File:  [gforth] / gforth / doc / gforth.ds
Revision 1.125: download - view: text, annotated - select for diffs
Mon May 31 12:59:20 2004 UTC (19 years, 10 months ago) by anton
Branches: MAIN
CVS tags: HEAD
AMD64 1-byte opcodes done (mostly, and not debugged)
minor documentation change

    1: \input texinfo   @c -*-texinfo-*-
    2: @comment The source is gforth.ds, from which gforth.texi is generated
    3: 
    4: @comment TODO: nac29jan99 - a list of things to add in the next edit:
    5: @comment 1. x-ref all ambiguous or implementation-defined features?
    6: @comment 2. Describe the use of Auser Avariable AConstant A, etc.
    7: @comment 3. words in miscellaneous section need a home.
    8: @comment 4. search for TODO for other minor and major works required.
    9: @comment 5. [rats] change all @var to @i in Forth source so that info
   10: @comment    file looks decent.
   11: @c          Not an improvement IMO - anton
   12: @c          and anyway, this should be taken up
   13: @c          with Karl Berry (the texinfo guy) - anton
   14: @c
   15: @c Karl Berry writes:
   16: @c  If they don't like the all-caps for @var Info output, all I can say is
   17: @c  that it's always been that way, and the usage of all-caps for
   18: @c  metavariables has a long tradition.  I think it's best to just let it be
   19: @c  what it is, for the sake of consistency among manuals.
   20: @c
   21: @comment .. would be useful to have a word that identified all deferred words
   22: @comment should semantics stuff in intro be moved to another section
   23: 
   24: @c POSTPONE, COMPILE, [COMPILE], LITERAL should have their own section
   25: 
   26: @comment %**start of header (This is for running Texinfo on a region.)
   27: @setfilename gforth.info
   28: @include version.texi
   29: @settitle Gforth Manual
   30: @c @syncodeindex pg cp
   31: 
   32: @macro progstyle {}
   33: Programming style note:
   34: @end macro
   35: 
   36: @macro assignment {}
   37: @table @i
   38: @item Assignment:
   39: @end macro
   40: @macro endassignment {}
   41: @end table
   42: @end macro
   43: 
   44: @comment macros for beautifying glossary entries
   45: @macro GLOSS-START {}
   46: @iftex
   47: @ninerm
   48: @end iftex
   49: @end macro
   50: 
   51: @macro GLOSS-END {}
   52: @iftex
   53: @rm
   54: @end iftex
   55: @end macro
   56: 
   57: @comment %**end of header (This is for running Texinfo on a region.)
   58: @copying
   59: This manual is for Gforth (version @value{VERSION}, @value{UPDATED}),
   60: a fast and portable implementation of the ANS Forth language.  It
   61: serves as reference manual, but it also contains an introduction to
   62: Forth and a Forth tutorial.
   63: 
   64: Copyright @copyright{} 1995, 1996, 1997, 1998, 2000, 2003 Free Software Foundation, Inc.
   65: 
   66: @quotation
   67: Permission is granted to copy, distribute and/or modify this document
   68: under the terms of the GNU Free Documentation License, Version 1.1 or
   69: any later version published by the Free Software Foundation; with no
   70: Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
   71: and with the Back-Cover Texts as in (a) below.  A copy of the
   72: license is included in the section entitled ``GNU Free Documentation
   73: License.''
   74: 
   75: (a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
   76: this GNU Manual, like GNU software.  Copies published by the Free
   77: Software Foundation raise funds for GNU development.''
   78: @end quotation
   79: @end copying
   80: 
   81: @dircategory Software development
   82: @direntry
   83: * Gforth: (gforth).             A fast interpreter for the Forth language.
   84: @end direntry
   85: @c The Texinfo manual also recommends doing this, but for Gforth it may
   86: @c  not make much sense
   87: @c @dircategory Individual utilities
   88: @c @direntry
   89: @c * Gforth: (gforth)Invoking Gforth.      gforth, gforth-fast, gforthmi
   90: @c @end direntry
   91: 
   92: @titlepage
   93: @title Gforth
   94: @subtitle for version @value{VERSION}, @value{UPDATED}
   95: @author Neal Crook
   96: @author Anton Ertl
   97: @author David Kuehling
   98: @author Bernd Paysan
   99: @author Jens Wilke
  100: @page
  101: @vskip 0pt plus 1filll
  102: @insertcopying
  103: @end titlepage
  104: 
  105: @contents
  106: 
  107: @ifnottex
  108: @node Top, Goals, (dir), (dir)
  109: @top Gforth
  110: 
  111: @insertcopying
  112: @end ifnottex
  113: 
  114: @menu
  115: * Goals::                       About the Gforth Project
  116: * Gforth Environment::          Starting (and exiting) Gforth
  117: * Tutorial::                    Hands-on Forth Tutorial
  118: * Introduction::                An introduction to ANS Forth
  119: * Words::                       Forth words available in Gforth
  120: * Error messages::              How to interpret them
  121: * Tools::                       Programming tools
  122: * ANS conformance::             Implementation-defined options etc.
  123: * Standard vs Extensions::      Should I use extensions?
  124: * Model::                       The abstract machine of Gforth
  125: * Integrating Gforth::          Forth as scripting language for applications
  126: * Emacs and Gforth::            The Gforth Mode
  127: * Image Files::                 @code{.fi} files contain compiled code
  128: * Engine::                      The inner interpreter and the primitives
  129: * Cross Compiler::              The Cross Compiler
  130: * Bugs::                        How to report them
  131: * Origin::                      Authors and ancestors of Gforth
  132: * Forth-related information::   Books and places to look on the WWW
  133: * Licenses::                    
  134: * Word Index::                  An item for each Forth word
  135: * Concept Index::               A menu covering many topics
  136: 
  137: @detailmenu
  138:  --- The Detailed Node Listing ---
  139: 
  140: Gforth Environment
  141: 
  142: * Invoking Gforth::             Getting in
  143: * Leaving Gforth::              Getting out
  144: * Command-line editing::        
  145: * Environment variables::       that affect how Gforth starts up
  146: * Gforth Files::                What gets installed and where
  147: * Gforth in pipes::             
  148: * Startup speed::               When 35ms is not fast enough ...
  149: 
  150: Forth Tutorial
  151: 
  152: * Starting Gforth Tutorial::    
  153: * Syntax Tutorial::             
  154: * Crash Course Tutorial::       
  155: * Stack Tutorial::              
  156: * Arithmetics Tutorial::        
  157: * Stack Manipulation Tutorial::  
  158: * Using files for Forth code Tutorial::  
  159: * Comments Tutorial::           
  160: * Colon Definitions Tutorial::  
  161: * Decompilation Tutorial::      
  162: * Stack-Effect Comments Tutorial::  
  163: * Types Tutorial::              
  164: * Factoring Tutorial::          
  165: * Designing the stack effect Tutorial::  
  166: * Local Variables Tutorial::    
  167: * Conditional execution Tutorial::  
  168: * Flags and Comparisons Tutorial::  
  169: * General Loops Tutorial::      
  170: * Counted loops Tutorial::      
  171: * Recursion Tutorial::          
  172: * Leaving definitions or loops Tutorial::  
  173: * Return Stack Tutorial::       
  174: * Memory Tutorial::             
  175: * Characters and Strings Tutorial::  
  176: * Alignment Tutorial::          
  177: * Files Tutorial::              
  178: * Interpretation and Compilation Semantics and Immediacy Tutorial::  
  179: * Execution Tokens Tutorial::   
  180: * Exceptions Tutorial::         
  181: * Defining Words Tutorial::     
  182: * Arrays and Records Tutorial::  
  183: * POSTPONE Tutorial::           
  184: * Literal Tutorial::            
  185: * Advanced macros Tutorial::    
  186: * Compilation Tokens Tutorial::  
  187: * Wordlists and Search Order Tutorial::  
  188: 
  189: An Introduction to ANS Forth
  190: 
  191: * Introducing the Text Interpreter::  
  192: * Stacks and Postfix notation::  
  193: * Your first definition::       
  194: * How does that work?::         
  195: * Forth is written in Forth::   
  196: * Review - elements of a Forth system::  
  197: * Where to go next::            
  198: * Exercises::                   
  199: 
  200: Forth Words
  201: 
  202: * Notation::                    
  203: * Case insensitivity::          
  204: * Comments::                    
  205: * Boolean Flags::               
  206: * Arithmetic::                  
  207: * Stack Manipulation::          
  208: * Memory::                      
  209: * Control Structures::          
  210: * Defining Words::              
  211: * Interpretation and Compilation Semantics::  
  212: * Tokens for Words::            
  213: * Compiling words::             
  214: * The Text Interpreter::        
  215: * The Input Stream::            
  216: * Word Lists::                  
  217: * Environmental Queries::       
  218: * Files::                       
  219: * Blocks::                      
  220: * Other I/O::                   
  221: * OS command line arguments::   
  222: * Locals::                      
  223: * Structures::                  
  224: * Object-oriented Forth::       
  225: * Programming Tools::           
  226: * Assembler and Code Words::    
  227: * Threading Words::             
  228: * Passing Commands to the OS::  
  229: * Keeping track of Time::       
  230: * Miscellaneous Words::         
  231: 
  232: Arithmetic
  233: 
  234: * Single precision::            
  235: * Double precision::            Double-cell integer arithmetic
  236: * Bitwise operations::          
  237: * Numeric comparison::          
  238: * Mixed precision::             Operations with single and double-cell integers
  239: * Floating Point::              
  240: 
  241: Stack Manipulation
  242: 
  243: * Data stack::                  
  244: * Floating point stack::        
  245: * Return stack::                
  246: * Locals stack::                
  247: * Stack pointer manipulation::  
  248: 
  249: Memory
  250: 
  251: * Memory model::                
  252: * Dictionary allocation::       
  253: * Heap Allocation::             
  254: * Memory Access::               
  255: * Address arithmetic::          
  256: * Memory Blocks::               
  257: 
  258: Control Structures
  259: 
  260: * Selection::                   IF ... ELSE ... ENDIF
  261: * Simple Loops::                BEGIN ...
  262: * Counted Loops::               DO
  263: * Arbitrary control structures::  
  264: * Calls and returns::           
  265: * Exception Handling::          
  266: 
  267: Defining Words
  268: 
  269: * CREATE::                      
  270: * Variables::                   Variables and user variables
  271: * Constants::                   
  272: * Values::                      Initialised variables
  273: * Colon Definitions::           
  274: * Anonymous Definitions::       Definitions without names
  275: * Supplying names::             Passing definition names as strings
  276: * User-defined Defining Words::  
  277: * Deferred words::              Allow forward references
  278: * Aliases::                     
  279: 
  280: User-defined Defining Words
  281: 
  282: * CREATE..DOES> applications::  
  283: * CREATE..DOES> details::       
  284: * Advanced does> usage example::  
  285: * @code{Const-does>}::          
  286: 
  287: Interpretation and Compilation Semantics
  288: 
  289: * Combined words::              
  290: 
  291: Tokens for Words
  292: 
  293: * Execution token::             represents execution/interpretation semantics
  294: * Compilation token::           represents compilation semantics
  295: * Name token::                  represents named words
  296: 
  297: Compiling words
  298: 
  299: * Literals::                    Compiling data values
  300: * Macros::                      Compiling words
  301: 
  302: The Text Interpreter
  303: 
  304: * Input Sources::               
  305: * Number Conversion::           
  306: * Interpret/Compile states::    
  307: * Interpreter Directives::      
  308: 
  309: Word Lists
  310: 
  311: * Vocabularies::                
  312: * Why use word lists?::         
  313: * Word list example::           
  314: 
  315: Files
  316: 
  317: * Forth source files::          
  318: * General files::               
  319: * Search Paths::                
  320: 
  321: Search Paths
  322: 
  323: * Source Search Paths::         
  324: * General Search Paths::        
  325: 
  326: Other I/O
  327: 
  328: * Simple numeric output::       Predefined formats
  329: * Formatted numeric output::    Formatted (pictured) output
  330: * String Formats::              How Forth stores strings in memory
  331: * Displaying characters and strings::  Other stuff
  332: * Input::                       Input
  333: * Pipes::                       How to create your own pipes
  334: 
  335: Locals
  336: 
  337: * Gforth locals::               
  338: * ANS Forth locals::            
  339: 
  340: Gforth locals
  341: 
  342: * Where are locals visible by name?::  
  343: * How long do locals live?::    
  344: * Locals programming style::    
  345: * Locals implementation::       
  346: 
  347: Structures
  348: 
  349: * Why explicit structure support?::  
  350: * Structure Usage::             
  351: * Structure Naming Convention::  
  352: * Structure Implementation::    
  353: * Structure Glossary::          
  354: 
  355: Object-oriented Forth
  356: 
  357: * Why object-oriented programming?::  
  358: * Object-Oriented Terminology::  
  359: * Objects::                     
  360: * OOF::                         
  361: * Mini-OOF::                    
  362: * Comparison with other object models::  
  363: 
  364: The @file{objects.fs} model
  365: 
  366: * Properties of the Objects model::  
  367: * Basic Objects Usage::         
  368: * The Objects base class::      
  369: * Creating objects::            
  370: * Object-Oriented Programming Style::  
  371: * Class Binding::               
  372: * Method conveniences::         
  373: * Classes and Scoping::         
  374: * Dividing classes::            
  375: * Object Interfaces::           
  376: * Objects Implementation::      
  377: * Objects Glossary::            
  378: 
  379: The @file{oof.fs} model
  380: 
  381: * Properties of the OOF model::  
  382: * Basic OOF Usage::             
  383: * The OOF base class::          
  384: * Class Declaration::           
  385: * Class Implementation::        
  386: 
  387: The @file{mini-oof.fs} model
  388: 
  389: * Basic Mini-OOF Usage::        
  390: * Mini-OOF Example::            
  391: * Mini-OOF Implementation::     
  392: 
  393: Programming Tools
  394: 
  395: * Examining::                   
  396: * Forgetting words::            
  397: * Debugging::                   Simple and quick.
  398: * Assertions::                  Making your programs self-checking.
  399: * Singlestep Debugger::         Executing your program word by word.
  400: 
  401: Assembler and Code Words
  402: 
  403: * Code and ;code::              
  404: * Common Assembler::            Assembler Syntax
  405: * Common Disassembler::         
  406: * 386 Assembler::               Deviations and special cases
  407: * Alpha Assembler::             Deviations and special cases
  408: * MIPS assembler::              Deviations and special cases
  409: * Other assemblers::            How to write them
  410: 
  411: Tools
  412: 
  413: * ANS Report::                  Report the words used, sorted by wordset.
  414: 
  415: ANS conformance
  416: 
  417: * The Core Words::              
  418: * The optional Block word set::  
  419: * The optional Double Number word set::  
  420: * The optional Exception word set::  
  421: * The optional Facility word set::  
  422: * The optional File-Access word set::  
  423: * The optional Floating-Point word set::  
  424: * The optional Locals word set::  
  425: * The optional Memory-Allocation word set::  
  426: * The optional Programming-Tools word set::  
  427: * The optional Search-Order word set::  
  428: 
  429: The Core Words
  430: 
  431: * core-idef::                   Implementation Defined Options                   
  432: * core-ambcond::                Ambiguous Conditions                
  433: * core-other::                  Other System Documentation                  
  434: 
  435: The optional Block word set
  436: 
  437: * block-idef::                  Implementation Defined Options
  438: * block-ambcond::               Ambiguous Conditions               
  439: * block-other::                 Other System Documentation                 
  440: 
  441: The optional Double Number word set
  442: 
  443: * double-ambcond::              Ambiguous Conditions              
  444: 
  445: The optional Exception word set
  446: 
  447: * exception-idef::              Implementation Defined Options              
  448: 
  449: The optional Facility word set
  450: 
  451: * facility-idef::               Implementation Defined Options               
  452: * facility-ambcond::            Ambiguous Conditions            
  453: 
  454: The optional File-Access word set
  455: 
  456: * file-idef::                   Implementation Defined Options
  457: * file-ambcond::                Ambiguous Conditions                
  458: 
  459: The optional Floating-Point word set
  460: 
  461: * floating-idef::               Implementation Defined Options
  462: * floating-ambcond::            Ambiguous Conditions            
  463: 
  464: The optional Locals word set
  465: 
  466: * locals-idef::                 Implementation Defined Options                 
  467: * locals-ambcond::              Ambiguous Conditions              
  468: 
  469: The optional Memory-Allocation word set
  470: 
  471: * memory-idef::                 Implementation Defined Options                 
  472: 
  473: The optional Programming-Tools word set
  474: 
  475: * programming-idef::            Implementation Defined Options            
  476: * programming-ambcond::         Ambiguous Conditions         
  477: 
  478: The optional Search-Order word set
  479: 
  480: * search-idef::                 Implementation Defined Options                 
  481: * search-ambcond::              Ambiguous Conditions              
  482: 
  483: Emacs and Gforth
  484: 
  485: * Installing gforth.el::        Making Emacs aware of Forth.
  486: * Emacs Tags::                  Viewing the source of a word in Emacs.
  487: * Hilighting::                  Making Forth code look prettier.
  488: * Auto-Indentation::            Customizing auto-indentation.
  489: * Blocks Files::                Reading and writing blocks files.
  490: 
  491: Image Files
  492: 
  493: * Image Licensing Issues::      Distribution terms for images.
  494: * Image File Background::       Why have image files?
  495: * Non-Relocatable Image Files::  don't always work.
  496: * Data-Relocatable Image Files::  are better.
  497: * Fully Relocatable Image Files::  better yet.
  498: * Stack and Dictionary Sizes::  Setting the default sizes for an image.
  499: * Running Image Files::         @code{gforth -i @i{file}} or @i{file}.
  500: * Modifying the Startup Sequence::  and turnkey applications.
  501: 
  502: Fully Relocatable Image Files
  503: 
  504: * gforthmi::                    The normal way
  505: * cross.fs::                    The hard way
  506: 
  507: Engine
  508: 
  509: * Portability::                 
  510: * Threading::                   
  511: * Primitives::                  
  512: * Performance::                 
  513: 
  514: Threading
  515: 
  516: * Scheduling::                  
  517: * Direct or Indirect Threaded?::  
  518: * Dynamic Superinstructions::   
  519: * DOES>::                       
  520: 
  521: Primitives
  522: 
  523: * Automatic Generation::        
  524: * TOS Optimization::            
  525: * Produced code::               
  526: 
  527: Cross Compiler
  528: 
  529: * Using the Cross Compiler::    
  530: * How the Cross Compiler Works::  
  531: 
  532: Licenses
  533: 
  534: * GNU Free Documentation License::  License for copying this manual.
  535: * Copying::                         GPL (for copying this software).
  536: 
  537: @end detailmenu
  538: @end menu
  539: 
  540: @c ----------------------------------------------------------
  541: @iftex
  542: @unnumbered Preface
  543: @cindex Preface
  544: This manual documents Gforth. Some introductory material is provided for
  545: readers who are unfamiliar with Forth or who are migrating to Gforth
  546: from other Forth compilers. However, this manual is primarily a
  547: reference manual.
  548: @end iftex
  549: 
  550: @comment TODO much more blurb here.
  551: 
  552: @c ******************************************************************
  553: @node Goals, Gforth Environment, Top, Top
  554: @comment node-name,     next,           previous, up
  555: @chapter Goals of Gforth
  556: @cindex goals of the Gforth project
  557: The goal of the Gforth Project is to develop a standard model for
  558: ANS Forth. This can be split into several subgoals:
  559: 
  560: @itemize @bullet
  561: @item
  562: Gforth should conform to the ANS Forth Standard.
  563: @item
  564: It should be a model, i.e. it should define all the
  565: implementation-dependent things.
  566: @item
  567: It should become standard, i.e. widely accepted and used. This goal
  568: is the most difficult one.
  569: @end itemize
  570: 
  571: To achieve these goals Gforth should be
  572: @itemize @bullet
  573: @item
  574: Similar to previous models (fig-Forth, F83)
  575: @item
  576: Powerful. It should provide for all the things that are considered
  577: necessary today and even some that are not yet considered necessary.
  578: @item
  579: Efficient. It should not get the reputation of being exceptionally
  580: slow.
  581: @item
  582: Free.
  583: @item
  584: Available on many machines/easy to port.
  585: @end itemize
  586: 
  587: Have we achieved these goals? Gforth conforms to the ANS Forth
  588: standard. It may be considered a model, but we have not yet documented
  589: which parts of the model are stable and which parts we are likely to
  590: change. It certainly has not yet become a de facto standard, but it
  591: appears to be quite popular. It has some similarities to and some
  592: differences from previous models. It has some powerful features, but not
  593: yet everything that we envisioned. We certainly have achieved our
  594: execution speed goals (@pxref{Performance})@footnote{However, in 1998
  595: the bar was raised when the major commercial Forth vendors switched to
  596: native code compilers.}.  It is free and available on many machines.
  597: 
  598: @c ******************************************************************
  599: @node Gforth Environment, Tutorial, Goals, Top
  600: @chapter Gforth Environment
  601: @cindex Gforth environment
  602: 
  603: Note: ultimately, the Gforth man page will be auto-generated from the
  604: material in this chapter.
  605: 
  606: @menu
  607: * Invoking Gforth::             Getting in
  608: * Leaving Gforth::              Getting out
  609: * Command-line editing::        
  610: * Environment variables::       that affect how Gforth starts up
  611: * Gforth Files::                What gets installed and where
  612: * Gforth in pipes::             
  613: * Startup speed::               When 35ms is not fast enough ...
  614: @end menu
  615: 
  616: For related information about the creation of images see @ref{Image Files}.
  617: 
  618: @comment ----------------------------------------------
  619: @node Invoking Gforth, Leaving Gforth, Gforth Environment, Gforth Environment
  620: @section Invoking Gforth
  621: @cindex invoking Gforth
  622: @cindex running Gforth
  623: @cindex command-line options
  624: @cindex options on the command line
  625: @cindex flags on the command line
  626: 
  627: Gforth is made up of two parts; an executable ``engine'' (named
  628: @command{gforth} or @command{gforth-fast}) and an image file. To start it, you
  629: will usually just say @code{gforth} -- this automatically loads the
  630: default image file @file{gforth.fi}. In many other cases the default
  631: Gforth image will be invoked like this:
  632: @example
  633: gforth [file | -e forth-code] ...
  634: @end example
  635: @noindent
  636: This interprets the contents of the files and the Forth code in the order they
  637: are given.
  638: 
  639: In addition to the @command{gforth} engine, there is also an engine
  640: called @command{gforth-fast}, which is faster, but gives less
  641: informative error messages (@pxref{Error messages}) and may catch some
  642: stack underflows later or not at all.  You should use it for debugged,
  643: performance-critical programs.
  644: 
  645: Moreover, there is an engine called @command{gforth-itc}, which is
  646: useful in some backwards-compatibility situations (@pxref{Direct or
  647: Indirect Threaded?}).
  648: 
  649: In general, the command line looks like this:
  650: 
  651: @example
  652: gforth[-fast] [engine options] [image options]
  653: @end example
  654: 
  655: The engine options must come before the rest of the command
  656: line. They are:
  657: 
  658: @table @code
  659: @cindex -i, command-line option
  660: @cindex --image-file, command-line option
  661: @item --image-file @i{file}
  662: @itemx -i @i{file}
  663: Loads the Forth image @i{file} instead of the default
  664: @file{gforth.fi} (@pxref{Image Files}).
  665: 
  666: @cindex --appl-image, command-line option
  667: @item --appl-image @i{file}
  668: Loads the image @i{file} and leaves all further command-line arguments
  669: to the image (instead of processing them as engine options).  This is
  670: useful for building executable application images on Unix, built with
  671: @code{gforthmi --application ...}.
  672: 
  673: @cindex --path, command-line option
  674: @cindex -p, command-line option
  675: @item --path @i{path}
  676: @itemx -p @i{path}
  677: Uses @i{path} for searching the image file and Forth source code files
  678: instead of the default in the environment variable @code{GFORTHPATH} or
  679: the path specified at installation time (e.g.,
  680: @file{/usr/local/share/gforth/0.2.0:.}). A path is given as a list of
  681: directories, separated by @samp{:} (on Unix) or @samp{;} (on other OSs).
  682: 
  683: @cindex --dictionary-size, command-line option
  684: @cindex -m, command-line option
  685: @cindex @i{size} parameters for command-line options
  686: @cindex size of the dictionary and the stacks
  687: @item --dictionary-size @i{size}
  688: @itemx -m @i{size}
  689: Allocate @i{size} space for the Forth dictionary space instead of
  690: using the default specified in the image (typically 256K). The
  691: @i{size} specification for this and subsequent options consists of
  692: an integer and a unit (e.g.,
  693: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
  694: size, in this case Cells), @code{k} (kilobytes), @code{M} (Megabytes),
  695: @code{G} (Gigabytes), and @code{T} (Terabytes). If no unit is specified,
  696: @code{e} is used.
  697: 
  698: @cindex --data-stack-size, command-line option
  699: @cindex -d, command-line option
  700: @item --data-stack-size @i{size}
  701: @itemx -d @i{size}
  702: Allocate @i{size} space for the data stack instead of using the
  703: default specified in the image (typically 16K).
  704: 
  705: @cindex --return-stack-size, command-line option
  706: @cindex -r, command-line option
  707: @item --return-stack-size @i{size}
  708: @itemx -r @i{size}
  709: Allocate @i{size} space for the return stack instead of using the
  710: default specified in the image (typically 15K).
  711: 
  712: @cindex --fp-stack-size, command-line option
  713: @cindex -f, command-line option
  714: @item --fp-stack-size @i{size}
  715: @itemx -f @i{size}
  716: Allocate @i{size} space for the floating point stack instead of
  717: using the default specified in the image (typically 15.5K). In this case
  718: the unit specifier @code{e} refers to floating point numbers.
  719: 
  720: @cindex --locals-stack-size, command-line option
  721: @cindex -l, command-line option
  722: @item --locals-stack-size @i{size}
  723: @itemx -l @i{size}
  724: Allocate @i{size} space for the locals stack instead of using the
  725: default specified in the image (typically 14.5K).
  726: 
  727: @cindex -h, command-line option
  728: @cindex --help, command-line option
  729: @item --help
  730: @itemx -h
  731: Print a message about the command-line options
  732: 
  733: @cindex -v, command-line option
  734: @cindex --version, command-line option
  735: @item --version
  736: @itemx -v
  737: Print version and exit
  738: 
  739: @cindex --debug, command-line option
  740: @item --debug
  741: Print some information useful for debugging on startup.
  742: 
  743: @cindex --offset-image, command-line option
  744: @item --offset-image
  745: Start the dictionary at a slightly different position than would be used
  746: otherwise (useful for creating data-relocatable images,
  747: @pxref{Data-Relocatable Image Files}).
  748: 
  749: @cindex --no-offset-im, command-line option
  750: @item --no-offset-im
  751: Start the dictionary at the normal position.
  752: 
  753: @cindex --clear-dictionary, command-line option
  754: @item --clear-dictionary
  755: Initialize all bytes in the dictionary to 0 before loading the image
  756: (@pxref{Data-Relocatable Image Files}).
  757: 
  758: @cindex --die-on-signal, command-line-option
  759: @item --die-on-signal
  760: Normally Gforth handles most signals (e.g., the user interrupt SIGINT,
  761: or the segmentation violation SIGSEGV) by translating it into a Forth
  762: @code{THROW}. With this option, Gforth exits if it receives such a
  763: signal. This option is useful when the engine and/or the image might be
  764: severely broken (such that it causes another signal before recovering
  765: from the first); this option avoids endless loops in such cases.
  766: 
  767: @cindex --no-dynamic, command-line option
  768: @cindex --dynamic, command-line option
  769: @item --no-dynamic
  770: @item --dynamic
  771: Disable or enable dynamic superinstructions with replication
  772: (@pxref{Dynamic Superinstructions}).
  773: 
  774: @cindex --no-super, command-line option
  775: @item --no-super
  776: Disable dynamic superinstructions, use just dynamic replication; this is
  777: useful if you want to patch threaded code (@pxref{Dynamic
  778: Superinstructions}).
  779: 
  780: @cindex --ss-number, command-line option
  781: @item --ss-number=@var{N}
  782: Use only the first @var{N} static superinstructions compiled into the
  783: engine (default: use them all; note that only @code{gforth-fast} has
  784: any).  This option is useful for measuring the performance impact of
  785: static superinstructions.
  786: 
  787: @cindex --ss-min-..., command-line options
  788: @item --ss-min-codesize
  789: @item --ss-min-ls
  790: @item --ss-min-lsu
  791: @item --ss-min-nexts
  792: Use specified metric for determining the cost of a primitive or static
  793: superinstruction for static superinstruction selection.  @code{Codesize}
  794: is the native code size of the primive or static superinstruction,
  795: @code{ls} is the number of loads and stores, @code{lsu} is the number of
  796: loads, stores, and updates, and @code{nexts} is the number of dispatches
  797: (not taking dynamic superinstructions into account), i.e. every
  798: primitive or static superinstruction has cost 1. Default:
  799: @code{codesize} if you use dynamic code generation, otherwise
  800: @code{nexts}.
  801: 
  802: @cindex --ss-greedy, command-line option
  803: @item --ss-greedy
  804: This option is useful for measuring the performance impact of static
  805: superinstructions.  By default, an optimal shortest-path algorithm is
  806: used for selecting static superinstructions.  With @option{--ss-greedy}
  807: this algorithm is modified to assume that anything after the static
  808: superinstruction currently under consideration is not combined into
  809: static superinstructions.  With @option{--ss-min-nexts} this produces
  810: the same result as a greedy algorithm that always selects the longest
  811: superinstruction available at the moment.  E.g., if there are
  812: superinstructions AB and BCD, then for the sequence A B C D the optimal
  813: algorithm will select A BCD and the greedy algorithm will select AB C D.
  814: 
  815: @cindex --print-metrics, command-line option
  816: @item --print-metrics
  817: Prints some metrics used during static superinstruction selection:
  818: @code{code size} is the actual size of the dynamically generated code.
  819: @code{Metric codesize} is the sum of the codesize metrics as seen by
  820: static superinstruction selection; there is a difference from @code{code
  821: size}, because not all primitives and static superinstructions are
  822: compiled into dynamically generated code, and because of markers.  The
  823: other metrics correspond to the @option{ss-min-...} options.  This
  824: option is useful for evaluating the effects of the @option{--ss-...}
  825: options.
  826: 
  827: @end table
  828: 
  829: @cindex loading files at startup
  830: @cindex executing code on startup
  831: @cindex batch processing with Gforth
  832: As explained above, the image-specific command-line arguments for the
  833: default image @file{gforth.fi} consist of a sequence of filenames and
  834: @code{-e @var{forth-code}} options that are interpreted in the sequence
  835: in which they are given. The @code{-e @var{forth-code}} or
  836: @code{--evaluate @var{forth-code}} option evaluates the Forth code. This
  837: option takes only one argument; if you want to evaluate more Forth
  838: words, you have to quote them or use @code{-e} several times. To exit
  839: after processing the command line (instead of entering interactive mode)
  840: append @code{-e bye} to the command line.  You can also process the
  841: command-line arguments with a Forth program (@pxref{OS command line
  842: arguments}).
  843: 
  844: @cindex versions, invoking other versions of Gforth
  845: If you have several versions of Gforth installed, @code{gforth} will
  846: invoke the version that was installed last. @code{gforth-@i{version}}
  847: invokes a specific version. If your environment contains the variable
  848: @code{GFORTHPATH}, you may want to override it by using the
  849: @code{--path} option.
  850: 
  851: Not yet implemented:
  852: On startup the system first executes the system initialization file
  853: (unless the option @code{--no-init-file} is given; note that the system
  854: resulting from using this option may not be ANS Forth conformant). Then
  855: the user initialization file @file{.gforth.fs} is executed, unless the
  856: option @code{--no-rc} is given; this file is searched for in @file{.},
  857: then in @file{~}, then in the normal path (see above).
  858: 
  859: 
  860: 
  861: @comment ----------------------------------------------
  862: @node Leaving Gforth, Command-line editing, Invoking Gforth, Gforth Environment
  863: @section Leaving Gforth
  864: @cindex Gforth - leaving
  865: @cindex leaving Gforth
  866: 
  867: You can leave Gforth by typing @code{bye} or @kbd{Ctrl-d} (at the start
  868: of a line) or (if you invoked Gforth with the @code{--die-on-signal}
  869: option) @kbd{Ctrl-c}. When you leave Gforth, all of your definitions and
  870: data are discarded.  For ways of saving the state of the system before
  871: leaving Gforth see @ref{Image Files}.
  872: 
  873: doc-bye
  874: 
  875: 
  876: @comment ----------------------------------------------
  877: @node Command-line editing, Environment variables, Leaving Gforth, Gforth Environment
  878: @section Command-line editing
  879: @cindex command-line editing
  880: 
  881: Gforth maintains a history file that records every line that you type to
  882: the text interpreter. This file is preserved between sessions, and is
  883: used to provide a command-line recall facility; if you type @kbd{Ctrl-P}
  884: repeatedly you can recall successively older commands from this (or
  885: previous) session(s). The full list of command-line editing facilities is:
  886: 
  887: @itemize @bullet
  888: @item
  889: @kbd{Ctrl-p} (``previous'') (or up-arrow) to recall successively older
  890: commands from the history buffer.
  891: @item
  892: @kbd{Ctrl-n} (``next'') (or down-arrow) to recall successively newer commands
  893: from the history buffer.
  894: @item
  895: @kbd{Ctrl-f} (or right-arrow) to move the cursor right, non-destructively.
  896: @item
  897: @kbd{Ctrl-b} (or left-arrow) to move the cursor left, non-destructively.
  898: @item
  899: @kbd{Ctrl-h} (backspace) to delete the character to the left of the cursor,
  900: closing up the line.
  901: @item
  902: @kbd{Ctrl-k} to delete (``kill'') from the cursor to the end of the line.
  903: @item
  904: @kbd{Ctrl-a} to move the cursor to the start of the line.
  905: @item
  906: @kbd{Ctrl-e} to move the cursor to the end of the line.
  907: @item
  908: @key{RET} (@kbd{Ctrl-m}) or @key{LFD} (@kbd{Ctrl-j}) to submit the current
  909: line.
  910: @item
  911: @key{TAB} to step through all possible full-word completions of the word
  912: currently being typed.
  913: @item
  914: @kbd{Ctrl-d} on an empty line line to terminate Gforth (gracefully,
  915: using @code{bye}). 
  916: @item
  917: @kbd{Ctrl-x} (or @code{Ctrl-d} on a non-empty line) to delete the
  918: character under the cursor.
  919: @end itemize
  920: 
  921: When editing, displayable characters are inserted to the left of the
  922: cursor position; the line is always in ``insert'' (as opposed to
  923: ``overstrike'') mode.
  924: 
  925: @cindex history file
  926: @cindex @file{.gforth-history}
  927: On Unix systems, the history file is @file{~/.gforth-history} by
  928: default@footnote{i.e. it is stored in the user's home directory.}. You
  929: can find out the name and location of your history file using:
  930: 
  931: @example 
  932: history-file type \ Unix-class systems
  933: 
  934: history-file type \ Other systems
  935: history-dir  type
  936: @end example
  937: 
  938: If you enter long definitions by hand, you can use a text editor to
  939: paste them out of the history file into a Forth source file for reuse at
  940: a later time.
  941: 
  942: Gforth never trims the size of the history file, so you should do this
  943: periodically, if necessary.
  944: 
  945: @comment this is all defined in history.fs
  946: @comment NAC TODO the ctrl-D behaviour can either do a bye or a beep.. how is that option
  947: @comment chosen?
  948: 
  949: 
  950: @comment ----------------------------------------------
  951: @node Environment variables, Gforth Files, Command-line editing, Gforth Environment
  952: @section Environment variables
  953: @cindex environment variables
  954: 
  955: Gforth uses these environment variables:
  956: 
  957: @itemize @bullet
  958: @item
  959: @cindex @code{GFORTHHIST} -- environment variable
  960: @code{GFORTHHIST} -- (Unix systems only) specifies the directory in which to
  961: open/create the history file, @file{.gforth-history}. Default:
  962: @code{$HOME}.
  963: 
  964: @item
  965: @cindex @code{GFORTHPATH} -- environment variable
  966: @code{GFORTHPATH} -- specifies the path used when searching for the gforth image file and
  967: for Forth source-code files.
  968: 
  969: @item
  970: @cindex @code{GFORTH} -- environment variable
  971: @code{GFORTH} -- used by @file{gforthmi}, @xref{gforthmi}.
  972: 
  973: @item
  974: @cindex @code{GFORTHD} -- environment variable
  975: @code{GFORTHD} -- used by @file{gforthmi}, @xref{gforthmi}.
  976: 
  977: @item
  978: @cindex @code{TMP}, @code{TEMP} - environment variable
  979: @code{TMP}, @code{TEMP} - (non-Unix systems only) used as a potential
  980: location for the history file.
  981: @end itemize
  982: 
  983: @comment also POSIXELY_CORRECT LINES COLUMNS HOME but no interest in
  984: @comment mentioning these.
  985: 
  986: All the Gforth environment variables default to sensible values if they
  987: are not set.
  988: 
  989: 
  990: @comment ----------------------------------------------
  991: @node Gforth Files, Gforth in pipes, Environment variables, Gforth Environment
  992: @section Gforth files
  993: @cindex Gforth files
  994: 
  995: When you install Gforth on a Unix system, it installs files in these
  996: locations by default:
  997: 
  998: @itemize @bullet
  999: @item
 1000: @file{/usr/local/bin/gforth}
 1001: @item
 1002: @file{/usr/local/bin/gforthmi}
 1003: @item
 1004: @file{/usr/local/man/man1/gforth.1} - man page.
 1005: @item
 1006: @file{/usr/local/info} - the Info version of this manual.
 1007: @item
 1008: @file{/usr/local/lib/gforth/<version>/...} - Gforth @file{.fi} files.
 1009: @item
 1010: @file{/usr/local/share/gforth/<version>/TAGS} - Emacs TAGS file.
 1011: @item
 1012: @file{/usr/local/share/gforth/<version>/...} - Gforth source files.
 1013: @item
 1014: @file{.../emacs/site-lisp/gforth.el} - Emacs gforth mode.
 1015: @end itemize
 1016: 
 1017: You can select different places for installation by using
 1018: @code{configure} options (listed with @code{configure --help}).
 1019: 
 1020: @comment ----------------------------------------------
 1021: @node Gforth in pipes, Startup speed, Gforth Files, Gforth Environment
 1022: @section Gforth in pipes
 1023: @cindex pipes, Gforth as part of
 1024: 
 1025: Gforth can be used in pipes created elsewhere (described here).  It can
 1026: also create pipes on its own (@pxref{Pipes}).
 1027: 
 1028: @cindex input from pipes
 1029: If you pipe into Gforth, your program should read with @code{read-file}
 1030: or @code{read-line} from @code{stdin} (@pxref{General files}).
 1031: @code{Key} does not recognize the end of input.  Words like
 1032: @code{accept} echo the input and are therefore usually not useful for
 1033: reading from a pipe.  You have to invoke the Forth program with an OS
 1034: command-line option, as you have no chance to use the Forth command line
 1035: (the text interpreter would try to interpret the pipe input).
 1036: 
 1037: @cindex output in pipes
 1038: You can output to a pipe with @code{type}, @code{emit}, @code{cr} etc.
 1039: 
 1040: @cindex silent exiting from Gforth
 1041: When you write to a pipe that has been closed at the other end, Gforth
 1042: receives a SIGPIPE signal (``pipe broken'').  Gforth translates this
 1043: into the exception @code{broken-pipe-error}.  If your application does
 1044: not catch that exception, the system catches it and exits, usually
 1045: silently (unless you were working on the Forth command line; then it
 1046: prints an error message and exits).  This is usually the desired
 1047: behaviour.
 1048: 
 1049: If you do not like this behaviour, you have to catch the exception
 1050: yourself, and react to it.
 1051: 
 1052: Here's an example of an invocation of Gforth that is usable in a pipe:
 1053: 
 1054: @example
 1055: gforth -e ": foo begin pad dup 10 stdin read-file throw dup while \
 1056:  type repeat ; foo bye"
 1057: @end example
 1058: 
 1059: This example just copies the input verbatim to the output.  A very
 1060: simple pipe containing this example looks like this:
 1061: 
 1062: @example
 1063: cat startup.fs |
 1064: gforth -e ": foo begin pad dup 80 stdin read-file throw dup while \
 1065:  type repeat ; foo bye"|
 1066: head
 1067: @end example
 1068: 
 1069: @cindex stderr and pipes
 1070: Pipes involving Gforth's @code{stderr} output do not work.
 1071: 
 1072: @comment ----------------------------------------------
 1073: @node Startup speed,  , Gforth in pipes, Gforth Environment
 1074: @section Startup speed
 1075: @cindex Startup speed
 1076: @cindex speed, startup
 1077: 
 1078: If Gforth is used for CGI scripts or in shell scripts, its startup
 1079: speed may become a problem.  On a 300MHz 21064a under Linux-2.2.13 with
 1080: glibc-2.0.7, @code{gforth -e bye} takes about 24.6ms user and 11.3ms
 1081: system time.
 1082: 
 1083: If startup speed is a problem, you may consider the following ways to
 1084: improve it; or you may consider ways to reduce the number of startups
 1085: (for example, by using Fast-CGI).
 1086: 
 1087: An easy step that influences Gforth startup speed is the use of the
 1088: @option{--no-dynamic} option; this decreases image loading speed, but
 1089: increases compile-time and run-time.
 1090: 
 1091: Another step to improve startup speed is to statically link Gforth, by
 1092: building it with @code{XLDFLAGS=-static}.  This requires more memory for
 1093: the code and will therefore slow down the first invocation, but
 1094: subsequent invocations avoid the dynamic linking overhead.  Another
 1095: disadvantage is that Gforth won't profit from library upgrades.  As a
 1096: result, @code{gforth-static -e bye} takes about 17.1ms user and
 1097: 8.2ms system time.
 1098: 
 1099: The next step to improve startup speed is to use a non-relocatable image
 1100: (@pxref{Non-Relocatable Image Files}).  You can create this image with
 1101: @code{gforth -e "savesystem gforthnr.fi bye"} and later use it with
 1102: @code{gforth -i gforthnr.fi ...}.  This avoids the relocation overhead
 1103: and a part of the copy-on-write overhead.  The disadvantage is that the
 1104: non-relocatable image does not work if the OS gives Gforth a different
 1105: address for the dictionary, for whatever reason; so you better provide a
 1106: fallback on a relocatable image.  @code{gforth-static -i gforthnr.fi -e
 1107: bye} takes about 15.3ms user and 7.5ms system time.
 1108: 
 1109: The final step is to disable dictionary hashing in Gforth.  Gforth
 1110: builds the hash table on startup, which takes much of the startup
 1111: overhead. You can do this by commenting out the @code{include hash.fs}
 1112: in @file{startup.fs} and everything that requires @file{hash.fs} (at the
 1113: moment @file{table.fs} and @file{ekey.fs}) and then doing @code{make}.
 1114: The disadvantages are that functionality like @code{table} and
 1115: @code{ekey} is missing and that text interpretation (e.g., compiling)
 1116: now takes much longer. So, you should only use this method if there is
 1117: no significant text interpretation to perform (the script should be
 1118: compiled into the image, amongst other things).  @code{gforth-static -i
 1119: gforthnrnh.fi -e bye} takes about 2.1ms user and 6.1ms system time.
 1120: 
 1121: @c ******************************************************************
 1122: @node Tutorial, Introduction, Gforth Environment, Top
 1123: @chapter Forth Tutorial
 1124: @cindex Tutorial
 1125: @cindex Forth Tutorial
 1126: 
 1127: @c Topics from nac's Introduction that could be mentioned:
 1128: @c press <ret> after each line
 1129: @c Prompt
 1130: @c numbers vs. words in dictionary on text interpretation
 1131: @c what happens on redefinition
 1132: @c parsing words (in particular, defining words)
 1133: 
 1134: The difference of this chapter from the Introduction
 1135: (@pxref{Introduction}) is that this tutorial is more fast-paced, should
 1136: be used while sitting in front of a computer, and covers much more
 1137: material, but does not explain how the Forth system works.
 1138: 
 1139: This tutorial can be used with any ANS-compliant Forth; any
 1140: Gforth-specific features are marked as such and you can skip them if you
 1141: work with another Forth.  This tutorial does not explain all features of
 1142: Forth, just enough to get you started and give you some ideas about the
 1143: facilities available in Forth.  Read the rest of the manual and the
 1144: standard when you are through this.
 1145: 
 1146: The intended way to use this tutorial is that you work through it while
 1147: sitting in front of the console, take a look at the examples and predict
 1148: what they will do, then try them out; if the outcome is not as expected,
 1149: find out why (e.g., by trying out variations of the example), so you
 1150: understand what's going on.  There are also some assignments that you
 1151: should solve.
 1152: 
 1153: This tutorial assumes that you have programmed before and know what,
 1154: e.g., a loop is.
 1155: 
 1156: @c !! explain compat library
 1157: 
 1158: @menu
 1159: * Starting Gforth Tutorial::    
 1160: * Syntax Tutorial::             
 1161: * Crash Course Tutorial::       
 1162: * Stack Tutorial::              
 1163: * Arithmetics Tutorial::        
 1164: * Stack Manipulation Tutorial::  
 1165: * Using files for Forth code Tutorial::  
 1166: * Comments Tutorial::           
 1167: * Colon Definitions Tutorial::  
 1168: * Decompilation Tutorial::      
 1169: * Stack-Effect Comments Tutorial::  
 1170: * Types Tutorial::              
 1171: * Factoring Tutorial::          
 1172: * Designing the stack effect Tutorial::  
 1173: * Local Variables Tutorial::    
 1174: * Conditional execution Tutorial::  
 1175: * Flags and Comparisons Tutorial::  
 1176: * General Loops Tutorial::      
 1177: * Counted loops Tutorial::      
 1178: * Recursion Tutorial::          
 1179: * Leaving definitions or loops Tutorial::  
 1180: * Return Stack Tutorial::       
 1181: * Memory Tutorial::             
 1182: * Characters and Strings Tutorial::  
 1183: * Alignment Tutorial::          
 1184: * Files Tutorial::              
 1185: * Interpretation and Compilation Semantics and Immediacy Tutorial::  
 1186: * Execution Tokens Tutorial::   
 1187: * Exceptions Tutorial::         
 1188: * Defining Words Tutorial::     
 1189: * Arrays and Records Tutorial::  
 1190: * POSTPONE Tutorial::           
 1191: * Literal Tutorial::            
 1192: * Advanced macros Tutorial::    
 1193: * Compilation Tokens Tutorial::  
 1194: * Wordlists and Search Order Tutorial::  
 1195: @end menu
 1196: 
 1197: @node Starting Gforth Tutorial, Syntax Tutorial, Tutorial, Tutorial
 1198: @section Starting Gforth
 1199: @cindex starting Gforth tutorial
 1200: You can start Gforth by typing its name:
 1201: 
 1202: @example
 1203: gforth
 1204: @end example
 1205: 
 1206: That puts you into interactive mode; you can leave Gforth by typing
 1207: @code{bye}.  While in Gforth, you can edit the command line and access
 1208: the command line history with cursor keys, similar to bash.
 1209: 
 1210: 
 1211: @node Syntax Tutorial, Crash Course Tutorial, Starting Gforth Tutorial, Tutorial
 1212: @section Syntax
 1213: @cindex syntax tutorial
 1214: 
 1215: A @dfn{word} is a sequence of arbitrary characters (expcept white
 1216: space).  Words are separated by white space.  E.g., each of the
 1217: following lines contains exactly one word:
 1218: 
 1219: @example
 1220: word
 1221: !@@#$%^&*()
 1222: 1234567890
 1223: 5!a
 1224: @end example
 1225: 
 1226: A frequent beginner's error is to leave away necessary white space,
 1227: resulting in an error like @samp{Undefined word}; so if you see such an
 1228: error, check if you have put spaces wherever necessary.
 1229: 
 1230: @example
 1231: ." hello, world" \ correct
 1232: ."hello, world"  \ gives an "Undefined word" error
 1233: @end example
 1234: 
 1235: Gforth and most other Forth systems ignore differences in case (they are
 1236: case-insensitive), i.e., @samp{word} is the same as @samp{Word}.  If
 1237: your system is case-sensitive, you may have to type all the examples
 1238: given here in upper case.
 1239: 
 1240: 
 1241: @node Crash Course Tutorial, Stack Tutorial, Syntax Tutorial, Tutorial
 1242: @section Crash Course
 1243: 
 1244: Type
 1245: 
 1246: @example
 1247: 0 0 !
