completed string is specified by an address
and length and can be manipulated ('TYPE'ed, copied, modified) under
program control.

   All of the integer output words described in the previous section
(*note Simple numeric output::) are implemented in Gforth using pictured
numeric output.

   Three important things to remember about pictured numeric output:

   * It always operates on double-precision numbers; to display a
     single-precision number, convert it first (for ways of doing this
     *note Double precision::).
   * It always treats the double-precision number as though it were
     unsigned.  The examples below show ways of printing signed numbers.
   * The string is built up from right to left; least significant digit
     first.

'<#'       -         core       "less-number-sign"
   Initialise/clear the pictured numeric output string.

'<<#'       -         gforth       "less-less-number-sign"
   Start a hold area that ends with '#>>'.  Can be nested in each other
and in '<#'.  Note: if you do not match up the '<<#'s with '#>>'s, you
will eventually run out of hold area; you can reset the hold area to
empty with '<#'.

'#'       ud1 - ud2         core       "number-sign"
   Used within '<#' and '#>'.  Add the next least-significant digit to
the pictured numeric output string.  This is achieved by dividing UD1 by
the number in 'base' to leave quotient UD2 and remainder N; N is
converted to the appropriate display code (eg ASCII code) and appended
to the string.  If the number has been fully converted, UD1 will be 0
and '#' will append a "0" to the string.

'#s'       ud - 0 0         core       "number-sign-s"
   Used within '<#' and '#>'.  Convert all remaining digits using the
same algorithm as for '#'.  '#s' will convert at least one digit.
Therefore, if UD is 0, '#s' will append a "0" to the pictured numeric
output string.

'hold'       char -         core       "hold"
   Used within '<#' and '#>'.  Append the character CHAR to the pictured
numeric output string.

'sign'       n -         core       "sign"
   Used within '<#' and '#>'.  If N (a SINGLE number) is negative,
append the display code for a minus sign to the pictured numeric output
string.  Since the string is built up "backwards" this is usually used
immediately prior to '#>', as shown in the examples below.

'#>'       xd - addr u         core       "number-sign-greater"
   Complete the pictured numeric output string by discarding XD and
returning ADDR U; the address and length of the formatted string.  A
Standard program may modify characters within the string.

'#>>'       -         gforth       "number-sign-greater-greater"
   Release the hold area started with '<<#'.

'represent'       r c-addr u - n f1 f2        float       "represent"

'f>str-rdp'       rf +nr +nd +np - c-addr nr         gforth       "f>str-rdp"
   Convert rf into a string at c-addr nr.  The conversion rules and the
meanings of nr +nd np are the same as for 'f.rdp'.  The result in in the
pictured numeric output buffer and will be destroyed by anything
destroying that buffer.

'f>buf-rdp'       rf c-addr +nr +nd +np -         gforth       "f>buf-rdp"
   Convert rf into a string at c-addr nr.  The conversion rules and the
meanings of nr nd np are the same as for 'f.rdp'.

Here are some examples of using pictured numeric output:

     : my-u. ( u -- )
       \ Simplest use of pns.. behaves like Standard u.
       0              \ convert to unsigned double
       <<#            \ start conversion
       #s             \ convert all digits
       #>             \ complete conversion
       TYPE SPACE     \ display, with trailing space
       #>> ;          \ release hold area

     : cents-only ( u -- )
       0              \ convert to unsigned double
       <<#            \ start conversion
       # #            \ convert two least-significant digits
       #>             \ complete conversion, discard other digits
       TYPE SPACE     \ display, with trailing space
       #>> ;          \ release hold area

     : dollars-and-cents ( u -- )
       0              \ convert to unsigned double
       <<#            \ start conversion
       # #            \ convert two least-significant digits
       [char] . hold  \ insert decimal point
       #s             \ convert remaining digits
       [char] $ hold  \ append currency symbol
       #>             \ complete conversion
       TYPE SPACE     \ display, with trailing space
       #>> ;          \ release hold area

     : my-. ( n -- )
       \ handling negatives.. behaves like Standard .
       s>d            \ convert to signed double
       swap over dabs \ leave sign byte followed by unsigned double
       <<#            \ start conversion
       #s             \ convert all digits
       rot sign       \ get at sign byte, append "-" if needed
       #>             \ complete conversion
       TYPE SPACE     \ display, with trailing space
       #>> ;          \ release hold area

     : account. ( n -- )
       \ accountants don't like minus signs, they use parentheses
       \ for negative numbers
       s>d            \ convert to signed double
       swap over dabs \ leave sign byte followed by unsigned double
       <<#            \ start conversion
       2 pick         \ get copy of sign byte
       0< IF [char] ) hold THEN \ right-most character of output
       #s             \ convert all digits
       rot            \ get at sign byte
       0< IF [char] ( hold THEN
       #>             \ complete conversion
       TYPE SPACE     \ display, with trailing space
       #>> ;          \ release hold area


   Here are some examples of using these words:

     1 my-u. 1
     hex -1 my-u. decimal FFFFFFFF
     1 cents-only 01
     1234 cents-only 34
     2 dollars-and-cents $0.02
     1234 dollars-and-cents $12.34
     123 my-. 123
     -123 my. -123
     123 account. 123
     -456 account. (456)

5.19.3 String Formats
---------------------

Forth commonly uses two different methods for representing character
strings:

   * As a "counted string", represented by a c-addr.  The char addressed
     by c-addr contains a character-count, n, of the string and the
     string occupies the subsequent n char addresses in memory.
   * As cell pair on the stack; c-addr u, where u is the length of the
     string in characters, and c-addr is the address of the first byte
     of the string.

   ANS Forth encourages the use of the second format when representing
strings.

'count'       c-addr1 - c-addr2 u        core       "count"
   c-addr2 is the first character and u the length of the counted string
at c-addr1.

   For words that move, copy and search for strings see *note Memory
Blocks::.  For words that display characters and strings see *note
Displaying characters and strings::.

5.19.4 Displaying characters and strings
----------------------------------------

This section starts with a glossary of Forth words and ends with a set
of examples.

'bl'       - c-char         core       "b-l"
   c-char is the character value for a space.

'space'       -         core       "space"
   Display one space.

'spaces'       u -         core       "spaces"
   Display N spaces.

'emit'       c -         core       "emit"
   Display the character associated with character value c.

'toupper'       c1 - c2        gforth       "toupper"
   If c1 is a lower-case character (in the current locale), c2 is the
equivalent upper-case character.  All other characters are unchanged.

'."'       compilation 'ccc"' - ; run-time -         core       "dot-quote"
   Compilation: Parse a string ccc delimited by a " (double quote).  At
run-time, display the string.  Interpretation semantics for this word
are undefined in ANS Forth.  Gforth's interpretation semantics are to
display the string.  This is the simplest way to display a string from
within a definition; see examples below.

'.('       compilation&interpretation "ccc<paren>" -         core-ext       "dot-paren"
   Compilation and interpretation semantics: Parse a string ccc
delimited by a ')' (right parenthesis).  Display the string.  This is
often used to display progress information during compilation; see
examples below.

'.\"'       compilation 'ccc"' - ; run-time -         gforth       "dot-backslash-quote"
   Like '."', but translates C-like \-escape-sequences (see 'S\"').

'type'       c-addr u -         core       "type"
   If U>0, display U characters from a string starting with the
character stored at C-ADDR.

'typewhite'       addr n -         gforth       "typewhite"
   Like type, but white space is printed instead of the characters.

'cr'       -         core       "c-r"
   Output a newline (of the favourite kind of the host OS). Note that
due to the way the Forth command line interpreter inserts newlines, the
preferred way to use 'cr' is at the start of a piece of text; e.g., 'cr
." hello, world"'.

'S"'       compilation 'ccc"' - ; run-time - c-addr u         core,file       "s-quote"
   Compilation: Parse a string ccc delimited by a '"' (double quote).
At run-time, return the length, u, and the start address, c-addr of the
string.  Interpretation: parse the string as before, and return c-addr,
u.  Gforth 'allocate's the string.  The resulting memory leak is usually
not a problem; the exception is if you create strings containing 'S"'
and 'evaluate' them; then the leak is not bounded by the size of the
interpreted files and you may want to 'free' the strings.  ANS Forth
only guarantees one buffer of 80 characters, so in standard programs you
should assume that the string lives only until the next 's"'.

's\"'       compilation 'ccc"' - ; run-time - c-addr u         gforth       "s-backslash-quote"
   Like 'S"', but translates C-like \-escape-sequences, as follows: '\a'
BEL (alert), '\b' BS, '\e' ESC (not in C99), '\f' FF, '\n' newline, '\r'
CR, '\t' HT, '\v' VT, '\"' ", '\\' \, '\'[0-7]{1,3} octal numerical
character value (non-standard), '\x'[0-9a-f]{0,2} hex numerical
character value (standard only with two digits); a '\' before any other
character is reserved.

'C"'       compilation "ccc<quote>" - ; run-time  - c-addr         core-ext       "c-quote"
   Compilation: parse a string ccc delimited by a '"' (double quote).
At run-time, return c-addr which specifies the counted string ccc.
Interpretation semantics are undefined.

'char'       '<spaces>ccc' - c         core       "char"
   Skip leading spaces.  Parse the string ccc and return c, the display
code representing the first character of ccc.

'[Char]'       compilation '<spaces>ccc' - ; run-time - c         core       "bracket-char"
   Compilation: skip leading spaces.  Parse the string ccc.  Run-time:
return c, the display code representing the first character of ccc.
Interpretation semantics for this word are undefined.

As an example, consider the following text, stored in a file 'test.fs':

     .( text-1)
     : my-word
       ." text-2" cr
       .( text-3)
     ;

     ." text-4"

     : my-char
       [char] ALPHABET emit
       char emit
     ;

   When you load this code into Gforth, the following output is
generated:

     include test.fs <RET> text-1text-3text-4 ok

   * Messages 'text-1' and 'text-3' are displayed because '.(' is an
     immediate word; it behaves in the same way whether it is used
     inside or outside a colon definition.
   * Message 'text-4' is displayed because of Gforth's added
     interpretation semantics for '."'.
   * Message 'text-2' is not displayed, because the text interpreter
     performs the compilation semantics for '."' within the definition
     of 'my-word'.

   Here are some examples of executing 'my-word' and 'my-char':

     my-word <RET> text-2
      ok
     my-char fred <RET> Af ok
     my-char jim <RET> Aj ok

   * Message 'text-2' is displayed because of the run-time behaviour of
     '."'.
   * '[char]' compiles the "A" from "ALPHABET" and puts its display code
     on the stack at run-time.  'emit' always displays the character
     when 'my-char' is executed.
   * 'char' parses a string at run-time and the second 'emit' displays
     the first character of the string.
   * If you type 'see my-char' you can see that '[char]' discarded the
     text "LPHABET" and only compiled the display code for "A" into the
     definition of 'my-char'.

5.19.5 Terminal output
----------------------

If you are outputting to a terminal, you may want to control the
positioning of the cursor:

'at-xy'       u1 u2 -         facility       "at-x-y"
   Position the cursor so that subsequent text output will take place at
column U1, row U2 of the display.  (column 0, row 0 is the top left-hand
corner of the display).

   In order to know where to position the cursor, it is often helpful to
know the size of the screen:

'form'       - urows ucols        gforth       "form"
   The number of lines and columns in the terminal.  These numbers may
change with the window size.  Note that it depends on the OS whether
this reflects the actual size and changes with the window size
(currently only on Unix-like OSs).  On other OSs you just get a default,
and can tell Gforth the terminal size by setting the environment
variables 'COLUMNS' and 'LINES' before starting Gforth.

   And sometimes you want to use:

'page'       -         facility       "page"
   Clear the display and set the cursor to the top left-hand corner.

   Note that on non-terminals you should use '12 emit', not 'page', to
get a form feed.

5.19.6 Single-key input
-----------------------

If you want to get a single printable character, you can use 'key'; to
check whether a character is available for 'key', you can use 'key?'.

'key'       - char         core       "key"
   Receive (but do not display) one character, CHAR.

'key?'       - flag         facility       "key-question"
   Determine whether a character is available.  If a character is
available, FLAG is true; the next call to 'key' will yield the
character.  Once 'key?' returns true, subsequent calls to 'key?' before
calling 'key' or 'ekey' will also return true.

   If you want to process a mix of printable and non-printable
characters, you can do that with 'ekey' and friends.  'Ekey' produces a
keyboard event that you have to convert into a character with
'ekey>char' or into a key identifier with 'ekey>fkey'.

   Typical code for using EKEY looks like this:

     ekey ekey>char if ( c )
       ... \ do something with the character
     else ekey>fkey if ( key-id )
       case
         k-up                                  of ... endof
         k-f1                                  of ... endof
         k-left k-shift-mask or k-ctrl-mask or of ... endof
         ...
       endcase
     else ( keyboard-event )
       drop \ just ignore an unknown keyboard event type
     then then

'ekey'       - u         facility-ext       "e-key"
   Receive a keyboard event U (encoding implementation-defined).

'ekey>char'       u - u false | c true         facility-ext       "e-key-to-char"
   Convert keyboard event U into character 'c' if possible.

'ekey>fkey'       u1 - u2 f         X:ekeys       "ekey>fkey"
   If u1 is a keyboard event in the special key set, convert keyboard
event U1 into key id U2 and return true; otherwise return U1 and false.

'ekey?'       - flag         facility-ext       "e-key-question"
   True if a keyboard event is available.

   The key identifiers for cursor keys are:

'k-left'       - u         X:ekeys       "k-left"

'k-right'       - u         X:ekeys       "k-right"

'k-up'       - u         X:ekeys       "k-up"

'k-down'       - u         X:ekeys       "k-down"

'k-home'       - u         X:ekeys       "k-home"
   aka Pos1

'k-end'       - u         X:ekeys       "k-end"

'k-prior'       - u         X:ekeys       "k-prior"
   aka PgUp

'k-next'       - u         X:ekeys       "k-next"
   aka PgDn

'k-insert'       - u         X:ekeys       "k-insert"

'k-delete'       - u         X:ekeys       "k-delete"

   The key identifiers for function keys (aka keypad keys) are:

'k-f1'       - u         X:ekeys       "k-f1"

'k-f2'       - u         X:ekeys       "k-f2"

'k-f3'       - u         X:ekeys       "k-f3"

'k-f4'       - u         X:ekeys       "k-f4"

'k-f5'       - u         X:ekeys       "k-f5"

'k-f6'       - u         X:ekeys       "k-f6"

'k-f7'       - u         X:ekeys       "k-f7"

'k-f8'       - u         X:ekeys       "k-f8"

'k-f9'       - u         X:ekeys       "k-f9"

'k-f10'       - u         X:ekeys       "k-f10"

'k-f11'       - u         X:ekeys       "k-f11"

'k-f12'       - u         X:ekeys       "k-f12"

   Note that 'k-f11' and 'k-f12' are not as widely available.

   You can combine these key identifiers with masks for various shift
keys:

'k-shift-mask'       - u         X:ekeys       "k-shift-mask"

'k-ctrl-mask'       - u         X:ekeys       "k-ctrl-mask"

'k-alt-mask'       - u         X:ekeys       "k-alt-mask"

   Note that, even if a Forth system has 'ekey>fkey' and the key
identifier words, the keys are not necessarily available or it may not
necessarily be able to report all the keys and all the possible
combinations with shift masks.  Therefore, write your programs in such a
way that they are still useful even if the keys and key combinations
cannot be pressed or are not recognized.

   Examples: Older keyboards often do not have an F11 and F12 key.  If
you run Gforth in an xterm, the xterm catches a number of combinations
(e.g., <Shift-Up>), and never passes it to Gforth.  Finally, Gforth
currently does not recognize and report combinations with multiple shift
keys (so the <shift-ctrl-left> case in the example above would never be
entered).

   Gforth recognizes various keys available on ANSI terminals (in MS-DOS
you need the ANSI.SYS driver to get that behaviour); it works by
recognizing the escape sequences that ANSI terminals send when such a
key is pressed.  If you have a terminal that sends other escape
sequences, you will not get useful results on Gforth.  Other Forth
systems may work in a different way.

5.19.7 Line input and conversion
--------------------------------

For ways of storing character strings in memory see *note String
Formats::.

   Words for inputting one line from the keyboard:

'accept'       c-addr +n1 - +n2         core       "accept"
   Get a string of up to N1 characters from the user input device and
store it at C-ADDR.  N2 is the length of the received string.  The user
indicates the end by pressing <RET>.  Gforth supports all the editing
functions available on the Forth command line (including history and
word completion) in 'accept'.

'edit-line'       c-addr n1 n2 - n3         gforth       "edit-line"
   edit the string with length N2 in the buffer C-ADDR N1, like
'accept'.

   Conversion words:

's>number?'       addr u - d f         gforth       "s>number?"
   converts string addr u into d, flag indicates success

's>unumber?'       c-addr u - ud flag         gforth       "s>unumber?"
   converts string c-addr u into ud, flag indicates success

'>number'       ud1 c-addr1 u1 - ud2 c-addr2 u2         core       "to-number"
   Attempt to convert the character string C-ADDR1 U1 to an unsigned
number in the current number base.  The double UD1 accumulates the
result of the conversion to form UD2.  Conversion continues,
left-to-right, until the whole string is converted or a character that
is not convertable in the current number base is encountered (including
+ or -).  For each convertable character, UD1 is first multiplied by the
value in 'BASE' and then incremented by the value represented by the
character.  C-ADDR2 is the location of the first unconverted character
(past the end of the string if the whole string was converted).  U2 is
the number of unconverted characters in the string.  Overflow is not
detected.

'>float'       c-addr u - f:... flag        float       "to-float"
   Actual stack effect: ( c_addr u - r t | f ).  Attempt to convert the
character string c-addr u to internal floating-point representation.  If
the string represents a valid floating-point number r is placed on the
floating-point stack and flag is true.  Otherwise, flag is false.  A
string of blanks is a special case and represents the floating-point
number 0.

   Obsolescent input and conversion words:

'convert'       ud1 c-addr1 - ud2 c-addr2         core-ext-obsolescent       "convert"
   Obsolescent: superseded by '>number'.

'expect'       c-addr +n -         core-ext-obsolescent       "expect"
   Receive a string of at most +n characters, and store it in memory
starting at c-addr.  The string is displayed.  Input terminates when the
<return> key is pressed or +n characters have been received.  The normal
Gforth line editing capabilites are available.  The length of the string
is stored in 'span'; it does not include the <return> character.
OBSOLESCENT: superceeded by 'accept'.

'span'       - c-addr         core-ext-obsolescent       "span"
   'Variable' - c-addr is the address of a cell that stores the length
of the last string received by 'expect'.  OBSOLESCENT.

5.19.8 Pipes
------------

In addition to using Gforth in pipes created by other processes (*note
Gforth in pipes::), you can create your own pipe with 'open-pipe', and
read from or write to it.

'open-pipe'       c-addr u wfam - wfileid wior        gforth       "open-pipe"

'close-pipe'       wfileid - wretval wior        gforth       "close-pipe"

   If you write to a pipe, Gforth can throw a 'broken-pipe-error'; if
you don't catch this exception, Gforth will catch it and exit, usually
silently (*note Gforth in pipes::).  Since you probably do not want
this, you should wrap a 'catch' or 'try' block around the code from
'open-pipe' to 'close-pipe', so you can deal with the problem yourself,
and then return to regular processing.

'broken-pipe-error'       - n         gforth       "broken-pipe-error"
   the error number for a broken pipe

5.19.9 Xchars and Unicode
-------------------------

ASCII is only appropriate for the English language.  Most western
languages however fit somewhat into the Forth frame, since a byte is
sufficient to encode the few special characters in each (though not
always the same encoding can be used; latin-1 is most widely used,
though).  For other languages, different char-sets have to be used,
several of them variable-width.  Most prominent representant is UTF-8.
Let's call these extended characters xchars.  The primitive fixed-size
characters stored as bytes are called pchars in this section.

   The xchar words add a few data types:

   * XC is an extended char (xchar) on the stack.  It occupies one cell,
     and is a subset of unsigned cell.  Note: UTF-8 can not store more
     that 31 bits; on 16 bit systems, only the UCS16 subset of the UTF-8
     character set can be used.

   * XC-ADDR is the address of an xchar in memory.  Alignment
     requirements are the same as C-ADDR.  The memory representation of
     an xchar differs from the stack representation, and depends on the
     encoding used.  An xchar may use a variable number of pchars in
     memory.

   * XC-ADDR U is a buffer of xchars in memory, starting at XC-ADDR, U
     pchars long.

'xc-size'       xc - u         xchar-ext       "xc-size"
   Computes the memory size of the xchar XC in pchars.

'x-size'       xc-addr u1 - u2         xchar       "x-size"
   Computes the memory size of the first xchar stored at XC-ADDR in
pchars.

'xc@+'       xc-addr1 - xc-addr2 xc         xchar-ext       "xc-fetch-plus"
   Fetchs the xchar XC at XC-ADDR1.  XC-ADDR2 points to the first memory
location after XC.

'xc!+?'       xc xc-addr1 u1 - xc-addr2 u2 f         xchar-ext       "xc-store-plus-query"
   Stores the xchar XC into the buffer starting at address XC-ADDR1, U1
pchars large.  XC-ADDR2 points to the first memory location after XC, U2
is the remaining size of the buffer.  If the xchar XC did fit into the
buffer, F is true, otherwise F is false, and XC-ADDR2 U2 equal XC-ADDR1
U1.  XC!+?  is safe for buffer overflows, and therefore preferred over
XC!+.

'xchar+'       xc-addr1 - xc-addr2         xchar-ext       "xchar+"
   Adds the size of the xchar stored at XC-ADDR1 to this address, giving
XC-ADDR2.

'xchar-'       xc-addr1 - xc-addr2         xchar-ext       "xchar-"
   Goes backward from XC_ADDR1 until it finds an xchar so that the size
of this xchar added to XC_ADDR2 gives XC_ADDR1.

'+x/string'       xc-addr1 u1 - xc-addr2 u2         xchar       "plus-x-slash-string"
   Step forward by one xchar in the buffer defined by address XC-ADDR1,
size U1 pchars.  XC-ADDR2 is the address and u2 the size in pchars of
the remaining buffer after stepping over the first xchar in the buffer.

'x\string-'       xc-addr1 u1 - xc-addr1 u2         xchar       "x-back-string-minus"
   Step backward by one xchar in the buffer defined by address XC-ADDR1
and size U1 in pchars, starting at the end of the buffer.  XC-ADDR1 is
the address and U2 the size in pchars of the remaining buffer after
stepping backward over the last xchar in the buffer.

'-trailing-garbage'       xc-addr u1 - addr u2         xchar-ext       "-trailing-garbage"
   Examine the last XCHAR in the buffer XC-ADDR U1--if the encoding is
correct and it repesents a full pchar, U2 equals U1, otherwise, U2
represents the string without the last (garbled) xchar.

'x-width'       xc-addr u - n         xchar-ext       "x-width"
   N is the number of monospace ASCII pchars that take the same space to
display as the the xchar string starting at XC-ADDR, using U pchars;
assuming a monospaced display font, i.e.  pchar width is always an
integer multiple of the width of an ASCII pchar.

'xkey'       - xc         xchar-ext       "xkey"
   Reads an xchar from the terminal.  This will discard all input events
up to the completion of the xchar.

'xemit'       xc -         xchar-ext       "xemit"
   Prints an xchar on the terminal.

   There's a new environment query

'xchar-encoding'       - addr u         xchar-ext       "xchar-encoding"
   Returns a printable ASCII string that reperesents the encoding, and
use the preferred MIME name (if any) or the name in
<http://www.iana.org/assignments/character-sets> like "ISO-LATIN-1" or
"UTF-8", with the exception of "ASCII", where we prefer the alias
"ASCII".

5.20 OS command line arguments
==============================

The usual way to pass arguments to Gforth programs on the command line
is via the '-e' option, e.g.

     gforth -e "123 456" foo.fs -e bye

   However, you may want to interpret the command-line arguments
directly.  In that case, you can access the (image-specific)
command-line arguments through 'next-arg':

'next-arg'       - addr u         gforth       "next-arg"
   get the next argument from the OS command line, consuming it; if
there is no argument left, return '0 0'.

   Here's an example program 'echo.fs' for 'next-arg':

     : echo ( -- )
         begin
     	next-arg 2dup 0 0 d<> while
     	    type space
         repeat
         2drop ;

     echo cr bye

   This can be invoked with

     gforth echo.fs hello world

   and it will print

     hello world

   The next lower level of dealing with the OS command line are the
following words:

'arg'       u - addr count         gforth       "arg"
   Return the string for the uth command-line argument; returns '0 0' if
the access is beyond the last argument.  '0 arg' is the program name
with which you started Gforth.  The next unprocessed argument is always
'1 arg', the one after that is '2 arg' etc.  All arguments already
processed by the system are deleted.  After you have processed an
argument, you can delete it with 'shift-args'.

'shift-args'       -         gforth       "shift-args"
   '1 arg' is deleted, shifting all following OS command line parameters
to the left by 1, and reducing 'argc @'.  This word can change 'argv @'.

   Finally, at the lowest level Gforth provides the following words:

'argc'       - addr         gforth       "argc"
   'Variable' - the number of command-line arguments (including the
command name).  Changed by 'next-arg' and 'shift-args'.

'argv'       - addr         gforth       "argv"
   'Variable' - a pointer to a vector of pointers to the command-line
arguments (including the command-name).  Each argument is represented as
a C-style zero-terminated string.  Changed by 'next-arg' and
'shift-args'.

5.21 Locals
===========

Local variables can make Forth programming more enjoyable and Forth
programs easier to read.  Unfortunately, the locals of ANS Forth are
laden with restrictions.  Therefore, we provide not only the ANS Forth
locals wordset, but also our own, more powerful locals wordset (we
implemented the ANS Forth locals wordset through our locals wordset).

   The ideas in this section have also been published in M. Anton Ertl,
'Automatic Scoping of Local Variables
(http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz)', EuroForth '94.

5.21.1 Gforth locals
--------------------

Locals can be defined with

     { local1 local2 ... -- comment }
   or
     { local1 local2 ... }

   E.g.,
     : max { n1 n2 -- n3 }
      n1 n2 > if
        n1
      else
        n2
      endif ;

   The similarity of locals definitions with stack comments is intended.
A locals definition often replaces the stack comment of a word.  The
order of the locals corresponds to the order in a stack comment and
everything after the '--' is really a comment.

   This similarity has one disadvantage: It is too easy to confuse
locals declarations with stack comments, causing bugs and making them
hard to find.  However, this problem can be avoided by appropriate
coding conventions: Do not use both notations in the same program.  If
you do, they should be distinguished using additional means, e.g.  by
position.

   The name of the local may be preceded by a type specifier, e.g., 'F:'
for a floating point value:

     : CX* { F: Ar F: Ai F: Br F: Bi -- Cr Ci }
     \ complex multiplication
      Ar Br f* Ai Bi f* f-
      Ar Bi f* Ai Br f* f+ ;

   Gforth currently supports cells ('W:', 'W^'), doubles ('D:', 'D^'),
floats ('F:', 'F^') and characters ('C:', 'C^') in two flavours: a
value-flavoured local (defined with 'W:', 'D:' etc.)  produces its value
and can be changed with 'TO'.  A variable-flavoured local (defined with
'W^' etc.)  produces its address (which becomes invalid when the
variable's scope is left).  E.g., the standard word 'emit' can be
defined in terms of 'type' like this:

     : emit { C^ char* -- }
         char* 1 type ;

   A local without type specifier is a 'W:' local.  Both flavours of
locals are initialized with values from the data or FP stack.

   Currently there is no way to define locals with user-defined data
structures, but we are working on it.

   Gforth allows defining locals everywhere in a colon definition.  This
poses the following questions:

5.21.1.1 Where are locals visible by name?
..........................................

Basically, the answer is that locals are visible where you would expect
it in block-structured languages, and sometimes a little longer.  If you
want to restrict the scope of a local, enclose its definition in
'SCOPE'...'ENDSCOPE'.

'scope'       compilation  - scope ; run-time  -         gforth       "scope"

'endscope'       compilation scope - ; run-time  -         gforth       "endscope"

   These words behave like control structure words, so you can use them
with 'CS-PICK' and 'CS-ROLL' to restrict the scope in arbitrary ways.

   If you want a more exact answer to the visibility question, here's
the basic principle: A local is visible in all places that can only be
reached through the definition of the local(1).  In other words, it is
not visible in places that can be reached without going through the
definition of the local.  E.g., locals defined in 'IF'...'ENDIF' are
visible until the 'ENDIF', locals defined in 'BEGIN'...'UNTIL' are
visible after the 'UNTIL' (until, e.g., a subsequent 'ENDSCOPE').

   The reasoning behind this solution is: We want to have the locals
visible as long as it is meaningful.  The user can always make the
visibility shorter by using explicit scoping.  In a place that can only
be reached through the definition of a local, the meaning of a local
name is clear.  In other places it is not: How is the local initialized
at the control flow path that does not contain the definition?  Which
local is meant, if the same name is defined twice in two independent
control flow paths?

   This should be enough detail for nearly all users, so you can skip
the rest of this section.  If you really must know all the gory details
and options, read on.

   In order to implement this rule, the compiler has to know which
places are unreachable.  It knows this automatically after 'AHEAD',
'AGAIN', 'EXIT' and 'LEAVE'; in other cases (e.g., after most 'THROW's),
you can use the word 'UNREACHABLE' to tell the compiler that the control
flow never reaches that place.  If 'UNREACHABLE' is not used where it
could, the only consequence is that the visibility of some locals is
more limited than the rule above says.  If 'UNREACHABLE' is used where
it should not (i.e., if you lie to the compiler), buggy code will be
produced.

'UNREACHABLE'       -         gforth       "UNREACHABLE"

   Another problem with this rule is that at 'BEGIN', the compiler does
not know which locals will be visible on the incoming back-edge.  All
problems discussed in the following are due to this ignorance of the
compiler (we discuss the problems using 'BEGIN' loops as examples; the
discussion also applies to '?DO' and other loops).  Perhaps the most
insidious example is:
     AHEAD
     BEGIN
       x
     [ 1 CS-ROLL ] THEN
       { x }
       ...
     UNTIL

   This should be legal according to the visibility rule.  The use of
'x' can only be reached through the definition; but that appears
textually below the use.

   From this example it is clear that the visibility rules cannot be
fully implemented without major headaches.  Our implementation treats
common cases as advertised and the exceptions are treated in a safe way:
The compiler makes a reasonable guess about the locals visible after a
'BEGIN'; if it is too pessimistic, the user will get a spurious error
about the local not being defined; if the compiler is too optimistic, it
will notice this later and issue a warning.  In the case above the
compiler would complain about 'x' being undefined at its use.  You can
see from the obscure examples in this section that it takes quite
unusual control structures to get the compiler into trouble, and even
then it will often do fine.

   If the 'BEGIN' is reachable from above, the most optimistic guess is
that all locals visible before the 'BEGIN' will also be visible after
the 'BEGIN'.  This guess is valid for all loops that are entered only
through the 'BEGIN', in particular, for normal
'BEGIN'...'WHILE'...'REPEAT' and 'BEGIN'...'UNTIL' loops and it is
implemented in our compiler.  When the branch to the 'BEGIN' is finally
generated by 'AGAIN' or 'UNTIL', the compiler checks the guess and warns
the user if it was too optimistic:
     IF
       { x }
     BEGIN
       \ x ?
     [ 1 cs-roll ] THEN
       ...
     UNTIL

   Here, 'x' lives only until the 'BEGIN', but the compiler
optimistically assumes that it lives until the 'THEN'.  It notices this
difference when it compiles the 'UNTIL' and issues a warning.  The user
can avoid the warning, and make sure that 'x' is not used in the wrong
area by using explicit scoping:
     IF
       SCOPE
       { x }
       ENDSCOPE
     BEGIN
     [ 1 cs-roll ] THEN
       ...
     UNTIL

   Since the guess is optimistic, there will be no spurious error
messages about undefined locals.

   If the 'BEGIN' is not reachable from above (e.g., after 'AHEAD' or
'EXIT'), the compiler cannot even make an optimistic guess, as the
locals visible after the 'BEGIN' may be defined later.  Therefore, the
compiler assumes that no locals are visible after the 'BEGIN'.  However,
the user can use 'ASSUME-LIVE' to make the compiler assume that the same
locals are visible at the BEGIN as at the point where the top
control-flow stack item was created.

'ASSUME-LIVE'       orig - orig         gforth       "ASSUME-LIVE"

E.g.,
     { x }
     AHEAD
     ASSUME-LIVE
     BEGIN
       x
     [ 1 CS-ROLL ] THEN
       ...
     UNTIL

   Other cases where the locals are defined before the 'BEGIN' can be
handled by inserting an appropriate 'CS-ROLL' before the 'ASSUME-LIVE'
(and changing the control-flow stack manipulation behind the
'ASSUME-LIVE').

   Cases where locals are defined after the 'BEGIN' (but should be
visible immediately after the 'BEGIN') can only be handled by
rearranging the loop.  E.g., the "most insidious" example above can be
arranged into:
     BEGIN
       { x }
       ... 0=
     WHILE
       x
     REPEAT

   ---------- Footnotes ----------

   (1) In compiler construction terminology, all places dominated by the
definition of the local.

5.21.1.2 How long do locals live?
.................................

The right answer for the lifetime question would be: A local lives at
least as long as it can be accessed.  For a value-flavoured local this
means: until the end of its visibility.  However, a variable-flavoured
local could be accessed through its address far beyond its visibility
scope.  Ultimately, this would mean that such locals would have to be
garbage collected.  Since this entails un-Forth-like implementation
complexities, I adopted the same cowardly solution as some other
languages (e.g., C): The local lives only as long as it is visible;
afterwards its address is invalid (and programs that access it
afterwards are erroneous).

5.21.1.3 Locals programming style
.................................

The freedom to define locals anywhere has the potential to change
programming styles dramatically.  In particular, the need to use the
return stack for intermediate storage vanishes.  Moreover, all stack
manipulations (except 'PICK's and 'ROLL's with run-time determined
arguments) can be eliminated: If the stack items are in the wrong order,
just write a locals definition for all of them; then write the items in
the order you want.

   This seems a little far-fetched and eliminating stack manipulations
is unlikely to become a conscious programming objective.  Still, the
number of stack manipulations will be reduced dramatically if local
variables are used liberally (e.g., compare 'max' (*note Gforth
locals::) with a traditional implementation of 'max').

   This shows one potential benefit of locals: making Forth programs
more readable.  Of course, this benefit will only be realized if the
programmers continue to honour the principle of factoring instead of
using the added latitude to make the words longer.

   Using 'TO' can and should be avoided.  Without 'TO', every
value-flavoured local has only a single assignment and many advantages
of functional languages apply to Forth.  I.e., programs are easier to
analyse, to optimize and to read: It is clear from the definition what
the local stands for, it does not turn into something different later.

   E.g., a definition using 'TO' might look like this:
     : strcmp { addr1 u1 addr2 u2 -- n }
      u1 u2 min 0
      ?do
        addr1 c@ addr2 c@ -
        ?dup-if
          unloop exit
        then
        addr1 char+ TO addr1
        addr2 char+ TO addr2
      loop
      u1 u2 - ;
   Here, 'TO' is used to update 'addr1' and 'addr2' at every loop
iteration.  'strcmp' is a typical example of the readability problems of
using 'TO'.  When you start reading 'strcmp', you think that 'addr1'
refers to the start of the string.  Only near the end of the loop you
realize that it is something else.

   This can be avoided by defining two locals at the start of the loop
that are initialized with the right value for the current iteration.
     : strcmp { addr1 u1 addr2 u2 -- n }
      addr1 addr2
      u1 u2 min 0
      ?do { s1 s2 }
        s1 c@ s2 c@ -
        ?dup-if
          unloop exit
        then
        s1 char+ s2 char+
      loop
      2drop
      u1 u2 - ;
   Here it is clear from the start that 's1' has a different value in
every loop iteration.

5.21.1.4 Locals implementation
..............................

Gforth uses an extra locals stack.  The most compelling reason for this
is that the return stack is not float-aligned; using an extra stack also
eliminates the problems and restrictions of using the return stack as
locals stack.  Like the other stacks, the locals stack grows toward
lower addresses.  A few primitives allow an efficient implementation:

'@local#'       #noffset - w        gforth       "fetch-local-number"

'f@local#'       #noffset - r        gforth       "f-fetch-local-number"

'laddr#'       #noffset - c-addr        gforth       "laddr-number"

'lp+!#'       #noffset -        gforth       "lp-plus-store-number"
   used with negative immediate values it allocates memory on the local
stack, a positive immediate argument drops memory from the local stack

'lp!'       c-addr -        gforth       "lp-store"

'>l'       w -        gforth       "to-l"

'f>l'       r -        gforth       "f-to-l"

   In addition to these primitives, some specializations of these
primitives for commonly occurring inline arguments are provided for
efficiency reasons, e.g., '@local0' as specialization of '@local#' for
the inline argument 0.  The following compiling words compile the right
specialized version, or the general version, as appropriate:

'compile-lp+!'       n -         gforth       "compile-l-p-plus-store"

   Combinations of conditional branches and 'lp+!#' like '?branch-lp+!#'
(the locals pointer is only changed if the branch is taken) are provided
for efficiency and correctness in loops.

   A special area in the dictionary space is reserved for keeping the
local variable names.  '{' switches the dictionary pointer to this area
and '}' switches it back and generates the locals initializing code.
'W:' etc. are normal defining words.  This special area is cleared at
the start of every colon definition.

   A special feature of Gforth's dictionary is used to implement the
definition of locals without type specifiers: every word list (aka
vocabulary) has its own methods for searching etc.  (*note Word
Lists::).  For the present purpose we defined a word list with a special
search method: When it is searched for a word, it actually creates that
word using 'W:'.  '{' changes the search order to first search the word
list containing '}', 'W:' etc., and then the word list for defining
locals without type specifiers.

   The lifetime rules support a stack discipline within a colon
definition: The lifetime of a local is either nested with other locals
lifetimes or it does not overlap them.

   At 'BEGIN', 'IF', and 'AHEAD' no code for locals stack pointer
manipulation is generated.  Between control structure words locals
definitions can push locals onto the locals stack.  'AGAIN' is the
simplest of the other three control flow words.  It has to restore the
locals stack depth of the corresponding 'BEGIN' before branching.  The
code looks like this:
'lp+!#' current-locals-size - dest-locals-size
'branch' <begin>

   'UNTIL' is a little more complicated: If it branches back, it must
adjust the stack just like 'AGAIN'.  But if it falls through, the locals
stack must not be changed.  The compiler generates the following code:
'?branch-lp+!#' <begin> current-locals-size - dest-locals-size
   The locals stack pointer is only adjusted if the branch is taken.

   'THEN' can produce somewhat inefficient code:
'lp+!#' current-locals-size - orig-locals-size
<orig target>:
'lp+!#' orig-locals-size - new-locals-size
   The second 'lp+!#' adjusts the locals stack pointer from the level at
the orig point to the level after the 'THEN'.  The first 'lp+!#' adjusts
the locals stack pointer from the current level to the level at the orig
point, so the complete effect is an adjustment from the current level to
the right level after the 'THEN'.

   In a conventional Forth implementation a dest control-flow stack
entry is just the target address and an orig entry is just the address
to be patched.  Our locals implementation adds a word list to every orig
or dest item.  It is the list of locals visible (or assumed visible) at
the point described by the entry.  Our implementation also adds a tag to
identify the kind of entry, in particular to differentiate between live
and dead (reachable and unreachable) orig entries.

   A few unusual operations have to be performed on locals word lists:

'common-list'       list1 list2 - list3         gforth-internal       "common-list"

'sub-list?'       list1 list2 - f         gforth-internal       "sub-list?"

'list-size'       list - u         gforth-internal       "list-size"

   Several features of our locals word list implementation make these
operations easy to implement: The locals word lists are organised as
linked lists; the tails of these lists are shared, if the lists contain
some of the same locals; and the address of a name is greater than the
address of the names behind it in the list.

   Another important implementation detail is the variable 'dead-code'.
It is used by 'BEGIN' and 'THEN' to determine if they can be reached
directly or only through the branch that they resolve.  'dead-code' is
set by 'UNREACHABLE', 'AHEAD', 'EXIT' etc., and cleared at the start of
a colon definition, by 'BEGIN' and usually by 'THEN'.

   Counted loops are similar to other loops in most respects, but
'LEAVE' requires special attention: It performs basically the same
service as 'AHEAD', but it does not create a control-flow stack entry.
Therefore the information has to be stored elsewhere; traditionally, the
information was stored in the target fields of the branches created by
the 'LEAVE's, by organizing these fields into a linked list.
Unfortunately, this clever trick does not provide enough space for
storing our extended control flow information.  Therefore, we introduce
another stack, the leave stack.  It contains the control-flow stack
entries for all unresolved 'LEAVE's.

   Local names are kept until the end of the colon definition, even if
they are no longer visible in any control-flow path.  In a few cases
this may lead to increased space needs for the locals name area, but
usually less than reclaiming this space would cost in code size.

5.21.2 ANS Forth locals
-----------------------

The ANS Forth locals wordset does not define a syntax for locals, but
words that make it possible to define various syntaxes.  One of the
possible syntaxes is a subset of the syntax we used in the Gforth locals
wordset, i.e.:

     { local1 local2 ... -- comment }
or
     { local1 local2 ... }

   The order of the locals corresponds to the order in a stack comment.
The restrictions are:

   * Locals can only be cell-sized values (no type specifiers are
     allowed).
   * Locals can be defined only outside control structures.
   * Locals can interfere with explicit usage of the return stack.  For
     the exact (and long) rules, see the standard.  If you don't use
     return stack accessing words in a definition using locals, you will
     be all right.  The purpose of this rule is to make locals
     implementation on the return stack easier.
   * The whole definition must be in one line.

   Locals defined in ANS Forth behave like 'VALUE's (*note Values::).
I.e., they are initialized from the stack.  Using their name produces
their value.  Their value can be changed using 'TO'.

   Since the syntax above is supported by Gforth directly, you need not
do anything to use it.  If you want to port a program using this syntax
to another ANS Forth system, use 'compat/anslocal.fs' to implement the
syntax on the other system.

   Note that a syntax shown in the standard, section A.13 looks similar,
but is quite different in having the order of locals reversed.  Beware!

   The ANS Forth locals wordset itself consists of one word:

'(local)'       addr u -         local       "paren-local-paren"

   The ANS Forth locals extension wordset defines a syntax using
'locals|', but it is so awful that we strongly recommend not to use it.
We have implemented this syntax to make porting to Gforth easy, but do
not document it here.  The problem with this syntax is that the locals
are defined in an order reversed with respect to the standard stack
comment notation, making programs harder to read, and easier to misread
and miswrite.  The only merit of this syntax is that it is easy to
implement using the ANS Forth locals wordset.

5.22 Structures
===============

This section presents the structure package that comes with Gforth.  A
version of the package implemented in ANS Forth is available in
'compat/struct.fs'.  This package was inspired by a posting on
comp.lang.forth in 1989 (unfortunately I don't remember, by whom;
possibly John Hayes).  A version of this section has been published in
M. Anton Ertl, Yet Another Forth Structures Package
(http://www.complang.tuwien.ac.at/forth/objects/structs.html), Forth
Dimensions 19(3), pages 13-16.  Marcel Hendrix provided helpful
comments.

5.22.1 Why explicit structure support?
--------------------------------------

If we want to use a structure containing several fields, we could simply
reserve memory for it, and access the fields using address arithmetic
(*note Address arithmetic::).  As an example, consider a structure with
the following fields

'a'
     is a float
'b'
     is a cell
'c'
     is a float

   Given the (float-aligned) base address of the structure we get the
address of the field

'a'
     without doing anything further.
'b'
     with 'float+'
'c'
     with 'float+ cell+ faligned'

   It is easy to see that this can become quite tiring.

   Moreover, it is not very readable, because seeing a 'cell+' tells us
neither which kind of structure is accessed nor what field is accessed;
we have to somehow infer the kind of structure, and then look up in the
documentation, which field of that structure corresponds to that offset.

   Finally, this kind of address arithmetic also causes maintenance
troubles: If you add or delete a field somewhere in the middle of the
structure, you have to find and change all computations for the fields
afterwards.

   So, instead of using 'cell+' and friends directly, how about storing
the offsets in constants:

     0 constant a-offset
     0 float+ constant b-offset
     0 float+ cell+ faligned c-offset

   Now we can get the address of field 'x' with 'x-offset +'.  This is
much better in all respects.  Of course, you still have to change all
later offset definitions if you add a field.  You can fix this by
declaring the offsets in the following way:

     0 constant a-offset
     a-offset float+ constant b-offset
     b-offset cell+ faligned constant c-offset

   Since we always use the offsets with '+', we could use a defining
word 'cfield' that includes the '+' in the action of the defined word:

     : cfield ( n "name" -- )
         create ,
     does> ( name execution: addr1 -- addr2 )
         @ + ;

     0 cfield a
     0 a float+ cfield b
     0 b cell+ faligned cfield c

   Instead of 'x-offset +', we now simply write 'x'.

   The structure field words now can be used quite nicely.  However,
their definition is still a bit cumbersome: We have to repeat the name,
the information about size and alignment is distributed before and after
the field definitions etc.  The structure package presented here
addresses these problems.

5.22.2 Structure Usage
----------------------

You can define a structure for a (data-less) linked list with:
     struct
         cell% field list-next
     end-struct list%

   With the address of the list node on the stack, you can compute the
address of the field that contains the address of the next node with
'list-next'.  E.g., you can determine the length of a list with:

     : list-length ( list -- n )
     \ "list" is a pointer to the first element of a linked list
     \ "n" is the length of the list
         0 BEGIN ( list1 n1 )
             over
         WHILE ( list1 n1 )
             1+ swap list-next @ swap
         REPEAT
         nip ;

   You can reserve memory for a list node in the dictionary with 'list%
%allot', which leaves the address of the list node on the stack.  For
the equivalent allocation on the heap you can use 'list% %alloc' (or,
for an 'allocate'-like stack effect (i.e., with ior), use 'list%
%allocate').  You can get the the size of a list node with 'list% %size'
and its alignment with 'list% %alignment'.

   Note that in ANS Forth the body of a 'create'd word is 'aligned' but
not necessarily 'faligned'; therefore, if you do a:

     create _name_ foo% %allot drop

then the memory alloted for 'foo%' is guaranteed to start at the body of
'_name_' only if 'foo%' contains only character, cell and double fields.
Therefore, if your structure contains floats, better use

     foo% %allot constant _name_

   You can include a structure 'foo%' as a field of another structure,
like this:
     struct
     ...
         foo% field ...
     ...
     end-struct ...

   Instead of starting with an empty structure, you can extend an
existing structure.  E.g., a plain linked list without data, as defined
above, is hardly useful; You can extend it to a linked list of integers,
like this:(1)

     list%
         cell% field intlist-int
     end-struct intlist%

   'intlist%' is a structure with two fields: 'list-next' and
'intlist-int'.

   You can specify an array type containing _n_ elements of type 'foo%'
like this:

     foo% _n_ *

   You can use this array type in any place where you can use a normal
type, e.g., when defining a 'field', or with '%allot'.

   The first field is at the base address of a structure and the word
for this field (e.g., 'list-next') actually does not change the address
on the stack.  You may be tempted to leave it away in the interest of
run-time and space efficiency.  This is not necessary, because the
structure package optimizes this case: If you compile a first-field
words, no code is generated.  So, in the interest of readability and
maintainability you should include the word for the field when accessing
the field.

   ---------- Footnotes ----------

   (1) This feature is also known as _extended records_.  It is the main
innovation in the Oberon language; in other words, adding this feature
to Modula-2 led Wirth to create a new language, write a new compiler
etc.  Adding this feature to Forth just required a few lines of code.

5.22.3 Structure Naming Convention
----------------------------------

The field names that come to (my) mind are often quite generic, and, if
used, would cause frequent name clashes.  E.g., many structures probably
contain a 'counter' field.  The structure names that come to (my) mind
are often also the logical choice for the names of words that create
such a structure.

   Therefore, I have adopted the following naming conventions:

   * The names of fields are of the form '_struct_-_field_', where
     '_struct_' is the basic name of the structure, and '_field_' is the
     basic name of the field.  You can think of field words as
     converting the (address of the) structure into the (address of the)
     field.

   * The names of structures are of the form '_struct_%', where
     '_struct_' is the basic name of the structure.

   This naming convention does not work that well for fields of extended
structures; e.g., the integer list structure has a field 'intlist-int',
but has 'list-next', not 'intlist-next'.

5.22.4 Structure Implementation
-------------------------------

The central idea in the implementation is to pass the data about the
structure being built on the stack, not in some global variable.
Everything else falls into place naturally once this design decision is
made.

   The type description on the stack is of the form _align size_.
Keeping the size on the top-of-stack makes dealing with arrays very
simple.

   'field' is a defining word that uses 'Create' and 'DOES>'.  The body
of the field contains the offset of the field, and the normal 'DOES>'
action is simply:

     @ +

i.e., add the offset to the address, giving the stack effect addr1 -
addr2 for a field.

   This simple structure is slightly complicated by the optimization for
fields with offset 0, which requires a different 'DOES>'-part (because
we cannot rely on there being something on the stack if such a field is
invoked during compilation).  Therefore, we put the different
'DOES>'-parts in separate words, and decide which one to invoke based on
the offset.  For a zero offset, the field is basically a noop; it is
immediate, and therefore no code is generated when it is compiled.

5.22.5 Structure Glossary
-------------------------

'%align'       align size -         gforth       "%align"
   Align the data space pointer to the alignment ALIGN.

'%alignment'       align size - align         gforth       "%alignment"
   The alignment of the structure.

'%alloc'       align size - addr         gforth       "%alloc"
   Allocate SIZE address units with alignment ALIGN, giving a data block
at ADDR; 'throw' an ior code if not successful.

'%allocate'       align size - addr ior         gforth       "%allocate"
   Allocate SIZE address units with alignment ALIGN, similar to
'allocate'.

'%allot'       align size - addr         gforth       "%allot"
   Allot SIZE address units of data space with alignment ALIGN; the
resulting block of data is found at ADDR.

'cell%'       - align size         gforth       "cell%"

'char%'       - align size         gforth       "char%"

'dfloat%'       - align size         gforth       "dfloat%"

'double%'       - align size         gforth       "double%"

'end-struct'       align size "name" -         gforth       "end-struct"
   Define a structure/type descriptor NAME with alignment ALIGN and size
SIZE1 (SIZE rounded up to be a multiple of ALIGN).
'name' execution: - ALIGN SIZE1

'field'       align1 offset1 align size "name" -  align2 offset2         gforth       "field"
   Create a field NAME with offset OFFSET1, and the type given by ALIGN
SIZE.  OFFSET2 is the offset of the next field, and ALIGN2 is the
alignment of all fields.
'name' execution: ADDR1 - ADDR2.
ADDR2=ADDR1+OFFSET1

'float%'       - align size         gforth       "float%"

'naligned'       addr1 n - addr2         gforth       "naligned"
   ADDR2 is the aligned version of ADDR1 with respect to the alignment
N.

'sfloat%'       - align size         gforth       "sfloat%"

'%size'       align size - size         gforth       "%size"
   The size of the structure.

'struct'       - align size         gforth       "struct"
   An empty structure, used to start a structure definition.

5.22.6 Forth200x Structures
---------------------------

The Forth 200x standard defines a slightly less convenient form of
structures.  In general (when using 'field+', you have to perform the
alignment yourself, but there are a number of convenience words (e.g.,
'field:' that perform the alignment for you.

   A typical usage example is:

     0
       field:                   s-a
       faligned 2 floats +field s-b
     constant s-struct

   An alternative way of writing this structure is:

     begin-structure s-struct
       field:                   s-a
       faligned 2 floats +field s-b
     end-structure

'begin-structure'       "name" - struct-sys 0         X:structures       "begin-structure"

'end-structure'       struct-sys +n -         X:structures       "end-structure"

'+field'       n1 n2 "name" - n3         X:structures       "plus-field"

'cfield:'       u1 "name" - u2         X:structures       "cfield:"

'field:'       u1 "name" - u2         X:structures       "field:"

'2field:'       u1 "name" - u2         gforth       "2field:"

'ffield:'       u1 "name" - u2         X:structures       "ffield:"

'sffield:'       u1 "name" - u2         X:structures       "sffield:"

'dffield:'       u1 "name" - u2         X:structures       "dffield:"

5.23 Object-oriented Forth
==========================

Gforth comes with three packages for object-oriented programming:
'objects.fs', 'oof.fs', and 'mini-oof.fs'; none of them is preloaded, so
you have to 'include' them before use.  The most important differences
between these packages (and others) are discussed in *note Comparison
with other object models::.  All packages are written in ANS Forth and
can be used with any other ANS Forth.

5.23.1 Why object-oriented programming?
---------------------------------------

Often we have to deal with several data structures (_objects_), that
have to be treated similarly in some respects, but differently in
others.  Graphical objects are the textbook example: circles, triangles,
dinosaurs, icons, and others, and we may want to add more during program
development.  We want to apply some operations to any graphical object,
e.g., 'draw' for displaying it on the screen.  However, 'draw' has to do
something different for every kind of object.

   We could implement 'draw' as a big 'CASE' control structure that
executes the appropriate code depending on the kind of object to be
drawn.  This would be not be very elegant, and, moreover, we would have
to change 'draw' every time we add a new kind of graphical object (say,
a spaceship).

   What we would rather do is: When defining spaceships, we would tell
the system: "Here's how you 'draw' a spaceship; you figure out the
rest".

   This is the problem that all systems solve that (rightfully) call
themselves object-oriented; the object-oriented packages presented here
solve this problem (and not much else).

5.23.2 Object-Oriented Terminology
----------------------------------

This section is mainly for reference, so you don't have to understand
all of it right away.  The terminology is mainly Smalltalk-inspired.  In
short:

_class_
     a data structure definition with some extras.

_object_
     an instance of the data structure described by the class
     definition.

_instance variables_
     fields of the data structure.

_selector_
     (or _method selector_) a word (e.g., 'draw') that performs an
     operation on a variety of data structures (classes).  A selector
     describes _what_ operation to perform.  In C++ terminology: a
     (pure) virtual function.

_method_
     the concrete definition that performs the operation described by
     the selector for a specific class.  A method specifies _how_ the
     operation is performed for a specific class.

_selector invocation_
     a call of a selector.  One argument of the call (the TOS
     (top-of-stack)) is used for determining which method is used.  In
     Smalltalk terminology: a message (consisting of the selector and
     the other arguments) is sent to the object.

_receiving object_
     the object used for determining the method executed by a selector
     invocation.  In the 'objects.fs' model, it is the object that is on
     the TOS when the selector is invoked.  (_Receiving_ comes from the
     Smalltalk _message_ terminology.)

_child class_
     a class that has (_inherits_) all properties (instance variables,
     selectors, methods) from a _parent class_.  In Smalltalk
     terminology: The subclass inherits from the superclass.  In C++
     terminology: The derived class inherits from the base class.

5.23.3 The 'objects.fs' model
-----------------------------

This section describes the 'objects.fs' package.  This material also has
been published in M. Anton Ertl, 'Yet Another Forth Objects Package
(http://www.complang.tuwien.ac.at/forth/objects/objects.html)', Forth
Dimensions 19(2), pages 37-43.

   This section assumes that you have read *note Structures::.

   The techniques on which this model is based have been used to
implement the parser generator, Gray, and have also been used in Gforth
for implementing the various flavours of word lists (hashed or not,
case-sensitive or not, special-purpose word lists for locals etc.).

   Marcel Hendrix provided helpful comments on this section.

5.23.3.1 Properties of the 'objects.fs' model
.............................................

   * It is straightforward to pass objects on the stack.  Passing
     selectors on the stack is a little less convenient, but possible.

   * Objects are just data structures in memory, and are referenced by
     their address.  You can create words for objects with normal
     defining words like 'constant'.  Likewise, there is no difference
     between instance variables that contain objects and those that
     contain other data.