 1248: here execute
 1249: ' catch >body 20 erase abort
 1250: ' (quit) >body 20 erase
 1251: @end example
 1252: 
 1253: The last two examples are guaranteed to destroy parts of Gforth (and
 1254: most other systems), so you better leave Gforth afterwards (if it has
 1255: not finished by itself).  On some systems you may have to kill gforth
 1256: from outside (e.g., in Unix with @code{kill}).
 1257: 
 1258: Now that you know how to produce crashes (and that there's not much to
 1259: them), let's learn how to produce meaningful programs.
 1260: 
 1261: 
 1262: @node Stack Tutorial, Arithmetics Tutorial, Crash Course Tutorial, Tutorial
 1263: @section Stack
 1264: @cindex stack tutorial
 1265: 
 1266: The most obvious feature of Forth is the stack.  When you type in a
 1267: number, it is pushed on the stack.  You can display the content of the
 1268: stack with @code{.s}.
 1269: 
 1270: @example
 1271: 1 2 .s
 1272: 3 .s
 1273: @end example
 1274: 
 1275: @code{.s} displays the top-of-stack to the right, i.e., the numbers
 1276: appear in @code{.s} output as they appeared in the input.
 1277: 
 1278: You can print the top of stack element with @code{.}.
 1279: 
 1280: @example
 1281: 1 2 3 . . .
 1282: @end example
 1283: 
 1284: In general, words consume their stack arguments (@code{.s} is an
 1285: exception).
 1286: 
 1287: @assignment
 1288: What does the stack contain after @code{5 6 7 .}?
 1289: @endassignment
 1290: 
 1291: 
 1292: @node Arithmetics Tutorial, Stack Manipulation Tutorial, Stack Tutorial, Tutorial
 1293: @section Arithmetics
 1294: @cindex arithmetics tutorial
 1295: 
 1296: The words @code{+}, @code{-}, @code{*}, @code{/}, and @code{mod} always
 1297: operate on the top two stack items:
 1298: 
 1299: @example
 1300: 2 2 .s
 1301: + .s
 1302: .
 1303: 2 1 - .
 1304: 7 3 mod .
 1305: @end example
 1306: 
 1307: The operands of @code{-}, @code{/}, and @code{mod} are in the same order
 1308: as in the corresponding infix expression (this is generally the case in
 1309: Forth).
 1310: 
 1311: Parentheses are superfluous (and not available), because the order of
 1312: the words unambiguously determines the order of evaluation and the
 1313: operands:
 1314: 
 1315: @example
 1316: 3 4 + 5 * .
 1317: 3 4 5 * + .
 1318: @end example
 1319: 
 1320: @assignment
 1321: What are the infix expressions corresponding to the Forth code above?
 1322: Write @code{6-7*8+9} in Forth notation@footnote{This notation is also
 1323: known as Postfix or RPN (Reverse Polish Notation).}.
 1324: @endassignment
 1325: 
 1326: To change the sign, use @code{negate}:
 1327: 
 1328: @example
 1329: 2 negate .
 1330: @end example
 1331: 
 1332: @assignment
 1333: Convert -(-3)*4-5 to Forth.
 1334: @endassignment
 1335: 
 1336: @code{/mod} performs both @code{/} and @code{mod}.
 1337: 
 1338: @example
 1339: 7 3 /mod . .
 1340: @end example
 1341: 
 1342: Reference: @ref{Arithmetic}.
 1343: 
 1344: 
 1345: @node Stack Manipulation Tutorial, Using files for Forth code Tutorial, Arithmetics Tutorial, Tutorial
 1346: @section Stack Manipulation
 1347: @cindex stack manipulation tutorial
 1348: 
 1349: Stack manipulation words rearrange the data on the stack.
 1350: 
 1351: @example
 1352: 1 .s drop .s
 1353: 1 .s dup .s drop drop .s
 1354: 1 2 .s over .s drop drop drop
 1355: 1 2 .s swap .s drop drop
 1356: 1 2 3 .s rot .s drop drop drop
 1357: @end example
 1358: 
 1359: These are the most important stack manipulation words.  There are also
 1360: variants that manipulate twice as many stack items:
 1361: 
 1362: @example
 1363: 1 2 3 4 .s 2swap .s 2drop 2drop
 1364: @end example
 1365: 
 1366: Two more stack manipulation words are:
 1367: 
 1368: @example
 1369: 1 2 .s nip .s drop
 1370: 1 2 .s tuck .s 2drop drop
 1371: @end example
 1372: 
 1373: @assignment
 1374: Replace @code{nip} and @code{tuck} with combinations of other stack
 1375: manipulation words.
 1376: 
 1377: @example
 1378: Given:          How do you get:
 1379: 1 2 3           3 2 1           
 1380: 1 2 3           1 2 3 2                 
 1381: 1 2 3           1 2 3 3                 
 1382: 1 2 3           1 3 3           
 1383: 1 2 3           2 1 3           
 1384: 1 2 3 4         4 3 2 1         
 1385: 1 2 3           1 2 3 1 2 3             
 1386: 1 2 3 4         1 2 3 4 1 2             
 1387: 1 2 3
 1388: 1 2 3           1 2 3 4                 
 1389: 1 2 3           1 3             
 1390: @end example
 1391: @endassignment
 1392: 
 1393: @example
 1394: 5 dup * .
 1395: @end example
 1396: 
 1397: @assignment
 1398: Write 17^3 and 17^4 in Forth, without writing @code{17} more than once.
 1399: Write a piece of Forth code that expects two numbers on the stack
 1400: (@var{a} and @var{b}, with @var{b} on top) and computes
 1401: @code{(a-b)(a+1)}.
 1402: @endassignment
 1403: 
 1404: Reference: @ref{Stack Manipulation}.
 1405: 
 1406: 
 1407: @node Using files for Forth code Tutorial, Comments Tutorial, Stack Manipulation Tutorial, Tutorial
 1408: @section Using files for Forth code
 1409: @cindex loading Forth code, tutorial
 1410: @cindex files containing Forth code, tutorial
 1411: 
 1412: While working at the Forth command line is convenient for one-line
 1413: examples and short one-off code, you probably want to store your source
 1414: code in files for convenient editing and persistence.  You can use your
 1415: favourite editor (Gforth includes Emacs support, @pxref{Emacs and
 1416: Gforth}) to create @var{file.fs} and use
 1417: 
 1418: @example
 1419: s" @var{file.fs}" included
 1420: @end example
 1421: 
 1422: to load it into your Forth system.  The file name extension I use for
 1423: Forth files is @samp{.fs}.
 1424: 
 1425: You can easily start Gforth with some files loaded like this:
 1426: 
 1427: @example
 1428: gforth @var{file1.fs} @var{file2.fs}
 1429: @end example
 1430: 
 1431: If an error occurs during loading these files, Gforth terminates,
 1432: whereas an error during @code{INCLUDED} within Gforth usually gives you
 1433: a Gforth command line.  Starting the Forth system every time gives you a
 1434: clean start every time, without interference from the results of earlier
 1435: tries.
 1436: 
 1437: I often put all the tests in a file, then load the code and run the
 1438: tests with
 1439: 
 1440: @example
 1441: gforth @var{code.fs} @var{tests.fs} -e bye
 1442: @end example
 1443: 
 1444: (often by performing this command with @kbd{C-x C-e} in Emacs).  The
 1445: @code{-e bye} ensures that Gforth terminates afterwards so that I can
 1446: restart this command without ado.
 1447: 
 1448: The advantage of this approach is that the tests can be repeated easily
 1449: every time the program ist changed, making it easy to catch bugs
 1450: introduced by the change.
 1451: 
 1452: Reference: @ref{Forth source files}.
 1453: 
 1454: 
 1455: @node Comments Tutorial, Colon Definitions Tutorial, Using files for Forth code Tutorial, Tutorial
 1456: @section Comments
 1457: @cindex comments tutorial
 1458: 
 1459: @example
 1460: \ That's a comment; it ends at the end of the line
 1461: ( Another comment; it ends here: )  .s
 1462: @end example
 1463: 
 1464: @code{\} and @code{(} are ordinary Forth words and therefore have to be
 1465: separated with white space from the following text.
 1466: 
 1467: @example
 1468: \This gives an "Undefined word" error
 1469: @end example
 1470: 
 1471: The first @code{)} ends a comment started with @code{(}, so you cannot
 1472: nest @code{(}-comments; and you cannot comment out text containing a
 1473: @code{)} with @code{( ... )}@footnote{therefore it's a good idea to
 1474: avoid @code{)} in word names.}.
 1475: 
 1476: I use @code{\}-comments for descriptive text and for commenting out code
 1477: of one or more line; I use @code{(}-comments for describing the stack
 1478: effect, the stack contents, or for commenting out sub-line pieces of
 1479: code.
 1480: 
 1481: The Emacs mode @file{gforth.el} (@pxref{Emacs and Gforth}) supports
 1482: these uses by commenting out a region with @kbd{C-x \}, uncommenting a
 1483: region with @kbd{C-u C-x \}, and filling a @code{\}-commented region
 1484: with @kbd{M-q}.
 1485: 
 1486: Reference: @ref{Comments}.
 1487: 
 1488: 
 1489: @node Colon Definitions Tutorial, Decompilation Tutorial, Comments Tutorial, Tutorial
 1490: @section Colon Definitions
 1491: @cindex colon definitions, tutorial
 1492: @cindex definitions, tutorial
 1493: @cindex procedures, tutorial
 1494: @cindex functions, tutorial
 1495: 
 1496: are similar to procedures and functions in other programming languages.
 1497: 
 1498: @example
 1499: : squared ( n -- n^2 )
 1500:    dup * ;
 1501: 5 squared .
 1502: 7 squared .
 1503: @end example
 1504: 
 1505: @code{:} starts the colon definition; its name is @code{squared}.  The
 1506: following comment describes its stack effect.  The words @code{dup *}
 1507: are not executed, but compiled into the definition.  @code{;} ends the
 1508: colon definition.
 1509: 
 1510: The newly-defined word can be used like any other word, including using
 1511: it in other definitions:
 1512: 
 1513: @example
 1514: : cubed ( n -- n^3 )
 1515:    dup squared * ;
 1516: -5 cubed .
 1517: : fourth-power ( n -- n^4 )
 1518:    squared squared ;
 1519: 3 fourth-power .
 1520: @end example
 1521: 
 1522: @assignment
 1523: Write colon definitions for @code{nip}, @code{tuck}, @code{negate}, and
 1524: @code{/mod} in terms of other Forth words, and check if they work (hint:
 1525: test your tests on the originals first).  Don't let the
 1526: @samp{redefined}-Messages spook you, they are just warnings.
 1527: @endassignment
 1528: 
 1529: Reference: @ref{Colon Definitions}.
 1530: 
 1531: 
 1532: @node Decompilation Tutorial, Stack-Effect Comments Tutorial, Colon Definitions Tutorial, Tutorial
 1533: @section Decompilation
 1534: @cindex decompilation tutorial
 1535: @cindex see tutorial
 1536: 
 1537: You can decompile colon definitions with @code{see}:
 1538: 
 1539: @example
 1540: see squared
 1541: see cubed
 1542: @end example
 1543: 
 1544: In Gforth @code{see} shows you a reconstruction of the source code from
 1545: the executable code.  Informations that were present in the source, but
 1546: not in the executable code, are lost (e.g., comments).
 1547: 
 1548: You can also decompile the predefined words:
 1549: 
 1550: @example
 1551: see .
 1552: see +
 1553: @end example
 1554: 
 1555: 
 1556: @node Stack-Effect Comments Tutorial, Types Tutorial, Decompilation Tutorial, Tutorial
 1557: @section Stack-Effect Comments
 1558: @cindex stack-effect comments, tutorial
 1559: @cindex --, tutorial
 1560: By convention the comment after the name of a definition describes the
 1561: stack effect: The part in from of the @samp{--} describes the state of
 1562: the stack before the execution of the definition, i.e., the parameters
 1563: that are passed into the colon definition; the part behind the @samp{--}
 1564: is the state of the stack after the execution of the definition, i.e.,
 1565: the results of the definition.  The stack comment only shows the top
 1566: stack items that the definition accesses and/or changes.
 1567: 
 1568: You should put a correct stack effect on every definition, even if it is
 1569: just @code{( -- )}.  You should also add some descriptive comment to
 1570: more complicated words (I usually do this in the lines following
 1571: @code{:}).  If you don't do this, your code becomes unreadable (because
 1572: you have to work through every definition before you can understand
 1573: any).
 1574: 
 1575: @assignment
 1576: The stack effect of @code{swap} can be written like this: @code{x1 x2 --
 1577: x2 x1}.  Describe the stack effect of @code{-}, @code{drop}, @code{dup},
 1578: @code{over}, @code{rot}, @code{nip}, and @code{tuck}.  Hint: When you
 1579: are done, you can compare your stack effects to those in this manual
 1580: (@pxref{Word Index}).
 1581: @endassignment
 1582: 
 1583: Sometimes programmers put comments at various places in colon
 1584: definitions that describe the contents of the stack at that place (stack
 1585: comments); i.e., they are like the first part of a stack-effect
 1586: comment. E.g.,
 1587: 
 1588: @example
 1589: : cubed ( n -- n^3 )
 1590:    dup squared  ( n n^2 ) * ;
 1591: @end example
 1592: 
 1593: In this case the stack comment is pretty superfluous, because the word
 1594: is simple enough.  If you think it would be a good idea to add such a
 1595: comment to increase readability, you should also consider factoring the
 1596: word into several simpler words (@pxref{Factoring Tutorial,,
 1597: Factoring}), which typically eliminates the need for the stack comment;
 1598: however, if you decide not to refactor it, then having such a comment is
 1599: better than not having it.
 1600: 
 1601: The names of the stack items in stack-effect and stack comments in the
 1602: standard, in this manual, and in many programs specify the type through
 1603: a type prefix, similar to Fortran and Hungarian notation.  The most
 1604: frequent prefixes are:
 1605: 
 1606: @table @code
 1607: @item n
 1608: signed integer
 1609: @item u
 1610: unsigned integer
 1611: @item c
 1612: character
 1613: @item f
 1614: Boolean flags, i.e. @code{false} or @code{true}.
 1615: @item a-addr,a-
 1616: Cell-aligned address
 1617: @item c-addr,c-
 1618: Char-aligned address (note that a Char may have two bytes in Windows NT)
 1619: @item xt
 1620: Execution token, same size as Cell
 1621: @item w,x
 1622: Cell, can contain an integer or an address.  It usually takes 32, 64 or
 1623: 16 bits (depending on your platform and Forth system). A cell is more
 1624: commonly known as machine word, but the term @emph{word} already means
 1625: something different in Forth.
 1626: @item d
 1627: signed double-cell integer
 1628: @item ud
 1629: unsigned double-cell integer
 1630: @item r
 1631: Float (on the FP stack)
 1632: @end table
 1633: 
 1634: You can find a more complete list in @ref{Notation}.
 1635: 
 1636: @assignment
 1637: Write stack-effect comments for all definitions you have written up to
 1638: now.
 1639: @endassignment
 1640: 
 1641: 
 1642: @node Types Tutorial, Factoring Tutorial, Stack-Effect Comments Tutorial, Tutorial
 1643: @section Types
 1644: @cindex types tutorial
 1645: 
 1646: In Forth the names of the operations are not overloaded; so similar
 1647: operations on different types need different names; e.g., @code{+} adds
 1648: integers, and you have to use @code{f+} to add floating-point numbers.
 1649: The following prefixes are often used for related operations on
 1650: different types:
 1651: 
 1652: @table @code
 1653: @item (none)
 1654: signed integer
 1655: @item u
 1656: unsigned integer
 1657: @item c
 1658: character
 1659: @item d
 1660: signed double-cell integer
 1661: @item ud, du
 1662: unsigned double-cell integer
 1663: @item 2
 1664: two cells (not-necessarily double-cell numbers)
 1665: @item m, um
 1666: mixed single-cell and double-cell operations
 1667: @item f
 1668: floating-point (note that in stack comments @samp{f} represents flags,
 1669: and @samp{r} represents FP numbers).
 1670: @end table
 1671: 
 1672: If there are no differences between the signed and the unsigned variant
 1673: (e.g., for @code{+}), there is only the prefix-less variant.
 1674: 
 1675: Forth does not perform type checking, neither at compile time, nor at
 1676: run time.  If you use the wrong oeration, the data are interpreted
 1677: incorrectly:
 1678: 
 1679: @example
 1680: -1 u.
 1681: @end example
 1682: 
 1683: If you have only experience with type-checked languages until now, and
 1684: have heard how important type-checking is, don't panic!  In my
 1685: experience (and that of other Forthers), type errors in Forth code are
 1686: usually easy to find (once you get used to it), the increased vigilance
 1687: of the programmer tends to catch some harder errors in addition to most
 1688: type errors, and you never have to work around the type system, so in
 1689: most situations the lack of type-checking seems to be a win (projects to
 1690: add type checking to Forth have not caught on).
 1691: 
 1692: 
 1693: @node Factoring Tutorial, Designing the stack effect Tutorial, Types Tutorial, Tutorial
 1694: @section Factoring
 1695: @cindex factoring tutorial
 1696: 
 1697: If you try to write longer definitions, you will soon find it hard to
 1698: keep track of the stack contents.  Therefore, good Forth programmers
 1699: tend to write only short definitions (e.g., three lines).  The art of
 1700: finding meaningful short definitions is known as factoring (as in
 1701: factoring polynomials).
 1702: 
 1703: Well-factored programs offer additional advantages: smaller, more
 1704: general words, are easier to test and debug and can be reused more and
 1705: better than larger, specialized words.
 1706: 
 1707: So, if you run into difficulties with stack management, when writing
 1708: code, try to define meaningful factors for the word, and define the word
 1709: in terms of those.  Even if a factor contains only two words, it is
 1710: often helpful.
 1711: 
 1712: Good factoring is not easy, and it takes some practice to get the knack
 1713: for it; but even experienced Forth programmers often don't find the
 1714: right solution right away, but only when rewriting the program.  So, if
 1715: you don't come up with a good solution immediately, keep trying, don't
 1716: despair.
 1717: 
 1718: @c example !!
 1719: 
 1720: 
 1721: @node Designing the stack effect Tutorial, Local Variables Tutorial, Factoring Tutorial, Tutorial
 1722: @section Designing the stack effect
 1723: @cindex Stack effect design, tutorial
 1724: @cindex design of stack effects, tutorial
 1725: 
 1726: In other languages you can use an arbitrary order of parameters for a
 1727: function; and since there is only one result, you don't have to deal with
 1728: the order of results, either.
 1729: 
 1730: In Forth (and other stack-based languages, e.g., PostScript) the
 1731: parameter and result order of a definition is important and should be
 1732: designed well.  The general guideline is to design the stack effect such
 1733: that the word is simple to use in most cases, even if that complicates
 1734: the implementation of the word.  Some concrete rules are:
 1735: 
 1736: @itemize @bullet
 1737: 
 1738: @item
 1739: Words consume all of their parameters (e.g., @code{.}).
 1740: 
 1741: @item
 1742: If there is a convention on the order of parameters (e.g., from
 1743: mathematics or another programming language), stick with it (e.g.,
 1744: @code{-}).
 1745: 
 1746: @item
 1747: If one parameter usually requires only a short computation (e.g., it is
 1748: a constant), pass it on the top of the stack.  Conversely, parameters
 1749: that usually require a long sequence of code to compute should be passed
 1750: as the bottom (i.e., first) parameter.  This makes the code easier to
 1751: read, because reader does not need to keep track of the bottom item
 1752: through a long sequence of code (or, alternatively, through stack
 1753: manipulations). E.g., @code{!} (store, @pxref{Memory}) expects the
 1754: address on top of the stack because it is usually simpler to compute
 1755: than the stored value (often the address is just a variable).
 1756: 
 1757: @item
 1758: Similarly, results that are usually consumed quickly should be returned
 1759: on the top of stack, whereas a result that is often used in long
 1760: computations should be passed as bottom result.  E.g., the file words
 1761: like @code{open-file} return the error code on the top of stack, because
 1762: it is usually consumed quickly by @code{throw}; moreover, the error code
 1763: has to be checked before doing anything with the other results.
 1764: 
 1765: @end itemize
 1766: 
 1767: These rules are just general guidelines, don't lose sight of the overall
 1768: goal to make the words easy to use.  E.g., if the convention rule
 1769: conflicts with the computation-length rule, you might decide in favour
 1770: of the convention if the word will be used rarely, and in favour of the
 1771: computation-length rule if the word will be used frequently (because
 1772: with frequent use the cost of breaking the computation-length rule would
 1773: be quite high, and frequent use makes it easier to remember an
 1774: unconventional order).
 1775: 
 1776: @c example !! structure package
 1777: 
 1778: 
 1779: @node Local Variables Tutorial, Conditional execution Tutorial, Designing the stack effect Tutorial, Tutorial
 1780: @section Local Variables
 1781: @cindex local variables, tutorial
 1782: 
 1783: You can define local variables (@emph{locals}) in a colon definition:
 1784: 
 1785: @example
 1786: : swap @{ a b -- b a @}
 1787:   b a ;
 1788: 1 2 swap .s 2drop
 1789: @end example
 1790: 
 1791: (If your Forth system does not support this syntax, include
 1792: @file{compat/anslocals.fs} first).
 1793: 
 1794: In this example @code{@{ a b -- b a @}} is the locals definition; it
 1795: takes two cells from the stack, puts the top of stack in @code{b} and
 1796: the next stack element in @code{a}.  @code{--} starts a comment ending
 1797: with @code{@}}.  After the locals definition, using the name of the
 1798: local will push its value on the stack.  You can leave the comment
 1799: part (@code{-- b a}) away:
 1800: 
 1801: @example
 1802: : swap ( x1 x2 -- x2 x1 )
 1803:   @{ a b @} b a ;
 1804: @end example
 1805: 
 1806: In Gforth you can have several locals definitions, anywhere in a colon
 1807: definition; in contrast, in a standard program you can have only one
 1808: locals definition per colon definition, and that locals definition must
 1809: be outside any controll structure.
 1810: 
 1811: With locals you can write slightly longer definitions without running
 1812: into stack trouble.  However, I recommend trying to write colon
 1813: definitions without locals for exercise purposes to help you gain the
 1814: essential factoring skills.
 1815: 
 1816: @assignment
 1817: Rewrite your definitions until now with locals
 1818: @endassignment
 1819: 
 1820: Reference: @ref{Locals}.
 1821: 
 1822: 
 1823: @node Conditional execution Tutorial, Flags and Comparisons Tutorial, Local Variables Tutorial, Tutorial
 1824: @section Conditional execution
 1825: @cindex conditionals, tutorial
 1826: @cindex if, tutorial
 1827: 
 1828: In Forth you can use control structures only inside colon definitions.
 1829: An @code{if}-structure looks like this:
 1830: 
 1831: @example
 1832: : abs ( n1 -- +n2 )
 1833:     dup 0 < if
 1834:         negate
 1835:     endif ;
 1836: 5 abs .
 1837: -5 abs .
 1838: @end example
 1839: 
 1840: @code{if} takes a flag from the stack.  If the flag is non-zero (true),
 1841: the following code is performed, otherwise execution continues after the
 1842: @code{endif} (or @code{else}).  @code{<} compares the top two stack
 1843: elements and prioduces a flag:
 1844: 
 1845: @example
 1846: 1 2 < .
 1847: 2 1 < .
 1848: 1 1 < .
 1849: @end example
 1850: 
 1851: Actually the standard name for @code{endif} is @code{then}.  This
 1852: tutorial presents the examples using @code{endif}, because this is often
 1853: less confusing for people familiar with other programming languages
 1854: where @code{then} has a different meaning.  If your system does not have
 1855: @code{endif}, define it with
 1856: 
 1857: @example
 1858: : endif postpone then ; immediate
 1859: @end example
 1860: 
 1861: You can optionally use an @code{else}-part:
 1862: 
 1863: @example
 1864: : min ( n1 n2 -- n )
 1865:   2dup < if
 1866:     drop
 1867:   else
 1868:     nip
 1869:   endif ;
 1870: 2 3 min .
 1871: 3 2 min .
 1872: @end example
 1873: 
 1874: @assignment
 1875: Write @code{min} without @code{else}-part (hint: what's the definition
 1876: of @code{nip}?).
 1877: @endassignment
 1878: 
 1879: Reference: @ref{Selection}.
 1880: 
 1881: 
 1882: @node Flags and Comparisons Tutorial, General Loops Tutorial, Conditional execution Tutorial, Tutorial
 1883: @section Flags and Comparisons
 1884: @cindex flags tutorial
 1885: @cindex comparison tutorial
 1886: 
 1887: In a false-flag all bits are clear (0 when interpreted as integer).  In
 1888: a canonical true-flag all bits are set (-1 as a twos-complement signed
 1889: integer); in many contexts (e.g., @code{if}) any non-zero value is
 1890: treated as true flag.
 1891: 
 1892: @example
 1893: false .
 1894: true .
 1895: true hex u. decimal
 1896: @end example
 1897: 
 1898: Comparison words produce canonical flags:
 1899: 
 1900: @example
 1901: 1 1 = .
 1902: 1 0= .
 1903: 0 1 < .
 1904: 0 0 < .
 1905: -1 1 u< . \ type error, u< interprets -1 as large unsigned number
 1906: -1 1 < .
 1907: @end example
 1908: 
 1909: Gforth supports all combinations of the prefixes @code{0 u d d0 du f f0}
 1910: (or none) and the comparisons @code{= <> < > <= >=}.  Only a part of
 1911: these combinations are standard (for details see the standard,
 1912: @ref{Numeric comparison}, @ref{Floating Point} or @ref{Word Index}).
 1913: 
 1914: You can use @code{and or xor invert} can be used as operations on
 1915: canonical flags.  Actually they are bitwise operations:
 1916: 
 1917: @example
 1918: 1 2 and .
 1919: 1 2 or .
 1920: 1 3 xor .
 1921: 1 invert .
 1922: @end example
 1923: 
 1924: You can convert a zero/non-zero flag into a canonical flag with
 1925: @code{0<>} (and complement it on the way with @code{0=}).
 1926: 
 1927: @example
 1928: 1 0= .
 1929: 1 0<> .
 1930: @end example
 1931: 
 1932: You can use the all-bits-set feature of canonical flags and the bitwise
 1933: operation of the Boolean operations to avoid @code{if}s:
 1934: 
 1935: @example
 1936: : foo ( n1 -- n2 )
 1937:   0= if
 1938:     14
 1939:   else
 1940:     0
 1941:   endif ;
 1942: 0 foo .
 1943: 1 foo .
 1944: 
 1945: : foo ( n1 -- n2 )
 1946:   0= 14 and ;
 1947: 0 foo .
 1948: 1 foo .
 1949: @end example
 1950: 
 1951: @assignment
 1952: Write @code{min} without @code{if}.
 1953: @endassignment
 1954: 
 1955: For reference, see @ref{Boolean Flags}, @ref{Numeric comparison}, and
 1956: @ref{Bitwise operations}.
 1957: 
 1958: 
 1959: @node General Loops Tutorial, Counted loops Tutorial, Flags and Comparisons Tutorial, Tutorial
 1960: @section General Loops
 1961: @cindex loops, indefinite, tutorial
 1962: 
 1963: The endless loop is the most simple one:
 1964: 
 1965: @example
 1966: : endless ( -- )
 1967:   0 begin
 1968:     dup . 1+
 1969:   again ;
 1970: endless
 1971: @end example
 1972: 
 1973: Terminate this loop by pressing @kbd{Ctrl-C} (in Gforth).  @code{begin}
 1974: does nothing at run-time, @code{again} jumps back to @code{begin}.
 1975: 
 1976: A loop with one exit at any place looks like this:
 1977: 
 1978: @example
 1979: : log2 ( +n1 -- n2 )
 1980: \ logarithmus dualis of n1>0, rounded down to the next integer
 1981:   assert( dup 0> )
 1982:   2/ 0 begin
 1983:     over 0> while
 1984:       1+ swap 2/ swap
 1985:   repeat
 1986:   nip ;
 1987: 7 log2 .
 1988: 8 log2 .
 1989: @end example
 1990: 
 1991: At run-time @code{while} consumes a flag; if it is 0, execution
 1992: continues behind the @code{repeat}; if the flag is non-zero, execution
 1993: continues behind the @code{while}.  @code{Repeat} jumps back to
 1994: @code{begin}, just like @code{again}.
 1995: 
 1996: In Forth there are many combinations/abbreviations, like @code{1+}.
 1997: However, @code{2/} is not one of them; it shifts its argument right by
 1998: one bit (arithmetic shift right):
 1999: 
 2000: @example
 2001: -5 2 / .
 2002: -5 2/ .
 2003: @end example
 2004: 
 2005: @code{assert(} is no standard word, but you can get it on systems other
 2006: then Gforth by including @file{compat/assert.fs}.  You can see what it
 2007: does by trying
 2008: 
 2009: @example
 2010: 0 log2 .
 2011: @end example
 2012: 
 2013: Here's a loop with an exit at the end:
 2014: 
 2015: @example
 2016: : log2 ( +n1 -- n2 )
 2017: \ logarithmus dualis of n1>0, rounded down to the next integer
 2018:   assert( dup 0 > )
 2019:   -1 begin
 2020:     1+ swap 2/ swap
 2021:     over 0 <=
 2022:   until
 2023:   nip ;
 2024: @end example
 2025: 
 2026: @code{Until} consumes a flag; if it is non-zero, execution continues at
 2027: the @code{begin}, otherwise after the @code{until}.
 2028: 
 2029: @assignment
 2030: Write a definition for computing the greatest common divisor.
 2031: @endassignment
 2032: 
 2033: Reference: @ref{Simple Loops}.
 2034: 
 2035: 
 2036: @node Counted loops Tutorial, Recursion Tutorial, General Loops Tutorial, Tutorial
 2037: @section Counted loops
 2038: @cindex loops, counted, tutorial
 2039: 
 2040: @example
 2041: : ^ ( n1 u -- n )
 2042: \ n = the uth power of u1
 2043:   1 swap 0 u+do
 2044:     over *
 2045:   loop
 2046:   nip ;
 2047: 3 2 ^ .
 2048: 4 3 ^ .
 2049: @end example
 2050: 
 2051: @code{U+do} (from @file{compat/loops.fs}, if your Forth system doesn't
 2052: have it) takes two numbers of the stack @code{( u3 u4 -- )}, and then
 2053: performs the code between @code{u+do} and @code{loop} for @code{u3-u4}
 2054: times (or not at all, if @code{u3-u4<0}).
 2055: 
 2056: You can see the stack effect design rules at work in the stack effect of
 2057: the loop start words: Since the start value of the loop is more
 2058: frequently constant than the end value, the start value is passed on
 2059: the top-of-stack.
 2060: 
 2061: You can access the counter of a counted loop with @code{i}:
 2062: 
 2063: @example
 2064: : fac ( u -- u! )
 2065:   1 swap 1+ 1 u+do
 2066:     i *
 2067:   loop ;
 2068: 5 fac .
 2069: 7 fac .
 2070: @end example
 2071: 
 2072: There is also @code{+do}, which expects signed numbers (important for
 2073: deciding whether to enter the loop).
 2074: 
 2075: @assignment
 2076: Write a definition for computing the nth Fibonacci number.
 2077: @endassignment
 2078: 
 2079: You can also use increments other than 1:
 2080: 
 2081: @example
 2082: : up2 ( n1 n2 -- )
 2083:   +do
 2084:     i .
 2085:   2 +loop ;
 2086: 10 0 up2
 2087: 
 2088: : down2 ( n1 n2 -- )
 2089:   -do
 2090:     i .
 2091:   2 -loop ;
 2092: 0 10 down2
 2093: @end example
 2094: 
 2095: Reference: @ref{Counted Loops}.
 2096: 
 2097: 
 2098: @node Recursion Tutorial, Leaving definitions or loops Tutorial, Counted loops Tutorial, Tutorial
 2099: @section Recursion
 2100: @cindex recursion tutorial
 2101: 
 2102: Usually the name of a definition is not visible in the definition; but
 2103: earlier definitions are usually visible:
 2104: 
 2105: @example
 2106: 1 0 / . \ "Floating-point unidentified fault" in Gforth on most platforms
 2107: : / ( n1 n2 -- n )
 2108:   dup 0= if
 2109:     -10 throw \ report division by zero
 2110:   endif
 2111:   /           \ old version
 2112: ;
 2113: 1 0 /
 2114: @end example
 2115: 
 2116: For recursive definitions you can use @code{recursive} (non-standard) or
 2117: @code{recurse}:
 2118: 
 2119: @example
 2120: : fac1 ( n -- n! ) recursive
 2121:  dup 0> if
 2122:    dup 1- fac1 *
 2123:  else
 2124:    drop 1
 2125:  endif ;
 2126: 7 fac1 .
 2127: 
 2128: : fac2 ( n -- n! )
 2129:  dup 0> if
 2130:    dup 1- recurse *
 2131:  else
 2132:    drop 1
 2133:  endif ;
 2134: 8 fac2 .
 2135: @end example
 2136: 
 2137: @assignment
 2138: Write a recursive definition for computing the nth Fibonacci number.
 2139: @endassignment
 2140: 
 2141: Reference (including indirect recursion): @xref{Calls and returns}.
 2142: 
 2143: 
 2144: @node Leaving definitions or loops Tutorial, Return Stack Tutorial, Recursion Tutorial, Tutorial
 2145: @section Leaving definitions or loops
 2146: @cindex leaving definitions, tutorial
 2147: @cindex leaving loops, tutorial
 2148: 
 2149: @code{EXIT} exits the current definition right away.  For every counted
 2150: loop that is left in this way, an @code{UNLOOP} has to be performed
 2151: before the @code{EXIT}:
 2152: 
 2153: @c !! real examples
 2154: @example
 2155: : ...
 2156:  ... u+do
 2157:    ... if
 2158:      ... unloop exit
 2159:    endif
 2160:    ...
 2161:  loop
 2162:  ... ;
 2163: @end example
 2164: 
 2165: @code{LEAVE} leaves the innermost counted loop right away:
 2166: 
 2167: @example
 2168: : ...
 2169:  ... u+do
 2170:    ... if
 2171:      ... leave
 2172:    endif
 2173:    ...
 2174:  loop
 2175:  ... ;
 2176: @end example
 2177: 
 2178: @c !! example
 2179: 
 2180: Reference: @ref{Calls and returns}, @ref{Counted Loops}.
 2181: 
 2182: 
 2183: @node Return Stack Tutorial, Memory Tutorial, Leaving definitions or loops Tutorial, Tutorial
 2184: @section Return Stack
 2185: @cindex return stack tutorial
 2186: 
 2187: In addition to the data stack Forth also has a second stack, the return
 2188: stack; most Forth systems store the return addresses of procedure calls
 2189: there (thus its name).  Programmers can also use this stack:
 2190: 
 2191: @example
 2192: : foo ( n1 n2 -- )
 2193:  .s
 2194:  >r .s
 2195:  r@@ .
 2196:  >r .s
 2197:  r@@ .
 2198:  r> .
 2199:  r@@ .
 2200:  r> . ;
 2201: 1 2 foo
 2202: @end example
 2203: 
 2204: @code{>r} takes an element from the data stack and pushes it onto the
 2205: return stack; conversely, @code{r>} moves an elementm from the return to
 2206: the data stack; @code{r@@} pushes a copy of the top of the return stack
 2207: on the return stack.
 2208: 
 2209: Forth programmers usually use the return stack for storing data
 2210: temporarily, if using the data stack alone would be too complex, and
 2211: factoring and locals are not an option:
 2212: 
 2213: @example
 2214: : 2swap ( x1 x2 x3 x4 -- x3 x4 x1 x2 )
 2215:  rot >r rot r> ;
 2216: @end example
 2217: 
 2218: The return address of the definition and the loop control parameters of
 2219: counted loops usually reside on the return stack, so you have to take
 2220: all items, that you have pushed on the return stack in a colon
 2221: definition or counted loop, from the return stack before the definition
 2222: or loop ends.  You cannot access items that you pushed on the return
 2223: stack outside some definition or loop within the definition of loop.
 2224: 
 2225: If you miscount the return stack items, this usually ends in a crash:
 2226: 
 2227: @example
 2228: : crash ( n -- )
 2229:   >r ;
 2230: 5 crash
 2231: @end example
 2232: 
 2233: You cannot mix using locals and using the return stack (according to the
 2234: standard; Gforth has no problem).  However, they solve the same
 2235: problems, so this shouldn't be an issue.
 2236: 
 2237: @assignment
 2238: Can you rewrite any of the definitions you wrote until now in a better
 2239: way using the return stack?
 2240: @endassignment
 2241: 
 2242: Reference: @ref{Return stack}.
 2243: 
 2244: 
 2245: @node Memory Tutorial, Characters and Strings Tutorial, Return Stack Tutorial, Tutorial
 2246: @section Memory
 2247: @cindex memory access/allocation tutorial
 2248: 
 2249: You can create a global variable @code{v} with
 2250: 
 2251: @example
 2252: variable v ( -- addr )
 2253: @end example
 2254: 
 2255: @code{v} pushes the address of a cell in memory on the stack.  This cell
 2256: was reserved by @code{variable}.  You can use @code{!} (store) to store
 2257: values into this cell and @code{@@} (fetch) to load the value from the
 2258: stack into memory:
 2259: 
 2260: @example
 2261: v .
 2262: 5 v ! .s
 2263: v @@ .
 2264: @end example
 2265: 
 2266: You can see a raw dump of memory with @code{dump}:
 2267: 
 2268: @example
 2269: v 1 cells .s dump
 2270: @end example
 2271: 
 2272: @code{Cells ( n1 -- n2 )} gives you the number of bytes (or, more
 2273: generally, address units (aus)) that @code{n1 cells} occupy.  You can
 2274: also reserve more memory:
 2275: 
 2276: @example
 2277: create v2 20 cells allot
 2278: v2 20 cells dump
 2279: @end example
 2280: 
 2281: creates a word @code{v2} and reserves 20 uninitialized cells; the
 2282: address pushed by @code{v2} points to the start of these 20 cells.  You
 2283: can use address arithmetic to access these cells:
 2284: 
 2285: @example
 2286: 3 v2 5 cells + !
 2287: v2 20 cells dump
 2288: @end example
 2289: 
 2290: You can reserve and initialize memory with @code{,}:
 2291: 
 2292: @example
 2293: create v3
 2294:   5 , 4 , 3 , 2 , 1 ,
 2295: v3 @@ .
 2296: v3 cell+ @@ .
 2297: v3 2 cells + @@ .
 2298: v3 5 cells dump
 2299: @end example
 2300: 
 2301: @assignment
 2302: Write a definition @code{vsum ( addr u -- n )} that computes the sum of
 2303: @code{u} cells, with the first of these cells at @code{addr}, the next
 2304: one at @code{addr cell+} etc.
 2305: @endassignment
 2306: 
 2307: You can also reserve memory without creating a new word:
 2308: 
 2309: @example
 2310: here 10 cells allot .
 2311: here .
 2312: @end example
 2313: 
 2314: @code{Here} pushes the start address of the memory area.  You should
 2315: store it somewhere, or you will have a hard time finding the memory area
 2316: again.
 2317: 
 2318: @code{Allot} manages dictionary memory.  The dictionary memory contains
 2319: the system's data structures for words etc. on Gforth and most other
 2320: Forth systems.  It is managed like a stack: You can free the memory that
 2321: you have just @code{allot}ed with
 2322: 
 2323: @example
 2324: -10 cells allot
 2325: here .
 2326: @end example
 2327: 
 2328: Note that you cannot do this if you have created a new word in the
 2329: meantime (because then your @code{allot}ed memory is no longer on the
 2330: top of the dictionary ``stack'').
 2331: 
 2332: Alternatively, you can use @code{allocate} and @code{free} which allow
 2333: freeing memory in any order:
 2334: 
 2335: @example
 2336: 10 cells allocate throw .s
 2337: 20 cells allocate throw .s
 2338: swap
 2339: free throw
 2340: free throw
 2341: @end example
 2342: 
 2343: The @code{throw}s deal with errors (e.g., out of memory).
 2344: 
 2345: And there is also a
 2346: @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
 2347: garbage collector}, which eliminates the need to @code{free} memory
 2348: explicitly.
 2349: 
 2350: Reference: @ref{Memory}.
 2351: 
 2352: 
 2353: @node Characters and Strings Tutorial, Alignment Tutorial, Memory Tutorial, Tutorial
 2354: @section Characters and Strings
 2355: @cindex strings tutorial
 2356: @cindex characters tutorial
 2357: 
 2358: On the stack characters take up a cell, like numbers.  In memory they
 2359: have their own size (one 8-bit byte on most systems), and therefore
 2360: require their own words for memory access:
 2361: 
 2362: @example
 2363: create v4 
 2364:   104 c, 97 c, 108 c, 108 c, 111 c,
 2365: v4 4 chars + c@@ .
 2366: v4 5 chars dump
 2367: @end example
 2368: 
 2369: The preferred representation of strings on the stack is @code{addr
 2370: u-count}, where @code{addr} is the address of the first character and
 2371: @code{u-count} is the number of characters in the string.
 2372: 
 2373: @example
 2374: v4 5 type
 2375: @end example
 2376: 
 2377: You get a string constant with
 2378: 
 2379: @example
 2380: s" hello, world" .s
 2381: type
 2382: @end example
 2383: 
 2384: Make sure you have a space between @code{s"} and the string; @code{s"}
 2385: is a normal Forth word and must be delimited with white space (try what
 2386: happens when you remove the space).
 2387: 
 2388: However, this interpretive use of @code{s"} is quite restricted: the
 2389: string exists only until the next call of @code{s"} (some Forth systems
 2390: keep more than one of these strings, but usually they still have a
 2391: limited lifetime).
 2392: 
 2393: @example
 2394: s" hello," s" world" .s
 2395: type
 2396: type
 2397: @end example
 2398: 
 2399: You can also use @code{s"} in a definition, and the resulting
 2400: strings then live forever (well, for as long as the definition):
 2401: 
 2402: @example
 2403: : foo s" hello," s" world" ;
 2404: foo .s
 2405: type
 2406: type
 2407: @end example
 2408: 
 2409: @assignment
 2410: @code{Emit ( c -- )} types @code{c} as character (not a number).
 2411: Implement @code{type ( addr u -- )}.
 2412: @endassignment
 2413: 
 2414: Reference: @ref{Memory Blocks}.
 2415: 
 2416: 
 2417: @node Alignment Tutorial, Files Tutorial, Characters and Strings Tutorial, Tutorial
 2418: @section Alignment
 2419: @cindex alignment tutorial
 2420: @cindex memory alignment tutorial
 2421: 
 2422: On many processors cells have to be aligned in memory, if you want to
 2423: access them with @code{@@} and @code{!} (and even if the processor does
 2424: not require alignment, access to aligned cells is faster).
 2425: 
 2426: @code{Create} aligns @code{here} (i.e., the place where the next
 2427: allocation will occur, and that the @code{create}d word points to).
 2428: Likewise, the memory produced by @code{allocate} starts at an aligned
 2429: address.  Adding a number of @code{cells} to an aligned address produces
 2430: another aligned address.
 2431: 
 2432: However, address arithmetic involving @code{char+} and @code{chars} can
 2433: create an address that is not cell-aligned.  @code{Aligned ( addr --
 2434: a-addr )} produces the next aligned address:
 2435: 
 2436: @example
 2437: v3 char+ aligned .s @@ .
 2438: v3 char+ .s @@ .
 2439: @end example
 2440: 
 2441: Similarly, @code{align} advances @code{here} to the next aligned
 2442: address:
 2443: 
 2444: @example
 2445: create v5 97 c,
 2446: here .
 2447: align here .
 2448: 1000 ,
 2449: @end example
 2450: 
 2451: Note that you should use aligned addresses even if your processor does
 2452: not require them, if you want your program to be portable.
 2453: 
 2454: Reference: @ref{Address arithmetic}.
 2455: 
 2456: 
 2457: @node Files Tutorial, Interpretation and Compilation Semantics and Immediacy Tutorial, Alignment Tutorial, Tutorial
 2458: @section Files
 2459: @cindex files tutorial
 2460: 
 2461: This section gives a short introduction into how to use files inside
 2462: Forth. It's broken up into five easy steps:
 2463: 
 2464: @enumerate 1
 2465: @item Opened an ASCII text file for input
 2466: @item Opened a file for output
 2467: @item Read input file until string matched (or some other condition matched)
 2468: @item Wrote some lines from input ( modified or not) to output
 2469: @item Closed the files.
 2470: @end enumerate
 2471: 
 2472: @subsection Open file for input
 2473: 
 2474: @example
 2475: s" foo.in"  r/o open-file throw Value fd-in
 2476: @end example
 2477: 
 2478: @subsection Create file for output
 2479: 
 2480: @example
 2481: s" foo.out" w/o create-file throw Value fd-out
 2482: @end example
 2483: 
 2484: The available file modes are r/o for read-only access, r/w for
 2485: read-write access, and w/o for write-only access. You could open both
 2486: files with r/w, too, if you like. All file words return error codes; for
 2487: most applications, it's best to pass there error codes with @code{throw}
 2488: to the outer error handler.
 2489: 
 2490: If you want words for opening and assigning, define them as follows:
 2491: 
 2492: @example
 2493: 0 Value fd-in
 2494: 0 Value fd-out
 2495: : open-input ( addr u -- )  r/o open-file throw to fd-in ;
 2496: : open-output ( addr u -- )  w/o create-file throw to fd-out ;
 2497: @end example
 2498: 
 2499: Usage example:
 2500: 
 2501: @example
 2502: s" foo.in" open-input
 2503: s" foo.out" open-output
 2504: @end example
 2505: 
 2506: @subsection Scan file for a particular line
 2507: 
 2508: @example
 2509: 256 Constant max-line
 2510: Create line-buffer  max-line 2 + allot
 2511: 
 2512: : scan-file ( addr u -- )
 2513:   begin
 2514:       line-buffer max-line fd-in read-line throw
 2515:   while
 2516:          >r 2dup line-buffer r> compare 0=
 2517:      until
 2518:   else
 2519:      drop
 2520:   then
 2521:   2drop ;
 2522: @end example
 2523: 
 2524: @code{read-line ( addr u1 fd -- u2 flag ior )} reads up to u1 bytes into
 2525: the buffer at addr, and returns the number of bytes read, a flag that is
 2526: false when the end of file is reached, and an error code.
 2527: 
 2528: @code{compare ( addr1 u1 addr2 u2 -- n )} compares two strings and
 2529: returns zero if both strings are equal. It returns a positive number if
 2530: the first string is lexically greater, a negative if the second string
 2531: is lexically greater.
 2532: 
 2533: We haven't seen this loop here; it has two exits. Since the @code{while}
 2534: exits with the number of bytes read on the stack, we have to clean up
 2535: that separately; that's after the @code{else}.
 2536: 
 2537: Usage example:
 2538: 
 2539: @example
 2540: s" The text I search is here" scan-file
 2541: @end example
 2542: 
 2543: @subsection Copy input to output
 2544: 
 2545: @example
 2546: : copy-file ( -- )
 2547:   begin
 2548:       line-buffer max-line fd-in read-line throw
 2549:   while
 2550:       line-buffer swap fd-out write-file throw
 2551:   repeat ;
 2552: @end example
 2553: 
 2554: @subsection Close files
 2555: 
 2556: @example
 2557: fd-in close-file throw
 2558: fd-out close-file throw
 2559: @end example
 2560: 
 2561: Likewise, you can put that into definitions, too:
 2562: 
 2563: @example
 2564: : close-input ( -- )  fd-in close-file throw ;
 2565: : close-output ( -- )  fd-out close-file throw ;
 2566: @end example
 2567: 
 2568: @assignment
 2569: How could you modify @code{copy-file} so that it copies until a second line is
 2570: matched? Can you write a program that extracts a section of a text file,
 2571: given the line that starts and the line that terminates that section?
 2572: @endassignment
 2573: 
 2574: @node Interpretation and Compilation Semantics and Immediacy Tutorial, Execution Tokens Tutorial, Files Tutorial, Tutorial
 2575: @section Interpretation and Compilation Semantics and Immediacy
 2576: @cindex semantics tutorial
 2577: @cindex interpretation semantics tutorial
 2578: @cindex compilation semantics tutorial
 2579: @cindex immediate, tutorial
 2580: 
 2581: When a word is compiled, it behaves differently from being interpreted.
 2582: E.g., consider @code{+}:
 2583: 
 2584: @example
 2585: 1 2 + .