   * Late binding is efficient and easy to use.

   * It avoids parsing, and thus avoids problems with state-smartness
     and reduced extensibility; for convenience there are a few parsing
     words, but they have non-parsing counterparts.  There are also a
     few defining words that parse.  This is hard to avoid, because all
     standard defining words parse (except ':noname'); however, such
     words are not as bad as many other parsing words, because they are
     not state-smart.

   * It does not try to incorporate everything.  It does a few things
     and does them well (IMO). In particular, this model was not
     designed to support information hiding (although it has features
     that may help); you can use a separate package for achieving this.

   * It is layered; you don't have to learn and use all features to use
     this model.  Only a few features are necessary (*note Basic Objects
     Usage::, *note The Objects base class::, *note Creating
     objects::.), the others are optional and independent of each other.

   * An implementation in ANS Forth is available.

5.23.3.2 Basic 'objects.fs' Usage
.................................

You can define a class for graphical objects like this:

     object class \ "object" is the parent class
       selector draw ( x y graphical -- )
     end-class graphical

   This code defines a class 'graphical' with an operation 'draw'.  We
can perform the operation 'draw' on any 'graphical' object, e.g.:

     100 100 t-rex draw

where 't-rex' is a word (say, a constant) that produces a graphical
object.

   How do we create a graphical object?  With the present definitions,
we cannot create a useful graphical object.  The class 'graphical'
describes graphical objects in general, but not any concrete graphical
object type (C++ users would call it an _abstract class_); e.g., there
is no method for the selector 'draw' in the class 'graphical'.

   For concrete graphical objects, we define child classes of the class
'graphical', e.g.:

     graphical class \ "graphical" is the parent class
       cell% field circle-radius

     :noname ( x y circle -- )
       circle-radius @ draw-circle ;
     overrides draw

     :noname ( n-radius circle -- )
       circle-radius ! ;
     overrides construct

     end-class circle

   Here we define a class 'circle' as a child of 'graphical', with field
'circle-radius' (which behaves just like a field (*note Structures::);
it defines (using 'overrides') new methods for the selectors 'draw' and
'construct' ('construct' is defined in 'object', the parent class of
'graphical').

   Now we can create a circle on the heap (i.e., 'allocate'd memory)
with:

     50 circle heap-new constant my-circle

'heap-new' invokes 'construct', thus initializing the field
'circle-radius' with 50.  We can draw this new circle at (100,100) with:

     100 100 my-circle draw

   Note: You can only invoke a selector if the object on the TOS (the
receiving object) belongs to the class where the selector was defined or
one of its descendents; e.g., you can invoke 'draw' only for objects
belonging to 'graphical' or its descendents (e.g., 'circle').
Immediately before 'end-class', the search order has to be the same as
immediately after 'class'.

5.23.3.3 The 'object.fs' base class
...................................

When you define a class, you have to specify a parent class.  So how do
you start defining classes?  There is one class available from the
start: 'object'.  It is ancestor for all classes and so is the only
class that has no parent.  It has two selectors: 'construct' and
'print'.

5.23.3.4 Creating objects
.........................

You can create and initialize an object of a class on the heap with
'heap-new' ( ...  class - object ) and in the dictionary (allocation
with 'allot') with 'dict-new' ( ...  class - object ).  Both words
invoke 'construct', which consumes the stack items indicated by "..."
above.

   If you want to allocate memory for an object yourself, you can get
its alignment and size with 'class-inst-size 2@' ( class - align size ).
Once you have memory for an object, you can initialize it with
'init-object' ( ...  class object - ); 'construct' does only a part of
the necessary work.

5.23.3.5 Object-Oriented Programming Style
..........................................

This section is not exhaustive.

   In general, it is a good idea to ensure that all methods for the same
selector have the same stack effect: when you invoke a selector, you
often have no idea which method will be invoked, so, unless all methods
have the same stack effect, you will not know the stack effect of the
selector invocation.

   One exception to this rule is methods for the selector 'construct'.
We know which method is invoked, because we specify the class to be
constructed at the same place.  Actually, I defined 'construct' as a
selector only to give the users a convenient way to specify
initialization.  The way it is used, a mechanism different from selector
invocation would be more natural (but probably would take more code and
more space to explain).

5.23.3.6 Class Binding
......................

Normal selector invocations determine the method at run-time depending
on the class of the receiving object.  This run-time selection is called
late binding.

   Sometimes it's preferable to invoke a different method.  For example,
you might want to use the simple method for 'print'ing 'object's instead
of the possibly long-winded 'print' method of the receiver class.  You
can achieve this by replacing the invocation of 'print' with:

     [bind] object print

in compiled code or:

     bind object print

in interpreted code.  Alternatively, you can define the method with a
name (e.g., 'print-object'), and then invoke it through the name.  Class
binding is just a (often more convenient) way to achieve the same
effect; it avoids name clutter and allows you to invoke methods directly
without naming them first.

   A frequent use of class binding is this: When we define a method for
a selector, we often want the method to do what the selector does in the
parent class, and a little more.  There is a special word for this
purpose: '[parent]'; '[parent] _selector_' is equivalent to '[bind]
_parent selector_', where '_parent_' is the parent class of the current
class.  E.g., a method definition might look like:

     :noname
       dup [parent] foo \ do parent's foo on the receiving object
       ... \ do some more
     ; overrides foo

   In 'Object-oriented programming in ANS Forth' (Forth Dimensions,
March 1997), Andrew McKewan presents class binding as an optimization
technique.  I recommend not using it for this purpose unless you are in
an emergency.  Late binding is pretty fast with this model anyway, so
the benefit of using class binding is small; the cost of using class
binding where it is not appropriate is reduced maintainability.

   While we are at programming style questions: You should bind
selectors only to ancestor classes of the receiving object.  E.g., say,
you know that the receiving object is of class 'foo' or its descendents;
then you should bind only to 'foo' and its ancestors.

5.23.3.7 Method conveniences
............................

In a method you usually access the receiving object pretty often.  If
you define the method as a plain colon definition (e.g., with
':noname'), you may have to do a lot of stack gymnastics.  To avoid
this, you can define the method with 'm: ... ;m'.  E.g., you could
define the method for 'draw'ing a 'circle' with

     m: ( x y circle -- )
       ( x y ) this circle-radius @ draw-circle ;m

   When this method is executed, the receiver object is removed from the
stack; you can access it with 'this' (admittedly, in this example the
use of 'm: ... ;m' offers no advantage).  Note that I specify the stack
effect for the whole method (i.e.  including the receiver object), not
just for the code between 'm:' and ';m'.  You cannot use 'exit' in
'm:...;m'; instead, use 'exitm'.(1)

   You will frequently use sequences of the form 'this _field_' (in the
example above: 'this circle-radius').  If you use the field only in this
way, you can define it with 'inst-var' and eliminate the 'this' before
the field name.  E.g., the 'circle' class above could also be defined
with:

     graphical class
       cell% inst-var radius

     m: ( x y circle -- )
       radius @ draw-circle ;m
     overrides draw

     m: ( n-radius circle -- )
       radius ! ;m
     overrides construct

     end-class circle

   'radius' can only be used in 'circle' and its descendent classes and
inside 'm:...;m'.

   You can also define fields with 'inst-value', which is to 'inst-var'
what 'value' is to 'variable'.  You can change the value of such a field
with '[to-inst]'.  E.g., we could also define the class 'circle' like
this:

     graphical class
       inst-value radius

     m: ( x y circle -- )
       radius draw-circle ;m
     overrides draw

     m: ( n-radius circle -- )
       [to-inst] radius ;m
     overrides construct

     end-class circle

   ---------- Footnotes ----------

   (1) Moreover, for any word that calls 'catch' and was defined before
loading 'objects.fs', you have to redefine it like I redefined 'catch':
': catch this >r catch r> to-this ;'

5.23.3.8 Classes and Scoping
............................

Inheritance is frequent, unlike structure extension.  This exacerbates
the problem with the field name convention (*note Structure Naming
Convention::): One always has to remember in which class the field was
originally defined; changing a part of the class structure would require
changes for renaming in otherwise unaffected code.

   To solve this problem, I added a scoping mechanism (which was not in
my original charter): A field defined with 'inst-var' (or 'inst-value')
is visible only in the class where it is defined and in the descendent
classes of this class.  Using such fields only makes sense in
'm:'-defined methods in these classes anyway.

   This scoping mechanism allows us to use the unadorned field name,
because name clashes with unrelated words become much less likely.

   Once we have this mechanism, we can also use it for controlling the
visibility of other words: All words defined after 'protected' are
visible only in the current class and its descendents.  'public'
restores the compilation (i.e.  'current') word list that was in effect
before.  If you have several 'protected's without an intervening
'public' or 'set-current', 'public' will restore the compilation word
list in effect before the first of these 'protected's.

5.23.3.9 Dividing classes
.........................

You may want to do the definition of methods separate from the
definition of the class, its selectors, fields, and instance variables,
i.e., separate the implementation from the definition.  You can do this
in the following way:

     graphical class
       inst-value radius
     end-class circle

     ... \ do some other stuff

     circle methods \ now we are ready

     m: ( x y circle -- )
       radius draw-circle ;m
     overrides draw

     m: ( n-radius circle -- )
       [to-inst] radius ;m
     overrides construct

     end-methods

   You can use several 'methods'...'end-methods' sections.  The only
things you can do to the class in these sections are: defining methods,
and overriding the class's selectors.  You must not define new selectors
or fields.

   Note that you often have to override a selector before using it.  In
particular, you usually have to override 'construct' with a new method
before you can invoke 'heap-new' and friends.  E.g., you must not create
a circle before the 'overrides construct' sequence in the example above.

5.23.3.10 Object Interfaces
...........................

In this model you can only call selectors defined in the class of the
receiving objects or in one of its ancestors.  If you call a selector
with a receiving object that is not in one of these classes, the result
is undefined; if you are lucky, the program crashes immediately.

   Now consider the case when you want to have a selector (or several)
available in two classes: You would have to add the selector to a common
ancestor class, in the worst case to 'object'.  You may not want to do
this, e.g., because someone else is responsible for this ancestor class.

   The solution for this problem is interfaces.  An interface is a
collection of selectors.  If a class implements an interface, the
selectors become available to the class and its descendents.  A class
can implement an unlimited number of interfaces.  For the problem
discussed above, we would define an interface for the selector(s), and
both classes would implement the interface.

   As an example, consider an interface 'storage' for writing objects to
disk and getting them back, and a class 'foo' that implements it.  The
code would look like this:

     interface
       selector write ( file object -- )
       selector read1 ( file object -- )
     end-interface storage

     bar class
       storage implementation

     ... overrides write
     ... overrides read1
     ...
     end-class foo

(I would add a word 'read' ( file - object ) that uses 'read1'
internally, but that's beyond the point illustrated here.)

   Note that you cannot use 'protected' in an interface; and of course
you cannot define fields.

   In the Neon model, all selectors are available for all classes;
therefore it does not need interfaces.  The price you pay in this model
is slower late binding, and therefore, added complexity to avoid late
binding.

5.23.3.11 'objects.fs' Implementation
.....................................

An object is a piece of memory, like one of the data structures
described with 'struct...end-struct'.  It has a field 'object-map' that
points to the method map for the object's class.

   The _method map_(1) is an array that contains the execution tokens
(xts) of the methods for the object's class.  Each selector contains an
offset into a method map.

   'selector' is a defining word that uses 'CREATE' and 'DOES>'.  The
body of the selector contains the offset; the 'DOES>' action for a class
selector is, basically:

     ( object addr ) @ over object-map @ + @ execute

   Since 'object-map' is the first field of the object, it does not
generate any code.  As you can see, calling a selector has a small,
constant cost.

   A class is basically a 'struct' combined with a method map.  During
the class definition the alignment and size of the class are passed on
the stack, just as with 'struct's, so 'field' can also be used for
defining class fields.  However, passing more items on the stack would
be inconvenient, so 'class' builds a data structure in memory, which is
accessed through the variable 'current-interface'.  After its definition
is complete, the class is represented on the stack by a pointer (e.g.,
as parameter for a child class definition).

   A new class starts off with the alignment and size of its parent, and
a copy of the parent's method map.  Defining new fields extends the size
and alignment; likewise, defining new selectors extends the method map.
'overrides' just stores a new xt in the method map at the offset given
by the selector.

   Class binding just gets the xt at the offset given by the selector
from the class's method map and 'compile,'s (in the case of '[bind]')
it.

   I implemented 'this' as a 'value'.  At the start of an 'm:...;m'
method the old 'this' is stored to the return stack and restored at the
end; and the object on the TOS is stored 'TO this'.  This technique has
one disadvantage: If the user does not leave the method via ';m', but
via 'throw' or 'exit', 'this' is not restored (and 'exit' may crash).
To deal with the 'throw' problem, I have redefined 'catch' to save and
restore 'this'; the same should be done with any word that can catch an
exception.  As for 'exit', I simply forbid it (as a replacement, there
is 'exitm').

   'inst-var' is just the same as 'field', with a different 'DOES>'
action:
     @ this +
   Similar for 'inst-value'.

   Each class also has a word list that contains the words defined with
'inst-var' and 'inst-value', and its protected words.  It also has a
pointer to its parent.  'class' pushes the word lists of the class and
all its ancestors onto the search order stack, and 'end-class' drops
them.

   An interface is like a class without fields, parent and protected
words; i.e., it just has a method map.  If a class implements an
interface, its method map contains a pointer to the method map of the
interface.  The positive offsets in the map are reserved for class
methods, therefore interface map pointers have negative offsets.
Interfaces have offsets that are unique throughout the system, unlike
class selectors, whose offsets are only unique for the classes where the
selector is available (invokable).

   This structure means that interface selectors have to perform one
indirection more than class selectors to find their method.  Their body
contains the interface map pointer offset in the class method map, and
the method offset in the interface method map.  The 'does>' action for
an interface selector is, basically:

     ( object selector-body )
     2dup selector-interface @ ( object selector-body object interface-offset )
     swap object-map @ + @ ( object selector-body map )
     swap selector-offset @ + @ execute

   where 'object-map' and 'selector-offset' are first fields and
generate no code.

   As a concrete example, consider the following code:

     interface
       selector if1sel1
       selector if1sel2
     end-interface if1

     object class
       if1 implementation
       selector cl1sel1
       cell% inst-var cl1iv1

     ' m1 overrides construct
     ' m2 overrides if1sel1
     ' m3 overrides if1sel2
     ' m4 overrides cl1sel2
     end-class cl1

     create obj1 object dict-new drop
     create obj2 cl1    dict-new drop

   The data structure created by this code (including the data structure
for 'object') is shown in the figure (objects-implementation.eps),
assuming a cell size of 4.

   ---------- Footnotes ----------

   (1) This is Self terminology; in C++ terminology: virtual function
table.

5.23.3.12 'objects.fs' Glossary
...............................

'bind'       ... "class" "selector" - ...         objects       "bind"
   Execute the method for SELECTOR in CLASS.

'<bind>'       class selector-xt - xt         objects       "<bind>"
   XT is the method for the selector SELECTOR-XT in CLASS.

'bind''       "class" "selector" - xt         objects       "bind"'
   XT is the method for SELECTOR in CLASS.

'[bind]'       compile-time: "class" "selector" - ; run-time: ... object - ...         objects       "[bind]"
   Compile the method for SELECTOR in CLASS.

'class'       parent-class - align offset         objects       "class"
   Start a new class definition as a child of PARENT-CLASS.  ALIGN
OFFSET are for use by FIELD etc.

'class->map'       class - map         objects       "class->map"
   MAP is the pointer to CLASS's method map; it points to the place in
the map to which the selector offsets refer (i.e., where OBJECT-MAPs
point to).

'class-inst-size'       class - addr         objects       "class-inst-size"
   Give the size specification for an instance (i.e.  an object) of
CLASS; used as 'class-inst-size 2 ( class -- align size )'.

'class-override!'       xt sel-xt class-map -         objects       "class-override!"
   XT is the new method for the selector SEL-XT in CLASS-MAP.

'class-previous'       class -         objects       "class-previous"
   Drop CLASS's wordlists from the search order.  No checking is made
whether CLASS's wordlists are actually on the search order.

'class>order'       class -         objects       "class>order"
   Add CLASS's wordlists to the head of the search-order.

'construct'       ... object -         objects       "construct"
   Initialize the data fields of OBJECT.  The method for the class
OBJECT just does nothing: '( object -- )'.

'current''       "selector" - xt         objects       "current"'
   XT is the method for SELECTOR in the current class.

'[current]'       compile-time: "selector" - ; run-time: ... object - ...         objects       "[current]"
   Compile the method for SELECTOR in the current class.

'current-interface'       - addr         objects       "current-interface"
   Variable: contains the class or interface currently being defined.

'dict-new'       ... class - object         objects       "dict-new"
   'allot' and initialize an object of class CLASS in the dictionary.

'end-class'       align offset "name" -         objects       "end-class"
   NAME execution: '-- class'
End a class definition.  The resulting class is CLASS.

'end-class-noname'       align offset - class         objects       "end-class-noname"
   End a class definition.  The resulting class is CLASS.

'end-interface'       "name" -         objects       "end-interface"
   'name' execution: '-- interface'
End an interface definition.  The resulting interface is INTERFACE.

'end-interface-noname'       - interface         objects       "end-interface-noname"
   End an interface definition.  The resulting interface is INTERFACE.

'end-methods'       -         objects       "end-methods"
   Switch back from defining methods of a class to normal mode
(currently this just restores the old search order).

'exitm'       -         objects       "exitm"
   'exit' from a method; restore old 'this'.

'heap-new'       ... class - object         objects       "heap-new"
   'allocate' and initialize an object of class CLASS.

'implementation'       interface -         objects       "implementation"
   The current class implements INTERFACE.  I.e., you can use all
selectors of the interface in the current class and its descendents.

'init-object'       ... class object -         objects       "init-object"
   Initialize a chunk of memory (OBJECT) to an object of class CLASS;
then performs 'construct'.

'inst-value'       align1 offset1 "name" - align2 offset2         objects       "inst-value"
   NAME execution: '-- w'
W is the value of the field NAME in 'this' object.

'inst-var'       align1 offset1 align size "name" - align2 offset2         objects       "inst-var"
   NAME execution: '-- addr'
ADDR is the address of the field NAME in 'this' object.

'interface'       -         objects       "interface"
   Start an interface definition.

'm:'       - xt colon-sys; run-time: object -         objects       "m:"
   Start a method definition; OBJECT becomes new 'this'.

':m'       "name" - xt; run-time: object -         objects       ":m"
   Start a named method definition; OBJECT becomes new 'this'.  Has to
be ended with ';m'.

';m'       colon-sys -; run-time: -         objects       ";m"
   End a method definition; restore old 'this'.

'method'       xt "name" -         objects       "method"
   'name' execution: '... object -- ...'
Create selector NAME and makes XT its method in the current class.

'methods'       class -         objects       "methods"
   Makes CLASS the current class.  This is intended to be used for
defining methods to override selectors; you cannot define new fields or
selectors.

'object'       - class         objects       "object"
   the ancestor of all classes.

'overrides'       xt "selector" -         objects       "overrides"
   replace default method for SELECTOR in the current class with XT.
'overrides' must not be used during an interface definition.

'[parent]'       compile-time: "selector" - ; run-time: ... object - ...         objects       "[parent]"
   Compile the method for SELECTOR in the parent of the current class.

'print'       object -         objects       "print"
   Print the object.  The method for the class OBJECT prints the address
of the object and the address of its class.

'protected'       -         objects       "protected"
   Set the compilation wordlist to the current class's wordlist

'public'       -         objects       "public"
   Restore the compilation wordlist that was in effect before the last
'protected' that actually changed the compilation wordlist.

'selector'       "name" -         objects       "selector"
   NAME execution: '... object -- ...'
Create selector NAME for the current class and its descendents; you can
set a method for the selector in the current class with 'overrides'.

'this'       - object         objects       "this"
   the receiving object of the current method (aka active object).

'<to-inst>'       w xt -         objects       "<to-inst>"
   store W into the field XT in 'this' object.

'[to-inst]'       compile-time: "name" - ; run-time: w -         objects       "[to-inst]"
   store W into field NAME in 'this' object.

'to-this'       object -         objects       "to-this"
   Set 'this' (used internally, but useful when debugging).

'xt-new'       ... class xt - object         objects       "xt-new"
   Make a new object, using 'xt ( align size -- addr )' to get memory.

5.23.4 The 'oof.fs' model
-------------------------

This section describes the 'oof.fs' package.

   The package described in this section has been used in bigFORTH since
1991, and used for two large applications: a chromatographic system used
to create new medicaments, and a graphic user interface library (MINOS).

   You can find a description (in German) of 'oof.fs' in 'Object
oriented bigFORTH' by Bernd Paysan, published in 'Vierte Dimension'
10(2), 1994.

5.23.4.1 Properties of the 'oof.fs' model
.........................................

   * This model combines object oriented programming with information
     hiding.  It helps you writing large application, where scoping is
     necessary, because it provides class-oriented scoping.

   * Named objects, object pointers, and object arrays can be created,
     selector invocation uses the "object selector" syntax.  Selector
     invocation to objects and/or selectors on the stack is a bit less
     convenient, but possible.

   * Selector invocation and instance variable usage of the active
     object is straightforward, since both make use of the active
     object.

   * Late binding is efficient and easy to use.

   * State-smart objects parse selectors.  However, extensibility is
     provided using a (parsing) selector 'postpone' and a selector '''.

   * An implementation in ANS Forth is available.

5.23.4.2 Basic 'oof.fs' Usage
.............................

This section uses the same example as for 'objects' (*note Basic Objects
Usage::).

   You can define a class for graphical objects like this:

     object class graphical \ "object" is the parent class
       method draw ( x y -- )
     class;

   This code defines a class 'graphical' with an operation 'draw'.  We
can perform the operation 'draw' on any 'graphical' object, e.g.:

     100 100 t-rex draw

where 't-rex' is an object or object pointer, created with e.g.
'graphical : t-rex'.

   How do we create a graphical object?  With the present definitions,
we cannot create a useful graphical object.  The class 'graphical'
describes graphical objects in general, but not any concrete graphical
object type (C++ users would call it an _abstract class_); e.g., there
is no method for the selector 'draw' in the class 'graphical'.

   For concrete graphical objects, we define child classes of the class
'graphical', e.g.:

     graphical class circle \ "graphical" is the parent class
       cell var circle-radius
     how:
       : draw ( x y -- )
         circle-radius @ draw-circle ;

       : init ( n-radius -- )
         circle-radius ! ;
     class;

   Here we define a class 'circle' as a child of 'graphical', with a
field 'circle-radius'; it defines new methods for the selectors 'draw'
and 'init' ('init' is defined in 'object', the parent class of
'graphical').

   Now we can create a circle in the dictionary with:

     50 circle : my-circle

':' invokes 'init', thus initializing the field 'circle-radius' with 50.
We can draw this new circle at (100,100) with:

     100 100 my-circle draw

   Note: You can only invoke a selector if the receiving object belongs
to the class where the selector was defined or one of its descendents;
e.g., you can invoke 'draw' only for objects belonging to 'graphical' or
its descendents (e.g., 'circle').  The scoping mechanism will check if
you try to invoke a selector that is not defined in this class
hierarchy, so you'll get an error at compilation time.

5.23.4.3 The 'oof.fs' base class
................................

When you define a class, you have to specify a parent class.  So how do
you start defining classes?  There is one class available from the
start: 'object'.  You have to use it as ancestor for all classes.  It is
the only class that has no parent.  Classes are also objects, except
that they don't have instance variables; class manipulation such as
inheritance or changing definitions of a class is handled through
selectors of the class 'object'.

   'object' provides a number of selectors:

   * 'class' for subclassing, 'definitions' to add definitions later on,
     and 'class?' to get type informations (is the class a subclass of
     the class passed on the stack?).

     'class'       "name" -         oof       "class"

     'definitions'       -         oof       "definitions"

     'class?'       o - flag         oof       "class-query"

   * 'init' and 'dispose' as constructor and destructor of the object.
     'init' is invocated after the object's memory is allocated, while
     'dispose' also handles deallocation.  Thus if you redefine
     'dispose', you have to call the parent's dispose with 'super
     dispose', too.

     'init'       ... -         oof       "init"

     'dispose'       -         oof       "dispose"

   * 'new', 'new[]', ':', 'ptr', 'asptr', and '[]' to create named and
     unnamed objects and object arrays or object pointers.

     'new'       - o         oof       "new"

     'new[]'       n - o         oof       "new-array"

     ':'       "name" -         oof       "define"

     'ptr'       "name" -         oof       "ptr"

     'asptr'       o "name" -         oof       "asptr"

     '[]'       n "name" -         oof       "array"

   * '::' and 'super' for explicit scoping.  You should use explicit
     scoping only for super classes or classes with the same set of
     instance variables.  Explicitly-scoped selectors use early binding.

     '::'       "name" -         oof       "scope"

     'super'       "name" -         oof       "super"

   * 'self' to get the address of the object

     'self'       - o         oof       "self"

   * 'bind', 'bound', 'link', and 'is' to assign object pointers and
     instance defers.

     'bind'       o "name" -         oof       "bind"

     'bound'       class addr "name" -         oof       "bound"

     'link'       "name" - class addr         oof       "link"

     'is'       xt "name" -         oof       "is"

   * ''' to obtain selector tokens, 'send' to invocate selectors form
     the stack, and 'postpone' to generate selector invocation code.

     '''       "name" - xt         oof       "tick"

     'postpone'       "name" -         oof       "postpone"

   * 'with' and 'endwith' to select the active object from the stack,
     and enable its scope.  Using 'with' and 'endwith' also allows you
     to create code using selector 'postpone' without being trapped by
     the state-smart objects.

     'with'       o -         oof       "with"

     'endwith'       -         oof       "endwith"

5.23.4.4 Class Declaration
..........................

   * Instance variables

     'var'       size -         oof       "var"
     Create an instance variable

   * Object pointers

     'ptr'       -         oof       "ptr"
     Create an instance pointer

     'asptr'       class -         oof       "asptr"
     Create an alias to an instance pointer, cast to another class.

   * Instance defers

     'defer'       -         oof       "defer"
     Create an instance defer

   * Method selectors

     'early'       -         oof       "early"
     Create a method selector for early binding.

     'method'       -         oof       "method"
     Create a method selector.

   * Class-wide variables

     'static'       -         oof       "static"
     Create a class-wide cell-sized variable.

   * End declaration

     'how:'       -         oof       "how-to"
     End declaration, start implementation

     'class;'       -         oof       "end-class"
     End class declaration or implementation

5.23.4.5 Class Implementation
.............................

5.23.5 The 'mini-oof.fs' model
------------------------------

Gforth's third object oriented Forth package is a 12-liner.  It uses a
mixture of the 'objects.fs' and the 'oof.fs' syntax, and reduces to the
bare minimum of features.  This is based on a posting of Bernd Paysan in
comp.lang.forth.

5.23.5.1 Basic 'mini-oof.fs' Usage
..................................

There is a base class ('class', which allocates one cell for the object
pointer) plus seven other words: to define a method, a variable, a
class; to end a class, to resolve binding, to allocate an object and to
compile a class method.

'object'       - a-addr         mini-oof       "object"
   OBJECT is the base class of all objects.

'method'       m v "name" - m' v         mini-oof       "method"
   Define a selector.

'var'       m v size "name" - m v'         mini-oof       "var"
   Define a variable with SIZE bytes.

'class'       class - class selectors vars         mini-oof       "class"
   Start the definition of a class.

'end-class'       class selectors vars "name" -         mini-oof       "end-class"
   End the definition of a class.

'defines'       xt class "name" -         mini-oof       "defines"
   Bind XT to the selector NAME in class CLASS.

'new'       class - o         mini-oof       "new"
   Create a new incarnation of the class CLASS.

'::'       class "name" -         mini-oof       "colon-colon"
   Compile the method for the selector NAME of the class CLASS (not
immediate!).

5.23.5.2 Mini-OOF Example
.........................

A short example shows how to use this package.  This example, in
slightly extended form, is supplied as 'moof-exm.fs'

     object class
       method init
       method draw
     end-class graphical

   This code defines a class 'graphical' with an operation 'draw'.  We
can perform the operation 'draw' on any 'graphical' object, e.g.:

     100 100 t-rex draw

   where 't-rex' is an object or object pointer, created with e.g.
'graphical new Constant t-rex'.

   For concrete graphical objects, we define child classes of the class
'graphical', e.g.:

     graphical class
       cell var circle-radius
     end-class circle \ "graphical" is the parent class

     :noname ( x y -- )
       circle-radius @ draw-circle ; circle defines draw
     :noname ( r -- )
       circle-radius ! ; circle defines init

   There is no implicit init method, so we have to define one.  The
creation code of the object now has to call init explicitely.

     circle new Constant my-circle
     50 my-circle init

   It is also possible to add a function to create named objects with
automatic call of 'init', given that all objects have 'init' on the same
place:

     : new: ( .. o "name" -- )
         new dup Constant init ;
     80 circle new: large-circle

   We can draw this new circle at (100,100) with:

     100 100 my-circle draw

5.23.5.3 'mini-oof.fs' Implementation
.....................................

Object-oriented systems with late binding typically use a
"vtable"-approach: the first variable in each object is a pointer to a
table, which contains the methods as function pointers.  The vtable may
also contain other information.

   So first, let's declare selectors:

     : method ( m v "name" -- m' v ) Create  over , swap cell+ swap
       DOES> ( ... o -- ... ) @ over @ + @ execute ;

   During selector declaration, the number of selectors and instance
variables is on the stack (in address units).  'method' creates one
selector and increments the selector number.  To execute a selector, it
takes the object, fetches the vtable pointer, adds the offset, and
executes the method xt stored there.  Each selector takes the object it
is invoked with as top of stack parameter; it passes the parameters
(including the object) unchanged to the appropriate method which should
consume that object.

   Now, we also have to declare instance variables

     : var ( m v size "name" -- m v' ) Create  over , +
       DOES> ( o -- addr ) @ + ;

   As before, a word is created with the current offset.  Instance
variables can have different sizes (cells, floats, doubles, chars), so
all we do is take the size and add it to the offset.  If your machine
has alignment restrictions, put the proper 'aligned' or 'faligned'
before the variable, to adjust the variable offset.  That's why it is on
the top of stack.

   We need a starting point (the base object) and some syntactic sugar:

     Create object  1 cells , 2 cells ,
     : class ( class -- class selectors vars ) dup 2@ ;

   For inheritance, the vtable of the parent object has to be copied
when a new, derived class is declared.  This gives all the methods of
the parent class, which can be overridden, though.

     : end-class  ( class selectors vars "name" -- )
       Create  here >r , dup , 2 cells ?DO ['] noop , 1 cells +LOOP
       cell+ dup cell+ r> rot @ 2 cells /string move ;

   The first line creates the vtable, initialized with 'noop's.  The
second line is the inheritance mechanism, it copies the xts from the
parent vtable.

   We still have no way to define new methods, let's do that now:

     : defines ( xt class "name" -- ) ' >body @ + ! ;

   To allocate a new object, we need a word, too:

     : new ( class -- o )  here over @ allot swap over ! ;

   Sometimes derived classes want to access the method of the parent
object.  There are two ways to achieve this with Mini-OOF: first, you
could use named words, and second, you could look up the vtable of the
parent object.

     : :: ( class "name" -- ) ' >body @ + @ compile, ;

   Nothing can be more confusing than a good example, so here is one.
First let's declare a text object (called 'button'), that stores text
and position:

     object class
       cell var text
       cell var len
       cell var x
       cell var y
       method init
       method draw
     end-class button

Now, implement the two methods, 'draw' and 'init':

     :noname ( o -- )
      >r r@ x @ r@ y @ at-xy  r@ text @ r> len @ type ;
      button defines draw
     :noname ( addr u o -- )
      >r 0 r@ x ! 0 r@ y ! r@ len ! r> text ! ;
      button defines init

To demonstrate inheritance, we define a class 'bold-button', with no new
data and no new selectors:

     button class
     end-class bold-button

     : bold   27 emit ." [1m" ;
     : normal 27 emit ." [0m" ;

The class 'bold-button' has a different draw method to 'button', but the
new method is defined in terms of the draw method for 'button':

     :noname bold [ button :: draw ] normal ; bold-button defines draw

Finally, create two objects and apply selectors:

     button new Constant foo
     s" thin foo" foo init
     page
     foo draw
     bold-button new Constant bar
     s" fat bar" bar init
     1 bar y !
     bar draw

5.23.6 Comparison with other object models
------------------------------------------

Many object-oriented Forth extensions have been proposed ('A survey of
object-oriented Forths' (SIGPLAN Notices, April 1996) by Bradford J.
Rodriguez and W. F. S. Poehlman lists 17).  This section discusses the
relation of the object models described here to two well-known and two
closely-related (by the use of method maps) models.  Andras Zsoter
helped us with this section.

   The most popular model currently seems to be the Neon model (see
'Object-oriented programming in ANS Forth' (Forth Dimensions, March
1997) by Andrew McKewan) but this model has a number of limitations (1):

   * It uses a '_selector object_' syntax, which makes it unnatural to
     pass objects on the stack.

   * It requires that the selector parses the input stream (at compile
     time); this leads to reduced extensibility and to bugs that are
     hard to find.

   * It allows using every selector on every object; this eliminates the
     need for interfaces, but makes it harder to create efficient
     implementations.

   Another well-known publication is 'Object-Oriented Forth' (Academic
Press, London, 1987) by Dick Pountain.  However, it is not really about
object-oriented programming, because it hardly deals with late binding.
Instead, it focuses on features like information hiding and overloading
that are characteristic of modular languages like Ada (83).

   In Does late binding have to be slow?
(http://www.forth.org/oopf.html) (Forth Dimensions 18(1) 1996, pages
31-35) Andras Zsoter describes a model that makes heavy use of an active
object (like 'this' in 'objects.fs'): The active object is not only used
for accessing all fields, but also specifies the receiving object of
every selector invocation; you have to change the active object
explicitly with '{ ... }', whereas in 'objects.fs' it changes more or
less implicitly at 'm: ... ;m'.  Such a change at the method entry point
is unnecessary with Zsoter's model, because the receiving object is the
active object already.  On the other hand, the explicit change is
absolutely necessary in that model, because otherwise no one could ever
change the active object.  An ANS Forth implementation of this model is
available through <http://www.forth.org/oopf.html>.

   The 'oof.fs' model combines information hiding and overloading
resolution (by keeping names in various word lists) with object-oriented
programming.  It sets the active object implicitly on method entry, but
also allows explicit changing (with '>o...o>' or with 'with...endwith').
It uses parsing and state-smart objects and classes for resolving
overloading and for early binding: the object or class parses the
selector and determines the method from this.  If the selector is not
parsed by an object or class, it performs a call to the selector for the
active object (late binding), like Zsoter's model.  Fields are always
accessed through the active object.  The big disadvantage of this model
is the parsing and the state-smartness, which reduces extensibility and
increases the opportunities for subtle bugs; essentially, you are only
safe if you never tick or 'postpone' an object or class (Bernd
disagrees, but I (Anton) am not convinced).

   The 'mini-oof.fs' model is quite similar to a very stripped-down
version of the 'objects.fs' model, but syntactically it is a mixture of
the 'objects.fs' and 'oof.fs' models.

   ---------- Footnotes ----------

   (1) A longer version of this critique can be found in 'On
Standardizing Object-Oriented Forth Extensions' (Forth Dimensions, May
1997) by Anton Ertl.

5.24 Programming Tools
======================

5.24.1 Examining data and code
------------------------------

The following words inspect the stack non-destructively:

'.s'       -         tools       "dot-s"
   Display the number of items on the data stack, followed by a list of
the items (but not more than specified by 'maxdepth-.s'; TOS is the
right-most item.

'f.s'       -         gforth       "f-dot-s"
   Display the number of items on the floating-point stack, followed by
a list of the items (but not more than specified by 'maxdepth-.s'; TOS
is the right-most item.

'maxdepth-.s'       - addr         gforth       "maxdepth-dot-s"
   A variable containing 9 by default.  '.s' and 'f.s' display at most
that many stack items.

   There is a word '.r' but it does not display the return stack!  It is
used for formatted numeric output (*note Simple numeric output::).

'depth'       - +n         core       "depth"
   +N is the number of values that were on the data stack before +N
itself was placed on the stack.

'fdepth'       - +n         float       "f-depth"
   +n is the current number of (floating-point) values on the
floating-point stack.

'clearstack'       ... -         gforth       "clear-stack"
   remove and discard all/any items from the data stack.

'clearstacks'       ... -         gforth       "clear-stacks"
   empty data and FP stack

   The following words inspect memory.

'?'       a-addr -         tools       "question"
   Display the contents of address A-ADDR in the current number base.

'dump'       addr u -         tools       "dump"
   Display U lines of memory starting at address ADDR.  Each line
displays the contents of 16 bytes.  When Gforth is running under an
operating system you may get 'Invalid memory address' errors if you
attempt to access arbitrary locations.

   And finally, 'see' allows to inspect code:

'see'       "<spaces>name" -         tools       "see"
   Locate NAME using the current search order.  Display the definition
of NAME.  Since this is achieved by decompiling the definition, the
formatting is mechanised and some source information (comments,
interpreted sequences within definitions etc.)  is lost.

'xt-see'       xt -         gforth       "xt-see"
   Decompile the definition represented by xt.

'simple-see'       "name" -         gforth       "simple-see"
   a simple decompiler that's closer to 'dump' than 'see'.

'simple-see-range'       addr1 addr2 -         gforth       "simple-see-range"

'see-code'       "name" -         gforth       "see-code"
   like 'simple-see', but also shows the dynamic native code for the
inlined primitives (except for the last).

'see-code-range'       addr1 addr2 -         gforth       "see-code-range"

5.24.2 Forgetting words
-----------------------

Forth allows you to forget words (and everything that was alloted in the
dictonary after them) in a LIFO manner.

'marker'       "<spaces> name" -         core-ext       "marker"
   Create a definition, name (called a mark) whose execution semantics
are to remove itself and everything defined after it.

   The most common use of this feature is during progam development:
when you change a source file, forget all the words it defined and load
it again (since you also forget everything defined after the source file
was loaded, you have to reload that, too).  Note that effects like
storing to variables and destroyed system words are not undone when you
forget words.  With a system like Gforth, that is fast enough at
starting up and compiling, I find it more convenient to exit and restart
Gforth, as this gives me a clean slate.

   Here's an example of using 'marker' at the start of a source file
that you are debugging; it ensures that you only ever have one copy of
the file's definitions compiled at any time:

     [IFDEF] my-code
         my-code
     [ENDIF]

     marker my-code
     init-included-files

     \ .. definitions start here
     \ .
     \ .
     \ end

5.24.3 Debugging
----------------

Languages with a slow edit/compile/link/test development loop tend to
require sophisticated tracing/stepping debuggers to facilate debugging.

   A much better (faster) way in fast-compiling languages is to add
printing code at well-selected places, let the program run, look at the
output, see where things went wrong, add more printing code, etc., until
the bug is found.

   The simple debugging aids provided in 'debugs.fs' are meant to
support this style of debugging.

   The word '~~' prints debugging information (by default the source
location and the stack contents).  It is easy to insert.  If you use
Emacs it is also easy to remove ('C-x ~' in the Emacs Forth mode to
query-replace them with nothing).  The deferred words 'printdebugdata'
and '.debugline' control the output of '~~'.  The default source
location output format works well with Emacs' compilation mode, so you
can step through the program at the source level using 'C-x `' (the
advantage over a stepping debugger is that you can step in any direction
and you know where the crash has happened or where the strange data has
occurred).

'~~'       -         gforth       "tilde-tilde"
   Prints the source code location of the '~~' and the stack contents
with '.debugline'.

'printdebugdata'       -         gforth       "print-debug-data"

'.debugline'       nfile nline -         gforth       "print-debug-line"
   Print the source code location indicated by NFILE NLINE, and
additional debugging information; the default '.debugline' prints the
additional information with 'printdebugdata'.

   '~~' (and assertions) will usually print the wrong file name if a
marker is executed in the same file after their occurance.  They will
print '*somewhere*' as file name if a marker is executed in the same
file before their occurance.

5.24.4 Assertions
-----------------

It is a good idea to make your programs self-checking, especially if you
make an assumption that may become invalid during maintenance (for
example, that a certain field of a data structure is never zero).
Gforth supports "assertions" for this purpose.  They are used like this:

     assert( flag )

   The code between 'assert(' and ')' should compute a flag, that should
be true if everything is alright and false otherwise.  It should not
change anything else on the stack.  The overall stack effect of the
assertion is '( -- )'.  E.g.

     assert( 1 1 + 2 = ) \ what we learn in school
     assert( dup 0<> ) \ assert that the top of stack is not zero
     assert( false ) \ this code should not be reached

   The need for assertions is different at different times.  During
debugging, we want more checking, in production we sometimes care more
for speed.  Therefore, assertions can be turned off, i.e., the assertion
becomes a comment.  Depending on the importance of an assertion and the
time it takes to check it, you may want to turn off some assertions and
keep others turned on.  Gforth provides several levels of assertions for
this purpose:

'assert0('       -         gforth       "assert-zero"
   Important assertions that should always be turned on.

'assert1('       -         gforth       "assert-one"
   Normal assertions; turned on by default.

'assert2('       -         gforth       "assert-two"
   Debugging assertions.

'assert3('       -         gforth       "assert-three"
   Slow assertions that you may not want to turn on in normal debugging;
you would turn them on mainly for thorough checking.

'assert('       -         gforth       "assert("
   Equivalent to 'assert1('

')'       -         gforth       "close-paren"
   End an assertion.

   The variable 'assert-level' specifies the highest assertions that are
turned on.  I.e., at the default 'assert-level' of one, 'assert0(' and
'assert1(' assertions perform checking, while 'assert2(' and 'assert3('
assertions are treated as comments.

   The value of 'assert-level' is evaluated at compile-time, not at
run-time.  Therefore you cannot turn assertions on or off at run-time;
you have to set the 'assert-level' appropriately before compiling a
piece of code.  You can compile different pieces of code at different
'assert-level's (e.g., a trusted library at level 1 and newly-written
code at level 3).

'assert-level'       - a-addr         gforth       "assert-level"
   All assertions above this level are turned off.

   If an assertion fails, a message compatible with Emacs' compilation
mode is produced and the execution is aborted (currently with 'ABORT"'.
If there is interest, we will introduce a special throw code.  But if
you intend to 'catch' a specific condition, using 'throw' is probably
more appropriate than an assertion).

   Assertions (and '~~') will usually print the wrong file name if a
marker is executed in the same file after their occurance.  They will
print '*somewhere*' as file name if a marker is executed in the same
file before their occurance.

   Definitions in ANS Forth for these assertion words are provided in
'compat/assert.fs'.

5.24.5 Singlestep Debugger
--------------------------

The singlestep debugger works only with the engine 'gforth-itc'.

   When you create a new word there's often the need to check whether it
behaves correctly or not.  You can do this by typing 'dbg badword'.  A
debug session might look like this:

     : badword 0 DO i . LOOP ;  ok
     2 dbg badword
     : badword
     Scanning code...

     Nesting debugger ready!

     400D4738  8049BC4 0              -> [ 2 ] 00002 00000
     400D4740  8049F68 DO             -> [ 0 ]
     400D4744  804A0C8 i              -> [ 1 ] 00000
     400D4748 400C5E60 .              -> 0 [ 0 ]
     400D474C  8049D0C LOOP           -> [ 0 ]
     400D4744  804A0C8 i              -> [ 1 ] 00001
     400D4748 400C5E60 .              -> 1 [ 0 ]
     400D474C  8049D0C LOOP           -> [ 0 ]
     400D4758  804B384 ;              ->  ok

   Each line displayed is one step.  You always have to hit return to
execute the next word that is displayed.  If you don't want to execute
the next word in a whole, you have to type 'n' for 'nest'.  Here is an
overview what keys are available:

<RET>
     Next; Execute the next word.

n
     Nest; Single step through next word.

u
     Unnest; Stop debugging and execute rest of word.  If we got to this
     word with nest, continue debugging with the calling word.

d
     Done; Stop debugging and execute rest.

s
     Stop; Abort immediately.

   Debugging large application with this mechanism is very difficult,
because you have to nest very deeply into the program before the
interesting part begins.  This takes a lot of time.

   To do it more directly put a 'BREAK:' command into your source code.
When program execution reaches 'BREAK:' the single step debugger is
invoked and you have all the features described above.

   If you have more than one part to debug it is useful to know where
the program has stopped at the moment.  You can do this by the 'BREAK"
string"' command.  This behaves like 'BREAK:' except that string is
typed out when the "breakpoint" is reached.

'dbg'       "name" -         gforth       "dbg"

'break:'       -         gforth       "break:"

'break"'       'ccc"' -         gforth       "break""

5.25 C Interface
================

Note that the C interface is not yet complete; callbacks are missing, as
well as a way of declaring structs, unions, and their fields.

5.25.1 Calling C functions
--------------------------

Once a C function is declared (see *note Declaring C Functions::), you
can call it as follows: You push the arguments on the stack(s), and then
call the word for the C function.  The arguments have to be pushed in
the same order as the arguments appear in the C documentation (i.e., the
first argument is deepest on the stack).  Integer and pointer arguments
have to be pushed on the data stack, floating-point arguments on the FP
stack; these arguments are consumed by the called C function.

   On returning from the C function, the return value, if any, resides
on the appropriate stack: an integer return value is pushed on the data
stack, an FP return value on the FP stack, and a void return value
results in not pushing anything.  Note that most C functions have a
return value, even if that is often not used in C; in Forth, you have to
'drop' this return value explicitly if you do not use it.

   The C interface automatically converts between the C type and the
Forth type as necessary, on a best-effort basis (in some cases, there
may be some loss).

   As an example, consider the POSIX function 'lseek()':

     off_t lseek(int fd, off_t offset, int whence);

   This function takes three integer arguments, and returns an integer
argument, so a Forth call for setting the current file offset to the
start of the file could look like this:

     fd @ 0 SEEK_SET lseek -1 = if
       ... \ error handling
     then

   You might be worried that an 'off_t' does not fit into a cell, so you
could not pass larger offsets to lseek, and might get only a part of the
return values.  In that case, in your declaration of the function (*note
Declaring C Functions::) you should declare it to use double-cells for
the off_t argument and return value, and maybe give the resulting Forth
word a different name, like 'dlseek'; the result could be called like
this:

     fd @ 0. SEEK_SET dlseek -1. d= if
       ... \ error handling
     then

   Passing and returning structs or unions is currently not supported by
our interface(1).

   Calling functions with a variable number of arguments (_variadic_
functions, e.g., 'printf()') is only supported by having you declare one
function-calling word for each argument pattern, and calling the
appropriate word for the desired pattern.

   ---------- Footnotes ----------

   (1) If you know the calling convention of your C compiler, you
usually can call such functions in some way, but that way is usually not
portable between platforms, and sometimes not even between C compilers.

5.25.2 Declaring C Functions
----------------------------

Before you can call 'lseek' or 'dlseek', you have to declare it.  The
declaration consists of two parts:

The C part
     is the C declaration of the function, or more typically and
     portably, a C-style '#include' of a file that contains the
     declaration of the C function.

The Forth part
     declares the Forth types of the parameters and the Forth word name
     corresponding to the C function.

   For the words 'lseek' and 'dlseek' mentioned earlier, the
declarations are:

     \c #define _FILE_OFFSET_BITS 64
     \c #include <sys/types.h>
     \c #include <unistd.h>
     c-function lseek lseek n n n -- n
     c-function dlseek lseek n d n -- d

   The C part of the declarations is prefixed by '\c', and the rest of
the line is ordinary C code.  You can use as many lines of C
declarations as you like, and they are visible for all further function
declarations.

   The Forth part declares each interface word with 'c-function',
followed by the Forth name of the word, the C name of the called
function, and the stack effect of the word.  The stack effect contains
an arbitrary number of types of parameters, then '--', and then exactly
one type for the return value.  The possible types are:

'n'
     single-cell integer

'a'
     address (single-cell)

'd'
     double-cell integer

'r'
     floating-point value

'func'
     C function pointer

'void'
     no value (used as return type for void functions)

   To deal with variadic C functions, you can declare one Forth word for
every pattern you want to use, e.g.:

     \c #include <stdio.h>
     c-function printf-nr printf a n r -- n
     c-function printf-rn printf a r n -- n

   Note that with C functions declared as variadic (or if you don't
provide a prototype), the C interface has no C type to convert to, so no
automatic conversion happens, which may lead to portability problems in
some cases.  In such cases you can perform the conversion explicitly on
the C level, e.g., as follows:

     \c #define printfll(s,ll) printf(s,(long long)ll)
     c-function printfll printfll a n -- n

   Here, instead of calling 'printf()' directly, we define a macro that
casts (converts) the Forth single-cell integer into a C 'long long'
before calling 'printf()'.

'\c'       "rest-of-line" -         gforth       "backslash-c"
   One line of C declarations for the C interface

'c-function'       "forth-name" "c-name" "{type}" "-" "type" -         gforth       "c-function"
   Define a Forth word forth-name.  Forth-name has the specified stack
effect and calls the C function 'c-name'.

   In order to work, this C interface invokes GCC at run-time and uses
dynamic linking.  If these features are not available, there are other,
less convenient and less portable C interfaces in 'lib.fs' and
'oldlib.fs'.  These interfaces are mostly undocumented and mostly
incompatible with each other and with the documented C interface; you
can find some examples for the 'lib.fs' interface in 'lib.fs'.

5.25.3 Calling C function pointers from Forth
---------------------------------------------

If you come across a C function pointer (e.g., in some C-constructed
structure) and want to call it from your Forth program, you can also use
the features explained until now to achieve that, as follows:

   Let us assume that there is a C function pointer type 'func1' defined
in some header file 'func1.h', and you know that these functions take
one integer argument and return an integer result; and you want to call
functions through such pointers.  Just define

     \c #include <func1.h>
     \c #define call_func1(par1,fptr) ((func1)fptr)(par1)
     c-function call-func1 call_func1 n func -- n

   and then you can call a function pointed to by, say 'func1a' as
follows:

     -5 func1a call-func1 .

   In the C part, 'call_func' is defined as a macro to avoid having to
declare the exact parameter and return types, so the C compiler knows
them from the declaration of 'func1'.

   The Forth word 'call-func1' is similar to 'execute', except that it
takes a C 'func1' pointer instead of a Forth execution token, and it is
specific to 'func1' pointers.  For each type of function pointer you
want to call from Forth, you have to define a separate calling word.

5.25.4 Defining library interfaces
----------------------------------

You can give a name to a bunch of C function declarations (a library
interface), as follows:

     c-library lseek-lib
     \c #define _FILE_OFFSET_BITS 64
     ...
     end-c-library

   The effect of giving such a name to the interface is that the
generated files will contain that name, and when you use the interface a
second time, it will use the existing files instead of generating and
compiling them again, saving you time.  Note that even if you change the
declarations, the old (stale) files will be used, probably leading to
errors.  So, during development of the declarations we recommend not
using 'c-library'.

   Note that the library name is not allocated in the dictionary and
therefore does not shadow dictionary names.  It is used in the file
system, so you have to use naming conventions appropriate for file
systems.  Also, you must not call a function you declare after
'c-library' before you perform 'end-c-library'.

   A major benefit of these named library interfaces is that, once they
are generated, the tools used to generated them (in particular, the C
compiler and libtool) are no longer needed, so the interface can be used
even on machines that do not have the tools installed.

'c-library-name'       c-addr u -         gforth       "c-library-name"
   Start a C library interface with name c-addr u.

'c-library'       "name" -         gforth       "c-library"
   Parsing version of 'c-library-name'

'end-c-library'       -         gforth       "end-c-library"
   Finish and (if necessary) build the latest C library interface.

5.25.5 Declaring OS-level libraries
-----------------------------------

For calling some C functions, you need to link with a specific OS-level
library that contains that function.  E.g., the 'sin' function requires
linking a special library by using the command line switch '-lm'.  In
our C iterface you do the equivalent thing by calling 'add-lib' as
follows:

     clear-libs
     s" m" add-lib
     \c #include <math.h>
     c-function sin sin r -- r

   First, you clear any libraries that may have been declared earlier
(you don't need them for 'sin'); then you add the 'm' library (actually
'libm.so' or somesuch) to the currently declared libraries; you can add
as many as you need.  Finally you declare the function as shown above.
Typically you will use the same set of library declarations for many
function declarations; you need to write only one set for that, right at
the beginning.

   Note that you must not call 'clear-libs' inside
'c-library...end-c-library'; however, 'c-library' performs the function
of 'clear-libs', so 'clear-libs' is not necessary, and you usually want
to put 'add-lib' calls inside 'c-library...end-c-library'.

'clear-libs'       -         gforth       "clear-libs"
   Clear the list of libs

'add-lib'       c-addr u -         gforth       "add-lib"
   Add library libstring to the list of libraries, where string is
represented by c-addr u.

5.25.6 Callbacks
----------------

Callbacks are not yet supported by the documented C interface.  You can
use the undocumented 'lib.fs' interface for callbacks.

   In some cases you have to pass a function pointer to a C function,
i.e., the library wants to call back to your application (and the
pointed-to function is called a callback function).  You can pass the
address of an existing C function (that you get with 'lib-sym', *note
Low-Level C Interface Words::), but if there is no appropriate C
function, you probably want to define the function as a Forth word.

5.25.7 How the C interface works
--------------------------------

The documented C interface works by generating a C code out of the
declarations.

   In particular, for every Forth word declared with 'c-function', it
generates a wrapper function in C that takes the Forth data from the
Forth stacks, and calls the target C function with these data as
arguments.  The C compiler then performs an implicit conversion between
the Forth type from the stack, and the C type for the parameter, which
is given by the C function prototype.  After the C function returns, the
return value is likewise implicitly converted to a Forth type and
written back on the stack.

   The '\c' lines are literally included in the C code (but without the
'\c'), and provide the necessary declarations so that the C compiler
knows the C types and has enough information to perform the conversion.

   These wrapper functions are eventually compiled and dynamically
linked into Gforth, and then they can be called.

   The libraries added with 'add-lib' are used in the compile command
line to specify dependent libraries with '-lLIB', causing these
libraries to be dynamically linked when the wrapper function is linked.

5.25.8 Low-Level C Interface Words
----------------------------------

'open-lib'       c-addr1 u1 - u2        gforth       "open-lib"

'lib-sym'       c-addr1 u1 u2 - u3        gforth       "lib-sym"

'lib-error'       - c-addr u        gforth       "lib-error"
   Error message for last failed 'open-lib' or 'lib-sym'.

'call-c'       ... w - ...        gforth       "call-c"
   Call the C function pointed to by w.  The C function has to access
the stack itself.  The stack pointers are exported in the global
variables 'gforth_SP' and 'gforth_FP'.

5.26 Assembler and Code Words
=============================

5.26.1 'Code' and ';code'
-------------------------

Gforth provides some words for defining primitives (words written in
machine code), and for defining the machine-code equivalent of
'DOES>'-based defining words.  However, the machine-independent nature
of Gforth poses a few problems: First of all, Gforth runs on several
architectures, so it can provide no standard assembler.  What's worse is
that the register allocation not only depends on the processor, but also
on the 'gcc' version and options used.

   The words that Gforth offers encapsulate some system dependences
(e.g., the header structure), so a system-independent assembler may be
used in Gforth.  If you do not have an assembler, you can compile
machine code directly with ',' and 'c,'(1).

'assembler'       -         tools-ext       "assembler"

'init-asm'       -         gforth       "init-asm"

'code'       "name" - colon-sys         tools-ext       "code"

'end-code'       colon-sys -         gforth       "end-code"

';code'       compilation. colon-sys1 - colon-sys2         tools-ext       "semicolon-code"

'flush-icache'       c-addr u -        gforth       "flush-icache"
   Make sure that the instruction cache of the processor (if there is
one) does not contain stale data at c-addr and u bytes afterwards.
'END-CODE' performs a 'flush-icache' automatically.  Caveat:
'flush-icache' might not work on your installation; this is usually the
case if direct threading is not supported on your machine (take a look
at your 'machine.h') and your machine has a separate instruction cache.
In such cases, 'flush-icache' does nothing instead of flushing the
instruction cache.