 2586: : foo + ;
 2587: @end example
 2588: 
 2589: These two behaviours are known as compilation and interpretation
 2590: semantics.  For normal words (e.g., @code{+}), the compilation semantics
 2591: is to append the interpretation semantics to the currently defined word
 2592: (@code{foo} in the example above).  I.e., when @code{foo} is executed
 2593: later, the interpretation semantics of @code{+} (i.e., adding two
 2594: numbers) will be performed.
 2595: 
 2596: However, there are words with non-default compilation semantics, e.g.,
 2597: the control-flow words like @code{if}.  You can use @code{immediate} to
 2598: change the compilation semantics of the last defined word to be equal to
 2599: the interpretation semantics:
 2600: 
 2601: @example
 2602: : [FOO] ( -- )
 2603:  5 . ; immediate
 2604: 
 2605: [FOO]
 2606: : bar ( -- )
 2607:   [FOO] ;
 2608: bar
 2609: see bar
 2610: @end example
 2611: 
 2612: Two conventions to mark words with non-default compilation semnatics are
 2613: names with brackets (more frequently used) and to write them all in
 2614: upper case (less frequently used).
 2615: 
 2616: In Gforth (and many other systems) you can also remove the
 2617: interpretation semantics with @code{compile-only} (the compilation
 2618: semantics is derived from the original interpretation semantics):
 2619: 
 2620: @example
 2621: : flip ( -- )
 2622:  6 . ; compile-only \ but not immediate
 2623: flip
 2624: 
 2625: : flop ( -- )
 2626:  flip ;
 2627: flop
 2628: @end example
 2629: 
 2630: In this example the interpretation semantics of @code{flop} is equal to
 2631: the original interpretation semantics of @code{flip}.
 2632: 
 2633: The text interpreter has two states: in interpret state, it performs the
 2634: interpretation semantics of words it encounters; in compile state, it
 2635: performs the compilation semantics of these words.
 2636: 
 2637: Among other things, @code{:} switches into compile state, and @code{;}
 2638: switches back to interpret state.  They contain the factors @code{]}
 2639: (switch to compile state) and @code{[} (switch to interpret state), that
 2640: do nothing but switch the state.
 2641: 
 2642: @example
 2643: : xxx ( -- )
 2644:   [ 5 . ]
 2645: ;
 2646: 
 2647: xxx
 2648: see xxx
 2649: @end example
 2650: 
 2651: These brackets are also the source of the naming convention mentioned
 2652: above.
 2653: 
 2654: Reference: @ref{Interpretation and Compilation Semantics}.
 2655: 
 2656: 
 2657: @node Execution Tokens Tutorial, Exceptions Tutorial, Interpretation and Compilation Semantics and Immediacy Tutorial, Tutorial
 2658: @section Execution Tokens
 2659: @cindex execution tokens tutorial
 2660: @cindex XT tutorial
 2661: 
 2662: @code{' word} gives you the execution token (XT) of a word.  The XT is a
 2663: cell representing the interpretation semantics of a word.  You can
 2664: execute this semantics with @code{execute}:
 2665: 
 2666: @example
 2667: ' + .s
 2668: 1 2 rot execute .
 2669: @end example
 2670: 
 2671: The XT is similar to a function pointer in C.  However, parameter
 2672: passing through the stack makes it a little more flexible:
 2673: 
 2674: @example
 2675: : map-array ( ... addr u xt -- ... )
 2676: \ executes xt ( ... x -- ... ) for every element of the array starting
 2677: \ at addr and containing u elements
 2678:   @{ xt @}
 2679:   cells over + swap ?do
 2680:     i @@ xt execute
 2681:   1 cells +loop ;
 2682: 
 2683: create a 3 , 4 , 2 , -1 , 4 ,
 2684: a 5 ' . map-array .s
 2685: 0 a 5 ' + map-array .
 2686: s" max-n" environment? drop .s
 2687: a 5 ' min map-array .
 2688: @end example
 2689: 
 2690: You can use map-array with the XTs of words that consume one element
 2691: more than they produce.  In theory you can also use it with other XTs,
 2692: but the stack effect then depends on the size of the array, which is
 2693: hard to understand.
 2694: 
 2695: Since XTs are cell-sized, you can store them in memory and manipulate
 2696: them on the stack like other cells.  You can also compile the XT into a
 2697: word with @code{compile,}:
 2698: 
 2699: @example
 2700: : foo1 ( n1 n2 -- n )
 2701:    [ ' + compile, ] ;
 2702: see foo
 2703: @end example
 2704: 
 2705: This is non-standard, because @code{compile,} has no compilation
 2706: semantics in the standard, but it works in good Forth systems.  For the
 2707: broken ones, use
 2708: 
 2709: @example
 2710: : [compile,] compile, ; immediate
 2711: 
 2712: : foo1 ( n1 n2 -- n )
 2713:    [ ' + ] [compile,] ;
 2714: see foo
 2715: @end example
 2716: 
 2717: @code{'} is a word with default compilation semantics; it parses the
 2718: next word when its interpretation semantics are executed, not during
 2719: compilation:
 2720: 
 2721: @example
 2722: : foo ( -- xt )
 2723:   ' ;
 2724: see foo
 2725: : bar ( ... "word" -- ... )
 2726:   ' execute ;
 2727: see bar
 2728: 1 2 bar + .
 2729: @end example
 2730: 
 2731: You often want to parse a word during compilation and compile its XT so
 2732: it will be pushed on the stack at run-time.  @code{[']} does this:
 2733: 
 2734: @example
 2735: : xt-+ ( -- xt )
 2736:   ['] + ;
 2737: see xt-+
 2738: 1 2 xt-+ execute .
 2739: @end example
 2740: 
 2741: Many programmers tend to see @code{'} and the word it parses as one
 2742: unit, and expect it to behave like @code{[']} when compiled, and are
 2743: confused by the actual behaviour.  If you are, just remember that the
 2744: Forth system just takes @code{'} as one unit and has no idea that it is
 2745: a parsing word (attempts to convenience programmers in this issue have
 2746: usually resulted in even worse pitfalls, see
 2747: @uref{http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz,
 2748: @code{State}-smartness---Why it is evil and How to Exorcise it}).
 2749: 
 2750: Note that the state of the interpreter does not come into play when
 2751: creating and executing XTs.  I.e., even when you execute @code{'} in
 2752: compile state, it still gives you the interpretation semantics.  And
 2753: whatever that state is, @code{execute} performs the semantics
 2754: represented by the XT (i.e., for XTs produced with @code{'} the
 2755: interpretation semantics).
 2756: 
 2757: Reference: @ref{Tokens for Words}.
 2758: 
 2759: 
 2760: @node Exceptions Tutorial, Defining Words Tutorial, Execution Tokens Tutorial, Tutorial
 2761: @section Exceptions
 2762: @cindex exceptions tutorial
 2763: 
 2764: @code{throw ( n -- )} causes an exception unless n is zero.
 2765: 
 2766: @example
 2767: 100 throw .s
 2768: 0 throw .s
 2769: @end example
 2770: 
 2771: @code{catch ( ... xt -- ... n )} behaves similar to @code{execute}, but
 2772: it catches exceptions and pushes the number of the exception on the
 2773: stack (or 0, if the xt executed without exception).  If there was an
 2774: exception, the stacks have the same depth as when entering @code{catch}:
 2775: 
 2776: @example
 2777: .s
 2778: 3 0 ' / catch .s
 2779: 3 2 ' / catch .s
 2780: @end example
 2781: 
 2782: @assignment
 2783: Try the same with @code{execute} instead of @code{catch}.
 2784: @endassignment
 2785: 
 2786: @code{Throw} always jumps to the dynamically next enclosing
 2787: @code{catch}, even if it has to leave several call levels to achieve
 2788: this:
 2789: 
 2790: @example
 2791: : foo 100 throw ;
 2792: : foo1 foo ." after foo" ;
 2793: : bar ['] foo1 catch ;
 2794: bar .
 2795: @end example
 2796: 
 2797: It is often important to restore a value upon leaving a definition, even
 2798: if the definition is left through an exception.  You can ensure this
 2799: like this:
 2800: 
 2801: @example
 2802: : ...
 2803:    save-x
 2804:    ['] word-changing-x catch ( ... n )
 2805:    restore-x
 2806:    ( ... n ) throw ;
 2807: @end example
 2808: 
 2809: Gforth provides an alternative syntax in addition to @code{catch}:
 2810: @code{try ... recover ... endtry}.  If the code between @code{try} and
 2811: @code{recover} has an exception, the stack depths are restored, the
 2812: exception number is pushed on the stack, and the code between
 2813: @code{recover} and @code{endtry} is performed.  E.g., the definition for
 2814: @code{catch} is
 2815: 
 2816: @example
 2817: : catch ( x1 .. xn xt -- y1 .. ym 0 / z1 .. zn error ) \ exception
 2818:   try
 2819:     execute 0
 2820:   recover
 2821:     nip
 2822:   endtry ;
 2823: @end example
 2824: 
 2825: The equivalent to the restoration code above is
 2826: 
 2827: @example
 2828: : ...
 2829:   save-x
 2830:   try
 2831:     word-changing-x 0
 2832:   recover endtry
 2833:   restore-x
 2834:   throw ;
 2835: @end example
 2836: 
 2837: This works if @code{word-changing-x} does not change the stack depth,
 2838: otherwise you should add some code between @code{recover} and
 2839: @code{endtry} to balance the stack.
 2840: 
 2841: Reference: @ref{Exception Handling}.
 2842: 
 2843: 
 2844: @node Defining Words Tutorial, Arrays and Records Tutorial, Exceptions Tutorial, Tutorial
 2845: @section Defining Words
 2846: @cindex defining words tutorial
 2847: @cindex does> tutorial
 2848: @cindex create...does> tutorial
 2849: 
 2850: @c before semantics?
 2851: 
 2852: @code{:}, @code{create}, and @code{variable} are definition words: They
 2853: define other words.  @code{Constant} is another definition word:
 2854: 
 2855: @example
 2856: 5 constant foo
 2857: foo .
 2858: @end example
 2859: 
 2860: You can also use the prefixes @code{2} (double-cell) and @code{f}
 2861: (floating point) with @code{variable} and @code{constant}.
 2862: 
 2863: You can also define your own defining words.  E.g.:
 2864: 
 2865: @example
 2866: : variable ( "name" -- )
 2867:   create 0 , ;
 2868: @end example
 2869: 
 2870: You can also define defining words that create words that do something
 2871: other than just producing their address:
 2872: 
 2873: @example
 2874: : constant ( n "name" -- )
 2875:   create ,
 2876: does> ( -- n )
 2877:   ( addr ) @@ ;
 2878: 
 2879: 5 constant foo
 2880: foo .
 2881: @end example
 2882: 
 2883: The definition of @code{constant} above ends at the @code{does>}; i.e.,
 2884: @code{does>} replaces @code{;}, but it also does something else: It
 2885: changes the last defined word such that it pushes the address of the
 2886: body of the word and then performs the code after the @code{does>}
 2887: whenever it is called.
 2888: 
 2889: In the example above, @code{constant} uses @code{,} to store 5 into the
 2890: body of @code{foo}.  When @code{foo} executes, it pushes the address of
 2891: the body onto the stack, then (in the code after the @code{does>})
 2892: fetches the 5 from there.
 2893: 
 2894: The stack comment near the @code{does>} reflects the stack effect of the
 2895: defined word, not the stack effect of the code after the @code{does>}
 2896: (the difference is that the code expects the address of the body that
 2897: the stack comment does not show).
 2898: 
 2899: You can use these definition words to do factoring in cases that involve
 2900: (other) definition words.  E.g., a field offset is always added to an
 2901: address.  Instead of defining
 2902: 
 2903: @example
 2904: 2 cells constant offset-field1
 2905: @end example
 2906: 
 2907: and using this like
 2908: 
 2909: @example
 2910: ( addr ) offset-field1 +
 2911: @end example
 2912: 
 2913: you can define a definition word
 2914: 
 2915: @example
 2916: : simple-field ( n "name" -- )
 2917:   create ,
 2918: does> ( n1 -- n1+n )
 2919:   ( addr ) @@ + ;
 2920: @end example
 2921: 
 2922: Definition and use of field offsets now look like this:
 2923: 
 2924: @example
 2925: 2 cells simple-field field1
 2926: create mystruct 4 cells allot
 2927: mystruct .s field1 .s drop
 2928: @end example
 2929: 
 2930: If you want to do something with the word without performing the code
 2931: after the @code{does>}, you can access the body of a @code{create}d word
 2932: with @code{>body ( xt -- addr )}:
 2933: 
 2934: @example
 2935: : value ( n "name" -- )
 2936:   create ,
 2937: does> ( -- n1 )
 2938:   @@ ;
 2939: : to ( n "name" -- )
 2940:   ' >body ! ;
 2941: 
 2942: 5 value foo
 2943: foo .
 2944: 7 to foo
 2945: foo .
 2946: @end example
 2947: 
 2948: @assignment
 2949: Define @code{defer ( "name" -- )}, which creates a word that stores an
 2950: XT (at the start the XT of @code{abort}), and upon execution
 2951: @code{execute}s the XT.  Define @code{is ( xt "name" -- )} that stores
 2952: @code{xt} into @code{name}, a word defined with @code{defer}.  Indirect
 2953: recursion is one application of @code{defer}.
 2954: @endassignment
 2955: 
 2956: Reference: @ref{User-defined Defining Words}.
 2957: 
 2958: 
 2959: @node Arrays and Records Tutorial, POSTPONE Tutorial, Defining Words Tutorial, Tutorial
 2960: @section Arrays and Records
 2961: @cindex arrays tutorial
 2962: @cindex records tutorial
 2963: @cindex structs tutorial
 2964: 
 2965: Forth has no standard words for defining data structures such as arrays
 2966: and records (structs in C terminology), but you can build them yourself
 2967: based on address arithmetic.  You can also define words for defining
 2968: arrays and records (@pxref{Defining Words Tutorial,, Defining Words}).
 2969: 
 2970: One of the first projects a Forth newcomer sets out upon when learning
 2971: about defining words is an array defining word (possibly for
 2972: n-dimensional arrays).  Go ahead and do it, I did it, too; you will
 2973: learn something from it.  However, don't be disappointed when you later
 2974: learn that you have little use for these words (inappropriate use would
 2975: be even worse).  I have not yet found a set of useful array words yet;
 2976: the needs are just too diverse, and named, global arrays (the result of
 2977: naive use of defining words) are often not flexible enough (e.g.,
 2978: consider how to pass them as parameters).  Another such project is a set
 2979: of words to help dealing with strings.
 2980: 
 2981: On the other hand, there is a useful set of record words, and it has
 2982: been defined in @file{compat/struct.fs}; these words are predefined in
 2983: Gforth.  They are explained in depth elsewhere in this manual (see
 2984: @pxref{Structures}).  The @code{simple-field} example above is
 2985: simplified variant of fields in this package.
 2986: 
 2987: 
 2988: @node POSTPONE Tutorial, Literal Tutorial, Arrays and Records Tutorial, Tutorial
 2989: @section @code{POSTPONE}
 2990: @cindex postpone tutorial
 2991: 
 2992: You can compile the compilation semantics (instead of compiling the
 2993: interpretation semantics) of a word with @code{POSTPONE}:
 2994: 
 2995: @example
 2996: : MY-+ ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
 2997:  POSTPONE + ; immediate
 2998: : foo ( n1 n2 -- n )
 2999:  MY-+ ;
 3000: 1 2 foo .
 3001: see foo
 3002: @end example
 3003: 
 3004: During the definition of @code{foo} the text interpreter performs the
 3005: compilation semantics of @code{MY-+}, which performs the compilation
 3006: semantics of @code{+}, i.e., it compiles @code{+} into @code{foo}.
 3007: 
 3008: This example also displays separate stack comments for the compilation
 3009: semantics and for the stack effect of the compiled code.  For words with
 3010: default compilation semantics these stack effects are usually not
 3011: displayed; the stack effect of the compilation semantics is always
 3012: @code{( -- )} for these words, the stack effect for the compiled code is
 3013: the stack effect of the interpretation semantics.
 3014: 
 3015: Note that the state of the interpreter does not come into play when
 3016: performing the compilation semantics in this way.  You can also perform
 3017: it interpretively, e.g.:
 3018: 
 3019: @example
 3020: : foo2 ( n1 n2 -- n )
 3021:  [ MY-+ ] ;
 3022: 1 2 foo .
 3023: see foo
 3024: @end example
 3025: 
 3026: However, there are some broken Forth systems where this does not always
 3027: work, and therefore this practice was been declared non-standard in
 3028: 1999.
 3029: @c !! repair.fs
 3030: 
 3031: Here is another example for using @code{POSTPONE}:
 3032: 
 3033: @example
 3034: : MY-- ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
 3035:  POSTPONE negate POSTPONE + ; immediate compile-only
 3036: : bar ( n1 n2 -- n )
 3037:   MY-- ;
 3038: 2 1 bar .
 3039: see bar
 3040: @end example
 3041: 
 3042: You can define @code{ENDIF} in this way:
 3043: 
 3044: @example
 3045: : ENDIF ( Compilation: orig -- )
 3046:   POSTPONE then ; immediate
 3047: @end example
 3048: 
 3049: @assignment
 3050: Write @code{MY-2DUP} that has compilation semantics equivalent to
 3051: @code{2dup}, but compiles @code{over over}.
 3052: @endassignment
 3053: 
 3054: @c !! @xref{Macros} for reference
 3055: 
 3056: 
 3057: @node Literal Tutorial, Advanced macros Tutorial, POSTPONE Tutorial, Tutorial
 3058: @section @code{Literal}
 3059: @cindex literal tutorial
 3060: 
 3061: You cannot @code{POSTPONE} numbers:
 3062: 
 3063: @example
 3064: : [FOO] POSTPONE 500 ; immediate
 3065: @end example
 3066: 
 3067: Instead, you can use @code{LITERAL (compilation: n --; run-time: -- n )}:
 3068: 
 3069: @example
 3070: : [FOO] ( compilation: --; run-time: -- n )
 3071:   500 POSTPONE literal ; immediate
 3072: 
 3073: : flip [FOO] ;
 3074: flip .
 3075: see flip
 3076: @end example
 3077: 
 3078: @code{LITERAL} consumes a number at compile-time (when it's compilation
 3079: semantics are executed) and pushes it at run-time (when the code it
 3080: compiled is executed).  A frequent use of @code{LITERAL} is to compile a
 3081: number computed at compile time into the current word:
 3082: 
 3083: @example
 3084: : bar ( -- n )
 3085:   [ 2 2 + ] literal ;
 3086: see bar
 3087: @end example
 3088: 
 3089: @assignment
 3090: Write @code{]L} which allows writing the example above as @code{: bar (
 3091: -- n ) [ 2 2 + ]L ;}
 3092: @endassignment
 3093: 
 3094: @c !! @xref{Macros} for reference
 3095: 
 3096: 
 3097: @node Advanced macros Tutorial, Compilation Tokens Tutorial, Literal Tutorial, Tutorial
 3098: @section Advanced macros
 3099: @cindex macros, advanced tutorial
 3100: @cindex run-time code generation, tutorial
 3101: 
 3102: Reconsider @code{map-array} from @ref{Execution Tokens Tutorial,,
 3103: Execution Tokens}.  It frequently performs @code{execute}, a relatively
 3104: expensive operation in some Forth implementations.  You can use
 3105: @code{compile,} and @code{POSTPONE} to eliminate these @code{execute}s
 3106: and produce a word that contains the word to be performed directly:
 3107: 
 3108: @c use ]] ... [[
 3109: @example
 3110: : compile-map-array ( compilation: xt -- ; run-time: ... addr u -- ... )
 3111: \ at run-time, execute xt ( ... x -- ... ) for each element of the
 3112: \ array beginning at addr and containing u elements
 3113:   @{ xt @}
 3114:   POSTPONE cells POSTPONE over POSTPONE + POSTPONE swap POSTPONE ?do
 3115:     POSTPONE i POSTPONE @@ xt compile,
 3116:   1 cells POSTPONE literal POSTPONE +loop ;
 3117: 
 3118: : sum-array ( addr u -- n )
 3119:  0 rot rot [ ' + compile-map-array ] ;
 3120: see sum-array
 3121: a 5 sum-array .
 3122: @end example
 3123: 
 3124: You can use the full power of Forth for generating the code; here's an
 3125: example where the code is generated in a loop:
 3126: 
 3127: @example
 3128: : compile-vmul-step ( compilation: n --; run-time: n1 addr1 -- n2 addr2 )
 3129: \ n2=n1+(addr1)*n, addr2=addr1+cell
 3130:   POSTPONE tuck POSTPONE @@
 3131:   POSTPONE literal POSTPONE * POSTPONE +
 3132:   POSTPONE swap POSTPONE cell+ ;
 3133: 
 3134: : compile-vmul ( compilation: addr1 u -- ; run-time: addr2 -- n )
 3135: \ n=v1*v2 (inner product), where the v_i are represented as addr_i u
 3136:   0 postpone literal postpone swap
 3137:   [ ' compile-vmul-step compile-map-array ]
 3138:   postpone drop ;
 3139: see compile-vmul
 3140: 
 3141: : a-vmul ( addr -- n )
 3142: \ n=a*v, where v is a vector that's as long as a and starts at addr
 3143:  [ a 5 compile-vmul ] ;
 3144: see a-vmul
 3145: a a-vmul .
 3146: @end example
 3147: 
 3148: This example uses @code{compile-map-array} to show off, but you could
 3149: also use @code{map-array} instead (try it now!).
 3150: 
 3151: You can use this technique for efficient multiplication of large
 3152: matrices.  In matrix multiplication, you multiply every line of one
 3153: matrix with every column of the other matrix.  You can generate the code
 3154: for one line once, and use it for every column.  The only downside of
 3155: this technique is that it is cumbersome to recover the memory consumed
 3156: by the generated code when you are done (and in more complicated cases
 3157: it is not possible portably).
 3158: 
 3159: @c !! @xref{Macros} for reference
 3160: 
 3161: 
 3162: @node Compilation Tokens Tutorial, Wordlists and Search Order Tutorial, Advanced macros Tutorial, Tutorial
 3163: @section Compilation Tokens
 3164: @cindex compilation tokens, tutorial
 3165: @cindex CT, tutorial
 3166: 
 3167: This section is Gforth-specific.  You can skip it.
 3168: 
 3169: @code{' word compile,} compiles the interpretation semantics.  For words
 3170: with default compilation semantics this is the same as performing the
 3171: compilation semantics.  To represent the compilation semantics of other
 3172: words (e.g., words like @code{if} that have no interpretation
 3173: semantics), Gforth has the concept of a compilation token (CT,
 3174: consisting of two cells), and words @code{comp'} and @code{[comp']}.
 3175: You can perform the compilation semantics represented by a CT with
 3176: @code{execute}:
 3177: 
 3178: @example
 3179: : foo2 ( n1 n2 -- n )
 3180:    [ comp' + execute ] ;
 3181: see foo
 3182: @end example
 3183: 
 3184: You can compile the compilation semantics represented by a CT with
 3185: @code{postpone,}:
 3186: 
 3187: @example
 3188: : foo3 ( -- )
 3189:   [ comp' + postpone, ] ;
 3190: see foo3
 3191: @end example
 3192: 
 3193: @code{[ comp' word postpone, ]} is equivalent to @code{POSTPONE word}.
 3194: @code{comp'} is particularly useful for words that have no
 3195: interpretation semantics:
 3196: 
 3197: @example
 3198: ' if
 3199: comp' if .s 2drop
 3200: @end example
 3201: 
 3202: Reference: @ref{Tokens for Words}.
 3203: 
 3204: 
 3205: @node Wordlists and Search Order Tutorial,  , Compilation Tokens Tutorial, Tutorial
 3206: @section Wordlists and Search Order
 3207: @cindex wordlists tutorial
 3208: @cindex search order, tutorial
 3209: 
 3210: The dictionary is not just a memory area that allows you to allocate
 3211: memory with @code{allot}, it also contains the Forth words, arranged in
 3212: several wordlists.  When searching for a word in a wordlist,
 3213: conceptually you start searching at the youngest and proceed towards
 3214: older words (in reality most systems nowadays use hash-tables); i.e., if
 3215: you define a word with the same name as an older word, the new word
 3216: shadows the older word.
 3217: 
 3218: Which wordlists are searched in which order is determined by the search
 3219: order.  You can display the search order with @code{order}.  It displays
 3220: first the search order, starting with the wordlist searched first, then
 3221: it displays the wordlist that will contain newly defined words.
 3222: 
 3223: You can create a new, empty wordlist with @code{wordlist ( -- wid )}:
 3224: 
 3225: @example
 3226: wordlist constant mywords
 3227: @end example
 3228: 
 3229: @code{Set-current ( wid -- )} sets the wordlist that will contain newly
 3230: defined words (the @emph{current} wordlist):
 3231: 
 3232: @example
 3233: mywords set-current
 3234: order
 3235: @end example
 3236: 
 3237: Gforth does not display a name for the wordlist in @code{mywords}
 3238: because this wordlist was created anonymously with @code{wordlist}.
 3239: 
 3240: You can get the current wordlist with @code{get-current ( -- wid)}.  If
 3241: you want to put something into a specific wordlist without overall
 3242: effect on the current wordlist, this typically looks like this:
 3243: 
 3244: @example
 3245: get-current mywords set-current ( wid )
 3246: create someword
 3247: ( wid ) set-current
 3248: @end example
 3249: 
 3250: You can write the search order with @code{set-order ( wid1 .. widn n --
 3251: )} and read it with @code{get-order ( -- wid1 .. widn n )}.  The first
 3252: searched wordlist is topmost.
 3253: 
 3254: @example
 3255: get-order mywords swap 1+ set-order
 3256: order
 3257: @end example
 3258: 
 3259: Yes, the order of wordlists in the output of @code{order} is reversed
 3260: from stack comments and the output of @code{.s} and thus unintuitive.
 3261: 
 3262: @assignment
 3263: Define @code{>order ( wid -- )} with adds @code{wid} as first searched
 3264: wordlist to the search order.  Define @code{previous ( -- )}, which
 3265: removes the first searched wordlist from the search order.  Experiment
 3266: with boundary conditions (you will see some crashes or situations that
 3267: are hard or impossible to leave).
 3268: @endassignment
 3269: 
 3270: The search order is a powerful foundation for providing features similar
 3271: to Modula-2 modules and C++ namespaces.  However, trying to modularize
 3272: programs in this way has disadvantages for debugging and reuse/factoring
 3273: that overcome the advantages in my experience (I don't do huge projects,
 3274: though).  These disadvantages are not so clear in other
 3275: languages/programming environments, because these languages are not so
 3276: strong in debugging and reuse.
 3277: 
 3278: @c !! example
 3279: 
 3280: Reference: @ref{Word Lists}.
 3281: 
 3282: @c ******************************************************************
 3283: @node Introduction, Words, Tutorial, Top
 3284: @comment node-name,     next,           previous, up
 3285: @chapter An Introduction to ANS Forth
 3286: @cindex Forth - an introduction
 3287: 
 3288: The difference of this chapter from the Tutorial (@pxref{Tutorial}) is
 3289: that it is slower-paced in its examples, but uses them to dive deep into
 3290: explaining Forth internals (not covered by the Tutorial).  Apart from
 3291: that, this chapter covers far less material.  It is suitable for reading
 3292: without using a computer.
 3293: 
 3294: The primary purpose of this manual is to document Gforth. However, since
 3295: Forth is not a widely-known language and there is a lack of up-to-date
 3296: teaching material, it seems worthwhile to provide some introductory
 3297: material.  For other sources of Forth-related
 3298: information, see @ref{Forth-related information}.
 3299: 
 3300: The examples in this section should work on any ANS Forth; the
 3301: output shown was produced using Gforth. Each example attempts to
 3302: reproduce the exact output that Gforth produces. If you try out the
 3303: examples (and you should), what you should type is shown @kbd{like this}
 3304: and Gforth's response is shown @code{like this}. The single exception is
 3305: that, where the example shows @key{RET} it means that you should
 3306: press the ``carriage return'' key. Unfortunately, some output formats for
 3307: this manual cannot show the difference between @kbd{this} and
 3308: @code{this} which will make trying out the examples harder (but not
 3309: impossible).
 3310: 
 3311: Forth is an unusual language. It provides an interactive development
 3312: environment which includes both an interpreter and compiler. Forth
 3313: programming style encourages you to break a problem down into many
 3314: @cindex factoring
 3315: small fragments (@dfn{factoring}), and then to develop and test each
 3316: fragment interactively. Forth advocates assert that breaking the
 3317: edit-compile-test cycle used by conventional programming languages can
 3318: lead to great productivity improvements.
 3319: 
 3320: @menu
 3321: * Introducing the Text Interpreter::  
 3322: * Stacks and Postfix notation::  
 3323: * Your first definition::       
 3324: * How does that work?::         
 3325: * Forth is written in Forth::   
 3326: * Review - elements of a Forth system::  
 3327: * Where to go next::            
 3328: * Exercises::                   
 3329: @end menu
 3330: 
 3331: @comment ----------------------------------------------
 3332: @node Introducing the Text Interpreter, Stacks and Postfix notation, Introduction, Introduction
 3333: @section Introducing the Text Interpreter
 3334: @cindex text interpreter
 3335: @cindex outer interpreter
 3336: 
 3337: @c IMO this is too detailed and the pace is too slow for
 3338: @c an introduction.  If you know German, take a look at
 3339: @c http://www.complang.tuwien.ac.at/anton/lvas/skriptum-stack.html 
 3340: @c to see how I do it - anton 
 3341: 
 3342: @c nac-> Where I have accepted your comments 100% and modified the text
 3343: @c accordingly, I have deleted your comments. Elsewhere I have added a
 3344: @c response like this to attempt to rationalise what I have done. Of
 3345: @c course, this is a very clumsy mechanism for something that would be
 3346: @c done far more efficiently over a beer. Please delete any dialogue
 3347: @c you consider closed.
 3348: 
 3349: When you invoke the Forth image, you will see a startup banner printed
 3350: and nothing else (if you have Gforth installed on your system, try
 3351: invoking it now, by typing @kbd{gforth@key{RET}}). Forth is now running
 3352: its command line interpreter, which is called the @dfn{Text Interpreter}
 3353: (also known as the @dfn{Outer Interpreter}).  (You will learn a lot
 3354: about the text interpreter as you read through this chapter, for more
 3355: detail @pxref{The Text Interpreter}).
 3356: 
 3357: Although it's not obvious, Forth is actually waiting for your
 3358: input. Type a number and press the @key{RET} key:
 3359: 
 3360: @example
 3361: @kbd{45@key{RET}}  ok
 3362: @end example
 3363: 
 3364: Rather than give you a prompt to invite you to input something, the text
 3365: interpreter prints a status message @i{after} it has processed a line
 3366: of input. The status message in this case (``@code{ ok}'' followed by
 3367: carriage-return) indicates that the text interpreter was able to process
 3368: all of your input successfully. Now type something illegal:
 3369: 
 3370: @example
 3371: @kbd{qwer341@key{RET}}
 3372: :1: Undefined word
 3373: qwer341
 3374: ^^^^^^^
 3375: $400D2BA8 Bounce
 3376: $400DBDA8 no.extensions
 3377: @end example
 3378: 
 3379: The exact text, other than the ``Undefined word'' may differ slightly on
 3380: your system, but the effect is the same; when the text interpreter
 3381: detects an error, it discards any remaining text on a line, resets
 3382: certain internal state and prints an error message. For a detailed description of error messages see @ref{Error
 3383: messages}.
 3384: 
 3385: The text interpreter waits for you to press carriage-return, and then
 3386: processes your input line. Starting at the beginning of the line, it
 3387: breaks the line into groups of characters separated by spaces. For each
 3388: group of characters in turn, it makes two attempts to do something:
 3389: 
 3390: @itemize @bullet
 3391: @item
 3392: @cindex name dictionary
 3393: It tries to treat it as a command. It does this by searching a @dfn{name
 3394: dictionary}. If the group of characters matches an entry in the name
 3395: dictionary, the name dictionary provides the text interpreter with
 3396: information that allows the text interpreter perform some actions. In
 3397: Forth jargon, we say that the group
 3398: @cindex word
 3399: @cindex definition
 3400: @cindex execution token
 3401: @cindex xt
 3402: of characters names a @dfn{word}, that the dictionary search returns an
 3403: @dfn{execution token (xt)} corresponding to the @dfn{definition} of the
 3404: word, and that the text interpreter executes the xt. Often, the terms
 3405: @dfn{word} and @dfn{definition} are used interchangeably.
 3406: @item
 3407: If the text interpreter fails to find a match in the name dictionary, it
 3408: tries to treat the group of characters as a number in the current number
 3409: base (when you start up Forth, the current number base is base 10). If
 3410: the group of characters legitimately represents a number, the text
 3411: interpreter pushes the number onto a stack (we'll learn more about that
 3412: in the next section).
 3413: @end itemize
 3414: 
 3415: If the text interpreter is unable to do either of these things with any
 3416: group of characters, it discards the group of characters and the rest of
 3417: the line, then prints an error message. If the text interpreter reaches
 3418: the end of the line without error, it prints the status message ``@code{ ok}''
 3419: followed by carriage-return.
 3420: 
 3421: This is the simplest command we can give to the text interpreter:
 3422: 
 3423: @example
 3424: @key{RET}  ok
 3425: @end example
 3426: 
 3427: The text interpreter did everything we asked it to do (nothing) without
 3428: an error, so it said that everything is ``@code{ ok}''. Try a slightly longer
 3429: command:
 3430: 
 3431: @example
 3432: @kbd{12 dup fred dup@key{RET}}
 3433: :1: Undefined word
 3434: 12 dup fred dup
 3435:        ^^^^
 3436: $400D2BA8 Bounce
 3437: $400DBDA8 no.extensions
 3438: @end example
 3439: 
 3440: When you press the carriage-return key, the text interpreter starts to
 3441: work its way along the line:
 3442: 
 3443: @itemize @bullet
 3444: @item
 3445: When it gets to the space after the @code{2}, it takes the group of
 3446: characters @code{12} and looks them up in the name
 3447: dictionary@footnote{We can't tell if it found them or not, but assume
 3448: for now that it did not}. There is no match for this group of characters
 3449: in the name dictionary, so it tries to treat them as a number. It is
 3450: able to do this successfully, so it puts the number, 12, ``on the stack''
 3451: (whatever that means).
 3452: @item
 3453: The text interpreter resumes scanning the line and gets the next group
 3454: of characters, @code{dup}. It looks it up in the name dictionary and
 3455: (you'll have to take my word for this) finds it, and executes the word
 3456: @code{dup} (whatever that means).
 3457: @item
 3458: Once again, the text interpreter resumes scanning the line and gets the
 3459: group of characters @code{fred}. It looks them up in the name
 3460: dictionary, but can't find them. It tries to treat them as a number, but
 3461: they don't represent any legal number.
 3462: @end itemize
 3463: 
 3464: At this point, the text interpreter gives up and prints an error
 3465: message. The error message shows exactly how far the text interpreter
 3466: got in processing the line. In particular, it shows that the text
 3467: interpreter made no attempt to do anything with the final character
 3468: group, @code{dup}, even though we have good reason to believe that the
 3469: text interpreter would have no problem looking that word up and
 3470: executing it a second time.
 3471: 
 3472: 
 3473: @comment ----------------------------------------------
 3474: @node Stacks and Postfix notation, Your first definition, Introducing the Text Interpreter, Introduction
 3475: @section Stacks, postfix notation and parameter passing
 3476: @cindex text interpreter
 3477: @cindex outer interpreter
 3478: 
 3479: In procedural programming languages (like C and Pascal), the
 3480: building-block of programs is the @dfn{function} or @dfn{procedure}. These
 3481: functions or procedures are called with @dfn{explicit parameters}. For
 3482: example, in C we might write:
 3483: 
 3484: @example
 3485: total = total + new_volume(length,height,depth);
 3486: @end example
 3487: 
 3488: @noindent
 3489: where new_volume is a function-call to another piece of code, and total,
 3490: length, height and depth are all variables. length, height and depth are
 3491: parameters to the function-call.
 3492: 
 3493: In Forth, the equivalent of the function or procedure is the
 3494: @dfn{definition} and parameters are implicitly passed between
 3495: definitions using a shared stack that is visible to the
 3496: programmer. Although Forth does support variables, the existence of the
 3497: stack means that they are used far less often than in most other
 3498: programming languages. When the text interpreter encounters a number, it
 3499: will place (@dfn{push}) it on the stack. There are several stacks (the
 3500: actual number is implementation-dependent ...) and the particular stack
 3501: used for any operation is implied unambiguously by the operation being
 3502: performed. The stack used for all integer operations is called the @dfn{data
 3503: stack} and, since this is the stack used most commonly, references to
 3504: ``the data stack'' are often abbreviated to ``the stack''.
 3505: 
 3506: The stacks have a last-in, first-out (LIFO) organisation. If you type:
 3507: 
 3508: @example
 3509: @kbd{1 2 3@key{RET}}  ok
 3510: @end example
 3511: 
 3512: Then this instructs the text interpreter to placed three numbers on the
 3513: (data) stack. An analogy for the behaviour of the stack is to take a
 3514: pack of playing cards and deal out the ace (1), 2 and 3 into a pile on
 3515: the table. The 3 was the last card onto the pile (``last-in'') and if
 3516: you take a card off the pile then, unless you're prepared to fiddle a
 3517: bit, the card that you take off will be the 3 (``first-out''). The
 3518: number that will be first-out of the stack is called the @dfn{top of
 3519: stack}, which
 3520: @cindex TOS definition
 3521: is often abbreviated to @dfn{TOS}.
 3522: 
 3523: To understand how parameters are passed in Forth, consider the
 3524: behaviour of the definition @code{+} (pronounced ``plus''). You will not
 3525: be surprised to learn that this definition performs addition. More
 3526: precisely, it adds two number together and produces a result. Where does
 3527: it get the two numbers from? It takes the top two numbers off the
 3528: stack. Where does it place the result? On the stack. You can act-out the
 3529: behaviour of @code{+} with your playing cards like this:
 3530: 
 3531: @itemize @bullet
 3532: @item
 3533: Pick up two cards from the stack on the table
 3534: @item
 3535: Stare at them intently and ask yourself ``what @i{is} the sum of these two
 3536: numbers''
 3537: @item
 3538: Decide that the answer is 5
 3539: @item
 3540: Shuffle the two cards back into the pack and find a 5
 3541: @item
 3542: Put a 5 on the remaining ace that's on the table.
 3543: @end itemize
 3544: 
 3545: If you don't have a pack of cards handy but you do have Forth running,
 3546: you can use the definition @code{.s} to show the current state of the stack,
 3547: without affecting the stack. Type:
 3548: 
 3549: @example
 3550: @kbd{clearstacks 1 2 3@key{RET}} ok
 3551: @kbd{.s@key{RET}} <3> 1 2 3  ok
 3552: @end example
 3553: 
 3554: The text interpreter looks up the word @code{clearstacks} and executes
 3555: it; it tidies up the stacks and removes any entries that may have been
 3556: left on it by earlier examples. The text interpreter pushes each of the
 3557: three numbers in turn onto the stack. Finally, the text interpreter
 3558: looks up the word @code{.s} and executes it. The effect of executing
 3559: @code{.s} is to print the ``<3>'' (the total number of items on the stack)
 3560: followed by a list of all the items on the stack; the item on the far
 3561: right-hand side is the TOS.
 3562: 
 3563: You can now type:
 3564: 
 3565: @example
 3566: @kbd{+ .s@key{RET}} <2> 1 5  ok
 3567: @end example
 3568: 
 3569: @noindent
 3570: which is correct; there are now 2 items on the stack and the result of
 3571: the addition is 5.
 3572: 
 3573: If you're playing with cards, try doing a second addition: pick up the
 3574: two cards, work out that their sum is 6, shuffle them into the pack,
 3575: look for a 6 and place that on the table. You now have just one item on
 3576: the stack. What happens if you try to do a third addition? Pick up the
 3577: first card, pick up the second card -- ah! There is no second card. This
 3578: is called a @dfn{stack underflow} and consitutes an error. If you try to
 3579: do the same thing with Forth it often reports an error (probably a Stack
 3580: Underflow or an Invalid Memory Address error).
 3581: 
 3582: The opposite situation to a stack underflow is a @dfn{stack overflow},
 3583: which simply accepts that there is a finite amount of storage space
 3584: reserved for the stack. To stretch the playing card analogy, if you had
 3585: enough packs of cards and you piled the cards up on the table, you would
 3586: eventually be unable to add another card; you'd hit the ceiling. Gforth
 3587: allows you to set the maximum size of the stacks. In general, the only
 3588: time that you will get a stack overflow is because a definition has a
 3589: bug in it and is generating data on the stack uncontrollably.
 3590: 
 3591: There's one final use for the playing card analogy. If you model your
 3592: stack using a pack of playing cards, the maximum number of items on
 3593: your stack will be 52 (I assume you didn't use the Joker). The maximum
 3594: @i{value} of any item on the stack is 13 (the King). In fact, the only
 3595: possible numbers are positive integer numbers 1 through 13; you can't
 3596: have (for example) 0 or 27 or 3.52 or -2. If you change the way you
 3597: think about some of the cards, you can accommodate different
 3598: numbers. For example, you could think of the Jack as representing 0,
 3599: the Queen as representing -1 and the King as representing -2. Your
 3600: @i{range} remains unchanged (you can still only represent a total of 13
 3601: numbers) but the numbers that you can represent are -2 through 10.
 3602: 
 3603: In that analogy, the limit was the amount of information that a single
 3604: stack entry could hold, and Forth has a similar limit. In Forth, the
 3605: size of a stack entry is called a @dfn{cell}. The actual size of a cell is
 3606: implementation dependent and affects the maximum value that a stack
 3607: entry can hold. A Standard Forth provides a cell size of at least
 3608: 16-bits, and most desktop systems use a cell size of 32-bits.
 3609: 
 3610: Forth does not do any type checking for you, so you are free to
 3611: manipulate and combine stack items in any way you wish. A convenient way
 3612: of treating stack items is as 2's complement signed integers, and that
 3613: is what Standard words like @code{+} do. Therefore you can type:
 3614: 
 3615: @example
 3616: @kbd{-5 12 + .s@key{RET}} <1> 7  ok
 3617: @end example
 3618: 
 3619: If you use numbers and definitions like @code{+} in order to turn Forth
 3620: into a great big pocket calculator, you will realise that it's rather
 3621: different from a normal calculator. Rather than typing 2 + 3 = you had
 3622: to type 2 3 + (ignore the fact that you had to use @code{.s} to see the
 3623: result). The terminology used to describe this difference is to say that
 3624: your calculator uses @dfn{Infix Notation} (parameters and operators are
 3625: mixed) whilst Forth uses @dfn{Postfix Notation} (parameters and
 3626: operators are separate), also called @dfn{Reverse Polish Notation}.
 3627: 
 3628: Whilst postfix notation might look confusing to begin with, it has
 3629: several important advantages:
 3630: 
 3631: @itemize @bullet
 3632: @item
 3633: it is unambiguous
 3634: @item
 3635: it is more concise
 3636: @item
 3637: it fits naturally with a stack-based system
 3638: @end itemize
 3639: 
 3640: To examine these claims in more detail, consider these sums:
 3641: 
 3642: @example
 3643: 6 + 5 * 4 =
 3644: 4 * 5 + 6 =
 3645: @end example
 3646: 
 3647: If you're just learning maths or your maths is very rusty, you will
 3648: probably come up with the answer 44 for the first and 26 for the
 3649: second. If you are a bit of a whizz at maths you will remember the
 3650: @i{convention} that multiplication takes precendence over addition, and
 3651: you'd come up with the answer 26 both times. To explain the answer 26
 3652: to someone who got the answer 44, you'd probably rewrite the first sum
 3653: like this:
 3654: 
 3655: @example
 3656: 6 + (5 * 4) =
 3657: @end example
 3658: 
 3659: If what you really wanted was to perform the addition before the
 3660: multiplication, you would have to use parentheses to force it.
 3661: 
 3662: If you did the first two sums on a pocket calculator you would probably
 3663: get the right answers, unless you were very cautious and entered them using
 3664: these keystroke sequences:
 3665: 
 3666: 6 + 5 = * 4 =
 3667: 4 * 5 = + 6 =
 3668: 
 3669: Postfix notation is unambiguous because the order that the operators
 3670: are applied is always explicit; that also means that parentheses are
 3671: never required. The operators are @i{active} (the act of quoting the
 3672: operator makes the operation occur) which removes the need for ``=''.
 3673: 
 3674: The sum 6 + 5 * 4 can be written (in postfix notation) in two
 3675: equivalent ways:
 3676: 
 3677: @example
 3678: 6 5 4 * +      or:
 3679: 5 4 * 6 +
 3680: @end example
 3681: 
 3682: An important thing that you should notice about this notation is that
 3683: the @i{order} of the numbers does not change; if you want to subtract
 3684: 2 from 10 you type @code{10 2 -}.
 3685: 
 3686: The reason that Forth uses postfix notation is very simple to explain: it
 3687: makes the implementation extremely simple, and it follows naturally from
 3688: using the stack as a mechanism for passing parameters. Another way of
 3689: thinking about this is to realise that all Forth definitions are
 3690: @i{active}; they execute as they are encountered by the text
 3691: interpreter. The result of this is that the syntax of Forth is trivially
 3692: simple.
 3693: 
 3694: 
 3695: 
 3696: @comment ----------------------------------------------
 3697: @node Your first definition, How does that work?, Stacks and Postfix notation, Introduction
 3698: @section Your first Forth definition
 3699: @cindex first definition
 3700: 
 3701: Until now, the examples we've seen have been trivial; we've just been
 3702: using Forth as a bigger-than-pocket calculator. Also, each calculation
 3703: we've shown has been a ``one-off'' -- to repeat it we'd need to type it in
 3704: again@footnote{That's not quite true. If you press the up-arrow key on
 3705: your keyboard you should be able to scroll back to any earlier command,
 3706: edit it and re-enter it.} In this section we'll see how to add new
 3707: words to Forth's vocabulary.
 3708: 
 3709: The easiest way to create a new word is to use a @dfn{colon
 3710: definition}. We'll define a few and try them out before worrying too
 3711: much about how they work. Try typing in these examples; be careful to
 3712: copy the spaces accurately:
 3713: 
 3714: @example
 3715: : add-two 2 + . ;
 3716: : greet ." Hello and welcome" ;
 3717: : demo 5 add-two ;
 3718: @end example
 3719: 
 3720: @noindent
 3721: Now try them out:
 3722: 
 3723: @example
 3724: @kbd{greet@key{RET}} Hello and welcome  ok
 3725: @kbd{greet greet@key{RET}} Hello and welcomeHello and welcome  ok
 3726: @kbd{4 add-two@key{RET}} 6  ok
 3727: @kbd{demo@key{RET}} 7  ok
 3728: @kbd{9 greet demo add-two@key{RET}} Hello and welcome7 11  ok
 3729: @end example
 3730: 
 3731: The first new thing that we've introduced here is the pair of words
 3732: @code{:} and @code{;}. These are used to start and terminate a new
 3733: definition, respectively. The first word after the @code{:} is the name
 3734: for the new definition.
 3735: 
 3736: As you can see from the examples, a definition is built up of words that
 3737: have already been defined; Forth makes no distinction between
 3738: definitions that existed when you started the system up, and those that
 3739: you define yourself.
 3740: 
 3741: The examples also introduce the words @code{.} (dot), @code{."}
 3742: (dot-quote) and @code{dup} (dewp). Dot takes the value from the top of
 3743: the stack and displays it. It's like @code{.s} except that it only
 3744: displays the top item of the stack and it is destructive; after it has
 3745: executed, the number is no longer on the stack. There is always one
 3746: space printed after the number, and no spaces before it. Dot-quote
 3747: defines a string (a sequence of characters) that will be printed when
 3748: the word is executed. The string can contain any printable characters
 3749: except @code{"}. A @code{"} has a special function; it is not a Forth
 3750: word but it acts as a delimiter (the way that delimiters work is
 3751: described in the next section). Finally, @code{dup} duplicates the value
 3752: at the top of the stack. Try typing @code{5 dup .s} to see what it does.
 3753: 
 3754: We already know that the text interpreter searches through the
 3755: dictionary to locate names. If you've followed the examples earlier, you
 3756: will already have a definition called @code{add-two}. Lets try modifying
 3757: it by typing in a new definition:
 3758: 
 3759: @example
 3760: @kbd{: add-two dup . ." + 2 =" 2 + . ;@key{RET}} redefined add-two  ok
 3761: @end example
 3762: 
 3763: Forth recognised that we were defining a word that already exists, and
 3764: printed a message to warn us of that fact. Let's try out the new
 3765: definition:
 3766: 
 3767: @example
 3768: @kbd{9 add-two@key{RET}} 9 + 2 =11  ok
 3769: @end example
 3770: 
 3771: @noindent
 3772: All that we've actually done here, though, is to create a new
 3773: definition, with a particular name. The fact that there was already a
 3774: definition with the same name did not make any difference to the way
 3775: that the new definition was created (except that Forth printed a warning
 3776: message). The old definition of add-two still exists (try @code{demo}
 3777: again to see that this is true). Any new definition will use the new
 3778: definition of @code{add-two}, but old definitions continue to use the
 3779: version that already existed at the time that they were @code{compiled}.
 3780: 
 3781: Before you go on to the next section, try defining and redefining some
 3782: words of your own.
 3783: 
 3784: @comment ----------------------------------------------
 3785: @node How does that work?, Forth is written in Forth, Your first definition, Introduction
 3786: @section How does that work?
 3787: @cindex parsing words
 3788: 
 3789: @c That's pretty deep (IMO way too deep) for an introduction. - anton
 3790: 
 3791: @c Is it a good idea to talk about the interpretation semantics of a
 3792: @c number? We don't have an xt to go along with it. - anton
 3793: 
 3794: @c Now that I have eliminated execution semantics, I wonder if it would not
 3795: @c be better to keep them (or add run-time semantics), to make it easier to
 3796: @c explain what compilation semantics usually does. - anton
 3797: 
 3798: @c nac-> I removed the term ``default compilation sematics'' from the
 3799: @c introductory chapter. Removing ``execution semantics'' was making
 3800: @c everything simpler to explain, then I think the use of this term made
 3801: @c everything more complex again. I replaced it with ``default
 3802: @c semantics'' (which is used elsewhere in the manual) by which I mean
 3803: @c ``a definition that has neither the immediate nor the compile-only
 3804: @c flag set''.
 3805: 
 3806: @c anton: I have eliminated default semantics (except in one place where it
 3807: @c means "default interpretation and compilation semantics"), because it
 3808: @c makes no sense in the presence of combined words.  I reverted to
 3809: @c "execution semantics" where necessary.
 3810: 
 3811: @c nac-> I reworded big chunks of the ``how does that work''
 3812: @c section (and, unusually for me, I think I even made it shorter!).  See
 3813: @c what you think -- I know I have not addressed your primary concern
 3814: @c that it is too heavy-going for an introduction. From what I understood
 3815: @c of your course notes it looks as though they might be a good framework. 
 3816: @c Things that I've tried to capture here are some things that came as a
 3817: @c great revelation here when I first understood them. Also, I like the
 3818: @c fact that a very simple code example shows up almost all of the issues
 3819: @c that you need to understand to see how Forth works. That's unique and
 3820: @c worthwhile to emphasise.
 3821: 
 3822: @c anton: I think it's a good idea to present the details, especially those
 3823: @c that you found to be a revelation, and probably the tutorial tries to be
 3824: @c too superficial and does not get some of the things across that make
 3825: @c Forth special.  I do believe that most of the time these things should
 3826: @c be discussed at the end of a section or in separate sections instead of
 3827: @c in the middle of a section (e.g., the stuff you added in "User-defined
 3828: @c defining words" leads in a completely different direction from the rest
 3829: @c of the section).
 3830: 
 3831: Now we're going to take another look at the definition of @code{add-two}
 3832: from the previous section. From our knowledge of the way that the text
 3833: interpreter works, we would have expected this result when we tried to
 3834: define @code{add-two}:
 3835: 
 3836: @example
 3837: @kbd{: add-two 2 + . ;@key{RET}}
 3838:   ^^^^^^^
 3839: Error: Undefined word
 3840: @end example
 3841: 
 3842: The reason that this didn't happen is bound up in the way that @code{:}
 3843: works. The word @code{:} does two special things. The first special
 3844: thing that it does prevents the text interpreter from ever seeing the
 3845: characters @code{add-two}. The text interpreter uses a variable called
 3846: @cindex modifying >IN
 3847: @code{>IN} (pronounced ``to-in'') to keep track of where it is in the
 3848: input line. When it encounters the word @code{:} it behaves in exactly
 3849: the same way as it does for any other word; it looks it up in the name
 3850: dictionary, finds its xt and executes it. When @code{:} executes, it
 3851: looks at the input buffer, finds the word @code{add-two} and advances the
 3852: value of @code{>IN} to point past it. It then does some other stuff
 3853: associated with creating the new definition (including creating an entry
 3854: for @code{add-two} in the name dictionary). When the execution of @code{:}
 3855: completes, control returns to the text interpreter, which is oblivious
 3856: to the fact that it has been tricked into ignoring part of the input
 3857: line.
 3858: 
 3859: @cindex parsing words
 3860: Words like @code{:} -- words that advance the value of @code{>IN} and so
 3861: prevent the text interpreter from acting on the whole of the input line
 3862: -- are called @dfn{parsing words}.
 3863: 
 3864: @cindex @code{state} - effect on the text interpreter
 3865: @cindex text interpreter - effect of state
 3866: The second special thing that @code{:} does is change the value of a
 3867: variable called @code{state}, which affects the way that the text
 3868: interpreter behaves. When Gforth starts up, @code{state} has the value
 3869: 0, and the text interpreter is said to be @dfn{interpreting}. During a
 3870: colon definition (started with @code{:}), @code{state} is set to -1 and
 3871: the text interpreter is said to be @dfn{compiling}.
 3872: 
 3873: In this example, the text interpreter is compiling when it processes the
 3874: string ``@code{2 + . ;}''. It still breaks the string down into
 3875: character sequences in the same way. However, instead of pushing the
 3876: number @code{2} onto the stack, it lays down (@dfn{compiles}) some magic
 3877: into the definition of @code{add-two} that will make the number @code{2} get
 3878: pushed onto the stack when @code{add-two} is @dfn{executed}. Similarly,
 3879: the behaviours of @code{+} and @code{.} are also compiled into the
 3880: definition.
 3881: 
 3882: One category of words don't get compiled. These so-called @dfn{immediate
 3883: words} get executed (performed @i{now}) regardless of whether the text
 3884: interpreter is interpreting or compiling. The word @code{;} is an
 3885: immediate word. Rather than being compiled into the definition, it
 3886: executes. Its effect is to terminate the current definition, which
 3887: includes changing the value of @code{state} back to 0.
 3888: 
 3889: When you execute @code{add-two}, it has a @dfn{run-time effect} that is
 3890: exactly the same as if you had typed @code{2 + . @key{RET}} outside of a
 3891: definition.
 3892: 
 3893: In Forth, every word or number can be described in terms of two
 3894: properties:
 3895: 
 3896: @itemize @bullet
 3897: @item
 3898: @cindex interpretation semantics
 3899: Its @dfn{interpretation semantics} describe how it will behave when the
 3900: text interpreter encounters it in @dfn{interpret} state. The
 3901: interpretation semantics of a word are represented by an @dfn{execution
 3902: token}.
 3903: @item
 3904: @cindex compilation semantics
 3905: Its @dfn{compilation semantics} describe how it will behave when the
 3906: text interpreter encounters it in @dfn{compile} state. The compilation
 3907: semantics of a word are represented in an implementation-dependent way;
 3908: Gforth uses a @dfn{compilation token}.
 3909: @end itemize
 3910: 
 3911: @noindent
 3912: Numbers are always treated in a fixed way:
 3913: 
 3914: @itemize @bullet
 3915: @item
 3916: When the number is @dfn{interpreted}, its behaviour is to push the
 3917: number onto the stack.
 3918: @item
 3919: When the number is @dfn{compiled}, a piece of code is appended to the
 3920: current definition that pushes the number when it runs. (In other words,
 3921: the compilation semantics of a number are to postpone its interpretation
 3922: semantics until the run-time of the definition that it is being compiled
 3923: into.)
 3924: @end itemize
 3925: 
 3926: Words don't behave in such a regular way, but most have @i{default
 3927: semantics} which means that they behave like this:
 3928: 
 3929: @itemize @bullet
 3930: @item
 3931: The @dfn{interpretation semantics} of the word are to do something useful.
 3932: @item
 3933: The @dfn{compilation semantics} of the word are to append its
 3934: @dfn{interpretation semantics} to the current definition (so that its
 3935: run-time behaviour is to do something useful).
 3936: @end itemize
 3937: 
 3938: @cindex immediate words
 3939: The actual behaviour of any particular word can be controlled by using
 3940: the words @code{immediate} and @code{compile-only} when the word is
 3941: defined. These words set flags in the name dictionary entry of the most
 3942: recently defined word, and these flags are retrieved by the text
 3943: interpreter when it finds the word in the name dictionary.
 3944: 
 3945: A word that is marked as @dfn{immediate} has compilation semantics that
 3946: are identical to its interpretation semantics. In other words, it
 3947: behaves like this:
 3948: 
 3949: @itemize @bullet
 3950: @item
 3951: The @dfn{interpretation semantics} of the word are to do something useful.
 3952: @item
 3953: The @dfn{compilation semantics} of the word are to do something useful
 3954: (and actually the same thing); i.e., it is executed during compilation.
 3955: @end itemize
 3956: 
 3957: Marking a word as @dfn{compile-only} prohibits the text interpreter from
 3958: performing the interpretation semantics of the word directly; an attempt
 3959: to do so will generate an error. It is never necessary to use
 3960: @code{compile-only} (and it is not even part of ANS Forth, though it is
 3961: provided by many implementations) but it is good etiquette to apply it
 3962: to a word that will not behave correctly (and might have unexpected
 3963: side-effects) in interpret state. For example, it is only legal to use
 3964: the conditional word @code{IF} within a definition. If you forget this
 3965: and try to use it elsewhere, the fact that (in Gforth) it is marked as
 3966: @code{compile-only} allows the text interpreter to generate a helpful
 3967: error message rather than subjecting you to the consequences of your
 3968: folly.
 3969: 
 3970: This example shows the difference between an immediate and a
 3971: non-immediate word:
 3972: 
 3973: @example
 3974: : show-state state @@ . ;
 3975: : show-state-now show-state ; immediate
 3976: : word1 show-state ;
 3977: : word2 show-state-now ;
 3978: @end example
 3979: 
 3980: The word @code{immediate} after the definition of @code{show-state-now}
 3981: makes that word an immediate word. These definitions introduce a new
 3982: word: @code{@@} (pronounced ``fetch''). This word fetches the value of a
 3983: variable, and leaves it on the stack. Therefore, the behaviour of
 3984: @code{show-state} is to print a number that represents the current value
 3985: of @code{state}.
 3986: 
 3987: When you execute @code{word1}, it prints the number 0, indicating that
 3988: the system is interpreting. When the text interpreter compiled the
 3989: definition of @code{word1}, it encountered @code{show-state} whose
 3990: compilation semantics are to append its interpretation semantics to the
 3991: current definition. When you execute @code{word1}, it performs the
 3992: interpretation semantics of @code{show-state}.  At the time that @code{word1}
 3993: (and therefore @code{show-state}) are executed, the system is
 3994: interpreting.
 3995: 
 3996: When you pressed @key{RET} after entering the definition of @code{word2},
 3997: you should have seen the number -1 printed, followed by ``@code{
 3998: ok}''. When the text interpreter compiled the definition of
 3999: @code{word2}, it encountered @code{show-state-now}, an immediate word,
 4000: whose compilation semantics are therefore to perform its interpretation
 4001: semantics. It is executed straight away (even before the text
 4002: interpreter has moved on to process another group of characters; the
 4003: @code{;} in this example). The effect of executing it are to display the
 4004: value of @code{state} @i{at the time that the definition of}
 4005: @code{word2} @i{is being defined}. Printing -1 demonstrates that the
 4006: system is compiling at this time. If you execute @code{word2} it does
 4007: nothing at all.
 4008: 
 4009: @cindex @code{."}, how it works
 4010: Before leaving the subject of immediate words, consider the behaviour of
 4011: @code{."} in the definition of @code{greet}, in the previous
 4012: section. This word is both a parsing word and an immediate word. Notice
 4013: that there is a space between @code{."} and the start of the text
 4014: @code{Hello and welcome}, but that there is no space between the last
 4015: letter of @code{welcome} and the @code{"} character. The reason for this
 4016: is that @code{."} is a Forth word; it must have a space after it so that
 4017: the text interpreter can identify it. The @code{"} is not a Forth word;
 4018: it is a @dfn{delimiter}. The examples earlier show that, when the string
 4019: is displayed, there is neither a space before the @code{H} nor after the
 4020: @code{e}. Since @code{."} is an immediate word, it executes at the time
 4021: that @code{greet} is defined. When it executes, its behaviour is to
 4022: search forward in the input line looking for the delimiter. When it
 4023: finds the delimiter, it updates @code{>IN} to point past the
 4024: delimiter. It also compiles some magic code into the definition of
 4025: @code{greet}; the xt of a run-time routine that prints a text string. It
 4026: compiles the string @code{Hello and welcome} into memory so that it is
 4027: available to be printed later. When the text interpreter gains control,
 4028: the next word it finds in the input stream is @code{;} and so it
 4029: terminates the definition of @code{greet}.
 4030: 
 4031: 
 4032: @comment ----------------------------------------------
 4033: @node Forth is written in Forth, Review - elements of a Forth system, How does that work?, Introduction
 4034: @section Forth is written in Forth
 4035: @cindex structure of Forth programs
 4036: 
 4037: When you start up a Forth compiler, a large number of definitions
 4038: already exist. In Forth, you develop a new application using bottom-up
 4039: programming techniques to create new definitions that are defined in
 4040: terms of existing definitions. As you create each definition you can
 4041: test and debug it interactively.
 4042: 
 4043: If you have tried out the examples in this section, you will probably
 4044: have typed them in by hand; when you leave Gforth, your definitions will
 4045: be lost. You can avoid this by using a text editor to enter Forth source
 4046: code into a file, and then loading code from the file using
 4047: @code{include} (@pxref{Forth source files}). A Forth source file is
 4048: processed by the text interpreter, just as though you had typed it in by
 4049: hand@footnote{Actually, there are some subtle differences -- see
 4050: @ref{The Text Interpreter}.}.
 4051: 
 4052: Gforth also supports the traditional Forth alternative to using text
 4053: files for program entry (@pxref{Blocks}).
 4054: 
 4055: In common with many, if not most, Forth compilers, most of Gforth is
 4056: actually written in Forth. All of the @file{.fs} files in the
 4057: installation directory@footnote{For example,
 4058: @file{/usr/local/share/gforth...}} are Forth source files, which you can
 4059: study to see examples of Forth programming.
 4060: 
 4061: Gforth maintains a history file that records every line that you type to
 4062: the text interpreter. This file is preserved between sessions, and is
 4063: used to provide a command-line recall facility. If you enter long
 4064: definitions by hand, you can use a text editor to paste them out of the
 4065: history file into a Forth source file for reuse at a later time
 4066: (for more information @pxref{Command-line editing}).
 4067: 
 4068: 
 4069: @comment ----------------------------------------------
 4070: @node Review - elements of a Forth system, Where to go next, Forth is written in Forth, Introduction
 4071: @section Review - elements of a Forth system
 4072: @cindex elements of a Forth system
 4073: 
 4074: To summarise this chapter:
 4075: 
 4076: @itemize @bullet
 4077: @item
 4078: Forth programs use @dfn{factoring} to break a problem down into small
 4079: fragments called @dfn{words} or @dfn{definitions}.
 4080: @item
 4081: Forth program development is an interactive process.
 4082: @item
 4083: The main command loop that accepts input, and controls both
 4084: interpretation and compilation, is called the @dfn{text interpreter}
 4085: (also known as the @dfn{outer interpreter}).
 4086: @item
 4087: Forth has a very simple syntax, consisting of words and numbers
 4088: separated by spaces or carriage-return characters. Any additional syntax
 4089: is imposed by @dfn{parsing words}.
 4090: @item
 4091: Forth uses a stack to pass parameters between words. As a result, it
 4092: uses postfix notation.
 4093: @item
 4094: To use a word that has previously been defined, the text interpreter
 4095: searches for the word in the @dfn{name dictionary}.
 4096: @item
 4097: Words have @dfn{interpretation semantics} and @dfn{compilation semantics}.
 4098: @item
 4099: The text interpreter uses the value of @code{state} to select between
 4100: the use of the @dfn{interpretation semantics} and the  @dfn{compilation
 4101: semantics} of a word that it encounters.
 4102: @item
 4103: The relationship between the @dfn{interpretation semantics} and
 4104: @dfn{compilation semantics} for a word
 4105: depend upon the way in which the word was defined (for example, whether
 4106: it is an @dfn{immediate} word).
 4107: @item
 4108: Forth definitions can be implemented in Forth (called @dfn{high-level
 4109: definitions}) or in some other way (usually a lower-level language and
 4110: as a result often called @dfn{low-level definitions}, @dfn{code
 4111: definitions} or @dfn{primitives}).
 4112: @item
 4113: Many Forth systems are implemented mainly in Forth.
 4114: @end itemize
 4115: 
 4116: 
 4117: @comment ----------------------------------------------
 4118: @node Where to go next, Exercises, Review - elements of a Forth system, Introduction
 4119: @section Where To Go Next
 4120: @cindex where to go next
 4121: 
 4122: Amazing as it may seem, if you have read (and understood) this far, you
 4123: know almost all the fundamentals about the inner workings of a Forth
 4124: system. You certainly know enough to be able to read and understand the
 4125: rest of this manual and the ANS Forth document, to learn more about the
 4126: facilities that Forth in general and Gforth in particular provide. Even
 4127: scarier, you know almost enough to implement your own Forth system.
 4128: However, that's not a good idea just yet... better to try writing some
 4129: programs in Gforth.
 4130: 
 4131: Forth has such a rich vocabulary that it can be hard to know where to
 4132: start in learning it. This section suggests a few sets of words that are
 4133: enough to write small but useful programs. Use the word index in this
 4134: document to learn more about each word, then try it out and try to write
 4135: small definitions using it. Start by experimenting with these words:
 4136: 
 4137: @itemize @bullet
 4138: @item
 4139: Arithmetic: @code{+ - * / /MOD */ ABS INVERT}
 4140: @item
 4141: Comparison: @code{MIN MAX =}
 4142: @item
 4143: Logic: @code{AND OR XOR NOT}
 4144: @item
 4145: Stack manipulation: @code{DUP DROP SWAP OVER}
 4146: @item
 4147: Loops and decisions: @code{IF ELSE ENDIF ?DO I LOOP}
 4148: @item
 4149: Input/Output: @code{. ." EMIT CR KEY}
 4150: @item
 4151: Defining words: @code{: ; CREATE}
 4152: @item
 4153: Memory allocation words: @code{ALLOT ,}
 4154: @item
 4155: Tools: @code{SEE WORDS .S MARKER}
 4156: @end itemize
 4157: 
 4158: When you have mastered those, go on to:
 4159: 
 4160: @itemize @bullet
 4161: @item
 4162: More defining words: @code{VARIABLE CONSTANT VALUE TO CREATE DOES>}
 4163: @item
 4164: Memory access: @code{@@ !}
 4165: @end itemize
 4166: 
 4167: When you have mastered these, there's nothing for it but to read through
 4168: the whole of this manual and find out what you've missed.
 4169: 
 4170: @comment ----------------------------------------------
 4171: @node Exercises,  , Where to go next, Introduction
 4172: @section Exercises
 4173: @cindex exercises
 4174: 
 4175: TODO: provide a set of programming excercises linked into the stuff done
 4176: already and into other sections of the manual. Provide solutions to all
 4177: the exercises in a .fs file in the distribution.
 4178: 
 4179: @c Get some inspiration from Starting Forth and Kelly&Spies.
 4180: 
 4181: @c excercises:
 4182: @c 1. take inches and convert to feet and inches.
 4183: @c 2. take temperature and convert from fahrenheight to celcius;
 4184: @c    may need to care about symmetric vs floored??
 4185: @c 3. take input line and do character substitution
 4186: @c    to encipher or decipher
 4187: @c 4. as above but work on a file for in and out
 4188: @c 5. take input line and convert to pig-latin 
 4189: @c
 4190: @c thing of sets of things to exercise then come up with
 4191: @c problems that need those things.
 4192: 
 4193: 
 4194: @c ******************************************************************
 4195: @node Words, Error messages, Introduction, Top
 4196: @chapter Forth Words
 4197: @cindex words
 4198: 
 4199: @menu
 4200: * Notation::                    
 4201: * Case insensitivity::          
 4202: * Comments::                    
 4203: * Boolean Flags::               
 4204: * Arithmetic::                  
 4205: * Stack Manipulation::          
 4206: * Memory::                      
 4207: * Control Structures::          
 4208: * Defining Words::              
 4209: * Interpretation and Compilation Semantics::  
 4210: * Tokens for Words::            
 4211: * Compiling words::             
 4212: * The Text Interpreter::        
 4213: * The Input Stream::            
 4214: * Word Lists::                  
 4215: * Environmental Queries::       
 4216: * Files::                       
 4217: * Blocks::                      
 4218: * Other I/O::                   
 4219: * OS command line arguments::   
 4220: * Locals::                      
 4221: * Structures::                  
 4222: * Object-oriented Forth::       
 4223: * Programming Tools::           
 4224: * Assembler and Code Words::    
 4225: * Threading Words::             
 4226: * Passing Commands to the OS::  
 4227: * Keeping track of Time::       
 4228: * Miscellaneous Words::         
 4229: @end menu
 4230: 
 4231: @node Notation, Case insensitivity, Words, Words
 4232: @section Notation
 4233: @cindex notation of glossary entries
 4234: @cindex format of glossary entries
 4235: @cindex glossary notation format
 4236: @cindex word glossary entry format
 4237: 
 4238: The Forth words are described in this section in the glossary notation
 4239: that has become a de-facto standard for Forth texts:
 4240: 
 4241: @format
 4242: @i{word}     @i{Stack effect}   @i{wordset}   @i{pronunciation}
 4243: @end format
 4244: @i{Description}
 4245: 
 4246: @table @var
 4247: @item word
 4248: The name of the word.
 4249: 
 4250: @item Stack effect
 4251: @cindex stack effect
 4252: The stack effect is written in the notation @code{@i{before} --
 4253: @i{after}}, where @i{before} and @i{after} describe the top of
 4254: stack entries before and after the execution of the word. The rest of
 4255: the stack is not touched by the word. The top of stack is rightmost,
 4256: i.e., a stack sequence is written as it is typed in. Note that Gforth
 4257: uses a separate floating point stack, but a unified stack
 4258: notation. Also, return stack effects are not shown in @i{stack
 4259: effect}, but in @i{Description}. The name of a stack item describes
 4260: the type and/or the function of the item. See below for a discussion of
 4261: the types.
 4262: 
 4263: All words have two stack effects: A compile-time stack effect and a
 4264: run-time stack effect. The compile-time stack-effect of most words is
 4265: @i{ -- }. If the compile-time stack-effect of a word deviates from
 4266: this standard behaviour, or the word does other unusual things at
 4267: compile time, both stack effects are shown; otherwise only the run-time
 4268: stack effect is shown.
 4269: 
 4270: @cindex pronounciation of words
 4271: @item pronunciation
 4272: How the word is pronounced.
 4273: 
 4274: @cindex wordset
 4275: @cindex environment wordset
 4276: @item wordset
 4277: The ANS Forth standard is divided into several word sets. A standard
 4278: system need not support all of them. Therefore, in theory, the fewer
 4279: word sets your program uses the more portable it will be. However, we
 4280: suspect that most ANS Forth systems on personal machines will feature
 4281: all word sets. Words that are not defined in ANS Forth have
 4282: @code{gforth} or @code{gforth-internal} as word set. @code{gforth}
 4283: describes words that will work in future releases of Gforth;
 4284: @code{gforth-internal} words are more volatile. Environmental query
 4285: strings are also displayed like words; you can recognize them by the
 4286: @code{environment} in the word set field.
 4287: 
 4288: @item Description
 4289: A description of the behaviour of the word.
 4290: @end table
 4291: 
 4292: @cindex types of stack items
 4293: @cindex stack item types
 4294: The type of a stack item is specified by the character(s) the name
 4295: starts with:
 4296: 
 4297: @table @code
 4298: @item f
 4299: @cindex @code{f}, stack item type
 4300: Boolean flags, i.e. @code{false} or @code{true}.
 4301: @item c
 4302: @cindex @code{c}, stack item type
 4303: Char
 4304: @item w
 4305: @cindex @code{w}, stack item type
 4306: Cell, can contain an integer or an address
 4307: @item n
 4308: @cindex @code{n}, stack item type
 4309: signed integer
 4310: @item u
 4311: @cindex @code{u}, stack item type
 4312: unsigned integer
 4313: @item d
 4314: @cindex @code{d}, stack item type
 4315: double sized signed integer
 4316: @item ud
 4317: @cindex @code{ud}, stack item type
 4318: double sized unsigned integer
 4319: @item r
 4320: @cindex @code{r}, stack item type
 4321: Float (on the FP stack)
 4322: @item a-
 4323: @cindex @code{a_}, stack item type
 4324: Cell-aligned address
 4325: @item c-
 4326: @cindex @code{c_}, stack item type
 4327: Char-aligned address (note that a Char may have two bytes in Windows NT)
 4328: @item f-
 4329: @cindex @code{f_}, stack item type
 4330: Float-aligned address
 4331: @item df-
 4332: @cindex @code{df_}, stack item type
 4333: Address aligned for IEEE double precision float
 4334: @item sf-
 4335: @cindex @code{sf_}, stack item type
 4336: Address aligned for IEEE single precision float
 4337: @item xt
 4338: @cindex @code{xt}, stack item type
 4339: Execution token, same size as Cell
 4340: @item wid
 4341: @cindex @code{wid}, stack item type
 4342: Word list ID, same size as Cell
 4343: @item ior, wior
 4344: @cindex ior type description
 4345: @cindex wior type description
 4346: I/O result code, cell-sized.  In Gforth, you can @code{throw} iors.
 4347: @item f83name
 4348: @cindex @code{f83name}, stack item type
 4349: Pointer to a name structure
 4350: @item "
 4351: @cindex @code{"}, stack item type
 4352: string in the input stream (not on the stack). The terminating character
 4353: is a blank by default. If it is not a blank, it is shown in @code{<>}
 4354: quotes.
 4355: @end table
 4356: 
 4357: @comment ----------------------------------------------
 4358: @node Case insensitivity, Comments, Notation, Words
 4359: @section Case insensitivity
 4360: @cindex case sensitivity
 4361: @cindex upper and lower case
 4362: 
 4363: Gforth is case-insensitive; you can enter definitions and invoke
 4364: Standard words using upper, lower or mixed case (however,
 4365: @pxref{core-idef, Implementation-defined options, Implementation-defined
 4366: options}).
 4367: 
 4368: ANS Forth only @i{requires} implementations to recognise Standard words
 4369: when they are typed entirely in upper case. Therefore, a Standard
 4370: program must use upper case for all Standard words. You can use whatever
 4371: case you like for words that you define, but in a Standard program you
 4372: have to use the words in the same case that you defined them.
 4373: 
 4374: Gforth supports case sensitivity through @code{table}s (case-sensitive
 4375: wordlists, @pxref{Word Lists}).
 4376: 
 4377: Two people have asked how to convert Gforth to be case-sensitive; while
 4378: we think this is a bad idea, you can change all wordlists into tables
 4379: like this:
 4380: 
 4381: @example
 4382: ' table-find forth-wordlist wordlist-map @ !
 4383: @end example
 4384: 
 4385: Note that you now have to type the predefined words in the same case
 4386: that we defined them, which are varying.  You may want to convert them
 4387: to your favourite case before doing this operation (I won't explain how,
 4388: because if you are even contemplating doing this, you'd better have
 4389: enough knowledge of Forth systems to know this already).
 4390: 
 4391: @node Comments, Boolean Flags, Case insensitivity, Words
 4392: @section Comments
 4393: @cindex comments
 4394: 
 4395: Forth supports two styles of comment; the traditional @i{in-line} comment,
 4396: @code{(} and its modern cousin, the @i{comment to end of line}; @code{\}.
 4397: 
 4398: 
 4399: doc-(
 4400: doc-\
 4401: doc-\G
 4402: 
 4403: 
 4404: @node Boolean Flags, Arithmetic, Comments, Words
 4405: @section Boolean Flags
 4406: @cindex Boolean flags
 4407: 
 4408: A Boolean flag is cell-sized. A cell with all bits clear represents the
 4409: flag @code{false} and a flag with all bits set represents the flag
 4410: @code{true}. Words that check a flag (for example, @code{IF}) will treat
 4411: a cell that has @i{any} bit set as @code{true}.
 4412: @c on and off to Memory? 
 4413: @c true and false to "Bitwise operations" or "Numeric comparison"?
 4414: 
 4415: doc-true
 4416: doc-false
 4417: doc-on
 4418: doc-off
 4419: 
 4420: 
 4421: @node Arithmetic, Stack Manipulation, Boolean Flags, Words
 4422: @section Arithmetic
 4423: @cindex arithmetic words
 4424: 
 4425: @cindex division with potentially negative operands
 4426: Forth arithmetic is not checked, i.e., you will not hear about integer
 4427: overflow on addition or multiplication, you may hear about division by
 4428: zero if you are lucky. The operator is written after the operands, but
 4429: the operands are still in the original order. I.e., the infix @code{2-1}
 4430: corresponds to @code{2 1 -}. Forth offers a variety of division
 4431: operators. If you perform division with potentially negative operands,
 4432: you do not want to use @code{/} or @code{/mod} with its undefined
 4433: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
 4434: former, @pxref{Mixed precision}).
 4435: @comment TODO discuss the different division forms and the std approach
 4436: 
 4437: @menu
 4438: * Single precision::            
 4439: * Double precision::            Double-cell integer arithmetic
 4440: * Bitwise operations::          
 4441: * Numeric comparison::          
 4442: * Mixed precision::             Operations with single and double-cell integers
 4443: * Floating Point::              
 4444: @end menu
 4445: 
 4446: @node Single precision, Double precision, Arithmetic, Arithmetic
 4447: @subsection Single precision
 4448: @cindex single precision arithmetic words
 4449: 
 4450: @c !! cell undefined
 4451: 
 4452: By default, numbers in Forth are single-precision integers that are one
 4453: cell in size. They can be signed or unsigned, depending upon how you
 4454: treat them. For the rules used by the text interpreter for recognising
 4455: single-precision integers see @ref{Number Conversion}.
 4456: 
 4457: These words are all defined for signed operands, but some of them also
 4458: work for unsigned numbers: @code{+}, @code{1+}, @code{-}, @code{1-},
 4459: @code{*}.
 4460: 
 4461: doc-+
 4462: doc-1+
 4463: doc--
 4464: doc-1-
 4465: doc-*
 4466: doc-/
 4467: doc-mod
 4468: doc-/mod
 4469: doc-negate
 4470: doc-abs
 4471: doc-min
 4472: doc-max
 4473: doc-floored
 4474: 
 4475: 
 4476: @node Double precision, Bitwise operations, Single precision, Arithmetic
 4477: @subsection Double precision
 4478: @cindex double precision arithmetic words
 4479: 
 4480: For the rules used by the text interpreter for
 4481: recognising double-precision integers, see @ref{Number Conversion}.
 4482: 
 4483: A double precision number is represented by a cell pair, with the most
 4484: significant cell at the TOS. It is trivial to convert an unsigned single
 4485: to a double: simply push a @code{0} onto the TOS. Since numbers are
 4486: represented by Gforth using 2's complement arithmetic, converting a
 4487: signed single to a (signed) double requires sign-extension across the
 4488: most significant cell. This can be achieved using @code{s>d}. The moral
 4489: of the story is that you cannot convert a number without knowing whether
 4490: it represents an unsigned or a signed number.
 4491: 
 4492: These words are all defined for signed operands, but some of them also
 4493: work for unsigned numbers: @code{d+}, @code{d-}.
 4494: 
 4495: doc-s>d
 4496: doc-d>s
 4497: doc-d+
 4498: doc-d-
 4499: doc-dnegate
 4500: doc-dabs
 4501: doc-dmin
 4502: doc-dmax
 4503: 
 4504: 
 4505: @node Bitwise operations, Numeric comparison, Double precision, Arithmetic
 4506: @subsection Bitwise operations
 4507: @cindex bitwise operation words
 4508: 
 4509: 
 4510: doc-and
 4511: doc-or
 4512: doc-xor
 4513: doc-invert
 4514: doc-lshift
 4515: doc-rshift
 4516: doc-2*
 4517: doc-d2*
 4518: doc-2/
 4519: doc-d2/
 4520: 
 4521: 
 4522: @node Numeric comparison, Mixed precision, Bitwise operations, Arithmetic
 4523: @subsection Numeric comparison
 4524: @cindex numeric comparison words
 4525: 
 4526: Note that the words that compare for equality (@code{= <> 0= 0<> d= d<>
 4527: d0= d0<>}) work for for both signed and unsigned numbers.
 4528: 
 4529: doc-<
 4530: doc-<=
 4531: doc-<>
 4532: doc-=
 4533: doc->
 4534: doc->=
 4535: 
 4536: doc-0<
 4537: doc-0<=
 4538: doc-0<>
 4539: doc-0=
 4540: doc-0>
 4541: doc-0>=
 4542: 
 4543: doc-u<
 4544: doc-u<=
 4545: @c u<> and u= exist but are the same as <> and =
 4546: @c doc-u<>
 4547: @c doc-u=
 4548: doc-u>
 4549: doc-u>=
 4550: 
 4551: doc-within
 4552: 
 4553: doc-d<
 4554: doc-d<=
 4555: doc-d<>
 4556: doc-d=
 4557: doc-d>
 4558: doc-d>=
 4559: 
 4560: doc-d0<
 4561: doc-d0<=
 4562: doc-d0<>
 4563: doc-d0=
 4564: doc-d0>
 4565: doc-d0>=
 4566: 
 4567: doc-du<
 4568: doc-du<=
 4569: @c du<> and du= exist but are the same as d<> and d=
 4570: @c doc-du<>
 4571: @c doc-du=
 4572: doc-du>
 4573: doc-du>=
 4574: 
 4575: 
 4576: @node Mixed precision, Floating Point, Numeric comparison, Arithmetic
 4577: @subsection Mixed precision
 4578: @cindex mixed precision arithmetic words
 4579: 
 4580: 
 4581: doc-m+
 4582: doc-*/
 4583: doc-*/mod
 4584: doc-m*
 4585: doc-um*
 4586: doc-m*/
 4587: doc-um/mod
 4588: doc-fm/mod
 4589: doc-sm/rem
 4590: 
 4591: 
 4592: @node Floating Point,  , Mixed precision, Arithmetic
 4593: @subsection Floating Point
 4594: @cindex floating point arithmetic words
 4595: 
 4596: For the rules used by the text interpreter for
 4597: recognising floating-point numbers see @ref{Number Conversion}.
 4598: 
 4599: Gforth has a separate floating point stack, but the documentation uses
 4600: the unified notation.@footnote{It's easy to generate the separate
 4601: notation from that by just separating the floating-point numbers out:
 4602: e.g. @code{( n r1 u r2 -- r3 )} becomes @code{( n u -- ) ( F: r1 r2 --
 4603: r3 )}.}
 4604: 
 4605: @cindex floating-point arithmetic, pitfalls
 4606: Floating point numbers have a number of unpleasant surprises for the
 4607: unwary (e.g., floating point addition is not associative) and even a few
 4608: for the wary. You should not use them unless you know what you are doing
 4609: or you don't care that the results you get are totally bogus. If you
 4610: want to learn about the problems of floating point numbers (and how to
 4611: avoid them), you might start with @cite{David Goldberg,
 4612: @uref{http://www.validgh.com/goldberg/paper.ps,What Every Computer
 4613: Scientist Should Know About Floating-Point Arithmetic}, ACM Computing
 4614: Surveys 23(1):5@minus{}48, March 1991}.
 4615: 
 4616: 
 4617: doc-d>f
 4618: doc-f>d
 4619: doc-f+
 4620: doc-f-
 4621: doc-f*
 4622: doc-f/
 4623: doc-fnegate
 4624: doc-fabs
 4625: doc-fmax
 4626: doc-fmin
 4627: doc-floor
 4628: doc-fround
 4629: doc-f**
 4630: doc-fsqrt
 4631: doc-fexp
 4632: doc-fexpm1
 4633: doc-fln
 4634: doc-flnp1
 4635: doc-flog
 4636: doc-falog
 4637: doc-f2*
 4638: doc-f2/
 4639: doc-1/f
 4640: doc-precision
 4641: doc-set-precision
 4642: 
 4643: @cindex angles in trigonometric operations
 4644: @cindex trigonometric operations
 4645: Angles in floating point operations are given in radians (a full circle
 4646: has 2 pi radians).
 4647: 
 4648: doc-fsin
 4649: doc-fcos
 4650: doc-fsincos
 4651: doc-ftan
 4652: doc-fasin
 4653: doc-facos
 4654: doc-fatan
 4655: doc-fatan2
 4656: doc-fsinh
 4657: doc-fcosh
 4658: doc-ftanh
 4659: doc-fasinh
 4660: doc-facosh
 4661: doc-fatanh
 4662: doc-pi
 4663: 
 4664: @cindex equality of floats
 4665: @cindex floating-point comparisons
 4666: One particular problem with floating-point arithmetic is that comparison
 4667: for equality often fails when you would expect it to succeed.  For this
 4668: reason approximate equality is often preferred (but you still have to
 4669: know what you are doing).  Also note that IEEE NaNs may compare
 4670: differently from what you might expect.  The comparison words are:
 4671: 
 4672: doc-f~rel
 4673: doc-f~abs
 4674: doc-f~
 4675: doc-f=
 4676: doc-f<>
 4677: 
 4678: doc-f<
 4679: doc-f<=
 4680: doc-f>
 4681: doc-f>=
 4682: 
 4683: doc-f0<
 4684: doc-f0<=
 4685: doc-f0<>
 4686: doc-f0=
 4687: doc-f0>
 4688: doc-f0>=
 4689: 
 4690: 
 4691: @node Stack Manipulation, Memory, Arithmetic, Words
 4692: @section Stack Manipulation
 4693: @cindex stack manipulation words
 4694: 
 4695: @cindex floating-point stack in the standard
 4696: Gforth maintains a number of separate stacks:
 4697: 
 4698: @cindex data stack
 4699: @cindex parameter stack
 4700: @itemize @bullet
 4701: @item
 4702: A data stack (also known as the @dfn{parameter stack}) -- for
 4703: characters, cells, addresses, and double cells.
 4704: 
 4705: @cindex floating-point stack
 4706: @item
 4707: A floating point stack -- for holding floating point (FP) numbers.
 4708: 
 4709: @cindex return stack
 4710: @item
 4711: A return stack -- for holding the return addresses of colon
 4712: definitions and other (non-FP) data.
 4713: 
 4714: @cindex locals stack
 4715: @item
 4716: A locals stack -- for holding local variables.
 4717: @end itemize
 4718: 
 4719: @menu
 4720: * Data stack::                  
 4721: * Floating point stack::        
 4722: * Return stack::                
 4723: * Locals stack::                
 4724: * Stack pointer manipulation::  
 4725: @end menu
 4726: 
 4727: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
 4728: @subsection Data stack
 4729: @cindex data stack manipulation words
 4730: @cindex stack manipulations words, data stack
 4731: 
 4732: 
 4733: doc-drop
 4734: doc-nip
 4735: doc-dup
 4736: doc-over
 4737: doc-tuck
 4738: doc-swap
 4739: doc-pick
 4740: doc-rot
 4741: doc--rot
 4742: doc-?dup
 4743: doc-roll
 4744: doc-2drop
 4745: doc-2nip
 4746: doc-2dup
 4747: doc-2over
 4748: doc-2tuck
 4749: doc-2swap
 4750: doc-2rot
 4751: 
 4752: 
 4753: @node Floating point stack, Return stack, Data stack, Stack Manipulation
 4754: @subsection Floating point stack
 4755: @cindex floating-point stack manipulation words
 4756: @cindex stack manipulation words, floating-point stack
 4757: 
 4758: Whilst every sane Forth has a separate floating-point stack, it is not
 4759: strictly required; an ANS Forth system could theoretically keep
 4760: floating-point numbers on the data stack. As an additional difficulty,
 4761: you don't know how many cells a floating-point number takes. It is
 4762: reportedly possible to write words in a way that they work also for a
 4763: unified stack model, but we do not recommend trying it. Instead, just
 4764: say that your program has an environmental dependency on a separate
 4765: floating-point stack.
 4766: 
 4767: doc-floating-stack
 4768: 
 4769: doc-fdrop
 4770: doc-fnip
 4771: doc-fdup
 4772: doc-fover
 4773: doc-ftuck
 4774: doc-fswap
 4775: doc-fpick
 4776: doc-frot
 4777: 
 4778: 
 4779: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
 4780: @subsection Return stack
 4781: @cindex return stack manipulation words
 4782: @cindex stack manipulation words, return stack
 4783: 
 4784: @cindex return stack and locals
 4785: @cindex locals and return stack
 4786: A Forth system is allowed to keep local variables on the
 4787: return stack. This is reasonable, as local variables usually eliminate
 4788: the need to use the return stack explicitly. So, if you want to produce
 4789: a standard compliant program and you are using local variables in a
 4790: word, forget about return stack manipulations in that word (refer to the
 4791: standard document for the exact rules).
 4792: 
 4793: doc->r
 4794: doc-r>
 4795: doc-r@
 4796: doc-rdrop
 4797: doc-2>r
 4798: doc-2r>
 4799: doc-2r@
 4800: doc-2rdrop
 4801: 
 4802: 
 4803: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
 4804: @subsection Locals stack
 4805: 
 4806: Gforth uses an extra locals stack.  It is described, along with the
 4807: reasons for its existence, in @ref{Locals implementation}.
 4808: 
 4809: @node Stack pointer manipulation,  , Locals stack, Stack Manipulation
 4810: @subsection Stack pointer manipulation
 4811: @cindex stack pointer manipulation words
 4812: 
 4813: @c removed s0 r0 l0 -- they are obsolete aliases for sp0 rp0 lp0
 4814: doc-sp0
 4815: doc-sp@
 4816: doc-sp!
 4817: doc-fp0
 4818: doc-fp@
 4819: doc-fp!
 4820: doc-rp0
 4821: doc-rp@
 4822: doc-rp!
 4823: doc-lp0
 4824: doc-lp@
 4825: doc-lp!
 4826: 
 4827: 
 4828: @node Memory, Control Structures, Stack Manipulation, Words
 4829: @section Memory
 4830: @cindex memory words
 4831: 
 4832: @menu
 4833: * Memory model::                
 4834: * Dictionary allocation::       
 4835: * Heap Allocation::             
 4836: * Memory Access::               
 4837: * Address arithmetic::          
 4838: * Memory Blocks::               
 4839: @end menu
 4840: 
 4841: In addition to the standard Forth memory allocation words, there is also
 4842: a @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
 4843: garbage collector}.
 4844: 
 4845: @node Memory model, Dictionary allocation, Memory, Memory
 4846: @subsection ANS Forth and Gforth memory models
 4847: 
 4848: @c The ANS Forth description is a mess (e.g., is the heap part of
 4849: @c the dictionary?), so let's not stick to closely with it.
 4850: 
 4851: ANS Forth considers a Forth system as consisting of several address
 4852: spaces, of which only @dfn{data space} is managed and accessible with
 4853: the memory words.  Memory not necessarily in data space includes the
 4854: stacks, the code (called code space) and the headers (called name
 4855: space). In Gforth everything is in data space, but the code for the
 4856: primitives is usually read-only.
 4857: 
 4858: Data space is divided into a number of areas: The (data space portion of
 4859: the) dictionary@footnote{Sometimes, the term @dfn{dictionary} is used to
 4860: refer to the search data structure embodied in word lists and headers,
 4861: because it is used for looking up names, just as you would in a
 4862: conventional dictionary.}, the heap, and a number of system-allocated
 4863: buffers.
 4864: 
 4865: @cindex address arithmetic restrictions, ANS vs. Gforth
 4866: @cindex contiguous regions, ANS vs. Gforth
 4867: In ANS Forth data space is also divided into contiguous regions.  You
 4868: can only use address arithmetic within a contiguous region, not between
 4869: them.  Usually each allocation gives you one contiguous region, but the
 4870: dictionary allocation words have additional rules (@pxref{Dictionary
 4871: allocation}).
 4872: 
 4873: Gforth provides one big address space, and address arithmetic can be
 4874: performed between any addresses. However, in the dictionary headers or
 4875: code are interleaved with data, so almost the only contiguous data space
 4876: regions there are those described by ANS Forth as contiguous; but you
 4877: can be sure that the dictionary is allocated towards increasing
 4878: addresses even between contiguous regions.  The memory order of
 4879: allocations in the heap is platform-dependent (and possibly different
 4880: from one run to the next).
 4881: 
 4882: 
 4883: @node Dictionary allocation, Heap Allocation, Memory model, Memory
 4884: @subsection Dictionary allocation
 4885: @cindex reserving data space
 4886: @cindex data space - reserving some
 4887: 
 4888: Dictionary allocation is a stack-oriented allocation scheme, i.e., if
 4889: you want to deallocate X, you also deallocate everything
 4890: allocated after X.
 4891: 
 4892: @cindex contiguous regions in dictionary allocation
 4893: The allocations using the words below are contiguous and grow the region
 4894: towards increasing addresses.  Other words that allocate dictionary
 4895: memory of any kind (i.e., defining words including @code{:noname}) end
 4896: the contiguous region and start a new one.
 4897: 
 4898: In ANS Forth only @code{create}d words are guaranteed to produce an
 4899: address that is the start of the following contiguous region.  In
 4900: particular, the cell allocated by @code{variable} is not guaranteed to
 4901: be contiguous with following @code{allot}ed memory.
 4902: 
 4903: You can deallocate memory by using @code{allot} with a negative argument
 4904: (with some restrictions, see @code{allot}). For larger deallocations use
 4905: @code{marker}.
 4906: 
 4907: 
 4908: doc-here
 4909: doc-unused
 4910: doc-allot
 4911: doc-c,
 4912: doc-f,
 4913: doc-,
 4914: doc-2,
 4915: 
 4916: Memory accesses have to be aligned (@pxref{Address arithmetic}). So of
 4917: course you should allocate memory in an aligned way, too. I.e., before
 4918: allocating allocating a cell, @code{here} must be cell-aligned, etc.
 4919: The words below align @code{here} if it is not already.  Basically it is
 4920: only already aligned for a type, if the last allocation was a multiple
 4921: of the size of this type and if @code{here} was aligned for this type
 4922: before.
 4923: 
 4924: After freshly @code{create}ing a word, @code{here} is @code{align}ed in
 4925: ANS Forth (@code{maxalign}ed in Gforth).
 4926: 
 4927: doc-align
 4928: doc-falign
 4929: doc-sfalign
 4930: doc-dfalign
 4931: doc-maxalign
 4932: doc-cfalign
 4933: 
 4934: 
 4935: @node Heap Allocation, Memory Access, Dictionary allocation, Memory
 4936: @subsection Heap allocation
 4937: @cindex heap allocation
 4938: @cindex dynamic allocation of memory
 4939: @cindex memory-allocation word set
 4940: 
 4941: @cindex contiguous regions and heap allocation
 4942: Heap allocation supports deallocation of allocated memory in any
 4943: order. Dictionary allocation is not affected by it (i.e., it does not
 4944: end a contiguous region). In Gforth, these words are implemented using
 4945: the standard C library calls malloc(), free() and resize().
 4946: 
 4947: The memory region produced by one invocation of @code{allocate} or
 4948: @code{resize} is internally contiguous.  There is no contiguity between
 4949: such a region and any other region (including others allocated from the
 4950: heap).
 4951: 
 4952: doc-allocate
 4953: doc-free
 4954: doc-resize
 4955: 
 4956: 
 4957: @node Memory Access, Address arithmetic, Heap Allocation, Memory
 4958: @subsection Memory Access
 4959: @cindex memory access words
 4960: 
 4961: doc-@
 4962: doc-!
 4963: doc-+!
 4964: doc-c@
 4965: doc-c!
 4966: doc-2@
 4967: doc-2!
 4968: doc-f@
 4969: doc-f!
 4970: doc-sf@
 4971: doc-sf!
 4972: doc-df@
 4973: doc-df!
 4974: 
 4975: 
 4976: @node Address arithmetic, Memory Blocks, Memory Access, Memory
 4977: @subsection Address arithmetic
 4978: @cindex address arithmetic words
 4979: 
 4980: Address arithmetic is the foundation on which you can build data
 4981: structures like arrays, records (@pxref{Structures}) and objects
 4982: (@pxref{Object-oriented Forth}).