   If 'flush-icache' does not work correctly, 'code' words etc.  will
not work (reliably), either.

   The typical usage of these 'code' words can be shown most easily by
analogy to the equivalent high-level defining words:

     : foo                              code foo
        <high-level Forth words>              <assembler>
     ;                                  end-code

     : bar                              : bar
        <high-level Forth words>           <high-level Forth words>
        CREATE                             CREATE
           <high-level Forth words>           <high-level Forth words>
        DOES>                              ;code
           <high-level Forth words>           <assembler>
     ;                                  end-code

   In the assembly code you will want to refer to the inner
interpreter's registers (e.g., the data stack pointer) and you may want
to use other registers for temporary storage.  Unfortunately, the
register allocation is installation-dependent.

   In particular, 'ip' (Forth instruction pointer) and 'rp' (return
stack pointer) may be in different places in 'gforth' and 'gforth-fast',
or different installations.  This means that you cannot write a 'NEXT'
routine that works reliably on both versions or different installations;
so for doing 'NEXT', I recommend jumping to '' noop >code-address',
which contains nothing but a 'NEXT'.

   For general accesses to the inner interpreter's registers, the
easiest solution is to use explicit register declarations (*note
Variables in Specified Registers: (gcc.info)Explicit Reg Vars.) for all
of the inner interpreter's registers: You have to compile Gforth with
'-DFORCE_REG' (configure option '--enable-force-reg') and the
appropriate declarations must be present in the 'machine.h' file (see
'mips.h' for an example; you can find a full list of all declarable
register symbols with 'grep register engine.c').  If you give explicit
registers to all variables that are declared at the beginning of
'engine()', you should be able to use the other caller-saved registers
for temporary storage.  Alternatively, you can use the 'gcc' option
'-ffixed-REG' (*note Options for Code Generation Conventions:
(gcc.info)Code Gen Options.) to reserve a register (however, this
restriction on register allocation may slow Gforth significantly).

   If this solution is not viable (e.g., because 'gcc' does not allow
you to explicitly declare all the registers you need), you have to find
out by looking at the code where the inner interpreter's registers
reside and which registers can be used for temporary storage.  You can
get an assembly listing of the engine's code with 'make engine.s'.

   In any case, it is good practice to abstract your assembly code from
the actual register allocation.  E.g., if the data stack pointer resides
in register '$17', create an alias for this register called 'sp', and
use that in your assembly code.

   Another option for implementing normal and defining words efficiently
is to add the desired functionality to the source of Gforth.  For normal
words you just have to edit 'primitives' (*note Automatic Generation::).
Defining words (equivalent to ';CODE' words, for fast defined words) may
require changes in 'engine.c', 'kernel.fs', 'prims2x.fs', and possibly
'cross.fs'.

   ---------- Footnotes ----------

   (1) This isn't portable, because these words emit stuff in data
space; it works because Gforth has unified code/data spaces.  Assembler
isn't likely to be portable anyway.

5.26.2 Common Assembler
-----------------------

The assemblers in Gforth generally use a postfix syntax, i.e., the
instruction name follows the operands.

   The operands are passed in the usual order (the same that is used in
the manual of the architecture).  Since they all are Forth words, they
have to be separated by spaces; you can also use Forth words to compute
the operands.

   The instruction names usually end with a ','.  This makes it easier
to visually separate instructions if you put several of them on one
line; it also avoids shadowing other Forth words (e.g., 'and').

   Registers are usually specified by number; e.g., (decimal) '11'
specifies registers R11 and F11 on the Alpha architecture (which one,
depends on the instruction).  The usual names are also available, e.g.,
's2' for R11 on Alpha.

   Control flow is specified similar to normal Forth code (*note
Arbitrary control structures::), with 'if,', 'ahead,', 'then,',
'begin,', 'until,', 'again,', 'cs-roll', 'cs-pick', 'else,', 'while,',
and 'repeat,'.  The conditions are specified in a way specific to each
assembler.

   Note that the register assignments of the Gforth engine can change
between Gforth versions, or even between different compilations of the
same Gforth version (e.g., if you use a different GCC version).  So if
you want to refer to Gforth's registers (e.g., the stack pointer or
TOS), I recommend defining your own words for refering to these
registers, and using them later on; then you can easily adapt to a
changed register assignment.  The stability of the register assignment
is usually better if you build Gforth with '--enable-force-reg'.

   The most common use of these registers is to dispatch to the next
word (the 'next' routine).  A portable way to do this is to jump to ''
noop >code-address' (of course, this is less efficient than integrating
the 'next' code and scheduling it well).

   Another difference between Gforth version is that the top of stack is
kept in memory in 'gforth' and, on most platforms, in a register in
'gforth-fast'.

5.26.3 Common Disassembler
--------------------------

You can disassemble a 'code' word with 'see' (*note Debugging::).  You
can disassemble a section of memory with

'discode'       addr u -         gforth       "discode"
   hook for the disassembler: disassemble code at addr of length u

   There are two kinds of disassembler for Gforth: The Forth
disassembler (available on some CPUs) and the gdb disassembler
(available on platforms with 'gdb' and 'mktemp').  If both are
available, the Forth disassembler is used by default.  If you prefer the
gdb disassembler, say

     ' disasm-gdb is discode

   If neither is available, 'discode' performs 'dump'.

   The Forth disassembler generally produces output that can be fed into
the assembler (i.e., same syntax, etc.).  It also includes additional
information in comments.  In particular, the address of the instruction
is given in a comment before the instruction.

   The gdb disassembler produces output in the same format as the gdb
'disassemble' command (*note Source and machine code: (gdb)Machine
Code.), in the default flavour (AT&T syntax for the 386 and AMD64
architectures).

   'See' may display more or less than the actual code of the word,
because the recognition of the end of the code is unreliable.  You can
use 'discode' if it did not display enough.  It may display more, if the
code word is not immediately followed by a named word.  If you have
something else there, you can follow the word with 'align latest ,' to
ensure that the end is recognized.

5.26.4 386 Assembler
--------------------

The 386 assembler included in Gforth was written by Bernd Paysan, it's
available under GPL, and originally part of bigFORTH.

   The 386 disassembler included in Gforth was written by Andrew McKewan
and is in the public domain.

   The disassembler displays code in an Intel-like prefix syntax.

   The assembler uses a postfix syntax with reversed parameters.

   The assembler includes all instruction of the Athlon, i.e.  486 core
instructions, Pentium and PPro extensions, floating point, MMX, 3Dnow!,
but not ISSE. It's an integrated 16- and 32-bit assembler.  Default is
32 bit, you can switch to 16 bit with .86 and back to 32 bit with .386.

   There are several prefixes to switch between different operation
sizes, '.b' for byte accesses, '.w' for word accesses, '.d' for
double-word accesses.  Addressing modes can be switched with '.wa' for
16 bit addresses, and '.da' for 32 bit addresses.  You don't need a
prefix for byte register names ('AL' et al).

   For floating point operations, the prefixes are '.fs' (IEEE single),
'.fl' (IEEE double), '.fx' (extended), '.fw' (word), '.fd'
(double-word), and '.fq' (quad-word).

   The MMX opcodes don't have size prefixes, they are spelled out like
in the Intel assembler.  Instead of move from and to memory, there are
PLDQ/PLDD and PSTQ/PSTD.

   The registers lack the 'e' prefix; even in 32 bit mode, eax is called
ax.  Immediate values are indicated by postfixing them with '#', e.g.,
'3 #'.  Here are some examples of addressing modes in various syntaxes:

     Gforth          Intel (NASM)   AT&T (gas)      Name
     .w ax           ax             %ax             register (16 bit)
     ax              eax            %eax            register (32 bit)
     3 #             offset 3       $3              immediate
     1000 #)         byte ptr 1000  1000            displacement
     bx )            [ebx]          (%ebx)          base
     100 di d)       100[edi]       100(%edi)       base+displacement
     20 ax *4 i#)    20[eax*4]      20(,%eax,4)     (index*scale)+displacement
     di ax *4 i)     [edi][eax*4]   (%edi,%eax,4)   base+(index*scale)
     4 bx cx di)     4[ebx][ecx]    4(%ebx,%ecx)    base+index+displacement
     12 sp ax *2 di) 12[esp][eax*2] 12(%esp,%eax,2) base+(index*scale)+displacement

   You can use 'L)' and 'LI)' instead of 'D)' and 'DI)' to enforce
32-bit displacement fields (useful for later patching).

   Some example of instructions are:

     ax bx mov             \ move ebx,eax
     3 # ax mov            \ mov eax,3
     100 di d) ax mov      \ mov eax,100[edi]
     4 bx cx di) ax mov    \ mov eax,4[ebx][ecx]
     .w ax bx mov          \ mov bx,ax

   The following forms are supported for binary instructions:

     <reg> <reg> <inst>
     <n> # <reg> <inst>
     <mem> <reg> <inst>
     <reg> <mem> <inst>
     <n> # <mem> <inst>

   The shift/rotate syntax is:

     <reg/mem> 1 # shl \ shortens to shift without immediate
     <reg/mem> 4 # shl
     <reg/mem> cl shl

   Precede string instructions ('movs' etc.)  with '.b' to get the byte
version.

   The control structure words 'IF' 'UNTIL' etc.  must be preceded by
one of these conditions: 'vs vc u< u>= 0= 0<> u<= u> 0< 0>= ps pc < >=
<= >'.  (Note that most of these words shadow some Forth words when
'assembler' is in front of 'forth' in the search path, e.g., in 'code'
words).  Currently the control structure words use one stack item, so
you have to use 'roll' instead of 'cs-roll' to shuffle them (you can
also use 'swap' etc.).

   Here is an example of a 'code' word (assumes that the stack pointer
is in esi and the TOS is in ebx):

     code my+ ( n1 n2 -- n )
         4 si D) bx add
         4 # si add
         Next
     end-code

5.26.5 Alpha Assembler
----------------------

The Alpha assembler and disassembler were originally written by Bernd
Thallner.

   The register names 'a0'-'a5' are not available to avoid shadowing hex
numbers.

   Immediate forms of arithmetic instructions are distinguished by a '#'
just before the ',', e.g., 'and#,' (note: 'lda,' does not count as
arithmetic instruction).

   You have to specify all operands to an instruction, even those that
other assemblers consider optional, e.g., the destination register for
'br,', or the destination register and hint for 'jmp,'.

   You can specify conditions for 'if,' by removing the first 'b' and
the trailing ',' from a branch with a corresponding name; e.g.,

     11 fgt if, \ if F11>0e
       ...
     endif,

   'fbgt,' gives 'fgt'.

5.26.6 MIPS assembler
---------------------

The MIPS assembler was originally written by Christian Pirker.

   Currently the assembler and disassembler only cover the MIPS-I
architecture (R3000), and don't support FP instructions.

   The register names '$a0'-'$a3' are not available to avoid shadowing
hex numbers.

   Because there is no way to distinguish registers from immediate
values, you have to explicitly use the immediate forms of instructions,
i.e., 'addiu,', not just 'addu,' ('as' does this implicitly).

   If the architecture manual specifies several formats for the
instruction (e.g., for 'jalr,'), you usually have to use the one with
more arguments (i.e., two for 'jalr,').  When in doubt, see
'arch/mips/testasm.fs' for an example of correct use.

   Branches and jumps in the MIPS architecture have a delay slot.  You
have to fill it yourself (the simplest way is to use 'nop,'), the
assembler does not do it for you (unlike 'as').  Even 'if,', 'ahead,',
'until,', 'again,', 'while,', 'else,' and 'repeat,' need a delay slot.
Since 'begin,' and 'then,' just specify branch targets, they are not
affected.

   Note that you must not put branches, jumps, or 'li,' into the delay
slot: 'li,' may expand to several instructions, and control flow
instructions may not be put into the branch delay slot in any case.

   For branches the argument specifying the target is a relative
address; You have to add the address of the delay slot to get the
absolute address.

   The MIPS architecture also has load delay slots and restrictions on
using 'mfhi,' and 'mflo,'; you have to order the instructions yourself
to satisfy these restrictions, the assembler does not do it for you.

   You can specify the conditions for 'if,' etc.  by taking a
conditional branch and leaving away the 'b' at the start and the ',' at
the end.  E.g.,

     4 5 eq if,
       ... \ do something if $4 equals $5
     then,

5.26.7 PowerPC assembler
------------------------

The PowerPC assembler and disassembler were contributed by Michal
Revucky.

   This assembler does not follow the convention of ending mnemonic
names with a ",", so some mnemonic names shadow regular Forth words (in
particular: 'and or xor fabs'); so if you want to use the Forth words,
you have to make them visible first, e.g., with 'also forth'.

   Registers are referred to by their number, e.g., '9' means the
integer register 9 or the FP register 9 (depending on the instruction).

   Because there is no way to distinguish registers from immediate
values, you have to explicitly use the immediate forms of instructions,
i.e., 'addi,', not just 'add,'.

   The assembler and disassembler usually support the most general form
of an instruction, but usually not the shorter forms (especially for
branches).

5.26.8 ARM Assembler
--------------------

The ARM assembler included in Gforth was written from scratch by David
Kuehling.

   The assembler includes all instruction of ARM architecture version 4,
but does not (yet) have support for Thumb instructions.  It also lacks
support for any co-processors.

   The assembler uses a postfix syntax with the target operand specified
last.  For load/store instructions the last operand will be the
register(s) to be loaded from/stored to.

   Registers are specified by their names 'r0' through 'r15', with the
aliases 'pc', 'lr', 'sp', 'ip' and 'fp' provided for convenience.  Note
that 'ip' means intra procedure call scratch register ('r12') and does
not refer to the instruction pointer.

   Condition codes can be specified anywhere in the instruction, but
will be most readable if specified just in front of the mnemonic.  The
'S' flag is not a separate word, but encoded into instruction mnemonics,
ie.  just use 'adds,' instead of 'add,' if you want the status register
to be updated.

   The following table lists the syntax of operands for general
instructions:

     Gforth          normal assembler      description
     123 #           #123                  immediate
     r12             r12                   register
     r12 4 #LSL      r12, LSL #4           shift left by immediate
     r12 r1 #LSL     r12, LSL r1           shift left by register
     r12 4 #LSR      r12, LSR #4           shift right by immediate
     r12 r1 #LSR     r12, LSR r1           shift right by register
     r12 4 #ASR      r12, ASR #4           arithmetic shift right
     r12 r1 #ASR     r12, ASR r1           ... by register
     r12 4 #ROR      r12, ROR #4           rotate right by immediate
     r12 r1 #ROR     r12, ROR r1           ... by register
     r12 RRX         r12, RRX              rotate right with extend by 1

   Memory operand syntax is listed in this table:

     Gforth            normal assembler      description
     r4 ]              [r4]                  register
     r4 4 #]           [r4, #+4]             register with immediate offset
     r4 -4 #]          [r4, #-4]             with negative offset
     r4 r1 +]          [r4, +r1]             register with register offset
     r4 r1 -]          [r4, -r1]             with negated register offset
     r4 r1 2 #LSL -]   [r4, -r1, LSL #2]     with negated and shifted offset
     r4 4 #]!          [r4, #+4]!            immediate preincrement
     r4 r1 +]!         [r4, +r1]!            register preincrement
     r4 r1 -]!         [r4, +r1]!            register predecrement
     r4 r1 2 #LSL +]!  [r4, +r1, LSL #2]!    shifted preincrement
     r4 -4 ]#          [r4], #-4             immediate postdecrement
     r4 r1 ]+          [r4], r1              register postincrement
     r4 r1 ]-          [r4], -r1             register postdecrement
     r4 r1 2 #LSL ]-   [r4], -r1, LSL #2     shifted postdecrement
     ' xyz >body [#]   xyz                   PC-relative addressing

   Register lists for load/store multiple instructions are started and
terminated by using the words '{' and '}' respectivly.  Between braces,
register names can be listed one by one, or register ranges can be
formed by using the postfix operator 'r-r'.  The '^' flag is not encoded
in the register list operand, but instead directly encoded into the
instruction mnemonic, ie.  use '^ldm,' and '^stm,'.

   Addressing modes for load/store multiple are not encoded as
instruction suffixes, but instead specified after the register that
supplies the address.  Use one of 'DA', 'IA', 'DB', 'IB', 'DA!', 'IA!',
'DB!' or 'IB!'.

   The following table gives some examples:

     Gforth                           normal assembler
     { r0 r7 r8 }  r4 ia  stm,        stmia    {r0,r7,r8}, r4
     { r0 r7 r8 }  r4 db!  ldm,       ldmdb    {r0,r7,r8}, r4!
     { r0 r15 r-r }  sp ia!  ^ldm,    ldmfd    {r0-r15}^, sp!

   Conditions for control structure words are specified in front of a
word:

     r1 r2 cmp,    \ compare r1 and r2
     eq if,        \ equal?
        ...          \ code executed if r1 == r2
     then,

   Here is an example of a 'code' word (assumes that the stack pointer
is in 'r9', and that 'r2' and 'r3' can be clobbered):

     code my+ ( n1 n2 --  n3 )
        r9 IA!       { r2 r3 } ldm,  \ pop r2 = n2, r3 = n1
        r2   r3      r3        add,  \ r3 = n2+n1
        r9 -4 #]!    r3        str,  \ push r3
        next,
     end-code

   Look at 'arch/arm/asm-example.fs' for more examples.

5.26.9 Other assemblers
-----------------------

If you want to contribute another assembler/disassembler, please contact
us (<anton@mips.complang.tuwien.ac.at>) to check if we have such an
assembler already.  If you are writing them from scratch, please use a
similar syntax style as the one we use (i.e., postfix, commas at the end
of the instruction names, *note Common Assembler::); make the output of
the disassembler be valid input for the assembler, and keep the style
similar to the style we used.

   Hints on implementation: The most important part is to have a good
test suite that contains all instructions.  Once you have that, the rest
is easy.  For actual coding you can take a look at 'arch/mips/disasm.fs'
to get some ideas on how to use data for both the assembler and
disassembler, avoiding redundancy and some potential bugs.  You can also
look at that file (and *note Advanced does> usage example::) to get
ideas how to factor a disassembler.

   Start with the disassembler, because it's easier to reuse data from
the disassembler for the assembler than the other way round.

   For the assembler, take a look at 'arch/alpha/asm.fs', which shows
how simple it can be.

5.27 Threading Words
====================

These words provide access to code addresses and other threading stuff
in Gforth (and, possibly, other interpretive Forths).  It more or less
abstracts away the differences between direct and indirect threading
(and, for direct threading, the machine dependences).  However, at
present this wordset is still incomplete.  It is also pretty low-level;
some day it will hopefully be made unnecessary by an internals wordset
that abstracts implementation details away completely.

   The terminology used here stems from indirect threaded Forth systems;
in such a system, the XT of a word is represented by the CFA (code field
address) of a word; the CFA points to a cell that contains the code
address.  The code address is the address of some machine code that
performs the run-time action of invoking the word (e.g., the 'dovar:'
routine pushes the address of the body of the word (a variable) on the
stack ).

   In an indirect threaded Forth, you can get the code address of name
with '' name @'; in Gforth you can get it with '' name >code-address',
independent of the threading method.

'threading-method'       - n        gforth       "threading-method"
   0 if the engine is direct threaded.  Note that this may change during
the lifetime of an image.

'>code-address'       xt - c_addr         gforth       ">code-address"
   c-addr is the code address of the word xt.

'code-address!'       c_addr xt -         gforth       "code-address!"
   Create a code field with code address c-addr at xt.

   For a word defined with 'DOES>', the code address usually points to a
jump instruction (the "does-handler") that jumps to the dodoes routine
(in Gforth on some platforms, it can also point to the dodoes routine
itself).  What you are typically interested in, though, is whether a
word is a 'DOES>'-defined word, and what Forth code it executes;
'>does-code' tells you that.

'>does-code'       xt - a_addr         gforth       ">does-code"
   If xt is the execution token of a child of a 'DOES>' word, a-addr is
the start of the Forth code after the 'DOES>'; Otherwise a-addr is 0.

   To create a 'DOES>'-defined word with the following basic words, you
have to set up a 'DOES>'-handler with 'does-handler!'; '/does-handler'
aus behind you have to place your executable Forth code.  Finally you
have to create a word and modify its behaviour with 'does-handler!'.

'does-code!'       a_addr xt -         gforth       "does-code!"
   Create a code field at xt for a child of a 'DOES>'-word; a-addr is
the start of the Forth code after 'DOES>'.

'does-handler!'       a_addr -         gforth       "does-handler!"
   Create a 'DOES>'-handler at address a-addr.  Normally, a-addr points
just behind a 'DOES>'.

'/does-handler'       - n         gforth       "/does-handler"
   The size of a 'DOES>'-handler (includes possible padding).

   The code addresses produced by various defining words are produced by
the following words:

'docol:'       - addr         gforth       "docol:"
   The code address of a colon definition.

'docon:'       - addr         gforth       "docon:"
   The code address of a 'CONSTANT'.

'dovar:'       - addr         gforth       "dovar:"
   The code address of a 'CREATE'd word.

'douser:'       - addr         gforth       "douser:"
   The code address of a 'USER' variable.

'dodefer:'       - addr         gforth       "dodefer:"
   The code address of a 'defer'ed word.

'dofield:'       - addr         gforth       "dofield:"
   The code address of a 'field'.

   The following two words generalize '>code-address', '>does-code',
'code-address!', and 'does-code!':

'>definer'       xt - definer         gforth       ">definer"
   DEFINER is a unique identifier for the way the XT was defined.  Words
defined with different 'does>'-codes have different definers.  The
definer can be used for comparison and in 'definer!'.

'definer!'       definer xt -         gforth       "definer!"
   The word represented by XT changes its behaviour to the behaviour
associated with DEFINER.

5.28 Passing Commands to the Operating System
=============================================

Gforth allows you to pass an arbitrary string to the host operating
system shell (if such a thing exists) for execution.

'sh'       "..." -         gforth       "sh"
   Parse a string and use 'system' to pass it to the host operating
system for execution in a sub-shell.

'system'       c-addr u -         gforth       "system"
   Pass the string specified by C-ADDR U to the host operating system
for execution in a sub-shell.  The value of the environment variable
'GFORTHSYSTEMPREFIX' (or its default value) is prepended to the string
(mainly to support using 'command.com' as shell in Windows instead of
whatever shell Cygwin uses by default; *note Environment variables::).

'$?'       - n         gforth       "dollar-question"
   'Value' - the exit status returned by the most recently executed
'system' command.

'getenv'       c-addr1 u1 - c-addr2 u2        gforth       "getenv"
   The string c-addr1 u1 specifies an environment variable.  The string
c-addr2 u2 is the host operating system's expansion of that environment
variable.  If the environment variable does not exist, c-addr2 u2
specifies a string 0 characters in length.

5.29 Keeping track of Time
==========================

'ms'       u -        facility-ext       "ms"
   Wait at least n milli-second.

'time&date'       - nsec nmin nhour nday nmonth nyear        facility-ext       "time-and-date"
   Report the current time of day.  Seconds, minutes and hours are
numbered from 0.  Months are numbered from 1.

'utime'       - dtime        gforth       "utime"
   Report the current time in microseconds since some epoch.

'cputime'       - duser dsystem        gforth       "cputime"
   duser and dsystem are the respective user- and system-level CPU times
used since the start of the Forth system (excluding child processes), in
microseconds (the granularity may be much larger, however).  On
platforms without the getrusage call, it reports elapsed time (since
some epoch) for duser and 0 for dsystem.

5.30 Miscellaneous Words
========================

These section lists the ANS Forth words that are not documented
elsewhere in this manual.  Ultimately, they all need proper homes.

'quit'       ?? - ??         core       "quit"
   Empty the return stack, make the user input device the input source,
enter interpret state and start the text interpreter.

   The following ANS Forth words are not currently supported by Gforth
(*note ANS conformance::):

   'EDITOR' 'EMIT?'  'FORGET'

6 Error messages
****************

A typical Gforth error message looks like this:

     in file included from \evaluated string/:-1
     in file included from ./yyy.fs:1
     ./xxx.fs:4: Invalid memory address
     >>>bar<<<
     Backtrace:
     $400E664C @
     $400E6664 foo

   The message identifying the error is 'Invalid memory address'.  The
error happened when text-interpreting line 4 of the file './xxx.fs'.
This line is given (it contains 'bar'), and the word on the line where
the error happened, is pointed out (with '>>>' and '<<<').

   The file containing the error was included in line 1 of './yyy.fs',
and 'yyy.fs' was included from a non-file (in this case, by giving
'yyy.fs' as command-line parameter to Gforth).

   At the end of the error message you find a return stack dump that can
be interpreted as a backtrace (possibly empty).  On top you find the top
of the return stack when the 'throw' happened, and at the bottom you
find the return stack entry just above the return stack of the topmost
text interpreter.

   To the right of most return stack entries you see a guess for the
word that pushed that return stack entry as its return address.  This
gives a backtrace.  In our case we see that 'bar' called 'foo', and
'foo' called '@' (and '@' had an _Invalid memory address_ exception).

   Note that the backtrace is not perfect: We don't know which return
stack entries are return addresses (so we may get false positives); and
in some cases (e.g., for 'abort"') we cannot determine from the return
address the word that pushed the return address, so for some return
addresses you see no names in the return stack dump.

   The return stack dump represents the return stack at the time when a
specific 'throw' was executed.  In programs that make use of 'catch', it
is not necessarily clear which 'throw' should be used for the return
stack dump (e.g., consider one 'throw' that indicates an error, which is
caught, and during recovery another error happens; which 'throw' should
be used for the stack dump?).  Gforth presents the return stack dump for
the first 'throw' after the last executed (not returned-to) 'catch' or
'nothrow'; this works well in the usual case.  To get the right
backtrace, you usually want to insert 'nothrow' or '['] false catch
drop' after a 'catch' if the error is not rethrown.

   'Gforth' is able to do a return stack dump for throws generated from
primitives (e.g., invalid memory address, stack empty etc.);
'gforth-fast' is only able to do a return stack dump from a directly
called 'throw' (including 'abort' etc.).  Given an exception caused by a
primitive in 'gforth-fast', you will typically see no return stack dump
at all; however, if the exception is caught by 'catch' (e.g., for
restoring some state), and then 'throw'n again, the return stack dump
will be for the first such 'throw'.

7 Tools
*******

See also *note Emacs and Gforth::.

7.1 'ans-report.fs': Report the words used, sorted by wordset
=============================================================

If you want to label a Forth program as ANS Forth Program, you must
document which wordsets the program uses; for extension wordsets, it is
helpful to list the words the program requires from these wordsets
(because Forth systems are allowed to provide only some words of them).

   The 'ans-report.fs' tool makes it easy for you to determine which
words from which wordset and which non-ANS words your application uses.
You simply have to include 'ans-report.fs' before loading the program
you want to check.  After loading your program, you can get the report
with 'print-ans-report'.  A typical use is to run this as batch job like
this:
     gforth ans-report.fs myprog.fs -e "print-ans-report bye"

   The output looks like this (for 'compat/control.fs'):
     The program uses the following words
     from CORE :
     : POSTPONE THEN ; immediate ?dup IF 0=
     from BLOCK-EXT :
     \
     from FILE :
     (

7.1.1 Caveats
-------------

Note that 'ans-report.fs' just checks which words are used, not whether
they are used in an ANS Forth conforming way!

   Some words are defined in several wordsets in the standard.
'ans-report.fs' reports them for only one of the wordsets, and not
necessarily the one you expect.  It depends on usage which wordset is
the right one to specify.  E.g., if you only use the compilation
semantics of 'S"', it is a Core word; if you also use its interpretation
semantics, it is a File word.

7.2 Stack depth changes during interpretation
=============================================

Sometimes you notice that, after loading a file, there are items left on
the stack.  The tool 'depth-changes.fs' helps you find out quickly where
in the file these stack items are coming from.

   The simplest way of using 'depth-changes.fs' is to include it before
the file(s) you want to check, e.g.:

     gforth depth-changes.fs my-file.fs

   This will compare the stack depths of the data and FP stack at every
empty line (in interpretation state) against these depths at the last
empty line (in interpretation state).  If the depths are not equal, the
position in the file and the stack contents are printed with '~~' (*note
Debugging::).  This indicates that a stack depth change has occured in
the paragraph of non-empty lines before the indicated line.  It is a
good idea to leave an empty line at the end of the file, so the last
paragraph is checked, too.

   Checking only at empty lines usually works well, but sometimes you
have big blocks of non-empty lines (e.g., when building a big table),
and you want to know where in this block the stack depth changed.  You
can check all interpreted lines with

     gforth depth-changes.fs -e "' all-lines is depth-changes-filter" my-file.fs

   This checks the stack depth at every end-of-line.  So the depth
change occured in the line reported by the '~~' (not in the line
before).

   Note that, while this offers better accuracy in indicating where the
stack depth changes, it will often report many intentional stack depth
changes (e.g., when an interpreted computation stretches across several
lines).  You can suppress the checking of some lines by putting
backslashes at the end of these lines (not followed by white space), and
using

     gforth depth-changes.fs -e "' most-lines is depth-changes-filter" my-file.fs

8 ANS conformance
*****************

To the best of our knowledge, Gforth is an

   ANS Forth System
   * providing the Core Extensions word set
   * providing the Block word set
   * providing the Block Extensions word set
   * providing the Double-Number word set
   * providing the Double-Number Extensions word set
   * providing the Exception word set
   * providing the Exception Extensions word set
   * providing the Facility word set
   * providing 'EKEY', 'EKEY>CHAR', 'EKEY?', 'MS' and 'TIME&DATE' from
     the Facility Extensions word set
   * providing the File Access word set
   * providing the File Access Extensions word set
   * providing the Floating-Point word set
   * providing the Floating-Point Extensions word set
   * providing the Locals word set
   * providing the Locals Extensions word set
   * providing the Memory-Allocation word set
   * providing the Memory-Allocation Extensions word set (that one's
     easy)
   * providing the Programming-Tools word set
   * providing ';CODE', 'AHEAD', 'ASSEMBLER', 'BYE', 'CODE', 'CS-PICK',
     'CS-ROLL', 'STATE', '[ELSE]', '[IF]', '[THEN]' from the
     Programming-Tools Extensions word set
   * providing the Search-Order word set
   * providing the Search-Order Extensions word set
   * providing the String word set
   * providing the String Extensions word set (another easy one)

   Gforth has the following environmental restrictions:

   * While processing the OS command line, if an exception is not
     caught, Gforth exits with a non-zero exit code instyead of
     performing QUIT.

   * When an 'throw' is performed after a 'query', Gforth does not
     allways restore the input source specification in effect at the
     corresponding catch.

   In addition, ANS Forth systems are required to document certain
implementation choices.  This chapter tries to meet these requirements.
In many cases it gives a way to ask the system for the information
instead of providing the information directly, in particular, if the
information depends on the processor, the operating system or the
installation options chosen, or if they are likely to change during the
maintenance of Gforth.

8.1 The Core Words
==================

8.1.1 Implementation Defined Options
------------------------------------

(Cell) aligned addresses:
     processor-dependent.  Gforth's alignment words perform natural
     alignment (e.g., an address aligned for a datum of size 8 is
     divisible by 8).  Unaligned accesses usually result in a '-23
     THROW'.

'EMIT' and non-graphic characters:
     The character is output using the C library function (actually,
     macro) 'putc'.

character editing of 'ACCEPT' and 'EXPECT':
     This is modeled on the GNU readline library (*note Command Line
     Editing: (readline)Readline Interaction.) with Emacs-like key
     bindings.  'Tab' deviates a little by producing a full word
     completion every time you type it (instead of producing the common
     prefix of all completions).  *Note Command-line editing::.

character set:
     The character set of your computer and display device.  Gforth is
     8-bit-clean (but some other component in your system may make
     trouble).

Character-aligned address requirements:
     installation-dependent.  Currently a character is represented by a
     C 'unsigned char'; in the future we might switch to 'wchar_t'
     (Comments on that requested).

character-set extensions and matching of names:
     Any character except the ASCII NUL character can be used in a name.
     Matching is case-insensitive (except in 'TABLE's).  The matching is
     performed using the C library function 'strncasecmp', whose
     function is probably influenced by the locale.  E.g., the 'C'
     locale does not know about accents and umlauts, so they are matched
     case-sensitively in that locale.  For portability reasons it is
     best to write programs such that they work in the 'C' locale.  Then
     one can use libraries written by a Polish programmer (who might use
     words containing ISO Latin-2 encoded characters) and by a French
     programmer (ISO Latin-1) in the same program (of course, 'WORDS'
     will produce funny results for some of the words (which ones,
     depends on the font you are using)).  Also, the locale you prefer
     may not be available in other operating systems.  Hopefully,
     Unicode will solve these problems one day.

conditions under which control characters match a space delimiter:
     If 'word' is called with the space character as a delimiter, all
     white-space characters (as identified by the C macro 'isspace()')
     are delimiters.  'Parse', on the other hand, treats space like
     other delimiters.  'Parse-name', which is used by the outer
     interpreter (aka text interpreter) by default, treats all
     white-space characters as delimiters.

format of the control-flow stack:
     The data stack is used as control-flow stack.  The size of a
     control-flow stack item in cells is given by the constant
     'cs-item-size'.  At the time of this writing, an item consists of a
     (pointer to a) locals list (third), an address in the code
     (second), and a tag for identifying the item (TOS). The following
     tags are used: 'defstart', 'live-orig', 'dead-orig', 'dest',
     'do-dest', 'scopestart'.

conversion of digits > 35
     The characters '[\]^_'' are the digits with the decimal value
     36-41.  There is no way to input many of the larger digits.

display after input terminates in 'ACCEPT' and 'EXPECT':
     The cursor is moved to the end of the entered string.  If the input
     is terminated using the 'Return' key, a space is typed.

exception abort sequence of 'ABORT"':
     The error string is stored into the variable '"error' and a '-2
     throw' is performed.

input line terminator:
     For interactive input, 'C-m' (CR) and 'C-j' (LF) terminate lines.
     One of these characters is typically produced when you type the
     'Enter' or 'Return' key.

maximum size of a counted string:
     's" /counted-string" environment? drop .'.  Currently 255
     characters on all platforms, but this may change.

maximum size of a parsed string:
     Given by the constant '/line'.  Currently 255 characters.

maximum size of a definition name, in characters:
     MAXU/8

maximum string length for 'ENVIRONMENT?', in characters:
     MAXU/8

method of selecting the user input device:
     The user input device is the standard input.  There is currently no
     way to change it from within Gforth.  However, the input can
     typically be redirected in the command line that starts Gforth.

method of selecting the user output device:
     'EMIT' and 'TYPE' output to the file-id stored in the value
     'outfile-id' ('stdout' by default).  Gforth uses unbuffered output
     when the user output device is a terminal, otherwise the output is
     buffered.

methods of dictionary compilation:
     What are we expected to document here?

number of bits in one address unit:
     's" address-units-bits" environment? drop .'.  8 in all current
     platforms.

number representation and arithmetic:
     Processor-dependent.  Binary two's complement on all current
     platforms.

ranges for integer types:
     Installation-dependent.  Make environmental queries for 'MAX-N',
     'MAX-U', 'MAX-D' and 'MAX-UD'.  The lower bounds for unsigned (and
     positive) types is 0.  The lower bound for signed types on two's
     complement and one's complement machines machines can be computed
     by adding 1 to the upper bound.

read-only data space regions:
     The whole Forth data space is writable.

size of buffer at 'WORD':
     'PAD HERE - .'.  104 characters on 32-bit machines.  The buffer is
     shared with the pictured numeric output string.  If overwriting
     'PAD' is acceptable, it is as large as the remaining dictionary
     space, although only as much can be sensibly used as fits in a
     counted string.

size of one cell in address units:
     '1 cells .'.

size of one character in address units:
     '1 chars .'.  1 on all current platforms.

size of the keyboard terminal buffer:
     Varies.  You can determine the size at a specific time using 'lp@
     tib - .'.  It is shared with the locals stack and TIBs of files
     that include the current file.  You can change the amount of space
     for TIBs and locals stack at Gforth startup with the command line
     option '-l'.

size of the pictured numeric output buffer:
     'PAD HERE - .'.  104 characters on 32-bit machines.  The buffer is
     shared with 'WORD'.

size of the scratch area returned by 'PAD':
     The remainder of dictionary space.  'unused pad here - - .'.

system case-sensitivity characteristics:
     Dictionary searches are case-insensitive (except in 'TABLE's).
     However, as explained above under character-set extensions, the
     matching for non-ASCII characters is determined by the locale you
     are using.  In the default 'C' locale all non-ASCII characters are
     matched case-sensitively.

system prompt:
     ' ok' in interpret state, ' compiled' in compile state.

division rounding:
     The ordinary division words '/ mod /mod */ */mod' perform floored
     division (with the default installation of Gforth).  You can check
     this with 's" floored" environment? drop .'.  If you write programs
     that need a specific division rounding, best use 'fm/mod' or
     'sm/rem' for portability.

values of 'STATE' when true:
     -1.

values returned after arithmetic overflow:
     On two's complement machines, arithmetic is performed modulo
     2**bits-per-cell for single arithmetic and 4**bits-per-cell for
     double arithmetic (with appropriate mapping for signed types).
     Division by zero typically results in a '-55 throw' (Floating-point
     unidentified fault) or '-10 throw' (divide by zero).  Integer
     division overflow can result in these throws, or in '-11 throw'; in
     'gforth-fast' division overflow and divide by zero may also result
     in returning bogus results without producing an exception.

whether the current definition can be found after DOES>:
     No.

8.1.2 Ambiguous conditions
--------------------------

a name is neither a word nor a number:
     '-13 throw' (Undefined word).

a definition name exceeds the maximum length allowed:
     '-19 throw' (Word name too long)

addressing a region not inside the various data spaces of the forth system:
     The stacks, code space and header space are accessible.  Machine
     code space is typically readable.  Accessing other addresses gives
     results dependent on the operating system.  On decent systems: '-9
     throw' (Invalid memory address).

argument type incompatible with parameter:
     This is usually not caught.  Some words perform checks, e.g., the
     control flow words, and issue a 'ABORT"' or '-12 THROW' (Argument
     type mismatch).

attempting to obtain the execution token of a word with undefined execution semantics:
     '-14 throw' (Interpreting a compile-only word).  In some cases, you
     get an execution token for 'compile-only-error' (which performs a
     '-14 throw' when executed).

dividing by zero:
     On some platforms, this produces a '-10 throw' (Division by zero);
     on other systems, this typically results in a '-55 throw'
     (Floating-point unidentified fault).

insufficient data stack or return stack space:
     Depending on the operating system, the installation, and the
     invocation of Gforth, this is either checked by the memory
     management hardware, or it is not checked.  If it is checked, you
     typically get a '-3 throw' (Stack overflow), '-5 throw' (Return
     stack overflow), or '-9 throw' (Invalid memory address) (depending
     on the platform and how you achieved the overflow) as soon as the
     overflow happens.  If it is not checked, overflows typically result
     in mysterious illegal memory accesses, producing '-9 throw'
     (Invalid memory address) or '-23 throw' (Address alignment
     exception); they might also destroy the internal data structure of
     'ALLOCATE' and friends, resulting in various errors in these words.

insufficient space for loop control parameters:
     Like other return stack overflows.

insufficient space in the dictionary:
     If you try to allot (either directly with 'allot', or indirectly
     with ',', 'create' etc.)  more memory than available in the
     dictionary, you get a '-8 throw' (Dictionary overflow).  If you try
     to access memory beyond the end of the dictionary, the results are
     similar to stack overflows.

interpreting a word with undefined interpretation semantics:
     For some words, we have defined interpretation semantics.  For the
     others: '-14 throw' (Interpreting a compile-only word).

modifying the contents of the input buffer or a string literal:
     These are located in writable memory and can be modified.

overflow of the pictured numeric output string:
     '-17 throw' (Pictured numeric ouput string overflow).

parsed string overflow:
     'PARSE' cannot overflow.  'WORD' does not check for overflow.

producing a result out of range:
     On two's complement machines, arithmetic is performed modulo
     2**bits-per-cell for single arithmetic and 4**bits-per-cell for
     double arithmetic (with appropriate mapping for signed types).
     Division by zero typically results in a '-10 throw' (divide by
     zero) or '-55 throw' (floating point unidentified fault).  Overflow
     on division may result in these errors or in '-11 throw' (result
     out of range).  'Gforth-fast' may silently produce bogus results on
     division overflow or division by zero.  'Convert' and '>number'
     currently overflow silently.

reading from an empty data or return stack:
     The data stack is checked by the outer (aka text) interpreter after
     every word executed.  If it has underflowed, a '-4 throw' (Stack
     underflow) is performed.  Apart from that, stacks may be checked or
     not, depending on operating system, installation, and invocation.
     If they are caught by a check, they typically result in '-4 throw'
     (Stack underflow), '-6 throw' (Return stack underflow) or '-9
     throw' (Invalid memory address), depending on the platform and
     which stack underflows and by how much.  Note that even if the
     system uses checking (through the MMU), your program may have to
     underflow by a significant number of stack items to trigger the
     reaction (the reason for this is that the MMU, and therefore the
     checking, works with a page-size granularity).  If there is no
     checking, the symptoms resulting from an underflow are similar to
     those from an overflow.  Unbalanced return stack errors can result
     in a variety of symptoms, including '-9 throw' (Invalid memory
     address) and Illegal Instruction (typically '-260 throw').

unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
     'Create' and its descendants perform a '-16 throw' (Attempt to use
     zero-length string as a name).  Words like ''' probably will not
     find what they search.  Note that it is possible to create
     zero-length names with 'nextname' (should it not?).

'>IN' greater than input buffer:
     The next invocation of a parsing word returns a string with length
     0.

'RECURSE' appears after 'DOES>':
     Compiles a recursive call to the defining word, not to the defined
     word.

argument input source different than current input source for 'RESTORE-INPUT':
     '-12 THROW'.  Note that, once an input file is closed (e.g.,
     because the end of the file was reached), its source-id may be
     reused.  Therefore, restoring an input source specification
     referencing a closed file may lead to unpredictable results instead
     of a '-12 THROW'.

     In the future, Gforth may be able to restore input source
     specifications from other than the current input source.

data space containing definitions gets de-allocated:
     Deallocation with 'allot' is not checked.  This typically results
     in memory access faults or execution of illegal instructions.

data space read/write with incorrect alignment:
     Processor-dependent.  Typically results in a '-23 throw' (Address
     alignment exception).  Under Linux-Intel on a 486 or later
     processor with alignment turned on, incorrect alignment results in
     a '-9 throw' (Invalid memory address).  There are reportedly some
     processors with alignment restrictions that do not report
     violations.

data space pointer not properly aligned, ',', 'C,':
     Like other alignment errors.

less than u+2 stack items ('PICK' and 'ROLL'):
     Like other stack underflows.

loop control parameters not available:
     Not checked.  The counted loop words simply assume that the top of
     return stack items are loop control parameters and behave
     accordingly.

most recent definition does not have a name ('IMMEDIATE'):
     'abort" last word was headerless"'.

name not defined by 'VALUE' used by 'TO':
     '-32 throw' (Invalid name argument) (unless name is a local or was
     defined by 'CONSTANT'; in the latter case it just changes the
     constant).

name not found (''', 'POSTPONE', '[']', '[COMPILE]'):
     '-13 throw' (Undefined word)

parameters are not of the same type ('DO', '?DO', 'WITHIN'):
     Gforth behaves as if they were of the same type.  I.e., you can
     predict the behaviour by interpreting all parameters as, e.g.,
     signed.

'POSTPONE' or '[COMPILE]' applied to 'TO':
     Assume ': X POSTPONE TO ; IMMEDIATE'.  'X' performs the compilation
     semantics of 'TO'.

String longer than a counted string returned by 'WORD':
     Not checked.  The string will be ok, but the count will, of course,
     contain only the least significant bits of the length.

u greater than or equal to the number of bits in a cell ('LSHIFT', 'RSHIFT'):
     Processor-dependent.  Typical behaviours are returning 0 and using
     only the low bits of the shift count.

word not defined via 'CREATE':
     '>BODY' produces the PFA of the word no matter how it was defined.

     'DOES>' changes the execution semantics of the last defined word no
     matter how it was defined.  E.g., 'CONSTANT DOES>' is equivalent to
     'CREATE , DOES>'.

words improperly used outside '<#' and '#>':
     Not checked.  As usual, you can expect memory faults.

8.1.3 Other system documentation
--------------------------------

nonstandard words using 'PAD':
     None.

operator's terminal facilities available:
     After processing the OS's command line, Gforth goes into
     interactive mode, and you can give commands to Gforth
     interactively.  The actual facilities available depend on how you
     invoke Gforth.

program data space available:
     'UNUSED .' gives the remaining dictionary space.  The total
     dictionary space can be specified with the '-m' switch (*note
     Invoking Gforth::) when Gforth starts up.

return stack space available:
     You can compute the total return stack space in cells with 's"
     RETURN-STACK-CELLS" environment? drop .'.  You can specify it at
     startup time with the '-r' switch (*note Invoking Gforth::).

stack space available:
     You can compute the total data stack space in cells with 's"
     STACK-CELLS" environment? drop .'.  You can specify it at startup
     time with the '-d' switch (*note Invoking Gforth::).

system dictionary space required, in address units:
     Type 'here forthstart - .' after startup.  At the time of this
     writing, this gives 80080 (bytes) on a 32-bit system.

8.2 The optional Block word set
===============================

8.2.1 Implementation Defined Options
------------------------------------

the format for display by 'LIST':
     First the screen number is displayed, then 16 lines of 64
     characters, each line preceded by the line number.

the length of a line affected by '\':
     64 characters.

8.2.2 Ambiguous conditions
--------------------------

correct block read was not possible:
     Typically results in a 'throw' of some OS-derived value (between
     -512 and -2048).  If the blocks file was just not long enough,
     blanks are supplied for the missing portion.

I/O exception in block transfer:
     Typically results in a 'throw' of some OS-derived value (between
     -512 and -2048).

invalid block number:
     '-35 throw' (Invalid block number)

a program directly alters the contents of 'BLK':
     The input stream is switched to that other block, at the same
     position.  If the storing to 'BLK' happens when interpreting
     non-block input, the system will get quite confused when the block
     ends.

no current block buffer for 'UPDATE':
     'UPDATE' has no effect.

8.2.3 Other system documentation
--------------------------------

any restrictions a multiprogramming system places on the use of buffer addresses:
     No restrictions (yet).

the number of blocks available for source and data:
     depends on your disk space.

8.3 The optional Double Number word set
=======================================

8.3.1 Ambiguous conditions
--------------------------

d outside of range of n in 'D>S':
     The least significant cell of d is produced.

8.4 The optional Exception word set
===================================

8.4.1 Implementation Defined Options
------------------------------------

'THROW'-codes used in the system:
     The codes -256--511 are used for reporting signals.  The mapping
     from OS signal numbers to throw codes is -256-signal.  The codes
     -512--2047 are used for OS errors (for file and memory allocation
     operations).  The mapping from OS error numbers to throw codes is
     -512-'errno'.  One side effect of this mapping is that undefined OS
     errors produce a message with a strange number; e.g., '-1000 THROW'
     results in 'Unknown error 488' on my system.

8.5 The optional Facility word set
==================================

8.5.1 Implementation Defined Options
------------------------------------

encoding of keyboard events ('EKEY'):
     Keys corresponding to ASCII characters are encoded as ASCII
     characters.  Other keys are encoded with the constants 'k-left',
     'k-right', 'k-up', 'k-down', 'k-home', 'k-end', 'k1', 'k2', 'k3',
     'k4', 'k5', 'k6', 'k7', 'k8', 'k9', 'k10', 'k11', 'k12'.

duration of a system clock tick:
     System dependent.  With respect to 'MS', the time is specified in
     microseconds.  How well the OS and the hardware implement this, is
     another question.

repeatability to be expected from the execution of 'MS':
     System dependent.  On Unix, a lot depends on load.  If the system
     is lightly loaded, and the delay is short enough that Gforth does
     not get swapped out, the performance should be acceptable.  Under
     MS-DOS and other single-tasking systems, it should be good.

8.5.2 Ambiguous conditions
--------------------------

'AT-XY' can't be performed on user output device:
     Largely terminal dependent.  No range checks are done on the
     arguments.  No errors are reported.  You may see some garbage
     appearing, you may see simply nothing happen.

8.6 The optional File-Access word set
=====================================

8.6.1 Implementation Defined Options
------------------------------------

file access methods used:
     'R/O', 'R/W' and 'BIN' work as you would expect.  'W/O' translates
     into the C file opening mode 'w' (or 'wb'): The file is cleared, if
     it exists, and created, if it does not (with both 'open-file' and
     'create-file').  Under Unix 'create-file' creates a file with 666
     permissions modified by your umask.

file exceptions:
     The file words do not raise exceptions (except, perhaps, memory
     access faults when you pass illegal addresses or file-ids).

file line terminator:
     System-dependent.  Gforth uses C's newline character as line
     terminator.  What the actual character code(s) of this are is
     system-dependent.

file name format:
     System dependent.  Gforth just uses the file name format of your
     OS.

information returned by 'FILE-STATUS':
     'FILE-STATUS' returns the most powerful file access mode allowed
     for the file: Either 'R/O', 'W/O' or 'R/W'.  If the file cannot be
     accessed, 'R/O BIN' is returned.  'BIN' is applicable along with
     the returned mode.

input file state after an exception when including source:
     All files that are left via the exception are closed.

ior values and meaning:
     The iors returned by the file and memory allocation words are
     intended as throw codes.  They typically are in the range
     -512--2047 of OS errors.  The mapping from OS error numbers to iors
     is -512-errno.

maximum depth of file input nesting:
     limited by the amount of return stack, locals/TIB stack, and the
     number of open files available.  This should not give you troubles.

maximum size of input line:
     '/line'.  Currently 255.

methods of mapping block ranges to files:
     By default, blocks are accessed in the file 'blocks.fb' in the
     current working directory.  The file can be switched with 'USE'.

number of string buffers provided by 'S"':
     1

size of string buffer used by 'S"':
     '/line'.  currently 255.

8.6.2 Ambiguous conditions
--------------------------

attempting to position a file outside its boundaries:
     'REPOSITION-FILE' is performed as usual: Afterwards,
     'FILE-POSITION' returns the value given to 'REPOSITION-FILE'.

attempting to read from file positions not yet written:
     End-of-file, i.e., zero characters are read and no error is
     reported.

file-id is invalid ('INCLUDE-FILE'):
     An appropriate exception may be thrown, but a memory fault or other
     problem is more probable.

I/O exception reading or closing file-id ('INCLUDE-FILE', 'INCLUDED'):
     The ior produced by the operation, that discovered the problem, is
     thrown.

named file cannot be opened ('INCLUDED'):
     The ior produced by 'open-file' is thrown.

requesting an unmapped block number:
     There are no unmapped legal block numbers.  On some operating
     systems, writing a block with a large number may overflow the file
     system and have an error message as consequence.

using 'source-id' when 'blk' is non-zero:
     'source-id' performs its function.  Typically it will give the id
     of the source which loaded the block.  (Better ideas?)

8.7 The optional Floating-Point word set
========================================

8.7.1 Implementation Defined Options
------------------------------------

format and range of floating point numbers:
     System-dependent; the 'double' type of C.

results of 'REPRESENT' when float is out of range:
     System dependent; 'REPRESENT' is implemented using the C library
     function 'ecvt()' and inherits its behaviour in this respect.

rounding or truncation of floating-point numbers:
     System dependent; the rounding behaviour is inherited from the
     hosting C compiler.  IEEE-FP-based (i.e., most) systems by default
     round to nearest, and break ties by rounding to even (i.e., such
     that the last bit of the mantissa is 0).

size of floating-point stack:
     's" FLOATING-STACK" environment? drop .' gives the total size of
     the floating-point stack (in floats).  You can specify this on
     startup with the command-line option '-f' (*note Invoking
     Gforth::).

width of floating-point stack:
     '1 floats'.

8.7.2 Ambiguous conditions
--------------------------

'df@' or 'df!' used with an address that is not double-float aligned:
     System-dependent.  Typically results in a '-23 THROW' like other
     alignment violations.

'f@' or 'f!' used with an address that is not float aligned:
     System-dependent.  Typically results in a '-23 THROW' like other
     alignment violations.

floating-point result out of range:
     System-dependent.  Can result in a '-43 throw' (floating point
     overflow), '-54 throw' (floating point underflow), '-41 throw'
     (floating point inexact result), '-55 THROW' (Floating-point
     unidentified fault), or can produce a special value representing,
     e.g., Infinity.

'sf@' or 'sf!' used with an address that is not single-float aligned:
     System-dependent.  Typically results in an alignment fault like
     other alignment violations.

'base' is not decimal ('REPRESENT', 'F.', 'FE.', 'FS.'):
     The floating-point number is converted into decimal nonetheless.

Both arguments are equal to zero ('FATAN2'):
     System-dependent.  'FATAN2' is implemented using the C library
     function 'atan2()'.

Using 'FTAN' on an argument r1 where cos(r1) is zero:
     System-dependent.  Anyway, typically the cos of r1 will not be zero
     because of small errors and the tan will be a very large (or very
     small) but finite number.

d cannot be presented precisely as a float in 'D>F':
     The result is rounded to the nearest float.

dividing by zero:
     Platform-dependent; can produce an Infinity, NaN, '-42 throw'
     (floating point divide by zero) or '-55 throw' (Floating-point
     unidentified fault).

exponent too big for conversion ('DF!', 'DF@', 'SF!', 'SF@'):
     System dependent.  On IEEE-FP based systems the number is converted
     into an infinity.

float<1 ('FACOSH'):
     Platform-dependent; on IEEE-FP systems typically produces a NaN.

float=<-1 ('FLNP1'):
     Platform-dependent; on IEEE-FP systems typically produces a NaN (or
     a negative infinity for float=-1).

float=<0 ('FLN', 'FLOG'):
     Platform-dependent; on IEEE-FP systems typically produces a NaN (or
     a negative infinity for float=0).

float<0 ('FASINH', 'FSQRT'):
     Platform-dependent; for 'fsqrt' this typically gives a NaN, for
     'fasinh' some platforms produce a NaN, others a number (bug in the
     C library?).

|float|>1 ('FACOS', 'FASIN', 'FATANH'):
     Platform-dependent; IEEE-FP systems typically produce a NaN.

integer part of float cannot be represented by d in 'F>D':
     Platform-dependent; typically, some double number is produced and
     no error is reported.

string larger than pictured numeric output area ('f.', 'fe.', 'fs.'):
     'Precision' characters of the numeric output area are used.  If
     'precision' is too high, these words will smash the data or code
     close to 'here'.

8.8 The optional Locals word set
================================

8.8.1 Implementation Defined Options
------------------------------------

maximum number of locals in a definition:
     's" #locals" environment? drop .'.  Currently 15.  This is a lower
     bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
     characters.  The number of locals in a definition is bounded by the
     size of locals-buffer, which contains the names of the locals.

8.8.2 Ambiguous conditions
--------------------------

executing a named local in interpretation state:
     Locals have no interpretation semantics.  If you try to perform the
     interpretation semantics, you will get a '-14 throw' somewhere
     (Interpreting a compile-only word).  If you perform the compilation
     semantics, the locals access will be compiled (irrespective of
     state).

name not defined by 'VALUE' or '(LOCAL)' ('TO'):
     '-32 throw' (Invalid name argument)

8.9 The optional Memory-Allocation word set
===========================================

8.9.1 Implementation Defined Options
------------------------------------

values and meaning of ior:
     The iors returned by the file and memory allocation words are
     intended as throw codes.  They typically are in the range
     -512--2047 of OS errors.  The mapping from OS error numbers to iors
     is -512-errno.