 4983: 
 4984: @cindex address unit
 4985: @cindex au (address unit)
 4986: ANS Forth does not specify the sizes of the data types. Instead, it
 4987: offers a number of words for computing sizes and doing address
 4988: arithmetic. Address arithmetic is performed in terms of address units
 4989: (aus); on most systems the address unit is one byte. Note that a
 4990: character may have more than one au, so @code{chars} is no noop (on
 4991: platforms where it is a noop, it compiles to nothing).
 4992: 
 4993: The basic address arithmetic words are @code{+} and @code{-}.  E.g., if
 4994: you have the address of a cell, perform @code{1 cells +}, and you will
 4995: have the address of the next cell.
 4996: 
 4997: @cindex contiguous regions and address arithmetic
 4998: In ANS Forth you can perform address arithmetic only within a contiguous
 4999: region, i.e., if you have an address into one region, you can only add
 5000: and subtract such that the result is still within the region; you can
 5001: only subtract or compare addresses from within the same contiguous
 5002: region.  Reasons: several contiguous regions can be arranged in memory
 5003: in any way; on segmented systems addresses may have unusual
 5004: representations, such that address arithmetic only works within a
 5005: region.  Gforth provides a few more guarantees (linear address space,
 5006: dictionary grows upwards), but in general I have found it easy to stay
 5007: within contiguous regions (exception: computing and comparing to the
 5008: address just beyond the end of an array).
 5009: 
 5010: @cindex alignment of addresses for types
 5011: ANS Forth also defines words for aligning addresses for specific
 5012: types. Many computers require that accesses to specific data types
 5013: must only occur at specific addresses; e.g., that cells may only be
 5014: accessed at addresses divisible by 4. Even if a machine allows unaligned
 5015: accesses, it can usually perform aligned accesses faster. 
 5016: 
 5017: For the performance-conscious: alignment operations are usually only
 5018: necessary during the definition of a data structure, not during the
 5019: (more frequent) accesses to it.
 5020: 
 5021: ANS Forth defines no words for character-aligning addresses. This is not
 5022: an oversight, but reflects the fact that addresses that are not
 5023: char-aligned have no use in the standard and therefore will not be
 5024: created.
 5025: 
 5026: @cindex @code{CREATE} and alignment
 5027: ANS Forth guarantees that addresses returned by @code{CREATE}d words
 5028: are cell-aligned; in addition, Gforth guarantees that these addresses
 5029: are aligned for all purposes.
 5030: 
 5031: Note that the ANS Forth word @code{char} has nothing to do with address
 5032: arithmetic.
 5033: 
 5034: 
 5035: doc-chars
 5036: doc-char+
 5037: doc-cells
 5038: doc-cell+
 5039: doc-cell
 5040: doc-aligned
 5041: doc-floats
 5042: doc-float+
 5043: doc-float
 5044: doc-faligned
 5045: doc-sfloats
 5046: doc-sfloat+
 5047: doc-sfaligned
 5048: doc-dfloats
 5049: doc-dfloat+
 5050: doc-dfaligned
 5051: doc-maxaligned
 5052: doc-cfaligned
 5053: doc-address-unit-bits
 5054: 
 5055: 
 5056: @node Memory Blocks,  , Address arithmetic, Memory
 5057: @subsection Memory Blocks
 5058: @cindex memory block words
 5059: @cindex character strings - moving and copying
 5060: 
 5061: Memory blocks often represent character strings; For ways of storing
 5062: character strings in memory see @ref{String Formats}.  For other
 5063: string-processing words see @ref{Displaying characters and strings}.
 5064: 
 5065: A few of these words work on address unit blocks.  In that case, you
 5066: usually have to insert @code{CHARS} before the word when working on
 5067: character strings.  Most words work on character blocks, and expect a
 5068: char-aligned address.
 5069: 
 5070: When copying characters between overlapping memory regions, use
 5071: @code{chars move} or choose carefully between @code{cmove} and
 5072: @code{cmove>}.
 5073: 
 5074: doc-move
 5075: doc-erase
 5076: doc-cmove
 5077: doc-cmove>
 5078: doc-fill
 5079: doc-blank
 5080: doc-compare
 5081: doc-str=
 5082: doc-str<
 5083: doc-string-prefix?
 5084: doc-search
 5085: doc--trailing
 5086: doc-/string
 5087: doc-bounds
 5088: 
 5089: 
 5090: @comment TODO examples
 5091: 
 5092: 
 5093: @node Control Structures, Defining Words, Memory, Words
 5094: @section Control Structures
 5095: @cindex control structures
 5096: 
 5097: Control structures in Forth cannot be used interpretively, only in a
 5098: colon definition@footnote{To be precise, they have no interpretation
 5099: semantics (@pxref{Interpretation and Compilation Semantics}).}. We do
 5100: not like this limitation, but have not seen a satisfying way around it
 5101: yet, although many schemes have been proposed.
 5102: 
 5103: @menu
 5104: * Selection::                   IF ... ELSE ... ENDIF
 5105: * Simple Loops::                BEGIN ...
 5106: * Counted Loops::               DO
 5107: * Arbitrary control structures::  
 5108: * Calls and returns::           
 5109: * Exception Handling::          
 5110: @end menu
 5111: 
 5112: @node Selection, Simple Loops, Control Structures, Control Structures
 5113: @subsection Selection
 5114: @cindex selection control structures
 5115: @cindex control structures for selection
 5116: 
 5117: @cindex @code{IF} control structure
 5118: @example
 5119: @i{flag}
 5120: IF
 5121:   @i{code}
 5122: ENDIF
 5123: @end example
 5124: @noindent
 5125: 
 5126: If @i{flag} is non-zero (as far as @code{IF} etc. are concerned, a cell
 5127: with any bit set represents truth) @i{code} is executed.
 5128: 
 5129: @example
 5130: @i{flag}
 5131: IF
 5132:   @i{code1}
 5133: ELSE
 5134:   @i{code2}
 5135: ENDIF
 5136: @end example
 5137: 
 5138: If @var{flag} is true, @i{code1} is executed, otherwise @i{code2} is
 5139: executed.
 5140: 
 5141: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
 5142: standard, and @code{ENDIF} is not, although it is quite popular. We
 5143: recommend using @code{ENDIF}, because it is less confusing for people
 5144: who also know other languages (and is not prone to reinforcing negative
 5145: prejudices against Forth in these people). Adding @code{ENDIF} to a
 5146: system that only supplies @code{THEN} is simple:
 5147: @example
 5148: : ENDIF   POSTPONE then ; immediate
 5149: @end example
 5150: 
 5151: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
 5152: (adv.)}  has the following meanings:
 5153: @quotation
 5154: ... 2b: following next after in order ... 3d: as a necessary consequence
 5155: (if you were there, then you saw them).
 5156: @end quotation
 5157: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
 5158: and many other programming languages has the meaning 3d.]
 5159: 
 5160: Gforth also provides the words @code{?DUP-IF} and @code{?DUP-0=-IF}, so
 5161: you can avoid using @code{?dup}. Using these alternatives is also more
 5162: efficient than using @code{?dup}. Definitions in ANS Forth
 5163: for @code{ENDIF}, @code{?DUP-IF} and @code{?DUP-0=-IF} are provided in
 5164: @file{compat/control.fs}.
 5165: 
 5166: @cindex @code{CASE} control structure
 5167: @example
 5168: @i{n}
 5169: CASE
 5170:   @i{n1} OF @i{code1} ENDOF
 5171:   @i{n2} OF @i{code2} ENDOF
 5172:   @dots{}
 5173:   ( n ) @i{default-code} ( n )
 5174: ENDCASE
 5175: @end example
 5176: 
 5177: Executes the first @i{codei}, where the @i{ni} is equal to @i{n}.  If no
 5178: @i{ni} matches, the optional @i{default-code} is executed. The optional
 5179: default case can be added by simply writing the code after the last
 5180: @code{ENDOF}. It may use @i{n}, which is on top of the stack, but must
 5181: not consume it.
 5182: 
 5183: @progstyle
 5184: To keep the code understandable, you should ensure that on all paths
 5185: through a selection construct the stack is changed in the same way
 5186: (wrt. number and types of stack items consumed and pushed).
 5187: 
 5188: @node Simple Loops, Counted Loops, Selection, Control Structures
 5189: @subsection Simple Loops
 5190: @cindex simple loops
 5191: @cindex loops without count 
 5192: 
 5193: @cindex @code{WHILE} loop
 5194: @example
 5195: BEGIN
 5196:   @i{code1}
 5197:   @i{flag}
 5198: WHILE
 5199:   @i{code2}
 5200: REPEAT
 5201: @end example
 5202: 
 5203: @i{code1} is executed and @i{flag} is computed. If it is true,
 5204: @i{code2} is executed and the loop is restarted; If @i{flag} is
 5205: false, execution continues after the @code{REPEAT}.
 5206: 
 5207: @cindex @code{UNTIL} loop
 5208: @example
 5209: BEGIN
 5210:   @i{code}
 5211:   @i{flag}
 5212: UNTIL
 5213: @end example
 5214: 
 5215: @i{code} is executed. The loop is restarted if @code{flag} is false.
 5216: 
 5217: @progstyle
 5218: To keep the code understandable, a complete iteration of the loop should
 5219: not change the number and types of the items on the stacks.
 5220: 
 5221: @cindex endless loop
 5222: @cindex loops, endless
 5223: @example
 5224: BEGIN
 5225:   @i{code}
 5226: AGAIN
 5227: @end example
 5228: 
 5229: This is an endless loop.
 5230: 
 5231: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
 5232: @subsection Counted Loops
 5233: @cindex counted loops
 5234: @cindex loops, counted
 5235: @cindex @code{DO} loops
 5236: 
 5237: The basic counted loop is:
 5238: @example
 5239: @i{limit} @i{start}
 5240: ?DO
 5241:   @i{body}
 5242: LOOP
 5243: @end example
 5244: 
 5245: This performs one iteration for every integer, starting from @i{start}
 5246: and up to, but excluding @i{limit}. The counter, or @i{index}, can be
 5247: accessed with @code{i}. For example, the loop:
 5248: @example
 5249: 10 0 ?DO
 5250:   i .
 5251: LOOP
 5252: @end example
 5253: @noindent
 5254: prints @code{0 1 2 3 4 5 6 7 8 9}
 5255: 
 5256: The index of the innermost loop can be accessed with @code{i}, the index
 5257: of the next loop with @code{j}, and the index of the third loop with
 5258: @code{k}.
 5259: 
 5260: 
 5261: doc-i
 5262: doc-j
 5263: doc-k
 5264: 
 5265: 
 5266: The loop control data are kept on the return stack, so there are some
 5267: restrictions on mixing return stack accesses and counted loop words. In
 5268: particuler, if you put values on the return stack outside the loop, you
 5269: cannot read them inside the loop@footnote{well, not in a way that is
 5270: portable.}. If you put values on the return stack within a loop, you
 5271: have to remove them before the end of the loop and before accessing the
 5272: index of the loop.
 5273: 
 5274: There are several variations on the counted loop:
 5275: 
 5276: @itemize @bullet
 5277: @item
 5278: @code{LEAVE} leaves the innermost counted loop immediately; execution
 5279: continues after the associated @code{LOOP} or @code{NEXT}. For example:
 5280: 
 5281: @example
 5282: 10 0 ?DO  i DUP . 3 = IF LEAVE THEN LOOP
 5283: @end example
 5284: prints @code{0 1 2 3}
 5285: 
 5286: 
 5287: @item
 5288: @code{UNLOOP} prepares for an abnormal loop exit, e.g., via
 5289: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
 5290: return stack so @code{EXIT} can get to its return address. For example:
 5291: 
 5292: @example
 5293: : demo 10 0 ?DO i DUP . 3 = IF UNLOOP EXIT THEN LOOP ." Done" ;
 5294: @end example
 5295: prints @code{0 1 2 3}
 5296: 
 5297: 
 5298: @item
 5299: If @i{start} is greater than @i{limit}, a @code{?DO} loop is entered
 5300: (and @code{LOOP} iterates until they become equal by wrap-around
 5301: arithmetic). This behaviour is usually not what you want. Therefore,
 5302: Gforth offers @code{+DO} and @code{U+DO} (as replacements for
 5303: @code{?DO}), which do not enter the loop if @i{start} is greater than
 5304: @i{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for
 5305: unsigned loop parameters.
 5306: 
 5307: @item
 5308: @code{?DO} can be replaced by @code{DO}. @code{DO} always enters
 5309: the loop, independent of the loop parameters. Do not use @code{DO}, even
 5310: if you know that the loop is entered in any case. Such knowledge tends
 5311: to become invalid during maintenance of a program, and then the
 5312: @code{DO} will make trouble.
 5313: 
 5314: @item
 5315: @code{LOOP} can be replaced with @code{@i{n} +LOOP}; this updates the
 5316: index by @i{n} instead of by 1. The loop is terminated when the border
 5317: between @i{limit-1} and @i{limit} is crossed. E.g.:
 5318: 
 5319: @example
 5320: 4 0 +DO  i .  2 +LOOP
 5321: @end example
 5322: @noindent
 5323: prints @code{0 2}
 5324: 
 5325: @example
 5326: 4 1 +DO  i .  2 +LOOP
 5327: @end example
 5328: @noindent
 5329: prints @code{1 3}
 5330: 
 5331: @item
 5332: @cindex negative increment for counted loops
 5333: @cindex counted loops with negative increment
 5334: The behaviour of @code{@i{n} +LOOP} is peculiar when @i{n} is negative:
 5335: 
 5336: @example
 5337: -1 0 ?DO  i .  -1 +LOOP
 5338: @end example
 5339: @noindent
 5340: prints @code{0 -1}
 5341: 
 5342: @example
 5343: 0 0 ?DO  i .  -1 +LOOP
 5344: @end example
 5345: prints nothing.
 5346: 
 5347: Therefore we recommend avoiding @code{@i{n} +LOOP} with negative
 5348: @i{n}. One alternative is @code{@i{u} -LOOP}, which reduces the
 5349: index by @i{u} each iteration. The loop is terminated when the border
 5350: between @i{limit+1} and @i{limit} is crossed. Gforth also provides
 5351: @code{-DO} and @code{U-DO} for down-counting loops. E.g.:
 5352: 
 5353: @example
 5354: -2 0 -DO  i .  1 -LOOP
 5355: @end example
 5356: @noindent
 5357: prints @code{0 -1}
 5358: 
 5359: @example
 5360: -1 0 -DO  i .  1 -LOOP
 5361: @end example
 5362: @noindent
 5363: prints @code{0}
 5364: 
 5365: @example
 5366: 0 0 -DO  i .  1 -LOOP
 5367: @end example
 5368: @noindent
 5369: prints nothing.
 5370: 
 5371: @end itemize
 5372: 
 5373: Unfortunately, @code{+DO}, @code{U+DO}, @code{-DO}, @code{U-DO} and
 5374: @code{-LOOP} are not defined in ANS Forth. However, an implementation
 5375: for these words that uses only standard words is provided in
 5376: @file{compat/loops.fs}.
 5377: 
 5378: 
 5379: @cindex @code{FOR} loops
 5380: Another counted loop is:
 5381: @example
 5382: @i{n}
 5383: FOR
 5384:   @i{body}
 5385: NEXT
 5386: @end example
 5387: This is the preferred loop of native code compiler writers who are too
 5388: lazy to optimize @code{?DO} loops properly. This loop structure is not
 5389: defined in ANS Forth. In Gforth, this loop iterates @i{n+1} times;
 5390: @code{i} produces values starting with @i{n} and ending with 0. Other
 5391: Forth systems may behave differently, even if they support @code{FOR}
 5392: loops. To avoid problems, don't use @code{FOR} loops.
 5393: 
 5394: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
 5395: @subsection Arbitrary control structures
 5396: @cindex control structures, user-defined
 5397: 
 5398: @cindex control-flow stack
 5399: ANS Forth permits and supports using control structures in a non-nested
 5400: way. Information about incomplete control structures is stored on the
 5401: control-flow stack. This stack may be implemented on the Forth data
 5402: stack, and this is what we have done in Gforth.
 5403: 
 5404: @cindex @code{orig}, control-flow stack item
 5405: @cindex @code{dest}, control-flow stack item
 5406: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
 5407: entry represents a backward branch target. A few words are the basis for
 5408: building any control structure possible (except control structures that
 5409: need storage, like calls, coroutines, and backtracking).
 5410: 
 5411: 
 5412: doc-if
 5413: doc-ahead
 5414: doc-then
 5415: doc-begin
 5416: doc-until
 5417: doc-again
 5418: doc-cs-pick
 5419: doc-cs-roll
 5420: 
 5421: 
 5422: The Standard words @code{CS-PICK} and @code{CS-ROLL} allow you to
 5423: manipulate the control-flow stack in a portable way. Without them, you
 5424: would need to know how many stack items are occupied by a control-flow
 5425: entry (many systems use one cell. In Gforth they currently take three,
 5426: but this may change in the future).
 5427: 
 5428: Some standard control structure words are built from these words:
 5429: 
 5430: 
 5431: doc-else
 5432: doc-while
 5433: doc-repeat
 5434: 
 5435: 
 5436: @noindent
 5437: Gforth adds some more control-structure words:
 5438: 
 5439: 
 5440: doc-endif
 5441: doc-?dup-if
 5442: doc-?dup-0=-if
 5443: 
 5444: 
 5445: @noindent
 5446: Counted loop words constitute a separate group of words:
 5447: 
 5448: 
 5449: doc-?do
 5450: doc-+do
 5451: doc-u+do
 5452: doc--do
 5453: doc-u-do
 5454: doc-do
 5455: doc-for
 5456: doc-loop
 5457: doc-+loop
 5458: doc--loop
 5459: doc-next
 5460: doc-leave
 5461: doc-?leave
 5462: doc-unloop
 5463: doc-done
 5464: 
 5465: 
 5466: The standard does not allow using @code{CS-PICK} and @code{CS-ROLL} on
 5467: @i{do-sys}. Gforth allows it, but it's your job to ensure that for
 5468: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
 5469: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
 5470: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
 5471: resolved (by using one of the loop-ending words or @code{DONE}).
 5472: 
 5473: @noindent
 5474: Another group of control structure words are:
 5475: 
 5476: 
 5477: doc-case
 5478: doc-endcase
 5479: doc-of
 5480: doc-endof
 5481: 
 5482: 
 5483: @i{case-sys} and @i{of-sys} cannot be processed using @code{CS-PICK} and
 5484: @code{CS-ROLL}.
 5485: 
 5486: @subsubsection Programming Style
 5487: @cindex control structures programming style
 5488: @cindex programming style, arbitrary control structures
 5489: 
 5490: In order to ensure readability we recommend that you do not create
 5491: arbitrary control structures directly, but define new control structure
 5492: words for the control structure you want and use these words in your
 5493: program. For example, instead of writing:
 5494: 
 5495: @example
 5496: BEGIN
 5497:   ...
 5498: IF [ 1 CS-ROLL ]
 5499:   ...
 5500: AGAIN THEN
 5501: @end example
 5502: 
 5503: @noindent
 5504: we recommend defining control structure words, e.g.,
 5505: 
 5506: @example
 5507: : WHILE ( DEST -- ORIG DEST )
 5508:  POSTPONE IF
 5509:  1 CS-ROLL ; immediate
 5510: 
 5511: : REPEAT ( orig dest -- )
 5512:  POSTPONE AGAIN
 5513:  POSTPONE THEN ; immediate
 5514: @end example
 5515: 
 5516: @noindent
 5517: and then using these to create the control structure:
 5518: 
 5519: @example
 5520: BEGIN
 5521:   ...
 5522: WHILE
 5523:   ...
 5524: REPEAT
 5525: @end example
 5526: 
 5527: That's much easier to read, isn't it? Of course, @code{REPEAT} and
 5528: @code{WHILE} are predefined, so in this example it would not be
 5529: necessary to define them.
 5530: 
 5531: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
 5532: @subsection Calls and returns
 5533: @cindex calling a definition
 5534: @cindex returning from a definition
 5535: 
 5536: @cindex recursive definitions
 5537: A definition can be called simply be writing the name of the definition
 5538: to be called. Normally a definition is invisible during its own
 5539: definition. If you want to write a directly recursive definition, you
 5540: can use @code{recursive} to make the current definition visible, or
 5541: @code{recurse} to call the current definition directly.
 5542: 
 5543: 
 5544: doc-recursive
 5545: doc-recurse
 5546: 
 5547: 
 5548: @comment TODO add example of the two recursion methods
 5549: @quotation
 5550: @progstyle
 5551: I prefer using @code{recursive} to @code{recurse}, because calling the
 5552: definition by name is more descriptive (if the name is well-chosen) than
 5553: the somewhat cryptic @code{recurse}.  E.g., in a quicksort
 5554: implementation, it is much better to read (and think) ``now sort the
 5555: partitions'' than to read ``now do a recursive call''.
 5556: @end quotation
 5557: 
 5558: For mutual recursion, use @code{Defer}red words, like this:
 5559: 
 5560: @example
 5561: Defer foo
 5562: 
 5563: : bar ( ... -- ... )
 5564:  ... foo ... ;
 5565: 
 5566: :noname ( ... -- ... )
 5567:  ... bar ... ;
 5568: IS foo
 5569: @end example
 5570: 
 5571: Deferred words are discussed in more detail in @ref{Deferred words}.
 5572: 
 5573: The current definition returns control to the calling definition when
 5574: the end of the definition is reached or @code{EXIT} is encountered.
 5575: 
 5576: doc-exit
 5577: doc-;s
 5578: 
 5579: 
 5580: @node Exception Handling,  , Calls and returns, Control Structures
 5581: @subsection Exception Handling
 5582: @cindex exceptions
 5583: 
 5584: @c quit is a very bad idea for error handling, 
 5585: @c because it does not translate into a THROW
 5586: @c it also does not belong into this chapter
 5587: 
 5588: If a word detects an error condition that it cannot handle, it can
 5589: @code{throw} an exception.  In the simplest case, this will terminate
 5590: your program, and report an appropriate error.
 5591: 
 5592: doc-throw
 5593: 
 5594: @code{Throw} consumes a cell-sized error number on the stack. There are
 5595: some predefined error numbers in ANS Forth (see @file{errors.fs}).  In
 5596: Gforth (and most other systems) you can use the iors produced by various
 5597: words as error numbers (e.g., a typical use of @code{allocate} is
 5598: @code{allocate throw}).  Gforth also provides the word @code{exception}
 5599: to define your own error numbers (with decent error reporting); an ANS
 5600: Forth version of this word (but without the error messages) is available
 5601: in @code{compat/except.fs}.  And finally, you can use your own error
 5602: numbers (anything outside the range -4095..0), but won't get nice error
 5603: messages, only numbers.  For example, try:
 5604: 
 5605: @example
 5606: -10 throw                    \ ANS defined
 5607: -267 throw                   \ system defined
 5608: s" my error" exception throw \ user defined
 5609: 7 throw                      \ arbitrary number
 5610: @end example
 5611: 
 5612: doc---exception-exception
 5613: 
 5614: A common idiom to @code{THROW} a specific error if a flag is true is
 5615: this:
 5616: 
 5617: @example
 5618: @code{( flag ) 0<> @i{errno} and throw}
 5619: @end example
 5620: 
 5621: Your program can provide exception handlers to catch exceptions.  An
 5622: exception handler can be used to correct the problem, or to clean up
 5623: some data structures and just throw the exception to the next exception
 5624: handler.  Note that @code{throw} jumps to the dynamically innermost
 5625: exception handler.  The system's exception handler is outermost, and just
 5626: prints an error and restarts command-line interpretation (or, in batch
 5627: mode (i.e., while processing the shell command line), leaves Gforth).
 5628: 
 5629: The ANS Forth way to catch exceptions is @code{catch}:
 5630: 
 5631: doc-catch
 5632: 
 5633: The most common use of exception handlers is to clean up the state when
 5634: an error happens.  E.g.,
 5635: 
 5636: @example
 5637: base @ >r hex \ actually the hex should be inside foo, or we h
 5638: ['] foo catch ( nerror|0 )
 5639: r> base !
 5640: ( nerror|0 ) throw \ pass it on
 5641: @end example
 5642: 
 5643: A use of @code{catch} for handling the error @code{myerror} might look
 5644: like this:
 5645: 
 5646: @example
 5647: ['] foo catch
 5648: CASE
 5649:   myerror OF ... ( do something about it ) ENDOF
 5650:   dup throw \ default: pass other errors on, do nothing on non-errors
 5651: ENDCASE
 5652: @end example
 5653: 
 5654: Having to wrap the code into a separate word is often cumbersome,
 5655: therefore Gforth provides an alternative syntax:
 5656: 
 5657: @example
 5658: TRY
 5659:   @i{code1}
 5660: RECOVER     \ optional
 5661:   @i{code2} \ optional
 5662: ENDTRY
 5663: @end example
 5664: 
 5665: This performs @i{Code1}.  If @i{code1} completes normally, execution
 5666: continues after the @code{endtry}.  If @i{Code1} throws, the stacks are
 5667: reset to the state during @code{try}, the throw value is pushed on the
 5668: data stack, and execution constinues at @i{code2}, and finally falls
 5669: through the @code{endtry} into the following code.
 5670: 
 5671: doc-try
 5672: doc-recover
 5673: doc-endtry
 5674: 
 5675: The cleanup example from above in this syntax:
 5676: 
 5677: @example
 5678: base @ >r TRY
 5679:   hex foo \ now the hex is placed correctly
 5680:   0       \ value for throw
 5681: RECOVER ENDTRY
 5682: r> base ! throw
 5683: @end example
 5684: 
 5685: And here's the error handling example:
 5686: 
 5687: @example
 5688: TRY
 5689:   foo
 5690: RECOVER
 5691:   CASE
 5692:     myerror OF ... ( do something about it ) ENDOF
 5693:     throw \ pass other errors on
 5694:   ENDCASE
 5695: ENDTRY
 5696: @end example
 5697: 
 5698: @progstyle
 5699: As usual, you should ensure that the stack depth is statically known at
 5700: the end: either after the @code{throw} for passing on errors, or after
 5701: the @code{ENDTRY} (or, if you use @code{catch}, after the end of the
 5702: selection construct for handling the error).
 5703: 
 5704: There are two alternatives to @code{throw}: @code{Abort"} is conditional
 5705: and you can provide an error message.  @code{Abort} just produces an
 5706: ``Aborted'' error.
 5707: 
 5708: The problem with these words is that exception handlers cannot
 5709: differentiate between different @code{abort"}s; they just look like
 5710: @code{-2 throw} to them (the error message cannot be accessed by
 5711: standard programs).  Similar @code{abort} looks like @code{-1 throw} to
 5712: exception handlers.
 5713: 
 5714: doc-abort"
 5715: doc-abort
 5716: 
 5717: 
 5718: 
 5719: @c -------------------------------------------------------------
 5720: @node Defining Words, Interpretation and Compilation Semantics, Control Structures, Words
 5721: @section Defining Words
 5722: @cindex defining words
 5723: 
 5724: Defining words are used to extend Forth by creating new entries in the dictionary.
 5725: 
 5726: @menu
 5727: * CREATE::                      
 5728: * Variables::                   Variables and user variables
 5729: * Constants::                   
 5730: * Values::                      Initialised variables
 5731: * Colon Definitions::           
 5732: * Anonymous Definitions::       Definitions without names
 5733: * Supplying names::             Passing definition names as strings
 5734: * User-defined Defining Words::  
 5735: * Deferred words::              Allow forward references
 5736: * Aliases::                     
 5737: @end menu
 5738: 
 5739: @node CREATE, Variables, Defining Words, Defining Words
 5740: @subsection @code{CREATE}
 5741: @cindex simple defining words
 5742: @cindex defining words, simple
 5743: 
 5744: Defining words are used to create new entries in the dictionary. The
 5745: simplest defining word is @code{CREATE}. @code{CREATE} is used like
 5746: this:
 5747: 
 5748: @example
 5749: CREATE new-word1
 5750: @end example
 5751: 
 5752: @code{CREATE} is a parsing word, i.e., it takes an argument from the
 5753: input stream (@code{new-word1} in our example).  It generates a
 5754: dictionary entry for @code{new-word1}. When @code{new-word1} is
 5755: executed, all that it does is leave an address on the stack. The address
 5756: represents the value of the data space pointer (@code{HERE}) at the time
 5757: that @code{new-word1} was defined. Therefore, @code{CREATE} is a way of
 5758: associating a name with the address of a region of memory.
 5759: 
 5760: doc-create
 5761: 
 5762: Note that in ANS Forth guarantees only for @code{create} that its body
 5763: is in dictionary data space (i.e., where @code{here}, @code{allot}
 5764: etc. work, @pxref{Dictionary allocation}).  Also, in ANS Forth only
 5765: @code{create}d words can be modified with @code{does>}
 5766: (@pxref{User-defined Defining Words}).  And in ANS Forth @code{>body}
 5767: can only be applied to @code{create}d words.
 5768: 
 5769: By extending this example to reserve some memory in data space, we end
 5770: up with something like a @i{variable}. Here are two different ways to do
 5771: it:
 5772: 
 5773: @example
 5774: CREATE new-word2 1 cells allot  \ reserve 1 cell - initial value undefined
 5775: CREATE new-word3 4 ,            \ reserve 1 cell and initialise it (to 4)
 5776: @end example
 5777: 
 5778: The variable can be examined and modified using @code{@@} (``fetch'') and
 5779: @code{!} (``store'') like this:
 5780: 
 5781: @example
 5782: new-word2 @@ .      \ get address, fetch from it and display
 5783: 1234 new-word2 !   \ new value, get address, store to it
 5784: @end example
 5785: 
 5786: @cindex arrays
 5787: A similar mechanism can be used to create arrays. For example, an
 5788: 80-character text input buffer:
 5789: 
 5790: @example
 5791: CREATE text-buf 80 chars allot
 5792: 
 5793: text-buf 0 chars c@@ \ the 1st character (offset 0)
 5794: text-buf 3 chars c@@ \ the 4th character (offset 3)
 5795: @end example
 5796: 
 5797: You can build arbitrarily complex data structures by allocating
 5798: appropriate areas of memory. For further discussions of this, and to
 5799: learn about some Gforth tools that make it easier,
 5800: @xref{Structures}.
 5801: 
 5802: 
 5803: @node Variables, Constants, CREATE, Defining Words
 5804: @subsection Variables
 5805: @cindex variables
 5806: 
 5807: The previous section showed how a sequence of commands could be used to
 5808: generate a variable.  As a final refinement, the whole code sequence can
 5809: be wrapped up in a defining word (pre-empting the subject of the next
 5810: section), making it easier to create new variables:
 5811: 
 5812: @example
 5813: : myvariableX ( "name" -- a-addr ) CREATE 1 cells allot ;
 5814: : myvariable0 ( "name" -- a-addr ) CREATE 0 , ;
 5815: 
 5816: myvariableX foo \ variable foo starts off with an unknown value
 5817: myvariable0 joe \ whilst joe is initialised to 0
 5818: 
 5819: 45 3 * foo !   \ set foo to 135
 5820: 1234 joe !     \ set joe to 1234
 5821: 3 joe +!       \ increment joe by 3.. to 1237
 5822: @end example
 5823: 
 5824: Not surprisingly, there is no need to define @code{myvariable}, since
 5825: Forth already has a definition @code{Variable}. ANS Forth does not
 5826: guarantee that a @code{Variable} is initialised when it is created
 5827: (i.e., it may behave like @code{myvariableX}). In contrast, Gforth's
 5828: @code{Variable} initialises the variable to 0 (i.e., it behaves exactly
 5829: like @code{myvariable0}). Forth also provides @code{2Variable} and
 5830: @code{fvariable} for double and floating-point variables, respectively
 5831: -- they are initialised to 0. and 0e in Gforth. If you use a @code{Variable} to
 5832: store a boolean, you can use @code{on} and @code{off} to toggle its
 5833: state.
 5834: 
 5835: doc-variable
 5836: doc-2variable
 5837: doc-fvariable
 5838: 
 5839: @cindex user variables
 5840: @cindex user space
 5841: The defining word @code{User} behaves in the same way as @code{Variable}.
 5842: The difference is that it reserves space in @i{user (data) space} rather
 5843: than normal data space. In a Forth system that has a multi-tasker, each
 5844: task has its own set of user variables.
 5845: 
 5846: doc-user
 5847: @c doc-udp
 5848: @c doc-uallot
 5849: 
 5850: @comment TODO is that stuff about user variables strictly correct? Is it
 5851: @comment just terminal tasks that have user variables?
 5852: @comment should document tasker.fs (with some examples) elsewhere
 5853: @comment in this manual, then expand on user space and user variables.
 5854: 
 5855: @node Constants, Values, Variables, Defining Words
 5856: @subsection Constants
 5857: @cindex constants
 5858: 
 5859: @code{Constant} allows you to declare a fixed value and refer to it by
 5860: name. For example:
 5861: 
 5862: @example
 5863: 12 Constant INCHES-PER-FOOT
 5864: 3E+08 fconstant SPEED-O-LIGHT
 5865: @end example
 5866: 
 5867: A @code{Variable} can be both read and written, so its run-time
 5868: behaviour is to supply an address through which its current value can be
 5869: manipulated. In contrast, the value of a @code{Constant} cannot be
 5870: changed once it has been declared@footnote{Well, often it can be -- but
 5871: not in a Standard, portable way. It's safer to use a @code{Value} (read
 5872: on).} so it's not necessary to supply the address -- it is more
 5873: efficient to return the value of the constant directly. That's exactly
 5874: what happens; the run-time effect of a constant is to put its value on
 5875: the top of the stack (You can find one
 5876: way of implementing @code{Constant} in @ref{User-defined Defining Words}).
 5877: 
 5878: Forth also provides @code{2Constant} and @code{fconstant} for defining
 5879: double and floating-point constants, respectively.
 5880: 
 5881: doc-constant
 5882: doc-2constant
 5883: doc-fconstant
 5884: 
 5885: @c that's too deep, and it's not necessarily true for all ANS Forths. - anton
 5886: @c nac-> How could that not be true in an ANS Forth? You can't define a
 5887: @c constant, use it and then delete the definition of the constant..
 5888: 
 5889: @c anton->An ANS Forth system can compile a constant to a literal; On
 5890: @c decompilation you would see only the number, just as if it had been used
 5891: @c in the first place.  The word will stay, of course, but it will only be
 5892: @c used by the text interpreter (no run-time duties, except when it is 
 5893: @c POSTPONEd or somesuch).
 5894: 
 5895: @c nac:
 5896: @c I agree that it's rather deep, but IMO it is an important difference
 5897: @c relative to other programming languages.. often it's annoying: it
 5898: @c certainly changes my programming style relative to C.
 5899: 
 5900: @c anton: In what way?
 5901: 
 5902: Constants in Forth behave differently from their equivalents in other
 5903: programming languages. In other languages, a constant (such as an EQU in
 5904: assembler or a #define in C) only exists at compile-time; in the
 5905: executable program the constant has been translated into an absolute
 5906: number and, unless you are using a symbolic debugger, it's impossible to
 5907: know what abstract thing that number represents. In Forth a constant has
 5908: an entry in the header space and remains there after the code that uses
 5909: it has been defined. In fact, it must remain in the dictionary since it
 5910: has run-time duties to perform. For example:
 5911: 
 5912: @example
 5913: 12 Constant INCHES-PER-FOOT
 5914: : FEET-TO-INCHES ( n1 -- n2 ) INCHES-PER-FOOT * ;
 5915: @end example
 5916: 
 5917: @cindex in-lining of constants
 5918: When @code{FEET-TO-INCHES} is executed, it will in turn execute the xt
 5919: associated with the constant @code{INCHES-PER-FOOT}. If you use
 5920: @code{see} to decompile the definition of @code{FEET-TO-INCHES}, you can
 5921: see that it makes a call to @code{INCHES-PER-FOOT}. Some Forth compilers
 5922: attempt to optimise constants by in-lining them where they are used. You
 5923: can force Gforth to in-line a constant like this:
 5924: 
 5925: @example
 5926: : FEET-TO-INCHES ( n1 -- n2 ) [ INCHES-PER-FOOT ] LITERAL * ;
 5927: @end example
 5928: 
 5929: If you use @code{see} to decompile @i{this} version of
 5930: @code{FEET-TO-INCHES}, you can see that @code{INCHES-PER-FOOT} is no
 5931: longer present. To understand how this works, read
 5932: @ref{Interpret/Compile states}, and @ref{Literals}.
 5933: 
 5934: In-lining constants in this way might improve execution time
 5935: fractionally, and can ensure that a constant is now only referenced at
 5936: compile-time. However, the definition of the constant still remains in
 5937: the dictionary. Some Forth compilers provide a mechanism for controlling
 5938: a second dictionary for holding transient words such that this second
 5939: dictionary can be deleted later in order to recover memory
 5940: space. However, there is no standard way of doing this.
 5941: 
 5942: 
 5943: @node Values, Colon Definitions, Constants, Defining Words
 5944: @subsection Values
 5945: @cindex values
 5946: 
 5947: A @code{Value} behaves like a @code{Constant}, but it can be changed.
 5948: @code{TO} is a parsing word that changes a @code{Values}.  In Gforth
 5949: (not in ANS Forth) you can access (and change) a @code{value} also with
 5950: @code{>body}.
 5951: 
 5952: Here are some
 5953: examples:
 5954: 
 5955: @example
 5956: 12 Value APPLES     \ Define APPLES with an initial value of 12
 5957: 34 TO APPLES        \ Change the value of APPLES. TO is a parsing word
 5958: 1 ' APPLES >body +! \ Increment APPLES.  Non-standard usage.
 5959: APPLES              \ puts 35 on the top of the stack.
 5960: @end example
 5961: 
 5962: doc-value
 5963: doc-to
 5964: 
 5965: 
 5966: 
 5967: @node Colon Definitions, Anonymous Definitions, Values, Defining Words
 5968: @subsection Colon Definitions
 5969: @cindex colon definitions
 5970: 
 5971: @example
 5972: : name ( ... -- ... )
 5973:     word1 word2 word3 ;
 5974: @end example
 5975: 
 5976: @noindent
 5977: Creates a word called @code{name} that, upon execution, executes
 5978: @code{word1 word2 word3}. @code{name} is a @dfn{(colon) definition}.
 5979: 
 5980: The explanation above is somewhat superficial. For simple examples of
 5981: colon definitions see @ref{Your first definition}.  For an in-depth
 5982: discussion of some of the issues involved, @xref{Interpretation and
 5983: Compilation Semantics}.
 5984: 
 5985: doc-:
 5986: doc-;
 5987: 
 5988: 
 5989: @node Anonymous Definitions, Supplying names, Colon Definitions, Defining Words
 5990: @subsection Anonymous Definitions
 5991: @cindex colon definitions
 5992: @cindex defining words without name
 5993: 
 5994: Sometimes you want to define an @dfn{anonymous word}; a word without a
 5995: name. You can do this with:
 5996: 
 5997: doc-:noname
 5998: 
 5999: This leaves the execution token for the word on the stack after the
 6000: closing @code{;}. Here's an example in which a deferred word is
 6001: initialised with an @code{xt} from an anonymous colon definition:
 6002: 
 6003: @example
 6004: Defer deferred
 6005: :noname ( ... -- ... )
 6006:   ... ;
 6007: IS deferred
 6008: @end example
 6009: 
 6010: @noindent
 6011: Gforth provides an alternative way of doing this, using two separate
 6012: words:
 6013: 
 6014: doc-noname
 6015: @cindex execution token of last defined word
 6016: doc-latestxt
 6017: 
 6018: @noindent
 6019: The previous example can be rewritten using @code{noname} and
 6020: @code{latestxt}:
 6021: 
 6022: @example
 6023: Defer deferred
 6024: noname : ( ... -- ... )
 6025:   ... ;
 6026: latestxt IS deferred
 6027: @end example
 6028: 
 6029: @noindent
 6030: @code{noname} works with any defining word, not just @code{:}.
 6031: 
 6032: @code{latestxt} also works when the last word was not defined as
 6033: @code{noname}.  It does not work for combined words, though.  It also has
 6034: the useful property that is is valid as soon as the header for a
 6035: definition has been built. Thus:
 6036: 
 6037: @example
 6038: latestxt . : foo [ latestxt . ] ; ' foo .
 6039: @end example
 6040: 
 6041: @noindent
 6042: prints 3 numbers; the last two are the same.
 6043: 
 6044: @node Supplying names, User-defined Defining Words, Anonymous Definitions, Defining Words
 6045: @subsection Supplying the name of a defined word
 6046: @cindex names for defined words
 6047: @cindex defining words, name given in a string
 6048: 
 6049: By default, a defining word takes the name for the defined word from the
 6050: input stream. Sometimes you want to supply the name from a string. You
 6051: can do this with:
 6052: 
 6053: doc-nextname
 6054: 
 6055: For example:
 6056: 
 6057: @example
 6058: s" foo" nextname create
 6059: @end example
 6060: 
 6061: @noindent
 6062: is equivalent to:
 6063: 
 6064: @example
 6065: create foo
 6066: @end example
 6067: 
 6068: @noindent
 6069: @code{nextname} works with any defining word.
 6070: 
 6071: 
 6072: @node User-defined Defining Words, Deferred words, Supplying names, Defining Words
 6073: @subsection User-defined Defining Words
 6074: @cindex user-defined defining words
 6075: @cindex defining words, user-defined
 6076: 
 6077: You can create a new defining word by wrapping defining-time code around
 6078: an existing defining word and putting the sequence in a colon
 6079: definition. 
 6080: 
 6081: @c anton: This example is very complex and leads in a quite different
 6082: @c direction from the CREATE-DOES> stuff that follows.  It should probably
 6083: @c be done elsewhere, or as a subsubsection of this subsection (or as a
 6084: @c subsection of Defining Words)
 6085: 
 6086: For example, suppose that you have a word @code{stats} that
 6087: gathers statistics about colon definitions given the @i{xt} of the
 6088: definition, and you want every colon definition in your application to
 6089: make a call to @code{stats}. You can define and use a new version of
 6090: @code{:} like this:
 6091: 
 6092: @example
 6093: : stats ( xt -- ) DUP ." (Gathering statistics for " . ." )"
 6094:   ... ;  \ other code
 6095: 
 6096: : my: : latestxt postpone literal ['] stats compile, ;
 6097: 
 6098: my: foo + - ;
 6099: @end example
 6100: 
 6101: When @code{foo} is defined using @code{my:} these steps occur:
 6102: 
 6103: @itemize @bullet
 6104: @item
 6105: @code{my:} is executed.
 6106: @item
 6107: The @code{:} within the definition (the one between @code{my:} and
 6108: @code{latestxt}) is executed, and does just what it always does; it parses
 6109: the input stream for a name, builds a dictionary header for the name
 6110: @code{foo} and switches @code{state} from interpret to compile.
 6111: @item
 6112: The word @code{latestxt} is executed. It puts the @i{xt} for the word that is
 6113: being defined -- @code{foo} -- onto the stack.
 6114: @item
 6115: The code that was produced by @code{postpone literal} is executed; this
 6116: causes the value on the stack to be compiled as a literal in the code
 6117: area of @code{foo}.
 6118: @item
 6119: The code @code{['] stats} compiles a literal into the definition of
 6120: @code{my:}. When @code{compile,} is executed, that literal -- the
 6121: execution token for @code{stats} -- is layed down in the code area of
 6122: @code{foo} , following the literal@footnote{Strictly speaking, the
 6123: mechanism that @code{compile,} uses to convert an @i{xt} into something
 6124: in the code area is implementation-dependent. A threaded implementation
 6125: might spit out the execution token directly whilst another
 6126: implementation might spit out a native code sequence.}.
 6127: @item
 6128: At this point, the execution of @code{my:} is complete, and control
 6129: returns to the text interpreter. The text interpreter is in compile
 6130: state, so subsequent text @code{+ -} is compiled into the definition of
 6131: @code{foo} and the @code{;} terminates the definition as always.
 6132: @end itemize
 6133: 
 6134: You can use @code{see} to decompile a word that was defined using
 6135: @code{my:} and see how it is different from a normal @code{:}
 6136: definition. For example:
 6137: 
 6138: @example
 6139: : bar + - ;  \ like foo but using : rather than my:
 6140: see bar
 6141: : bar
 6142:   + - ;
 6143: see foo
 6144: : foo
 6145:   107645672 stats + - ;
 6146: 
 6147: \ use ' stats . to show that 107645672 is the xt for stats
 6148: @end example
 6149: 
 6150: You can use techniques like this to make new defining words in terms of
 6151: @i{any} existing defining word.
 6152: 
 6153: 
 6154: @cindex defining defining words
 6155: @cindex @code{CREATE} ... @code{DOES>}
 6156: If you want the words defined with your defining words to behave
 6157: differently from words defined with standard defining words, you can
 6158: write your defining word like this:
 6159: 
 6160: @example
 6161: : def-word ( "name" -- )
 6162:     CREATE @i{code1}
 6163: DOES> ( ... -- ... )
 6164:     @i{code2} ;
 6165: 
 6166: def-word name
 6167: @end example
 6168: 
 6169: @cindex child words
 6170: This fragment defines a @dfn{defining word} @code{def-word} and then
 6171: executes it.  When @code{def-word} executes, it @code{CREATE}s a new
 6172: word, @code{name}, and executes the code @i{code1}. The code @i{code2}
 6173: is not executed at this time. The word @code{name} is sometimes called a
 6174: @dfn{child} of @code{def-word}.
 6175: 
 6176: When you execute @code{name}, the address of the body of @code{name} is
 6177: put on the data stack and @i{code2} is executed (the address of the body
 6178: of @code{name} is the address @code{HERE} returns immediately after the
 6179: @code{CREATE}, i.e., the address a @code{create}d word returns by
 6180: default).
 6181: 
 6182: @c anton:
 6183: @c www.dictionary.com says:
 6184: @c at·a·vism: 1.The reappearance of a characteristic in an organism after
 6185: @c several generations of absence, usually caused by the chance
 6186: @c recombination of genes.  2.An individual or a part that exhibits
 6187: @c atavism. Also called throwback.  3.The return of a trait or recurrence
 6188: @c of previous behavior after a period of absence.
 6189: @c
 6190: @c Doesn't seem to fit.
 6191: 
 6192: @c @cindex atavism in child words
 6193: You can use @code{def-word} to define a set of child words that behave
 6194: similarly; they all have a common run-time behaviour determined by
 6195: @i{code2}. Typically, the @i{code1} sequence builds a data area in the
 6196: body of the child word. The structure of the data is common to all
 6197: children of @code{def-word}, but the data values are specific -- and
 6198: private -- to each child word. When a child word is executed, the
 6199: address of its private data area is passed as a parameter on TOS to be
 6200: used and manipulated@footnote{It is legitimate both to read and write to
 6201: this data area.} by @i{code2}.
 6202: 
 6203: The two fragments of code that make up the defining words act (are
 6204: executed) at two completely separate times:
 6205: 
 6206: @itemize @bullet
 6207: @item
 6208: At @i{define time}, the defining word executes @i{code1} to generate a
 6209: child word
 6210: @item
 6211: At @i{child execution time}, when a child word is invoked, @i{code2}
 6212: is executed, using parameters (data) that are private and specific to
 6213: the child word.
 6214: @end itemize
 6215: 
 6216: Another way of understanding the behaviour of @code{def-word} and
 6217: @code{name} is to say that, if you make the following definitions:
 6218: @example
 6219: : def-word1 ( "name" -- )
 6220:     CREATE @i{code1} ;
 6221: 
 6222: : action1 ( ... -- ... )
 6223:     @i{code2} ;
 6224: 
 6225: def-word1 name1
 6226: @end example
 6227: 
 6228: @noindent
 6229: Then using @code{name1 action1} is equivalent to using @code{name}.
 6230: 
 6231: The classic example is that you can define @code{CONSTANT} in this way:
 6232: 
 6233: @example
 6234: : CONSTANT ( w "name" -- )
 6235:     CREATE ,
 6236: DOES> ( -- w )
 6237:     @@ ;
 6238: @end example
 6239: 
 6240: @comment There is a beautiful description of how this works and what
 6241: @comment it does in the Forthwrite 100th edition.. as well as an elegant
 6242: @comment commentary on the Counting Fruits problem.
 6243: 
 6244: When you create a constant with @code{5 CONSTANT five}, a set of
 6245: define-time actions take place; first a new word @code{five} is created,
 6246: then the value 5 is laid down in the body of @code{five} with
 6247: @code{,}. When @code{five} is executed, the address of the body is put on
 6248: the stack, and @code{@@} retrieves the value 5. The word @code{five} has
 6249: no code of its own; it simply contains a data field and a pointer to the
 6250: code that follows @code{DOES>} in its defining word. That makes words
 6251: created in this way very compact.