8.10 The optional Programming-Tools word set
============================================

8.10.1 Implementation Defined Options
-------------------------------------

ending sequence for input following ';CODE' and 'CODE':
     'END-CODE'

manner of processing input following ';CODE' and 'CODE':
     The 'ASSEMBLER' vocabulary is pushed on the search order stack, and
     the input is processed by the text interpreter, (starting) in
     interpret state.

search order capability for 'EDITOR' and 'ASSEMBLER':
     The ANS Forth search order word set.

source and format of display by 'SEE':
     The source for 'see' is the executable code used by the inner
     interpreter.  The current 'see' tries to output Forth source code
     (and on some platforms, assembly code for primitives) as well as
     possible.

8.10.2 Ambiguous conditions
---------------------------

deleting the compilation word list ('FORGET'):
     Not implemented (yet).

fewer than u+1 items on the control-flow stack ('CS-PICK', 'CS-ROLL'):
     This typically results in an 'abort"' with a descriptive error
     message (may change into a '-22 throw' (Control structure mismatch)
     in the future).  You may also get a memory access error.  If you
     are unlucky, this ambiguous condition is not caught.

name can't be found ('FORGET'):
     Not implemented (yet).

name not defined via 'CREATE':
     ';CODE' behaves like 'DOES>' in this respect, i.e., it changes the
     execution semantics of the last defined word no matter how it was
     defined.

'POSTPONE' applied to '[IF]':
     After defining ': X POSTPONE [IF] ; IMMEDIATE'.  'X' is equivalent
     to '[IF]'.

reaching the end of the input source before matching '[ELSE]' or '[THEN]':
     Continue in the same state of conditional compilation in the next
     outer input source.  Currently there is no warning to the user
     about this.

removing a needed definition ('FORGET'):
     Not implemented (yet).

8.11 The optional Search-Order word set
=======================================

8.11.1 Implementation Defined Options
-------------------------------------

maximum number of word lists in search order:
     's" wordlists" environment? drop .'.  Currently 16.

minimum search order:
     'root root'.

8.11.2 Ambiguous conditions
---------------------------

changing the compilation word list (during compilation):
     The word is entered into the word list that was the compilation
     word list at the start of the definition.  Any changes to the name
     field (e.g., 'immediate') or the code field (e.g., when executing
     'DOES>') are applied to the latest defined word (as reported by
     'latest' or 'latestxt'), if possible, irrespective of the
     compilation word list.

search order empty ('previous'):
     'abort" Vocstack empty"'.

too many word lists in search order ('also'):
     'abort" Vocstack full"'.

9 Should I use Gforth extensions?
*********************************

As you read through the rest of this manual, you will see documentation
for Standard words, and documentation for some appealing Gforth
extensions.  You might ask yourself the question: "Should I restrict
myself to the standard, or should I use the extensions?"

   The answer depends on the goals you have for the program you are
working on:

   * Is it just for yourself or do you want to share it with others?

   * If you want to share it, do the others all use Gforth?

   * If it is just for yourself, do you want to restrict yourself to
     Gforth?

   If restricting the program to Gforth is ok, then there is no reason
not to use extensions.  It is still a good idea to keep to the standard
where it is easy, in case you want to reuse these parts in another
program that you want to be portable.

   If you want to be able to port the program to other Forth systems,
there are the following points to consider:

   * Most Forth systems that are being maintained support the ANS Forth
     standard.  So if your program complies with the standard, it will
     be portable among many systems.

   * A number of the Gforth extensions can be implemented in ANS Forth
     using public-domain files provided in the 'compat/' directory.
     These are mentioned in the text in passing.  There is no reason not
     to use these extensions, your program will still be ANS Forth
     compliant; just include the appropriate compat files with your
     program.

   * The tool 'ans-report.fs' (*note ANS Report::) makes it easy to
     analyse your program and determine what non-Standard words it
     relies upon.  However, it does not check whether you use standard
     words in a non-standard way.

   * Some techniques are not standardized by ANS Forth, and are hard or
     impossible to implement in a standard way, but can be implemented
     in most Forth systems easily, and usually in similar ways (e.g.,
     accessing word headers).  Forth has a rich historical precedent for
     programmers taking advantage of implementation-dependent features
     of their tools (for example, relying on a knowledge of the
     dictionary structure).  Sometimes these techniques are necessary to
     extract every last bit of performance from the hardware, sometimes
     they are just a programming shorthand.

   * Does using a Gforth extension save more work than the porting this
     part to other Forth systems (if any) will cost?

   * Is the additional functionality worth the reduction in portability
     and the additional porting problems?

   In order to perform these consideratios, you need to know what's
standard and what's not.  This manual generally states if something is
non-standard, but the authoritative source is the standard document
(http://www.taygeta.com/forth/dpans.html).  Appendix A of the Standard
(RATIONALE) provides a valuable insight into the thought processes of
the technical committee.

   Note also that portability between Forth systems is not the only
portability issue; there is also the issue of portability between
different platforms (processor/OS combinations).

10 Model
********

This chapter has yet to be written.  It will contain information, on
which internal structures you can rely.

11 Integrating Gforth into C programs
*************************************

This is not yet implemented.

   Several people like to use Forth as scripting language for
applications that are otherwise written in C, C++, or some other
language.

   The Forth system ATLAST provides facilities for embedding it into
applications; unfortunately it has several disadvantages: most
importantly, it is not based on ANS Forth, and it is apparently dead
(i.e., not developed further and not supported).  The facilities
provided by Gforth in this area are inspired by ATLAST's facilities, so
making the switch should not be hard.

   We also tried to design the interface such that it can easily be
implemented by other Forth systems, so that we may one day arrive at a
standardized interface.  Such a standard interface would allow you to
replace the Forth system without having to rewrite C code.

   You embed the Gforth interpreter by linking with the library
'libgforth.a' (give the compiler the option '-lgforth').  All global
symbols in this library that belong to the interface, have the prefix
'forth_'.  (Global symbols that are used internally have the prefix
'gforth_').

   You can include the declarations of Forth types and the functions and
variables of the interface with '#include <forth.h>'.

   Types.

   Variables.

   Data and FP Stack pointer.  Area sizes.

   functions.

   forth_init(imagefile) forth_evaluate(string) exceptions?
forth_goto(address) (or forth_execute(xt)?)  forth_continue() (a
corountining mechanism)

   Adding primitives.

   No checking.

   Signals?

   Accessing the Stacks

12 Emacs and Gforth
*******************

Gforth comes with 'gforth.el', an improved version of 'forth.el' by
Goran Rydqvist (included in the TILE package).  The improvements are:

   * A better handling of indentation.
   * A custom hilighting engine for Forth-code.
   * Comment paragraph filling ('M-q')
   * Commenting ('C-x \') and uncommenting ('C-u C-x \') of regions
   * Removal of debugging tracers ('C-x ~', *note Debugging::).
   * Support of the 'info-lookup' feature for looking up the
     documentation of a word.
   * Support for reading and writing blocks files.

   To get a basic description of these features, enter Forth mode and
type 'C-h m'.

   In addition, Gforth supports Emacs quite well: The source code
locations given in error messages, debugging output (from '~~') and
failed assertion messages are in the right format for Emacs' compilation
mode (*note Running Compilations under Emacs: (emacs)Compilation.) so
the source location corresponding to an error or other message is only a
few keystrokes away ('C-x `' for the next error, 'C-c C-c' for the error
under the cursor).

   Moreover, for words documented in this manual, you can look up the
glossary entry quickly by using 'C-h TAB' ('info-lookup-symbol', *note
Documentation Commands: (emacs)Documentation.).  This feature requires
Emacs 20.3 or later and does not work for words containing ':'.

12.1 Installing gforth.el
=========================

To make the features from 'gforth.el' available in Emacs, add the
following lines to your '.emacs' file:

     (autoload 'forth-mode "gforth.el")
     (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode)
     			    auto-mode-alist))
     (autoload 'forth-block-mode "gforth.el")
     (setq auto-mode-alist (cons '("\\.fb\\'" . forth-block-mode)
     			    auto-mode-alist))
     (add-hook 'forth-mode-hook (function (lambda ()
        ;; customize variables here:
        (setq forth-indent-level 4)
        (setq forth-minor-indent-level 2)
        (setq forth-hilight-level 3)
        ;;; ...
     )))

12.2 Emacs Tags
===============

If you 'require' 'etags.fs', a new 'TAGS' file will be produced (*note
Tags Tables: (emacs)Tags.) that contains the definitions of all words
defined afterwards.  You can then find the source for a word using
'M-.'.  Note that Emacs can use several tags files at the same time
(e.g., one for the Gforth sources and one for your program, *note
Selecting a Tags Table: (emacs)Select Tags Table.).  The TAGS file for
the preloaded words is '$(datadir)/gforth/$(VERSION)/TAGS' (e.g.,
'/usr/local/share/gforth/0.2.0/TAGS').  To get the best behaviour with
'etags.fs', you should avoid putting definitions both before and after
'require' etc., otherwise you will see the same file visited several
times by commands like 'tags-search'.

12.3 Hilighting
===============

'gforth.el' comes with a custom source hilighting engine.  When you open
a file in 'forth-mode', it will be completely parsed, assigning faces to
keywords, comments, strings etc.  While you edit the file, modified
regions get parsed and updated on-the-fly.

   Use the variable 'forth-hilight-level' to change the level of
decoration from 0 (no hilighting at all) to 3 (the default).  Even if
you set the hilighting level to 0, the parser will still work in the
background, collecting information about whether regions of text are
"compiled" or "interpreted".  Those information are required for
auto-indentation to work properly.  Set 'forth-disable-parser' to
non-nil if your computer is too slow to handle parsing.  This will have
an impact on the smartness of the auto-indentation engine, though.

   Sometimes Forth sources define new features that should be hilighted,
new control structures, defining-words etc.  You can use the variable
'forth-custom-words' to make 'forth-mode' hilight additional words and
constructs.  See the docstring of 'forth-words' for details (in Emacs,
type 'C-h v forth-words').

   'forth-custom-words' is meant to be customized in your '.emacs' file.
To customize hilighing in a file-specific manner, set
'forth-local-words' in a local-variables section at the end of your
source file (*note Variables: (emacs)Local Variables in Files.).

   Example:
     0 [IF]
        Local Variables:
        forth-local-words:
           ((("t:") definition-starter (font-lock-keyword-face . 1)
             "[ \t\n]" t name (font-lock-function-name-face . 3))
            ((";t") definition-ender (font-lock-keyword-face . 1)))
        End:
     [THEN]

12.4 Auto-Indentation
=====================

'forth-mode' automatically tries to indent lines in a smart way,
whenever you type <TAB> or break a line with 'C-m'.

   Simple customization can be achieved by setting 'forth-indent-level'
and 'forth-minor-indent-level' in your '.emacs' file.  For historical
reasons 'gforth.el' indents per default by multiples of 4 columns.  To
use the more traditional 3-column indentation, add the following lines
to your '.emacs':

     (add-hook 'forth-mode-hook (function (lambda ()
        ;; customize variables here:
        (setq forth-indent-level 3)
        (setq forth-minor-indent-level 1)
     )))

   If you want indentation to recognize non-default words, customize it
by setting 'forth-custom-indent-words' in your '.emacs'.  See the
docstring of 'forth-indent-words' for details (in Emacs, type 'C-h v
forth-indent-words').

   To customize indentation in a file-specific manner, set
'forth-local-indent-words' in a local-variables section at the end of
your source file (*note Variables: (emacs)Local Variables in Files.).

   Example:
     0 [IF]
        Local Variables:
        forth-local-indent-words:
           ((("t:") (0 . 2) (0 . 2))
            ((";t") (-2 . 0) (0 . -2)))
        End:
     [THEN]

12.5 Blocks Files
=================

'forth-mode' Autodetects blocks files by checking whether the length of
the first line exceeds 1023 characters.  It then tries to convert the
file into normal text format.  When you save the file, it will be
written to disk as normal stream-source file.

   If you want to write blocks files, use 'forth-blocks-mode'.  It
inherits all the features from 'forth-mode', plus some additions:

   * Files are written to disk in blocks file format.
   * Screen numbers are displayed in the mode line (enumerated beginning
     with the value of 'forth-block-base')
   * Warnings are displayed when lines exceed 64 characters.
   * The beginning of the currently edited block is marked with an
     overlay-arrow.

   There are some restrictions you should be aware of.  When you open a
blocks file that contains tabulator or newline characters, these
characters will be translated into spaces when the file is written back
to disk.  If tabs or newlines are encountered during blocks file
reading, an error is output to the echo area.  So have a look at the
'*Messages*' buffer, when Emacs' bell rings during reading.

   Please consult the docstring of 'forth-blocks-mode' for more
information by typing 'C-h v forth-blocks-mode').

13 Image Files
**************

An image file is a file containing an image of the Forth dictionary,
i.e., compiled Forth code and data residing in the dictionary.  By
convention, we use the extension '.fi' for image files.

13.1 Image Licensing Issues
===========================

An image created with 'gforthmi' (*note gforthmi::) or 'savesystem'
(*note Non-Relocatable Image Files::) includes the original image; i.e.,
according to copyright law it is a derived work of the original image.

   Since Gforth is distributed under the GNU GPL, the newly created
image falls under the GNU GPL, too.  In particular, this means that if
you distribute the image, you have to make all of the sources for the
image available, including those you wrote.  For details see *note GNU
General Public License (Section 3): Copying.

   If you create an image with 'cross' (*note cross.fs::), the image
contains only code compiled from the sources you gave it; if none of
these sources is under the GPL, the terms discussed above do not apply
to the image.  However, if your image needs an engine (a gforth binary)
that is under the GPL, you should make sure that you distribute both in
a way that is at most a _mere aggregation_, if you don't want the terms
of the GPL to apply to the image.

13.2 Image File Background
==========================

Gforth consists not only of primitives (in the engine), but also of
definitions written in Forth.  Since the Forth compiler itself belongs
to those definitions, it is not possible to start the system with the
engine and the Forth source alone.  Therefore we provide the Forth code
as an image file in nearly executable form.  When Gforth starts up, a C
routine loads the image file into memory, optionally relocates the
addresses, then sets up the memory (stacks etc.)  according to
information in the image file, and (finally) starts executing Forth
code.

   The image file variants represent different compromises between the
goals of making it easy to generate image files and making them
portable.

   Win32Forth 3.4 and Mitch Bradley's 'cforth' use relocation at
run-time.  This avoids many of the complications discussed below (image
files are data relocatable without further ado), but costs performance
(one addition per memory access).

   By contrast, the Gforth loader performs relocation at image load
time.  The loader also has to replace tokens that represent primitive
calls with the appropriate code-field addresses (or code addresses in
the case of direct threading).

   There are three kinds of image files, with different degrees of
relocatability: non-relocatable, data-relocatable, and fully relocatable
image files.

   These image file variants have several restrictions in common; they
are caused by the design of the image file loader:

   * There is only one segment; in particular, this means, that an image
     file cannot represent 'ALLOCATE'd memory chunks (and pointers to
     them).  The contents of the stacks are not represented, either.

   * The only kinds of relocation supported are: adding the same offset
     to all cells that represent data addresses; and replacing special
     tokens with code addresses or with pieces of machine code.

     If any complex computations involving addresses are performed, the
     results cannot be represented in the image file.  Several
     applications that use such computations come to mind:
        - Hashing addresses (or data structures which contain addresses)
          for table lookup.  If you use Gforth's 'table's or 'wordlist's
          for this purpose, you will have no problem, because the hash
          tables are recomputed automatically when the system is
          started.  If you use your own hash tables, you will have to do
          something similar.

        - There's a cute implementation of doubly-linked lists that uses
          'XOR'ed addresses.  You could represent such lists as
          singly-linked in the image file, and restore the doubly-linked
          representation on startup.(1)

        - The code addresses of run-time routines like 'docol:' cannot
          be represented in the image file (because their tokens would
          be replaced by machine code in direct threaded
          implementations).  As a workaround, compute these addresses at
          run-time with '>code-address' from the executions tokens of
          appropriate words (see the definitions of 'docol:' and friends
          in 'kernel/getdoers.fs').

        - On many architectures addresses are represented in machine
          code in some shifted or mangled form.  You cannot put 'CODE'
          words that contain absolute addresses in this form in a
          relocatable image file.  Workarounds are representing the
          address in some relative form (e.g., relative to the CFA,
          which is present in some register), or loading the address
          from a place where it is stored in a non-mangled form.

   ---------- Footnotes ----------

   (1) In my opinion, though, you should think thrice before using a
doubly-linked list (whatever implementation).

13.3 Non-Relocatable Image Files
================================

These files are simple memory dumps of the dictionary.  They are
specific to the executable (i.e., 'gforth' file) they were created with.
What's worse, they are specific to the place on which the dictionary
resided when the image was created.  Now, there is no guarantee that the
dictionary will reside at the same place the next time you start Gforth,
so there's no guarantee that a non-relocatable image will work the next
time (Gforth will complain instead of crashing, though).

   You can create a non-relocatable image file with

'savesystem'       "name" -         gforth       "savesystem"

13.4 Data-Relocatable Image Files
=================================

These files contain relocatable data addresses, but fixed code addresses
(instead of tokens).  They are specific to the executable (i.e.,
'gforth' file) they were created with.  For direct threading on some
architectures (e.g., the i386), data-relocatable images do not work.
You get a data-relocatable image, if you use 'gforthmi' with a Gforth
binary that is not doubly indirect threaded (*note Fully Relocatable
Image Files::).

13.5 Fully Relocatable Image Files
==================================

These image files have relocatable data addresses, and tokens for code
addresses.  They can be used with different binaries (e.g., with and
without debugging) on the same machine, and even across machines with
the same data formats (byte order, cell size, floating point format).
However, they are usually specific to the version of Gforth they were
created with.  The files 'gforth.fi' and 'kernl*.fi' are fully
relocatable.

   There are two ways to create a fully relocatable image file:

13.5.1 'gforthmi'
-----------------

You will usually use 'gforthmi'.  If you want to create an image file
that contains everything you would load by invoking Gforth with 'gforth
options', you simply say:
     gforthmi file options

   E.g., if you want to create an image 'asm.fi' that has the file
'asm.fs' loaded in addition to the usual stuff, you could do it like
this:

     gforthmi asm.fi asm.fs

   'gforthmi' is implemented as a sh script and works like this: It
produces two non-relocatable images for different addresses and then
compares them.  Its output reflects this: first you see the output (if
any) of the two Gforth invocations that produce the non-relocatable
image files, then you see the output of the comparing program: It
displays the offset used for data addresses and the offset used for code
addresses; moreover, for each cell that cannot be represented correctly
in the image files, it displays a line like this:

          78DC         BFFFFA50         BFFFFA40

   This means that at offset $78dc from 'forthstart', one input image
contains $bffffa50, and the other contains $bffffa40.  Since these cells
cannot be represented correctly in the output image, you should examine
these places in the dictionary and verify that these cells are dead
(i.e., not read before they are written).

   If you insert the option '--application' in front of the image file
name, you will get an image that uses the '--appl-image' option instead
of the '--image-file' option (*note Invoking Gforth::).  When you
execute such an image on Unix (by typing the image name as command), the
Gforth engine will pass all options to the image instead of trying to
interpret them as engine options.

   If you type 'gforthmi' with no arguments, it prints some usage
instructions.

   There are a few wrinkles: After processing the passed options, the
words 'savesystem' and 'bye' must be visible.  A special doubly indirect
threaded version of the 'gforth' executable is used for creating the
non-relocatable images; you can pass the exact filename of this
executable through the environment variable 'GFORTHD' (default:
'gforth-ditc'); if you pass a version that is not doubly indirect
threaded, you will not get a fully relocatable image, but a
data-relocatable image (because there is no code address offset).  The
normal 'gforth' executable is used for creating the relocatable image;
you can pass the exact filename of this executable through the
environment variable 'GFORTH'.

13.5.2 'cross.fs'
-----------------

You can also use 'cross', a batch compiler that accepts a Forth-like
programming language (*note Cross Compiler::).

   'cross' allows you to create image files for machines with different
data sizes and data formats than the one used for generating the image
file.  You can also use it to create an application image that does not
contain a Forth compiler.  These features are bought with restrictions
and inconveniences in programming.  E.g., addresses have to be stored in
memory with special words ('A!', 'A,', etc.)  in order to make the code
relocatable.

13.6 Stack and Dictionary Sizes
===============================

If you invoke Gforth with a command line flag for the size (*note
Invoking Gforth::), the size you specify is stored in the dictionary.
If you save the dictionary with 'savesystem' or create an image with
'gforthmi', this size will become the default for the resulting image
file.  E.g., the following will create a fully relocatable version of
'gforth.fi' with a 1MB dictionary:

     gforthmi gforth.fi -m 1M

   In other words, if you want to set the default size for the
dictionary and the stacks of an image, just invoke 'gforthmi' with the
appropriate options when creating the image.

   Note: For cache-friendly behaviour (i.e., good performance), you
should make the sizes of the stacks modulo, say, 2K, somewhat different.
E.g., the default stack sizes are: data: 16k (mod 2k=0); fp: 15.5k (mod
2k=1.5k); return: 15k(mod 2k=1k); locals: 14.5k (mod 2k=0.5k).

13.7 Running Image Files
========================

You can invoke Gforth with an image file image instead of the default
'gforth.fi' with the '-i' flag (*note Invoking Gforth::):
     gforth -i image

   If your operating system supports starting scripts with a line of the
form '#! ...', you just have to type the image file name to start Gforth
with this image file (note that the file extension '.fi' is just a
convention).  I.e., to run Gforth with the image file image, you can
just type image instead of 'gforth -i image'.  This works because every
'.fi' file starts with a line of this format:

     #! /usr/local/bin/gforth-0.4.0 -i

   The file and pathname for the Gforth engine specified on this line is
the specific Gforth executable that it was built against; i.e.  the
value of the environment variable 'GFORTH' at the time that 'gforthmi'
was executed.

   You can make use of the same shell capability to make a Forth source
file into an executable.  For example, if you place this text in a file:

     #! /usr/local/bin/gforth

     ." Hello, world" CR
     bye

and then make the file executable (chmod +x in Unix), you can run it
directly from the command line.  The sequence '#!' is used in two ways;
firstly, it is recognised as a "magic sequence" by the operating
system(1) secondly it is treated as a comment character by Gforth.
Because of the second usage, a space is required between '#!' and the
path to the executable (moreover, some Unixes require the sequence '#!
/').

   The disadvantage of this latter technique, compared with using
'gforthmi', is that it is slightly slower; the Forth source code is
compiled on-the-fly, each time the program is invoked.

'#!'       -         gforth       "hash-bang"
   An alias for '\'

   ---------- Footnotes ----------

   (1) The Unix kernel actually recognises two types of files:
executable files and files of data, where the data is processed by an
interpreter that is specified on the "interpreter line" - the first line
of the file, starting with the sequence #!.  There may be a small limit
(e.g., 32) on the number of characters that may be specified on the
interpreter line.

13.8 Modifying the Startup Sequence
===================================

You can add your own initialization to the startup sequence of an image
through the deferred word ''cold'.  ''cold' is invoked just before the
image-specific command line processing (i.e., loading files and
evaluating ('-e') strings) starts.

   A sequence for adding your initialization usually looks like this:

     :noname
         Defers 'cold \ do other initialization stuff (e.g., rehashing wordlists)
         ... \ your stuff
     ; IS 'cold

   After ''cold', Gforth processes the image options (*note Invoking
Gforth::), and then it performs 'bootmessage', another deferred word.
This normally prints Gforth's startup message and does nothing else.

   So, if you want to make a turnkey image (i.e., an image for an
application instead of an extended Forth system), you can do this in two
ways:

   * If you want to do your interpretation of the OS command-line
     arguments, hook into ''cold'.  In that case you probably also want
     to build the image with 'gforthmi --application' (*note gforthmi::)
     to keep the engine from processing OS command line options.  You
     can then do your own command-line processing with 'next-arg'

   * If you want to have the normal Gforth processing of OS command-line
     arguments, hook into 'bootmessage'.

   In either case, you probably do not want the word that you execute in
these hooks to exit normally, but use 'bye' or 'throw'.  Otherwise the
Gforth startup process would continue and eventually present the Forth
command line to the user.

''cold'       -         gforth       "tick-cold"
   Hook (deferred word) for things to do right before interpreting the
OS command-line arguments.  Normally does some initializations that you
also want to perform.

'bootmessage'       -         gforth       "bootmessage"
   Hook (deferred word) executed right after interpreting the OS
command-line arguments.  Normally prints the Gforth startup message.

14 Engine
*********

Reading this chapter is not necessary for programming with Gforth.  It
may be helpful for finding your way in the Gforth sources.

   The ideas in this section have also been published in the following
papers: Bernd Paysan, 'ANS fig/GNU/??? Forth' (in German), Forth-Tagung
'93; M. Anton Ertl, 'A Portable Forth Engine
(http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z)', EuroForth '93;
M. Anton Ertl, 'Threaded code variations and optimizations (extended
version) (http://www.complang.tuwien.ac.at/papers/ertl02.ps.gz)',
Forth-Tagung '02.

14.1 Portability
================

An important goal of the Gforth Project is availability across a wide
range of personal machines.  fig-Forth, and, to a lesser extent, F83,
achieved this goal by manually coding the engine in assembly language
for several then-popular processors.  This approach is very
labor-intensive and the results are short-lived due to progress in
computer architecture.

   Others have avoided this problem by coding in C, e.g., Mitch Bradley
(cforth), Mikael Patel (TILE) and Dirk Zoller (pfe).  This approach is
particularly popular for UNIX-based Forths due to the large variety of
architectures of UNIX machines.  Unfortunately an implementation in C
does not mix well with the goals of efficiency and with using
traditional techniques: Indirect or direct threading cannot be expressed
in C, and switch threading, the fastest technique available in C, is
significantly slower.  Another problem with C is that it is very
cumbersome to express double integer arithmetic.

   Fortunately, there is a portable language that does not have these
limitations: GNU C, the version of C processed by the GNU C compiler
(*note Extensions to the C Language Family: (gcc.info)C Extensions.).
Its labels as values feature (*note Labels as Values: (gcc.info)Labels
as Values.) makes direct and indirect threading possible, its 'long
long' type (*note Double-Word Integers: (gcc.info)Long Long.)
corresponds to Forth's double numbers on many systems.  GNU C is freely
available on all important (and many unimportant) UNIX machines, VMS,
80386s running MS-DOS, the Amiga, and the Atari ST, so a Forth written
in GNU C can run on all these machines.

   Writing in a portable language has the reputation of producing code
that is slower than assembly.  For our Forth engine we repeatedly looked
at the code produced by the compiler and eliminated most
compiler-induced inefficiencies by appropriate changes in the source
code.

   However, register allocation cannot be portably influenced by the
programmer, leading to some inefficiencies on register-starved machines.
We use explicit register declarations (*note Variables in Specified
Registers: (gcc.info)Explicit Reg Vars.) to improve the speed on some
machines.  They are turned on by using the configuration flag
'--enable-force-reg' ('gcc' switch '-DFORCE_REG').  Unfortunately, this
feature not only depends on the machine, but also on the compiler
version: On some machines some compiler versions produce incorrect code
when certain explicit register declarations are used.  So by default
'-DFORCE_REG' is not used.

14.2 Threading
==============

GNU C's labels as values extension (available since 'gcc-2.0', *note
Labels as Values: (gcc.info)Labels as Values.) makes it possible to take
the address of label by writing '&&label'.  This address can then be
used in a statement like 'goto *address'.  I.e., 'goto *&&x' is the same
as 'goto x'.

   With this feature an indirect threaded 'NEXT' looks like:
     cfa = *ip++;
     ca = *cfa;
     goto *ca;
   For those unfamiliar with the names: 'ip' is the Forth instruction
pointer; the 'cfa' (code-field address) corresponds to ANS Forths
execution token and points to the code field of the next word to be
executed; The 'ca' (code address) fetched from there points to some
executable code, e.g., a primitive or the colon definition handler
'docol'.

   Direct threading is even simpler:
     ca = *ip++;
     goto *ca;

   Of course we have packaged the whole thing neatly in macros called
'NEXT' and 'NEXT1' (the part of 'NEXT' after fetching the cfa).

14.2.1 Scheduling
-----------------

There is a little complication: Pipelined and superscalar processors,
i.e., RISC and some modern CISC machines can process independent
instructions while waiting for the results of an instruction.  The
compiler usually reorders (schedules) the instructions in a way that
achieves good usage of these delay slots.  However, on our first tries
the compiler did not do well on scheduling primitives.  E.g., for '+'
implemented as
     n=sp[0]+sp[1];
     sp++;
     sp[0]=n;
     NEXT;
   the 'NEXT' comes strictly after the other code, i.e., there is nearly
no scheduling.  After a little thought the problem becomes clear: The
compiler cannot know that 'sp' and 'ip' point to different addresses
(and the version of 'gcc' we used would not know it even if it was
possible), so it could not move the load of the cfa above the store to
the TOS. Indeed the pointers could be the same, if code on or very near
the top of stack were executed.  In the interest of speed we chose to
forbid this probably unused "feature" and helped the compiler in
scheduling: 'NEXT' is divided into several parts: 'NEXT_P0', 'NEXT_P1'
and 'NEXT_P2').  '+' now looks like:
     NEXT_P0;
     n=sp[0]+sp[1];
     sp++;
     NEXT_P1;
     sp[0]=n;
     NEXT_P2;

   There are various schemes that distribute the different operations of
NEXT between these parts in several ways; in general, different schemes
perform best on different processors.  We use a scheme for most
architectures that performs well for most processors of this
architecture; in the future we may switch to benchmarking and chosing
the scheme on installation time.

14.2.2 Direct or Indirect Threaded?
-----------------------------------

Threaded forth code consists of references to primitives (simple machine
code routines like '+') and to non-primitives (e.g., colon definitions,
variables, constants); for a specific class of non-primitives (e.g.,
variables) there is one code routine (e.g., 'dovar'), but each variable
needs a separate reference to its data.

   Traditionally Forth has been implemented as indirect threaded code,
because this allows to use only one cell to reference a non-primitive
(basically you point to the data, and find the code address there).

   However, threaded code in Gforth (since 0.6.0) uses two cells for
non-primitives, one for the code address, and one for the data address;
the data pointer is an immediate argument for the virtual machine
instruction represented by the code address.  We call this
_primitive-centric_ threaded code, because all code addresses point to
simple primitives.  E.g., for a variable, the code address is for 'lit'
(also used for integer literals like '99').

   Primitive-centric threaded code allows us to use (faster) direct
threading as dispatch method, completely portably (direct threaded code
in Gforth before 0.6.0 required architecture-specific code).  It also
eliminates the performance problems related to I-cache consistency that
386 implementations have with direct threaded code, and allows
additional optimizations.

   There is a catch, however: the XT parameter of 'execute' can occupy
only one cell, so how do we pass non-primitives with their code _and_
data addresses to them?  Our answer is to use indirect threaded dispatch
for 'execute' and other words that use a single-cell xt.  So, normal
threaded code in colon definitions uses direct threading, and 'execute'
and similar words, which dispatch to xts on the data stack, use indirect
threaded code.  We call this _hybrid direct/indirect_ threaded code.

   The engines 'gforth' and 'gforth-fast' use hybrid direct/indirect
threaded code.  This means that with these engines you cannot use ',' to
compile an xt.  Instead, you have to use 'compile,'.

   If you want to compile xts with ',', use 'gforth-itc'.  This engine
uses plain old indirect threaded code.  It still compiles in a
primitive-centric style, so you cannot use 'compile,' instead of ','
(e.g., for producing tables of xts with '] word1 word2 ... [').  If you
want to do that, you have to use 'gforth-itc' and execute '' , is
compile,'.  Your program can check if it is running on a hybrid
direct/indirect threaded engine or a pure indirect threaded engine with
'threading-method' (*note Threading Words::).

14.2.3 Dynamic Superinstructions
--------------------------------

The engines 'gforth' and 'gforth-fast' use another optimization: Dynamic
superinstructions with replication.  As an example, consider the
following colon definition:

     : squared ( n1 -- n2 )
       dup * ;

   Gforth compiles this into the threaded code sequence

     dup
     *
     ;s

   In normal direct threaded code there is a code address occupying one
cell for each of these primitives.  Each code address points to a
machine code routine, and the interpreter jumps to this machine code in
order to execute the primitive.  The routines for these three primitives
are (in 'gforth-fast' on the 386):

     Code dup
     ( $804B950 )  add     esi , # -4  \ $83 $C6 $FC
     ( $804B953 )  add     ebx , # 4  \ $83 $C3 $4
     ( $804B956 )  mov     dword ptr 4 [esi] , ecx  \ $89 $4E $4
     ( $804B959 )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC
     end-code
     Code *
     ( $804ACC4 )  mov     eax , dword ptr 4 [esi]  \ $8B $46 $4
     ( $804ACC7 )  add     esi , # 4  \ $83 $C6 $4
     ( $804ACCA )  add     ebx , # 4  \ $83 $C3 $4
     ( $804ACCD )  imul    ecx , eax  \ $F $AF $C8
     ( $804ACD0 )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC
     end-code
     Code ;s
     ( $804A693 )  mov     eax , dword ptr [edi]  \ $8B $7
     ( $804A695 )  add     edi , # 4  \ $83 $C7 $4
     ( $804A698 )  lea     ebx , dword ptr 4 [eax]  \ $8D $58 $4
     ( $804A69B )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC
     end-code

   With dynamic superinstructions and replication the compiler does not
just lay down the threaded code, but also copies the machine code
fragments, usually without the jump at the end.

     ( $4057D27D )  add     esi , # -4  \ $83 $C6 $FC
     ( $4057D280 )  add     ebx , # 4  \ $83 $C3 $4
     ( $4057D283 )  mov     dword ptr 4 [esi] , ecx  \ $89 $4E $4
     ( $4057D286 )  mov     eax , dword ptr 4 [esi]  \ $8B $46 $4
     ( $4057D289 )  add     esi , # 4  \ $83 $C6 $4
     ( $4057D28C )  add     ebx , # 4  \ $83 $C3 $4
     ( $4057D28F )  imul    ecx , eax  \ $F $AF $C8
     ( $4057D292 )  mov     eax , dword ptr [edi]  \ $8B $7
     ( $4057D294 )  add     edi , # 4  \ $83 $C7 $4
     ( $4057D297 )  lea     ebx , dword ptr 4 [eax]  \ $8D $58 $4
     ( $4057D29A )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC

   Only when a threaded-code control-flow change happens (e.g., in
';s'), the jump is appended.  This optimization eliminates many of these
jumps and makes the rest much more predictable.  The speedup depends on
the processor and the application; on the Athlon and Pentium III this
optimization typically produces a speedup by a factor of 2.

   The code addresses in the direct-threaded code are set to point to
the appropriate points in the copied machine code, in this example like
this:

     primitive  code address
        dup       $4057D27D
        *         $4057D286
        ;s        $4057D292

   Thus there can be threaded-code jumps to any place in this piece of
code.  This also simplifies decompilation quite a bit.

   You can disable this optimization with '--no-dynamic'.  You can use
the copying without eliminating the jumps (i.e., dynamic replication,
but without superinstructions) with '--no-super'; this gives the branch
prediction benefit alone; the effect on performance depends on the CPU;
on the Athlon and Pentium III the speedup is a little less than for
dynamic superinstructions with replication.

   One use of these options is if you want to patch the threaded code.
With superinstructions, many of the dispatch jumps are eliminated, so
patching often has no effect.  These options preserve all the dispatch
jumps.

   On some machines dynamic superinstructions are disabled by default,
because it is unsafe on these machines.  However, if you feel
adventurous, you can enable it with '--dynamic'.

14.2.4 DOES>
------------

One of the most complex parts of a Forth engine is 'dodoes', i.e., the
chunk of code executed by every word defined by a 'CREATE'...'DOES>'
pair; actually with primitive-centric code, this is only needed if the
xt of the word is 'execute'd.  The main problem here is: How to find the
Forth code to be executed, i.e.  the code after the 'DOES>' (the
'DOES>'-code)?  There are two solutions:

   In fig-Forth the code field points directly to the 'dodoes' and the
'DOES>'-code address is stored in the cell after the code address (i.e.
at 'CFA cell+').  It may seem that this solution is illegal in the
Forth-79 and all later standards, because in fig-Forth this address lies
in the body (which is illegal in these standards).  However, by making
the code field larger for all words this solution becomes legal again.
We use this approach.  Leaving a cell unused in most words is a bit
wasteful, but on the machines we are targeting this is hardly a problem.

14.3 Primitives
===============

14.3.1 Automatic Generation
---------------------------

Since the primitives are implemented in a portable language, there is no
longer any need to minimize the number of primitives.  On the contrary,
having many primitives has an advantage: speed.  In order to reduce the
number of errors in primitives and to make programming them easier, we
provide a tool, the primitive generator ('prims2x.fs' aka Vmgen, *note
Vmgen: (vmgen)Top.), that automatically generates most (and sometimes
all) of the C code for a primitive from the stack effect notation.  The
source for a primitive has the following form:

Forth-name  ( stack-effect )        category    [pronounc.]
['""'glossary entry'""']
C code
[':'
Forth code]

   The items in brackets are optional.  The category and glossary fields
are there for generating the documentation, the Forth code is there for
manual implementations on machines without GNU C. E.g., the source for
the primitive '+' is:
     +    ( n1 n2 -- n )   core    plus
     n = n1+n2;

   This looks like a specification, but in fact 'n = n1+n2' is C code.
Our primitive generation tool extracts a lot of information from the
stack effect notations(1): The number of items popped from and pushed on
the stack, their type, and by what name they are referred to in the C
code.  It then generates a C code prelude and postlude for each
primitive.  The final C code for '+' looks like this:

     I_plus: /* + ( n1 n2 -- n ) */  /* label, stack effect */
     /*  */                          /* documentation */
     NAME("+")                       /* debugging output (with -DDEBUG) */
     {
     DEF_CA                          /* definition of variable ca (indirect threading) */
     Cell n1;                        /* definitions of variables */
     Cell n2;
     Cell n;
     NEXT_P0;                        /* NEXT part 0 */
     n1 = (Cell) sp[1];              /* input */
     n2 = (Cell) TOS;
     sp += 1;                        /* stack adjustment */
     {
     n = n1+n2;                      /* C code taken from the source */
     }
     NEXT_P1;                        /* NEXT part 1 */
     TOS = (Cell)n;                  /* output */
     NEXT_P2;                        /* NEXT part 2 */
     }

   This looks long and inefficient, but the GNU C compiler optimizes
quite well and produces optimal code for '+' on, e.g., the R3000 and the
HP RISC machines: Defining the 'n's does not produce any code, and using
them as intermediate storage also adds no cost.

   There are also other optimizations that are not illustrated by this
example: assignments between simple variables are usually for free (copy
propagation).  If one of the stack items is not used by the primitive
(e.g.  in 'drop'), the compiler eliminates the load from the stack (dead
code elimination).  On the other hand, there are some things that the
compiler does not do, therefore they are performed by 'prims2x.fs': The
compiler does not optimize code away that stores a stack item to the
place where it just came from (e.g., 'over').

   While programming a primitive is usually easy, there are a few cases
where the programmer has to take the actions of the generator into
account, most notably '?dup', but also words that do not (always) fall
through to 'NEXT'.

   For more information

   ---------- Footnotes ----------

   (1) We use a one-stack notation, even though we have separate data
and floating-point stacks; The separate notation can be generated easily
from the unified notation.

14.3.2 TOS Optimization
-----------------------

An important optimization for stack machine emulators, e.g., Forth
engines, is keeping one or more of the top stack items in registers.  If
a word has the stack effect in1...inx '--' out1...outy, keeping the top
n items in registers
   * is better than keeping n-1 items, if x>=n and y>=n, due to fewer
     loads from and stores to the stack.
   * is slower than keeping n-1 items, if x<>y and x<n and y<n, due to
     additional moves between registers.

   In particular, keeping one item in a register is never a
disadvantage, if there are enough registers.  Keeping two items in
registers is a disadvantage for frequent words like '?branch',
constants, variables, literals and 'i'.  Therefore our generator only
produces code that keeps zero or one items in registers.  The generated
C code covers both cases; the selection between these alternatives is
made at C-compile time using the switch '-DUSE_TOS'.  'TOS' in the C
code for '+' is just a simple variable name in the one-item case,
otherwise it is a macro that expands into 'sp[0]'.  Note that the GNU C
compiler tries to keep simple variables like 'TOS' in registers, and it
usually succeeds, if there are enough registers.

   The primitive generator performs the TOS optimization for the
floating-point stack, too ('-DUSE_FTOS').  For floating-point operations
the benefit of this optimization is even larger: floating-point
operations take quite long on most processors, but can be performed in
parallel with other operations as long as their results are not used.
If the FP-TOS is kept in a register, this works.  If it is kept on the
stack, i.e., in memory, the store into memory has to wait for the result
of the floating-point operation, lengthening the execution time of the
primitive considerably.

   The TOS optimization makes the automatic generation of primitives a
bit more complicated.  Just replacing all occurrences of 'sp[0]' by
'TOS' is not sufficient.  There are some special cases to consider:
   * In the case of 'dup ( w -- w w )' the generator must not eliminate
     the store to the original location of the item on the stack, if the
     TOS optimization is turned on.
   * Primitives with stack effects of the form '--' out1...outy must
     store the TOS to the stack at the start.  Likewise, primitives with
     the stack effect in1...inx '--' must load the TOS from the stack at
     the end.  But for the null stack effect '--' no stores or loads
     should be generated.

14.3.3 Produced code
--------------------

To see what assembly code is produced for the primitives on your machine
with your compiler and your flag settings, type 'make engine.s' and look
at the resulting file 'engine.s'.  Alternatively, you can also
disassemble the code of primitives with 'see' on some architectures.

14.4 Performance
================

On RISCs the Gforth engine is very close to optimal; i.e., it is usually
impossible to write a significantly faster threaded-code engine.

   On register-starved machines like the 386 architecture processors
improvements are possible, because 'gcc' does not utilize the registers
as well as a human, even with explicit register declarations; e.g.,
Bernd Beuster wrote a Forth system fragment in assembly language and
hand-tuned it for the 486; this system is 1.19 times faster on the Sieve
benchmark on a 486DX2/66 than Gforth compiled with 'gcc-2.6.3' with
'-DFORCE_REG'.  The situation has improved with gcc-2.95 and
gforth-0.4.9; now the most important virtual machine registers fit in
real registers (and we can even afford to use the TOS optimization),
resulting in a speedup of 1.14 on the sieve over the earlier results.
And dynamic superinstructions provide another speedup (but only around a
factor 1.2 on the 486).

   The potential advantage of assembly language implementations is not
necessarily realized in complete Forth systems: We compared Gforth-0.5.9
(direct threaded, compiled with 'gcc-2.95.1' and '-DFORCE_REG') with
Win32Forth 1.2093 (newer versions are reportedly much faster), LMI's NT
Forth (Beta, May 1994) and Eforth (with and without peephole (aka
pinhole) optimization of the threaded code); all these systems were
written in assembly language.  We also compared Gforth with three
systems written in C: PFE-0.9.14 (compiled with 'gcc-2.6.3' with the
default configuration for Linux: '-O2 -fomit-frame-pointer -DUSE_REGS
-DUNROLL_NEXT'), ThisForth Beta (compiled with 'gcc-2.6.3 -O3
-fomit-frame-pointer'; ThisForth employs peephole optimization of the
threaded code) and TILE (compiled with 'make opt').  We benchmarked
Gforth, PFE, ThisForth and TILE on a 486DX2/66 under Linux.  Kenneth
O'Heskin kindly provided the results for Win32Forth and NT Forth on a
486DX2/66 with similar memory performance under Windows NT. Marcel
Hendrix ported Eforth to Linux, then extended it to run the benchmarks,
added the peephole optimizer, ran the benchmarks and reported the
results.

   We used four small benchmarks: the ubiquitous Sieve; bubble-sorting
and matrix multiplication come from the Stanford integer benchmarks and
have been translated into Forth by Martin Fraeman; we used the versions
included in the TILE Forth package, but with bigger data set sizes; and
a recursive Fibonacci number computation for benchmarking calling
performance.  The following table shows the time taken for the
benchmarks scaled by the time taken by Gforth (in other words, it shows
the speedup factor that Gforth achieved over the other systems).

     relative       Win32-    NT       eforth       This-
     time     Gforth Forth Forth eforth  +opt   PFE Forth  TILE
     sieve      1.00  2.16  1.78   2.16  1.32  2.46  4.96 13.37
     bubble     1.00  1.93  2.07   2.18  1.29  2.21        5.70
     matmul     1.00  1.92  1.76   1.90  0.96  2.06        5.32
     fib        1.00  2.32  2.03   1.86  1.31  2.64  4.55  6.54

   You may be quite surprised by the good performance of Gforth when
compared with systems written in assembly language.  One important
reason for the disappointing performance of these other systems is
probably that they are not written optimally for the 486 (e.g., they use
the 'lods' instruction).  In addition, Win32Forth uses a comfortable,
but costly method for relocating the Forth image: like 'cforth', it
computes the actual addresses at run time, resulting in two address
computations per 'NEXT' (*note Image File Background::).

   The speedup of Gforth over PFE, ThisForth and TILE can be easily
explained with the self-imposed restriction of the latter systems to
standard C, which makes efficient threading impossible (however, the
measured implementation of PFE uses a GNU C extension: *note Defining
Global Register Variables: (gcc.info)Global Reg Vars.).  Moreover,
current C compilers have a hard time optimizing other aspects of the
ThisForth and the TILE source.

   The performance of Gforth on 386 architecture processors varies
widely with the version of 'gcc' used.  E.g., 'gcc-2.5.8' failed to
allocate any of the virtual machine registers into real machine
registers by itself and would not work correctly with explicit register
declarations, giving a significantly slower engine (on a 486DX2/66
running the Sieve) than the one measured above.

   Note that there have been several releases of Win32Forth since the
release presented here, so the results presented above may have little
predictive value for the performance of Win32Forth today (results for
the current release on an i486DX2/66 are welcome).

   In 'Translating Forth to Efficient C
(http://www.complang.tuwien.ac.at/papers/ertl&maierhofer95.ps.gz)' by M.
Anton Ertl and Martin Maierhofer (presented at EuroForth '95), an
indirect threaded version of Gforth is compared with Win32Forth, NT
Forth, PFE, ThisForth, and several native code systems; that version of
Gforth is slower on a 486 than the version used here.  You can find a
newer version of these measurements at
<http://www.complang.tuwien.ac.at/forth/performance.html>.  You can find
numbers for Gforth on various machines in 'Benchres'.

15 Cross Compiler
*****************

The cross compiler is used to bootstrap a Forth kernel.  Since Gforth is
mostly written in Forth, including crucial parts like the outer
interpreter and compiler, it needs compiled Forth code to get started.
The cross compiler allows to create new images for other architectures,
even running under another Forth system.

15.1 Using the Cross Compiler
=============================

The cross compiler uses a language that resembles Forth, but isn't.  The
main difference is that you can execute Forth code after definition,
while you usually can't execute the code compiled by cross, because the
code you are compiling is typically for a different computer than the
one you are compiling on.

   The Makefile is already set up to allow you to create kernels for new
architectures with a simple make command.  The generic kernels using the
GCC compiled virtual machine are created in the normal build process
with 'make'.  To create a embedded Gforth executable for e.g.  the 8086
processor (running on a DOS machine), type

     make kernl-8086.fi

   This will use the machine description from the 'arch/8086' directory
to create a new kernel.  A machine file may look like that:

     \ Parameter for target systems                         06oct92py

         4 Constant cell             \ cell size in bytes
         2 Constant cell<<           \ cell shift to bytes
         5 Constant cell>bit         \ cell shift to bits
         8 Constant bits/char        \ bits per character
         8 Constant bits/byte        \ bits per byte [default: 8]
         8 Constant float            \ bytes per float
         8 Constant /maxalign        \ maximum alignment in bytes
     false Constant bigendian        \ byte order
     ( true=big, false=little )

     include machpc.fs               \ feature list

   This part is obligatory for the cross compiler itself, the feature
list is used by the kernel to conditionally compile some features in and
out, depending on whether the target supports these features.

   There are some optional features, if you define your own primitives,
have an assembler, or need special, nonstandard preparation to make the
boot process work.  'asm-include' includes an assembler, 'prims-include'
includes primitives, and '>boot' prepares for booting.

     : asm-include    ." Include assembler" cr
       s" arch/8086/asm.fs" included ;

     : prims-include  ." Include primitives" cr
       s" arch/8086/prim.fs" included ;

     : >boot          ." Prepare booting" cr
       s" ' boot >body into-forth 1+ !" evaluate ;

   These words are used as sort of macro during the cross compilation in
the file 'kernel/main.fs'.  Instead of using these macros, it would be
possible -- but more complicated -- to write a new kernel project file,
too.

   'kernel/main.fs' expects the machine description file name on the
stack; the cross compiler itself ('cross.fs') assumes that either
'mach-file' leaves a counted string on the stack, or 'machine-file'
leaves an address, count pair of the filename on the stack.

   The feature list is typically controlled using 'SetValue', generic
files that are used by several projects can use 'DefaultValue' instead.
Both functions work like 'Value', when the value isn't defined, but
'SetValue' works like 'to' if the value is defined, and 'DefaultValue'
doesn't set anything, if the value is defined.

     \ generic mach file for pc gforth                       03sep97jaw

     true DefaultValue NIL  \ relocating

     >ENVIRON

     true DefaultValue file          \ controls the presence of the
                                     \ file access wordset
     true DefaultValue OS            \ flag to indicate a operating system

     true DefaultValue prims         \ true: primitives are c-code

     true DefaultValue floating      \ floating point wordset is present

     true DefaultValue glocals       \ gforth locals are present
                                     \ will be loaded
     true DefaultValue dcomps        \ double number comparisons

     true DefaultValue hash          \ hashing primitives are loaded/present

     true DefaultValue xconds        \ used together with glocals,
                                     \ special conditionals supporting gforths'
                                     \ local variables
     true DefaultValue header        \ save a header information

     true DefaultValue backtrace     \ enables backtrace code

     false DefaultValue ec
     false DefaultValue crlf

     cell 2 = [IF] &32 [ELSE] &256 [THEN] KB DefaultValue kernel-size

     &16 KB          DefaultValue stack-size
     &15 KB &512 +   DefaultValue fstack-size
     &15 KB          DefaultValue rstack-size
     &14 KB &512 +   DefaultValue lstack-size

15.2 How the Cross Compiler Works
=================================

Appendix A Bugs
***************

Known bugs are described in the file 'BUGS' in the Gforth distribution.

   If you find a bug, please submit a bug report through
<https://savannah.gnu.org/bugs/?func=addbug&group=gforth>.

   * A program (or a sequence of keyboard commands) that reproduces the
     bug.
   * A description of what you think constitutes the buggy behaviour.
   * The Gforth version used (it is announced at the start of an
     interactive Gforth session).
   * The machine and operating system (on Unix systems 'uname -a' will
     report this information).
   * The installation options (you can find the configure options at the
     start of 'config.status') and configuration ('configure' output or
     'config.cache').
   * A complete list of changes (if any) you (or your installer) have
     made to the Gforth sources.

   For a thorough guide on reporting bugs read *note How to Report Bugs:
(gcc.info)Bug Reporting.

Appendix B Authors and Ancestors of Gforth
******************************************

B.1 Authors and Contributors
============================

The Gforth project was started in mid-1992 by Bernd Paysan and Anton
Ertl.  The third major author was Jens Wilke.  Neal Crook contributed a
lot to the manual.  Assemblers and disassemblers were contributed by
Andrew McKewan, Christian Pirker, Bernd Thallner, and Michal Revucky.
Lennart Benschop (who was one of Gforth's first users, in mid-1993) and
Stuart Ramsden inspired us with their continuous feedback.  Lennart
Benshop contributed 'glosgen.fs', while Stuart Ramsden has been working
on automatic support for calling C libraries.  Helpful comments also
came from Paul Kleinrubatscher, Christian Pirker, Dirk Zoller, Marcel
Hendrix, John Wavrik, Barrie Stott, Marc de Groot, Jorge Acerada, Bruce
Hoyt, Robert Epprecht, Dennis Ruffer and David N. Williams.  Since the
release of Gforth-0.2.1 there were also helpful comments from many
others; thank you all, sorry for not listing you here (but digging
through my mailbox to extract your names is on my to-do list).

   Gforth also owes a lot to the authors of the tools we used (GCC, CVS,
and autoconf, among others), and to the creators of the Internet: Gforth
was developed across the Internet, and its authors did not meet
physically for the first 4 years of development.

B.2 Pedigree
============

Gforth descends from bigFORTH (1993) and fig-Forth.  Of course, a
significant part of the design of Gforth was prescribed by ANS Forth.

   Bernd Paysan wrote bigFORTH, a descendent from TurboForth, an
unreleased 32 bit native code version of VolksForth for the Atari ST,
written mostly by Dietrich Weineck.

   VolksForth was written by Klaus Schleisiek, Bernd Pennemann, Georg
Rehfeld and Dietrich Weineck for the C64 (called UltraForth there) in
the mid-80s and ported to the Atari ST in 1986.  It descends from
fig-Forth.

   A team led by Bill Ragsdale implemented fig-Forth on many processors
in 1979.  Robert Selzer and Bill Ragsdale developed the original
implementation of fig-Forth for the 6502 based on microForth.

   The principal architect of microForth was Dean Sanderson.  microForth
was FORTH, Inc.'s first off-the-shelf product.  It was developed in 1976
for the 1802, and subsequently implemented on the 8080, the 6800 and the
Z80.

   All earlier Forth systems were custom-made, usually by Charles Moore,
who discovered (as he puts it) Forth during the late 60s.  The first
full Forth existed in 1971.

   A part of the information in this section comes from 'The Evolution
of Forth (http://www.forth.com/Content/History/History1.htm)' by
Elizabeth D. Rather, Donald R. Colburn and Charles H. Moore, presented
at the HOPL-II conference and preprinted in SIGPLAN Notices 28(3), 1993.
You can find more historical and genealogical information about Forth
there.  For a more general (and graphical) Forth family tree look see
'<http://www.complang.tuwien.ac.at/forth/family-tree/>, Forth Family
Tree and Timeline'.

Appendix C Other Forth-related information
******************************************

There is an active news group (comp.lang.forth) discussing Forth
(including Gforth) and Forth-related issues.  Its FAQs
(http://www.complang.tuwien.ac.at/forth/faq/faq-general-2.html)
(frequently asked questions and their answers) contains a lot of
information on Forth.  You should read it before posting to
comp.lang.forth.

   The ANS Forth standard is most usable in its HTML form
(http://www.taygeta.com/forth/dpans.html).

Appendix D Licenses
*******************

D.1 GNU Free Documentation License
==================================

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307, USA

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  0. PREAMBLE

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     This License is a kind of "copyleft", which means that derivative
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     It complements the GNU General Public License, which is a copyleft
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     We have designed this License in order to use it for manuals for
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       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          "History" section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version's
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses terminated
     so long as such parties remain in full compliance.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.

D.1.1 ADDENDUM: How to use this License for your documents
----------------------------------------------------------

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts."  line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

D.2 GNU GENERAL PUBLIC LICENSE
==============================

                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.

Preamble
========

The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program--to make sure it remains free
software for all its users.  We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to
any other work released this way by its authors.  You can apply it to
your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.

   To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights.  Therefore, you have
certain responsibilities if you distribute copies of the software, or if
you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
freedoms that you received.  You must make sure that they, too, receive
or can get the source code.  And you must show them these terms so they
know their rights.

   Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.

   For the developers' and authors' protection, the GPL clearly explains
that there is no warranty for this free software.  For both users' and
authors' sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.

   Some devices are designed to deny users access to install or run
modified versions of the software inside them, although the manufacturer
can do so.  This is fundamentally incompatible with the aim of
protecting users' freedom to change the software.  The systematic
pattern of such abuse occurs in the area of products for individuals to
use, which is precisely where it is most unacceptable.  Therefore, we
have designed this version of the GPL to prohibit the practice for those
products.  If such problems arise substantially in other domains, we
stand ready to extend this provision to those domains in future versions
of the GPL, as needed to protect the freedom of users.

   Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
software on general-purpose computers, but in those that do, we wish to
avoid the special danger that patents applied to a free program could
make it effectively proprietary.  To prevent this, the GPL assures that
patents cannot be used to render the program non-free.

   The precise terms and conditions for copying, distribution and
modification follow.

TERMS AND CONDITIONS
====================

  0. Definitions.

     "This License" refers to version 3 of the GNU General Public
     License.

     "Copyright" also means copyright-like laws that apply to other
     kinds of works, such as semiconductor masks.

     "The Program" refers to any copyrightable work licensed under this
     License.  Each licensee is addressed as "you".  "Licensees" and
     "recipients" may be individuals or organizations.

     To "modify" a work means to copy from or adapt all or part of the
     work in a fashion requiring copyright permission, other than the
     making of an exact copy.  The resulting work is called a "modified
     version" of the earlier work or a work "based on" the earlier work.

     A "covered work" means either the unmodified Program or a work
     based on the Program.

     To "propagate" a work means to do anything with it that, without
     permission, would make you directly or secondarily liable for
     infringement under applicable copyright law, except executing it on
     a computer or modifying a private copy.  Propagation includes
     copying, distribution (with or without modification), making
     available to the public, and in some countries other activities as
     well.

     To "convey" a work means any kind of propagation that enables other
     parties to make or receive copies.  Mere interaction with a user
     through a computer network, with no transfer of a copy, is not
     conveying.

     An interactive user interface displays "Appropriate Legal Notices"
     to the extent that it includes a convenient and prominently visible
     feature that (1) displays an appropriate copyright notice, and (2)
     tells the user that there is no warranty for the work (except to
     the extent that warranties are provided), that licensees may convey
     the work under this License, and how to view a copy of this
     License.  If the interface presents a list of user commands or
     options, such as a menu, a prominent item in the list meets this
     criterion.

  1. Source Code.

     The "source code" for a work means the preferred form of the work
     for making modifications to it.  "Object code" means any non-source
     form of a work.

     A "Standard Interface" means an interface that either is an
     official standard defined by a recognized standards body, or, in
     the case of interfaces specified for a particular programming
     language, one that is widely used among developers working in that
     language.

     The "System Libraries" of an executable work include anything,
     other than the work as a whole, that (a) is included in the normal
     form of packaging a Major Component, but which is not part of that
     Major Component, and (b) serves only to enable use of the work with
     that Major Component, or to implement a Standard Interface for
     which an implementation is available to the public in source code
     form.  A "Major Component", in this context, means a major
     essential component (kernel, window system, and so on) of the
     specific operating system (if any) on which the executable work
     runs, or a compiler used to produce the work, or an object code
     interpreter used to run it.

     The "Corresponding Source" for a work in object code form means all
     the source code needed to generate, install, and (for an executable
     work) run the object code and to modify the work, including scripts
     to control those activities.  However, it does not include the
     work's System Libraries, or general-purpose tools or generally
     available free programs which are used unmodified in performing
     those activities but which are not part of the work.  For example,
     Corresponding Source includes interface definition files associated
     with source files for the work, and the source code for shared
     libraries and dynamically linked subprograms that the work is
     specifically designed to require, such as by intimate data
     communication or control flow between those subprograms and other
     parts of the work.

     The Corresponding Source need not include anything that users can
     regenerate automatically from other parts of the Corresponding
     Source.

     The Corresponding Source for a work in source code form is that
     same work.

  2. Basic Permissions.

     All rights granted under this License are granted for the term of
     copyright on the Program, and are irrevocable provided the stated
     conditions are met.  This License explicitly affirms your unlimited
     permission to run the unmodified Program.  The output from running
     a covered work is covered by this License only if the output, given
     its content, constitutes a covered work.  This License acknowledges
     your rights of fair use or other equivalent, as provided by
     copyright law.

     You may make, run and propagate covered works that you do not
     convey, without conditions so long as your license otherwise
     remains in force.  You may convey covered works to others for the
     sole purpose of having them make modifications exclusively for you,
     or provide you with facilities for running those works, provided
     that you comply with the terms of this License in conveying all
     material for which you do not control copyright.  Those thus making
     or running the covered works for you must do so exclusively on your
     behalf, under your direction and control, on terms that prohibit
     them from making any copies of your copyrighted material outside
     their relationship with you.

     Conveying under any other circumstances is permitted solely under
     the conditions stated below.  Sublicensing is not allowed; section
     10 makes it unnecessary.

  3. Protecting Users' Legal Rights From Anti-Circumvention Law.

     No covered work shall be deemed part of an effective technological
     measure under any applicable law fulfilling obligations under
     article 11 of the WIPO copyright treaty adopted on 20 December
     1996, or similar laws prohibiting or restricting circumvention of
     such measures.

     When you convey a covered work, you waive any legal power to forbid
     circumvention of technological measures to the extent such
     circumvention is effected by exercising rights under this License
     with respect to the covered work, and you disclaim any intention to
     limit operation or modification of the work as a means of
     enforcing, against the work's users, your or third parties' legal
     rights to forbid circumvention of technological measures.

  4. Conveying Verbatim Copies.

     You may convey verbatim copies of the Program's source code as you
     receive it, in any medium, provided that you conspicuously and
     appropriately publish on each copy an appropriate copyright notice;
     keep intact all notices stating that this License and any
     non-permissive terms added in accord with section 7 apply to the
     code; keep intact all notices of the absence of any warranty; and
     give all recipients a copy of this License along with the Program.

     You may charge any price or no price for each copy that you convey,
     and you may offer support or warranty protection for a fee.

  5. Conveying Modified Source Versions.

     You may convey a work based on the Program, or the modifications to
     produce it from the Program, in the form of source code under the
     terms of section 4, provided that you also meet all of these
     conditions:

       a. The work must carry prominent notices stating that you
          modified it, and giving a relevant date.

       b. The work must carry prominent notices stating that it is
          released under this License and any conditions added under
          section 7.  This requirement modifies the requirement in
          section 4 to "keep intact all notices".

       c. You must license the entire work, as a whole, under this
          License to anyone who comes into possession of a copy.  This
          License will therefore apply, along with any applicable
          section 7 additional terms, to the whole of the work, and all
          its parts, regardless of how they are packaged.  This License
          gives no permission to license the work in any other way, but
          it does not invalidate such permission if you have separately
          received it.

       d. If the work has interactive user interfaces, each must display
          Appropriate Legal Notices; however, if the Program has
          interactive interfaces that do not display Appropriate Legal
          Notices, your work need not make them do so.

     A compilation of a covered work with other separate and independent
     works, which are not by their nature extensions of the covered
     work, and which are not combined with it such as to form a larger
     program, in or on a volume of a storage or distribution medium, is
     called an "aggregate" if the compilation and its resulting
     copyright are not used to limit the access or legal rights of the
     compilation's users beyond what the individual works permit.
     Inclusion of a covered work in an aggregate does not cause this
     License to apply to the other parts of the aggregate.

  6. Conveying Non-Source Forms.

     You may convey a covered work in object code form under the terms
     of sections 4 and 5, provided that you also convey the
     machine-readable Corresponding Source under the terms of this
     License, in one of these ways:

       a. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by the
          Corresponding Source fixed on a durable physical medium
          customarily used for software interchange.

       b. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by a
          written offer, valid for at least three years and valid for as
          long as you offer spare parts or customer support for that
          product model, to give anyone who possesses the object code
          either (1) a copy of the Corresponding Source for all the
          software in the product that is covered by this License, on a
          durable physical medium customarily used for software
          interchange, for a price no more than your reasonable cost of
          physically performing this conveying of source, or (2) access
          to copy the Corresponding Source from a network server at no
          charge.

       c. Convey individual copies of the object code with a copy of the
          written offer to provide the Corresponding Source.  This
          alternative is allowed only occasionally and noncommercially,
          and only if you received the object code with such an offer,
          in accord with subsection 6b.

       d. Convey the object code by offering access from a designated
          place (gratis or for a charge), and offer equivalent access to
          the Corresponding Source in the same way through the same
          place at no further charge.  You need not require recipients
          to copy the Corresponding Source along with the object code.
          If the place to copy the object code is a network server, the
          Corresponding Source may be on a different server (operated by
          you or a third party) that supports equivalent copying
          facilities, provided you maintain clear directions next to the
          object code saying where to find the Corresponding Source.
          Regardless of what server hosts the Corresponding Source, you
          remain obligated to ensure that it is available for as long as
          needed to satisfy these requirements.

       e. Convey the object code using peer-to-peer transmission,
          provided you inform other peers where the object code and
          Corresponding Source of the work are being offered to the
          general public at no charge under subsection 6d.

     A separable portion of the object code, whose source code is
     excluded from the Corresponding Source as a System Library, need
     not be included in conveying the object code work.

     A "User Product" is either (1) a "consumer product", which means
     any tangible personal property which is normally used for personal,
     family, or household purposes, or (2) anything designed or sold for
     incorporation into a dwelling.  In determining whether a product is
     a consumer product, doubtful cases shall be resolved in favor of
     coverage.  For a particular product received by a particular user,
     "normally used" refers to a typical or common use of that class of
     product, regardless of the status of the particular user or of the
     way in which the particular user actually uses, or expects or is
     expected to use, the product.  A product is a consumer product
     regardless of whether the product has substantial commercial,
     industrial or non-consumer uses, unless such uses represent the
     only significant mode of use of the product.

     "Installation Information" for a User Product means any methods,
     procedures, authorization keys, or other information required to
     install and execute modified versions of a covered work in that
     User Product from a modified version of its Corresponding Source.
     The information must suffice to ensure that the continued
     functioning of the modified object code is in no case prevented or
     interfered with solely because modification has been made.

     If you convey an object code work under this section in, or with,
     or specifically for use in, a User Product, and the conveying
     occurs as part of a transaction in which the right of possession
     and use of the User Product is transferred to the recipient in
     perpetuity or for a fixed term (regardless of how the transaction
     is characterized), the Corresponding Source conveyed under this
     section must be accompanied by the Installation Information.  But
     this requirement does not apply if neither you nor any third party
     retains the ability to install modified object code on the User
     Product (for example, the work has been installed in ROM).

     The requirement to provide Installation Information does not
     include a requirement to continue to provide support service,
     warranty, or updates for a work that has been modified or installed
     by the recipient, or for the User Product in which it has been
     modified or installed.  Access to a network may be denied when the
     modification itself materially and adversely affects the operation
     of the network or violates the rules and protocols for
     communication across the network.

     Corresponding Source conveyed, and Installation Information
     provided, in accord with this section must be in a format that is
     publicly documented (and with an implementation available to the
     public in source code form), and must require no special password
     or key for unpacking, reading or copying.

  7. Additional Terms.

     "Additional permissions" are terms that supplement the terms of
     this License by making exceptions from one or more of its
     conditions.  Additional permissions that are applicable to the
     entire Program shall be treated as though they were included in
     this License, to the extent that they are valid under applicable
     law.  If additional permissions apply only to part of the Program,
     that part may be used separately under those permissions, but the
     entire Program remains governed by this License without regard to
     the additional permissions.

     When you convey a copy of a covered work, you may at your option
     remove any additional permissions from that copy, or from any part
     of it.  (Additional permissions may be written to require their own
     removal in certain cases when you modify the work.)  You may place
     additional permissions on material, added by you to a covered work,
     for which you have or can give appropriate copyright permission.

     Notwithstanding any other provision of this License, for material
     you add to a covered work, you may (if authorized by the copyright
     holders of that material) supplement the terms of this License with
     terms:

       a. Disclaiming warranty or limiting liability differently from
          the terms of sections 15 and 16 of this License; or

       b. Requiring preservation of specified reasonable legal notices
          or author attributions in that material or in the Appropriate
          Legal Notices displayed by works containing it; or

       c. Prohibiting misrepresentation of the origin of that material,
          or requiring that modified versions of such material be marked
          in reasonable ways as different from the original version; or

       d. Limiting the use for publicity purposes of names of licensors
          or authors of the material; or

       e. Declining to grant rights under trademark law for use of some
          trade names, trademarks, or service marks; or

       f. Requiring indemnification of licensors and authors of that
          material by anyone who conveys the material (or modified
          versions of it) with contractual assumptions of liability to
          the recipient, for any liability that these contractual
          assumptions directly impose on those licensors and authors.

     All other non-permissive additional terms are considered "further
     restrictions" within the meaning of section 10.  If the Program as
     you received it, or any part of it, contains a notice stating that
     it is governed by this License along with a term that is a further
     restriction, you may remove that term.  If a license document
     contains a further restriction but permits relicensing or conveying
     under this License, you may add to a covered work material governed
     by the terms of that license document, provided that the further
     restriction does not survive such relicensing or conveying.

     If you add terms to a covered work in accord with this section, you
     must place, in the relevant source files, a statement of the
     additional terms that apply to those files, or a notice indicating
     where to find the applicable terms.

     Additional terms, permissive or non-permissive, may be stated in
     the form of a separately written license, or stated as exceptions;
     the above requirements apply either way.

  8. Termination.

     You may not propagate or modify a covered work except as expressly
     provided under this License.  Any attempt otherwise to propagate or
     modify it is void, and will automatically terminate your rights
     under this License (including any patent licenses granted under the
     third paragraph of section 11).

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, you do not qualify to receive new licenses
     for the same material under section 10.

  9. Acceptance Not Required for Having Copies.

     You are not required to accept this License in order to receive or
     run a copy of the Program.  Ancillary propagation of a covered work
     occurring solely as a consequence of using peer-to-peer
     transmission to receive a copy likewise does not require
     acceptance.  However, nothing other than this License grants you
     permission to propagate or modify any covered work.  These actions
     infringe copyright if you do not accept this License.  Therefore,
     by modifying or propagating a covered work, you indicate your
     acceptance of this License to do so.

  10. Automatic Licensing of Downstream Recipients.

     Each time you convey a covered work, the recipient automatically
     receives a license from the original licensors, to run, modify and
     propagate that work, subject to this License.  You are not
     responsible for enforcing compliance by third parties with this
     License.

     An "entity transaction" is a transaction transferring control of an
     organization, or substantially all assets of one, or subdividing an
     organization, or merging organizations.  If propagation of a
     covered work results from an entity transaction, each party to that
     transaction who receives a copy of the work also receives whatever
     licenses to the work the party's predecessor in interest had or
     could give under the previous paragraph, plus a right to possession
     of the Corresponding Source of the work from the predecessor in
     interest, if the predecessor has it or can get it with reasonable
     efforts.

     You may not impose any further restrictions on the exercise of the
     rights granted or affirmed under this License.  For example, you
     may not impose a license fee, royalty, or other charge for exercise
     of rights granted under this License, and you may not initiate
     litigation (including a cross-claim or counterclaim in a lawsuit)
     alleging that any patent claim is infringed by making, using,
     selling, offering for sale, or importing the Program or any portion
     of it.

  11. Patents.

     A "contributor" is a copyright holder who authorizes use under this
     License of the Program or a work on which the Program is based.
     The work thus licensed is called the contributor's "contributor
     version".

     A contributor's "essential patent claims" are all patent claims
     owned or controlled by the contributor, whether already acquired or
     hereafter acquired, that would be infringed by some manner,
     permitted by this License, of making, using, or selling its
     contributor version, but do not include claims that would be
     infringed only as a consequence of further modification of the
     contributor version.  For purposes of this definition, "control"
     includes the right to grant patent sublicenses in a manner
     consistent with the requirements of this License.

     Each contributor grants you a non-exclusive, worldwide,
     royalty-free patent license under the contributor's essential
     patent claims, to make, use, sell, offer for sale, import and
     otherwise run, modify and propagate the contents of its contributor
     version.

     In the following three paragraphs, a "patent license" is any
     express agreement or commitment, however denominated, not to
     enforce a patent (such as an express permission to practice a
     patent or covenant not to sue for patent infringement).  To "grant"
     such a patent license to a party means to make such an agreement or
     commitment not to enforce a patent against the party.

     If you convey a covered work, knowingly relying on a patent
     license, and the Corresponding Source of the work is not available
     for anyone to copy, free of charge and under the terms of this
     License, through a publicly available network server or other
     readily accessible means, then you must either (1) cause the
     Corresponding Source to be so available, or (2) arrange to deprive
     yourself of the benefit of the patent license for this particular
     work, or (3) arrange, in a manner consistent with the requirements
     of this License, to extend the patent license to downstream
     recipients.  "Knowingly relying" means you have actual knowledge
     that, but for the patent license, your conveying the covered work
     in a country, or your recipient's use of the covered work in a
     country, would infringe one or more identifiable patents in that
     country that you have reason to believe are valid.

     If, pursuant to or in connection with a single transaction or
     arrangement, you convey, or propagate by procuring conveyance of, a
     covered work, and grant a patent license to some of the parties
     receiving the covered work authorizing them to use, propagate,
     modify or convey a specific copy of the covered work, then the
     patent license you grant is automatically extended to all
     recipients of the covered work and works based on it.

     A patent license is "discriminatory" if it does not include within
     the scope of its coverage, prohibits the exercise of, or is
     conditioned on the non-exercise of one or more of the rights that
     are specifically granted under this License.  You may not convey a
     covered work if you are a party to an arrangement with a third
     party that is in the business of distributing software, under which
     you make payment to the third party based on the extent of your
     activity of conveying the work, and under which the third party
     grants, to any of the parties who would receive the covered work
     from you, a discriminatory patent license (a) in connection with
     copies of the covered work conveyed by you (or copies made from
     those copies), or (b) primarily for and in connection with specific
     products or compilations that contain the covered work, unless you
     entered into that arrangement, or that patent license was granted,
     prior to 28 March 2007.

     Nothing in this License shall be construed as excluding or limiting
     any implied license or other defenses to infringement that may
     otherwise be available to you under applicable patent law.

  12. No Surrender of Others' Freedom.

     If conditions are imposed on you (whether by court order, agreement
     or otherwise) that contradict the conditions of this License, they
     do not excuse you from the conditions of this License.  If you
     cannot convey a covered work so as to satisfy simultaneously your
     obligations under this License and any other pertinent obligations,
     then as a consequence you may not convey it at all.  For example,
     if you agree to terms that obligate you to collect a royalty for
     further conveying from those to whom you convey the Program, the
     only way you could satisfy both those terms and this License would
     be to refrain entirely from conveying the Program.

  13. Use with the GNU Affero General Public License.

     Notwithstanding any other provision of this License, you have
     permission to link or combine any covered work with a work licensed
     under version 3 of the GNU Affero General Public License into a
     single combined work, and to convey the resulting work.  The terms
     of this License will continue to apply to the part which is the
     covered work, but the special requirements of the GNU Affero
     General Public License, section 13, concerning interaction through
     a network will apply to the combination as such.

  14. Revised Versions of this License.

     The Free Software Foundation may publish revised and/or new
     versions of the GNU General Public License from time to time.  Such
     new versions will be similar in spirit to the present version, but
     may differ in detail to address new problems or concerns.

     Each version is given a distinguishing version number.  If the
     Program specifies that a certain numbered version of the GNU
     General Public License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that numbered version or of any later version published by the Free
     Software Foundation.  If the Program does not specify a version
     number of the GNU General Public License, you may choose any
     version ever published by the Free Software Foundation.

     If the Program specifies that a proxy can decide which future
     versions of the GNU General Public License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Program.

     Later license versions may give you additional or different
     permissions.  However, no additional obligations are imposed on any
     author or copyright holder as a result of your choosing to follow a
     later version.

  15. Disclaimer of Warranty.

     THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
     APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE
     COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS"
     WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,
     INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
     MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE
     RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.
     SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
     NECESSARY SERVICING, REPAIR OR CORRECTION.

  16. Limitation of Liability.

     IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
     WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES
     AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR
     DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
     CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE
     THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA
     BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
     PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
     PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF
     THE POSSIBILITY OF SUCH DAMAGES.

  17. Interpretation of Sections 15 and 16.

     If the disclaimer of warranty and limitation of liability provided
     above cannot be given local legal effect according to their terms,
     reviewing courts shall apply local law that most closely
     approximates an absolute waiver of all civil liability in
     connection with the Program, unless a warranty or assumption of
     liability accompanies a copy of the Program in return for a fee.

END OF TERMS AND CONDITIONS
===========================

How to Apply These Terms to Your New Programs
=============================================

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
     Copyright (C) YEAR NAME OF AUTHOR

     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation, either version 3 of the License, or (at
     your option) any later version.

     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.

   Also add information on how to contact you by electronic and paper
mail.

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

     PROGRAM Copyright (C) YEAR NAME OF AUTHOR
     This program comes with ABSOLUTELY NO WARRANTY; for details type 'show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type 'show c' for details.

   The hypothetical commands 'show w' and 'show c' should show the
appropriate parts of the General Public License.  Of course, your
program's commands might be different; for a GUI interface, you would
use an "about box".

   You should also get your employer (if you work as a programmer) or
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  For more information on this, and how to apply and follow
the GNU GPL, see <http://www.gnu.org/licenses/>.

   The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
GNU Lesser General Public License instead of this License.  But first,
please read <http://www.gnu.org/philosophy/why-not-lgpl.html>.

Word Index
**********

This index is a list of Forth words that have "glossary" entries within
this manual.  Each word is listed with its stack effect and wordset.

* Menu:

* ! W A-ADDR -- core:                    Memory Access.     (line  4135)
* # UD1 -- UD2 core:                     Formatted numeric output.
                                                            (line  7801)
* #! -- gforth:                          Running Image Files.
                                                            (line 13324)
* #> XD -- ADDR U core:                  Formatted numeric output.
                                                            (line  7825)
* #>> -- gforth:                         Formatted numeric output.
                                                            (line  7830)
* #s UD -- 0 0 core:                     Formatted numeric output.
                                                            (line  7809)
* #tib -- ADDR core-ext-obsolescent:     The Text Interpreter.
                                                            (line  6479)
* $? -- N gforth:                        Passing Commands to the OS.
                                                            (line 11705)
* %align ALIGN SIZE -- gforth:           Structure Glossary.
                                                            (line  9172)
* %alignment ALIGN SIZE -- ALIGN gforth: Structure Glossary.
                                                            (line  9175)
* %alloc ALIGN SIZE -- ADDR gforth:      Structure Glossary.
                                                            (line  9178)
* %allocate ALIGN SIZE -- ADDR IOR gforth: Structure Glossary.
                                                            (line  9182)
* %allot ALIGN SIZE -- ADDR gforth:      Structure Glossary.
                                                            (line  9186)
* %size ALIGN SIZE -- SIZE gforth:       Structure Glossary.
                                                            (line  9218)
* ' "NAME" -- XT core:                   Execution token.   (line  5988)
* ' "NAME" -- XT oof:                    The OOF base class.
                                                            (line 10165)
* 'cold -- gforth:                       Modifying the Startup Sequence.
                                                            (line 13373)
* ( COMPILATION 'CCC<CLOSE-PAREN>' -- ; RUN-TIME -- core,file: Comments.
                                                            (line  3386)
* (local) ADDR U -- local:               ANS Forth locals.  (line  8935)
* ) -- gforth:                           Assertions.        (line 10687)
* * N1 N2 -- N core:                     Single precision.  (line  3457)
* */ N1 N2 N3 -- N4 core:                Mixed precision.   (line  3624)
* */mod N1 N2 N3 -- N4 N5 core:          Mixed precision.   (line  3627)
* + N1 N2 -- N core:                     Single precision.  (line  3446)
* +! N A-ADDR -- core:                   Memory Access.     (line  4138)
* +DO COMPILATION -- DO-SYS ; RUN-TIME N1 N2 -- | LOOP-SYS gforth: Arbitrary control structures.
                                                            (line  4668)
* +field N1 N2 "NAME" -- N3 X:structures: Forth200x Structures.
                                                            (line  9250)
* +load I*X N -- J*X gforth:             Blocks.            (line  7646)
* +LOOP COMPILATION DO-SYS -- ; RUN-TIME LOOP-SYS1 N -- | LOOP-SYS2 core: Arbitrary control structures.
                                                            (line  4682)
* +thru I*X N1 N2 -- J*X gforth:         Blocks.            (line  7650)
* +x/string XC-ADDR1 U1 -- XC-ADDR2 U2 xchar: Xchars and Unicode.
                                                            (line  8372)
* , W -- core:                           Dictionary allocation.
                                                            (line  4055)
* - N1 N2 -- N core:                     Single precision.  (line  3453)
* --> -- gforth:                         Blocks.            (line  7654)
* -DO COMPILATION -- DO-SYS ; RUN-TIME N1 N2 -- | LOOP-SYS gforth: Arbitrary control structures.
                                                            (line  4672)
* -LOOP COMPILATION DO-SYS -- ; RUN-TIME LOOP-SYS1 U -- | LOOP-SYS2 gforth: Arbitrary control structures.
                                                            (line  4684)
* -rot W1 W2 W3 -- W3 W1 W2 gforth:      Data stack.        (line  3850)
* -trailing C_ADDR U1 -- C_ADDR U2 string: Memory Blocks.   (line  4375)
* -trailing-garbage XC-ADDR U1 -- ADDR U2 xchar-ext: Xchars and Unicode.
                                                            (line  8383)
* . N -- core:                           Simple numeric output.
                                                            (line  7688)
* ." COMPILATION 'CCC"' -- ; RUN-TIME -- core: Displaying characters and strings.
                                                            (line  7961)
* .( COMPILATION&INTERPRETATION "CCC<PAREN>" -- core-ext: Displaying characters and strings.
                                                            (line  7968)
* .debugline NFILE NLINE -- gforth:      Debugging.         (line 10634)
* .id NT -- F83:                         Name token.        (line  6122)
* .name NT -- gforth-obsolete:           Name token.        (line  6119)
* .path PATH-ADDR -- gforth:             General Search Paths.
                                                            (line  7444)
* .r N1 N2 -- core-ext:                  Simple numeric output.
                                                            (line  7703)
* .s -- tools:                           Examining.         (line 10504)
* .\" COMPILATION 'CCC"' -- ; RUN-TIME -- gforth: Displaying characters and strings.
                                                            (line  7974)
* / N1 N2 -- N core:                     Single precision.  (line  3459)
* /does-handler -- N gforth:             Threading Words.   (line 11652)
* /l -- U gforth:                        Address arithmetic.
                                                            (line  4314)
* /mod N1 N2 -- N3 N4 core:              Single precision.  (line  3463)
* /string C-ADDR1 U1 N -- C-ADDR2 U2 string: Memory Blocks. (line  4379)
* /w -- U gforth:                        Address arithmetic.
                                                            (line  4311)
* 0< N -- F core:                        Numeric comparison.
                                                            (line  3558)
* 0<= N -- F gforth:                     Numeric comparison.
                                                            (line  3560)
* 0<> N -- F core-ext:                   Numeric comparison.
                                                            (line  3562)
* 0= N -- F core:                        Numeric comparison.
                                                            (line  3564)
* 0> N -- F core-ext:                    Numeric comparison.
                                                            (line  3566)
* 0>= N -- F gforth:                     Numeric comparison.
                                                            (line  3568)
* 1+ N1 -- N2 core:                      Single precision.  (line  3448)
* 1- N1 -- N2 core:                      Single precision.  (line  3455)
* 1/f R1 -- R2 gforth:                   Floating Point.    (line  3720)
* 2! W1 W2 A-ADDR -- core:               Memory Access.     (line  4151)
* 2* N1 -- N2 core:                      Bitwise operations.
                                                            (line  3526)
* 2, W1 W2 -- gforth:                    Dictionary allocation.
                                                            (line  4058)
* 2/ N1 -- N2 core:                      Bitwise operations.
                                                            (line  3532)
* 2>r D -- R:D core-ext:                 Return stack.      (line  3923)
* 2@ A-ADDR -- W1 W2 core:               Memory Access.     (line  4147)
* 2Constant W1 W2 "NAME" -- double:      Constants.         (line  5130)
* 2drop W1 W2 -- core:                   Data stack.        (line  3858)
* 2dup W1 W2 -- W1 W2 W1 W2 core:        Data stack.        (line  3862)
* 2field: U1 "NAME" -- U2 gforth:        Forth200x Structures.
                                                            (line  9256)
* 2Literal COMPILATION W1 W2 -- ; RUN-TIME -- W1 W2 double: Literals.
                                                            (line  6186)
* 2nip W1 W2 W3 W4 -- W3 W4 gforth:      Data stack.        (line  3860)
* 2over W1 W2 W3 W4 -- W1 W2 W3 W4 W1 W2 core: Data stack.  (line  3864)
* 2r> R:D -- D core-ext:                 Return stack.      (line  3925)
* 2r@ R:D -- R:D D core-ext:             Return stack.      (line  3927)
* 2rdrop R:D -- gforth:                  Return stack.      (line  3929)
* 2rot W1 W2 W3 W4 W5 W6 -- W3 W4 W5 W6 W1 W2 double-ext: Data stack.
                                                            (line  3870)
* 2swap W1 W2 W3 W4 -- W3 W4 W1 W2 core: Data stack.        (line  3868)
* 2tuck W1 W2 W3 W4 -- W3 W4 W1 W2 W3 W4 gforth: Data stack.
                                                            (line  3866)
* 2Variable "NAME" -- double:            Variables.         (line  5093)
* : "NAME" -- oof:                       The OOF base class.
                                                            (line 10131)
* : "NAME" -- COLON-SYS core:            Colon Definitions. (line  5205)
* :: "NAME" -- oof:                      The OOF base class.
                                                            (line 10143)
* :: CLASS "NAME" -- mini-oof:           Basic Mini-OOF Usage.
                                                            (line 10260)
* :m "NAME" -- XT; RUN-TIME: OBJECT -- objects: Objects Glossary.
                                                            (line  9938)
* :noname -- XT COLON-SYS core-ext:      Anonymous Definitions.
                                                            (line  5215)
* ; COMPILATION COLON-SYS -- ; RUN-TIME NEST-SYS core: Colon Definitions.
                                                            (line  5207)
* ;code COMPILATION. COLON-SYS1 -- COLON-SYS2 tools-ext: Code and ;code.
                                                            (line 11122)
* ;m COLON-SYS --; RUN-TIME: -- objects: Objects Glossary.  (line  9942)
* ;s R:W -- gforth:                      Calls and returns. (line  4796)
* < N1 N2 -- F core:                     Numeric comparison.
                                                            (line  3546)
* <# -- core:                            Formatted numeric output.
                                                            (line  7792)
* <<# -- gforth:                         Formatted numeric output.
                                                            (line  7795)
* <= N1 N2 -- F gforth:                  Numeric comparison.
                                                            (line  3548)
* <> N1 N2 -- F core-ext:                Numeric comparison.
                                                            (line  3550)
* <bind> CLASS SELECTOR-XT -- XT objects: Objects Glossary. (line  9844)
* <compilation COMPILATION. ORIG COLON-SYS -- gforth: Combined words.
                                                            (line  5954)
* <interpretation COMPILATION. ORIG COLON-SYS -- gforth: Combined words.
                                                            (line  5950)
* <to-inst> W XT -- objects:             Objects Glossary.  (line  9983)
* = N1 N2 -- F core:                     Numeric comparison.
                                                            (line  3552)
* > N1 N2 -- F core:                     Numeric comparison.
                                                            (line  3554)
* >= N1 N2 -- F gforth:                  Numeric comparison.
                                                            (line  3556)
* >body XT -- A_ADDR core:               CREATE..DOES> details.
                                                            (line  5525)
* >code-address XT -- C_ADDR gforth:     Threading Words.   (line 11622)
* >definer XT -- DEFINER gforth:         Threading Words.   (line 11679)
* >does-code XT -- A_ADDR gforth:        Threading Words.   (line 11635)
* >float C-ADDR U -- F:... FLAG float:   Line input and conversion.
                                                            (line  8271)
* >in -- ADDR core:                      The Text Interpreter.
                                                            (line  6469)
* >l W -- gforth:                        Locals implementation.
                                                            (line  8792)
* >name XT -- NT|0 gforth:               Name token.        (line  6095)
* >number UD1 C-ADDR1 U1 -- UD2 C-ADDR2 U2 core: Line input and conversion.
                                                            (line  8258)
* >order WID -- gforth:                  Word Lists.        (line  6914)
* >r W -- R:W core:                      Return stack.      (line  3915)
* ? A-ADDR -- tools:                     Examining.         (line 10537)
* ?DO COMPILATION -- DO-SYS ; RUN-TIME W1 W2 -- | LOOP-SYS core-ext: Arbitrary control structures.
                                                            (line  4666)
* ?dup W -- S:... W core:                Data stack.        (line  3852)
* ?DUP-0=-IF COMPILATION -- ORIG ; RUN-TIME N -- N| gforth: Arbitrary control structures.
                                                            (line  4662)
* ?DUP-IF COMPILATION -- ORIG ; RUN-TIME N -- N| gforth: Arbitrary control structures.
                                                            (line  4657)
* ?LEAVE COMPILATION -- ; RUN-TIME F | F LOOP-SYS -- gforth: Arbitrary control structures.
                                                            (line  4690)
* @ A-ADDR -- W core:                    Memory Access.     (line  4132)
* @local# #NOFFSET -- W gforth:          Locals implementation.
                                                            (line  8780)
* [ -- core:                             Literals.          (line  6169)
* ['] COMPILATION. "NAME" -- ; RUN-TIME. -- XT core: Execution token.
                                                            (line  6000)
* [+LOOP] N -- gforth:                   Interpreter Directives.
                                                            (line  6764)
* [?DO] N-LIMIT N-INDEX -- gforth:       Interpreter Directives.
                                                            (line  6756)
* [AGAIN] -- gforth:                     Interpreter Directives.
                                                            (line  6772)
* [BEGIN] -- gforth:                     Interpreter Directives.
                                                            (line  6768)
* [bind] COMPILE-TIME: "CLASS" "SELECTOR" -- ; RUN-TIME: ... OBJECT -- ... objects: Objects Glossary.
                                                            (line  9850)
* [Char] COMPILATION '<SPACES>CCC' -- ; RUN-TIME -- C core: Displaying characters and strings.
                                                            (line  8018)
* [COMP'] COMPILATION "NAME" -- ; RUN-TIME -- W XT gforth: Compilation token.
                                                            (line  6053)
* [compile] COMPILATION "NAME" -- ; RUN-TIME ? -- ? core-ext: Macros.
                                                            (line  6235)
* [current] COMPILE-TIME: "SELECTOR" -- ; RUN-TIME: ... OBJECT -- ... objects: Objects Glossary.
                                                            (line  9883)
* [DO] N-LIMIT N-INDEX -- gforth:        Interpreter Directives.
                                                            (line  6758)
* [ELSE] -- tools-ext:                   Interpreter Directives.
                                                            (line  6730)
* [ENDIF] -- gforth:                     Interpreter Directives.
                                                            (line  6743)
* [FOR] N -- gforth:                     Interpreter Directives.
                                                            (line  6760)
* [IFDEF] "<SPACES>NAME" -- gforth:      Interpreter Directives.
                                                            (line  6746)
* [IFUNDEF] "<SPACES>NAME" -- gforth:    Interpreter Directives.
                                                            (line  6751)
* [IF] FLAG -- tools-ext:                Interpreter Directives.
                                                            (line  6722)
* [LOOP] -- gforth:                      Interpreter Directives.
                                                            (line  6762)
* [NEXT] N -- gforth:                    Interpreter Directives.
                                                            (line  6766)
* [parent] COMPILE-TIME: "SELECTOR" -- ; RUN-TIME: ... OBJECT -- ... objects: Objects Glossary.
                                                            (line  9961)
* [REPEAT] -- gforth:                    Interpreter Directives.
                                                            (line  6776)
* [THEN] -- tools-ext:                   Interpreter Directives.
                                                            (line  6739)
* [to-inst] COMPILE-TIME: "NAME" -- ; RUN-TIME: W -- objects: Objects Glossary.
                                                            (line  9986)
* [UNTIL] FLAG -- gforth:                Interpreter Directives.
                                                            (line  6770)
* [WHILE] FLAG -- gforth:                Interpreter Directives.
                                                            (line  6774)
* [] N "NAME" -- oof:                    The OOF base class.
                                                            (line 10137)
* \ COMPILATION 'CCC<NEWLINE>' -- ; RUN-TIME -- core-ext,block-ext: Comments.
                                                            (line  3393)
* \c "REST-OF-LINE" -- gforth:           Declaring C Functions.
                                                            (line 10924)
* \G COMPILATION 'CCC<NEWLINE>' -- ; RUN-TIME -- gforth: Comments.
                                                            (line  3399)
* ] -- core:                             Literals.          (line  6172)
* ]L COMPILATION: N -- ; RUN-TIME: -- N gforth: Literals.   (line  6180)
* ~~ -- gforth:                          Debugging.         (line 10628)
* abort ?? -- ?? core,exception-ext:     Exception Handling.
                                                            (line  5005)
* ABORT" COMPILATION 'CCC"' -- ; RUN-TIME F -- core,exception-ext: Exception Handling.
                                                            (line  5000)
* abs N -- U core:                       Single precision.  (line  3467)
* accept C-ADDR +N1 -- +N2 core:         Line input and conversion.
                                                            (line  8239)
* action-of INTERPRETATION "NAME" -- XT; COMPILATION "NAME" -- ; RUN-TIME -- XT gforth: Deferred Words.
                                                            (line  5759)
* add-lib C-ADDR U -- gforth:            Declaring OS-level libraries.
                                                            (line 11037)
* ADDRESS-UNIT-BITS -- N environment:    Address arithmetic.
                                                            (line  4308)
* AGAIN COMPILATION DEST -- ; RUN-TIME -- core-ext: Arbitrary control structures.
                                                            (line  4633)
* AHEAD COMPILATION -- ORIG ; RUN-TIME -- tools-ext: Arbitrary control structures.
                                                            (line  4625)
* Alias XT "NAME" -- gforth:             Aliases.           (line  5800)
* align -- core:                         Dictionary allocation.
                                                            (line  4072)
* aligned C-ADDR -- A-ADDR core:         Address arithmetic.
                                                            (line  4261)
* allocate U -- A-ADDR WIOR memory:      Heap Allocation.   (line  4108)
* allot N -- core:                       Dictionary allocation.
                                                            (line  4041)
* also -- search-ext:                    Word Lists.        (line  6920)
* also-path C-ADDR LEN PATH-ADDR -- gforth: General Search Paths.
                                                            (line  7441)
* and W1 W2 -- W core:                   Bitwise operations.
                                                            (line  3513)
* arg U -- ADDR COUNT gforth:            OS command line arguments.
                                                            (line  8448)
* argc -- ADDR gforth:                   OS command line arguments.
                                                            (line  8462)
* argv -- ADDR gforth:                   OS command line arguments.
                                                            (line  8466)
* asptr CLASS -- oof:                    Class Declaration. (line 10191)
* asptr O "NAME" -- oof:                 The OOF base class.
                                                            (line 10135)
* assembler -- tools-ext:                Code and ;code.    (line 11114)
* assert( -- gforth:                     Assertions.        (line 10684)
* assert-level -- A-ADDR gforth:         Assertions.        (line 10702)
* assert0( -- gforth:                    Assertions.        (line 10671)
* assert1( -- gforth:                    Assertions.        (line 10674)
* assert2( -- gforth:                    Assertions.        (line 10677)
* assert3( -- gforth:                    Assertions.        (line 10680)
* ASSUME-LIVE ORIG -- ORIG gforth:       Where are locals visible by name?.
                                                            (line  8661)
* at-xy U1 U2 -- facility:               Terminal output.   (line  8076)
* base -- A-ADDR core:                   Number Conversion. (line  6611)
* base-execute I*X XT U -- J*X gforth:   Number Conversion. (line  6607)
* BEGIN COMPILATION -- DEST ; RUN-TIME -- core: Arbitrary control structures.
                                                            (line  4629)
* begin-structure "NAME" -- STRUCT-SYS 0 X:structures: Forth200x Structures.
                                                            (line  9246)
* bin FAM1 -- FAM2 file:                 General files.     (line  7280)
* bind ... "CLASS" "SELECTOR" -- ... objects: Objects Glossary.
                                                            (line  9841)
* bind O "NAME" -- oof:                  The OOF base class.
                                                            (line 10154)
* bind' "CLASS" "SELECTOR" -- XT objects: Objects Glossary. (line  9847)
* bl -- C-CHAR core:                     Displaying characters and strings.
                                                            (line  7945)
* blank C-ADDR U -- string:              Memory Blocks.     (line  4352)
* blk -- ADDR block:                     Input Sources.     (line  6524)
* block U -- A-ADDR block:               Blocks.            (line  7600)
* block-included A-ADDR U -- gforth:     Blocks.            (line  7661)
* block-offset -- ADDR gforth:           Blocks.            (line  7579)
* block-position U -- block:             Blocks.            (line  7589)
* bootmessage -- gforth:                 Modifying the Startup Sequence.
                                                            (line 13378)
* bound CLASS ADDR "NAME" -- oof:        The OOF base class.
                                                            (line 10156)
* bounds ADDR U -- ADDR+U ADDR gforth:   Memory Blocks.     (line  4383)
* break" 'CCC"' -- gforth:               Singlestep Debugger.
                                                            (line 10783)
* break: -- gforth:                      Singlestep Debugger.
                                                            (line 10781)
* broken-pipe-error -- N gforth:         Pipes.             (line  8314)
* buffer U -- A-ADDR block:              Blocks.            (line  7607)
* bye -- tools-ext:                      Leaving Gforth.    (line   581)
* c! C C-ADDR -- core:                   Memory Access.     (line  4144)
* C" COMPILATION "CCC<QUOTE>" -- ; RUN-TIME -- C-ADDR core-ext: Displaying characters and strings.
                                                            (line  8009)
* c, C -- core:                          Dictionary allocation.
                                                            (line  4048)
* c-function "FORTH-NAME" "C-NAME" "{TYPE}" "--" "TYPE" -- gforth: Declaring C Functions.
                                                            (line 10927)
* c-library "NAME" -- gforth:            Defining library interfaces.
                                                            (line 11001)
* c-library-name C-ADDR U -- gforth:     Defining library interfaces.
                                                            (line 10998)
* c@ C-ADDR -- C core:                   Memory Access.     (line  4141)
* call-c ... W -- ... gforth:            Low-Level C Interface Words.
                                                            (line 11090)
* case COMPILATION -- CASE-SYS ; RUN-TIME -- core-ext: Arbitrary control structures.
                                                            (line  4705)
* catch ... XT -- ... N exception:       Exception Handling.
                                                            (line  4846)
* cell -- U gforth:                      Address arithmetic.
                                                            (line  4258)
* cell% -- ALIGN SIZE gforth:            Structure Glossary.
                                                            (line  9190)
* cell+ A-ADDR1 -- A-ADDR2 core:         Address arithmetic.
                                                            (line  4255)
* cells N1 -- N2 core:                   Address arithmetic.
                                                            (line  4252)
* cfalign -- gforth:                     Dictionary allocation.
                                                            (line  4091)
* cfaligned ADDR1 -- ADDR2 gforth:       Address arithmetic.
                                                            (line  4304)
* cfield: U1 "NAME" -- U2 X:structures:  Forth200x Structures.
                                                            (line  9252)
* char '<SPACES>CCC' -- C core:          Displaying characters and strings.
                                                            (line  8014)
* char% -- ALIGN SIZE gforth:            Structure Glossary.
                                                            (line  9192)
* char+ C-ADDR1 -- C-ADDR2 core:         Address arithmetic.
                                                            (line  4249)
* chars N1 -- N2 core:                   Address arithmetic.
                                                            (line  4246)
* class "NAME" -- oof:                   The OOF base class.
                                                            (line 10108)
* class CLASS -- CLASS SELECTORS VARS mini-oof: Basic Mini-OOF Usage.
                                                            (line 10248)
* class PARENT-CLASS -- ALIGN OFFSET objects: Objects Glossary.
                                                            (line  9853)
* class->map CLASS -- MAP objects:       Objects Glossary.  (line  9857)
* class-inst-size CLASS -- ADDR objects: Objects Glossary.  (line  9862)
* class-override! XT SEL-XT CLASS-MAP -- objects: Objects Glossary.
                                                            (line  9866)
* class-previous CLASS -- objects:       Objects Glossary.  (line  9869)
* class; -- oof:                         Class Declaration. (line 10217)
* class>order CLASS -- objects:          Objects Glossary.  (line  9873)
* class? O -- FLAG oof:                  The OOF base class.
                                                            (line 10112)
* clear-libs -- gforth:                  Declaring OS-level libraries.
                                                            (line 11034)
* clear-path PATH-ADDR -- gforth:        General Search Paths.
                                                            (line  7438)
* clearstack ... -- gforth:              Examining.         (line 10529)
* clearstacks ... -- gforth:             Examining.         (line 10532)
* close-file WFILEID -- WIOR file:       General files.     (line  7291)
* close-pipe WFILEID -- WRETVAL WIOR gforth: Pipes.         (line  8305)
* cmove C-FROM C-TO U -- string:         Memory Blocks.     (line  4339)
* cmove> C-FROM C-TO U -- string:        Memory Blocks.     (line  4344)
* code "NAME" -- COLON-SYS tools-ext:    Code and ;code.    (line 11118)
* code-address! C_ADDR XT -- gforth:     Threading Words.   (line 11625)
* common-list LIST1 LIST2 -- LIST3 gforth-internal: Locals implementation.
                                                            (line  8862)
* COMP' "NAME" -- W XT gforth:           Compilation token. (line  6056)
* compare C-ADDR1 U1 C-ADDR2 U2 -- N string: Memory Blocks. (line  4355)
* compilation> COMPILATION. -- ORIG COLON-SYS gforth: Combined words.
                                                            (line  5952)
* compile, XT -- core-ext:               Macros.            (line  6299)
* compile-lp+! N -- gforth:              Locals implementation.
                                                            (line  8802)
* compile-only -- gforth:                Interpretation and Compilation Semantics.
                                                            (line  5835)
* const-does> RUN-TIME: W*UW R*UR UW UR "NAME" -- gforth: Const-does>.
                                                            (line  5641)
* Constant W "NAME" -- core:             Constants.         (line  5125)
* construct ... OBJECT -- objects:       Objects Glossary.  (line  9876)
* context -- ADDR gforth:                Word Lists.        (line  6990)
* convert UD1 C-ADDR1 -- UD2 C-ADDR2 core-ext-obsolescent: Line input and conversion.
                                                            (line  8281)
* count C-ADDR1 -- C-ADDR2 U core:       String Formats.    (line  7931)
* cputime -- DUSER DSYSTEM gforth:       Keeping track of Time.
                                                            (line 11728)
* cr -- core:                            Displaying characters and strings.
                                                            (line  7984)
* Create "NAME" -- core:                 CREATE.            (line  5030)
* create-file C-ADDR U WFAM -- WFILEID WIOR file: General files.
                                                            (line  7289)
* create-interpret/compile "NAME" -- gforth: Combined words.
                                                            (line  5946)
* CS-PICK ... U -- ... DESTU tools-ext:  Arbitrary control structures.
                                                            (line  4635)
* CS-ROLL DESTU/ORIGU .. DEST0/ORIG0 U -- .. DEST0/ORIG0 DESTU/ORIGU tools-ext: Arbitrary control structures.
                                                            (line  4637)
* current -- ADDR gforth:                Word Lists.        (line  6987)
* current' "SELECTOR" -- XT objects:     Objects Glossary.  (line  9880)
* current-interface -- ADDR objects:     Objects Glossary.  (line  9886)
* d+ D1 D2 -- D double:                  Double precision.  (line  3498)
* d- D1 D2 -- D double:                  Double precision.  (line  3500)
* d. D -- double:                        Simple numeric output.
                                                            (line  7713)
* d.r D N -- double:                     Simple numeric output.
                                                            (line  7721)
* d0< D -- F double:                     Numeric comparison.
                                                            (line  3599)
* d0<= D -- F gforth:                    Numeric comparison.
                                                            (line  3601)
* d0<> D -- F gforth:                    Numeric comparison.
                                                            (line  3603)
* d0= D -- F double:                     Numeric comparison.
                                                            (line  3605)
* d0> D -- F gforth:                     Numeric comparison.
                                                            (line  3607)
* d0>= D -- F gforth:                    Numeric comparison.
                                                            (line  3609)
* d2* D1 -- D2 double:                   Bitwise operations.
                                                            (line  3529)
* d2/ D1 -- D2 double:                   Bitwise operations.
                                                            (line  3536)
* d< D1 D2 -- F double:                  Numeric comparison.
                                                            (line  3587)
* d<= D1 D2 -- F gforth:                 Numeric comparison.
                                                            (line  3589)
* d<> D1 D2 -- F gforth:                 Numeric comparison.
                                                            (line  3591)
* d= D1 D2 -- F double:                  Numeric comparison.
                                                            (line  3593)
* d> D1 D2 -- F gforth:                  Numeric comparison.
                                                            (line  3595)
* d>= D1 D2 -- F gforth:                 Numeric comparison.
                                                            (line  3597)
* d>f D -- R float:                      Floating Point.    (line  3666)
* d>s D -- N double:                     Double precision.  (line  3496)
* dabs D -- UD double:                   Double precision.  (line  3504)
* dbg "NAME" -- gforth:                  Singlestep Debugger.
                                                            (line 10779)
* dec. N -- gforth:                      Simple numeric output.
                                                            (line  7692)
* decimal -- core:                       Number Conversion. (line  6620)
* Defer "NAME" -- gforth:                Deferred Words.    (line  5744)
* defer -- oof:                          Class Declaration. (line 10196)
* defer! XT XT-DEFERRED -- gforth:       Deferred Words.    (line  5748)
* defer@ XT-DEFERRED -- XT gforth:       Deferred Words.    (line  5755)
* defers COMPILATION "NAME" -- ; RUN-TIME ... -- ... gforth: Deferred Words.
                                                            (line  5762)
* definer! DEFINER XT -- gforth:         Threading Words.   (line 11684)
* defines XT CLASS "NAME" -- mini-oof:   Basic Mini-OOF Usage.
                                                            (line 10254)
* definitions -- oof:                    The OOF base class.
                                                            (line 10110)
* definitions -- search:                 Word Lists.        (line  6884)
* delete-file C-ADDR U -- WIOR file:     General files.     (line  7293)
* depth -- +N core:                      Examining.         (line 10521)
* df! R DF-ADDR -- float-ext:            Memory Access.     (line  4172)
* df@ DF-ADDR -- R float-ext:            Memory Access.     (line  4168)
* dfalign -- float-ext:                  Dictionary allocation.
                                                            (line  4084)
* dfaligned C-ADDR -- DF-ADDR float-ext: Address arithmetic.
                                                            (line  4296)
* dffield: U1 "NAME" -- U2 X:structures: Forth200x Structures.
                                                            (line  9262)
* dfloat% -- ALIGN SIZE gforth:          Structure Glossary.
                                                            (line  9194)
* dfloat+ DF-ADDR1 -- DF-ADDR2 float-ext: Address arithmetic.
                                                            (line  4293)
* dfloats N1 -- N2 float-ext:            Address arithmetic.
                                                            (line  4289)
* dict-new ... CLASS -- OBJECT objects:  Objects Glossary.  (line  9889)
* discode ADDR U -- gforth:              Common Disassembler.
                                                            (line 11253)
* dispose -- oof:                        The OOF base class.
                                                            (line 10122)
* dmax D1 D2 -- D double:                Double precision.  (line  3508)
* dmin D1 D2 -- D double:                Double precision.  (line  3506)
* dnegate D1 -- D2 double:               Double precision.  (line  3502)
* DO COMPILATION -- DO-SYS ; RUN-TIME W1 W2 -- LOOP-SYS core: Arbitrary control structures.
                                                            (line  4676)
* docol: -- ADDR gforth:                 Threading Words.   (line 11658)
* docon: -- ADDR gforth:                 Threading Words.   (line 11661)
* dodefer: -- ADDR gforth:               Threading Words.   (line 11670)
* does-code! A_ADDR XT -- gforth:        Threading Words.   (line 11644)
* does-handler! A_ADDR -- gforth:        Threading Words.   (line 11648)
* DOES> COMPILATION COLON-SYS1 -- COLON-SYS2 ; RUN-TIME NEST-SYS -- core: CREATE..DOES> details.
                                                            (line  5482)
* dofield: -- ADDR gforth:               Threading Words.   (line 11673)
* DONE COMPILATION ORIG -- ; RUN-TIME -- gforth: Arbitrary control structures.
                                                            (line  4694)
* double% -- ALIGN SIZE gforth:          Structure Glossary.
                                                            (line  9196)
* douser: -- ADDR gforth:                Threading Words.   (line 11667)
* dovar: -- ADDR gforth:                 Threading Words.   (line 11664)
* dpl -- A-ADDR gforth:                  Number Conversion. (line  6600)
* drop W -- core:                        Data stack.        (line  3833)
* du< UD1 UD2 -- F double-ext:           Numeric comparison.
                                                            (line  3611)
* du<= UD1 UD2 -- F gforth:              Numeric comparison.
                                                            (line  3613)
* du> UD1 UD2 -- F gforth:               Numeric comparison.
                                                            (line  3615)
* du>= UD1 UD2 -- F gforth:              Numeric comparison.
                                                            (line  3617)
* dump ADDR U -- tools:                  Examining.         (line 10540)
* dup W -- W W core:                     Data stack.        (line  3837)
* early -- oof:                          Class Declaration. (line 10201)
* edit-line C-ADDR N1 N2 -- N3 gforth:   Line input and conversion.
                                                            (line  8246)
* ekey -- U facility-ext:                Single-key input.  (line  8135)
* ekey>char U -- U FALSE | C TRUE facility-ext: Single-key input.
                                                            (line  8138)
* ekey>fkey U1 -- U2 F X:ekeys:          Single-key input.  (line  8141)
* ekey? -- FLAG facility-ext:            Single-key input.  (line  8145)
* ELSE COMPILATION ORIG1 -- ORIG2 ; RUN-TIME -- core: Arbitrary control structures.
                                                            (line  4647)
* emit C -- core:                        Displaying characters and strings.
                                                            (line  7954)
* emit-file C WFILEID -- WIOR gforth:    General files.     (line  7319)
* empty-buffer BUFFER -- gforth:         Blocks.            (line  7620)
* empty-buffers -- block-ext:            Blocks.            (line  7616)
* end-c-library -- gforth:               Defining library interfaces.
                                                            (line 11004)
* end-class ALIGN OFFSET "NAME" -- objects: Objects Glossary.
                                                            (line  9892)
* end-class CLASS SELECTORS VARS "NAME" -- mini-oof: Basic Mini-OOF Usage.
                                                            (line 10251)
* end-class-noname ALIGN OFFSET -- CLASS objects: Objects Glossary.
                                                            (line  9896)
* end-code COLON-SYS -- gforth:          Code and ;code.    (line 11120)
* end-interface "NAME" -- objects:       Objects Glossary.  (line  9899)
* end-interface-noname -- INTERFACE objects: Objects Glossary.
                                                            (line  9903)
* end-methods -- objects:                Objects Glossary.  (line  9906)
* end-struct ALIGN SIZE "NAME" -- gforth: Structure Glossary.
                                                            (line  9198)
* end-structure STRUCT-SYS +N -- X:structures: Forth200x Structures.
                                                            (line  9248)
* endcase COMPILATION CASE-SYS -- ; RUN-TIME X -- core-ext: Arbitrary control structures.
                                                            (line  4707)
* ENDIF COMPILATION ORIG -- ; RUN-TIME -- gforth: Arbitrary control structures.
                                                            (line  4655)
* endof COMPILATION CASE-SYS1 OF-SYS -- CASE-SYS2 ; RUN-TIME -- core-ext: Arbitrary control structures.
                                                            (line  4711)
* endscope COMPILATION SCOPE -- ; RUN-TIME -- gforth: Where are locals visible by name?.
                                                            (line  8552)
* endtry COMPILATION -- ; RUN-TIME R:SYS1 -- gforth: Exception Handling.
                                                            (line  4890)
* endtry-iferror COMPILATION ORIG1 -- ORIG2 ; RUN-TIME R:SYS1 -- gforth: Exception Handling.
                                                            (line  4969)
* endwith -- oof:                        The OOF base class.
                                                            (line 10176)
* environment-wordlist -- WID gforth:    Environmental Queries.
                                                            (line  7118)
* environment? C-ADDR U -- FALSE / ... TRUE core: Environmental Queries.
                                                            (line  7113)
* erase ADDR U -- core-ext:              Memory Blocks.     (line  4336)
* evaluate ... ADDR U -- ... core,block: Input Sources.     (line  6541)
* exception ADDR U -- N gforth:          Exception Handling.
                                                            (line  4827)
* execute XT -- core:                    Execution token.   (line  6026)
* execute-parsing ... ADDR U XT -- ... gforth: The Input Stream.
                                                            (line  6840)
* execute-parsing-file I*X FILEID XT -- J*X gforth: The Input Stream.
                                                            (line  6850)
* EXIT COMPILATION -- ; RUN-TIME NEST-SYS -- core: Calls and returns.
                                                            (line  4791)
* exitm -- objects:                      Objects Glossary.  (line  9910)
* expect C-ADDR +N -- core-ext-obsolescent: Line input and conversion.
                                                            (line  8284)
* f! R F-ADDR -- float:                  Memory Access.     (line  4157)
* f* R1 R2 -- R3 float:                  Floating Point.    (line  3674)
* f** R1 R2 -- R3 float-ext:             Floating Point.    (line  3693)
* f+ R1 R2 -- R3 float:                  Floating Point.    (line  3670)
* f, F -- gforth:                        Dictionary allocation.
                                                            (line  4051)
* f- R1 R2 -- R3 float:                  Floating Point.    (line  3672)
* f. R -- float-ext:                     Simple numeric output.
                                                            (line  7730)
* f.rdp RF +NR +ND +NP -- gforth:        Simple numeric output.
                                                            (line  7742)
* f.s -- gforth:                         Examining.         (line 10509)
* f/ R1 R2 -- R3 float:                  Floating Point.    (line  3676)
* f0< R -- F float:                      Floating Point.    (line  3797)
* f0<= R -- F gforth:                    Floating Point.    (line  3799)
* f0<> R -- F gforth:                    Floating Point.    (line  3801)
* f0= R -- F float:                      Floating Point.    (line  3803)
* f0> R -- F gforth:                     Floating Point.    (line  3805)
* f0>= R -- F gforth:                    Floating Point.    (line  3807)
* f2* R1 -- R2 gforth:                   Floating Point.    (line  3714)
* f2/ R1 -- R2 gforth:                   Floating Point.    (line  3717)
* f< R1 R2 -- F float:                   Floating Point.    (line  3789)
* f<= R1 R2 -- F gforth:                 Floating Point.    (line  3791)
* f<> R1 R2 -- F gforth:                 Floating Point.    (line  3787)
* f= R1 R2 -- F gforth:                  Floating Point.    (line  3785)
* f> R1 R2 -- F gforth:                  Floating Point.    (line  3793)
* f>= R1 R2 -- F gforth:                 Floating Point.    (line  3795)
* f>buf-rdp RF C-ADDR +NR +ND +NP -- gforth: Formatted numeric output.
                                                            (line  7841)
* f>d R -- D float:                      Floating Point.    (line  3668)
* f>l R -- gforth:                       Locals implementation.
                                                            (line  8794)
* f>str-rdp RF +NR +ND +NP -- C-ADDR NR gforth: Formatted numeric output.
                                                            (line  7835)
* f@ F-ADDR -- R float:                  Memory Access.     (line  4154)
* f@local# #NOFFSET -- R gforth:         Locals implementation.
                                                            (line  8782)
* fabs R1 -- R2 float-ext:               Floating Point.    (line  3680)
* facos R1 -- R2 float-ext:              Floating Point.    (line  3745)
* facosh R1 -- R2 float-ext:             Floating Point.    (line  3761)
* falign -- float:                       Dictionary allocation.
                                                            (line  4076)
* faligned C-ADDR -- F-ADDR float:       Address arithmetic.
                                                            (line  4274)
* falog R1 -- R2 float-ext:              Floating Point.    (line  3711)
* false -- F core-ext:                   Boolean Flags.     (line  3414)
* fasin R1 -- R2 float-ext:              Floating Point.    (line  3743)
* fasinh R1 -- R2 float-ext:             Floating Point.    (line  3759)
* fatan R1 -- R2 float-ext:              Floating Point.    (line  3747)
* fatan2 R1 R2 -- R3 float-ext:          Floating Point.    (line  3749)
* fatanh R1 -- R2 float-ext:             Floating Point.    (line  3763)
* fconstant R "NAME" -- float:           Constants.         (line  5132)
* fcos R1 -- R2 float-ext:               Floating Point.    (line  3736)
* fcosh R1 -- R2 float-ext:              Floating Point.    (line  3755)
* fdepth -- +N float:                    Examining.         (line 10525)
* fdrop R -- float:                      Floating point stack.
                                                            (line  3888)
* fdup R -- R R float:                   Floating point stack.
                                                            (line  3892)
* fe. R -- float-ext:                    Simple numeric output.
                                                            (line  7734)
* fexp R1 -- R2 float-ext:               Floating Point.    (line  3698)
* fexpm1 R1 -- R2 float-ext:             Floating Point.    (line  3700)
* ffield: U1 "NAME" -- U2 X:structures:  Forth200x Structures.
                                                            (line  9258)
* field ALIGN1 OFFSET1 ALIGN SIZE "NAME" -- ALIGN2 OFFSET2 gforth: Structure Glossary.
                                                            (line  9203)
* field: U1 "NAME" -- U2 X:structures:   Forth200x Structures.
                                                            (line  9254)
* file-position WFILEID -- UD WIOR file: General files.     (line  7325)
* file-size WFILEID -- UD WIOR file:     General files.     (line  7329)
* file-status C-ADDR U -- WFAM WIOR file-ext: General files.
                                                            (line  7323)
* fill C-ADDR U C -- core:               Memory Blocks.     (line  4349)
* find C-ADDR -- XT +-1 | C-ADDR 0 core,search: Word Lists. (line  6939)
* find-name C-ADDR U -- NT | 0 gforth:   Name token.        (line  6087)
* FLiteral COMPILATION R -- ; RUN-TIME -- R float: Literals.
                                                            (line  6190)
* fln R1 -- R2 float-ext:                Floating Point.    (line  3703)
* flnp1 R1 -- R2 float-ext:              Floating Point.    (line  3705)
* float -- U gforth:                     Address arithmetic.
                                                            (line  4270)
* float% -- ALIGN SIZE gforth:           Structure Glossary.
                                                            (line  9210)
* float+ F-ADDR1 -- F-ADDR2 float:       Address arithmetic.
                                                            (line  4267)
* floating-stack -- N environment:       Floating point stack.
                                                            (line  3884)
* floats N1 -- N2 float:                 Address arithmetic.
                                                            (line  4264)
* flog R1 -- R2 float-ext:               Floating Point.    (line  3708)
* floor R1 -- R2 float:                  Floating Point.    (line  3686)
* FLOORED -- F environment:              Single precision.  (line  3473)
* flush -- block:                        Blocks.            (line  7635)
* flush-file WFILEID -- WIOR file-ext:   General files.     (line  7321)
* flush-icache C-ADDR U -- gforth:       Code and ;code.    (line 11124)
* fm/mod D1 N1 -- N2 N3 core:            Mixed precision.   (line  3641)
* fmax R1 R2 -- R3 float:                Floating Point.    (line  3682)
* fmin R1 R2 -- R3 float:                Floating Point.    (line  3684)
* fnegate R1 -- R2 float:                Floating Point.    (line  3678)
* fnip R1 R2 -- R2 gforth:               Floating point stack.
                                                            (line  3890)
* FOR COMPILATION -- DO-SYS ; RUN-TIME U -- LOOP-SYS gforth: Arbitrary control structures.
                                                            (line  4678)
* form -- UROWS UCOLS gforth:            Terminal output.   (line  8084)
* Forth -- search-ext:                   Word Lists.        (line  6925)
* forth-wordlist -- WID search:          Word Lists.        (line  6879)
* fover R1 R2 -- R1 R2 R1 float:         Floating point stack.
                                                            (line  3894)
* fp! F-ADDR -- F:... gforth:            Stack pointer manipulation.
                                                            (line  3953)
* fp0 -- A-ADDR gforth:                  Stack pointer manipulation.
                                                            (line  3948)
* fp@ F:... -- F-ADDR gforth:            Stack pointer manipulation.
                                                            (line  3951)
* fpath -- PATH-ADDR gforth:             Source Search Paths.
                                                            (line  7414)
* fpick F:... U -- F:... R gforth:       Floating point stack.
                                                            (line  3900)
* free A-ADDR -- WIOR memory:            Heap Allocation.   (line  4115)
* frot R1 R2 R3 -- R2 R3 R1 float:       Floating point stack.
                                                            (line  3903)
* fround R1 -- R2 float:                 Floating Point.    (line  3690)
* fs. R -- float-ext:                    Simple numeric output.
                                                            (line  7738)
* fsin R1 -- R2 float-ext:               Floating Point.    (line  3734)
* fsincos R1 -- R2 R3 float-ext:         Floating Point.    (line  3738)
* fsinh R1 -- R2 float-ext:              Floating Point.    (line  3753)
* fsqrt R1 -- R2 float-ext:              Floating Point.    (line  3696)
* fswap R1 R2 -- R2 R1 float:            Floating point stack.
                                                            (line  3898)
* ftan R1 -- R2 float-ext:               Floating Point.    (line  3741)
* ftanh R1 -- R2 float-ext:              Floating Point.    (line  3757)
* ftuck R1 R2 -- R2 R1 R2 gforth:        Floating point stack.
                                                            (line  3896)
* fvariable "NAME" -- float:             Variables.         (line  5095)
* f~ R1 R2 R3 -- FLAG float-ext:         Floating Point.    (line  3781)
* f~abs R1 R2 R3 -- FLAG gforth:         Floating Point.    (line  3778)
* f~rel R1 R2 R3 -- FLAG gforth:         Floating Point.    (line  3775)
* get-block-fid -- WFILEID gforth:       Blocks.            (line  7585)
* get-current -- WID search:             Word Lists.        (line  6888)
* get-order -- WIDN .. WID1 N search:    Word Lists.        (line  6894)
* getenv C-ADDR1 U1 -- C-ADDR2 U2 gforth: Passing Commands to the OS.
                                                            (line 11709)
* gforth -- C-ADDR U gforth-environment: Environmental Queries.
                                                            (line  7122)
* heap-new ... CLASS -- OBJECT objects:  Objects Glossary.  (line  9913)
* here -- ADDR core:                     Dictionary allocation.
                                                            (line  4034)
* hex -- core-ext:                       Number Conversion. (line  6616)
* hex. U -- gforth:                      Simple numeric output.
                                                            (line  7695)
* hold CHAR -- core:                     Formatted numeric output.
                                                            (line  7815)
* how: -- oof:                           Class Declaration. (line 10214)
* i R:N -- R:N N core:                   Counted Loops.     (line  4516)
* id. NT -- gforth:                      Name token.        (line  6116)
* IF COMPILATION -- ORIG ; RUN-TIME F -- core: Arbitrary control structures.
                                                            (line  4623)
* iferror COMPILATION ORIG1 -- ORIG2 ; RUN-TIME -- gforth: Exception Handling.
                                                            (line  4893)
* immediate -- core:                     Interpretation and Compilation Semantics.
                                                            (line  5831)
* implementation INTERFACE -- objects:   Objects Glossary.  (line  9916)
* include ... "FILE" -- ... gforth:      Forth source files.
                                                            (line  7238)
* include-file I*X WFILEID -- J*X file:  Forth source files.
                                                            (line  7224)
* included I*X C-ADDR U -- J*X file:     Forth source files.
                                                            (line  7228)
* included? C-ADDR U -- F gforth:        Forth source files.
                                                            (line  7231)
* infile-execute ... XT FILE-ID -- ... gforth: Redirection. (line  7369)
* init ... -- oof:                       The OOF base class.
                                                            (line 10120)
* init-asm -- gforth:                    Code and ;code.    (line 11116)
* init-object ... CLASS OBJECT -- objects: Objects Glossary.
                                                            (line  9920)
* inst-value ALIGN1 OFFSET1 "NAME" -- ALIGN2 OFFSET2 objects: Objects Glossary.
                                                            (line  9924)
* inst-var ALIGN1 OFFSET1 ALIGN SIZE "NAME" -- ALIGN2 OFFSET2 objects: Objects Glossary.
                                                            (line  9928)
* interface -- objects:                  Objects Glossary.  (line  9932)
* interpret/compile: INTERP-XT COMP-XT "NAME" -- gforth: Combined words.
                                                            (line  5863)
* interpretation> COMPILATION. -- ORIG COLON-SYS gforth: Combined words.
                                                            (line  5948)
* invert W1 -- W2 core:                  Bitwise operations.
                                                            (line  3519)
* IS COMPILATION/INTERPRETATION "NAME-DEFERRED" -- ; RUN-TIME XT -- gforth: Deferred Words.
                                                            (line  5751)
* is XT "NAME" -- oof:                   The OOF base class.
                                                            (line 10160)
* j R:W R:W1 R:W2 -- W R:W R:W1 R:W2 core: Counted Loops.   (line  4518)
* k R:W R:W1 R:W2 R:W3 R:W4 -- W R:W R:W1 R:W2 R:W3 R:W4 gforth: Counted Loops.
                                                            (line  4520)
* k-alt-mask -- U X:ekeys:               Single-key input.  (line  8208)
* k-ctrl-mask -- U X:ekeys:              Single-key input.  (line  8206)
* k-delete -- U X:ekeys:                 Single-key input.  (line  8171)
* k-down -- U X:ekeys:                   Single-key input.  (line  8156)
* k-end -- U X:ekeys:                    Single-key input.  (line  8161)
* k-f1 -- U X:ekeys:                     Single-key input.  (line  8175)
* k-f10 -- U X:ekeys:                    Single-key input.  (line  8193)
* k-f11 -- U X:ekeys:                    Single-key input.  (line  8195)
* k-f12 -- U X:ekeys:                    Single-key input.  (line  8197)
* k-f2 -- U X:ekeys:                     Single-key input.  (line  8177)
* k-f3 -- U X:ekeys:                     Single-key input.  (line  8179)
* k-f4 -- U X:ekeys:                     Single-key input.  (line  8181)
* k-f5 -- U X:ekeys:                     Single-key input.  (line  8183)
* k-f6 -- U X:ekeys:                     Single-key input.  (line  8185)
* k-f7 -- U X:ekeys:                     Single-key input.  (line  8187)
* k-f8 -- U X:ekeys:                     Single-key input.  (line  8189)
* k-f9 -- U X:ekeys:                     Single-key input.  (line  8191)
* k-home -- U X:ekeys:                   Single-key input.  (line  8158)
* k-insert -- U X:ekeys:                 Single-key input.  (line  8169)
* k-left -- U X:ekeys:                   Single-key input.  (line  8150)
* k-next -- U X:ekeys:                   Single-key input.  (line  8166)
* k-prior -- U X:ekeys:                  Single-key input.  (line  8163)
* k-right -- U X:ekeys:                  Single-key input.  (line  8152)
* k-shift-mask -- U X:ekeys:             Single-key input.  (line  8204)
* k-up -- U X:ekeys:                     Single-key input.  (line  8154)
* key -- CHAR core:                      Single-key input.  (line  8106)
* key-file WFILEID -- C gforth:          General files.     (line  7302)
* key? -- FLAG facility:                 Single-key input.  (line  8109)
* key?-file WFILEID -- F gforth:         General files.     (line  7309)
* l! W C-ADDR -- gforth:                 Memory Access.     (line  4191)
* laddr# #NOFFSET -- C-ADDR gforth:      Locals implementation.
                                                            (line  8784)
* latest -- NT gforth:                   Name token.        (line  6091)
* latestxt -- XT gforth:                 Anonymous Definitions.
                                                            (line  5234)
* LEAVE COMPILATION -- ; RUN-TIME LOOP-SYS -- core: Arbitrary control structures.
                                                            (line  4688)
* lib-error -- C-ADDR U gforth:          Low-Level C Interface Words.
                                                            (line 11087)
* lib-sym C-ADDR1 U1 U2 -- U3 gforth:    Low-Level C Interface Words.
                                                            (line 11085)
* link "NAME" -- CLASS ADDR oof:         The OOF base class.
                                                            (line 10158)
* list U -- block-ext:                   Blocks.            (line  7592)
* list-size LIST -- U gforth-internal:   Locals implementation.
                                                            (line  8866)
* Literal COMPILATION N -- ; RUN-TIME -- N core: Literals.  (line  6175)
* load I*X N -- J*X block:               Blocks.            (line  7638)
* LOOP COMPILATION DO-SYS -- ; RUN-TIME LOOP-SYS1 -- | LOOP-SYS2 core: Arbitrary control structures.
                                                            (line  4680)
* lp! C-ADDR -- gforth:                  Stack pointer manipulation.
                                                            (line  3969)
* lp! C-ADDR -- gforth <1>:              Locals implementation.
                                                            (line  8790)
* lp+!# #NOFFSET -- gforth:              Locals implementation.
                                                            (line  8786)
* lp0 -- A-ADDR gforth:                  Stack pointer manipulation.
                                                            (line  3963)
* lp@ -- ADDR gforth:                    Stack pointer manipulation.
                                                            (line  3967)
* lshift U1 N -- U2 core:                Bitwise operations.
                                                            (line  3521)
* m* N1 N2 -- D core:                    Mixed precision.   (line  3630)
* m*/ D1 N2 U3 -- DQUOT double:          Mixed precision.   (line  3634)
* m+ D1 N -- D2 double:                  Mixed precision.   (line  3622)
* m: -- XT COLON-SYS; RUN-TIME: OBJECT -- objects: Objects Glossary.
                                                            (line  9935)
* marker "<SPACES> NAME" -- core-ext:    Forgetting words.  (line 10574)
* max N1 N2 -- N core:                   Single precision.  (line  3471)
* maxalign -- gforth:                    Dictionary allocation.
                                                            (line  4088)
* maxaligned ADDR1 -- ADDR2 gforth:      Address arithmetic.
                                                            (line  4300)
* maxdepth-.s -- ADDR gforth:            Examining.         (line 10514)
* method -- oof:                         Class Declaration. (line 10204)
* method M V "NAME" -- M' V mini-oof:    Basic Mini-OOF Usage.
                                                            (line 10242)
* method XT "NAME" -- objects:           Objects Glossary.  (line  9945)
* methods CLASS -- objects:              Objects Glossary.  (line  9949)
* min N1 N2 -- N core:                   Single precision.  (line  3469)
* mod N1 N2 -- N core:                   Single precision.  (line  3461)
* move C-FROM C-TO UCOUNT -- core:       Memory Blocks.     (line  4332)
* ms U -- facility-ext:                  Keeping track of Time.
                                                            (line 11718)
* naligned ADDR1 N -- ADDR2 gforth:      Structure Glossary.
                                                            (line  9212)
* name -- C-ADDR U gforth-obsolete:      The Input Stream.  (line  6811)
* name>comp NT -- W XT gforth:           Name token.        (line  6110)
* name>int NT -- XT gforth:              Name token.        (line  6100)
* name>string NT -- ADDR COUNT gforth:   Name token.        (line  6113)
* name?int NT -- XT gforth:              Name token.        (line  6106)
* needs ... "NAME" -- ... gforth:        Forth source files.
                                                            (line  7250)
* negate N1 -- N2 core:                  Single precision.  (line  3465)
* new -- O oof:                          The OOF base class.
                                                            (line 10127)
* new CLASS -- O mini-oof:               Basic Mini-OOF Usage.
                                                            (line 10257)
* new[] N -- O oof:                      The OOF base class.
                                                            (line 10129)
* NEXT COMPILATION DO-SYS -- ; RUN-TIME LOOP-SYS1 -- | LOOP-SYS2 gforth: Arbitrary control structures.
                                                            (line  4686)
* next-arg -- ADDR U gforth:             OS command line arguments.
                                                            (line  8422)
* nextname C-ADDR U -- gforth:           Supplying names.   (line  5262)
* nip W1 W2 -- W2 core-ext:              Data stack.        (line  3835)
* noname -- gforth:                      Anonymous Definitions.
                                                            (line  5229)
* nothrow -- gforth:                     Exception Handling.
                                                            (line  4848)
* object -- A-ADDR mini-oof:             Basic Mini-OOF Usage.
                                                            (line 10239)
* object -- CLASS objects:               Objects Glossary.  (line  9954)
* of COMPILATION -- OF-SYS ; RUN-TIME X1 X2 -- |X1 core-ext: Arbitrary control structures.
                                                            (line  4709)
* off A-ADDR -- gforth:                  Boolean Flags.     (line  3420)
* on A-ADDR -- gforth:                   Boolean Flags.     (line  3417)
* Only -- search-ext:                    Word Lists.        (line  6929)
* open-blocks C-ADDR U -- gforth:        Blocks.            (line  7573)
* open-file C-ADDR U WFAM -- WFILEID WIOR file: General files.
                                                            (line  7287)
* open-lib C-ADDR1 U1 -- U2 gforth:      Low-Level C Interface Words.
                                                            (line 11083)
* open-path-file ADDR1 U1 PATH-ADDR -- WFILEID ADDR2 U2 0 | IOR gforth: General Search Paths.
                                                            (line  7429)
* open-pipe C-ADDR U WFAM -- WFILEID WIOR gforth: Pipes.    (line  8303)
* or W1 W2 -- W core:                    Bitwise operations.
                                                            (line  3515)
* order -- search-ext:                   Word Lists.        (line  6933)
* os-class -- C-ADDR U gforth-environment: Environmental Queries.
                                                            (line  7127)
* outfile-execute ... XT FILE-ID -- ... gforth: Redirection.
                                                            (line  7366)
* over W1 W2 -- W1 W2 W1 core:           Data stack.        (line  3839)
* overrides XT "SELECTOR" -- objects:    Objects Glossary.  (line  9957)
* pad -- C-ADDR core-ext:                Memory Blocks.     (line  4388)
* page -- facility:                      Terminal output.   (line  8094)
* parse CHAR "CCC<CHAR>" -- C-ADDR U core-ext: The Input Stream.
                                                            (line  6800)
* parse-name "NAME" -- C-ADDR U gforth:  The Input Stream.  (line  6805)
* parse-word -- C-ADDR U gforth-obsolete: The Input Stream. (line  6808)
* path+ PATH-ADDR "DIR" -- gforth:       General Search Paths.
                                                            (line  7447)
* path-allot UMAX -- gforth:             General Search Paths.
                                                            (line  7435)
* path= PATH-ADDR "DIR1|DIR2|DIR3" gforth: General Search Paths.
                                                            (line  7450)
* perform A-ADDR -- gforth:              Execution token.   (line  6029)
* pi -- R gforth:                        Floating Point.    (line  3765)
* pick S:... U -- S:... W core-ext:      Data stack.        (line  3845)
* postpone "NAME" -- core:               Macros.            (line  6232)
* postpone "NAME" -- oof:                The OOF base class.
                                                            (line 10167)
* postpone, W XT -- gforth:              Compilation token. (line  6059)
* precision -- U float-ext:              Floating Point.    (line  3723)
* previous -- search-ext:                Word Lists.        (line  6917)
* print OBJECT -- objects:               Objects Glossary.  (line  9964)
* printdebugdata -- gforth:              Debugging.         (line 10632)
* protected -- objects:                  Objects Glossary.  (line  9968)
* ptr "NAME" -- oof:                     The OOF base class.
                                                            (line 10133)
* ptr -- oof:                            Class Declaration. (line 10188)
* public -- objects:                     Objects Glossary.  (line  9971)
* query -- core-ext-obsolescent:         Input Sources.     (line  6547)
* quit ?? -- ?? core:                    Miscellaneous Words.
                                                            (line 11741)
* r/o -- FAM file:                       General files.     (line  7274)
* r/w -- FAM file:                       General files.     (line  7276)
* r> R:W -- W core:                      Return stack.      (line  3917)
* r@ -- W ; R: W -- W core:              Return stack.      (line  3919)
* rdrop R:W -- gforth:                   Return stack.      (line  3921)
* read-file C-ADDR U1 WFILEID -- U2 WIOR file: General files.
                                                            (line  7298)
* read-line C_ADDR U1 WFILEID -- U2 FLAG WIOR file: General files.
                                                            (line  7300)
* recurse COMPILATION -- ; RUN-TIME ?? -- ?? core: Calls and returns.
                                                            (line  4765)
* recursive COMPILATION -- ; RUN-TIME -- gforth: Calls and returns.
                                                            (line  4761)
* refill -- FLAG core-ext,block-ext,file-ext: The Input Stream.
                                                            (line  6823)
* rename-file C-ADDR1 U1 C-ADDR2 U2 -- WIOR file-ext: General files.
                                                            (line  7295)
* REPEAT COMPILATION ORIG DEST -- ; RUN-TIME -- core: Arbitrary control structures.
                                                            (line  4651)
* reposition-file UD WFILEID -- WIOR file: General files.   (line  7327)
* represent R C-ADDR U -- N F1 F2 float: Formatted numeric output.
                                                            (line  7833)
* require ... "FILE" -- ... gforth:      Forth source files.
                                                            (line  7247)
* required I*X ADDR U -- I*X gforth:     Forth source files.
                                                            (line  7241)
* resize A-ADDR1 U -- A-ADDR2 WIOR memory: Heap Allocation. (line  4121)
* resize-file UD WFILEID -- WIOR file:   General files.     (line  7331)
* restore COMPILATION ORIG1 -- ; RUN-TIME -- gforth: Exception Handling.
                                                            (line  4965)
* restore-input X1 .. XN N -- FLAG core-ext: Input Sources. (line  6532)
* restrict -- gforth:                    Interpretation and Compilation Semantics.
                                                            (line  5838)
* roll X0 X1 .. XN N -- X1 .. XN X0 core-ext: Data stack.   (line  3856)
* Root -- gforth:                        Word Lists.        (line  6970)
* rot W1 W2 W3 -- W2 W3 W1 core:         Data stack.        (line  3848)
* rp! A-ADDR -- gforth:                  Stack pointer manipulation.
                                                            (line  3961)
* rp0 -- A-ADDR gforth:                  Stack pointer manipulation.
                                                            (line  3955)
* rp@ -- A-ADDR gforth:                  Stack pointer manipulation.
                                                            (line  3959)
* rshift U1 N -- U2 core:                Bitwise operations.
                                                            (line  3523)
* S" COMPILATION 'CCC"' -- ; RUN-TIME -- C-ADDR U core,file: Displaying characters and strings.
                                                            (line  7990)
* s>d N -- D core:                       Double precision.  (line  3494)
* s>number? ADDR U -- D F gforth:        Line input and conversion.
                                                            (line  8252)
* s>unumber? C-ADDR U -- UD FLAG gforth: Line input and conversion.
                                                            (line  8255)
* save-buffer BUFFER -- gforth:          Blocks.            (line  7633)
* save-buffers -- block:                 Blocks.            (line  7629)
* save-input -- X1 .. XN N core-ext:     Input Sources.     (line  6527)
* savesystem "NAME" -- gforth:           Non-Relocatable Image Files.
                                                            (line 13169)
* scope COMPILATION -- SCOPE ; RUN-TIME -- gforth: Where are locals visible by name?.
                                                            (line  8550)
* scr -- A-ADDR block-ext:               Blocks.            (line  7596)
* seal -- gforth:                        Word Lists.        (line  6980)
* search C-ADDR1 U1 C-ADDR2 U2 -- C-ADDR3 U3 FLAG string: Memory Blocks.
                                                            (line  4369)
* search-wordlist C-ADDR COUNT WID -- 0 | XT +-1 search: Word Lists.
                                                            (line  6955)
* see "<SPACES>NAME" -- tools:           Examining.         (line 10548)
* see-code "NAME" -- gforth:             Examining.         (line 10562)
* see-code-range ADDR1 ADDR2 -- gforth:  Examining.         (line 10566)
* selector "NAME" -- objects:            Objects Glossary.  (line  9975)
* self -- O oof:                         The OOF base class.
                                                            (line 10149)
* set-current WID -- search:             Word Lists.        (line  6891)
* set-order WIDN .. WID1 N -- search:    Word Lists.        (line  6900)
* set-precision U -- float-ext:          Floating Point.    (line  3727)
* sf! R SF-ADDR -- float-ext:            Memory Access.     (line  4164)
* sf@ SF-ADDR -- R float-ext:            Memory Access.     (line  4160)
* sfalign -- float-ext:                  Dictionary allocation.
                                                            (line  4080)
* sfaligned C-ADDR -- SF-ADDR float-ext: Address arithmetic.
                                                            (line  4285)
* sffield: U1 "NAME" -- U2 X:structures: Forth200x Structures.
                                                            (line  9260)
* sfloat% -- ALIGN SIZE gforth:          Structure Glossary.
                                                            (line  9216)
* sfloat+ SF-ADDR1 -- SF-ADDR2 float-ext: Address arithmetic.
                                                            (line  4282)
* sfloats N1 -- N2 float-ext:            Address arithmetic.
                                                            (line  4278)
* sh "..." -- gforth:                    Passing Commands to the OS.
                                                            (line 11694)
* shift-args -- gforth:                  OS command line arguments.
                                                            (line  8456)
* sign N -- core:                        Formatted numeric output.
                                                            (line  7819)
* simple-see "NAME" -- gforth:           Examining.         (line 10557)
* simple-see-range ADDR1 ADDR2 -- gforth: Examining.        (line 10560)
* sl@ C-ADDR -- N gforth:                Memory Access.     (line  4185)
* SLiteral COMPILATION C-ADDR1 U ; RUN-TIME -- C-ADDR2 U string: Literals.
                                                            (line  6194)
* slurp-fid FID -- ADDR U gforth:        General files.     (line  7336)
* slurp-file C-ADDR1 U1 -- C-ADDR2 U2 gforth: General files.
                                                            (line  7333)
* sm/rem D1 N1 -- N2 N3 core:            Mixed precision.   (line  3644)
* source -- ADDR U core:                 The Text Interpreter.
                                                            (line  6474)
* source-id -- 0 | -1 | FILEID core-ext,file: Input Sources.
                                                            (line  6519)
* sourcefilename -- C-ADDR U gforth:     Forth source files.
                                                            (line  7253)
* sourceline# -- U gforth:               Forth source files.
                                                            (line  7260)
* sp! A-ADDR -- S:... gforth:            Stack pointer manipulation.
                                                            (line  3946)
* sp0 -- A-ADDR gforth:                  Stack pointer manipulation.
                                                            (line  3940)
* sp@ S:... -- A-ADDR gforth:            Stack pointer manipulation.
                                                            (line  3944)
* space -- core:                         Displaying characters and strings.
                                                            (line  7948)
* spaces U -- core:                      Displaying characters and strings.
                                                            (line  7951)
* span -- C-ADDR core-ext-obsolescent:   Line input and conversion.
                                                            (line  8292)
* static -- oof:                         Class Declaration. (line 10209)
* stderr -- WFILEID gforth:              General files.     (line  7345)
* stdin -- WFILEID gforth:               General files.     (line  7339)
* stdout -- WFILEID gforth:              General files.     (line  7342)
* str< C-ADDR1 U1 C-ADDR2 U2 -- F gforth: Memory Blocks.    (line  4364)
* str= C-ADDR1 U1 C-ADDR2 U2 -- F gforth: Memory Blocks.    (line  4362)
* string-prefix? C-ADDR1 U1 C-ADDR2 U2 -- F gforth: Memory Blocks.
                                                            (line  4366)
* struct -- ALIGN SIZE gforth:           Structure Glossary.
                                                            (line  9221)
* sub-list? LIST1 LIST2 -- F gforth-internal: Locals implementation.
                                                            (line  8864)
* super "NAME" -- oof:                   The OOF base class.
                                                            (line 10145)
* sw@ C-ADDR -- N gforth:                Memory Access.     (line  4176)
* swap W1 W2 -- W2 W1 core:              Data stack.        (line  3843)
* system C-ADDR U -- gforth:             Passing Commands to the OS.
                                                            (line 11698)
* s\" COMPILATION 'CCC"' -- ; RUN-TIME -- C-ADDR U gforth: Displaying characters and strings.
                                                            (line  8001)
* table -- WID gforth:                   Word Lists.        (line  6911)
* THEN COMPILATION ORIG -- ; RUN-TIME -- core: Arbitrary control structures.
                                                            (line  4627)
* this -- OBJECT objects:                Objects Glossary.  (line  9980)
* threading-method -- N gforth:          Threading Words.   (line 11618)
* throw Y1 .. YM NERROR -- Y1 .. YM / Z1 .. ZN ERROR exception: Exception Handling.
                                                            (line  4806)
* thru I*X N1 N2 -- J*X block-ext:       Blocks.            (line  7643)
* tib -- ADDR core-ext-obsolescent:      The Text Interpreter.
                                                            (line  6477)
* time&date -- NSEC NMIN NHOUR NDAY NMONTH NYEAR facility-ext: Keeping track of Time.
                                                            (line 11721)
* TO C|W|D|R "NAME" -- core-ext,local:   Values.            (line  5189)
* to-this OBJECT -- objects:             Objects Glossary.  (line  9989)
* toupper C1 -- C2 gforth:               Displaying characters and strings.
                                                            (line  7957)
* true -- F core-ext:                    Boolean Flags.     (line  3411)
* try COMPILATION -- ORIG ; RUN-TIME -- R:SYS1 gforth: Exception Handling.
                                                            (line  4887)
* tuck W1 W2 -- W2 W1 W2 core-ext:       Data stack.        (line  3841)
* type C-ADDR U -- core:                 Displaying characters and strings.
                                                            (line  7977)
* typewhite ADDR N -- gforth:            Displaying characters and strings.
                                                            (line  7981)
* U+DO COMPILATION -- DO-SYS ; RUN-TIME U1 U2 -- | LOOP-SYS gforth: Arbitrary control structures.
                                                            (line  4670)
* U-DO COMPILATION -- DO-SYS ; RUN-TIME U1 U2 -- | LOOP-SYS gforth: Arbitrary control structures.
                                                            (line  4674)
* u. U -- core:                          Simple numeric output.
                                                            (line  7699)
* u.r U N -- core-ext:                   Simple numeric output.
                                                            (line  7709)
* u< U1 U2 -- F core:                    Numeric comparison.
                                                            (line  3570)
* u<= U1 U2 -- F gforth:                 Numeric comparison.
                                                            (line  3572)
* u> U1 U2 -- F core-ext:                Numeric comparison.
                                                            (line  3574)
* u>= U1 U2 -- F gforth:                 Numeric comparison.
                                                            (line  3576)
* ud. UD -- gforth:                      Simple numeric output.
                                                            (line  7717)
* ud.r UD N -- gforth:                   Simple numeric output.
                                                            (line  7726)
* ul@ C-ADDR -- U gforth:                Memory Access.     (line  4188)
* um* U1 U2 -- UD core:                  Mixed precision.   (line  3632)
* um/mod UD U1 -- U2 U3 core:            Mixed precision.   (line  3638)
* under+ N1 N2 N3 -- N N2 gforth:        Single precision.  (line  3450)
* unloop R:W1 R:W2 -- core:              Arbitrary control structures.
                                                            (line  4692)
* UNREACHABLE -- gforth:                 Where are locals visible by name?.
                                                            (line  8588)
* UNTIL COMPILATION DEST -- ; RUN-TIME F -- core: Arbitrary control structures.
                                                            (line  4631)
* unused -- U core-ext:                  Dictionary allocation.
                                                            (line  4037)
* update -- block:                       Blocks.            (line  7622)
* updated? N -- F gforth:                Blocks.            (line  7625)
* use "FILE" -- gforth:                  Blocks.            (line  7576)
* User "NAME" -- gforth:                 Variables.         (line  5102)
* utime -- DTIME gforth:                 Keeping track of Time.
                                                            (line 11725)
* uw@ C-ADDR -- U gforth:                Memory Access.     (line  4179)
* Value W "NAME" -- core-ext:            Values.            (line  5187)
* var M V SIZE "NAME" -- M V' mini-oof:  Basic Mini-OOF Usage.
                                                            (line 10245)
* var SIZE -- oof:                       Class Declaration. (line 10183)
* Variable "NAME" -- core:               Variables.         (line  5091)
* vlist -- gforth:                       Word Lists.        (line  6967)
* Vocabulary "NAME" -- gforth:           Word Lists.        (line  6975)
* vocs -- gforth:                        Word Lists.        (line  6984)
* w! W C-ADDR -- gforth:                 Memory Access.     (line  4182)
* w/o -- FAM file:                       General files.     (line  7278)
* WHILE COMPILATION DEST -- ORIG DEST ; RUN-TIME F -- core: Arbitrary control structures.
                                                            (line  4649)
* with O -- oof:                         The OOF base class.
                                                            (line 10174)
* within U1 U2 U3 -- F core-ext:         Numeric comparison.
                                                            (line  3578)
* word CHAR "<CHARS>CCC<CHAR>-- C-ADDR core: The Input Stream.
                                                            (line  6814)
* wordlist -- WID search:                Word Lists.        (line  6908)
* words -- tools:                        Word Lists.        (line  6963)
* write-file C-ADDR U1 WFILEID -- WIOR file: General files. (line  7315)
* write-line C-ADDR U FILEID -- IOR file: General files.    (line  7317)
* x-size XC-ADDR U1 -- U2 xchar:         Xchars and Unicode.
                                                            (line  8348)
* x-width XC-ADDR U -- N xchar-ext:      Xchars and Unicode.
                                                            (line  8388)
* xc!+? XC XC-ADDR1 U1 -- XC-ADDR2 U2 F xchar-ext: Xchars and Unicode.
                                                            (line  8356)
* xc-size XC -- U xchar-ext:             Xchars and Unicode.
                                                            (line  8345)
* xc@+ XC-ADDR1 -- XC-ADDR2 XC xchar-ext: Xchars and Unicode.
                                                            (line  8352)
* xchar+ XC-ADDR1 -- XC-ADDR2 xchar-ext: Xchars and Unicode.
                                                            (line  8364)
* xchar- XC-ADDR1 -- XC-ADDR2 xchar-ext: Xchars and Unicode.
                                                            (line  8368)
* xchar-encoding -- ADDR U xchar-ext:    Xchars and Unicode.
                                                            (line  8403)
* xemit XC -- xchar-ext:                 Xchars and Unicode.
                                                            (line  8398)
* xkey -- XC xchar-ext:                  Xchars and Unicode.
                                                            (line  8394)
* xor W1 W2 -- W core:                   Bitwise operations.
                                                            (line  3517)
* xt-new ... CLASS XT -- OBJECT objects: Objects Glossary.  (line  9992)
* xt-see XT -- gforth:                   Examining.         (line 10554)
* x\string- XC-ADDR1 U1 -- XC-ADDR1 U2 xchar: Xchars and Unicode.
                                                            (line  8377)

Concept and Word Index
**********************

Not all entries listed in this index are present verbatim in the text.
This index also duplicates, in abbreviated form, all of the words listed
in the Word Index (only the names are listed for the words here).

* Menu:

* '!':                                   Memory Access.     (line  4135)
* '"', stack item type:                  Notation.          (line  3347)
* '#':                                   Formatted numeric output.
                                                            (line  7801)
* '#!':                                  Running Image Files.
                                                            (line 13324)
* #-prefix for decimal numbers:          Number Conversion. (line  6624)
* '#>':                                  Formatted numeric output.
                                                            (line  7825)
* '#>>':                                 Formatted numeric output.
                                                            (line  7830)
* '#s':                                  Formatted numeric output.
                                                            (line  7809)
* '#tib':                                The Text Interpreter.
                                                            (line  6479)
* $-prefix for hexadecimal numbers:      Number Conversion. (line  6624)
* '$?':                                  Passing Commands to the OS.
                                                            (line 11705)
* %-prefix for binary numbers:           Number Conversion. (line  6624)
* '%align':                              Structure Glossary.
                                                            (line  9172)
* '%alignment':                          Structure Glossary.
                                                            (line  9175)
* '%alloc':                              Structure Glossary.
                                                            (line  9178)
* '%allocate':                           Structure Glossary.
                                                            (line  9182)
* '%allot':                              Structure Glossary.
                                                            (line  9186)
* '%size':                               Structure Glossary.
                                                            (line  9218)
* &-prefix for decimal numbers:          Number Conversion. (line  6624)
* ''':                                   Execution token.   (line  5988)
* ''' <1>:                               The OOF base class.
                                                            (line 10165)
* '-prefix for character strings:        Number Conversion. (line  6624)
* ''cold':                               Modifying the Startup Sequence.
                                                            (line 13373)
* '(':                                   Comments.          (line  3386)
* '(local)':                             ANS Forth locals.  (line  8935)
* ')':                                   Assertions.        (line 10687)
* '*':                                   Single precision.  (line  3457)
* '*/':                                  Mixed precision.   (line  3624)
* '*/mod':                               Mixed precision.   (line  3627)
* '+':                                   Single precision.  (line  3446)
* '+!':                                  Memory Access.     (line  4138)
* '+DO':                                 Arbitrary control structures.
                                                            (line  4668)
* '+field':                              Forth200x Structures.
                                                            (line  9250)
* '+load':                               Blocks.            (line  7646)
* '+LOOP':                               Arbitrary control structures.
                                                            (line  4682)
* '+thru':                               Blocks.            (line  7650)
* '+x/string':                           Xchars and Unicode.
                                                            (line  8372)
* ',':                                   Dictionary allocation.
                                                            (line  4055)
* '-':                                   Single precision.  (line  3453)
* -, tutorial:                           Stack-Effect Comments Tutorial.
                                                            (line  1093)
* '-->':                                 Blocks.            (line  7654)
* -appl-image, command-line option:      Invoking Gforth.   (line   406)
* -application, 'gforthmi' option:       gforthmi.          (line 13226)
* -clear-dictionary, command-line option: Invoking Gforth.  (line   482)
* -d, command-line option:               Invoking Gforth.   (line   430)
* -data-stack-size, command-line option: Invoking Gforth.   (line   430)
* -debug, command-line option:           Invoking Gforth.   (line   471)
* -DFORCE_REG:                           Portability.       (line 13433)
* -dictionary-size, command-line option: Invoking Gforth.   (line   420)
* -die-on-signal, command-line-option:   Invoking Gforth.   (line   486)
* '-DO':                                 Arbitrary control structures.
                                                            (line  4672)
* -DUSE_FTOS:                            TOS Optimization.  (line 13775)
* -DUSE_NO_FTOS:                         TOS Optimization.  (line 13775)
* -DUSE_NO_TOS:                          TOS Optimization.  (line 13763)
* -DUSE_TOS:                             TOS Optimization.  (line 13763)
* -dynamic command-line option:          Dynamic Superinstructions.
                                                            (line 13646)
* -dynamic, command-line option:         Invoking Gforth.   (line   495)
* -enable-force-reg, configuration flag: Portability.       (line 13433)
* -f, command-line option:               Invoking Gforth.   (line   440)
* -fp-stack-size, command-line option:   Invoking Gforth.   (line   440)
* -h, command-line option:               Invoking Gforth.   (line   463)
* -help, command-line option:            Invoking Gforth.   (line   463)
* -i, command-line option:               Invoking Gforth.   (line   401)
* -i, invoke image file:                 Running Image Files.
                                                            (line 13286)
* -image file, invoke image file:        Running Image Files.
                                                            (line 13286)
* -image-file, command-line option:      Invoking Gforth.   (line   401)
* -l, command-line option:               Invoking Gforth.   (line   446)
* -locals-stack-size, command-line option: Invoking Gforth. (line   446)
* '-LOOP':                               Arbitrary control structures.
                                                            (line  4684)
* -m, command-line option:               Invoking Gforth.   (line   420)
* -no-dynamic command-line option:       Dynamic Superinstructions.
                                                            (line 13634)
* -no-dynamic, command-line option:      Invoking Gforth.   (line   495)
* -no-offset-im, command-line option:    Invoking Gforth.   (line   479)
* -no-super command-line option:         Dynamic Superinstructions.
                                                            (line 13634)
* -no-super, command-line option:        Invoking Gforth.   (line   500)
* -offset-image, command-line option:    Invoking Gforth.   (line   474)
* -p, command-line option:               Invoking Gforth.   (line   412)
* -path, command-line option:            Invoking Gforth.   (line   412)
* -print-metrics, command-line option:   Invoking Gforth.   (line   538)
* -r, command-line option:               Invoking Gforth.   (line   435)
* -return-stack-size, command-line option: Invoking Gforth. (line   435)
* '-rot':                                Data stack.        (line  3850)
* -ss-greedy, command-line option:       Invoking Gforth.   (line   525)
* -ss-min-..., command-line options:     Invoking Gforth.   (line   511)
* -ss-number, command-line option:       Invoking Gforth.   (line   505)
* '-trailing':                           Memory Blocks.     (line  4375)
* '-trailing-garbage':                   Xchars and Unicode.
                                                            (line  8383)
* -v, command-line option:               Invoking Gforth.   (line   467)
* -version, command-line option:         Invoking Gforth.   (line   467)
* -vm-commit, command-line option:       Invoking Gforth.   (line   451)
* '.':                                   Simple numeric output.
                                                            (line  7688)
* '."':                                  Displaying characters and strings.
                                                            (line  7961)
* '."', how it works:                    How does that work?.
                                                            (line  3123)
* '.(':                                  Displaying characters and strings.
                                                            (line  7968)
* '.debugline':                          Debugging.         (line 10634)
* '.emacs':                              Installing gforth.el.
                                                            (line 12916)
* '.fi' files:                           Image Files.       (line 13053)
* '.gforth-history':                     Command-line editing.
                                                            (line   619)
* '.id':                                 Name token.        (line  6122)
* '.name':                               Name token.        (line  6119)
* '.path':                               General Search Paths.
                                                            (line  7444)
* '.r':                                  Simple numeric output.
                                                            (line  7703)
* '.s':                                  Examining.         (line 10504)
* '.\"':                                 Displaying characters and strings.
                                                            (line  7974)
* '/':                                   Single precision.  (line  3459)
* '/does-handler':                       Threading Words.   (line 11652)
* '/l':                                  Address arithmetic.
                                                            (line  4314)
* '/mod':                                Single precision.  (line  3463)
* '/string':                             Memory Blocks.     (line  4379)
* '/w':                                  Address arithmetic.
                                                            (line  4311)
* '0<':                                  Numeric comparison.
                                                            (line  3558)
* '0<=':                                 Numeric comparison.
                                                            (line  3560)
* '0<>':                                 Numeric comparison.
                                                            (line  3562)
* '0=':                                  Numeric comparison.
                                                            (line  3564)
* '0>':                                  Numeric comparison.
                                                            (line  3566)
* '0>=':                                 Numeric comparison.
                                                            (line  3568)
* 0x-prefix for hexadecimal numbers:     Number Conversion. (line  6624)
* '1+':                                  Single precision.  (line  3448)
* '1-':                                  Single precision.  (line  3455)
* '1/f':                                 Floating Point.    (line  3720)
* '2!':                                  Memory Access.     (line  4151)
* '2*':                                  Bitwise operations.
                                                            (line  3526)
* '2,':                                  Dictionary allocation.
                                                            (line  4058)
* '2/':                                  Bitwise operations.
                                                            (line  3532)
* '2>r':                                 Return stack.      (line  3923)
* '2@':                                  Memory Access.     (line  4147)
* '2Constant':                           Constants.         (line  5130)
* '2drop':                               Data stack.        (line  3858)
* '2dup':                                Data stack.        (line  3862)
* '2field:':                             Forth200x Structures.
                                                            (line  9256)
* '2Literal':                            Literals.          (line  6186)
* '2nip':                                Data stack.        (line  3860)
* '2over':                               Data stack.        (line  3864)
* '2r>':                                 Return stack.      (line  3925)
* '2r@':                                 Return stack.      (line  3927)
* '2rdrop':                              Return stack.      (line  3929)
* '2rot':                                Data stack.        (line  3870)
* '2swap':                               Data stack.        (line  3868)
* '2tuck':                               Data stack.        (line  3866)
* '2Variable':                           Variables.         (line  5093)
* ':':                                   Colon Definitions. (line  5205)
* ':' <1>:                               The OOF base class.
                                                            (line 10131)
* ':', passing data across:              Literals.          (line  6199)
* '::':                                  The OOF base class.
                                                            (line 10143)
* '::' <1>:                              Basic Mini-OOF Usage.
                                                            (line 10260)
* ':m':                                  Objects Glossary.  (line  9938)
* ':noname':                             Anonymous Definitions.
                                                            (line  5215)
* ';':                                   Colon Definitions. (line  5207)
* ';code':                               Code and ;code.    (line 11122)
* ';CODE' ending sequence:               programming-idef.  (line 12680)
* ';CODE', name not defined via 'CREATE': programming-ambcond.
                                                            (line 12712)
* ';CODE', processing input:             programming-idef.  (line 12683)
* ';m':                                  Objects Glossary.  (line  9942)
* ';m' usage:                            Method conveniences.
                                                            (line  9558)
* ';s':                                  Calls and returns. (line  4796)
* '<':                                   Numeric comparison.
                                                            (line  3546)
* '<#':                                  Formatted numeric output.
                                                            (line  7792)
* '<<#':                                 Formatted numeric output.
                                                            (line  7795)
* '<=':                                  Numeric comparison.
                                                            (line  3548)
* '<>':                                  Numeric comparison.
                                                            (line  3550)
* '<bind>':                              Objects Glossary.  (line  9844)
* '<compilation':                        Combined words.    (line  5954)
* '<interpretation':                     Combined words.    (line  5950)
* '<to-inst>':                           Objects Glossary.  (line  9983)
* '=':                                   Numeric comparison.
                                                            (line  3552)
* '>':                                   Numeric comparison.
                                                            (line  3554)
* '>=':                                  Numeric comparison.
                                                            (line  3556)
* '>body':                               CREATE..DOES> details.
                                                            (line  5525)
* '>BODY' of non-'CREATE'd words:        core-ambcond.      (line 12304)
* '>code-address':                       Threading Words.   (line 11622)
* '>definer':                            Threading Words.   (line 11679)
* '>does-code':                          Threading Words.   (line 11635)
* '>float':                              Line input and conversion.
                                                            (line  8271)
* '>in':                                 The Text Interpreter.
                                                            (line  6469)
* '>IN' greater than input buffer:       core-ambcond.      (line 12235)
* '>l':                                  Locals implementation.
                                                            (line  8792)
* '>name':                               Name token.        (line  6095)
* '>number':                             Line input and conversion.
                                                            (line  8258)
* '>order':                              Word Lists.        (line  6914)
* '>r':                                  Return stack.      (line  3915)
* '?':                                   Examining.         (line 10537)
* '?DO':                                 Arbitrary control structures.
                                                            (line  4666)
* '?dup':                                Data stack.        (line  3852)
* '?DUP-0=-IF':                          Arbitrary control structures.
                                                            (line  4662)
* '?DUP-IF':                             Arbitrary control structures.
                                                            (line  4657)
* '?LEAVE':                              Arbitrary control structures.
                                                            (line  4690)
* '@':                                   Memory Access.     (line  4132)
* '@local#':                             Locals implementation.
                                                            (line  8780)
* '[':                                   Literals.          (line  6169)
* '[']':                                 Execution token.   (line  6000)
* '[+LOOP]':                             Interpreter Directives.
                                                            (line  6764)
* '[?DO]':                               Interpreter Directives.
                                                            (line  6756)
* '[AGAIN]':                             Interpreter Directives.
                                                            (line  6772)
* '[BEGIN]':                             Interpreter Directives.
                                                            (line  6768)
* '[bind]':                              Objects Glossary.  (line  9850)
* '[bind]' usage:                        Class Binding.     (line  9513)
* '[Char]':                              Displaying characters and strings.
                                                            (line  8018)
* '[COMP']':                             Compilation token. (line  6053)
* '[compile]':                           Macros.            (line  6235)
* '[current]':                           Objects Glossary.  (line  9883)
* '[DO]':                                Interpreter Directives.
                                                            (line  6758)
* '[ELSE]':                              Interpreter Directives.
                                                            (line  6730)
* '[ENDIF]':                             Interpreter Directives.
                                                            (line  6743)
* '[FOR]':                               Interpreter Directives.
                                                            (line  6760)
* '[IFDEF]':                             Interpreter Directives.
                                                            (line  6746)
* '[IFUNDEF]':                           Interpreter Directives.
                                                            (line  6751)
* '[IF]':                                Interpreter Directives.
                                                            (line  6722)
* '[IF]' and 'POSTPONE':                 programming-ambcond.
                                                            (line 12717)
* '[IF]', end of the input source before matching '[ELSE]' or '[THEN]': programming-ambcond.
                                                            (line 12721)
* '[LOOP]':                              Interpreter Directives.
                                                            (line  6762)
* '[NEXT]':                              Interpreter Directives.
                                                            (line  6766)
* '[parent]':                            Objects Glossary.  (line  9961)
* '[parent]' usage:                      Class Binding.     (line  9532)
* '[REPEAT]':                            Interpreter Directives.
                                                            (line  6776)
* '[THEN]':                              Interpreter Directives.
                                                            (line  6739)
* '[to-inst]':                           Objects Glossary.  (line  9986)
* '[UNTIL]':                             Interpreter Directives.
                                                            (line  6770)
* '[WHILE]':                             Interpreter Directives.
                                                            (line  6774)
* '[]':                                  The OOF base class.
                                                            (line 10137)
* '\':                                   Comments.          (line  3393)
* '\', editing with Emacs:               Emacs and Gforth.  (line 12885)
* '\', line length in blocks:            block-idef.        (line 12355)
* '\c':                                  Declaring C Functions.
                                                            (line 10924)
* '\G':                                  Comments.          (line  3399)
* ']':                                   Literals.          (line  6172)
* ']L':                                  Literals.          (line  6180)
* '~~':                                  Debugging.         (line 10628)
* '~~', removal with Emacs:              Emacs and Gforth.  (line 12885)
* 'abort':                               Exception Handling.
                                                            (line  5005)
* 'ABORT"':                              Exception Handling.
                                                            (line  5000)
* 'ABORT"', exception abort sequence:    core-idef.         (line 12020)
* 'abs':                                 Single precision.  (line  3467)
* abstract class:                        Basic Objects Usage.
                                                            (line  9413)
* abstract class <1>:                    Basic OOF Usage.   (line 10051)
* 'accept':                              Line input and conversion.
                                                            (line  8239)
* 'ACCEPT', display after end of input:  core-idef.         (line 12016)
* 'ACCEPT', editing:                     core-idef.         (line 11962)
* 'action-of':                           Deferred Words.    (line  5759)
* 'add-lib':                             Declaring OS-level libraries.
                                                            (line 11037)
* address alignment exception:           core-ambcond.      (line 12257)
* address alignment exception, stack overflow: core-ambcond.
                                                            (line 12164)
* address arithmetic for structures:     Why explicit structure support?.
                                                            (line  8962)
* address arithmetic restrictions, ANS vs. Gforth: Memory model.
                                                            (line  3992)
* address arithmetic words:              Address arithmetic.
                                                            (line  4197)
* address of counted string:             String Formats.    (line  7921)
* address unit:                          Address arithmetic.
                                                            (line  4201)
* address unit, size in bits:            core-idef.         (line 12056)
* 'ADDRESS-UNIT-BITS':                   Address arithmetic.
                                                            (line  4308)
* 'AGAIN':                               Arbitrary control structures.
                                                            (line  4633)
* 'AHEAD':                               Arbitrary control structures.
                                                            (line  4625)
* 'Alias':                               Aliases.           (line  5800)
* aliases:                               Aliases.           (line  5773)
* 'align':                               Dictionary allocation.
                                                            (line  4072)
* 'aligned':                             Address arithmetic.
                                                            (line  4261)
* aligned addresses:                     core-idef.         (line 11952)
* alignment faults:                      core-ambcond.      (line 12257)
* alignment of addresses for types:      Address arithmetic.
                                                            (line  4224)
* alignment tutorial:                    Alignment Tutorial.
                                                            (line  1798)
* 'allocate':                            Heap Allocation.   (line  4108)
* 'allot':                               Dictionary allocation.
                                                            (line  4041)
* 'also':                                Word Lists.        (line  6920)
* 'also', too many word lists in search order: search-ambcond.
                                                            (line 12755)
* 'also-path':                           General Search Paths.
                                                            (line  7441)
* ambiguous conditions, block words:     block-ambcond.     (line 12360)
* ambiguous conditions, core words:      core-ambcond.      (line 12136)
* ambiguous conditions, double words:    double-ambcond.    (line 12396)
* ambiguous conditions, facility words:  facility-ambcond.  (line 12440)
* ambiguous conditions, file words:      file-ambcond.      (line 12506)
* ambiguous conditions, floating-point words: floating-ambcond.
                                                            (line 12565)
* ambiguous conditions, locals words:    locals-ambcond.    (line 12651)
* ambiguous conditions, programming-tools words: programming-ambcond.
                                                            (line 12699)
* ambiguous conditions, search-order words: search-ambcond. (line 12743)
* 'and':                                 Bitwise operations.
                                                            (line  3513)
* angles in trigonometric operations:    Floating Point.    (line  3731)
* ANS conformance of Gforth:             ANS conformance.   (line 11896)
* 'ans-report.fs':                       ANS Report.        (line 11817)
* 'arg':                                 OS command line arguments.
                                                            (line  8448)
* 'argc':                                OS command line arguments.
                                                            (line  8462)
* argument input source different than current input source for 'RESTORE-INPUT': core-ambcond.
                                                            (line 12243)
* argument type mismatch:                core-ambcond.      (line 12149)
* argument type mismatch, 'RESTORE-INPUT': core-ambcond.    (line 12243)
* arguments, OS command line:            OS command line arguments.
                                                            (line  8413)
* 'argv':                                OS command line arguments.
                                                            (line  8466)
* arithmetic words:                      Arithmetic.        (line  3426)
* arithmetics tutorial:                  Arithmetics Tutorial.
                                                            (line   889)
* arrays:                                CREATE.            (line  5051)
* arrays tutorial:                       Arrays and Records Tutorial.
                                                            (line  2312)
* 'asptr':                               The OOF base class.
                                                            (line 10135)
* 'asptr' <1>:                           Class Declaration. (line 10191)
* assembler:                             Assembler and Code Words.
                                                            (line 11098)
* 'assembler':                           Code and ;code.    (line 11114)
* 'ASSEMBLER', search order capability:  programming-idef.  (line 12688)
* 'assert(':                             Assertions.        (line 10684)
* 'assert-level':                        Assertions.        (line 10702)
* 'assert0(':                            Assertions.        (line 10671)
* 'assert1(':                            Assertions.        (line 10674)
* 'assert2(':                            Assertions.        (line 10677)
* 'assert3(':                            Assertions.        (line 10680)
* assertions:                            Assertions.        (line 10647)
* 'ASSUME-LIVE':                         Where are locals visible by name?.
                                                            (line  8661)
* 'at-xy':                               Terminal output.   (line  8076)
* 'AT-XY' can't be performed on user output device: facility-ambcond.
                                                            (line 12441)
* Attempt to use zero-length string as a name: core-ambcond.
                                                            (line 12229)
* au (address unit):                     Address arithmetic.
                                                            (line  4201)
* authors of Gforth:                     Origin.            (line 14045)
* auto-indentation of Forth code in Emacs: Auto-Indentation.
                                                            (line 12989)
* 'a_', stack item type:                 Notation.          (line  3328)
* backtrace:                             Error messages.    (line 11753)
* backtraces with 'gforth-fast':         Error messages.    (line 11800)
* 'base':                                Number Conversion. (line  6611)
* 'base' is not decimal ('REPRESENT', 'F.', 'FE.', 'FS.'): floating-ambcond.
                                                            (line 12585)
* 'base-execute':                        Number Conversion. (line  6607)
* basic objects usage:                   Basic Objects Usage.
                                                            (line  9399)
* batch processing with Gforth:          Invoking Gforth.   (line   549)
* 'BEGIN':                               Arbitrary control structures.
                                                            (line  4629)
* 'begin-structure':                     Forth200x Structures.
                                                            (line  9246)
* benchmarking Forth systems:            Performance.       (line 13808)
* 'Benchres':                            Performance.       (line 13888)
* 'bin':                                 General files.     (line  7280)
* 'bind':                                Objects Glossary.  (line  9841)
* 'bind' <1>:                            The OOF base class.
                                                            (line 10154)
* 'bind' usage:                          Class Binding.     (line  9517)
* 'bind'':                               Objects Glossary.  (line  9847)
* bitwise operation words:               Bitwise operations.
                                                            (line  3513)
* 'bl':                                  Displaying characters and strings.
                                                            (line  7945)
* 'blank':                               Memory Blocks.     (line  4352)
* 'blk':                                 Input Sources.     (line  6524)
* 'BLK', altering 'BLK':                 block-ambcond.     (line 12373)
* 'block':                               Blocks.            (line  7600)
* block buffers:                         Blocks.            (line  7493)
* block number invalid:                  block-ambcond.     (line 12370)
* block read not possible:               block-ambcond.     (line 12361)
* block transfer, I/O exception:         block-ambcond.     (line 12366)
* block words, ambiguous conditions:     block-ambcond.     (line 12360)
* block words, implementation-defined options: block-idef.  (line 12350)
* block words, other system documentation: block-other.     (line 12384)
* block words, system documentation:     The optional Block word set.
                                                            (line 12347)
* 'block-included':                      Blocks.            (line  7661)
* 'block-offset':                        Blocks.            (line  7579)
* 'block-position':                      Blocks.            (line  7589)
* blocks:                                Blocks.            (line  7459)
* blocks file:                           Blocks.            (line  7482)
* blocks files, use with Emacs:          Blocks Files.      (line 13025)
* blocks in files:                       file-idef.         (line 12494)
* 'blocks.fb':                           Blocks.            (line  7488)
* Boolean flags:                         Boolean Flags.     (line  3406)
* 'bootmessage':                         Modifying the Startup Sequence.
                                                            (line 13378)
* 'bound':                               The OOF base class.
                                                            (line 10156)
* 'bounds':                              Memory Blocks.     (line  4383)
* 'break"':                              Singlestep Debugger.
                                                            (line 10783)
* 'break:':                              Singlestep Debugger.
                                                            (line 10781)
* 'broken-pipe-error':                   Pipes.             (line  8314)
* 'buffer':                              Blocks.            (line  7607)
* bug reporting:                         Bugs.              (line 14018)
* 'bye':                                 Leaving Gforth.    (line   581)
* 'bye' during 'gforthmi':               gforthmi.          (line 13236)
* C function pointers to Forth words:    Callbacks.         (line 11044)
* C function pointers, calling from Forth: Calling C function pointers.
                                                            (line 10941)
* C functions, calls to:                 Calling C Functions.
                                                            (line 10794)
* C functions, declarations:             Declaring C Functions.
                                                            (line 10854)
* C interface:                           C Interface.       (line 10788)
* 'c!':                                  Memory Access.     (line  4144)
* 'C"':                                  Displaying characters and strings.
                                                            (line  8009)
* 'c,':                                  Dictionary allocation.
                                                            (line  4048)
* 'c', stack item type:                  Notation.          (line  3314)
* C, using C for the engine:             Portability.       (line 13406)
* 'c-function':                          Declaring C Functions.
                                                            (line 10927)
* 'c-library':                           Defining library interfaces.
                                                            (line 11001)
* 'c-library-name':                      Defining library interfaces.
                                                            (line 10998)
* 'c@':                                  Memory Access.     (line  4141)
* 'call-c':                              Low-Level C Interface Words.
                                                            (line 11090)
* Callback functions written in Forth:   Callbacks.         (line 11044)
* calling a definition:                  Calls and returns. (line  4755)
* calling C functions:                   Calling C Functions.
                                                            (line 10794)
* 'case':                                Arbitrary control structures.
                                                            (line  4705)
* 'CASE' control structure:              Selection.         (line  4445)
* case sensitivity:                      Case insensitivity.
                                                            (line  3354)
* case-sensitivity characteristics:      core-idef.         (line 12101)
* case-sensitivity for name lookup:      core-idef.         (line 11979)
* 'catch':                               Exception Handling.
                                                            (line  4846)
* 'catch' and backtraces:                Error messages.    (line 11789)
* 'catch' and 'this':                    Objects Implementation.
                                                            (line  9764)
* 'catch' in 'm: ... ;m':                Method conveniences.
                                                            (line  9561)
* 'cell':                                Address arithmetic.
                                                            (line  4258)
* cell size:                             core-idef.         (line 12081)
* 'cell%':                               Structure Glossary.
                                                            (line  9190)
* 'cell+':                               Address arithmetic.
                                                            (line  4255)
* cell-aligned addresses:                core-idef.         (line 11952)
* 'cells':                               Address arithmetic.
                                                            (line  4252)
* CFA:                                   Execution token.   (line  6022)
* 'cfalign':                             Dictionary allocation.
                                                            (line  4091)
* 'cfaligned':                           Address arithmetic.
                                                            (line  4304)
* 'cfield:':                             Forth200x Structures.
                                                            (line  9252)
* changing the compilation word list (during compilation): search-ambcond.
                                                            (line 12744)
* 'char':                                Displaying characters and strings.
                                                            (line  8014)
* char size:                             core-idef.         (line 12084)
* 'char%':                               Structure Glossary.
                                                            (line  9192)
* 'char+':                               Address arithmetic.
                                                            (line  4249)
* character editing of 'ACCEPT' and 'EXPECT': core-idef.    (line 11962)
* character set:                         core-idef.         (line 11969)
* character strings - compiling and displaying: Displaying characters and strings.
                                                            (line  7942)
* character strings - formats:           String Formats.    (line  7918)
* character strings - moving and copying: Memory Blocks.    (line  4320)
* character-aligned address requirements: core-idef.        (line 11974)
* character-set extensions and matching of names: core-idef.
                                                            (line 11979)
* characters - compiling and displaying: Displaying characters and strings.
                                                            (line  7942)
* characters tutorial:                   Characters and Strings Tutorial.
                                                            (line  1749)
* 'chars':                               Address arithmetic.
                                                            (line  4246)
* child class:                           Object-Oriented Terminology.
                                                            (line  9339)
* child words:                           User-defined Defining Words.
                                                            (line  5344)
* class:                                 Object-Oriented Terminology.
                                                            (line  9306)
* 'class':                               Objects Glossary.  (line  9853)
* 'class' <1>:                           The OOF base class.
                                                            (line 10108)
* 'class' <2>:                           Basic Mini-OOF Usage.
                                                            (line 10248)
* class binding:                         Class Binding.     (line  9504)
* class binding as optimization:         Class Binding.     (line  9537)
* class binding, alternative to:         Class Binding.     (line  9519)
* class binding, implementation:         Objects Implementation.
                                                            (line  9760)
* class declaration:                     Class Declaration. (line 10181)
* class definition, restrictions:        Basic Objects Usage.
                                                            (line  9451)
* class definition, restrictions <1>:    Basic OOF Usage.   (line 10084)
* class implementation:                  Class Implementation.
                                                            (line 10222)
* class implementation and representation: Objects Implementation.
                                                            (line  9745)
* class scoping implementation:          Objects Implementation.
                                                            (line  9779)
* 'class' usage:                         Basic Objects Usage.
                                                            (line  9401)
* 'class' usage <1>:                     Basic OOF Usage.   (line 10039)
* 'class->map':                          Objects Glossary.  (line  9857)
* 'class-inst-size':                     Objects Glossary.  (line  9862)
* 'class-inst-size' discussion:          Creating objects.  (line  9476)
* 'class-override!':                     Objects Glossary.  (line  9866)
* 'class-previous':                      Objects Glossary.  (line  9869)
* 'class;':                              Class Declaration. (line 10217)
* 'class;' usage:                        Basic OOF Usage.   (line 10039)
* 'class>order':                         Objects Glossary.  (line  9873)
* 'class?':                              The OOF base class.
                                                            (line 10112)
* classes and scoping:                   Classes and Scoping.
                                                            (line  9617)
* clear screen:                          Terminal output.   (line  8092)
* 'clear-libs':                          Declaring OS-level libraries.
                                                            (line 11034)
* 'clear-path':                          General Search Paths.
                                                            (line  7438)
* 'clearstack':                          Examining.         (line 10529)
* 'clearstacks':                         Examining.         (line 10532)
* clock tick duration:                   facility-idef.     (line 12427)
* 'close-file':                          General files.     (line  7291)
* 'close-pipe':                          Pipes.             (line  8305)
* 'cmove':                               Memory Blocks.     (line  4339)
* 'cmove>':                              Memory Blocks.     (line  4344)
* 'code':                                Code and ;code.    (line 11118)
* code address:                          Threading Words.   (line 11598)
* code address <1>:                      Threading Words.   (line 11614)
* 'CODE' ending sequence:                programming-idef.  (line 12680)
* code examination:                      Examining.         (line 10502)
* code field address:                    Execution token.   (line  6022)
* code field address <1>:                Threading Words.   (line 11614)
* code words:                            Assembler and Code Words.
                                                            (line 11098)
* code words, portable:                  Code and ;code.    (line 11190)
* 'CODE', processing input:              programming-idef.  (line 12683)
* 'code-address!':                       Threading Words.   (line 11625)
* colon definitions:                     Colon Definitions. (line  5194)
* colon definitions <1>:                 Anonymous Definitions.
                                                            (line  5212)
* colon definitions, tutorial:           Colon Definitions Tutorial.
                                                            (line  1044)
* colon-sys, passing data across ':':    Literals.          (line  6199)
* combined words:                        Combined words.    (line  5860)
* command line arguments, OS:            OS command line arguments.
                                                            (line  8413)
* command-line editing:                  Command-line editing.
                                                            (line   587)
* command-line options:                  Invoking Gforth.   (line   375)
* comment editing commands:              Emacs and Gforth.  (line 12885)
* comments:                              Comments.          (line  3383)
* comments tutorial:                     Comments Tutorial. (line  1015)
* 'common-list':                         Locals implementation.
                                                            (line  8862)
* 'COMP'':                               Compilation token. (line  6056)
* 'comp-i.fs':                           gforthmi.          (line 13198)
* comp.lang.forth:                       Forth-related information.
                                                            (line 14105)
* 'compare':                             Memory Blocks.     (line  4355)
* comparison of object models:           Comparison with other object models.
                                                            (line 10429)
* comparison tutorial:                   Flags and Comparisons Tutorial.
                                                            (line  1366)
* compilation semantics:                 How does that work?.
                                                            (line  3037)
* compilation semantics <1>:             Interpretation and Compilation Semantics.
                                                            (line  5812)
* compilation semantics tutorial:        Interpretation and Compilation Semantics and Immediacy Tutorial.
                                                            (line  2008)
* compilation token:                     Compilation token. (line  6035)
* compilation tokens, tutorial:          Compilation Tokens Tutorial.
                                                            (line  2474)
* compilation word list:                 Word Lists.        (line  6866)
* compilation word list, change before definition ends: search-ambcond.
                                                            (line 12744)
* 'compilation>':                        Combined words.    (line  5952)
* compile state:                         The Text Interpreter.
                                                            (line  6357)
* 'compile,':                            Macros.            (line  6299)
* 'compile-lp+!':                        Locals implementation.
                                                            (line  8802)
* 'compile-only':                        Interpretation and Compilation Semantics.
                                                            (line  5835)
* compile-only words:                    Interpretation and Compilation Semantics.
                                                            (line  5829)
* compiling compilation semantics:       Macros.            (line  6215)
* compiling words:                       Compiling words.   (line  6128)
* conditional compilation:               Interpreter Directives.
                                                            (line  6698)
* conditionals, tutorial:                Conditional execution Tutorial.
                                                            (line  1320)
* 'const-does>':                         Const-does>.       (line  5641)
* 'Constant':                            Constants.         (line  5125)
* constants:                             Constants.         (line  5107)
* 'construct':                           Objects Glossary.  (line  9876)
* 'construct' discussion:                Creating objects.  (line  9470)
* 'context':                             Word Lists.        (line  6990)
* context-sensitive help:                Emacs and Gforth.  (line 12908)
* contiguous regions and address arithmetic: Address arithmetic.
                                                            (line  4212)
* contiguous regions and heap allocation: Heap Allocation.  (line  4098)
* contiguous regions in dictionary allocation: Dictionary allocation.
                                                            (line  4020)
* contiguous regions, ANS vs. Gforth:    Memory model.      (line  3992)
* contributors to Gforth:                Origin.            (line 14045)
* control characters as delimiters:      core-idef.         (line 11995)
* control structures:                    Control Structures.
                                                            (line  4395)
* control structures for selection:      Selection.         (line  4408)
* control structures programming style:  Arbitrary control structures.
                                                            (line  4719)
* control structures, user-defined:      Arbitrary control structures.
                                                            (line  4613)
* control-flow stack:                    Arbitrary control structures.
                                                            (line  4613)
* control-flow stack items, locals information: Locals implementation.
                                                            (line  8852)
* control-flow stack underflow:          programming-ambcond.
                                                            (line 12703)
* control-flow stack, format:            core-idef.         (line 12003)
* 'convert':                             Line input and conversion.
                                                            (line  8281)
* convertin strings to numbers:          Line input and conversion.
                                                            (line  8234)
* core words, ambiguous conditions:      core-ambcond.      (line 12136)
* core words, implementation-defined options: core-idef.    (line 11951)
* core words, other system documentation: core-other.       (line 12316)
* core words, system documentation:      The Core Words.    (line 11948)
* 'count':                               String Formats.    (line  7931)
* counted loops:                         Counted Loops.     (line  4499)
* counted loops with negative increment: Counted Loops.     (line  4567)
* counted string:                        String Formats.    (line  7921)
* counted string, maximum size:          core-idef.         (line 12029)
* counted strings:                       String Formats.    (line  7918)
* 'cputime':                             Keeping track of Time.
                                                            (line 11728)
* 'cr':                                  Displaying characters and strings.
                                                            (line  7984)
* 'Create':                              CREATE.            (line  5030)
* 'CREATE' ... 'DOES>':                  User-defined Defining Words.
                                                            (line  5333)
* 'CREATE' ... 'DOES>', applications:    CREATE..DOES> applications.
                                                            (line  5445)
* 'CREATE' ... 'DOES>', details:         CREATE..DOES> details.
                                                            (line  5482)
* 'CREATE' and alignment:                Address arithmetic.
                                                            (line  4239)
* 'create-file':                         General files.     (line  7289)
* 'create-interpret/compile':            Combined words.    (line  5946)
* create...does> tutorial:               Defining Words Tutorial.
                                                            (line  2223)
* creating objects:                      Creating objects.  (line  9470)
* cross-compiler:                        cross.fs.          (line 13251)
* cross-compiler <1>:                    Cross Compiler.    (line 13901)
* 'cross.fs':                            cross.fs.          (line 13251)
* 'cross.fs' <1>:                        Cross Compiler.    (line 13901)
* 'CS-PICK':                             Arbitrary control structures.
                                                            (line  4635)
* 'CS-PICK', fewer than u+1 items on the control flow-stack: programming-ambcond.
                                                            (line 12703)
* 'CS-ROLL':                             Arbitrary control structures.
                                                            (line  4637)
* 'CS-ROLL', fewer than u+1 items on the control flow-stack: programming-ambcond.
                                                            (line 12703)
* CT (compilation token):                Compilation token. (line  6035)
* CT, tutorial:                          Compilation Tokens Tutorial.
                                                            (line  2474)
* 'current':                             Word Lists.        (line  6987)
* 'current'':                            Objects Glossary.  (line  9880)
* 'current-interface':                   Objects Glossary.  (line  9886)
* 'current-interface' discussion:        Objects Implementation.
                                                            (line  9745)
* currying:                              CREATE..DOES> applications.
                                                            (line  5466)
* cursor control:                        Displaying characters and strings.
                                                            (line  7990)
* cursor positioning:                    Terminal output.   (line  8074)
* 'c_', stack item type:                 Notation.          (line  3330)
* 'd+':                                  Double precision.  (line  3498)
* 'd', stack item type:                  Notation.          (line  3322)
* 'd-':                                  Double precision.  (line  3500)
* 'd.':                                  Simple numeric output.
                                                            (line  7713)
* 'd.r':                                 Simple numeric output.
                                                            (line  7721)
* 'd0<':                                 Numeric comparison.
                                                            (line  3599)
* 'd0<=':                                Numeric comparison.
                                                            (line  3601)
* 'd0<>':                                Numeric comparison.
                                                            (line  3603)
* 'd0=':                                 Numeric comparison.
                                                            (line  3605)
* 'd0>':                                 Numeric comparison.
                                                            (line  3607)
* 'd0>=':                                Numeric comparison.
                                                            (line  3609)
* 'd2*':                                 Bitwise operations.
                                                            (line  3529)
* 'd2/':                                 Bitwise operations.
                                                            (line  3536)
* 'd<':                                  Numeric comparison.
                                                            (line  3587)
* 'd<=':                                 Numeric comparison.
                                                            (line  3589)
* 'd<>':                                 Numeric comparison.
                                                            (line  3591)
* 'd=':                                  Numeric comparison.
                                                            (line  3593)
* 'd>':                                  Numeric comparison.
                                                            (line  3595)
* 'd>=':                                 Numeric comparison.
                                                            (line  3597)
* 'd>f':                                 Floating Point.    (line  3666)
* 'D>F', d cannot be presented precisely as a float: floating-ambcond.
                                                            (line 12597)
* 'd>s':                                 Double precision.  (line  3496)
* 'D>S', d out of range of n:            double-ambcond.    (line 12397)
* 'dabs':                                Double precision.  (line  3504)
* data examination:                      Examining.         (line 10502)
* data space - reserving some:           Dictionary allocation.
                                                            (line  4016)
* data space available:                  core-other.        (line 12326)
* data space containing definitions gets de-allocated: core-ambcond.
                                                            (line 12253)
* data space pointer not properly aligned, ',', 'C,': core-ambcond.
                                                            (line 12265)
* data space read/write with incorrect alignment: core-ambcond.
                                                            (line 12257)
* data stack:                            Stack Manipulation.
                                                            (line  3820)
* data stack manipulation words:         Data stack.        (line  3833)
* data-relocatable image files:          Data-Relocatable Image Files.
                                                            (line 13174)
* data-space, read-only regions:         core-idef.         (line 12071)
* 'dbg':                                 Singlestep Debugger.
                                                            (line 10779)
* debug tracer editing commands:         Emacs and Gforth.  (line 12885)
* debugging:                             Debugging.         (line 10606)
* debugging output, finding the source location in Emacs: Emacs and Gforth.
                                                            (line 12900)
* debugging Singlestep:                  Singlestep Debugger.
                                                            (line 10722)
* 'dec.':                                Simple numeric output.
                                                            (line  7692)
* 'decimal':                             Number Conversion. (line  6620)
* declaring C functions:                 Declaring C Functions.
                                                            (line 10854)
* decompilation tutorial:                Decompilation Tutorial.
                                                            (line  1076)
* default type of locals:                Gforth locals.     (line  8533)
* 'Defer':                               Deferred Words.    (line  5744)
* 'defer':                               Class Declaration. (line 10196)
* 'defer!':                              Deferred Words.    (line  5748)
* 'defer@':                              Deferred Words.    (line  5755)
* deferred words:                        Deferred Words.    (line  5673)
* 'defers':                              Deferred Words.    (line  5762)
* definer:                               Threading Words.   (line 11676)
* 'definer!':                            Threading Words.   (line 11684)
* 'defines':                             Basic Mini-OOF Usage.
                                                            (line 10254)
* defining defining words:               User-defined Defining Words.
                                                            (line  5333)
* defining words:                        Defining Words.    (line  5011)
* defining words tutorial:               Defining Words Tutorial.
                                                            (line  2223)
* defining words with arbitrary semantics combinations: Combined words.
                                                            (line  5916)
* defining words without name:           Anonymous Definitions.
                                                            (line  5212)
* defining words, name given in a string: Supplying names.  (line  5258)
* defining words, simple:                CREATE.            (line  5017)
* defining words, user-defined:          User-defined Defining Words.
                                                            (line  5279)
* definition:                            Introducing the Text Interpreter.
                                                            (line  2644)
* 'definitions':                         Word Lists.        (line  6884)
* 'definitions' <1>:                     The OOF base class.
                                                            (line 10110)
* definitions, tutorial:                 Colon Definitions Tutorial.
                                                            (line  1044)
* 'delete-file':                         General files.     (line  7293)
* 'depth':                               Examining.         (line 10521)
* depth changes during interpretation:   Stack depth changes.
                                                            (line 11855)
* 'depth-changes.fs':                    Stack depth changes.
                                                            (line 11855)
* design of stack effects, tutorial:     Designing the stack effect Tutorial.
                                                            (line  1238)
* 'dest', control-flow stack item:       Arbitrary control structures.
                                                            (line  4618)
* 'df!':                                 Memory Access.     (line  4172)
* 'df@':                                 Memory Access.     (line  4168)
* 'df@' or 'df!' used with an address that is not double-float aligned: floating-ambcond.
                                                            (line 12566)
* 'dfalign':                             Dictionary allocation.
                                                            (line  4084)
* 'dfaligned':                           Address arithmetic.
                                                            (line  4296)
* 'dffield:':                            Forth200x Structures.
                                                            (line  9262)
* 'dfloat%':                             Structure Glossary.
                                                            (line  9194)
* 'dfloat+':                             Address arithmetic.
                                                            (line  4293)
* 'dfloats':                             Address arithmetic.
                                                            (line  4289)
* 'df_', stack item type:                Notation.          (line  3335)
* 'dict-new':                            Objects Glossary.  (line  9889)
* 'dict-new' discussion:                 Creating objects.  (line  9470)
* dictionary:                            The Text Interpreter.
                                                            (line  6374)
* dictionary in persistent form:         Image Files.       (line 13053)
* dictionary overflow:                   core-ambcond.      (line 12180)
* dictionary size default:               Stack and Dictionary Sizes.
                                                            (line 13265)
* digits > 35:                           core-idef.         (line 12012)
* direct threaded inner interpreter:     Threading.         (line 13464)
* disassembler, general:                 Common Disassembler.
                                                            (line 11250)
* 'discode':                             Common Disassembler.
                                                            (line 11253)
* 'dispose':                             The OOF base class.
                                                            (line 10122)
* dividing by zero:                      core-ambcond.      (line 12159)
* dividing by zero, floating-point:      floating-ambcond.  (line 12600)
* Dividing classes:                      Dividing classes.  (line  9643)
* division rounding:                     core-idef.         (line 12111)
* division with potentially negative operands: Arithmetic.  (line  3426)
* 'dmax':                                Double precision.  (line  3508)
* 'dmin':                                Double precision.  (line  3506)
* 'dnegate':                             Double precision.  (line  3502)
* 'DO':                                  Arbitrary control structures.
                                                            (line  4676)
* 'DO' loops:                            Counted Loops.     (line  4499)
* 'docol:':                              Threading Words.   (line 11658)
* 'docon:':                              Threading Words.   (line 11661)
* 'dodefer:':                            Threading Words.   (line 11670)
* 'dodoes' routine:                      DOES>.             (line 13653)
* 'does-code!':                          Threading Words.   (line 11644)
* 'does-handler!':                       Threading Words.   (line 11648)
* 'DOES>':                               CREATE..DOES> details.
                                                            (line  5482)
* 'DOES>' implementation:                DOES>.             (line 13653)
* 'DOES>' in a separate definition:      CREATE..DOES> details.
                                                            (line  5484)
* 'DOES>' in interpretation state:       CREATE..DOES> details.
                                                            (line  5507)
* 'DOES>' of non-'CREATE'd words:        core-ambcond.      (line 12306)
* does> tutorial:                        Defining Words Tutorial.
                                                            (line  2223)
* DOES>, visibility of current definition: core-idef.       (line 12131)
* 'does>'-code:                          Threading Words.   (line 11628)
* 'DOES>'-code:                          DOES>.             (line 13653)
* 'does>'-handler:                       Threading Words.   (line 11628)
* 'DOES>'-parts, stack effect:           User-defined Defining Words.
                                                            (line  5422)
* 'dofield:':                            Threading Words.   (line 11673)
* 'DONE':                                Arbitrary control structures.
                                                            (line  4694)
* double precision arithmetic words:     Double precision.  (line  3479)
* double words, ambiguous conditions:    double-ambcond.    (line 12396)
* double words, system documentation:    The optional Double Number word set.
                                                            (line 12393)
* 'double%':                             Structure Glossary.
                                                            (line  9196)
* double-cell numbers, input format:     Number Conversion. (line  6555)
* doubly indirect threaded code:         gforthmi.          (line 13236)
* 'douser:':                             Threading Words.   (line 11667)
* 'dovar:':                              Threading Words.   (line 11664)
* 'dpl':                                 Number Conversion. (line  6600)
* 'drop':                                Data stack.        (line  3833)
* 'du<':                                 Numeric comparison.
                                                            (line  3611)
* 'du<=':                                Numeric comparison.
                                                            (line  3613)
* 'du>':                                 Numeric comparison.
                                                            (line  3615)
* 'du>=':                                Numeric comparison.
                                                            (line  3617)
* 'dump':                                Examining.         (line 10540)
* 'dup':                                 Data stack.        (line  3837)
* duration of a system clock tick:       facility-idef.     (line 12427)
* dynamic allocation of memory:          Heap Allocation.   (line  4098)
* Dynamic superinstructions with replication: Dynamic Superinstructions.
                                                            (line 13561)
* Dynamically linked libraries in C interface: Declaring OS-level libraries.
                                                            (line 11010)
* 'early':                               Class Declaration. (line 10201)
* early binding:                         Class Binding.     (line  9504)
* 'edit-line':                           Line input and conversion.
                                                            (line  8246)
* editing in 'ACCEPT' and 'EXPECT':      core-idef.         (line 11962)
* eforth performance:                    Performance.       (line 13824)
* 'ekey':                                Single-key input.  (line  8135)
* 'EKEY', encoding of keyboard events:   facility-idef.     (line 12421)
* 'ekey>char':                           Single-key input.  (line  8138)
* 'ekey>fkey':                           Single-key input.  (line  8141)
* 'ekey?':                               Single-key input.  (line  8145)
* elements of a Forth system:            Review - elements of a Forth system.
                                                            (line  3185)
* 'ELSE':                                Arbitrary control structures.
                                                            (line  4647)
* Emacs and Gforth:                      Emacs and Gforth.  (line 12885)
* 'emit':                                Displaying characters and strings.
                                                            (line  7954)
* 'EMIT' and non-graphic characters:     core-idef.         (line 11958)
* 'emit-file':                           General files.     (line  7319)
* 'empty-buffer':                        Blocks.            (line  7620)
* 'empty-buffers':                       Blocks.            (line  7616)
* 'end-c-library':                       Defining library interfaces.
                                                            (line 11004)
* 'end-class':                           Objects Glossary.  (line  9892)
* 'end-class' <1>:                       Basic Mini-OOF Usage.
                                                            (line 10251)
* 'end-class' usage:                     Basic Objects Usage.
                                                            (line  9401)
* 'end-class-noname':                    Objects Glossary.  (line  9896)
* 'end-code':                            Code and ;code.    (line 11120)
* 'end-interface':                       Objects Glossary.  (line  9899)
* 'end-interface' usage:                 Object Interfaces. (line  9700)
* 'end-interface-noname':                Objects Glossary.  (line  9903)
* 'end-methods':                         Objects Glossary.  (line  9906)
* 'end-struct':                          Structure Glossary.
                                                            (line  9198)
* 'end-struct' usage:                    Structure Usage.   (line  9035)
* 'end-structure':                       Forth200x Structures.
                                                            (line  9248)
* 'endcase':                             Arbitrary control structures.
                                                            (line  4707)
* 'ENDIF':                               Arbitrary control structures.
                                                            (line  4655)
* endless loop:                          Simple Loops.      (line  4490)
* 'endof':                               Arbitrary control structures.
                                                            (line  4711)
* 'endscope':                            Where are locals visible by name?.
                                                            (line  8552)
* 'endtry':                              Exception Handling.
                                                            (line  4890)
* 'endtry-iferror':                      Exception Handling.
                                                            (line  4969)
* 'endwith':                             The OOF base class.
                                                            (line 10176)
* engine:                                Engine.            (line 13385)
* engine performance:                    Performance.       (line 13808)
* engine portability:                    Portability.       (line 13399)
* 'engine.s':                            Produced code.     (line 13800)
* engines, gforth vs. gforth-fast vs. gforth-itc: Direct or Indirect Threaded?.
                                                            (line 13545)
* environment variables:                 Environment variables.
                                                            (line   642)
* environment variables <1>:             gforthmi.          (line 13236)
* environment wordset:                   Notation.          (line  3293)
* 'environment-wordlist':                Environmental Queries.
                                                            (line  7118)
* 'environment?':                        Environmental Queries.
                                                            (line  7113)
* 'ENVIRONMENT?' string length, maximum: core-idef.         (line 12039)
* environmental queries:                 Environmental Queries.
                                                            (line  7095)
* environmental restrictions:            ANS conformance.   (line 11929)
* equality of floats:                    Floating Point.    (line  3769)
* 'erase':                               Memory Blocks.     (line  4336)
* error messages:                        Error messages.    (line 11753)
* error output, finding the source location in Emacs: Emacs and Gforth.
                                                            (line 12900)
* 'etags.fs':                            Emacs Tags.        (line 12936)
* 'evaluate':                            Input Sources.     (line  6541)
* examining data and code:               Examining.         (line 10502)
* 'exception':                           Exception Handling.
                                                            (line  4827)
* exception abort sequence of 'ABORT"':  core-idef.         (line 12020)
* exception when including source:       file-idef.         (line 12478)
* exception words, implementation-defined options: exception-idef.
                                                            (line 12405)
* exception words, system documentation: The optional Exception word set.
                                                            (line 12402)
* exceptions:                            Exception Handling.
                                                            (line  4802)
* exceptions tutorial:                   Exceptions Tutorial.
                                                            (line  2164)
* executable image file:                 Running Image Files.
                                                            (line 13290)
* 'execute':                             Execution token.   (line  6026)
* 'execute-parsing':                     The Input Stream.  (line  6840)
* 'execute-parsing-file':                The Input Stream.  (line  6850)
* executing code on startup:             Invoking Gforth.   (line   549)
* execution semantics:                   Interpretation and Compilation Semantics.
                                                            (line  5817)
* execution token:                       Introducing the Text Interpreter.
                                                            (line  2644)
* execution token <1>:                   Execution token.   (line  5979)
* execution token of last defined word:  Anonymous Definitions.
                                                            (line  5234)
* execution token of words with undefined execution semantics: core-ambcond.
                                                            (line 12154)
* execution tokens tutorial:             Execution Tokens Tutorial.
                                                            (line  2078)
* exercises:                             Exercises.         (line  3253)
* 'EXIT':                                Calls and returns. (line  4791)
* 'exit' in 'm: ... ;m':                 Method conveniences.
                                                            (line  9561)
* 'exitm':                               Objects Glossary.  (line  9910)
* 'exitm' discussion:                    Method conveniences.
                                                            (line  9561)
* 'expect':                              Line input and conversion.
                                                            (line  8284)
* 'EXPECT', display after end of input:  core-idef.         (line 12016)
* 'EXPECT', editing:                     core-idef.         (line 11962)
* explicit register declarations:        Portability.       (line 13433)
* exponent too big for conversion ('DF!', 'DF@', 'SF!', 'SF@'): floating-ambcond.
                                                            (line 12605)
* extended records:                      Structure Usage.   (line  9080)
* 'f!':                                  Memory Access.     (line  4157)
* 'f!' used with an address that is not float aligned: floating-ambcond.
                                                            (line 12570)
* 'f*':                                  Floating Point.    (line  3674)
* 'f**':                                 Floating Point.    (line  3693)
* 'f+':                                  Floating Point.    (line  3670)
* 'f,':                                  Dictionary allocation.
                                                            (line  4051)
* 'f', stack item type:                  Notation.          (line  3312)
* 'f-':                                  Floating Point.    (line  3672)
* 'f.':                                  Simple numeric output.
                                                            (line  7730)
* 'f.rdp':                               Simple numeric output.
                                                            (line  7742)
* 'f.s':                                 Examining.         (line 10509)
* 'f/':                                  Floating Point.    (line  3676)
* 'f0<':                                 Floating Point.    (line  3797)
* 'f0<=':                                Floating Point.    (line  3799)
* 'f0<>':                                Floating Point.    (line  3801)
* 'f0=':                                 Floating Point.    (line  3803)
* 'f0>':                                 Floating Point.    (line  3805)
* 'f0>=':                                Floating Point.    (line  3807)
* 'f2*':                                 Floating Point.    (line  3714)
* 'f2/':                                 Floating Point.    (line  3717)
* 'f83name', stack item type:            Notation.          (line  3345)
* 'f<':                                  Floating Point.    (line  3789)
* 'f<=':                                 Floating Point.    (line  3791)
* 'f<>':                                 Floating Point.    (line  3787)
* 'f=':                                  Floating Point.    (line  3785)
* 'f>':                                  Floating Point.    (line  3793)
* 'f>=':                                 Floating Point.    (line  3795)
* 'f>buf-rdp':                           Formatted numeric output.
                                                            (line  7841)
* 'f>d':                                 Floating Point.    (line  3668)
* 'F>D', integer part of float cannot be represented by d: floating-ambcond.
                                                            (line 12628)
* 'f>l':                                 Locals implementation.
                                                            (line  8794)
* 'f>str-rdp':                           Formatted numeric output.
                                                            (line  7835)
* 'f@':                                  Memory Access.     (line  4154)
* 'f@' used with an address that is not float aligned: floating-ambcond.
                                                            (line 12570)
* 'f@local#':                            Locals implementation.
                                                            (line  8782)
* 'fabs':                                Floating Point.    (line  3680)
* facility words, ambiguous conditions:  facility-ambcond.  (line 12440)
* facility words, implementation-defined options: facility-idef.
                                                            (line 12420)
* facility words, system documentation:  The optional Facility word set.
                                                            (line 12417)
* 'facos':                               Floating Point.    (line  3745)
* 'FACOS', |float|>1:                    floating-ambcond.  (line 12625)
* 'facosh':                              Floating Point.    (line  3761)
* 'FACOSH', float<1:                     floating-ambcond.  (line 12609)
* factoring:                             Introduction.      (line  2594)
* factoring similar colon definitions:   CREATE..DOES> applications.
                                                            (line  5447)
* factoring tutorial:                    Factoring Tutorial.
                                                            (line  1214)
* 'falign':                              Dictionary allocation.
                                                            (line  4076)
* 'faligned':                            Address arithmetic.
                                                            (line  4274)
* 'falog':                               Floating Point.    (line  3711)
* 'false':                               Boolean Flags.     (line  3414)
* fam (file access method):              General files.     (line  7274)
* 'fasin':                               Floating Point.    (line  3743)
* 'FASIN', |float|>1:                    floating-ambcond.  (line 12625)
* 'fasinh':                              Floating Point.    (line  3759)
* 'FASINH', float<0:                     floating-ambcond.  (line 12620)
* 'fatan':                               Floating Point.    (line  3747)
* 'fatan2':                              Floating Point.    (line  3749)
* 'FATAN2', both arguments are equal to zero: floating-ambcond.
                                                            (line 12588)
* 'fatanh':                              Floating Point.    (line  3763)
* 'FATANH', |float|>1:                   floating-ambcond.  (line 12625)
* 'fconstant':                           Constants.         (line  5132)
* 'fcos':                                Floating Point.    (line  3736)
* 'fcosh':                               Floating Point.    (line  3755)
* 'fdepth':                              Examining.         (line 10525)
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
                                                            (line 14121)
* 'fdrop':                               Floating point stack.
                                                            (line  3888)
* 'fdup':                                Floating point stack.
                                                            (line  3892)
* 'fe.':                                 Simple numeric output.
                                                            (line  7734)
* 'fexp':                                Floating Point.    (line  3698)
* 'fexpm1':                              Floating Point.    (line  3700)
* 'ffield:':                             Forth200x Structures.
                                                            (line  9258)
* 'field':                               Structure Glossary.
                                                            (line  9203)
* field naming convention:               Structure Naming Convention.
                                                            (line  9127)
* 'field' usage:                         Structure Usage.   (line  9035)
* 'field' usage in class definition:     Basic Objects Usage.
                                                            (line  9422)
* 'field:':                              Forth200x Structures.
                                                            (line  9254)
* file access methods used:              file-idef.         (line 12452)
* file exceptions:                       file-idef.         (line 12459)
* file input nesting, maximum depth:     file-idef.         (line 12487)
* file line terminator:                  file-idef.         (line 12463)
* file name format:                      file-idef.         (line 12468)
* file search path:                      Search Paths.      (line  7382)
* file words, ambiguous conditions:      file-ambcond.      (line 12506)
* file words, implementation-defined options: file-idef.    (line 12451)
* file words, system documentation:      The optional File-Access word set.
                                                            (line 12448)
* file-handling:                         General files.     (line  7271)
* 'file-position':                       General files.     (line  7325)
* 'file-size':                           General files.     (line  7329)
* 'file-status':                         General files.     (line  7323)
* 'FILE-STATUS', returned information:   file-idef.         (line 12472)
* filenames in assertion output:         Assertions.        (line 10711)
* filenames in '~~' output:              Debugging.         (line 10639)
* files:                                 Files.             (line  7187)
* files containing blocks:               file-idef.         (line 12494)
* files containing Forth code, tutorial: Using files for Forth code Tutorial.
                                                            (line   976)
* files tutorial:                        Files Tutorial.    (line  1905)
* 'fill':                                Memory Blocks.     (line  4349)
* 'find':                                Word Lists.        (line  6939)
* 'find-name':                           Name token.        (line  6087)
* first definition:                      Your first definition.
                                                            (line  2896)
* first field optimization:              Structure Usage.   (line  9100)
* first field optimization, implementation: Structure Implementation.
                                                            (line  9161)
* flags on the command line:             Invoking Gforth.   (line   375)
* flags tutorial:                        Flags and Comparisons Tutorial.
                                                            (line  1366)
* flavours of locals:                    Gforth locals.     (line  8522)
* 'FLiteral':                            Literals.          (line  6190)
* 'fln':                                 Floating Point.    (line  3703)
* 'FLN', float=<0:                       floating-ambcond.  (line 12616)
* 'flnp1':                               Floating Point.    (line  3705)
* 'FLNP1', float=<-1:                    floating-ambcond.  (line 12612)
* 'float':                               Address arithmetic.
                                                            (line  4270)
* 'float%':                              Structure Glossary.
                                                            (line  9210)
* 'float+':                              Address arithmetic.
                                                            (line  4267)
* floating point arithmetic words:       Floating Point.    (line  3650)
* floating point numbers, format and range: floating-idef.  (line 12541)
* floating point tutorial:               Floating Point Tutorial.
                                                            (line  1830)
* floating point unidentified fault, integer division: core-ambcond.
                                                            (line 12159)
* floating-point arithmetic, pitfalls:   Floating Point.    (line  3656)
* floating-point comparisons:            Floating Point.    (line  3769)
* floating-point dividing by zero:       floating-ambcond.  (line 12600)
* floating-point numbers, input format:  Number Conversion. (line  6555)
* floating-point numbers, rounding or truncation: floating-idef.
                                                            (line 12548)
* floating-point result out of range:    floating-ambcond.  (line 12574)
* floating-point stack:                  Stack Manipulation.
                                                            (line  3823)
* floating-point stack in the standard:  Stack Manipulation.
                                                            (line  3818)
* floating-point stack manipulation words: Floating point stack.
                                                            (line  3875)
* floating-point stack size:             floating-idef.     (line 12554)
* floating-point stack width:            floating-idef.     (line 12560)
* floating-point unidentified fault, 'F>D': floating-ambcond.
                                                            (line 12628)
* floating-point unidentified fault, 'FACOS', 'FASIN' or 'FATANH': floating-ambcond.
                                                            (line 12625)
* floating-point unidentified fault, 'FACOSH': floating-ambcond.
                                                            (line 12609)
* floating-point unidentified fault, 'FASINH' or 'FSQRT': floating-ambcond.
                                                            (line 12620)
* floating-point unidentified fault, 'FLN' or 'FLOG': floating-ambcond.
                                                            (line 12616)
* floating-point unidentified fault, 'FLNP1': floating-ambcond.
                                                            (line 12612)
* floating-point unidentified fault, FP divide-by-zero: floating-ambcond.
                                                            (line 12600)
* floating-point words, ambiguous conditions: floating-ambcond.
                                                            (line 12565)
* floating-point words, implementation-defined options: floating-idef.
                                                            (line 12540)
* floating-point words, system documentation: The optional Floating-Point word set.
                                                            (line 12537)
* 'floating-stack':                      Floating point stack.
                                                            (line  3884)
* 'floats':                              Address arithmetic.
                                                            (line  4264)
* 'flog':                                Floating Point.    (line  3708)
* 'FLOG', float=<0:                      floating-ambcond.  (line 12616)
* 'floor':                               Floating Point.    (line  3686)
* 'FLOORED':                             Single precision.  (line  3473)
* 'flush':                               Blocks.            (line  7635)
* 'flush-file':                          General files.     (line  7321)
* 'flush-icache':                        Code and ;code.    (line 11124)
* 'fm/mod':                              Mixed precision.   (line  3641)
* 'fmax':                                Floating Point.    (line  3682)
* 'fmin':                                Floating Point.    (line  3684)
* 'fnegate':                             Floating Point.    (line  3678)
* 'fnip':                                Floating point stack.
                                                            (line  3890)
* 'FOR':                                 Arbitrary control structures.
                                                            (line  4678)
* 'FOR' loops:                           Counted Loops.     (line  4594)
* foreign language interface:            C Interface.       (line 10788)
* 'FORGET', deleting the compilation word list: programming-ambcond.
                                                            (line 12700)
* 'FORGET', name can't be found:         programming-ambcond.
                                                            (line 12709)
* 'FORGET', removing a needed definition: programming-ambcond.
                                                            (line 12726)
* forgeting words:                       Forgetting words.  (line 10571)
* 'form':                                Terminal output.   (line  8084)
* format and range of floating point numbers: floating-idef.
                                                            (line 12541)
* format of glossary entries:            Notation.          (line  3263)
* formatted numeric output:              Formatted numeric output.
                                                            (line  7768)
* 'Forth':                               Word Lists.        (line  6925)
* Forth - an introduction:               Introduction.      (line  2570)
* Forth mode in Emacs:                   Emacs and Gforth.  (line 12885)
* Forth source files:                    Forth source files.
                                                            (line  7198)
* Forth Tutorial:                        Tutorial.          (line   789)
* Forth-related information:             Forth-related information.
                                                            (line 14105)
* 'forth-wordlist':                      Word Lists.        (line  6879)
* 'forth.el':                            Emacs and Gforth.  (line 12885)
* 'fover':                               Floating point stack.
                                                            (line  3894)
* FP tutorial:                           Floating Point Tutorial.
                                                            (line  1830)
* 'fp!':                                 Stack pointer manipulation.
                                                            (line  3953)
* 'fp0':                                 Stack pointer manipulation.
                                                            (line  3948)
* 'fp@':                                 Stack pointer manipulation.
                                                            (line  3951)
* 'fpath':                               Source Search Paths.
                                                            (line  7414)
* 'fpick':                               Floating point stack.
                                                            (line  3900)
* 'free':                                Heap Allocation.   (line  4115)
* frequently asked questions:            Forth-related information.
                                                            (line 14105)
* 'frot':                                Floating point stack.
                                                            (line  3903)