 6252: 
 6253: The final example in this section is intended to remind you that space
 6254: reserved in @code{CREATE}d words is @i{data} space and therefore can be
 6255: both read and written by a Standard program@footnote{Exercise: use this
 6256: example as a starting point for your own implementation of @code{Value}
 6257: and @code{TO} -- if you get stuck, investigate the behaviour of @code{'} and
 6258: @code{[']}.}:
 6259: 
 6260: @example
 6261: : foo ( "name" -- )
 6262:     CREATE -1 ,
 6263: DOES> ( -- )
 6264:     @@ . ;
 6265: 
 6266: foo first-word
 6267: foo second-word
 6268: 
 6269: 123 ' first-word >BODY !
 6270: @end example
 6271: 
 6272: If @code{first-word} had been a @code{CREATE}d word, we could simply
 6273: have executed it to get the address of its data field. However, since it
 6274: was defined to have @code{DOES>} actions, its execution semantics are to
 6275: perform those @code{DOES>} actions. To get the address of its data field
 6276: it's necessary to use @code{'} to get its xt, then @code{>BODY} to
 6277: translate the xt into the address of the data field.  When you execute
 6278: @code{first-word}, it will display @code{123}. When you execute
 6279: @code{second-word} it will display @code{-1}.
 6280: 
 6281: @cindex stack effect of @code{DOES>}-parts
 6282: @cindex @code{DOES>}-parts, stack effect
 6283: In the examples above the stack comment after the @code{DOES>} specifies
 6284: the stack effect of the defined words, not the stack effect of the
 6285: following code (the following code expects the address of the body on
 6286: the top of stack, which is not reflected in the stack comment). This is
 6287: the convention that I use and recommend (it clashes a bit with using
 6288: locals declarations for stack effect specification, though).
 6289: 
 6290: @menu
 6291: * CREATE..DOES> applications::  
 6292: * CREATE..DOES> details::       
 6293: * Advanced does> usage example::  
 6294: * @code{Const-does>}::          
 6295: @end menu
 6296: 
 6297: @node CREATE..DOES> applications, CREATE..DOES> details, User-defined Defining Words, User-defined Defining Words
 6298: @subsubsection Applications of @code{CREATE..DOES>}
 6299: @cindex @code{CREATE} ... @code{DOES>}, applications
 6300: 
 6301: You may wonder how to use this feature. Here are some usage patterns:
 6302: 
 6303: @cindex factoring similar colon definitions
 6304: When you see a sequence of code occurring several times, and you can
 6305: identify a meaning, you will factor it out as a colon definition. When
 6306: you see similar colon definitions, you can factor them using
 6307: @code{CREATE..DOES>}. E.g., an assembler usually defines several words
 6308: that look very similar:
 6309: @example
 6310: : ori, ( reg-target reg-source n -- )
 6311:     0 asm-reg-reg-imm ;
 6312: : andi, ( reg-target reg-source n -- )
 6313:     1 asm-reg-reg-imm ;
 6314: @end example
 6315: 
 6316: @noindent
 6317: This could be factored with:
 6318: @example
 6319: : reg-reg-imm ( op-code -- )
 6320:     CREATE ,
 6321: DOES> ( reg-target reg-source n -- )
 6322:     @@ asm-reg-reg-imm ;
 6323: 
 6324: 0 reg-reg-imm ori,
 6325: 1 reg-reg-imm andi,
 6326: @end example
 6327: 
 6328: @cindex currying
 6329: Another view of @code{CREATE..DOES>} is to consider it as a crude way to
 6330: supply a part of the parameters for a word (known as @dfn{currying} in
 6331: the functional language community). E.g., @code{+} needs two
 6332: parameters. Creating versions of @code{+} with one parameter fixed can
 6333: be done like this:
 6334: 
 6335: @example
 6336: : curry+ ( n1 "name" -- )
 6337:     CREATE ,
 6338: DOES> ( n2 -- n1+n2 )
 6339:     @@ + ;
 6340: 
 6341:  3 curry+ 3+
 6342: -2 curry+ 2-
 6343: @end example
 6344: 
 6345: 
 6346: @node CREATE..DOES> details, Advanced does> usage example, CREATE..DOES> applications, User-defined Defining Words
 6347: @subsubsection The gory details of @code{CREATE..DOES>}
 6348: @cindex @code{CREATE} ... @code{DOES>}, details
 6349: 
 6350: doc-does>
 6351: 
 6352: @cindex @code{DOES>} in a separate definition
 6353: This means that you need not use @code{CREATE} and @code{DOES>} in the
 6354: same definition; you can put the @code{DOES>}-part in a separate
 6355: definition. This allows us to, e.g., select among different @code{DOES>}-parts:
 6356: @example
 6357: : does1 
 6358: DOES> ( ... -- ... )
 6359:     ... ;
 6360: 
 6361: : does2
 6362: DOES> ( ... -- ... )
 6363:     ... ;
 6364: 
 6365: : def-word ( ... -- ... )
 6366:     create ...
 6367:     IF
 6368:        does1
 6369:     ELSE
 6370:        does2
 6371:     ENDIF ;
 6372: @end example
 6373: 
 6374: In this example, the selection of whether to use @code{does1} or
 6375: @code{does2} is made at definition-time; at the time that the child word is
 6376: @code{CREATE}d.
 6377: 
 6378: @cindex @code{DOES>} in interpretation state
 6379: In a standard program you can apply a @code{DOES>}-part only if the last
 6380: word was defined with @code{CREATE}. In Gforth, the @code{DOES>}-part
 6381: will override the behaviour of the last word defined in any case. In a
 6382: standard program, you can use @code{DOES>} only in a colon
 6383: definition. In Gforth, you can also use it in interpretation state, in a
 6384: kind of one-shot mode; for example:
 6385: @example
 6386: CREATE name ( ... -- ... )
 6387:   @i{initialization}
 6388: DOES>
 6389:   @i{code} ;
 6390: @end example
 6391: 
 6392: @noindent
 6393: is equivalent to the standard:
 6394: @example
 6395: :noname
 6396: DOES>
 6397:     @i{code} ;
 6398: CREATE name EXECUTE ( ... -- ... )
 6399:     @i{initialization}
 6400: @end example
 6401: 
 6402: doc->body
 6403: 
 6404: @node Advanced does> usage example, @code{Const-does>}, CREATE..DOES> details, User-defined Defining Words
 6405: @subsubsection Advanced does> usage example
 6406: 
 6407: The MIPS disassembler (@file{arch/mips/disasm.fs}) contains many words
 6408: for disassembling instructions, that follow a very repetetive scheme:
 6409: 
 6410: @example
 6411: :noname @var{disasm-operands} s" @var{inst-name}" type ;
 6412: @var{entry-num} cells @var{table} + !
 6413: @end example
 6414: 
 6415: Of course, this inspires the idea to factor out the commonalities to
 6416: allow a definition like
 6417: 
 6418: @example
 6419: @var{disasm-operands} @var{entry-num} @var{table} define-inst @var{inst-name}
 6420: @end example
 6421: 
 6422: The parameters @var{disasm-operands} and @var{table} are usually
 6423: correlated.  Moreover, before I wrote the disassembler, there already
 6424: existed code that defines instructions like this:
 6425: 
 6426: @example
 6427: @var{entry-num} @var{inst-format} @var{inst-name}
 6428: @end example
 6429: 
 6430: This code comes from the assembler and resides in
 6431: @file{arch/mips/insts.fs}.
 6432: 
 6433: So I had to define the @var{inst-format} words that performed the scheme
 6434: above when executed.  At first I chose to use run-time code-generation:
 6435: 
 6436: @example
 6437: : @var{inst-format} ( entry-num "name" -- ; compiled code: addr w -- )
 6438:   :noname Postpone @var{disasm-operands}
 6439:   name Postpone sliteral Postpone type Postpone ;
 6440:   swap cells @var{table} + ! ;
 6441: @end example
 6442: 
 6443: Note that this supplies the other two parameters of the scheme above.
 6444: 
 6445: An alternative would have been to write this using
 6446: @code{create}/@code{does>}:
 6447: 
 6448: @example
 6449: : @var{inst-format} ( entry-num "name" -- )
 6450:   here name string, ( entry-num c-addr ) \ parse and save "name"
 6451:   noname create , ( entry-num )
 6452:   latestxt swap cells @var{table} + !
 6453: does> ( addr w -- )
 6454:   \ disassemble instruction w at addr
 6455:   @@ >r 
 6456:   @var{disasm-operands}
 6457:   r> count type ;
 6458: @end example
 6459: 
 6460: Somehow the first solution is simpler, mainly because it's simpler to
 6461: shift a string from definition-time to use-time with @code{sliteral}
 6462: than with @code{string,} and friends.
 6463: 
 6464: I wrote a lot of words following this scheme and soon thought about
 6465: factoring out the commonalities among them.  Note that this uses a
 6466: two-level defining word, i.e., a word that defines ordinary defining
 6467: words.
 6468: 
 6469: This time a solution involving @code{postpone} and friends seemed more
 6470: difficult (try it as an exercise), so I decided to use a
 6471: @code{create}/@code{does>} word; since I was already at it, I also used
 6472: @code{create}/@code{does>} for the lower level (try using
 6473: @code{postpone} etc. as an exercise), resulting in the following
 6474: definition:
 6475: 
 6476: @example
 6477: : define-format ( disasm-xt table-xt -- )
 6478:     \ define an instruction format that uses disasm-xt for
 6479:     \ disassembling and enters the defined instructions into table
 6480:     \ table-xt
 6481:     create 2,
 6482: does> ( u "inst" -- )
 6483:     \ defines an anonymous word for disassembling instruction inst,
 6484:     \ and enters it as u-th entry into table-xt
 6485:     2@@ swap here name string, ( u table-xt disasm-xt c-addr ) \ remember string
 6486:     noname create 2,      \ define anonymous word
 6487:     execute latestxt swap ! \ enter xt of defined word into table-xt
 6488: does> ( addr w -- )
 6489:     \ disassemble instruction w at addr
 6490:     2@@ >r ( addr w disasm-xt R: c-addr )
 6491:     execute ( R: c-addr ) \ disassemble operands
 6492:     r> count type ; \ print name 
 6493: @end example
 6494: 
 6495: Note that the tables here (in contrast to above) do the @code{cells +}
 6496: by themselves (that's why you have to pass an xt).  This word is used in
 6497: the following way:
 6498: 
 6499: @example
 6500: ' @var{disasm-operands} ' @var{table} define-format @var{inst-format}
 6501: @end example
 6502: 
 6503: As shown above, the defined instruction format is then used like this:
 6504: 
 6505: @example
 6506: @var{entry-num} @var{inst-format} @var{inst-name}
 6507: @end example
 6508: 
 6509: In terms of currying, this kind of two-level defining word provides the
 6510: parameters in three stages: first @var{disasm-operands} and @var{table},
 6511: then @var{entry-num} and @var{inst-name}, finally @code{addr w}, i.e.,
 6512: the instruction to be disassembled.  
 6513: 
 6514: Of course this did not quite fit all the instruction format names used
 6515: in @file{insts.fs}, so I had to define a few wrappers that conditioned
 6516: the parameters into the right form.
 6517: 
 6518: If you have trouble following this section, don't worry.  First, this is
 6519: involved and takes time (and probably some playing around) to
 6520: understand; second, this is the first two-level
 6521: @code{create}/@code{does>} word I have written in seventeen years of
 6522: Forth; and if I did not have @file{insts.fs} to start with, I may well
 6523: have elected to use just a one-level defining word (with some repeating
 6524: of parameters when using the defining word). So it is not necessary to
 6525: understand this, but it may improve your understanding of Forth.
 6526: 
 6527: 
 6528: @node @code{Const-does>},  , Advanced does> usage example, User-defined Defining Words
 6529: @subsubsection @code{Const-does>}
 6530: 
 6531: A frequent use of @code{create}...@code{does>} is for transferring some
 6532: values from definition-time to run-time.  Gforth supports this use with
 6533: 
 6534: doc-const-does>
 6535: 
 6536: A typical use of this word is:
 6537: 
 6538: @example
 6539: : curry+ ( n1 "name" -- )
 6540: 1 0 CONST-DOES> ( n2 -- n1+n2 )
 6541:     + ;
 6542: 
 6543: 3 curry+ 3+
 6544: @end example
 6545: 
 6546: Here the @code{1 0} means that 1 cell and 0 floats are transferred from
 6547: definition to run-time.
 6548: 
 6549: The advantages of using @code{const-does>} are:
 6550: 
 6551: @itemize
 6552: 
 6553: @item
 6554: You don't have to deal with storing and retrieving the values, i.e.,
 6555: your program becomes more writable and readable.
 6556: 
 6557: @item
 6558: When using @code{does>}, you have to introduce a @code{@@} that cannot
 6559: be optimized away (because you could change the data using
 6560: @code{>body}...@code{!}); @code{const-does>} avoids this problem.
 6561: 
 6562: @end itemize
 6563: 
 6564: An ANS Forth implementation of @code{const-does>} is available in
 6565: @file{compat/const-does.fs}.
 6566: 
 6567: 
 6568: @node Deferred words, Aliases, User-defined Defining Words, Defining Words
 6569: @subsection Deferred words
 6570: @cindex deferred words
 6571: 
 6572: The defining word @code{Defer} allows you to define a word by name
 6573: without defining its behaviour; the definition of its behaviour is
 6574: deferred. Here are two situation where this can be useful:
 6575: 
 6576: @itemize @bullet
 6577: @item
 6578: Where you want to allow the behaviour of a word to be altered later, and
 6579: for all precompiled references to the word to change when its behaviour
 6580: is changed.
 6581: @item
 6582: For mutual recursion; @xref{Calls and returns}.
 6583: @end itemize
 6584: 
 6585: In the following example, @code{foo} always invokes the version of
 6586: @code{greet} that prints ``@code{Good morning}'' whilst @code{bar}
 6587: always invokes the version that prints ``@code{Hello}''. There is no way
 6588: of getting @code{foo} to use the later version without re-ordering the
 6589: source code and recompiling it.
 6590: 
 6591: @example
 6592: : greet ." Good morning" ;
 6593: : foo ... greet ... ;
 6594: : greet ." Hello" ;
 6595: : bar ... greet ... ;
 6596: @end example
 6597: 
 6598: This problem can be solved by defining @code{greet} as a @code{Defer}red
 6599: word. The behaviour of a @code{Defer}red word can be defined and
 6600: redefined at any time by using @code{IS} to associate the xt of a
 6601: previously-defined word with it. The previous example becomes:
 6602: 
 6603: @example
 6604: Defer greet ( -- )
 6605: : foo ... greet ... ;
 6606: : bar ... greet ... ;
 6607: : greet1 ( -- ) ." Good morning" ;
 6608: : greet2 ( -- ) ." Hello" ;
 6609: ' greet2 <IS> greet  \ make greet behave like greet2
 6610: @end example
 6611: 
 6612: @progstyle
 6613: You should write a stack comment for every deferred word, and put only
 6614: XTs into deferred words that conform to this stack effect.  Otherwise
 6615: it's too difficult to use the deferred word.
 6616: 
 6617: A deferred word can be used to improve the statistics-gathering example
 6618: from @ref{User-defined Defining Words}; rather than edit the
 6619: application's source code to change every @code{:} to a @code{my:}, do
 6620: this:
 6621: 
 6622: @example
 6623: : real: : ;     \ retain access to the original
 6624: defer :         \ redefine as a deferred word
 6625: ' my: <IS> :      \ use special version of :
 6626: \
 6627: \ load application here
 6628: \
 6629: ' real: <IS> :    \ go back to the original
 6630: @end example
 6631: 
 6632: 
 6633: One thing to note is that @code{<IS>} consumes its name when it is
 6634: executed.  If you want to specify the name at compile time, use
 6635: @code{[IS]}:
 6636: 
 6637: @example
 6638: : set-greet ( xt -- )
 6639:   [IS] greet ;
 6640: 
 6641: ' greet1 set-greet
 6642: @end example
 6643: 
 6644: A deferred word can only inherit execution semantics from the xt
 6645: (because that is all that an xt can represent -- for more discussion of
 6646: this @pxref{Tokens for Words}); by default it will have default
 6647: interpretation and compilation semantics deriving from this execution
 6648: semantics.  However, you can change the interpretation and compilation
 6649: semantics of the deferred word in the usual ways:
 6650: 
 6651: @example
 6652: : bar .... ; compile-only
 6653: Defer fred immediate
 6654: Defer jim
 6655: 
 6656: ' bar <IS> jim  \ jim has default semantics
 6657: ' bar <IS> fred \ fred is immediate
 6658: @end example
 6659: 
 6660: doc-defer
 6661: doc-<is>
 6662: doc-[is]
 6663: doc-is
 6664: @comment TODO document these: what's defers [is]
 6665: doc-what's
 6666: doc-defers
 6667: 
 6668: @c Use @code{words-deferred} to see a list of deferred words.
 6669: 
 6670: Definitions in ANS Forth for @code{defer}, @code{<is>} and @code{[is]}
 6671: are provided in @file{compat/defer.fs}.
 6672: 
 6673: 
 6674: @node Aliases,  , Deferred words, Defining Words
 6675: @subsection Aliases
 6676: @cindex aliases
 6677: 
 6678: The defining word @code{Alias} allows you to define a word by name that
 6679: has the same behaviour as some other word. Here are two situation where
 6680: this can be useful:
 6681: 
 6682: @itemize @bullet
 6683: @item
 6684: When you want access to a word's definition from a different word list
 6685: (for an example of this, see the definition of the @code{Root} word list
 6686: in the Gforth source).
 6687: @item
 6688: When you want to create a synonym; a definition that can be known by
 6689: either of two names (for example, @code{THEN} and @code{ENDIF} are
 6690: aliases).
 6691: @end itemize
 6692: 
 6693: Like deferred words, an alias has default compilation and interpretation
 6694: semantics at the beginning (not the modifications of the other word),
 6695: but you can change them in the usual ways (@code{immediate},
 6696: @code{compile-only}). For example:
 6697: 
 6698: @example
 6699: : foo ... ; immediate
 6700: 
 6701: ' foo Alias bar \ bar is not an immediate word
 6702: ' foo Alias fooby immediate \ fooby is an immediate word
 6703: @end example
 6704: 
 6705: Words that are aliases have the same xt, different headers in the
 6706: dictionary, and consequently different name tokens (@pxref{Tokens for
 6707: Words}) and possibly different immediate flags.  An alias can only have
 6708: default or immediate compilation semantics; you can define aliases for
 6709: combined words with @code{interpret/compile:} -- see @ref{Combined words}.
 6710: 
 6711: doc-alias
 6712: 
 6713: 
 6714: @node Interpretation and Compilation Semantics, Tokens for Words, Defining Words, Words
 6715: @section Interpretation and Compilation Semantics
 6716: @cindex semantics, interpretation and compilation
 6717: 
 6718: @c !! state and ' are used without explanation
 6719: @c example for immediate/compile-only? or is the tutorial enough
 6720: 
 6721: @cindex interpretation semantics
 6722: The @dfn{interpretation semantics} of a (named) word are what the text
 6723: interpreter does when it encounters the word in interpret state. It also
 6724: appears in some other contexts, e.g., the execution token returned by
 6725: @code{' @i{word}} identifies the interpretation semantics of @i{word}
 6726: (in other words, @code{' @i{word} execute} is equivalent to
 6727: interpret-state text interpretation of @code{@i{word}}).
 6728: 
 6729: @cindex compilation semantics
 6730: The @dfn{compilation semantics} of a (named) word are what the text
 6731: interpreter does when it encounters the word in compile state. It also
 6732: appears in other contexts, e.g, @code{POSTPONE @i{word}}
 6733: compiles@footnote{In standard terminology, ``appends to the current
 6734: definition''.} the compilation semantics of @i{word}.
 6735: 
 6736: @cindex execution semantics
 6737: The standard also talks about @dfn{execution semantics}. They are used
 6738: only for defining the interpretation and compilation semantics of many
 6739: words. By default, the interpretation semantics of a word are to
 6740: @code{execute} its execution semantics, and the compilation semantics of
 6741: a word are to @code{compile,} its execution semantics.@footnote{In
 6742: standard terminology: The default interpretation semantics are its
 6743: execution semantics; the default compilation semantics are to append its
 6744: execution semantics to the execution semantics of the current
 6745: definition.}
 6746: 
 6747: Unnamed words (@pxref{Anonymous Definitions}) cannot be encountered by
 6748: the text interpreter, ticked, or @code{postpone}d, so they have no
 6749: interpretation or compilation semantics.  Their behaviour is represented
 6750: by their XT (@pxref{Tokens for Words}), and we call it execution
 6751: semantics, too.
 6752: 
 6753: @comment TODO expand, make it co-operate with new sections on text interpreter.
 6754: 
 6755: @cindex immediate words
 6756: @cindex compile-only words
 6757: You can change the semantics of the most-recently defined word:
 6758: 
 6759: 
 6760: doc-immediate
 6761: doc-compile-only
 6762: doc-restrict
 6763: 
 6764: By convention, words with non-default compilation semantics (e.g.,
 6765: immediate words) often have names surrounded with brackets (e.g.,
 6766: @code{[']}, @pxref{Execution token}).
 6767: 
 6768: Note that ticking (@code{'}) a compile-only word gives an error
 6769: (``Interpreting a compile-only word'').
 6770: 
 6771: @menu
 6772: * Combined words::              
 6773: @end menu
 6774: 
 6775: 
 6776: @node Combined words,  , Interpretation and Compilation Semantics, Interpretation and Compilation Semantics
 6777: @subsection Combined Words
 6778: @cindex combined words
 6779: 
 6780: Gforth allows you to define @dfn{combined words} -- words that have an
 6781: arbitrary combination of interpretation and compilation semantics.
 6782: 
 6783: doc-interpret/compile:
 6784: 
 6785: This feature was introduced for implementing @code{TO} and @code{S"}. I
 6786: recommend that you do not define such words, as cute as they may be:
 6787: they make it hard to get at both parts of the word in some contexts.
 6788: E.g., assume you want to get an execution token for the compilation
 6789: part. Instead, define two words, one that embodies the interpretation
 6790: part, and one that embodies the compilation part.  Once you have done
 6791: that, you can define a combined word with @code{interpret/compile:} for
 6792: the convenience of your users.
 6793: 
 6794: You might try to use this feature to provide an optimizing
 6795: implementation of the default compilation semantics of a word. For
 6796: example, by defining:
 6797: @example
 6798: :noname
 6799:    foo bar ;
 6800: :noname
 6801:    POSTPONE foo POSTPONE bar ;
 6802: interpret/compile: opti-foobar
 6803: @end example
 6804: 
 6805: @noindent
 6806: as an optimizing version of:
 6807: 
 6808: @example
 6809: : foobar
 6810:     foo bar ;
 6811: @end example
 6812: 
 6813: Unfortunately, this does not work correctly with @code{[compile]},
 6814: because @code{[compile]} assumes that the compilation semantics of all
 6815: @code{interpret/compile:} words are non-default. I.e., @code{[compile]
 6816: opti-foobar} would compile compilation semantics, whereas
 6817: @code{[compile] foobar} would compile interpretation semantics.
 6818: 
 6819: @cindex state-smart words (are a bad idea)
 6820: @anchor{state-smartness}
 6821: Some people try to use @dfn{state-smart} words to emulate the feature provided
 6822: by @code{interpret/compile:} (words are state-smart if they check
 6823: @code{STATE} during execution). E.g., they would try to code
 6824: @code{foobar} like this:
 6825: 
 6826: @example
 6827: : foobar
 6828:   STATE @@
 6829:   IF ( compilation state )
 6830:     POSTPONE foo POSTPONE bar
 6831:   ELSE
 6832:     foo bar
 6833:   ENDIF ; immediate
 6834: @end example
 6835: 
 6836: Although this works if @code{foobar} is only processed by the text
 6837: interpreter, it does not work in other contexts (like @code{'} or
 6838: @code{POSTPONE}). E.g., @code{' foobar} will produce an execution token
 6839: for a state-smart word, not for the interpretation semantics of the
 6840: original @code{foobar}; when you execute this execution token (directly
 6841: with @code{EXECUTE} or indirectly through @code{COMPILE,}) in compile
 6842: state, the result will not be what you expected (i.e., it will not
 6843: perform @code{foo bar}). State-smart words are a bad idea. Simply don't
 6844: write them@footnote{For a more detailed discussion of this topic, see
 6845: M. Anton Ertl,
 6846: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz,@code{State}-smartness---Why
 6847: it is Evil and How to Exorcise it}}, EuroForth '98.}!
 6848: 
 6849: @cindex defining words with arbitrary semantics combinations
 6850: It is also possible to write defining words that define words with
 6851: arbitrary combinations of interpretation and compilation semantics. In
 6852: general, they look like this:
 6853: 
 6854: @example
 6855: : def-word
 6856:     create-interpret/compile
 6857:     @i{code1}
 6858: interpretation>
 6859:     @i{code2}
 6860: <interpretation
 6861: compilation>
 6862:     @i{code3}
 6863: <compilation ;
 6864: @end example
 6865: 
 6866: For a @i{word} defined with @code{def-word}, the interpretation
 6867: semantics are to push the address of the body of @i{word} and perform
 6868: @i{code2}, and the compilation semantics are to push the address of
 6869: the body of @i{word} and perform @i{code3}. E.g., @code{constant}
 6870: can also be defined like this (except that the defined constants don't
 6871: behave correctly when @code{[compile]}d):
 6872: 
 6873: @example
 6874: : constant ( n "name" -- )
 6875:     create-interpret/compile
 6876:     ,
 6877: interpretation> ( -- n )
 6878:     @@
 6879: <interpretation
 6880: compilation> ( compilation. -- ; run-time. -- n )
 6881:     @@ postpone literal
 6882: <compilation ;
 6883: @end example
 6884: 
 6885: 
 6886: doc-create-interpret/compile
 6887: doc-interpretation>
 6888: doc-<interpretation
 6889: doc-compilation>
 6890: doc-<compilation
 6891: 
 6892: 
 6893: Words defined with @code{interpret/compile:} and
 6894: @code{create-interpret/compile} have an extended header structure that
 6895: differs from other words; however, unless you try to access them with
 6896: plain address arithmetic, you should not notice this. Words for
 6897: accessing the header structure usually know how to deal with this; e.g.,
 6898: @code{'} @i{word} @code{>body} also gives you the body of a word created
 6899: with @code{create-interpret/compile}.
 6900: 
 6901: 
 6902: @c -------------------------------------------------------------
 6903: @node Tokens for Words, Compiling words, Interpretation and Compilation Semantics, Words
 6904: @section Tokens for Words
 6905: @cindex tokens for words
 6906: 
 6907: This section describes the creation and use of tokens that represent
 6908: words.
 6909: 
 6910: @menu
 6911: * Execution token::             represents execution/interpretation semantics
 6912: * Compilation token::           represents compilation semantics
 6913: * Name token::                  represents named words
 6914: @end menu
 6915: 
 6916: @node Execution token, Compilation token, Tokens for Words, Tokens for Words
 6917: @subsection Execution token
 6918: 
 6919: @cindex xt
 6920: @cindex execution token
 6921: An @dfn{execution token} (@i{XT}) represents some behaviour of a word.
 6922: You can use @code{execute} to invoke this behaviour.
 6923: 
 6924: @cindex tick (')
 6925: You can use @code{'} to get an execution token that represents the
 6926: interpretation semantics of a named word:
 6927: 
 6928: @example
 6929: 5 ' .   ( n xt ) 
 6930: execute ( )      \ execute the xt (i.e., ".")
 6931: @end example
 6932: 
 6933: doc-'
 6934: 
 6935: @code{'} parses at run-time; there is also a word @code{[']} that parses
 6936: when it is compiled, and compiles the resulting XT:
 6937: 
 6938: @example
 6939: : foo ['] . execute ;
 6940: 5 foo
 6941: : bar ' execute ; \ by contrast,
 6942: 5 bar .           \ ' parses "." when bar executes
 6943: @end example
 6944: 
 6945: doc-[']
 6946: 
 6947: If you want the execution token of @i{word}, write @code{['] @i{word}}
 6948: in compiled code and @code{' @i{word}} in interpreted code.  Gforth's
 6949: @code{'} and @code{[']} behave somewhat unusually by complaining about
 6950: compile-only words (because these words have no interpretation
 6951: semantics).  You might get what you want by using @code{COMP' @i{word}
 6952: DROP} or @code{[COMP'] @i{word} DROP} (for details @pxref{Compilation
 6953: token}).
 6954: 
 6955: Another way to get an XT is @code{:noname} or @code{latestxt}
 6956: (@pxref{Anonymous Definitions}).  For anonymous words this gives an xt
 6957: for the only behaviour the word has (the execution semantics).  For
 6958: named words, @code{latestxt} produces an XT for the same behaviour it
 6959: would produce if the word was defined anonymously.
 6960: 
 6961: @example
 6962: :noname ." hello" ;
 6963: execute
 6964: @end example
 6965: 
 6966: An XT occupies one cell and can be manipulated like any other cell.
 6967: 
 6968: @cindex code field address
 6969: @cindex CFA
 6970: In ANS Forth the XT is just an abstract data type (i.e., defined by the
 6971: operations that produce or consume it).  For old hands: In Gforth, the
 6972: XT is implemented as a code field address (CFA).
 6973: 
 6974: doc-execute
 6975: doc-perform
 6976: 
 6977: @node Compilation token, Name token, Execution token, Tokens for Words
 6978: @subsection Compilation token
 6979: 
 6980: @cindex compilation token
 6981: @cindex CT (compilation token)
 6982: Gforth represents the compilation semantics of a named word by a
 6983: @dfn{compilation token} consisting of two cells: @i{w xt}. The top cell
 6984: @i{xt} is an execution token. The compilation semantics represented by
 6985: the compilation token can be performed with @code{execute}, which
 6986: consumes the whole compilation token, with an additional stack effect
 6987: determined by the represented compilation semantics.
 6988: 
 6989: At present, the @i{w} part of a compilation token is an execution token,
 6990: and the @i{xt} part represents either @code{execute} or
 6991: @code{compile,}@footnote{Depending upon the compilation semantics of the
 6992: word. If the word has default compilation semantics, the @i{xt} will
 6993: represent @code{compile,}. Otherwise (e.g., for immediate words), the
 6994: @i{xt} will represent @code{execute}.}. However, don't rely on that
 6995: knowledge, unless necessary; future versions of Gforth may introduce
 6996: unusual compilation tokens (e.g., a compilation token that represents
 6997: the compilation semantics of a literal).
 6998: 
 6999: You can perform the compilation semantics represented by the compilation
 7000: token with @code{execute}.  You can compile the compilation semantics
 7001: with @code{postpone,}. I.e., @code{COMP' @i{word} postpone,} is
 7002: equivalent to @code{postpone @i{word}}.
 7003: 
 7004: doc-[comp']
 7005: doc-comp'
 7006: doc-postpone,
 7007: 
 7008: @node Name token,  , Compilation token, Tokens for Words
 7009: @subsection Name token
 7010: 
 7011: @cindex name token
 7012: Gforth represents named words by the @dfn{name token}, (@i{nt}).  Name
 7013: token is an abstract data type that occurs as argument or result of the
 7014: words below.
 7015: 
 7016: @c !! put this elswhere?
 7017: @cindex name field address
 7018: @cindex NFA
 7019: The closest thing to the nt in older Forth systems is the name field
 7020: address (NFA), but there are significant differences: in older Forth
 7021: systems each word had a unique NFA, LFA, CFA and PFA (in this order, or
 7022: LFA, NFA, CFA, PFA) and there were words for getting from one to the
 7023: next.  In contrast, in Gforth 0@dots{}n nts correspond to one xt; there
 7024: is a link field in the structure identified by the name token, but
 7025: searching usually uses a hash table external to these structures; the
 7026: name in Gforth has a cell-wide count-and-flags field, and the nt is not
 7027: implemented as the address of that count field.
 7028: 
 7029: doc-find-name
 7030: doc-latest
 7031: doc->name
 7032: doc-name>int
 7033: doc-name?int
 7034: doc-name>comp
 7035: doc-name>string
 7036: doc-id.
 7037: doc-.name
 7038: doc-.id
 7039: 
 7040: @c ----------------------------------------------------------
 7041: @node Compiling words, The Text Interpreter, Tokens for Words, Words
 7042: @section Compiling words
 7043: @cindex compiling words
 7044: @cindex macros
 7045: 
 7046: In contrast to most other languages, Forth has no strict boundary
 7047: between compilation and run-time.  E.g., you can run arbitrary code
 7048: between defining words (or for computing data used by defining words
 7049: like @code{constant}). Moreover, @code{Immediate} (@pxref{Interpretation
 7050: and Compilation Semantics} and @code{[}...@code{]} (see below) allow
 7051: running arbitrary code while compiling a colon definition (exception:
 7052: you must not allot dictionary space).
 7053: 
 7054: @menu
 7055: * Literals::                    Compiling data values
 7056: * Macros::                      Compiling words
 7057: @end menu
 7058: 
 7059: @node Literals, Macros, Compiling words, Compiling words
 7060: @subsection Literals
 7061: @cindex Literals
 7062: 
 7063: The simplest and most frequent example is to compute a literal during
 7064: compilation.  E.g., the following definition prints an array of strings,
 7065: one string per line:
 7066: 
 7067: @example
 7068: : .strings ( addr u -- ) \ gforth
 7069:     2* cells bounds U+DO
 7070: 	cr i 2@@ type
 7071:     2 cells +LOOP ;  
 7072: @end example
 7073: 
 7074: With a simple-minded compiler like Gforth's, this computes @code{2
 7075: cells} on every loop iteration.  You can compute this value once and for
 7076: all at compile time and compile it into the definition like this:
 7077: 
 7078: @example
 7079: : .strings ( addr u -- ) \ gforth
 7080:     2* cells bounds U+DO
 7081: 	cr i 2@@ type
 7082:     [ 2 cells ] literal +LOOP ;  
 7083: @end example
 7084: 
 7085: @code{[} switches the text interpreter to interpret state (you will get
 7086: an @code{ok} prompt if you type this example interactively and insert a
 7087: newline between @code{[} and @code{]}), so it performs the
 7088: interpretation semantics of @code{2 cells}; this computes a number.
 7089: @code{]} switches the text interpreter back into compile state.  It then
 7090: performs @code{Literal}'s compilation semantics, which are to compile
 7091: this number into the current word.  You can decompile the word with
 7092: @code{see .strings} to see the effect on the compiled code.
 7093: 
 7094: You can also optimize the @code{2* cells} into @code{[ 2 cells ] literal
 7095: *} in this way.
 7096: 
 7097: doc-[
 7098: doc-]
 7099: doc-literal
 7100: doc-]L
 7101: 
 7102: There are also words for compiling other data types than single cells as
 7103: literals:
 7104: 
 7105: doc-2literal
 7106: doc-fliteral
 7107: doc-sliteral
 7108: 
 7109: @cindex colon-sys, passing data across @code{:}
 7110: @cindex @code{:}, passing data across
 7111: You might be tempted to pass data from outside a colon definition to the
 7112: inside on the data stack.  This does not work, because @code{:} puhes a
 7113: colon-sys, making stuff below unaccessible.  E.g., this does not work:
 7114: 
 7115: @example
 7116: 5 : foo literal ; \ error: "unstructured"
 7117: @end example
 7118: 
 7119: Instead, you have to pass the value in some other way, e.g., through a
 7120: variable:
 7121: 
 7122: @example
 7123: variable temp
 7124: 5 temp !
 7125: : foo [ temp @@ ] literal ;
 7126: @end example
 7127: 
 7128: 
 7129: @node Macros,  , Literals, Compiling words
 7130: @subsection Macros
 7131: @cindex Macros
 7132: @cindex compiling compilation semantics
 7133: 
 7134: @code{Literal} and friends compile data values into the current
 7135: definition.  You can also write words that compile other words into the
 7136: current definition.  E.g.,
 7137: 
 7138: @example
 7139: : compile-+ ( -- ) \ compiled code: ( n1 n2 -- n )
 7140:   POSTPONE + ;
 7141: 
 7142: : foo ( n1 n2 -- n )
 7143:   [ compile-+ ] ;
 7144: 1 2 foo .
 7145: @end example
 7146: 
 7147: This is equivalent to @code{: foo + ;} (@code{see foo} to check this).
 7148: What happens in this example?  @code{Postpone} compiles the compilation
 7149: semantics of @code{+} into @code{compile-+}; later the text interpreter
 7150: executes @code{compile-+} and thus the compilation semantics of +, which
 7151: compile (the execution semantics of) @code{+} into
 7152: @code{foo}.@footnote{A recent RFI answer requires that compiling words
 7153: should only be executed in compile state, so this example is not
 7154: guaranteed to work on all standard systems, but on any decent system it
 7155: will work.}
 7156: 
 7157: doc-postpone
 7158: doc-[compile]
 7159: 
 7160: Compiling words like @code{compile-+} are usually immediate (or similar)
 7161: so you do not have to switch to interpret state to execute them;
 7162: mopifying the last example accordingly produces:
 7163: 
 7164: @example
 7165: : [compile-+] ( compilation: --; interpretation: -- )
 7166:   \ compiled code: ( n1 n2 -- n )
 7167:   POSTPONE + ; immediate
 7168: 
 7169: : foo ( n1 n2 -- n )
 7170:   [compile-+] ;
 7171: 1 2 foo .
 7172: @end example
 7173: 
 7174: Immediate compiling words are similar to macros in other languages (in
 7175: particular, Lisp).  The important differences to macros in, e.g., C are:
 7176: 
 7177: @itemize @bullet
 7178: 
 7179: @item
 7180: You use the same language for defining and processing macros, not a
 7181: separate preprocessing language and processor.
 7182: 
 7183: @item
 7184: Consequently, the full power of Forth is available in macro definitions.
 7185: E.g., you can perform arbitrarily complex computations, or generate
 7186: different code conditionally or in a loop (e.g., @pxref{Advanced macros
 7187: Tutorial}).  This power is very useful when writing a parser generators
 7188: or other code-generating software.
 7189: 
 7190: @item
 7191: Macros defined using @code{postpone} etc. deal with the language at a
 7192: higher level than strings; name binding happens at macro definition
 7193: time, so you can avoid the pitfalls of name collisions that can happen
 7194: in C macros.  Of course, Forth is a liberal language and also allows to
 7195: shoot yourself in the foot with text-interpreted macros like
 7196: 
 7197: @example
 7198: : [compile-+] s" +" evaluate ; immediate
 7199: @end example
 7200: 
 7201: Apart from binding the name at macro use time, using @code{evaluate}
 7202: also makes your definition @code{state}-smart (@pxref{state-smartness}).
 7203: @end itemize
 7204: 
 7205: You may want the macro to compile a number into a word.  The word to do
 7206: it is @code{literal}, but you have to @code{postpone} it, so its
 7207: compilation semantics take effect when the macro is executed, not when
 7208: it is compiled:
 7209: 
 7210: @example
 7211: : [compile-5] ( -- ) \ compiled code: ( -- n )
 7212:   5 POSTPONE literal ; immediate
 7213: 
 7214: : foo [compile-5] ;
 7215: foo .
 7216: @end example
 7217: 
 7218: You may want to pass parameters to a macro, that the macro should
 7219: compile into the current definition.  If the parameter is a number, then
 7220: you can use @code{postpone literal} (similar for other values).
 7221: 
 7222: If you want to pass a word that is to be compiled, the usual way is to
 7223: pass an execution token and @code{compile,} it:
 7224: 
 7225: @example
 7226: : twice1 ( xt -- ) \ compiled code: ... -- ...
 7227:   dup compile, compile, ;
 7228: 
 7229: : 2+ ( n1 -- n2 )
 7230:   [ ' 1+ twice1 ] ;
 7231: @end example
 7232: 
 7233: doc-compile,
 7234: 
 7235: An alternative available in Gforth, that allows you to pass compile-only
 7236: words as parameters is to use the compilation token (@pxref{Compilation
 7237: token}).  The same example in this technique:
 7238: 
 7239: @example
 7240: : twice ( ... ct -- ... ) \ compiled code: ... -- ...
 7241:   2dup 2>r execute 2r> execute ;
 7242: 
 7243: : 2+ ( n1 -- n2 )
 7244:   [ comp' 1+ twice ] ;
 7245: @end example
 7246: 
 7247: In the example above @code{2>r} and @code{2r>} ensure that @code{twice}
 7248: works even if the executed compilation semantics has an effect on the
 7249: data stack.
 7250: 
 7251: You can also define complete definitions with these words; this provides
 7252: an alternative to using @code{does>} (@pxref{User-defined Defining
 7253: Words}).  E.g., instead of
 7254: 
 7255: @example
 7256: : curry+ ( n1 "name" -- )
 7257:     CREATE ,
 7258: DOES> ( n2 -- n1+n2 )
 7259:     @@ + ;
 7260: @end example
 7261: 
 7262: you could define
 7263: 
 7264: @example
 7265: : curry+ ( n1 "name" -- )
 7266:   \ name execution: ( n2 -- n1+n2 )
 7267:   >r : r> POSTPONE literal POSTPONE + POSTPONE ; ;
 7268: 
 7269: -3 curry+ 3-
 7270: see 3-
 7271: @end example
 7272: 
 7273: The sequence @code{>r : r>} is necessary, because @code{:} puts a
 7274: colon-sys on the data stack that makes everything below it unaccessible.
 7275: 
 7276: This way of writing defining words is sometimes more, sometimes less
 7277: convenient than using @code{does>} (@pxref{Advanced does> usage
 7278: example}).  One advantage of this method is that it can be optimized
 7279: better, because the compiler knows that the value compiled with
 7280: @code{literal} is fixed, whereas the data associated with a
 7281: @code{create}d word can be changed.
 7282: 
 7283: @c ----------------------------------------------------------
 7284: @node The Text Interpreter, The Input Stream, Compiling words, Words
 7285: @section  The Text Interpreter
 7286: @cindex interpreter - outer
 7287: @cindex text interpreter
 7288: @cindex outer interpreter
 7289: 
 7290: @c Should we really describe all these ugly details?  IMO the text
 7291: @c interpreter should be much cleaner, but that may not be possible within
 7292: @c ANS Forth. - anton
 7293: @c nac-> I wanted to explain how it works to show how you can exploit
 7294: @c it in your own programs. When I was writing a cross-compiler, figuring out
 7295: @c some of these gory details was very helpful to me. None of the textbooks
 7296: @c I've seen cover it, and the most modern Forth textbook -- Forth Inc's,
 7297: @c seems to positively avoid going into too much detail for some of
 7298: @c the internals.
 7299: 
 7300: @c anton: ok.  I wonder, though, if this is the right place; for some stuff
 7301: @c it is; for the ugly details, I would prefer another place.  I wonder
 7302: @c whether we should have a chapter before "Words" that describes some
 7303: @c basic concepts referred to in words, and a chapter after "Words" that
 7304: @c describes implementation details.
 7305: 
 7306: The text interpreter@footnote{This is an expanded version of the
 7307: material in @ref{Introducing the Text Interpreter}.} is an endless loop
 7308: that processes input from the current input device. It is also called
 7309: the outer interpreter, in contrast to the inner interpreter
 7310: (@pxref{Engine}) which executes the compiled Forth code on interpretive
 7311: implementations.
 7312: 
 7313: @cindex interpret state
 7314: @cindex compile state
 7315: The text interpreter operates in one of two states: @dfn{interpret
 7316: state} and @dfn{compile state}. The current state is defined by the
 7317: aptly-named variable @code{state}.
 7318: 
 7319: This section starts by describing how the text interpreter behaves when
 7320: it is in interpret state, processing input from the user input device --
 7321: the keyboard. This is the mode that a Forth system is in after it starts
 7322: up.
 7323: 
 7324: @cindex input buffer
 7325: @cindex terminal input buffer
 7326: The text interpreter works from an area of memory called the @dfn{input
 7327: buffer}@footnote{When the text interpreter is processing input from the
 7328: keyboard, this area of memory is called the @dfn{terminal input buffer}
 7329: (TIB) and is addressed by the (obsolescent) words @code{TIB} and
 7330: @code{#TIB}.}, which stores your keyboard input when you press the
 7331: @key{RET} key. Starting at the beginning of the input buffer, it skips
 7332: leading spaces (called @dfn{delimiters}) then parses a string (a
 7333: sequence of non-space characters) until it reaches either a space
 7334: character or the end of the buffer. Having parsed a string, it makes two
 7335: attempts to process it:
 7336: 
 7337: @cindex dictionary
 7338: @itemize @bullet
 7339: @item
 7340: It looks for the string in a @dfn{dictionary} of definitions. If the
 7341: string is found, the string names a @dfn{definition} (also known as a
 7342: @dfn{word}) and the dictionary search returns information that allows
 7343: the text interpreter to perform the word's @dfn{interpretation
 7344: semantics}. In most cases, this simply means that the word will be
 7345: executed.
 7346: @item
 7347: If the string is not found in the dictionary, the text interpreter
 7348: attempts to treat it as a number, using the rules described in
 7349: @ref{Number Conversion}. If the string represents a legal number in the
 7350: current radix, the number is pushed onto a parameter stack (the data
 7351: stack for integers, the floating-point stack for floating-point
 7352: numbers).
 7353: @end itemize
 7354: 
 7355: If both attempts fail, or if the word is found in the dictionary but has
 7356: no interpretation semantics@footnote{This happens if the word was
 7357: defined as @code{COMPILE-ONLY}.} the text interpreter discards the
 7358: remainder of the input buffer, issues an error message and waits for
 7359: more input. If one of the attempts succeeds, the text interpreter
 7360: repeats the parsing process until the whole of the input buffer has been
 7361: processed, at which point it prints the status message ``@code{ ok}''
 7362: and waits for more input.
 7363: 
 7364: @c anton: this should be in the input stream subsection (or below it)
 7365: 
 7366: @cindex parse area
 7367: The text interpreter keeps track of its position in the input buffer by
 7368: updating a variable called @code{>IN} (pronounced ``to-in''). The value
 7369: of @code{>IN} starts out as 0, indicating an offset of 0 from the start
 7370: of the input buffer. The region from offset @code{>IN @@} to the end of
 7371: the input buffer is called the @dfn{parse area}@footnote{In other words,
 7372: the text interpreter processes the contents of the input buffer by
 7373: parsing strings from the parse area until the parse area is empty.}.
 7374: This example shows how @code{>IN} changes as the text interpreter parses
 7375: the input buffer:
 7376: 
 7377: @example
 7378: : remaining >IN @@ SOURCE 2 PICK - -ROT + SWAP
 7379:   CR ." ->" TYPE ." <-" ; IMMEDIATE 
 7380: 
 7381: 1 2 3 remaining + remaining . 
 7382: 
 7383: : foo 1 2 3 remaining SWAP remaining ;
 7384: @end example
 7385: 
 7386: @noindent
 7387: The result is:
 7388: 
 7389: @example
 7390: ->+ remaining .<-
 7391: ->.<-5  ok
 7392: 
 7393: ->SWAP remaining ;-<
 7394: ->;<-  ok
 7395: @end example
 7396: 
 7397: @cindex parsing words
 7398: The value of @code{>IN} can also be modified by a word in the input
 7399: buffer that is executed by the text interpreter.  This means that a word
 7400: can ``trick'' the text interpreter into either skipping a section of the
 7401: input buffer@footnote{This is how parsing words work.} or into parsing a
 7402: section twice. For example:
 7403: 
 7404: @example
 7405: : lat ." <<foo>>" ;
 7406: : flat ." <<bar>>" >IN DUP @@ 3 - SWAP ! ;
 7407: @end example
 7408: 
 7409: @noindent
 7410: When @code{flat} is executed, this output is produced@footnote{Exercise
 7411: for the reader: what would happen if the @code{3} were replaced with
 7412: @code{4}?}:
 7413: 
 7414: @example
 7415: <<bar>><<foo>>
 7416: @end example
 7417: 
 7418: This technique can be used to work around some of the interoperability
 7419: problems of parsing words.  Of course, it's better to avoid parsing
 7420: words where possible.
 7421: 
 7422: @noindent
 7423: Two important notes about the behaviour of the text interpreter:
 7424: 
 7425: @itemize @bullet
 7426: @item
 7427: It processes each input string to completion before parsing additional
 7428: characters from the input buffer.
 7429: @item
 7430: It treats the input buffer as a read-only region (and so must your code).
 7431: @end itemize
 7432: 
 7433: @noindent
 7434: When the text interpreter is in compile state, its behaviour changes in
 7435: these ways:
 7436: 
 7437: @itemize @bullet
 7438: @item
 7439: If a parsed string is found in the dictionary, the text interpreter will
 7440: perform the word's @dfn{compilation semantics}. In most cases, this
 7441: simply means that the execution semantics of the word will be appended
 7442: to the current definition.
 7443: @item
 7444: When a number is encountered, it is compiled into the current definition
 7445: (as a literal) rather than being pushed onto a parameter stack.
 7446: @item
 7447: If an error occurs, @code{state} is modified to put the text interpreter
 7448: back into interpret state.
 7449: @item
 7450: Each time a line is entered from the keyboard, Gforth prints
 7451: ``@code{ compiled}'' rather than `` @code{ok}''.
 7452: @end itemize
 7453: 
 7454: @cindex text interpreter - input sources
 7455: When the text interpreter is using an input device other than the
 7456: keyboard, its behaviour changes in these ways:
 7457: 
 7458: @itemize @bullet
 7459: @item
 7460: When the parse area is empty, the text interpreter attempts to refill
 7461: the input buffer from the input source. When the input source is
 7462: exhausted, the input source is set back to the previous input source.
 7463: @item
 7464: It doesn't print out ``@code{ ok}'' or ``@code{ compiled}'' messages each
 7465: time the parse area is emptied.
 7466: @item
 7467: If an error occurs, the input source is set back to the user input
 7468: device.
 7469: @end itemize
 7470: 
 7471: You can read about this in more detail in @ref{Input Sources}.
 7472: 
 7473: doc->in
 7474: doc-source
 7475: 
 7476: doc-tib
 7477: doc-#tib
 7478: 
 7479: 
 7480: @menu
 7481: * Input Sources::               
 7482: * Number Conversion::           
 7483: * Interpret/Compile states::    
 7484: * Interpreter Directives::      
 7485: @end menu
 7486: 
 7487: @node Input Sources, Number Conversion, The Text Interpreter, The Text Interpreter
 7488: @subsection Input Sources
 7489: @cindex input sources
 7490: @cindex text interpreter - input sources
 7491: 
 7492: By default, the text interpreter processes input from the user input
 7493: device (the keyboard) when Forth starts up. The text interpreter can
 7494: process input from any of these sources:
 7495: 
 7496: @itemize @bullet
 7497: @item
 7498: The user input device -- the keyboard.
 7499: @item
 7500: A file, using the words described in @ref{Forth source files}.
 7501: @item
 7502: A block, using the words described in @ref{Blocks}.
 7503: @item
 7504: A text string, using @code{evaluate}.
 7505: @end itemize
 7506: 
 7507: A program can identify the current input device from the values of
 7508: @code{source-id} and @code{blk}.
 7509: 
 7510: 
 7511: doc-source-id
 7512: doc-blk
 7513: 
 7514: doc-save-input
 7515: doc-restore-input
 7516: 
 7517: doc-evaluate
 7518: doc-query
 7519: 
 7520: 
 7521: 
 7522: @node Number Conversion, Interpret/Compile states, Input Sources, The Text Interpreter
 7523: @subsection Number Conversion
 7524: @cindex number conversion
 7525: @cindex double-cell numbers, input format
 7526: @cindex input format for double-cell numbers
 7527: @cindex single-cell numbers, input format
 7528: @cindex input format for single-cell numbers
 7529: @cindex floating-point numbers, input format
 7530: @cindex input format for floating-point numbers
 7531: 
 7532: This section describes the rules that the text interpreter uses when it
 7533: tries to convert a string into a number.
 7534: 
 7535: Let <digit> represent any character that is a legal digit in the current
 7536: number base@footnote{For example, 0-9 when the number base is decimal or
 7537: 0-9, A-F when the number base is hexadecimal.}.
 7538: 
 7539: Let <decimal digit> represent any character in the range 0-9.
 7540: 
 7541: Let @{@i{a b}@} represent the @i{optional} presence of any of the characters
 7542: in the braces (@i{a} or @i{b} or neither).
 7543: 
 7544: Let * represent any number of instances of the previous character
 7545: (including none).
 7546: 
 7547: Let any other character represent itself.
 7548: 
 7549: @noindent
 7550: Now, the conversion rules are:
 7551: 
 7552: @itemize @bullet
 7553: @item
 7554: A string of the form <digit><digit>* is treated as a single-precision
 7555: (cell-sized) positive integer. Examples are 0 123 6784532 32343212343456 42
 7556: @item
 7557: A string of the form -<digit><digit>* is treated as a single-precision
 7558: (cell-sized) negative integer, and is represented using 2's-complement
 7559: arithmetic. Examples are -45 -5681 -0
 7560: @item
 7561: A string of the form <digit><digit>*.<digit>* is treated as a double-precision
 7562: (double-cell-sized) positive integer. Examples are 3465. 3.465 34.65
 7563: (all three of these represent the same number).
 7564: @item
 7565: A string of the form -<digit><digit>*.<digit>* is treated as a
 7566: double-precision (double-cell-sized) negative integer, and is
 7567: represented using 2's-complement arithmetic. Examples are -3465. -3.465
 7568: -34.65 (all three of these represent the same number).
 7569: @item
 7570: A string of the form @{+ -@}<decimal digit>@{.@}<decimal digit>*@{e
 7571: E@}@{+ -@}<decimal digit><decimal digit>* is treated as a floating-point
 7572: number. Examples are 1e 1e0 1.e 1.e0 +1e+0 (which all represent the same
 7573: number) +12.E-4
 7574: @end itemize
 7575: 
 7576: By default, the number base used for integer number conversion is given
 7577: by the contents of the variable @code{base}.  Note that a lot of
 7578: confusion can result from unexpected values of @code{base}.  If you
 7579: change @code{base} anywhere, make sure to save the old value and restore
 7580: it afterwards.  In general I recommend keeping @code{base} decimal, and
 7581: using the prefixes described below for the popular non-decimal bases.
 7582: 
 7583: doc-dpl
 7584: doc-base
 7585: doc-hex
 7586: doc-decimal
 7587: 
 7588: 
 7589: @cindex '-prefix for character strings
 7590: @cindex &-prefix for decimal numbers
 7591: @cindex %-prefix for binary numbers
 7592: @cindex $-prefix for hexadecimal numbers
 7593: Gforth allows you to override the value of @code{base} by using a
 7594: prefix@footnote{Some Forth implementations provide a similar scheme by
 7595: implementing @code{$} etc. as parsing words that process the subsequent
 7596: number in the input stream and push it onto the stack. For example, see
 7597: @cite{Number Conversion and Literals}, by Wil Baden; Forth Dimensions
 7598: 20(3) pages 26--27. In such implementations, unlike in Gforth, a space
 7599: is required between the prefix and the number.} before the first digit
 7600: of an (integer) number. Four prefixes are supported:
 7601: 
 7602: @itemize @bullet
 7603: @item
 7604: @code{&} -- decimal
 7605: @item
 7606: @code{%} -- binary
 7607: @item
 7608: @code{$} -- hexadecimal
 7609: @item
 7610: @code{'} -- base @code{max-char+1}
 7611: @end itemize
 7612: 
 7613: Here are some examples, with the equivalent decimal number shown after
 7614: in braces:
 7615: 
 7616: -$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision number),
 7617: 'AB (16706; ascii A is 65, ascii B is 66, number is 65*256 + 66),
 7618: 'ab (24930; ascii a is 97, ascii B is 98, number is 97*256 + 98),
 7619: &905 (905), $abc (2478), $ABC (2478).
 7620: 
 7621: @cindex number conversion - traps for the unwary
 7622: @noindent
 7623: Number conversion has a number of traps for the unwary:
 7624: 
 7625: @itemize @bullet
 7626: @item
 7627: You cannot determine the current number base using the code sequence
 7628: @code{base @@ .} -- the number base is always 10 in the current number
 7629: base. Instead, use something like @code{base @@ dec.}
 7630: @item
 7631: If the number base is set to a value greater than 14 (for example,
 7632: hexadecimal), the number 123E4 is ambiguous; the conversion rules allow
 7633: it to be intepreted as either a single-precision integer or a
 7634: floating-point number (Gforth treats it as an integer). The ambiguity
 7635: can be resolved by explicitly stating the sign of the mantissa and/or
 7636: exponent: 123E+4 or +123E4 -- if the number base is decimal, no
 7637: ambiguity arises; either representation will be treated as a
 7638: floating-point number.
 7639: @item
 7640: There is a word @code{bin} but it does @i{not} set the number base!
 7641: It is used to specify file types.
 7642: @item
 7643: ANS Forth requires the @code{.} of a double-precision number to be the
 7644: final character in the string.  Gforth allows the @code{.} to be
 7645: anywhere after the first digit.
 7646: @item
 7647: The number conversion process does not check for overflow.
 7648: @item
 7649: In an ANS Forth program @code{base} is required to be decimal when
 7650: converting floating-point numbers.  In Gforth, number conversion to
 7651: floating-point numbers always uses base &10, irrespective of the value
 7652: of @code{base}.
 7653: @end itemize
 7654: 
 7655: You can read numbers into your programs with the words described in
 7656: @ref{Input}.
 7657: 
 7658: @node Interpret/Compile states, Interpreter Directives, Number Conversion, The Text Interpreter
 7659: @subsection Interpret/Compile states
 7660: @cindex Interpret/Compile states
 7661: 
 7662: A standard program is not permitted to change @code{state}
 7663: explicitly. However, it can change @code{state} implicitly, using the
 7664: words @code{[} and @code{]}. When @code{[} is executed it switches
 7665: @code{state} to interpret state, and therefore the text interpreter
 7666: starts interpreting. When @code{]} is executed it switches @code{state}
 7667: to compile state and therefore the text interpreter starts
 7668: compiling. The most common usage for these words is for switching into
 7669: interpret state and back from within a colon definition; this technique
 7670: can be used to compile a literal (for an example, @pxref{Literals}) or
 7671: for conditional compilation (for an example, @pxref{Interpreter
 7672: Directives}).
 7673: 
 7674: 
 7675: @c This is a bad example: It's non-standard, and it's not necessary.
 7676: @c However, I can't think of a good example for switching into compile
 7677: @c state when there is no current word (@code{state}-smart words are not a
 7678: @c good reason).  So maybe we should use an example for switching into
 7679: @c interpret @code{state} in a colon def. - anton
 7680: @c nac-> I agree. I started out by putting in the example, then realised
 7681: @c that it was non-ANS, so wrote more words around it. I hope this
 7682: @c re-written version is acceptable to you. I do want to keep the example
 7683: @c as it is helpful for showing what is and what is not portable, particularly
 7684: @c where it outlaws a style in common use.
 7685: 
 7686: @c anton: it's more important to show what's portable.  After we have done
 7687: @c that, we can also show what's not.  In any case, I have written a
 7688: @c section Compiling Words which also deals with [ ].
 7689: 
 7690: @c  !! The following example does not work in Gforth 0.5.9 or later.
 7691: 
 7692: @c  @code{[} and @code{]} also give you the ability to switch into compile
 7693: @c  state and back, but we cannot think of any useful Standard application
 7694: @c  for this ability. Pre-ANS Forth textbooks have examples like this:
 7695: 
 7696: @c  @example
 7697: @c  : AA ." this is A" ;
 7698: @c  : BB ." this is B" ;
 7699: @c  : CC ." this is C" ;
 7700: 
 7701: @c  create table ] aa bb cc [
 7702: 
 7703: @c  : go ( n -- ) \ n is offset into table.. 0 for 1st entry
 7704: @c    cells table + @@ execute ;
 7705: @c  @end example
 7706: 
 7707: @c  This example builds a jump table; @code{0 go} will display ``@code{this
 7708: @c  is A}''. Using @code{[} and @code{]} in this example is equivalent to
 7709: @c  defining @code{table} like this:
 7710: 
 7711: @c  @example
 7712: @c  create table ' aa COMPILE, ' bb COMPILE, ' cc COMPILE,
 7713: @c  @end example
 7714: 
 7715: @c  The problem with this code is that the definition of @code{table} is not
 7716: @c  portable -- it @i{compile}s execution tokens into code space. Whilst it
 7717: @c  @i{may} work on systems where code space and data space co-incide, the
 7718: @c  Standard only allows data space to be assigned for a @code{CREATE}d
 7719: @c  word. In addition, the Standard only allows @code{@@} to access data
 7720: @c  space, whilst this example is using it to access code space. The only
 7721: @c  portable, Standard way to build this table is to build it in data space,
 7722: @c  like this:
 7723: 
 7724: @c  @example
 7725: @c  create table ' aa , ' bb , ' cc ,
 7726: @c  @end example
 7727: 
 7728: @c  doc-state
 7729: 
 7730: 
 7731: @node Interpreter Directives,  , Interpret/Compile states, The Text Interpreter
 7732: @subsection Interpreter Directives
 7733: @cindex interpreter directives
 7734: @cindex conditional compilation
 7735: 
 7736: These words are usually used in interpret state; typically to control
 7737: which parts of a source file are processed by the text
 7738: interpreter. There are only a few ANS Forth Standard words, but Gforth
 7739: supplements these with a rich set of immediate control structure words
 7740: to compensate for the fact that the non-immediate versions can only be
 7741: used in compile state (@pxref{Control Structures}). Typical usages:
 7742: 
 7743: @example
 7744: FALSE Constant HAVE-ASSEMBLER
 7745: .
 7746: .
 7747: HAVE-ASSEMBLER [IF]
 7748: : ASSEMBLER-FEATURE
 7749:   ...
 7750: ;
 7751: [ENDIF]
 7752: .
 7753: .
 7754: : SEE
 7755:   ... \ general-purpose SEE code
 7756:   [ HAVE-ASSEMBLER [IF] ]
 7757:   ... \ assembler-specific SEE code
 7758:   [ [ENDIF] ]
 7759: ;
 7760: @end example
 7761: 
 7762: 
 7763: doc-[IF]
 7764: doc-[ELSE]
 7765: doc-[THEN]
 7766: doc-[ENDIF]
 7767: 
 7768: doc-[IFDEF]
 7769: doc-[IFUNDEF]
 7770: 
 7771: doc-[?DO]
 7772: doc-[DO]
 7773: doc-[FOR]
 7774: doc-[LOOP]
 7775: doc-[+LOOP]
 7776: doc-[NEXT]
 7777: 
 7778: doc-[BEGIN]
 7779: doc-[UNTIL]
 7780: doc-[AGAIN]
 7781: doc-[WHILE]
 7782: doc-[REPEAT]
 7783: 
 7784: 
 7785: @c -------------------------------------------------------------
 7786: @node The Input Stream, Word Lists, The Text Interpreter, Words
 7787: @section The Input Stream
 7788: @cindex input stream
 7789: 
 7790: @c !! integrate this better with the "Text Interpreter" section
 7791: The text interpreter reads from the input stream, which can come from
 7792: several sources (@pxref{Input Sources}).  Some words, in particular
 7793: defining words, but also words like @code{'}, read parameters from the
 7794: input stream instead of from the stack.
 7795: 
 7796: Such words are called parsing words, because they parse the input
 7797: stream.  Parsing words are hard to use in other words, because it is
 7798: hard to pass program-generated parameters through the input stream.
 7799: They also usually have an unintuitive combination of interpretation and
 7800: compilation semantics when implemented naively, leading to various
 7801: approaches that try to produce a more intuitive behaviour
 7802: (@pxref{Combined words}).
 7803: 
 7804: It should be obvious by now that parsing words are a bad idea.  If you
 7805: want to implement a parsing word for convenience, also provide a factor
 7806: of the word that does not parse, but takes the parameters on the stack.
 7807: To implement the parsing word on top if it, you can use the following
 7808: words:
 7809: 
 7810: @c anton: these belong in the input stream section
 7811: doc-parse
 7812: doc-parse-word
 7813: doc-name
 7814: doc-word
 7815: doc-\"-parse
 7816: doc-refill
 7817: 
 7818: Conversely, if you have the bad luck (or lack of foresight) to have to
 7819: deal with parsing words without having such factors, how do you pass a
 7820: string that is not in the input stream to it?
 7821: 
 7822: doc-execute-parsing
 7823: 
 7824: If you want to run a parsing word on a file, the following word should
 7825: help:
 7826: 
 7827: doc-execute-parsing-file
 7828: 
 7829: @c -------------------------------------------------------------
 7830: @node Word Lists, Environmental Queries, The Input Stream, Words
 7831: @section Word Lists
 7832: @cindex word lists
 7833: @cindex header space
 7834: 
 7835: A wordlist is a list of named words; you can add new words and look up
 7836: words by name (and you can remove words in a restricted way with
 7837: markers).  Every named (and @code{reveal}ed) word is in one wordlist.
 7838: 
 7839: @cindex search order stack
 7840: The text interpreter searches the wordlists present in the search order
 7841: (a stack of wordlists), from the top to the bottom.  Within each
 7842: wordlist, the search starts conceptually at the newest word; i.e., if
 7843: two words in a wordlist have the same name, the newer word is found.
 7844: 
 7845: @cindex compilation word list
 7846: New words are added to the @dfn{compilation wordlist} (aka current
 7847: wordlist).
 7848: 
 7849: @cindex wid
 7850: A word list is identified by a cell-sized word list identifier (@i{wid})
 7851: in much the same way as a file is identified by a file handle. The
 7852: numerical value of the wid has no (portable) meaning, and might change
 7853: from session to session.
 7854: 
 7855: The ANS Forth ``Search order'' word set is intended to provide a set of
 7856: low-level tools that allow various different schemes to be
 7857: implemented. Gforth also provides @code{vocabulary}, a traditional Forth
 7858: word.  @file{compat/vocabulary.fs} provides an implementation in ANS
 7859: Forth.
 7860: 
 7861: @comment TODO: locals section refers to here, saying that every word list (aka
 7862: @comment vocabulary) has its own methods for searching etc. Need to document that.
 7863: @c anton: but better in a separate subsection on wordlist internals
 7864: 
 7865: @comment TODO: document markers, reveal, tables, mappedwordlist
 7866: 
 7867: @comment the gforthman- prefix is used to pick out the true definition of a
 7868: @comment word from the source files, rather than some alias.
 7869: 
 7870: doc-forth-wordlist
 7871: doc-definitions
 7872: doc-get-current
 7873: doc-set-current
 7874: doc-get-order
 7875: doc---gforthman-set-order
 7876: doc-wordlist
 7877: doc-table
 7878: doc->order
 7879: doc-previous
 7880: doc-also
 7881: doc---gforthman-forth
 7882: doc-only
 7883: doc---gforthman-order
 7884: 
 7885: doc-find
 7886: doc-search-wordlist
 7887: 
 7888: doc-words
 7889: doc-vlist
 7890: @c doc-words-deferred
 7891: 
 7892: @c doc-mappedwordlist @c map-structure undefined, implemantation-specific
 7893: doc-root
 7894: doc-vocabulary
 7895: doc-seal
 7896: doc-vocs
 7897: doc-current
 7898: doc-context
 7899: 
 7900: 
 7901: @menu
 7902: * Vocabularies::                
 7903: * Why use word lists?::         
 7904: * Word list example::           
 7905: @end menu
 7906: 
 7907: @node Vocabularies, Why use word lists?, Word Lists, Word Lists
 7908: @subsection Vocabularies
 7909: @cindex Vocabularies, detailed explanation
 7910: 
 7911: Here is an example of creating and using a new wordlist using ANS
 7912: Forth words:
 7913: 
 7914: @example
 7915: wordlist constant my-new-words-wordlist
 7916: : my-new-words get-order nip my-new-words-wordlist swap set-order ;
 7917: 
 7918: \ add it to the search order
 7919: also my-new-words
 7920: 
 7921: \ alternatively, add it to the search order and make it
 7922: \ the compilation word list
 7923: also my-new-words definitions
 7924: \ type "order" to see the problem
 7925: @end example
 7926: 
 7927: The problem with this example is that @code{order} has no way to
 7928: associate the name @code{my-new-words} with the wid of the word list (in
 7929: Gforth, @code{order} and @code{vocs} will display @code{???}  for a wid
 7930: that has no associated name). There is no Standard way of associating a
 7931: name with a wid.
 7932: 
 7933: In Gforth, this example can be re-coded using @code{vocabulary}, which
 7934: associates a name with a wid:
 7935: 
 7936: @example
 7937: vocabulary my-new-words
 7938: 
 7939: \ add it to the search order
 7940: also my-new-words
 7941: 
 7942: \ alternatively, add it to the search order and make it
 7943: \ the compilation word list
 7944: my-new-words definitions
 7945: \ type "order" to see that the problem is solved
 7946: @end example
 7947: 
 7948: 
 7949: @node Why use word lists?, Word list example, Vocabularies, Word Lists
 7950: @subsection Why use word lists?
 7951: @cindex word lists - why use them?
 7952: 
 7953: Here are some reasons why people use wordlists:
 7954: 
 7955: @itemize @bullet
 7956: 
 7957: @c anton: Gforth's hashing implementation makes the search speed
 7958: @c independent from the number of words.  But it is linear with the number
 7959: @c of wordlists that have to be searched, so in effect using more wordlists
 7960: @c actually slows down compilation.
 7961: 
 7962: @c @item
 7963: @c To improve compilation speed by reducing the number of header space
 7964: @c entries that must be searched. This is achieved by creating a new
 7965: @c word list that contains all of the definitions that are used in the
 7966: @c definition of a Forth system but which would not usually be used by
 7967: @c programs running on that system. That word list would be on the search
 7968: @c list when the Forth system was compiled but would be removed from the
 7969: @c search list for normal operation. This can be a useful technique for
 7970: @c low-performance systems (for example, 8-bit processors in embedded
 7971: @c systems) but is unlikely to be necessary in high-performance desktop
 7972: @c systems.
 7973: 
 7974: @item
 7975: To prevent a set of words from being used outside the context in which
 7976: they are valid. Two classic examples of this are an integrated editor
 7977: (all of the edit commands are defined in a separate word list; the
 7978: search order is set to the editor word list when the editor is invoked;
 7979: the old search order is restored when the editor is terminated) and an
 7980: integrated assembler (the op-codes for the machine are defined in a
 7981: separate word list which is used when a @code{CODE} word is defined).
 7982: 
 7983: @item
 7984: To organize the words of an application or library into a user-visible
 7985: set (in @code{forth-wordlist} or some other common wordlist) and a set
 7986: of helper words used just for the implementation (hidden in a separate
 7987: wordlist).  This keeps @code{words}' output smaller, separates
 7988: implementation and interface, and reduces the chance of name conflicts
 7989: within the common wordlist.
 7990: 
 7991: @item
 7992: To prevent a name-space clash between multiple definitions with the same
 7993: name. For example, when building a cross-compiler you might have a word
 7994: @code{IF} that generates conditional code for your target system. By
 7995: placing this definition in a different word list you can control whether
 7996: the host system's @code{IF} or the target system's @code{IF} get used in
 7997: any particular context by controlling the order of the word lists on the
 7998: search order stack.
 7999: 
 8000: @end itemize
 8001: 
 8002: The downsides of using wordlists are:
 8003: 
 8004: @itemize
 8005: 
 8006: @item
 8007: Debugging becomes more cumbersome.
 8008: 
 8009: @item
 8010: Name conflicts worked around with wordlists are still there, and you
 8011: have to arrange the search order carefully to get the desired results;
 8012: if you forget to do that, you get hard-to-find errors (as in any case
 8013: where you read the code differently from the compiler; @code{see} can
 8014: help seeing which of several possible words the name resolves to in such
 8015: cases).  @code{See} displays just the name of the words, not what
 8016: wordlist they belong to, so it might be misleading.  Using unique names
 8017: is a better approach to avoid name conflicts.
 8018: 
 8019: @item
 8020: You have to explicitly undo any changes to the search order.  In many
 8021: cases it would be more convenient if this happened implicitly.  Gforth
 8022: currently does not provide such a feature, but it may do so in the
 8023: future.
 8024: @end itemize
 8025: 
 8026: 
 8027: @node Word list example,  , Why use word lists?, Word Lists
 8028: @subsection Word list example
 8029: @cindex word lists - example
 8030: 
 8031: The following example is from the
 8032: @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
 8033: garbage collector} and uses wordlists to separate public words from
 8034: helper words:
 8035: 
 8036: @example
 8037: get-current ( wid )
 8038: vocabulary garbage-collector also garbage-collector definitions
 8039: ... \ define helper words
 8040: ( wid ) set-current \ restore original (i.e., public) compilation wordlist
 8041: ... \ define the public (i.e., API) words
 8042:     \ they can refer to the helper words
 8043: previous \ restore original search order (helper words become invisible)
 8044: @end example
 8045: 
 8046: @c -------------------------------------------------------------
 8047: @node Environmental Queries, Files, Word Lists, Words
 8048: @section Environmental Queries
 8049: @cindex environmental queries
 8050: 
 8051: ANS Forth introduced the idea of ``environmental queries'' as a way
 8052: for a program running on a system to determine certain characteristics of the system.
 8053: The Standard specifies a number of strings that might be recognised by a system.
 8054: 
 8055: The Standard requires that the header space used for environmental queries
 8056: be distinct from the header space used for definitions.
 8057: 
 8058: Typically, environmental queries are supported by creating a set of
 8059: definitions in a word list that is @i{only} used during environmental
 8060: queries; that is what Gforth does. There is no Standard way of adding
 8061: definitions to the set of recognised environmental queries, but any
 8062: implementation that supports the loading of optional word sets must have
 8063: some mechanism for doing this (after loading the word set, the
 8064: associated environmental query string must return @code{true}). In
 8065: Gforth, the word list used to honour environmental queries can be
 8066: manipulated just like any other word list.
 8067: 
 8068: 
 8069: doc-environment?
 8070: doc-environment-wordlist
 8071: 
 8072: doc-gforth
 8073: doc-os-class
 8074: 
 8075: 
 8076: Note that, whilst the documentation for (e.g.) @code{gforth} shows it
 8077: returning two items on the stack, querying it using @code{environment?}
 8078: will return an additional item; the @code{true} flag that shows that the
 8079: string was recognised.
 8080: 
 8081: @comment TODO Document the standard strings or note where they are documented herein
 8082: 
 8083: Here are some examples of using environmental queries:
 8084: 
 8085: @example
 8086: s" address-unit-bits" environment? 0=
 8087: [IF]
 8088:      cr .( environmental attribute address-units-bits unknown... ) cr
 8089: [ELSE]
 8090:      drop \ ensure balanced stack effect
 8091: [THEN]
 8092: 
 8093: \ this might occur in the prelude of a standard program that uses THROW
 8094: s" exception" environment? [IF]
 8095:    0= [IF]
 8096:       : throw abort" exception thrown" ;
 8097:    [THEN]
 8098: [ELSE] \ we don't know, so make sure
 8099:    : throw abort" exception thrown" ;
 8100: [THEN]
 8101: 
 8102: s" gforth" environment? [IF] .( Gforth version ) TYPE
 8103:                         [ELSE] .( Not Gforth..) [THEN]
 8104: 
 8105: \ a program using v*
 8106: s" gforth" environment? [IF]
 8107:   s" 0.5.0" compare 0< [IF] \ v* is a primitive since 0.5.0
 8108:    : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
 8109:      >r swap 2swap swap 0e r> 0 ?DO
 8110:        dup f@ over + 2swap dup f@ f* f+ over + 2swap
 8111:      LOOP
 8112:      2drop 2drop ; 
 8113:   [THEN]
 8114: [ELSE] \ 
 8115:   : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
 8116:   ...
 8117: [THEN]
 8118: @end example
 8119: 
 8120: Here is an example of adding a definition to the environment word list:
 8121: 
 8122: @example
 8123: get-current environment-wordlist set-current
 8124: true constant block
 8125: true constant block-ext
 8126: set-current
 8127: @end example
 8128: 
 8129: You can see what definitions are in the environment word list like this:
 8130: 
 8131: @example
 8132: environment-wordlist >order words previous
 8133: @end example
 8134: 
 8135: 
 8136: @c -------------------------------------------------------------
 8137: @node Files, Blocks, Environmental Queries, Words
 8138: @section Files
 8139: @cindex files
 8140: @cindex I/O - file-handling
 8141: 
 8142: Gforth provides facilities for accessing files that are stored in the
 8143: host operating system's file-system. Files that are processed by Gforth
 8144: can be divided into two categories:
 8145: 
 8146: @itemize @bullet
 8147: @item
 8148: Files that are processed by the Text Interpreter (@dfn{Forth source files}).
 8149: @item
 8150: Files that are processed by some other program (@dfn{general files}).
 8151: @end itemize
 8152: 
 8153: @menu
 8154: * Forth source files::          
 8155: * General files::               
 8156: * Search Paths::                
 8157: @end menu
 8158: 
 8159: @c -------------------------------------------------------------
 8160: @node Forth source files, General files, Files, Files
 8161: @subsection Forth source files
 8162: @cindex including files
 8163: @cindex Forth source files
 8164: 
 8165: The simplest way to interpret the contents of a file is to use one of
 8166: these two formats:
 8167: 
 8168: @example
 8169: include mysource.fs
 8170: s" mysource.fs" included
 8171: @end example
 8172: 
 8173: You usually want to include a file only if it is not included already
 8174: (by, say, another source file). In that case, you can use one of these
 8175: three formats:
 8176: 
 8177: @example
 8178: require mysource.fs
 8179: needs mysource.fs
 8180: s" mysource.fs" required
 8181: @end example
 8182: 
 8183: @cindex stack effect of included files
 8184: @cindex including files, stack effect
 8185: It is good practice to write your source files such that interpreting them
 8186: does not change the stack. Source files designed in this way can be used with
 8187: @code{required} and friends without complications. For example:
 8188: 
 8189: @example
 8190: 1024 require foo.fs drop
 8191: @end example
 8192: 
 8193: Here you want to pass the argument 1024 (e.g., a buffer size) to
 8194: @file{foo.fs}.  Interpreting @file{foo.fs} has the stack effect ( n -- n
 8195: ), which allows its use with @code{require}.  Of course with such
 8196: parameters to required files, you have to ensure that the first
 8197: @code{require} fits for all uses (i.e., @code{require} it early in the
 8198: master load file).
 8199: 
 8200: doc-include-file
 8201: doc-included
 8202: doc-included?
 8203: doc-include
 8204: doc-required
 8205: doc-require
 8206: doc-needs
 8207: @c doc-init-included-files @c internal
 8208: doc-sourcefilename
 8209: doc-sourceline#
 8210: 
 8211: A definition in ANS Forth for @code{required} is provided in
 8212: @file{compat/required.fs}.
 8213: 
 8214: @c -------------------------------------------------------------
 8215: @node General files, Search Paths, Forth source files, Files
 8216: @subsection General files
 8217: @cindex general files
 8218: @cindex file-handling
 8219: 
 8220: Files are opened/created by name and type. The following file access
 8221: methods (FAMs) are recognised:
 8222: 
 8223: @cindex fam (file access method)
 8224: doc-r/o
 8225: doc-r/w
 8226: doc-w/o
 8227: doc-bin
 8228: 
 8229: 
 8230: When a file is opened/created, it returns a file identifier,
 8231: @i{wfileid} that is used for all other file commands. All file
 8232: commands also return a status value, @i{wior}, that is 0 for a
 8233: successful operation and an implementation-defined non-zero value in the
 8234: case of an error.
 8235: 
 8236: 
 8237: doc-open-file
 8238: doc-create-file
 8239: 
 8240: doc-close-file
 8241: doc-delete-file
 8242: doc-rename-file
 8243: doc-read-file
 8244: doc-read-line
 8245: doc-write-file
 8246: doc-write-line
 8247: doc-emit-file
 8248: doc-flush-file
 8249: 
 8250: doc-file-status
 8251: doc-file-position
 8252: doc-reposition-file
 8253: doc-file-size
 8254: doc-resize-file
 8255: 
 8256: doc-slurp-file
 8257: doc-slurp-fid
 8258: doc-stdin
 8259: doc-stdout
 8260: doc-stderr
 8261: 
 8262: @c ---------------------------------------------------------
 8263: @node Search Paths,  , General files, Files
 8264: @subsection Search Paths
 8265: @cindex path for @code{included}
 8266: @cindex file search path
 8267: @cindex @code{include} search path
 8268: @cindex search path for files
 8269: 
 8270: If you specify an absolute filename (i.e., a filename starting with
 8271: @file{/} or @file{~}, or with @file{:} in the second position (as in
 8272: @samp{C:...})) for @code{included} and friends, that file is included
 8273: just as you would expect.
 8274: 
 8275: If the filename starts with @file{./}, this refers to the directory that
 8276: the present file was @code{included} from.  This allows files to include
 8277: other files relative to their own position (irrespective of the current
 8278: working directory or the absolute position).  This feature is essential
 8279: for libraries consisting of several files, where a file may include
 8280: other files from the library.  It corresponds to @code{#include "..."}
 8281: in C. If the current input source is not a file, @file{.} refers to the
 8282: directory of the innermost file being included, or, if there is no file
 8283: being included, to the current working directory.
 8284: 
 8285: For relative filenames (not starting with @file{./}), Gforth uses a
 8286: search path similar to Forth's search order (@pxref{Word Lists}). It
 8287: tries to find the given filename in the directories present in the path,
 8288: and includes the first one it finds. There are separate search paths for
 8289: Forth source files and general files.  If the search path contains the
 8290: directory @file{.}, this refers to the directory of the current file, or
 8291: the working directory, as if the file had been specified with @file{./}.
 8292: 
 8293: Use @file{~+} to refer to the current working directory (as in the
 8294: @code{bash}).
 8295: 
 8296: @c anton: fold the following subsubsections into this subsection?
 8297: 
 8298: @menu
 8299: * Source Search Paths::         
 8300: * General Search Paths::        
 8301: @end menu
 8302: 
 8303: @c ---------------------------------------------------------
 8304: @node Source Search Paths, General Search Paths, Search Paths, Search Paths
 8305: @subsubsection Source Search Paths
 8306: @cindex search path control, source files
 8307: 
 8308: The search path is initialized when you start Gforth (@pxref{Invoking
 8309: Gforth}). You can display it and change it using @code{fpath} in
 8310: combination with the general path handling words.
 8311: 
 8312: doc-fpath
 8313: @c the functionality of the following words is easily available through
 8314: @c   fpath and the general path words.  The may go away.
 8315: @c doc-.fpath
 8316: @c doc-fpath+
 8317: @c doc-fpath=
 8318: @c doc-open-fpath-file
 8319: 
 8320: @noindent
 8321: Here is an example of using @code{fpath} and @code{require}:
 8322: 
 8323: @example
 8324: fpath path= /usr/lib/forth/|./
 8325: require timer.fs
 8326: @end example
 8327: 
 8328: 
 8329: @c ---------------------------------------------------------
 8330: @node General Search Paths,  , Source Search Paths, Search Paths
 8331: @subsubsection General Search Paths
 8332: @cindex search path control, source files
 8333: 
 8334: Your application may need to search files in several directories, like
 8335: @code{included} does. To facilitate this, Gforth allows you to define
 8336: and use your own search paths, by providing generic equivalents of the
 8337: Forth search path words:
 8338: 
 8339: doc-open-path-file
 8340: doc-path-allot
 8341: doc-clear-path
 8342: doc-also-path
 8343: doc-.path
 8344: doc-path+
 8345: doc-path=
 8346: 
 8347: @c anton: better define a word for it, say "path-allot ( ucount -- path-addr )
 8348: 
 8349: Here's an example of creating an empty search path:
 8350: @c
 8351: @example
 8352: create mypath 500 path-allot \ maximum length 500 chars (is checked)
 8353: @end example
 8354: 
 8355: @c -------------------------------------------------------------
 8356: @node Blocks, Other I/O, Files, Words
 8357: @section Blocks
 8358: @cindex I/O - blocks
 8359: @cindex blocks
 8360: 
 8361: When you run Gforth on a modern desk-top computer, it runs under the
 8362: control of an operating system which provides certain services.  One of
 8363: these services is @var{file services}, which allows Forth source code
 8364: and data to be stored in files and read into Gforth (@pxref{Files}).
 8365: 
 8366: Traditionally, Forth has been an important programming language on
 8367: systems where it has interfaced directly to the underlying hardware with
 8368: no intervening operating system. Forth provides a mechanism, called
 8369: @dfn{blocks}, for accessing mass storage on such systems.
 8370: 
 8371: A block is a 1024-byte data area, which can be used to hold data or
 8372: Forth source code. No structure is imposed on the contents of the
 8373: block. A block is identified by its number; blocks are numbered
 8374: contiguously from 1 to an implementation-defined maximum.
 8375: 
 8376: A typical system that used blocks but no operating system might use a
 8377: single floppy-disk drive for mass storage, with the disks formatted to
 8378: provide 256-byte sectors. Blocks would be implemented by assigning the
 8379: first four sectors of the disk to block 1, the second four sectors to
 8380: block 2 and so on, up to the limit of the capacity of the disk. The disk
 8381: would not contain any file system information, just the set of blocks.
 8382: 
 8383: @cindex blocks file
 8384: On systems that do provide file services, blocks are typically
 8385: implemented by storing a sequence of blocks within a single @dfn{blocks
 8386: file}.  The size of the blocks file will be an exact multiple of 1024
 8387: bytes, corresponding to the number of blocks it contains. This is the
 8388: mechanism that Gforth uses.
 8389: 
 8390: @cindex @file{blocks.fb}
 8391: Only one blocks file can be open at a time. If you use block words without
 8392: having specified a blocks file, Gforth defaults to the blocks file
 8393: @file{blocks.fb}. Gforth uses the Forth search path when attempting to
 8394: locate a blocks file (@pxref{Source Search Paths}).
 8395: 
 8396: @cindex block buffers
 8397: When you read and write blocks under program control, Gforth uses a
 8398: number of @dfn{block buffers} as intermediate storage. These buffers are
 8399: not used when you use @code{load} to interpret the contents of a block.
 8400: 
 8401: The behaviour of the block buffers is analagous to that of a cache.
 8402: Each block buffer has three states:
 8403: 
 8404: @itemize @bullet
 8405: @item
 8406: Unassigned
 8407: @item
 8408: Assigned-clean
 8409: @item
 8410: Assigned-dirty
 8411: @end itemize
 8412: 
 8413: Initially, all block buffers are @i{unassigned}. In order to access a
 8414: block, the block (specified by its block number) must be assigned to a
 8415: block buffer.
 8416: 
 8417: The assignment of a block to a block buffer is performed by @code{block}
 8418: or @code{buffer}. Use @code{block} when you wish to modify the existing
 8419: contents of a block. Use @code{buffer} when you don't care about the
 8420: existing contents of the block@footnote{The ANS Forth definition of
 8421: @code{buffer} is intended not to cause disk I/O; if the data associated
 8422: with the particular block is already stored in a block buffer due to an
 8423: earlier @code{block} command, @code{buffer} will return that block
 8424: buffer and the existing contents of the block will be
 8425: available. Otherwise, @code{buffer} will simply assign a new, empty
 8426: block buffer for the block.}.
 8427: 
 8428: Once a block has been assigned to a block buffer using @code{block} or
 8429: @code{buffer}, that block buffer becomes the @i{current block
 8430: buffer}. Data may only be manipulated (read or written) within the
 8431: current block buffer.
 8432: 
 8433: When the contents of the current block buffer has been modified it is
 8434: necessary, @emph{before calling @code{block} or @code{buffer} again}, to
 8435: either abandon the changes (by doing nothing) or mark the block as
 8436: changed (assigned-dirty), using @code{update}. Using @code{update} does
 8437: not change the blocks file; it simply changes a block buffer's state to
 8438: @i{assigned-dirty}.  The block will be written implicitly when it's
 8439: buffer is needed for another block, or explicitly by @code{flush} or
 8440: @code{save-buffers}.
 8441: 
 8442: word @code{Flush} writes all @i{assigned-dirty} blocks back to the
 8443: blocks file on disk. Leaving Gforth with @code{bye} also performs a
 8444: @code{flush}.
 8445: 
 8446: In Gforth, @code{block} and @code{buffer} use a @i{direct-mapped}
 8447: algorithm to assign a block buffer to a block. That means that any
 8448: particular block can only be assigned to one specific block buffer,
 8449: called (for the particular operation) the @i{victim buffer}. If the
 8450: victim buffer is @i{unassigned} or @i{assigned-clean} it is allocated to
 8451: the new block immediately. If it is @i{assigned-dirty} its current
 8452: contents are written back to the blocks file on disk before it is
 8453: allocated to the new block.
 8454: 
 8455: Although no structure is imposed on the contents of a block, it is
 8456: traditional to display the contents as 16 lines each of 64 characters.  A
 8457: block provides a single, continuous stream of input (for example, it
 8458: acts as a single parse area) -- there are no end-of-line characters
 8459: within a block, and no end-of-file character at the end of a
 8460: block. There are two consequences of this:
 8461: 
 8462: @itemize @bullet
 8463: @item
 8464: The last character of one line wraps straight into the first character
 8465: of the following line
 8466: @item
 8467: The word @code{\} -- comment to end of line -- requires special
 8468: treatment; in the context of a block it causes all characters until the
 8469: end of the current 64-character ``line'' to be ignored.
 8470: @end itemize
 8471: 
 8472: In Gforth, when you use @code{block} with a non-existent block number,
 8473: the current blocks file will be extended to the appropriate size and the
 8474: block buffer will be initialised with spaces.
 8475: 
 8476: Gforth includes a simple block editor (type @code{use blocked.fb 0 list}
 8477: for details) but doesn't encourage the use of blocks; the mechanism is
 8478: only provided for backward compatibility -- ANS Forth requires blocks to
 8479: be available when files are.
 8480: 
 8481: Common techniques that are used when working with blocks include:
 8482: 
 8483: @itemize @bullet
 8484: @item
 8485: A screen editor that allows you to edit blocks without leaving the Forth
 8486: environment.
 8487: @item
 8488: Shadow screens; where every code block has an associated block
 8489: containing comments (for example: code in odd block numbers, comments in
 8490: even block numbers). Typically, the block editor provides a convenient
 8491: mechanism to toggle between code and comments.
 8492: @item
 8493: Load blocks; a single block (typically block 1) contains a number of
 8494: @code{thru} commands which @code{load} the whole of the application.
 8495: @end itemize
 8496: 
 8497: See Frank Sergeant's Pygmy Forth to see just how well blocks can be
 8498: integrated into a Forth programming environment.
 8499: 
 8500: @comment TODO what about errors on open-blocks?
 8501: 
 8502: doc-open-blocks
 8503: doc-use
 8504: doc-block-offset
 8505: doc-get-block-fid
 8506: doc-block-position
 8507: 
 8508: doc-list
 8509: doc-scr
 8510: 
 8511: doc---gforthman-block
 8512: doc-buffer
 8513: 
 8514: doc-empty-buffers
 8515: doc-empty-buffer
 8516: doc-update
 8517: doc-updated?
 8518: doc-save-buffers
 8519: doc-save-buffer
 8520: doc-flush
 8521: 
 8522: doc-load
 8523: doc-thru
 8524: doc-+load
 8525: doc-+thru
 8526: doc---gforthman--->
 8527: doc-block-included
 8528: 
 8529: 
 8530: @c -------------------------------------------------------------
 8531: @node Other I/O, Locals, Blocks, Words
 8532: @section Other I/O
 8533: @cindex I/O - keyboard and display
 8534: 
 8535: @menu
 8536: * Simple numeric output::       Predefined formats
 8537: * Formatted numeric output::    Formatted (pictured) output
 8538: * String Formats::              How Forth stores strings in memory
 8539: * Displaying characters and strings::  Other stuff
 8540: * Input::                       Input
 8541: * Pipes::                       How to create your own pipes
 8542: @end menu
 8543: 
 8544: @node Simple numeric output, Formatted numeric output, Other I/O, Other I/O
 8545: @subsection Simple numeric output
 8546: @cindex numeric output - simple/free-format
 8547: 
 8548: The simplest output functions are those that display numbers from the
 8549: data or floating-point stacks. Floating-point output is always displayed
 8550: using base 10. Numbers displayed from the data stack use the value stored
 8551: in @code{base}.
 8552: 
 8553: 
 8554: doc-.
 8555: doc-dec.
 8556: doc-hex.
 8557: doc-u.
 8558: doc-.r
 8559: doc-u.r
 8560: doc-d.
 8561: doc-ud.
 8562: doc-d.r
 8563: doc-ud.r
 8564: doc-f.
 8565: doc-fe.
 8566: doc-fs.
 8567: doc-f.rdp
 8568: 
 8569: Examples of printing the number 1234.5678E23 in the different floating-point output
 8570: formats are shown below:
 8571: 
 8572: @example
 8573: f. 123456779999999000000000000.
 8574: fe. 123.456779999999E24
 8575: fs. 1.23456779999999E26
 8576: @end example
 8577: 
 8578: 
 8579: @node Formatted numeric output, String Formats, Simple numeric output, Other I/O
 8580: @subsection Formatted numeric output
 8581: @cindex formatted numeric output
 8582: @cindex pictured numeric output
 8583: @cindex numeric output - formatted
 8584: 
 8585: Forth traditionally uses a technique called @dfn{pictured numeric
 8586: output} for formatted printing of integers.  In this technique, digits
 8587: are extracted from the number (using the current output radix defined by
 8588: @code{base}), converted to ASCII codes and appended to a string that is
 8589: built in a scratch-pad area of memory (@pxref{core-idef,
 8590: Implementation-defined options, Implementation-defined
 8591: options}). Arbitrary characters can be appended to the string during the
 8592: extraction process. The completed string is specified by an address
 8593: and length and can be manipulated (@code{TYPE}ed, copied, modified)
 8594: under program control.
 8595: 
 8596: All of the integer output words described in the previous section
 8597: (@pxref{Simple numeric output}) are implemented in Gforth using pictured
 8598: numeric output.
 8599: 
 8600: Three important things to remember about pictured numeric output:
 8601: 
 8602: @itemize @bullet
 8603: @item
 8604: It always operates on double-precision numbers; to display a
 8605: single-precision number, convert it first (for ways of doing this
 8606: @pxref{Double precision}).
 8607: @item
 8608: It always treats the double-precision number as though it were
 8609: unsigned. The examples below show ways of printing signed numbers.
 8610: @item
 8611: The string is built up from right to left; least significant digit first.
 8612: @end itemize
 8613: 
 8614: 
 8615: doc-<#
 8616: doc-<<#
 8617: doc-#
 8618: doc-#s
 8619: doc-hold
 8620: doc-sign
 8621: doc-#>
 8622: doc-#>>
 8623: 
 8624: doc-represent
 8625: doc-f>str-rdp
 8626: doc-f>buf-rdp
 8627: 
 8628: 
 8629: @noindent
 8630: Here are some examples of using pictured numeric output:
 8631: 
 8632: @example
 8633: : my-u. ( u -- )
 8634:   \ Simplest use of pns.. behaves like Standard u. 
 8635:   0              \ convert to unsigned double
 8636:   <<#            \ start conversion
 8637:   #s             \ convert all digits
 8638:   #>             \ complete conversion
 8639:   TYPE SPACE     \ display, with trailing space
 8640:   #>> ;          \ release hold area
 8641: 
 8642: : cents-only ( u -- )
 8643:   0              \ convert to unsigned double
 8644:   <<#            \ start conversion
 8645:   # #            \ convert two least-significant digits
 8646:   #>             \ complete conversion, discard other digits
 8647:   TYPE SPACE     \ display, with trailing space
 8648:   #>> ;          \ release hold area
 8649: 
 8650: : dollars-and-cents ( u -- )
 8651:   0              \ convert to unsigned double
 8652:   <<#            \ start conversion
 8653:   # #            \ convert two least-significant digits
 8654:   [char] . hold  \ insert decimal point
 8655:   #s             \ convert remaining digits
 8656:   [char] $ hold  \ append currency symbol
 8657:   #>             \ complete conversion
 8658:   TYPE SPACE     \ display, with trailing space
 8659:   #>> ;          \ release hold area
 8660: 
 8661: : my-. ( n -- )
 8662:   \ handling negatives.. behaves like Standard .