* 'fround':                              Floating Point.    (line  3690)
* 'fs.':                                 Simple numeric output.
                                                            (line  7738)
* 'fsin':                                Floating Point.    (line  3734)
* 'fsincos':                             Floating Point.    (line  3738)
* 'fsinh':                               Floating Point.    (line  3753)
* 'fsqrt':                               Floating Point.    (line  3696)
* 'FSQRT', float<0:                      floating-ambcond.  (line 12620)
* 'fswap':                               Floating point stack.
                                                            (line  3898)
* 'ftan':                                Floating Point.    (line  3741)
* 'FTAN' on an argument r1 where cos(r1) is zero: floating-ambcond.
                                                            (line 12592)
* 'ftanh':                               Floating Point.    (line  3757)
* 'ftuck':                               Floating point stack.
                                                            (line  3896)
* fully relocatable image files:         Fully Relocatable Image Files.
                                                            (line 13185)
* functions, tutorial:                   Colon Definitions Tutorial.
                                                            (line  1044)
* 'fvariable':                           Variables.         (line  5095)
* 'f_', stack item type:                 Notation.          (line  3333)
* 'f~':                                  Floating Point.    (line  3781)
* 'f~abs':                               Floating Point.    (line  3778)
* 'f~rel':                               Floating Point.    (line  3775)
* gdb disassembler:                      Common Disassembler.
                                                            (line 11250)
* general files:                         General files.     (line  7271)
* 'get-block-fid':                       Blocks.            (line  7585)
* 'get-current':                         Word Lists.        (line  6888)
* 'get-order':                           Word Lists.        (line  6894)
* 'getenv':                              Passing Commands to the OS.
                                                            (line 11709)
* 'gforth':                              Environmental Queries.
                                                            (line  7122)
* 'GFORTH' - environment variable:       Environment variables.
                                                            (line   669)
* 'GFORTH' - environment variable <1>:   gforthmi.          (line 13236)
* Gforth - leaving:                      Leaving Gforth.    (line   575)
* gforth engine:                         Direct or Indirect Threaded?.
                                                            (line 13545)
* Gforth environment:                    Gforth Environment.
                                                            (line   366)
* Gforth extensions:                     Standard vs Extensions.
                                                            (line 12760)
* Gforth files:                          Gforth Files.      (line   682)
* Gforth locals:                         Gforth locals.     (line  8488)
* Gforth performance:                    Performance.       (line 13808)
* 'gforth-ditc':                         gforthmi.          (line 13236)
* 'gforth-fast' and backtraces:          Error messages.    (line 11800)
* gforth-fast engine:                    Direct or Indirect Threaded?.
                                                            (line 13545)
* 'gforth-fast', difference from 'gforth': Error messages.  (line 11800)
* gforth-itc engine:                     Direct or Indirect Threaded?.
                                                            (line 13549)
* 'gforth.el':                           Emacs and Gforth.  (line 12885)
* 'gforth.el', installation:             Installing gforth.el.
                                                            (line 12916)
* 'gforth.fi', relocatability:           Fully Relocatable Image Files.
                                                            (line 13185)
* 'GFORTHD' - environment variable:      Environment variables.
                                                            (line   671)
* 'GFORTHD' - environment variable <1>:  gforthmi.          (line 13236)
* 'GFORTHHIST' - environment variable:   Environment variables.
                                                            (line   644)
* 'gforthmi':                            gforthmi.          (line 13198)
* 'GFORTHPATH' - environment variable:   Environment variables.
                                                            (line   648)
* 'GFORTHSYSTEMPREFIX' - environment variable: Environment variables.
                                                            (line   662)
* giving a name to a library interface:  Defining library interfaces.
                                                            (line 10971)
* glossary notation format:              Notation.          (line  3263)
* GNU C for the engine:                  Portability.       (line 13416)
* goals of the Gforth project:           Goals.             (line   329)
* header space:                          Word Lists.        (line  6857)
* heap allocation:                       Heap Allocation.   (line  4098)
* 'heap-new':                            Objects Glossary.  (line  9913)
* 'heap-new' discussion:                 Creating objects.  (line  9470)
* 'heap-new' usage:                      Basic Objects Usage.
                                                            (line  9444)
* 'here':                                Dictionary allocation.
                                                            (line  4034)
* 'hex':                                 Number Conversion. (line  6616)
* 'hex.':                                Simple numeric output.
                                                            (line  7695)
* highlighting Forth code in Emacs:      Hilighting.        (line 12951)
* hilighting Forth code in Emacs:        Hilighting.        (line 12951)
* history file:                          Command-line editing.
                                                            (line   619)
* 'hold':                                Formatted numeric output.
                                                            (line  7815)
* 'how:':                                Class Declaration. (line 10214)
* hybrid direct/indirect threaded code:  Direct or Indirect Threaded?.
                                                            (line 13537)
* 'i':                                   Counted Loops.     (line  4516)
* I/O - blocks:                          Blocks.            (line  7459)
* I/O - file-handling:                   Files.             (line  7187)
* I/O - keyboard and display:            Other I/O.         (line  7680)
* I/O - see character strings:           String Formats.    (line  7918)
* I/O - see input:                       Line input and conversion.
                                                            (line  8234)
* I/O exception in block transfer:       block-ambcond.     (line 12366)
* 'id.':                                 Name token.        (line  6116)
* 'IF':                                  Arbitrary control structures.
                                                            (line  4623)
* 'IF' control structure:                Selection.         (line  4408)
* if, tutorial:                          Conditional execution Tutorial.
                                                            (line  1320)
* 'iferror':                             Exception Handling.
                                                            (line  4893)
* image file:                            Image Files.       (line 13053)
* image file background:                 Image File Background.
                                                            (line 13081)
* image file initialization sequence:    Modifying the Startup Sequence.
                                                            (line 13339)
* image file invocation:                 Running Image Files.
                                                            (line 13286)
* image file loader:                     Image File Background.
                                                            (line 13109)
* image file, data-relocatable:          Data-Relocatable Image Files.
                                                            (line 13174)
* image file, executable:                Running Image Files.
                                                            (line 13290)
* image file, fully relocatable:         Fully Relocatable Image Files.
                                                            (line 13185)
* image file, non-relocatable:           Non-Relocatable Image Files.
                                                            (line 13159)
* image file, stack and dictionary sizes: Stack and Dictionary Sizes.
                                                            (line 13265)
* image file, turnkey applications:      Modifying the Startup Sequence.
                                                            (line 13355)
* image license:                         Image Licensing Issues.
                                                            (line 13060)
* 'immediate':                           Interpretation and Compilation Semantics.
                                                            (line  5831)
* immediate words:                       How does that work?.
                                                            (line  3061)
* immediate words <1>:                   Interpretation and Compilation Semantics.
                                                            (line  5829)
* immediate, tutorial:                   Interpretation and Compilation Semantics and Immediacy Tutorial.
                                                            (line  2008)
* 'implementation':                      Objects Glossary.  (line  9916)
* implementation of locals:              Locals implementation.
                                                            (line  8774)
* implementation of structures:          Structure Implementation.
                                                            (line  9143)
* 'implementation' usage:                Object Interfaces. (line  9700)
* implementation-defined options, block words: block-idef.  (line 12350)
* implementation-defined options, core words: core-idef.    (line 11951)
* implementation-defined options, exception words: exception-idef.
                                                            (line 12405)
* implementation-defined options, facility words: facility-idef.
                                                            (line 12420)
* implementation-defined options, file words: file-idef.    (line 12451)
* implementation-defined options, floating-point words: floating-idef.
                                                            (line 12540)
* implementation-defined options, locals words: locals-idef.
                                                            (line 12642)
* implementation-defined options, memory-allocation words: memory-idef.
                                                            (line 12667)
* implementation-defined options, programming-tools words: programming-idef.
                                                            (line 12679)
* implementation-defined options, search-order words: search-idef.
                                                            (line 12734)
* in-lining of constants:                Constants.         (line  5147)
* 'include':                             Forth source files.
                                                            (line  7238)
* 'include' search path:                 Search Paths.      (line  7382)
* 'include', placement in files:         Emacs Tags.        (line 12936)
* 'include-file':                        Forth source files.
                                                            (line  7224)
* 'INCLUDE-FILE', file-id is invalid:    file-ambcond.      (line 12515)
* 'INCLUDE-FILE', I/O exception reading or closing file-id: file-ambcond.
                                                            (line 12519)
* 'included':                            Forth source files.
                                                            (line  7228)
* 'INCLUDED', I/O exception reading or closing file-id: file-ambcond.
                                                            (line 12519)
* 'INCLUDED', named file cannot be opened: file-ambcond.    (line 12523)
* 'included?':                           Forth source files.
                                                            (line  7231)
* including files:                       Forth source files.
                                                            (line  7198)
* including files, stack effect:         Forth source files.
                                                            (line  7212)
* indentation of Forth code in Emacs:    Auto-Indentation.  (line 12989)
* indirect threaded inner interpreter:   Threading.         (line 13453)
* 'infile-execute':                      Redirection.       (line  7369)
* inheritance:                           Object-Oriented Terminology.
                                                            (line  9339)
* 'init':                                The OOF base class.
                                                            (line 10120)
* 'init-asm':                            Code and ;code.    (line 11116)
* 'init-object':                         Objects Glossary.  (line  9920)
* 'init-object' discussion:              Creating objects.  (line  9476)
* initialization sequence of image file: Modifying the Startup Sequence.
                                                            (line 13339)
* inner interpreter implementation:      Threading.         (line 13447)
* inner interpreter optimization:        Scheduling.        (line 13474)
* inner interpreter, direct threaded:    Threading.         (line 13464)
* inner interpreter, indirect threaded:  Threading.         (line 13453)
* input buffer:                          The Text Interpreter.
                                                            (line  6366)
* input format for double-cell numbers:  Number Conversion. (line  6555)
* input format for floating-point numbers: Number Conversion.
                                                            (line  6555)
* input format for single-cell numbers:  Number Conversion. (line  6555)
* input from pipes:                      Gforth in pipes.   (line   703)
* input line size, maximum:              file-idef.         (line 12491)
* input line terminator:                 core-idef.         (line 12024)
* Input Redirection:                     Redirection.       (line  7351)
* input sources:                         Input Sources.     (line  6507)
* input stream:                          The Input Stream.  (line  6781)
* input, linewise from terminal:         Line input and conversion.
                                                            (line  8234)
* input, single-key:                     Single-key input.  (line  8103)
* 'inst-value':                          Objects Glossary.  (line  9924)
* 'inst-value' usage:                    Method conveniences.
                                                            (line  9590)
* 'inst-value' visibility:               Classes and Scoping.
                                                            (line  9623)
* 'inst-var':                            Objects Glossary.  (line  9928)
* 'inst-var' implementation:             Objects Implementation.
                                                            (line  9774)
* 'inst-var' usage:                      Method conveniences.
                                                            (line  9568)
* 'inst-var' visibility:                 Classes and Scoping.
                                                            (line  9623)
* instance variables:                    Object-Oriented Terminology.
                                                            (line  9313)
* instruction pointer:                   Threading.         (line 13457)
* insufficient data stack or return stack space: core-ambcond.
                                                            (line 12164)
* insufficient space for loop control parameters: core-ambcond.
                                                            (line 12177)
* insufficient space in the dictionary:  core-ambcond.      (line 12180)
* integer types, ranges:                 core-idef.         (line 12064)
* 'interface':                           Objects Glossary.  (line  9932)
* interface implementation:              Objects Implementation.
                                                            (line  9785)
* interface to C functions:              C Interface.       (line 10788)
* 'interface' usage:                     Object Interfaces. (line  9700)
* interfaces for objects:                Object Interfaces. (line  9679)
* interpret state:                       The Text Interpreter.
                                                            (line  6357)
* Interpret/Compile states:              Interpret/Compile states.
                                                            (line  6684)
* 'interpret/compile:':                  Combined words.    (line  5863)
* interpretation semantics:              How does that work?.
                                                            (line  3033)
* interpretation semantics <1>:          Interpretation and Compilation Semantics.
                                                            (line  5805)
* interpretation semantics tutorial:     Interpretation and Compilation Semantics and Immediacy Tutorial.
                                                            (line  2008)
* 'interpretation>':                     Combined words.    (line  5948)
* interpreter - outer:                   The Text Interpreter.
                                                            (line  6352)
* interpreter directives:                Interpreter Directives.
                                                            (line  6698)
* Interpreting a compile-only word:      core-ambcond.      (line 12187)
* Interpreting a compile-only word, for ''' etc.: core-ambcond.
                                                            (line 12154)
* Interpreting a compile-only word, for a local: locals-ambcond.
                                                            (line 12652)
* interpreting a word with undefined interpretation semantics: core-ambcond.
                                                            (line 12187)
* invalid block number:                  block-ambcond.     (line 12370)
* Invalid memory address:                core-ambcond.      (line 12143)
* Invalid memory address, stack overflow: core-ambcond.     (line 12164)
* Invalid name argument, 'TO':           core-ambcond.      (line 12279)
* Invalid name argument, 'TO' <1>:       locals-ambcond.    (line 12659)
* 'invert':                              Bitwise operations.
                                                            (line  3519)
* invoking a selector:                   Object-Oriented Terminology.
                                                            (line  9327)
* invoking Gforth:                       Invoking Gforth.   (line   375)
* invoking image files:                  Running Image Files.
                                                            (line 13286)
* ior type description:                  Notation.          (line  3343)
* ior values and meaning:                file-idef.         (line 12481)
* ior values and meaning <1>:            memory-idef.       (line 12668)
* 'IS':                                  Deferred Words.    (line  5751)
* 'is':                                  The OOF base class.
                                                            (line 10160)
* items on the stack after interpretation: Stack depth changes.
                                                            (line 11855)
* 'j':                                   Counted Loops.     (line  4518)
* 'k':                                   Counted Loops.     (line  4520)
* 'k-alt-mask':                          Single-key input.  (line  8208)
* 'k-ctrl-mask':                         Single-key input.  (line  8206)
* 'k-delete':                            Single-key input.  (line  8171)
* 'k-down':                              Single-key input.  (line  8156)
* 'k-end':                               Single-key input.  (line  8161)
* 'k-f1':                                Single-key input.  (line  8175)
* 'k-f10':                               Single-key input.  (line  8193)
* 'k-f11':                               Single-key input.  (line  8195)
* 'k-f12':                               Single-key input.  (line  8197)
* 'k-f2':                                Single-key input.  (line  8177)
* 'k-f3':                                Single-key input.  (line  8179)
* 'k-f4':                                Single-key input.  (line  8181)
* 'k-f5':                                Single-key input.  (line  8183)
* 'k-f6':                                Single-key input.  (line  8185)
* 'k-f7':                                Single-key input.  (line  8187)
* 'k-f8':                                Single-key input.  (line  8189)
* 'k-f9':                                Single-key input.  (line  8191)
* 'k-home':                              Single-key input.  (line  8158)
* 'k-insert':                            Single-key input.  (line  8169)
* 'k-left':                              Single-key input.  (line  8150)
* 'k-next':                              Single-key input.  (line  8166)
* 'k-prior':                             Single-key input.  (line  8163)
* 'k-right':                             Single-key input.  (line  8152)
* 'k-shift-mask':                        Single-key input.  (line  8204)
* 'k-up':                                Single-key input.  (line  8154)
* 'kern*.fi', relocatability:            Fully Relocatable Image Files.
                                                            (line 13185)
* 'key':                                 Single-key input.  (line  8106)
* 'key-file':                            General files.     (line  7302)
* 'key?':                                Single-key input.  (line  8109)
* 'key?-file':                           General files.     (line  7309)
* keyboard events, encoding in 'EKEY':   facility-idef.     (line 12421)
* Kuehling, David:                       Emacs and Gforth.  (line 12885)
* 'l!':                                  Memory Access.     (line  4191)
* labels as values:                      Threading.         (line 13447)
* 'laddr#':                              Locals implementation.
                                                            (line  8784)
* 'LANG' - environment variable:         Environment variables.
                                                            (line   651)
* last word was headerless:              core-ambcond.      (line 12276)
* late binding:                          Class Binding.     (line  9504)
* 'latest':                              Name token.        (line  6091)
* 'latestxt':                            Anonymous Definitions.
                                                            (line  5234)
* 'LC_ALL' - environment variable:       Environment variables.
                                                            (line   653)
* 'LC_CTYPE' - environment variable:     Environment variables.
                                                            (line   655)
* 'LEAVE':                               Arbitrary control structures.
                                                            (line  4688)
* leaving definitions, tutorial:         Leaving definitions or loops Tutorial.
                                                            (line  1583)
* leaving Gforth:                        Leaving Gforth.    (line   575)
* leaving loops, tutorial:               Leaving definitions or loops Tutorial.
                                                            (line  1583)
* length of a line affected by '\':      block-idef.        (line 12355)
* 'lib-error':                           Low-Level C Interface Words.
                                                            (line 11087)
* 'lib-sym':                             Low-Level C Interface Words.
                                                            (line 11085)
* Libraries in C interface:              Declaring OS-level libraries.
                                                            (line 11010)
* library interface names:               Defining library interfaces.
                                                            (line 10971)
* license for images:                    Image Licensing Issues.
                                                            (line 13060)
* lifetime of locals:                    How long do locals live?.
                                                            (line  8697)
* line input from terminal:              Line input and conversion.
                                                            (line  8234)
* line terminator on input:              core-idef.         (line 12024)
* 'link':                                The OOF base class.
                                                            (line 10158)
* 'list':                                Blocks.            (line  7592)
* 'LIST' display format:                 block-idef.        (line 12351)
* 'list-size':                           Locals implementation.
                                                            (line  8866)
* 'Literal':                             Literals.          (line  6175)
* literal tutorial:                      Literal Tutorial.  (line  2391)
* Literals:                              Literals.          (line  6139)
* 'load':                                Blocks.            (line  7638)
* loader for image files:                Image File Background.
                                                            (line 13109)
* loading files at startup:              Invoking Gforth.   (line   549)
* loading Forth code, tutorial:          Using files for Forth code Tutorial.
                                                            (line   976)
* local in interpretation state:         locals-ambcond.    (line 12652)
* local variables, tutorial:             Local Variables Tutorial.
                                                            (line  1285)
* locale and case-sensitivity:           core-idef.         (line 11979)
* locals:                                Locals.            (line  8475)
* locals and return stack:               Return stack.      (line  3908)
* locals flavours:                       Gforth locals.     (line  8522)
* locals implementation:                 Locals implementation.
                                                            (line  8774)
* locals information on the control-flow stack: Locals implementation.
                                                            (line  8852)
* locals lifetime:                       How long do locals live?.
                                                            (line  8697)
* locals programming style:              Locals programming style.
                                                            (line  8711)
* locals stack:                          Stack Manipulation.
                                                            (line  3828)
* locals stack <1>:                      Locals implementation.
                                                            (line  8774)
* locals types:                          Gforth locals.     (line  8514)
* locals visibility:                     Where are locals visible by name?.
                                                            (line  8545)
* locals words, ambiguous conditions:    locals-ambcond.    (line 12651)
* locals words, implementation-defined options: locals-idef.
                                                            (line 12642)
* locals words, system documentation:    The optional Locals word set.
                                                            (line 12639)
* locals, ANS Forth style:               ANS Forth locals.  (line  8899)
* locals, default type:                  Gforth locals.     (line  8533)
* locals, Gforth style:                  Gforth locals.     (line  8488)
* locals, maximum number in a definition: locals-idef.      (line 12643)
* long long:                             Portability.       (line 13416)
* 'LOOP':                                Arbitrary control structures.
                                                            (line  4680)
* loop control parameters not available: core-ambcond.      (line 12271)
* loops without count:                   Simple Loops.      (line  4468)
* loops, counted:                        Counted Loops.     (line  4499)
* loops, counted, tutorial:              Counted loops Tutorial.
                                                            (line  1491)
* loops, endless:                        Simple Loops.      (line  4490)
* loops, indefinite, tutorial:           General Loops Tutorial.
                                                            (line  1428)
* 'lp!':                                 Stack pointer manipulation.
                                                            (line  3969)
* 'lp!' <1>:                             Locals implementation.
                                                            (line  8790)
* 'lp+!#':                               Locals implementation.
                                                            (line  8786)
* 'lp0':                                 Stack pointer manipulation.
                                                            (line  3963)
* 'lp@':                                 Stack pointer manipulation.
                                                            (line  3967)
* 'lshift':                              Bitwise operations.
                                                            (line  3521)
* 'LSHIFT', large shift counts:          core-ambcond.      (line 12300)
* 'm*':                                  Mixed precision.   (line  3630)
* 'm*/':                                 Mixed precision.   (line  3634)
* 'm+':                                  Mixed precision.   (line  3622)
* 'm:':                                  Objects Glossary.  (line  9935)
* 'm:' usage:                            Method conveniences.
                                                            (line  9558)
* macros:                                Compiling words.   (line  6128)
* Macros:                                Macros.            (line  6215)
* macros, advanced tutorial:             Advanced macros Tutorial.
                                                            (line  2419)
* mapping block ranges to files:         file-idef.         (line 12494)
* 'marker':                              Forgetting words.  (line 10574)
* 'max':                                 Single precision.  (line  3471)
* 'maxalign':                            Dictionary allocation.
                                                            (line  4088)
* 'maxaligned':                          Address arithmetic.
                                                            (line  4300)
* 'maxdepth-.s':                         Examining.         (line 10514)
* maximum depth of file input nesting:   file-idef.         (line 12487)
* maximum number of locals in a definition: locals-idef.    (line 12643)
* maximum number of word lists in search order: search-idef.
                                                            (line 12735)
* maximum size of a counted string:      core-idef.         (line 12029)
* maximum size of a definition name, in characters: core-idef.
                                                            (line 12036)
* maximum size of a parsed string:       core-idef.         (line 12033)
* maximum size of input line:            file-idef.         (line 12491)
* maximum string length for 'ENVIRONMENT?', in characters: core-idef.
                                                            (line 12039)
* memory access words:                   Memory Access.     (line  4132)
* memory access/allocation tutorial:     Memory Tutorial.   (line  1664)
* memory alignment tutorial:             Alignment Tutorial.
                                                            (line  1798)
* memory block words:                    Memory Blocks.     (line  4320)
* memory overcommit for dictionary and stacks: Invoking Gforth.
                                                            (line   451)
* memory words:                          Memory.            (line  3974)
* memory-allocation word set:            Heap Allocation.   (line  4098)
* memory-allocation words, implementation-defined options: memory-idef.
                                                            (line 12667)
* memory-allocation words, system documentation: The optional Memory-Allocation word set.
                                                            (line 12664)
* message send:                          Object-Oriented Terminology.
                                                            (line  9327)
* metacompiler:                          cross.fs.          (line 13251)
* metacompiler <1>:                      Cross Compiler.    (line 13901)
* method:                                Object-Oriented Terminology.
                                                            (line  9322)
* 'method':                              Objects Glossary.  (line  9945)
* 'method' <1>:                          Class Declaration. (line 10204)
* 'method' <2>:                          Basic Mini-OOF Usage.
                                                            (line 10242)
* method conveniences:                   Method conveniences.
                                                            (line  9552)
* method map:                            Objects Implementation.
                                                            (line  9731)
* method selector:                       Object-Oriented Terminology.
                                                            (line  9316)
* 'method' usage:                        Basic OOF Usage.   (line 10039)
* 'methods':                             Objects Glossary.  (line  9949)
* 'methods'...'end-methods':             Dividing classes.  (line  9643)
* 'min':                                 Single precision.  (line  3469)
* mini-oof:                              Mini-OOF.          (line 10226)
* mini-oof example:                      Mini-OOF Example.  (line 10267)
* mini-oof usage:                        Basic Mini-OOF Usage.
                                                            (line 10234)
* 'mini-oof.fs', differences to other models: Comparison with other object models.
                                                            (line 10486)
* minimum search order:                  search-idef.       (line 12738)
* miscellaneous words:                   Miscellaneous Words.
                                                            (line 11738)
* mixed precision arithmetic words:      Mixed precision.   (line  3622)
* 'mod':                                 Single precision.  (line  3461)
* modifying >IN:                         How does that work?.
                                                            (line  2988)
* modifying the contents of the input buffer or a string literal: core-ambcond.
                                                            (line 12191)
* most recent definition does not have a name ('IMMEDIATE'): core-ambcond.
                                                            (line 12276)
* motivation for object-oriented programming: Why object-oriented programming?.
                                                            (line  9277)
* 'move':                                Memory Blocks.     (line  4332)
* 'ms':                                  Keeping track of Time.
                                                            (line 11718)
* 'MS', repeatability to be expected:    facility-idef.     (line 12432)
* 'n', stack item type:                  Notation.          (line  3318)
* 'naligned':                            Structure Glossary.
                                                            (line  9212)
* 'name':                                The Input Stream.  (line  6811)
* name dictionary:                       Introducing the Text Interpreter.
                                                            (line  2640)
* name field address:                    Name token.        (line  6077)
* name lookup, case-sensitivity:         core-idef.         (line 11979)
* name not defined by 'VALUE' or '(LOCAL)' used by 'TO': locals-ambcond.
                                                            (line 12659)
* name not defined by 'VALUE' used by 'TO': core-ambcond.   (line 12279)
* name not found:                        core-ambcond.      (line 12137)
* name not found (''', 'POSTPONE', '[']', '[COMPILE]'): core-ambcond.
                                                            (line 12284)
* name token:                            Name token.        (line  6073)
* name, maximum length:                  core-idef.         (line 12036)
* 'name>comp':                           Name token.        (line  6110)
* 'name>int':                            Name token.        (line  6100)
* 'name>string':                         Name token.        (line  6113)
* 'name?int':                            Name token.        (line  6106)
* names for defined words:               Supplying names.   (line  5258)
* 'needs':                               Forth source files.
                                                            (line  7250)
* 'negate':                              Single precision.  (line  3465)
* negative increment for counted loops:  Counted Loops.     (line  4567)
* Neon model:                            Comparison with other object models.
                                                            (line 10436)
* 'new':                                 The OOF base class.
                                                            (line 10127)
* 'new' <1>:                             Basic Mini-OOF Usage.
                                                            (line 10257)
* newline character on input:            core-idef.         (line 12024)
* 'new[]':                               The OOF base class.
                                                            (line 10129)
* 'NEXT':                                Arbitrary control structures.
                                                            (line  4686)
* 'NEXT', direct threaded:               Threading.         (line 13464)
* 'NEXT', indirect threaded:             Threading.         (line 13453)
* 'next-arg':                            OS command line arguments.
                                                            (line  8422)
* 'nextname':                            Supplying names.   (line  5262)
* NFA:                                   Name token.        (line  6077)
* 'nip':                                 Data stack.        (line  3835)
* non-graphic characters and 'EMIT':     core-idef.         (line 11958)
* non-relocatable image files:           Non-Relocatable Image Files.
                                                            (line 13159)
* 'noname':                              Anonymous Definitions.
                                                            (line  5229)
* notation of glossary entries:          Notation.          (line  3263)
* 'nothrow':                             Exception Handling.
                                                            (line  4848)
* NT Forth performance:                  Performance.       (line 13824)
* number conversion:                     Number Conversion. (line  6555)
* number conversion - traps for the unwary: Number Conversion.
                                                            (line  6642)
* number of bits in one address unit:    core-idef.         (line 12056)
* number representation and arithmetic:  core-idef.         (line 12060)
* numeric comparison words:              Numeric comparison.
                                                            (line  3543)
* numeric output - formatted:            Formatted numeric output.
                                                            (line  7768)
* numeric output - simple/free-format:   Simple numeric output.
                                                            (line  7683)
* object:                                Object-Oriented Terminology.
                                                            (line  9309)
* 'object':                              Objects Glossary.  (line  9954)
* 'object' <1>:                          Basic Mini-OOF Usage.
                                                            (line 10239)
* object allocation options:             Creating objects.  (line  9470)
* 'object' class:                        The Objects base class.
                                                            (line  9461)
* object creation:                       Creating objects.  (line  9470)
* object interfaces:                     Object Interfaces. (line  9679)
* object models, comparison:             Comparison with other object models.
                                                            (line 10429)
* 'object-map' discussion:               Objects Implementation.
                                                            (line  9727)
* object-oriented programming:           Objects.           (line  9348)
* object-oriented programming <1>:       OOF.               (line  9998)
* object-oriented programming motivation: Why object-oriented programming?.
                                                            (line  9277)
* object-oriented programming style:     Object-Oriented Programming Style.
                                                            (line  9485)
* object-oriented terminology:           Object-Oriented Terminology.
                                                            (line  9302)
* objects:                               Objects.           (line  9348)
* objects, basic usage:                  Basic Objects Usage.
                                                            (line  9399)
* 'objects.fs':                          Objects.           (line  9348)
* 'objects.fs' <1>:                      OOF.               (line  9998)
* 'objects.fs' Glossary:                 Objects Glossary.  (line  9841)
* 'objects.fs' implementation:           Objects Implementation.
                                                            (line  9727)
* 'objects.fs' properties:               Properties of the Objects model.
                                                            (line  9365)
* 'of':                                  Arbitrary control structures.
                                                            (line  4709)
* 'off':                                 Boolean Flags.     (line  3420)
* 'on':                                  Boolean Flags.     (line  3417)
* 'Only':                                Word Lists.        (line  6929)
* oof:                                   OOF.               (line  9998)
* 'oof.fs':                              Objects.           (line  9348)
* 'oof.fs' <1>:                          OOF.               (line  9998)
* 'oof.fs' base class:                   The OOF base class.
                                                            (line 10094)
* 'oof.fs' properties:                   Properties of the OOF model.
                                                            (line 10011)
* 'oof.fs' usage:                        Basic OOF Usage.   (line 10034)
* 'oof.fs', differences to other models: Comparison with other object models.
                                                            (line 10471)
* 'open-blocks':                         Blocks.            (line  7573)
* 'open-file':                           General files.     (line  7287)
* 'open-lib':                            Low-Level C Interface Words.
                                                            (line 11083)
* 'open-path-file':                      General Search Paths.
                                                            (line  7429)
* 'open-pipe':                           Pipes.             (line  8303)
* operating system - passing commands:   Passing Commands to the OS.
                                                            (line 11691)
* operator's terminal facilities available: core-other.     (line 12320)
* options on the command line:           Invoking Gforth.   (line   375)
* 'or':                                  Bitwise operations.
                                                            (line  3515)
* 'order':                               Word Lists.        (line  6933)
* 'orig', control-flow stack item:       Arbitrary control structures.
                                                            (line  4618)
* OS command line arguments:             OS command line arguments.
                                                            (line  8413)
* 'os-class':                            Environmental Queries.
                                                            (line  7127)
* other system documentation, block words: block-other.     (line 12384)
* other system documentation, core words: core-other.       (line 12316)
* outer interpreter:                     Introducing the Text Interpreter.
                                                            (line  2603)
* outer interpreter <1>:                 Stacks and Postfix notation.
                                                            (line  2711)
* outer interpreter <2>:                 The Text Interpreter.
                                                            (line  6352)
* 'outfile-execute':                     Redirection.       (line  7366)
* output in pipes:                       Gforth in pipes.   (line   711)
* Output Redirection:                    Redirection.       (line  7351)
* output to terminal:                    Terminal output.   (line  8073)
* 'over':                                Data stack.        (line  3839)
* overcommit memory for dictionary and stacks: Invoking Gforth.
                                                            (line   451)
* overflow of the pictured numeric output string: core-ambcond.
                                                            (line 12194)
* 'overrides':                           Objects Glossary.  (line  9957)
* 'overrides' usage:                     Basic Objects Usage.
                                                            (line  9422)
* 'pad':                                 Memory Blocks.     (line  4388)
* 'PAD' size:                            core-idef.         (line 12098)
* 'PAD' use by nonstandard words:        core-other.        (line 12317)
* 'page':                                Terminal output.   (line  8094)
* parameter stack:                       Stack Manipulation.
                                                            (line  3820)
* parameters are not of the same type ('DO', '?DO', 'WITHIN'): core-ambcond.
                                                            (line 12287)
* parent class:                          Object-Oriented Terminology.
                                                            (line  9339)
* parent class binding:                  Class Binding.     (line  9525)
* 'parse':                               The Input Stream.  (line  6800)
* parse area:                            The Text Interpreter.
                                                            (line  6395)
* 'parse-name':                          The Input Stream.  (line  6805)
* 'parse-word':                          The Input Stream.  (line  6808)
* parsed string overflow:                core-ambcond.      (line 12197)
* parsed string, maximum size:           core-idef.         (line 12033)
* parsing words:                         How does that work?.
                                                            (line  2976)
* parsing words <1>:                     How does that work?.
                                                            (line  3000)
* parsing words <2>:                     The Text Interpreter.
                                                            (line  6417)
* patching threaded code:                Dynamic Superinstructions.
                                                            (line 13641)
* path for 'included':                   Search Paths.      (line  7382)
* 'path+':                               General Search Paths.
                                                            (line  7447)
* 'path-allot':                          General Search Paths.
                                                            (line  7435)
* 'path=':                               General Search Paths.
                                                            (line  7450)
* pedigree of Gforth:                    Origin.            (line 14068)
* 'perform':                             Execution token.   (line  6029)
* performance of some Forth interpreters: Performance.      (line 13808)
* persistent form of dictionary:         Image Files.       (line 13053)
* PFE performance:                       Performance.       (line 13824)
* 'pi':                                  Floating Point.    (line  3765)
* 'pick':                                Data stack.        (line  3845)
* pictured numeric output:               Formatted numeric output.
                                                            (line  7768)
* pictured numeric output buffer, size:  core-idef.         (line 12094)
* pictured numeric output string, overflow: core-ambcond.   (line 12194)
* pipes, creating your own:              Pipes.             (line  8299)
* pipes, Gforth as part of:              Gforth in pipes.   (line   700)
* 'postpone':                            Macros.            (line  6232)
* 'postpone' <1>:                        The OOF base class.
                                                            (line 10167)
* 'POSTPONE' applied to '[IF]':          programming-ambcond.
                                                            (line 12717)
* 'POSTPONE' or '[COMPILE]' applied to 'TO': core-ambcond.  (line 12292)
* postpone tutorial:                     POSTPONE Tutorial. (line  2337)
* 'postpone,':                           Compilation token. (line  6059)
* Pountain's object-oriented model:      Comparison with other object models.
                                                            (line 10451)
* 'precision':                           Floating Point.    (line  3723)
* precompiled Forth code:                Image Files.       (line 13053)
* 'previous':                            Word Lists.        (line  6917)
* 'previous', search order empty:        search-ambcond.    (line 12752)
* primitive source format:               Automatic Generation.
                                                            (line 13684)
* primitive-centric threaded code:       Direct or Indirect Threaded?.
                                                            (line 13522)
* primitives, assembly code listing:     Produced code.     (line 13800)
* primitives, automatic generation:      Automatic Generation.
                                                            (line 13675)
* primitives, implementation:            Primitives.        (line 13672)
* primitives, keeping the TOS in a register: TOS Optimization.
                                                            (line 13754)
* 'prims2x.fs':                          Automatic Generation.
                                                            (line 13675)
* 'print':                               Objects Glossary.  (line  9964)
* 'printdebugdata':                      Debugging.         (line 10632)
* 'private' discussion:                  Classes and Scoping.
                                                            (line  9632)
* procedures, tutorial:                  Colon Definitions Tutorial.
                                                            (line  1044)
* program data space available:          core-other.        (line 12326)
* programming style, arbitrary control structures: Arbitrary control structures.
                                                            (line  4719)
* programming style, locals:             Locals programming style.
                                                            (line  8711)
* programming style, object-oriented:    Object-Oriented Programming Style.
                                                            (line  9485)
* programming tools:                     Programming Tools. (line 10499)
* programming-tools words, ambiguous conditions: programming-ambcond.
                                                            (line 12699)
* programming-tools words, implementation-defined options: programming-idef.
                                                            (line 12679)
* programming-tools words, system documentation: The optional Programming-Tools word set.
                                                            (line 12676)
* prompt:                                core-idef.         (line 12108)
* pronounciation of words:               Notation.          (line  3290)
* 'protected':                           Objects Glossary.  (line  9968)
* 'protected' discussion:                Classes and Scoping.
                                                            (line  9632)
* 'ptr':                                 The OOF base class.
                                                            (line 10133)
* 'ptr' <1>:                             Class Declaration. (line 10188)
* 'public':                              Objects Glossary.  (line  9971)
* 'query':                               Input Sources.     (line  6547)
* 'quit':                                Miscellaneous Words.
                                                            (line 11741)
* 'r', stack item type:                  Notation.          (line  3326)
* 'r/o':                                 General files.     (line  7274)
* 'r/w':                                 General files.     (line  7276)
* 'r>':                                  Return stack.      (line  3917)
* 'r@':                                  Return stack.      (line  3919)
* ranges for integer types:              core-idef.         (line 12064)
* 'rdrop':                               Return stack.      (line  3921)
* 'read-file':                           General files.     (line  7298)
* 'read-line':                           General files.     (line  7300)
* read-only data space regions:          core-idef.         (line 12071)
* reading from file positions not yet written: file-ambcond.
                                                            (line 12511)
* receiving object:                      Object-Oriented Terminology.
                                                            (line  9333)
* records:                               Structures.        (line  8949)
* records tutorial:                      Arrays and Records Tutorial.
                                                            (line  2312)
* 'recover' (old Gforth versions):       Exception Handling.
                                                            (line  4932)
* 'recurse':                             Calls and returns. (line  4765)
* 'RECURSE' appears after 'DOES>':       core-ambcond.      (line 12239)
* recursion tutorial:                    Recursion Tutorial.
                                                            (line  1544)
* 'recursive':                           Calls and returns. (line  4761)
* recursive definitions:                 Calls and returns. (line  4755)
* Redirection:                           Redirection.       (line  7351)
* 'refill':                              The Input Stream.  (line  6823)
* registers of the inner interpreter:    Code and ;code.    (line 11152)
* relocating loader:                     Image File Background.
                                                            (line 13109)
* relocation at load-time:               Image File Background.
                                                            (line 13100)
* relocation at run-time:                Image File Background.
                                                            (line 13095)
* 'rename-file':                         General files.     (line  7295)
* 'REPEAT':                              Arbitrary control structures.
                                                            (line  4651)
* repeatability to be expected from the execution of 'MS': facility-idef.
                                                            (line 12432)
* Replication:                           Dynamic Superinstructions.
                                                            (line 13561)
* report the words used in your program: ANS Report.        (line 11817)
* 'reposition-file':                     General files.     (line  7327)
* 'REPOSITION-FILE', outside the file's boundaries: file-ambcond.
                                                            (line 12507)
* 'represent':                           Formatted numeric output.
                                                            (line  7833)
* 'REPRESENT', results when float is out of range: floating-idef.
                                                            (line 12544)
* 'require':                             Forth source files.
                                                            (line  7247)
* 'require', placement in files:         Emacs Tags.        (line 12936)
* 'required':                            Forth source files.
                                                            (line  7241)
* reserving data space:                  Dictionary allocation.
                                                            (line  4016)
* 'resize':                              Heap Allocation.   (line  4121)
* 'resize-file':                         General files.     (line  7331)
* 'restore':                             Exception Handling.
                                                            (line  4965)
* 'restore-input':                       Input Sources.     (line  6532)
* 'RESTORE-INPUT', Argument type mismatch: core-ambcond.    (line 12243)
* 'restrict':                            Interpretation and Compilation Semantics.
                                                            (line  5838)
* result out of range:                   core-ambcond.      (line 12200)
* return stack:                          Stack Manipulation.
                                                            (line  3825)
* return stack and locals:               Return stack.      (line  3908)
* return stack dump with 'gforth-fast':  Error messages.    (line 11800)
* return stack manipulation words:       Return stack.      (line  3908)
* return stack space available:          core-other.        (line 12331)
* return stack tutorial:                 Return Stack Tutorial.
                                                            (line  1612)
* return stack underflow:                core-ambcond.      (line 12211)
* returning from a definition:           Calls and returns. (line  4755)
* 'roll':                                Data stack.        (line  3856)
* 'Root':                                Word Lists.        (line  6970)
* 'rot':                                 Data stack.        (line  3848)
* rounding of floating-point numbers:    floating-idef.     (line 12548)
* 'rp!':                                 Stack pointer manipulation.
                                                            (line  3961)
* 'rp0':                                 Stack pointer manipulation.
                                                            (line  3955)
* 'rp@':                                 Stack pointer manipulation.
                                                            (line  3959)
* 'rshift':                              Bitwise operations.
                                                            (line  3523)
* 'RSHIFT', large shift counts:          core-ambcond.      (line 12300)
* run-time code generation, tutorial:    Advanced macros Tutorial.
                                                            (line  2419)
* running Gforth:                        Invoking Gforth.   (line   375)
* running image files:                   Running Image Files.
                                                            (line 13286)
* Rydqvist, Goran:                       Emacs and Gforth.  (line 12885)
* 'S"':                                  Displaying characters and strings.
                                                            (line  7990)
* 'S"', number of string buffers:        file-idef.         (line 12498)
* 'S"', size of string buffer:           file-idef.         (line 12501)
* 's>d':                                 Double precision.  (line  3494)
* 's>number?':                           Line input and conversion.
                                                            (line  8252)
* 's>unumber?':                          Line input and conversion.
                                                            (line  8255)
* 'save-buffer':                         Blocks.            (line  7633)
* 'save-buffers':                        Blocks.            (line  7629)
* 'save-input':                          Input Sources.     (line  6527)
* 'savesystem':                          Non-Relocatable Image Files.
                                                            (line 13169)
* 'savesystem' during 'gforthmi':        gforthmi.          (line 13236)
* 'scope':                               Where are locals visible by name?.
                                                            (line  8550)
* scope of locals:                       Where are locals visible by name?.
                                                            (line  8545)
* scoping and classes:                   Classes and Scoping.
                                                            (line  9617)
* 'scr':                                 Blocks.            (line  7596)
* 'seal':                                Word Lists.        (line  6980)
* 'search':                              Memory Blocks.     (line  4369)
* search order stack:                    Word Lists.        (line  6861)
* search order, maximum depth:           search-idef.       (line 12735)
* search order, minimum:                 search-idef.       (line 12738)
* search order, tutorial:                Wordlists and Search Order Tutorial.
                                                            (line  2506)
* search path control, source files:     Source Search Paths.
                                                            (line  7410)
* search path control, source files <1>: General Search Paths.
                                                            (line  7424)
* search path for files:                 Search Paths.      (line  7382)
* search-order words, ambiguous conditions: search-ambcond. (line 12743)
* search-order words, implementation-defined options: search-idef.
                                                            (line 12734)
* search-order words, system documentation: The optional Search-Order word set.
                                                            (line 12731)
* 'search-wordlist':                     Word Lists.        (line  6955)
* 'see':                                 Examining.         (line 10548)
* see tutorial:                          Decompilation Tutorial.
                                                            (line  1076)
* 'SEE', source and format of output:    programming-idef.  (line 12691)
* 'see-code':                            Examining.         (line 10562)
* 'see-code-range':                      Examining.         (line 10566)
* selection control structures:          Selection.         (line  4408)
* selector:                              Object-Oriented Terminology.
                                                            (line  9316)
* 'selector':                            Objects Glossary.  (line  9975)
* 'selector' implementation, class:      Objects Implementation.
                                                            (line  9735)
* selector invocation:                   Object-Oriented Terminology.
                                                            (line  9327)
* selector invocation, restrictions:     Basic Objects Usage.
                                                            (line  9451)
* selector invocation, restrictions <1>: Basic OOF Usage.   (line 10084)
* 'selector' usage:                      Basic Objects Usage.
                                                            (line  9401)
* selectors and stack effects:           Object-Oriented Programming Style.
                                                            (line  9487)
* selectors common to hardly-related classes: Object Interfaces.
                                                            (line  9684)
* 'self':                                The OOF base class.
                                                            (line 10149)
* semantics tutorial:                    Interpretation and Compilation Semantics and Immediacy Tutorial.
                                                            (line  2008)
* semantics, interpretation and compilation: Interpretation and Compilation Semantics.
                                                            (line  5805)
* 'set-current':                         Word Lists.        (line  6891)
* 'set-order':                           Word Lists.        (line  6900)
* 'set-precision':                       Floating Point.    (line  3727)
* 'sf!':                                 Memory Access.     (line  4164)
* 'sf@':                                 Memory Access.     (line  4160)
* 'sf@' or 'sf!' used with an address that is not single-float aligned: floating-ambcond.
                                                            (line 12581)
* 'sfalign':                             Dictionary allocation.
                                                            (line  4080)
* 'sfaligned':                           Address arithmetic.
                                                            (line  4285)
* 'sffield:':                            Forth200x Structures.
                                                            (line  9260)
* 'sfloat%':                             Structure Glossary.
                                                            (line  9216)
* 'sfloat+':                             Address arithmetic.
                                                            (line  4282)
* 'sfloats':                             Address arithmetic.
                                                            (line  4278)
* 'sf_', stack item type:                Notation.          (line  3337)
* 'sh':                                  Passing Commands to the OS.
                                                            (line 11694)
* Shared libraries in C interface:       Declaring OS-level libraries.
                                                            (line 11010)
* shell commands:                        Passing Commands to the OS.
                                                            (line 11691)
* 'shift-args':                          OS command line arguments.
                                                            (line  8456)
* 'sign':                                Formatted numeric output.
                                                            (line  7819)
* silent exiting from Gforth:            Gforth in pipes.   (line   713)
* simple defining words:                 CREATE.            (line  5017)
* simple loops:                          Simple Loops.      (line  4468)
* 'simple-see':                          Examining.         (line 10557)
* 'simple-see-range':                    Examining.         (line 10560)
* single precision arithmetic words:     Single precision.  (line  3438)
* single-assignment style for locals:    Locals programming style.
                                                            (line  8730)
* single-cell numbers, input format:     Number Conversion. (line  6555)
* single-key input:                      Single-key input.  (line  8103)
* singlestep Debugger:                   Singlestep Debugger.
                                                            (line 10722)
* size of buffer at 'WORD':              core-idef.         (line 12074)
* size of the dictionary and the stacks: Invoking Gforth.   (line   420)
* size of the keyboard terminal buffer:  core-idef.         (line 12087)
* size of the pictured numeric output buffer: core-idef.    (line 12094)
* size of the scratch area returned by 'PAD': core-idef.    (line 12098)
* size parameters for command-line options: Invoking Gforth.
                                                            (line   420)
* 'sl@':                                 Memory Access.     (line  4185)
* 'SLiteral':                            Literals.          (line  6194)
* 'slurp-fid':                           General files.     (line  7336)
* 'slurp-file':                          General files.     (line  7333)
* 'sm/rem':                              Mixed precision.   (line  3644)
* 'source':                              The Text Interpreter.
                                                            (line  6474)
* source location of error or debugging output in Emacs: Emacs and Gforth.
                                                            (line 12900)
* 'source-id':                           Input Sources.     (line  6519)
* 'SOURCE-ID', behaviour when 'BLK' is non-zero: file-ambcond.
                                                            (line 12531)
* 'sourcefilename':                      Forth source files.
                                                            (line  7253)
* 'sourceline#':                         Forth source files.
                                                            (line  7260)
* 'sp!':                                 Stack pointer manipulation.
                                                            (line  3946)
* 'sp0':                                 Stack pointer manipulation.
                                                            (line  3940)
* 'sp@':                                 Stack pointer manipulation.
                                                            (line  3944)
* 'space':                               Displaying characters and strings.
                                                            (line  7948)
* space delimiters:                      core-idef.         (line 11995)
* 'spaces':                              Displaying characters and strings.
                                                            (line  7951)
* 'span':                                Line input and conversion.
                                                            (line  8292)
* speed, startup:                        Startup speed.     (line   743)
* stack depth changes during interpretation: Stack depth changes.
                                                            (line 11855)
* stack effect:                          Notation.          (line  3273)
* Stack effect design, tutorial:         Designing the stack effect Tutorial.
                                                            (line  1238)
* stack effect of 'DOES>'-parts:         User-defined Defining Words.
                                                            (line  5422)
* stack effect of included files:        Forth source files.
                                                            (line  7212)
* stack effects of selectors:            Object-Oriented Programming Style.
                                                            (line  9487)
* stack empty:                           core-ambcond.      (line 12211)
* stack item types:                      Notation.          (line  3308)
* stack manipulation tutorial:           Stack Manipulation Tutorial.
                                                            (line   931)
* stack manipulation words:              Stack Manipulation.
                                                            (line  3818)
* stack manipulation words, floating-point stack: Floating point stack.
                                                            (line  3875)
* stack manipulation words, return stack: Return stack.     (line  3908)
* stack manipulations words, data stack: Data stack.        (line  3833)
* stack overflow:                        core-ambcond.      (line 12164)
* stack pointer manipulation words:      Stack pointer manipulation.
                                                            (line  3940)
* stack size default:                    Stack and Dictionary Sizes.
                                                            (line 13265)
* stack size, cache-friendly:            Stack and Dictionary Sizes.
                                                            (line 13278)
* stack space available:                 core-other.        (line 12336)
* stack tutorial:                        Stack Tutorial.    (line   867)
* stack underflow:                       core-ambcond.      (line 12211)
* stack-effect comments, tutorial:       Stack-Effect Comments Tutorial.
                                                            (line  1093)
* starting Gforth tutorial:              Starting Gforth Tutorial.
                                                            (line   814)
* startup sequence for image file:       Modifying the Startup Sequence.
                                                            (line 13339)
* Startup speed:                         Startup speed.     (line   743)
* 'state' - effect on the text interpreter: How does that work?.
                                                            (line  3004)
* 'STATE' values:                        core-idef.         (line 12118)
* state-smart words (are a bad idea):    Combined words.    (line  5894)
* 'static':                              Class Declaration. (line 10209)
* 'stderr':                              General files.     (line  7345)
* stderr and pipes:                      Gforth in pipes.   (line   738)
* 'stdin':                               General files.     (line  7339)
* 'stdout':                              General files.     (line  7342)
* 'str<':                                Memory Blocks.     (line  4364)
* 'str=':                                Memory Blocks.     (line  4362)
* string larger than pictured numeric output area ('f.', 'fe.', 'fs.'): floating-ambcond.
                                                            (line 12632)
* string longer than a counted string returned by 'WORD': core-ambcond.
                                                            (line 12296)
* 'string-prefix?':                      Memory Blocks.     (line  4366)
* strings - see character strings:       String Formats.    (line  7918)
* strings tutorial:                      Characters and Strings Tutorial.
                                                            (line  1749)
* 'struct':                              Structure Glossary.
                                                            (line  9221)
* 'struct' usage:                        Structure Usage.   (line  9035)
* structs tutorial:                      Arrays and Records Tutorial.
                                                            (line  2312)
* structure extension:                   Structure Usage.   (line  9080)
* structure glossary:                    Structure Glossary.
                                                            (line  9172)
* structure implementation:              Structure Implementation.
                                                            (line  9143)
* structure naming convention:           Structure Naming Convention.
                                                            (line  9119)
* structure naming convention <1>:       Structure Naming Convention.
                                                            (line  9133)
* structure of Forth programs:           Forth is written in Forth.
                                                            (line  3146)
* structure usage:                       Structure Usage.   (line  9035)
* structures:                            Structures.        (line  8949)
* structures containing arrays:          Structure Usage.   (line  9092)
* structures containing structures:      Structure Usage.   (line  9072)
* Structures in Forth200x:               Forth200x Structures.
                                                            (line  9227)
* structures using address arithmetic:   Why explicit structure support?.
                                                            (line  8962)
* 'sub-list?':                           Locals implementation.
                                                            (line  8864)
* 'super':                               The OOF base class.
                                                            (line 10145)
* superclass binding:                    Class Binding.     (line  9525)
* Superinstructions:                     Dynamic Superinstructions.
                                                            (line 13561)
* 'sw@':                                 Memory Access.     (line  4176)
* 'swap':                                Data stack.        (line  3843)
* syntax tutorial:                       Syntax Tutorial.   (line   825)
* 'system':                              Passing Commands to the OS.
                                                            (line 11698)
* system dictionary space required, in address units: core-other.
                                                            (line 12341)
* system documentation:                  ANS conformance.   (line 11937)
* system documentation, block words:     The optional Block word set.
                                                            (line 12347)
* system documentation, core words:      The Core Words.    (line 11948)
* system documentation, double words:    The optional Double Number word set.
                                                            (line 12393)
* system documentation, exception words: The optional Exception word set.
                                                            (line 12402)
* system documentation, facility words:  The optional Facility word set.
                                                            (line 12417)
* system documentation, file words:      The optional File-Access word set.
                                                            (line 12448)
* system documentation, floating-point words: The optional Floating-Point word set.
                                                            (line 12537)
* system documentation, locals words:    The optional Locals word set.
                                                            (line 12639)
* system documentation, memory-allocation words: The optional Memory-Allocation word set.
                                                            (line 12664)
* system documentation, programming-tools words: The optional Programming-Tools word set.
                                                            (line 12676)
* system documentation, search-order words: The optional Search-Order word set.
                                                            (line 12731)
* system prompt:                         core-idef.         (line 12108)
* 's\"':                                 Displaying characters and strings.
                                                            (line  8001)
* 'table':                               Word Lists.        (line  6911)
* 'TAGS' file:                           Emacs Tags.        (line 12936)
* target compiler:                       cross.fs.          (line 13251)
* target compiler <1>:                   Cross Compiler.    (line 13901)
* terminal buffer, size:                 core-idef.         (line 12087)
* terminal input buffer:                 The Text Interpreter.
                                                            (line  6366)
* terminal output:                       Terminal output.   (line  8073)
* terminal size:                         Terminal output.   (line  8082)
* terminology for object-oriented programming: Object-Oriented Terminology.
                                                            (line  9302)
* text interpreter:                      Introducing the Text Interpreter.
                                                            (line  2603)
* text interpreter <1>:                  Stacks and Postfix notation.
                                                            (line  2711)
* text interpreter <2>:                  The Text Interpreter.
                                                            (line  6352)
* text interpreter - effect of state:    How does that work?.
                                                            (line  3004)
* text interpreter - input sources:      The Text Interpreter.
                                                            (line  6455)
* text interpreter - input sources <1>:  Input Sources.     (line  6507)
* 'THEN':                                Arbitrary control structures.
                                                            (line  4627)
* 'this':                                Objects Glossary.  (line  9980)
* 'this' and 'catch':                    Objects Implementation.
                                                            (line  9764)
* 'this' implementation:                 Objects Implementation.
                                                            (line  9764)
* 'this' usage:                          Method conveniences.
                                                            (line  9558)
* ThisForth performance:                 Performance.       (line 13824)
* threaded code implementation:          Threading.         (line 13447)
* threading words:                       Threading Words.   (line 11598)
* threading, direct or indirect?:        Direct or Indirect Threaded?.
                                                            (line 13512)
* 'threading-method':                    Threading Words.   (line 11618)
* 'throw':                               Exception Handling.
                                                            (line  4806)
* 'THROW'-codes used in the system:      exception-idef.    (line 12406)
* 'thru':                                Blocks.            (line  7643)
* 'tib':                                 The Text Interpreter.
                                                            (line  6477)
* tick ('):                              Execution token.   (line  5982)
* TILE performance:                      Performance.       (line 13824)
* 'time&date':                           Keeping track of Time.
                                                            (line 11721)
* time-related words:                    Keeping track of Time.
                                                            (line 11718)
* 'TMP', 'TEMP' - environment variable:  Environment variables.
                                                            (line   673)
* 'TO':                                  Values.            (line  5189)
* 'TO' on non-'VALUE's:                  core-ambcond.      (line 12279)
* 'TO' on non-'VALUE's and non-locals:   locals-ambcond.    (line 12659)
* 'to-this':                             Objects Glossary.  (line  9989)
* tokens for words:                      Tokens for Words.  (line  5973)
* TOS definition:                        Stacks and Postfix notation.
                                                            (line  2746)
* TOS optimization for primitives:       TOS Optimization.  (line 13754)
* 'toupper':                             Displaying characters and strings.
                                                            (line  7957)
* trigonometric operations:              Floating Point.    (line  3731)
* 'true':                                Boolean Flags.     (line  3411)
* truncation of floating-point numbers:  floating-idef.     (line 12548)
* 'try':                                 Exception Handling.
                                                            (line  4887)
* 'tuck':                                Data stack.        (line  3841)
* turnkey image files:                   Modifying the Startup Sequence.
                                                            (line 13355)
* Tutorial:                              Tutorial.          (line   789)
* 'type':                                Displaying characters and strings.
                                                            (line  7977)
* types of locals:                       Gforth locals.     (line  8514)
* types of stack items:                  Notation.          (line  3308)
* types tutorial:                        Types Tutorial.    (line  1169)
* 'typewhite':                           Displaying characters and strings.
                                                            (line  7981)
* 'U+DO':                                Arbitrary control structures.
                                                            (line  4670)
* 'u', stack item type:                  Notation.          (line  3320)
* 'U-DO':                                Arbitrary control structures.
                                                            (line  4674)
* 'u.':                                  Simple numeric output.
                                                            (line  7699)
* 'u.r':                                 Simple numeric output.
                                                            (line  7709)
* 'u<':                                  Numeric comparison.
                                                            (line  3570)
* 'u<=':                                 Numeric comparison.
                                                            (line  3572)
* 'u>':                                  Numeric comparison.
                                                            (line  3574)
* 'u>=':                                 Numeric comparison.
                                                            (line  3576)
* 'ud', stack item type:                 Notation.          (line  3324)
* 'ud.':                                 Simple numeric output.
                                                            (line  7717)
* 'ud.r':                                Simple numeric output.
                                                            (line  7726)
* 'ul@':                                 Memory Access.     (line  4188)
* 'um*':                                 Mixed precision.   (line  3632)
* 'um/mod':                              Mixed precision.   (line  3638)
* undefined word:                        core-ambcond.      (line 12137)
* undefined word, ''', 'POSTPONE', '[']', '[COMPILE]': core-ambcond.
                                                            (line 12284)
* 'under+':                              Single precision.  (line  3450)
* unexpected end of the input buffer:    core-ambcond.      (line 12229)
* 'unloop':                              Arbitrary control structures.
                                                            (line  4692)
* unmapped block numbers:                file-ambcond.      (line 12526)
* 'UNREACHABLE':                         Where are locals visible by name?.
                                                            (line  8588)
* 'UNTIL':                               Arbitrary control structures.
                                                            (line  4631)
* 'UNTIL' loop:                          Simple Loops.      (line  4479)
* 'unused':                              Dictionary allocation.
                                                            (line  4037)
* unwind-protect:                        Exception Handling.
                                                            (line  4906)
* 'update':                              Blocks.            (line  7622)
* 'UPDATE', no current block buffer:     block-ambcond.     (line 12379)
* 'updated?':                            Blocks.            (line  7625)
* upper and lower case:                  Case insensitivity.
                                                            (line  3354)
* 'use':                                 Blocks.            (line  7576)
* 'User':                                Variables.         (line  5102)
* user input device, method of selecting: core-idef.        (line 12042)
* user output device, method of selecting: core-idef.       (line 12047)
* user space:                            Variables.         (line  5097)
* user variables:                        Variables.         (line  5097)
* user-defined defining words:           User-defined Defining Words.
                                                            (line  5279)
* 'utime':                               Keeping track of Time.
                                                            (line 11725)
* 'uw@':                                 Memory Access.     (line  4179)
* 'Value':                               Values.            (line  5187)
* value-flavoured locals:                Gforth locals.     (line  8522)
* values:                                Values.            (line  5176)
* 'var':                                 Class Declaration. (line 10183)
* 'var' <1>:                             Basic Mini-OOF Usage.
                                                            (line 10245)
* 'Variable':                            Variables.         (line  5091)
* variable-flavoured locals:             Gforth locals.     (line  8522)
* variables:                             Variables.         (line  5066)
* variadic C functions:                  Declaring C Functions.
                                                            (line 10904)
* versions, invoking other versions of Gforth: Invoking Gforth.
                                                            (line   560)
* viewing the documentation of a word in Emacs: Emacs and Gforth.
                                                            (line 12908)
* viewing the source of a word in Emacs: Emacs Tags.        (line 12936)
* virtual function:                      Object-Oriented Terminology.
                                                            (line  9316)
* virtual function table:                Objects Implementation.
                                                            (line  9731)
* virtual machine:                       Engine.            (line 13385)
* virtual machine instructions, implementation: Primitives. (line 13672)
* visibility of locals:                  Where are locals visible by name?.
                                                            (line  8545)
* 'vlist':                               Word Lists.        (line  6967)
* Vocabularies, detailed explanation:    Vocabularies.      (line  6997)
* 'Vocabulary':                          Word Lists.        (line  6975)
* 'vocs':                                Word Lists.        (line  6984)
* vocstack empty, 'previous':            search-ambcond.    (line 12752)
* vocstack full, 'also':                 search-ambcond.    (line 12755)
* 'w!':                                  Memory Access.     (line  4182)
* 'w', stack item type:                  Notation.          (line  3316)
* 'w/o':                                 General files.     (line  7278)
* where to go next:                      Where to go next.  (line  3216)
* 'WHILE':                               Arbitrary control structures.
                                                            (line  4649)
* 'WHILE' loop:                          Simple Loops.      (line  4468)
* wid:                                   Word Lists.        (line  6869)
* 'wid', stack item type:                Notation.          (line  3341)
* Win32Forth performance:                Performance.       (line 13824)
* wior type description:                 Notation.          (line  3343)
* wior values and meaning:               file-idef.         (line 12481)
* 'with':                                The OOF base class.
                                                            (line 10174)
* 'within':                              Numeric comparison.
                                                            (line  3578)
* word:                                  Introducing the Text Interpreter.
                                                            (line  2644)
* 'word':                                The Input Stream.  (line  6814)
* 'WORD' buffer size:                    core-idef.         (line 12074)
* word glossary entry format:            Notation.          (line  3263)
* word list for defining locals:         Locals implementation.
                                                            (line  8814)
* word lists:                            Word Lists.        (line  6857)
* word lists - example:                  Word list example. (line  7080)
* word lists - why use them?:            Why use word lists?.
                                                            (line  7032)
* word name too long:                    core-ambcond.      (line 12140)
* 'WORD', string overflow:               core-ambcond.      (line 12296)
* 'wordlist':                            Word Lists.        (line  6908)
* wordlists tutorial:                    Wordlists and Search Order Tutorial.
                                                            (line  2506)
* words:                                 Words.             (line  3260)
* 'words':                               Word Lists.        (line  6963)
* words used in your program:            ANS Report.        (line 11817)
* words, forgetting:                     Forgetting words.  (line 10571)
* wordset:                               Notation.          (line  3293)
* 'write-file':                          General files.     (line  7315)
* 'write-line':                          General files.     (line  7317)
* 'x-size':                              Xchars and Unicode.
                                                            (line  8348)
* 'x-width':                             Xchars and Unicode.
                                                            (line  8388)
* 'xc!+?':                               Xchars and Unicode.
                                                            (line  8356)
* 'xc-size':                             Xchars and Unicode.
                                                            (line  8345)
* 'xc@+':                                Xchars and Unicode.
                                                            (line  8352)
* 'xchar+':                              Xchars and Unicode.
                                                            (line  8364)
* 'xchar-':                              Xchars and Unicode.
                                                            (line  8368)
* 'xchar-encoding':                      Xchars and Unicode.
                                                            (line  8403)
* 'xemit':                               Xchars and Unicode.
                                                            (line  8398)
* 'xkey':                                Xchars and Unicode.
                                                            (line  8394)
* 'xor':                                 Bitwise operations.
                                                            (line  3517)
* xt:                                    Introducing the Text Interpreter.
                                                            (line  2644)
* xt <1>:                                Execution token.   (line  5979)
* XT tutorial:                           Execution Tokens Tutorial.
                                                            (line  2078)
* 'xt', stack item type:                 Notation.          (line  3339)
* 'xt-new':                              Objects Glossary.  (line  9992)
* 'xt-see':                              Examining.         (line 10554)
* 'x\string-':                           Xchars and Unicode.
                                                            (line  8377)
* zero-length string as a name:          core-ambcond.      (line 12229)
* Zsoter's object-oriented model:        Comparison with other object models.
                                                            (line 10457)