 8663:   s>d            \ convert to signed double
 8664:   swap over dabs \ leave sign byte followed by unsigned double
 8665:   <<#            \ start conversion
 8666:   #s             \ convert all digits
 8667:   rot sign       \ get at sign byte, append "-" if needed
 8668:   #>             \ complete conversion
 8669:   TYPE SPACE     \ display, with trailing space
 8670:   #>> ;          \ release hold area
 8671: 
 8672: : account. ( n -- )
 8673:   \ accountants don't like minus signs, they use parentheses
 8674:   \ for negative numbers
 8675:   s>d            \ convert to signed double
 8676:   swap over dabs \ leave sign byte followed by unsigned double
 8677:   <<#            \ start conversion
 8678:   2 pick         \ get copy of sign byte
 8679:   0< IF [char] ) hold THEN \ right-most character of output
 8680:   #s             \ convert all digits
 8681:   rot            \ get at sign byte
 8682:   0< IF [char] ( hold THEN
 8683:   #>             \ complete conversion
 8684:   TYPE SPACE     \ display, with trailing space
 8685:   #>> ;          \ release hold area
 8686: 
 8687: @end example
 8688: 
 8689: Here are some examples of using these words:
 8690: 
 8691: @example
 8692: 1 my-u. 1
 8693: hex -1 my-u. decimal FFFFFFFF
 8694: 1 cents-only 01
 8695: 1234 cents-only 34
 8696: 2 dollars-and-cents $0.02
 8697: 1234 dollars-and-cents $12.34
 8698: 123 my-. 123
 8699: -123 my. -123
 8700: 123 account. 123
 8701: -456 account. (456)
 8702: @end example
 8703: 
 8704: 
 8705: @node String Formats, Displaying characters and strings, Formatted numeric output, Other I/O
 8706: @subsection String Formats
 8707: @cindex strings - see character strings
 8708: @cindex character strings - formats
 8709: @cindex I/O - see character strings
 8710: @cindex counted strings
 8711: 
 8712: @c anton: this does not really belong here; maybe the memory section,
 8713: @c  or the principles chapter
 8714: 
 8715: Forth commonly uses two different methods for representing character
 8716: strings:
 8717: 
 8718: @itemize @bullet
 8719: @item
 8720: @cindex address of counted string
 8721: @cindex counted string
 8722: As a @dfn{counted string}, represented by a @i{c-addr}. The char
 8723: addressed by @i{c-addr} contains a character-count, @i{n}, of the
 8724: string and the string occupies the subsequent @i{n} char addresses in
 8725: memory.
 8726: @item
 8727: As cell pair on the stack; @i{c-addr u}, where @i{u} is the length
 8728: of the string in characters, and @i{c-addr} is the address of the
 8729: first byte of the string.
 8730: @end itemize
 8731: 
 8732: ANS Forth encourages the use of the second format when representing
 8733: strings.
 8734: 
 8735: 
 8736: doc-count
 8737: 
 8738: 
 8739: For words that move, copy and search for strings see @ref{Memory
 8740: Blocks}. For words that display characters and strings see
 8741: @ref{Displaying characters and strings}.
 8742: 
 8743: @node Displaying characters and strings, Input, String Formats, Other I/O
 8744: @subsection Displaying characters and strings
 8745: @cindex characters - compiling and displaying
 8746: @cindex character strings - compiling and displaying
 8747: 
 8748: This section starts with a glossary of Forth words and ends with a set
 8749: of examples.
 8750: 
 8751: 
 8752: doc-bl
 8753: doc-space
 8754: doc-spaces
 8755: doc-emit
 8756: doc-toupper
 8757: doc-."
 8758: doc-.(
 8759: doc-.\"
 8760: doc-type
 8761: doc-typewhite
 8762: doc-cr
 8763: @cindex cursor control
 8764: doc-at-xy
 8765: doc-page
 8766: doc-s"
 8767: doc-s\"
 8768: doc-c"
 8769: doc-char
 8770: doc-[char]
 8771: 
 8772: 
 8773: @noindent
 8774: As an example, consider the following text, stored in a file @file{test.fs}:
 8775: 
 8776: @example
 8777: .( text-1)
 8778: : my-word
 8779:   ." text-2" cr
 8780:   .( text-3)
 8781: ;
 8782: 
 8783: ." text-4"
 8784: 
 8785: : my-char
 8786:   [char] ALPHABET emit
 8787:   char emit
 8788: ;
 8789: @end example
 8790: 
 8791: When you load this code into Gforth, the following output is generated:
 8792: 
 8793: @example
 8794: @kbd{include test.fs @key{RET}} text-1text-3text-4 ok
 8795: @end example
 8796: 
 8797: @itemize @bullet
 8798: @item
 8799: Messages @code{text-1} and @code{text-3} are displayed because @code{.(} 
 8800: is an immediate word; it behaves in the same way whether it is used inside
 8801: or outside a colon definition.
 8802: @item
 8803: Message @code{text-4} is displayed because of Gforth's added interpretation
 8804: semantics for @code{."}.
 8805: @item
 8806: Message @code{text-2} is @i{not} displayed, because the text interpreter
 8807: performs the compilation semantics for @code{."} within the definition of
 8808: @code{my-word}.
 8809: @end itemize
 8810: 
 8811: Here are some examples of executing @code{my-word} and @code{my-char}:
 8812: 
 8813: @example
 8814: @kbd{my-word @key{RET}} text-2
 8815:  ok
 8816: @kbd{my-char fred @key{RET}} Af ok
 8817: @kbd{my-char jim @key{RET}} Aj ok
 8818: @end example
 8819: 
 8820: @itemize @bullet
 8821: @item
 8822: Message @code{text-2} is displayed because of the run-time behaviour of
 8823: @code{."}.
 8824: @item
 8825: @code{[char]} compiles the ``A'' from ``ALPHABET'' and puts its display code
 8826: on the stack at run-time. @code{emit} always displays the character
 8827: when @code{my-char} is executed.
 8828: @item
 8829: @code{char} parses a string at run-time and the second @code{emit} displays
 8830: the first character of the string.
 8831: @item
 8832: If you type @code{see my-char} you can see that @code{[char]} discarded
 8833: the text ``LPHABET'' and only compiled the display code for ``A'' into the
 8834: definition of @code{my-char}.
 8835: @end itemize
 8836: 
 8837: 
 8838: 
 8839: @node Input, Pipes, Displaying characters and strings, Other I/O
 8840: @subsection Input
 8841: @cindex input
 8842: @cindex I/O - see input
 8843: @cindex parsing a string
 8844: 
 8845: For ways of storing character strings in memory see @ref{String Formats}.
 8846: 
 8847: @comment TODO examples for >number >float accept key key? pad parse word refill
 8848: @comment then index them
 8849: 
 8850: 
 8851: doc-key
 8852: doc-key?
 8853: doc-ekey
 8854: doc-ekey?
 8855: doc-ekey>char
 8856: doc->number
 8857: doc->float
 8858: doc-accept
 8859: doc-edit-line
 8860: doc-pad
 8861: @comment obsolescent words..
 8862: doc-convert
 8863: doc-expect
 8864: doc-span
 8865: 
 8866: 
 8867: @node Pipes,  , Input, Other I/O
 8868: @subsection Pipes
 8869: @cindex pipes, creating your own
 8870: 
 8871: In addition to using Gforth in pipes created by other processes
 8872: (@pxref{Gforth in pipes}), you can create your own pipe with
 8873: @code{open-pipe}, and read from or write to it.
 8874: 
 8875: doc-open-pipe
 8876: doc-close-pipe
 8877: 
 8878: If you write to a pipe, Gforth can throw a @code{broken-pipe-error}; if
 8879: you don't catch this exception, Gforth will catch it and exit, usually
 8880: silently (@pxref{Gforth in pipes}).  Since you probably do not want
 8881: this, you should wrap a @code{catch} or @code{try} block around the code
 8882: from @code{open-pipe} to @code{close-pipe}, so you can deal with the
 8883: problem yourself, and then return to regular processing.
 8884: 
 8885: doc-broken-pipe-error
 8886: 
 8887: 
 8888: @node OS command line arguments, Locals, Other I/O, Words
 8889: @section OS command line arguments
 8890: @cindex OS command line arguments
 8891: @cindex command line arguments, OS
 8892: @cindex arguments, OS command line
 8893: 
 8894: The usual way to pass arguments to Gforth programs on the command line
 8895: is via the @option{-e} option, e.g.
 8896: 
 8897: @example
 8898: gforth -e "123 456" foo.fs -e bye
 8899: @end example
 8900: 
 8901: However, you may want to interpret the command-line arguments directly.
 8902: In that case, you can access the (image-specific) command-line arguments
 8903: through @code{next-arg}:
 8904: 
 8905: doc-next-arg
 8906: 
 8907: Here's an example program @file{echo.fs} for @code{next-arg}:
 8908: 
 8909: @example
 8910: : echo ( -- )
 8911:     begin
 8912: 	next-arg 2dup 0 0 d<> while
 8913: 	    type space
 8914:     repeat
 8915:     2drop ;
 8916: 
 8917: echo cr bye
 8918: @end example
 8919: 
 8920: This can be invoked with
 8921: 
 8922: @example
 8923: gforth echo.fs hello world
 8924: @end example
 8925: 
 8926: and it will print
 8927: 
 8928: @example
 8929: hello world
 8930: @end example
 8931: 
 8932: The next lower level of dealing with the OS command line are the
 8933: following words:
 8934: 
 8935: doc-arg
 8936: doc-shift-args
 8937: 
 8938: Finally, at the lowest level Gforth provides the following words:
 8939: 
 8940: doc-argc
 8941: doc-argv
 8942: 
 8943: @c -------------------------------------------------------------
 8944: @node Locals, Structures, Other I/O, Words
 8945: @section Locals
 8946: @cindex locals
 8947: 
 8948: Local variables can make Forth programming more enjoyable and Forth
 8949: programs easier to read. Unfortunately, the locals of ANS Forth are
 8950: laden with restrictions. Therefore, we provide not only the ANS Forth
 8951: locals wordset, but also our own, more powerful locals wordset (we
 8952: implemented the ANS Forth locals wordset through our locals wordset).
 8953: 
 8954: The ideas in this section have also been published in M. Anton Ertl,
 8955: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz,
 8956: Automatic Scoping of Local Variables}}, EuroForth '94.
 8957: 
 8958: @menu
 8959: * Gforth locals::               
 8960: * ANS Forth locals::            
 8961: @end menu
 8962: 
 8963: @node Gforth locals, ANS Forth locals, Locals, Locals
 8964: @subsection Gforth locals
 8965: @cindex Gforth locals
 8966: @cindex locals, Gforth style
 8967: 
 8968: Locals can be defined with
 8969: 
 8970: @example
 8971: @{ local1 local2 ... -- comment @}
 8972: @end example
 8973: or
 8974: @example
 8975: @{ local1 local2 ... @}
 8976: @end example
 8977: 
 8978: E.g.,
 8979: @example
 8980: : max @{ n1 n2 -- n3 @}
 8981:  n1 n2 > if
 8982:    n1
 8983:  else
 8984:    n2
 8985:  endif ;
 8986: @end example
 8987: 
 8988: The similarity of locals definitions with stack comments is intended. A
 8989: locals definition often replaces the stack comment of a word. The order
 8990: of the locals corresponds to the order in a stack comment and everything
 8991: after the @code{--} is really a comment.
 8992: 
 8993: This similarity has one disadvantage: It is too easy to confuse locals
 8994: declarations with stack comments, causing bugs and making them hard to
 8995: find. However, this problem can be avoided by appropriate coding
 8996: conventions: Do not use both notations in the same program. If you do,
 8997: they should be distinguished using additional means, e.g. by position.
 8998: 
 8999: @cindex types of locals
 9000: @cindex locals types
 9001: The name of the local may be preceded by a type specifier, e.g.,
 9002: @code{F:} for a floating point value:
 9003: 
 9004: @example
 9005: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
 9006: \ complex multiplication
 9007:  Ar Br f* Ai Bi f* f-
 9008:  Ar Bi f* Ai Br f* f+ ;
 9009: @end example
 9010: 
 9011: @cindex flavours of locals
 9012: @cindex locals flavours
 9013: @cindex value-flavoured locals
 9014: @cindex variable-flavoured locals
 9015: Gforth currently supports cells (@code{W:}, @code{W^}), doubles
 9016: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
 9017: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
 9018: with @code{W:}, @code{D:} etc.) produces its value and can be changed
 9019: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
 9020: produces its address (which becomes invalid when the variable's scope is
 9021: left). E.g., the standard word @code{emit} can be defined in terms of
 9022: @code{type} like this:
 9023: 
 9024: @example
 9025: : emit @{ C^ char* -- @}
 9026:     char* 1 type ;
 9027: @end example
 9028: 
 9029: @cindex default type of locals
 9030: @cindex locals, default type
 9031: A local without type specifier is a @code{W:} local. Both flavours of
 9032: locals are initialized with values from the data or FP stack.
 9033: 
 9034: Currently there is no way to define locals with user-defined data
 9035: structures, but we are working on it.
 9036: 
 9037: Gforth allows defining locals everywhere in a colon definition. This
 9038: poses the following questions:
 9039: 
 9040: @menu
 9041: * Where are locals visible by name?::  
 9042: * How long do locals live?::    
 9043: * Locals programming style::    
 9044: * Locals implementation::       
 9045: @end menu
 9046: 
 9047: @node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
 9048: @subsubsection Where are locals visible by name?
 9049: @cindex locals visibility
 9050: @cindex visibility of locals
 9051: @cindex scope of locals
 9052: 
 9053: Basically, the answer is that locals are visible where you would expect
 9054: it in block-structured languages, and sometimes a little longer. If you
 9055: want to restrict the scope of a local, enclose its definition in
 9056: @code{SCOPE}...@code{ENDSCOPE}.
 9057: 
 9058: 
 9059: doc-scope
 9060: doc-endscope
 9061: 
 9062: 
 9063: These words behave like control structure words, so you can use them
 9064: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
 9065: arbitrary ways.
 9066: 
 9067: If you want a more exact answer to the visibility question, here's the
 9068: basic principle: A local is visible in all places that can only be
 9069: reached through the definition of the local@footnote{In compiler
 9070: construction terminology, all places dominated by the definition of the
 9071: local.}. In other words, it is not visible in places that can be reached
 9072: without going through the definition of the local. E.g., locals defined
 9073: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
 9074: defined in @code{BEGIN}...@code{UNTIL} are visible after the
 9075: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
 9076: 
 9077: The reasoning behind this solution is: We want to have the locals
 9078: visible as long as it is meaningful. The user can always make the
 9079: visibility shorter by using explicit scoping. In a place that can
 9080: only be reached through the definition of a local, the meaning of a
 9081: local name is clear. In other places it is not: How is the local
 9082: initialized at the control flow path that does not contain the
 9083: definition? Which local is meant, if the same name is defined twice in
 9084: two independent control flow paths?
 9085: 
 9086: This should be enough detail for nearly all users, so you can skip the
 9087: rest of this section. If you really must know all the gory details and
 9088: options, read on.
 9089: 
 9090: In order to implement this rule, the compiler has to know which places
 9091: are unreachable. It knows this automatically after @code{AHEAD},
 9092: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
 9093: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
 9094: compiler that the control flow never reaches that place. If
 9095: @code{UNREACHABLE} is not used where it could, the only consequence is
 9096: that the visibility of some locals is more limited than the rule above
 9097: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
 9098: lie to the compiler), buggy code will be produced.
 9099: 
 9100: 
 9101: doc-unreachable
 9102: 
 9103: 
 9104: Another problem with this rule is that at @code{BEGIN}, the compiler
 9105: does not know which locals will be visible on the incoming
 9106: back-edge. All problems discussed in the following are due to this
 9107: ignorance of the compiler (we discuss the problems using @code{BEGIN}
 9108: loops as examples; the discussion also applies to @code{?DO} and other
 9109: loops). Perhaps the most insidious example is:
 9110: @example
 9111: AHEAD
 9112: BEGIN
 9113:   x
 9114: [ 1 CS-ROLL ] THEN
 9115:   @{ x @}
 9116:   ...
 9117: UNTIL
 9118: @end example
 9119: 
 9120: This should be legal according to the visibility rule. The use of
 9121: @code{x} can only be reached through the definition; but that appears
 9122: textually below the use.
 9123: 
 9124: From this example it is clear that the visibility rules cannot be fully
 9125: implemented without major headaches. Our implementation treats common
 9126: cases as advertised and the exceptions are treated in a safe way: The
 9127: compiler makes a reasonable guess about the locals visible after a
 9128: @code{BEGIN}; if it is too pessimistic, the
 9129: user will get a spurious error about the local not being defined; if the
 9130: compiler is too optimistic, it will notice this later and issue a
 9131: warning. In the case above the compiler would complain about @code{x}
 9132: being undefined at its use. You can see from the obscure examples in
 9133: this section that it takes quite unusual control structures to get the
 9134: compiler into trouble, and even then it will often do fine.
 9135: 
 9136: If the @code{BEGIN} is reachable from above, the most optimistic guess
 9137: is that all locals visible before the @code{BEGIN} will also be
 9138: visible after the @code{BEGIN}. This guess is valid for all loops that
 9139: are entered only through the @code{BEGIN}, in particular, for normal
 9140: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
 9141: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
 9142: compiler. When the branch to the @code{BEGIN} is finally generated by
 9143: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
 9144: warns the user if it was too optimistic:
 9145: @example
 9146: IF
 9147:   @{ x @}
 9148: BEGIN
 9149:   \ x ? 
 9150: [ 1 cs-roll ] THEN
 9151:   ...
 9152: UNTIL
 9153: @end example
 9154: 
 9155: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
 9156: optimistically assumes that it lives until the @code{THEN}. It notices
 9157: this difference when it compiles the @code{UNTIL} and issues a
 9158: warning. The user can avoid the warning, and make sure that @code{x}
 9159: is not used in the wrong area by using explicit scoping:
 9160: @example
 9161: IF
 9162:   SCOPE
 9163:   @{ x @}
 9164:   ENDSCOPE
 9165: BEGIN
 9166: [ 1 cs-roll ] THEN
 9167:   ...
 9168: UNTIL
 9169: @end example
 9170: 
 9171: Since the guess is optimistic, there will be no spurious error messages
 9172: about undefined locals.
 9173: 
 9174: If the @code{BEGIN} is not reachable from above (e.g., after
 9175: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
 9176: optimistic guess, as the locals visible after the @code{BEGIN} may be
 9177: defined later. Therefore, the compiler assumes that no locals are
 9178: visible after the @code{BEGIN}. However, the user can use
 9179: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
 9180: visible at the BEGIN as at the point where the top control-flow stack
 9181: item was created.
 9182: 
 9183: 
 9184: doc-assume-live
 9185: 
 9186: 
 9187: @noindent
 9188: E.g.,
 9189: @example
 9190: @{ x @}
 9191: AHEAD
 9192: ASSUME-LIVE
 9193: BEGIN
 9194:   x
 9195: [ 1 CS-ROLL ] THEN
 9196:   ...
 9197: UNTIL
 9198: @end example
 9199: 
 9200: Other cases where the locals are defined before the @code{BEGIN} can be
 9201: handled by inserting an appropriate @code{CS-ROLL} before the
 9202: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
 9203: behind the @code{ASSUME-LIVE}).
 9204: 
 9205: Cases where locals are defined after the @code{BEGIN} (but should be
 9206: visible immediately after the @code{BEGIN}) can only be handled by
 9207: rearranging the loop. E.g., the ``most insidious'' example above can be
 9208: arranged into:
 9209: @example
 9210: BEGIN
 9211:   @{ x @}
 9212:   ... 0=
 9213: WHILE
 9214:   x
 9215: REPEAT
 9216: @end example
 9217: 
 9218: @node How long do locals live?, Locals programming style, Where are locals visible by name?, Gforth locals
 9219: @subsubsection How long do locals live?
 9220: @cindex locals lifetime
 9221: @cindex lifetime of locals
 9222: 
 9223: The right answer for the lifetime question would be: A local lives at
 9224: least as long as it can be accessed. For a value-flavoured local this
 9225: means: until the end of its visibility. However, a variable-flavoured
 9226: local could be accessed through its address far beyond its visibility
 9227: scope. Ultimately, this would mean that such locals would have to be
 9228: garbage collected. Since this entails un-Forth-like implementation
 9229: complexities, I adopted the same cowardly solution as some other
 9230: languages (e.g., C): The local lives only as long as it is visible;
 9231: afterwards its address is invalid (and programs that access it
 9232: afterwards are erroneous).
 9233: 
 9234: @node Locals programming style, Locals implementation, How long do locals live?, Gforth locals
 9235: @subsubsection Locals programming style
 9236: @cindex locals programming style
 9237: @cindex programming style, locals
 9238: 
 9239: The freedom to define locals anywhere has the potential to change
 9240: programming styles dramatically. In particular, the need to use the
 9241: return stack for intermediate storage vanishes. Moreover, all stack
 9242: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
 9243: determined arguments) can be eliminated: If the stack items are in the
 9244: wrong order, just write a locals definition for all of them; then
 9245: write the items in the order you want.
 9246: 
 9247: This seems a little far-fetched and eliminating stack manipulations is
 9248: unlikely to become a conscious programming objective. Still, the number
 9249: of stack manipulations will be reduced dramatically if local variables
 9250: are used liberally (e.g., compare @code{max} (@pxref{Gforth locals}) with
 9251: a traditional implementation of @code{max}).
 9252: 
 9253: This shows one potential benefit of locals: making Forth programs more
 9254: readable. Of course, this benefit will only be realized if the
 9255: programmers continue to honour the principle of factoring instead of
 9256: using the added latitude to make the words longer.
 9257: 
 9258: @cindex single-assignment style for locals
 9259: Using @code{TO} can and should be avoided.  Without @code{TO},
 9260: every value-flavoured local has only a single assignment and many
 9261: advantages of functional languages apply to Forth. I.e., programs are
 9262: easier to analyse, to optimize and to read: It is clear from the
 9263: definition what the local stands for, it does not turn into something
 9264: different later.
 9265: 
 9266: E.g., a definition using @code{TO} might look like this:
 9267: @example
 9268: : strcmp @{ addr1 u1 addr2 u2 -- n @}
 9269:  u1 u2 min 0
 9270:  ?do
 9271:    addr1 c@@ addr2 c@@ -
 9272:    ?dup-if
 9273:      unloop exit
 9274:    then
 9275:    addr1 char+ TO addr1
 9276:    addr2 char+ TO addr2
 9277:  loop
 9278:  u1 u2 - ;
 9279: @end example
 9280: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
 9281: every loop iteration. @code{strcmp} is a typical example of the
 9282: readability problems of using @code{TO}. When you start reading
 9283: @code{strcmp}, you think that @code{addr1} refers to the start of the
 9284: string. Only near the end of the loop you realize that it is something
 9285: else.
 9286: 
 9287: This can be avoided by defining two locals at the start of the loop that
 9288: are initialized with the right value for the current iteration.
 9289: @example
 9290: : strcmp @{ addr1 u1 addr2 u2 -- n @}
 9291:  addr1 addr2
 9292:  u1 u2 min 0 
 9293:  ?do @{ s1 s2 @}
 9294:    s1 c@@ s2 c@@ -
 9295:    ?dup-if
 9296:      unloop exit
 9297:    then
 9298:    s1 char+ s2 char+
 9299:  loop
 9300:  2drop
 9301:  u1 u2 - ;
 9302: @end example
 9303: Here it is clear from the start that @code{s1} has a different value
 9304: in every loop iteration.
 9305: 
 9306: @node Locals implementation,  , Locals programming style, Gforth locals
 9307: @subsubsection Locals implementation
 9308: @cindex locals implementation
 9309: @cindex implementation of locals
 9310: 
 9311: @cindex locals stack
 9312: Gforth uses an extra locals stack. The most compelling reason for
 9313: this is that the return stack is not float-aligned; using an extra stack
 9314: also eliminates the problems and restrictions of using the return stack
 9315: as locals stack. Like the other stacks, the locals stack grows toward
 9316: lower addresses. A few primitives allow an efficient implementation:
 9317: 
 9318: 
 9319: doc-@local#
 9320: doc-f@local#
 9321: doc-laddr#
 9322: doc-lp+!#
 9323: doc-lp!
 9324: doc->l
 9325: doc-f>l
 9326: 
 9327: 
 9328: In addition to these primitives, some specializations of these
 9329: primitives for commonly occurring inline arguments are provided for
 9330: efficiency reasons, e.g., @code{@@local0} as specialization of
 9331: @code{@@local#} for the inline argument 0. The following compiling words
 9332: compile the right specialized version, or the general version, as
 9333: appropriate:
 9334: 
 9335: 
 9336: @c doc-compile-@local
 9337: @c doc-compile-f@local
 9338: doc-compile-lp+!
 9339: 
 9340: 
 9341: Combinations of conditional branches and @code{lp+!#} like
 9342: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
 9343: is taken) are provided for efficiency and correctness in loops.
 9344: 
 9345: A special area in the dictionary space is reserved for keeping the
 9346: local variable names. @code{@{} switches the dictionary pointer to this
 9347: area and @code{@}} switches it back and generates the locals
 9348: initializing code. @code{W:} etc.@ are normal defining words. This
 9349: special area is cleared at the start of every colon definition.
 9350: 
 9351: @cindex word list for defining locals
 9352: A special feature of Gforth's dictionary is used to implement the
 9353: definition of locals without type specifiers: every word list (aka
 9354: vocabulary) has its own methods for searching
 9355: etc. (@pxref{Word Lists}). For the present purpose we defined a word list
 9356: with a special search method: When it is searched for a word, it
 9357: actually creates that word using @code{W:}. @code{@{} changes the search
 9358: order to first search the word list containing @code{@}}, @code{W:} etc.,
 9359: and then the word list for defining locals without type specifiers.
 9360: 
 9361: The lifetime rules support a stack discipline within a colon
 9362: definition: The lifetime of a local is either nested with other locals
 9363: lifetimes or it does not overlap them.
 9364: 
 9365: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
 9366: pointer manipulation is generated. Between control structure words
 9367: locals definitions can push locals onto the locals stack. @code{AGAIN}
 9368: is the simplest of the other three control flow words. It has to
 9369: restore the locals stack depth of the corresponding @code{BEGIN}
 9370: before branching. The code looks like this:
 9371: @format
 9372: @code{lp+!#} current-locals-size @minus{} dest-locals-size
 9373: @code{branch} <begin>
 9374: @end format
 9375: 
 9376: @code{UNTIL} is a little more complicated: If it branches back, it
 9377: must adjust the stack just like @code{AGAIN}. But if it falls through,
 9378: the locals stack must not be changed. The compiler generates the
 9379: following code:
 9380: @format
 9381: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
 9382: @end format
 9383: The locals stack pointer is only adjusted if the branch is taken.
 9384: 
 9385: @code{THEN} can produce somewhat inefficient code:
 9386: @format
 9387: @code{lp+!#} current-locals-size @minus{} orig-locals-size
 9388: <orig target>:
 9389: @code{lp+!#} orig-locals-size @minus{} new-locals-size
 9390: @end format
 9391: The second @code{lp+!#} adjusts the locals stack pointer from the
 9392: level at the @i{orig} point to the level after the @code{THEN}. The
 9393: first @code{lp+!#} adjusts the locals stack pointer from the current
 9394: level to the level at the orig point, so the complete effect is an
 9395: adjustment from the current level to the right level after the
 9396: @code{THEN}.
 9397: 
 9398: @cindex locals information on the control-flow stack
 9399: @cindex control-flow stack items, locals information
 9400: In a conventional Forth implementation a dest control-flow stack entry
 9401: is just the target address and an orig entry is just the address to be
 9402: patched. Our locals implementation adds a word list to every orig or dest
 9403: item. It is the list of locals visible (or assumed visible) at the point
 9404: described by the entry. Our implementation also adds a tag to identify
 9405: the kind of entry, in particular to differentiate between live and dead
 9406: (reachable and unreachable) orig entries.
 9407: 
 9408: A few unusual operations have to be performed on locals word lists:
 9409: 
 9410: 
 9411: doc-common-list
 9412: doc-sub-list?
 9413: doc-list-size
 9414: 
 9415: 
 9416: Several features of our locals word list implementation make these
 9417: operations easy to implement: The locals word lists are organised as
 9418: linked lists; the tails of these lists are shared, if the lists
 9419: contain some of the same locals; and the address of a name is greater
 9420: than the address of the names behind it in the list.
 9421: 
 9422: Another important implementation detail is the variable
 9423: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
 9424: determine if they can be reached directly or only through the branch
 9425: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
 9426: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
 9427: definition, by @code{BEGIN} and usually by @code{THEN}.
 9428: 
 9429: Counted loops are similar to other loops in most respects, but
 9430: @code{LEAVE} requires special attention: It performs basically the same
 9431: service as @code{AHEAD}, but it does not create a control-flow stack
 9432: entry. Therefore the information has to be stored elsewhere;
 9433: traditionally, the information was stored in the target fields of the
 9434: branches created by the @code{LEAVE}s, by organizing these fields into a
 9435: linked list. Unfortunately, this clever trick does not provide enough
 9436: space for storing our extended control flow information. Therefore, we
 9437: introduce another stack, the leave stack. It contains the control-flow
 9438: stack entries for all unresolved @code{LEAVE}s.
 9439: 
 9440: Local names are kept until the end of the colon definition, even if
 9441: they are no longer visible in any control-flow path. In a few cases
 9442: this may lead to increased space needs for the locals name area, but
 9443: usually less than reclaiming this space would cost in code size.
 9444: 
 9445: 
 9446: @node ANS Forth locals,  , Gforth locals, Locals
 9447: @subsection ANS Forth locals
 9448: @cindex locals, ANS Forth style
 9449: 
 9450: The ANS Forth locals wordset does not define a syntax for locals, but
 9451: words that make it possible to define various syntaxes. One of the
 9452: possible syntaxes is a subset of the syntax we used in the Gforth locals
 9453: wordset, i.e.:
 9454: 
 9455: @example
 9456: @{ local1 local2 ... -- comment @}
 9457: @end example
 9458: @noindent
 9459: or
 9460: @example
 9461: @{ local1 local2 ... @}
 9462: @end example
 9463: 
 9464: The order of the locals corresponds to the order in a stack comment. The
 9465: restrictions are:
 9466: 
 9467: @itemize @bullet
 9468: @item
 9469: Locals can only be cell-sized values (no type specifiers are allowed).
 9470: @item
 9471: Locals can be defined only outside control structures.
 9472: @item
 9473: Locals can interfere with explicit usage of the return stack. For the
 9474: exact (and long) rules, see the standard. If you don't use return stack
 9475: accessing words in a definition using locals, you will be all right. The
 9476: purpose of this rule is to make locals implementation on the return
 9477: stack easier.
 9478: @item
 9479: The whole definition must be in one line.
 9480: @end itemize
 9481: 
 9482: Locals defined in ANS Forth behave like @code{VALUE}s
 9483: (@pxref{Values}). I.e., they are initialized from the stack. Using their
 9484: name produces their value. Their value can be changed using @code{TO}.
 9485: 
 9486: Since the syntax above is supported by Gforth directly, you need not do
 9487: anything to use it. If you want to port a program using this syntax to
 9488: another ANS Forth system, use @file{compat/anslocal.fs} to implement the
 9489: syntax on the other system.
 9490: 
 9491: Note that a syntax shown in the standard, section A.13 looks
 9492: similar, but is quite different in having the order of locals
 9493: reversed. Beware!
 9494: 
 9495: The ANS Forth locals wordset itself consists of one word:
 9496: 
 9497: doc-(local)
 9498: 
 9499: The ANS Forth locals extension wordset defines a syntax using
 9500: @code{locals|}, but it is so awful that we strongly recommend not to use
 9501: it. We have implemented this syntax to make porting to Gforth easy, but
 9502: do not document it here. The problem with this syntax is that the locals
 9503: are defined in an order reversed with respect to the standard stack
 9504: comment notation, making programs harder to read, and easier to misread
 9505: and miswrite. The only merit of this syntax is that it is easy to
 9506: implement using the ANS Forth locals wordset.
 9507: 
 9508: 
 9509: @c ----------------------------------------------------------
 9510: @node Structures, Object-oriented Forth, Locals, Words
 9511: @section  Structures
 9512: @cindex structures
 9513: @cindex records
 9514: 
 9515: This section presents the structure package that comes with Gforth. A
 9516: version of the package implemented in ANS Forth is available in
 9517: @file{compat/struct.fs}. This package was inspired by a posting on
 9518: comp.lang.forth in 1989 (unfortunately I don't remember, by whom;
 9519: possibly John Hayes). A version of this section has been published in
 9520: M. Anton Ertl,
 9521: @uref{http://www.complang.tuwien.ac.at/forth/objects/structs.html, Yet
 9522: Another Forth Structures Package}, Forth Dimensions 19(3), pages
 9523: 13--16. Marcel Hendrix provided helpful comments.
 9524: 
 9525: @menu
 9526: * Why explicit structure support?::  
 9527: * Structure Usage::             
 9528: * Structure Naming Convention::  
 9529: * Structure Implementation::    
 9530: * Structure Glossary::          
 9531: @end menu
 9532: 
 9533: @node Why explicit structure support?, Structure Usage, Structures, Structures
 9534: @subsection Why explicit structure support?
 9535: 
 9536: @cindex address arithmetic for structures
 9537: @cindex structures using address arithmetic
 9538: If we want to use a structure containing several fields, we could simply
 9539: reserve memory for it, and access the fields using address arithmetic
 9540: (@pxref{Address arithmetic}). As an example, consider a structure with
 9541: the following fields
 9542: 
 9543: @table @code
 9544: @item a
 9545: is a float
 9546: @item b
 9547: is a cell
 9548: @item c
 9549: is a float
 9550: @end table
 9551: 
 9552: Given the (float-aligned) base address of the structure we get the
 9553: address of the field
 9554: 
 9555: @table @code
 9556: @item a
 9557: without doing anything further.
 9558: @item b
 9559: with @code{float+}
 9560: @item c
 9561: with @code{float+ cell+ faligned}
 9562: @end table
 9563: 
 9564: It is easy to see that this can become quite tiring. 
 9565: 
 9566: Moreover, it is not very readable, because seeing a
 9567: @code{cell+} tells us neither which kind of structure is
 9568: accessed nor what field is accessed; we have to somehow infer the kind
 9569: of structure, and then look up in the documentation, which field of
 9570: that structure corresponds to that offset.
 9571: 
 9572: Finally, this kind of address arithmetic also causes maintenance
 9573: troubles: If you add or delete a field somewhere in the middle of the
 9574: structure, you have to find and change all computations for the fields
 9575: afterwards.
 9576: 
 9577: So, instead of using @code{cell+} and friends directly, how
 9578: about storing the offsets in constants:
 9579: 
 9580: @example
 9581: 0 constant a-offset
 9582: 0 float+ constant b-offset
 9583: 0 float+ cell+ faligned c-offset
 9584: @end example
 9585: 
 9586: Now we can get the address of field @code{x} with @code{x-offset
 9587: +}. This is much better in all respects. Of course, you still
 9588: have to change all later offset definitions if you add a field. You can
 9589: fix this by declaring the offsets in the following way:
 9590: 
 9591: @example
 9592: 0 constant a-offset
 9593: a-offset float+ constant b-offset
 9594: b-offset cell+ faligned constant c-offset
 9595: @end example
 9596: 
 9597: Since we always use the offsets with @code{+}, we could use a defining
 9598: word @code{cfield} that includes the @code{+} in the action of the
 9599: defined word:
 9600: 
 9601: @example
 9602: : cfield ( n "name" -- )
 9603:     create ,
 9604: does> ( name execution: addr1 -- addr2 )
 9605:     @@ + ;
 9606: 
 9607: 0 cfield a
 9608: 0 a float+ cfield b
 9609: 0 b cell+ faligned cfield c
 9610: @end example
 9611: 
 9612: Instead of @code{x-offset +}, we now simply write @code{x}.
 9613: 
 9614: The structure field words now can be used quite nicely. However,
 9615: their definition is still a bit cumbersome: We have to repeat the
 9616: name, the information about size and alignment is distributed before
 9617: and after the field definitions etc.  The structure package presented
 9618: here addresses these problems.
 9619: 
 9620: @node Structure Usage, Structure Naming Convention, Why explicit structure support?, Structures
 9621: @subsection Structure Usage
 9622: @cindex structure usage
 9623: 
 9624: @cindex @code{field} usage
 9625: @cindex @code{struct} usage
 9626: @cindex @code{end-struct} usage
 9627: You can define a structure for a (data-less) linked list with:
 9628: @example
 9629: struct
 9630:     cell% field list-next
 9631: end-struct list%
 9632: @end example
 9633: 
 9634: With the address of the list node on the stack, you can compute the
 9635: address of the field that contains the address of the next node with
 9636: @code{list-next}. E.g., you can determine the length of a list
 9637: with:
 9638: 
 9639: @example
 9640: : list-length ( list -- n )
 9641: \ "list" is a pointer to the first element of a linked list
 9642: \ "n" is the length of the list
 9643:     0 BEGIN ( list1 n1 )
 9644:         over
 9645:     WHILE ( list1 n1 )
 9646:         1+ swap list-next @@ swap
 9647:     REPEAT
 9648:     nip ;
 9649: @end example
 9650: 
 9651: You can reserve memory for a list node in the dictionary with
 9652: @code{list% %allot}, which leaves the address of the list node on the
 9653: stack. For the equivalent allocation on the heap you can use @code{list%
 9654: %alloc} (or, for an @code{allocate}-like stack effect (i.e., with ior),
 9655: use @code{list% %allocate}). You can get the the size of a list
 9656: node with @code{list% %size} and its alignment with @code{list%
 9657: %alignment}.
 9658: 
 9659: Note that in ANS Forth the body of a @code{create}d word is
 9660: @code{aligned} but not necessarily @code{faligned};
 9661: therefore, if you do a:
 9662: 
 9663: @example
 9664: create @emph{name} foo% %allot drop
 9665: @end example
 9666: 
 9667: @noindent
 9668: then the memory alloted for @code{foo%} is guaranteed to start at the
 9669: body of @code{@emph{name}} only if @code{foo%} contains only character,
 9670: cell and double fields.  Therefore, if your structure contains floats,
 9671: better use
 9672: 
 9673: @example
 9674: foo% %allot constant @emph{name}
 9675: @end example
 9676: 
 9677: @cindex structures containing structures
 9678: You can include a structure @code{foo%} as a field of
 9679: another structure, like this:
 9680: @example
 9681: struct
 9682: ...
 9683:     foo% field ...
 9684: ...
 9685: end-struct ...
 9686: @end example
 9687: 
 9688: @cindex structure extension
 9689: @cindex extended records
 9690: Instead of starting with an empty structure, you can extend an
 9691: existing structure. E.g., a plain linked list without data, as defined
 9692: above, is hardly useful; You can extend it to a linked list of integers,
 9693: like this:@footnote{This feature is also known as @emph{extended
 9694: records}. It is the main innovation in the Oberon language; in other
 9695: words, adding this feature to Modula-2 led Wirth to create a new
 9696: language, write a new compiler etc.  Adding this feature to Forth just
 9697: required a few lines of code.}
 9698: 
 9699: @example
 9700: list%
 9701:     cell% field intlist-int
 9702: end-struct intlist%
 9703: @end example
 9704: 
 9705: @code{intlist%} is a structure with two fields:
 9706: @code{list-next} and @code{intlist-int}.
 9707: 
 9708: @cindex structures containing arrays
 9709: You can specify an array type containing @emph{n} elements of
 9710: type @code{foo%} like this:
 9711: 
 9712: @example
 9713: foo% @emph{n} *
 9714: @end example
 9715: 
 9716: You can use this array type in any place where you can use a normal
 9717: type, e.g., when defining a @code{field}, or with
 9718: @code{%allot}.
 9719: 
 9720: @cindex first field optimization
 9721: The first field is at the base address of a structure and the word for
 9722: this field (e.g., @code{list-next}) actually does not change the address
 9723: on the stack. You may be tempted to leave it away in the interest of
 9724: run-time and space efficiency. This is not necessary, because the
 9725: structure package optimizes this case: If you compile a first-field
 9726: words, no code is generated. So, in the interest of readability and
 9727: maintainability you should include the word for the field when accessing
 9728: the field.
 9729: 
 9730: 
 9731: @node Structure Naming Convention, Structure Implementation, Structure Usage, Structures
 9732: @subsection Structure Naming Convention
 9733: @cindex structure naming convention
 9734: 
 9735: The field names that come to (my) mind are often quite generic, and,
 9736: if used, would cause frequent name clashes. E.g., many structures
 9737: probably contain a @code{counter} field. The structure names
 9738: that come to (my) mind are often also the logical choice for the names
 9739: of words that create such a structure.
 9740: 
 9741: Therefore, I have adopted the following naming conventions: 
 9742: 
 9743: @itemize @bullet
 9744: @cindex field naming convention
 9745: @item
 9746: The names of fields are of the form
 9747: @code{@emph{struct}-@emph{field}}, where
 9748: @code{@emph{struct}} is the basic name of the structure, and
 9749: @code{@emph{field}} is the basic name of the field. You can
 9750: think of field words as converting the (address of the)
 9751: structure into the (address of the) field.
 9752: 
 9753: @cindex structure naming convention
 9754: @item
 9755: The names of structures are of the form
 9756: @code{@emph{struct}%}, where
 9757: @code{@emph{struct}} is the basic name of the structure.
 9758: @end itemize
 9759: 
 9760: This naming convention does not work that well for fields of extended
 9761: structures; e.g., the integer list structure has a field
 9762: @code{intlist-int}, but has @code{list-next}, not
 9763: @code{intlist-next}.
 9764: 
 9765: @node Structure Implementation, Structure Glossary, Structure Naming Convention, Structures
 9766: @subsection Structure Implementation
 9767: @cindex structure implementation
 9768: @cindex implementation of structures
 9769: 
 9770: The central idea in the implementation is to pass the data about the
 9771: structure being built on the stack, not in some global
 9772: variable. Everything else falls into place naturally once this design
 9773: decision is made.
 9774: 
 9775: The type description on the stack is of the form @emph{align
 9776: size}. Keeping the size on the top-of-stack makes dealing with arrays
 9777: very simple.
 9778: 
 9779: @code{field} is a defining word that uses @code{Create}
 9780: and @code{DOES>}. The body of the field contains the offset
 9781: of the field, and the normal @code{DOES>} action is simply:
 9782: 
 9783: @example
 9784: @@ +
 9785: @end example
 9786: 
 9787: @noindent
 9788: i.e., add the offset to the address, giving the stack effect
 9789: @i{addr1 -- addr2} for a field.
 9790: 
 9791: @cindex first field optimization, implementation
 9792: This simple structure is slightly complicated by the optimization
 9793: for fields with offset 0, which requires a different
 9794: @code{DOES>}-part (because we cannot rely on there being
 9795: something on the stack if such a field is invoked during
 9796: compilation). Therefore, we put the different @code{DOES>}-parts
 9797: in separate words, and decide which one to invoke based on the
 9798: offset. For a zero offset, the field is basically a noop; it is
 9799: immediate, and therefore no code is generated when it is compiled.
 9800: 
 9801: @node Structure Glossary,  , Structure Implementation, Structures
 9802: @subsection Structure Glossary
 9803: @cindex structure glossary
 9804: 
 9805: 
 9806: doc-%align
 9807: doc-%alignment
 9808: doc-%alloc
 9809: doc-%allocate
 9810: doc-%allot
 9811: doc-cell%
 9812: doc-char%
 9813: doc-dfloat%
 9814: doc-double%
 9815: doc-end-struct
 9816: doc-field
 9817: doc-float%
 9818: doc-naligned
 9819: doc-sfloat%
 9820: doc-%size
 9821: doc-struct
 9822: 
 9823: 
 9824: @c -------------------------------------------------------------
 9825: @node Object-oriented Forth, Programming Tools, Structures, Words
 9826: @section Object-oriented Forth
 9827: 
 9828: Gforth comes with three packages for object-oriented programming:
 9829: @file{objects.fs}, @file{oof.fs}, and @file{mini-oof.fs}; none of them
 9830: is preloaded, so you have to @code{include} them before use. The most
 9831: important differences between these packages (and others) are discussed
 9832: in @ref{Comparison with other object models}. All packages are written
 9833: in ANS Forth and can be used with any other ANS Forth.
 9834: 
 9835: @menu
 9836: * Why object-oriented programming?::  
 9837: * Object-Oriented Terminology::  
 9838: * Objects::                     
 9839: * OOF::                         
 9840: * Mini-OOF::                    
 9841: * Comparison with other object models::  
 9842: @end menu
 9843: 
 9844: @c ----------------------------------------------------------------
 9845: @node Why object-oriented programming?, Object-Oriented Terminology, Object-oriented Forth, Object-oriented Forth
 9846: @subsection Why object-oriented programming?
 9847: @cindex object-oriented programming motivation
 9848: @cindex motivation for object-oriented programming
 9849: 
 9850: Often we have to deal with several data structures (@emph{objects}),
 9851: that have to be treated similarly in some respects, but differently in
 9852: others. Graphical objects are the textbook example: circles, triangles,
 9853: dinosaurs, icons, and others, and we may want to add more during program
 9854: development. We want to apply some operations to any graphical object,
 9855: e.g., @code{draw} for displaying it on the screen. However, @code{draw}
 9856: has to do something different for every kind of object.
 9857: @comment TODO add some other operations eg perimeter, area
 9858: @comment and tie in to concrete examples later..
 9859: 
 9860: We could implement @code{draw} as a big @code{CASE}
 9861: control structure that executes the appropriate code depending on the
 9862: kind of object to be drawn. This would be not be very elegant, and,
 9863: moreover, we would have to change @code{draw} every time we add
 9864: a new kind of graphical object (say, a spaceship).
 9865: 
 9866: What we would rather do is: When defining spaceships, we would tell
 9867: the system: ``Here's how you @code{draw} a spaceship; you figure
 9868: out the rest''.
 9869: 
 9870: This is the problem that all systems solve that (rightfully) call
 9871: themselves object-oriented; the object-oriented packages presented here
 9872: solve this problem (and not much else).
 9873: @comment TODO ?list properties of oo systems.. oo vs o-based?
 9874: 
 9875: @c ------------------------------------------------------------------------
 9876: @node Object-Oriented Terminology, Objects, Why object-oriented programming?, Object-oriented Forth
 9877: @subsection Object-Oriented Terminology
 9878: @cindex object-oriented terminology
 9879: @cindex terminology for object-oriented programming
 9880: 
 9881: This section is mainly for reference, so you don't have to understand
 9882: all of it right away.  The terminology is mainly Smalltalk-inspired.  In
 9883: short:
 9884: 
 9885: @table @emph
 9886: @cindex class
 9887: @item class
 9888: a data structure definition with some extras.
 9889: 
 9890: @cindex object
 9891: @item object
 9892: an instance of the data structure described by the class definition.
 9893: 
 9894: @cindex instance variables
 9895: @item instance variables
 9896: fields of the data structure.
 9897: 
 9898: @cindex selector
 9899: @cindex method selector
 9900: @cindex virtual function
 9901: @item selector
 9902: (or @emph{method selector}) a word (e.g.,
 9903: @code{draw}) that performs an operation on a variety of data
 9904: structures (classes). A selector describes @emph{what} operation to
 9905: perform. In C++ terminology: a (pure) virtual function.
 9906: 
 9907: @cindex method
 9908: @item method
 9909: the concrete definition that performs the operation
 9910: described by the selector for a specific class. A method specifies
 9911: @emph{how} the operation is performed for a specific class.
 9912: 
 9913: @cindex selector invocation
 9914: @cindex message send
 9915: @cindex invoking a selector
 9916: @item selector invocation
 9917: a call of a selector. One argument of the call (the TOS (top-of-stack))
 9918: is used for determining which method is used. In Smalltalk terminology:
 9919: a message (consisting of the selector and the other arguments) is sent
 9920: to the object.
 9921: 
 9922: @cindex receiving object
 9923: @item receiving object
 9924: the object used for determining the method executed by a selector
 9925: invocation. In the @file{objects.fs} model, it is the object that is on
 9926: the TOS when the selector is invoked. (@emph{Receiving} comes from
 9927: the Smalltalk @emph{message} terminology.)
 9928: 
 9929: @cindex child class
 9930: @cindex parent class
 9931: @cindex inheritance
 9932: @item child class
 9933: a class that has (@emph{inherits}) all properties (instance variables,
 9934: selectors, methods) from a @emph{parent class}. In Smalltalk
 9935: terminology: The subclass inherits from the superclass. In C++
 9936: terminology: The derived class inherits from the base class.
 9937: 
 9938: @end table
 9939: 
 9940: @c If you wonder about the message sending terminology, it comes from
 9941: @c a time when each object had it's own task and objects communicated via
 9942: @c message passing; eventually the Smalltalk developers realized that
 9943: @c they can do most things through simple (indirect) calls. They kept the
 9944: @c terminology.
 9945: 
 9946: @c --------------------------------------------------------------
 9947: @node Objects, OOF, Object-Oriented Terminology, Object-oriented Forth
 9948: @subsection The @file{objects.fs} model
 9949: @cindex objects
 9950: @cindex object-oriented programming
 9951: 
 9952: @cindex @file{objects.fs}
 9953: @cindex @file{oof.fs}
 9954: 
 9955: This section describes the @file{objects.fs} package. This material also
 9956: has been published in M. Anton Ertl,
 9957: @cite{@uref{http://www.complang.tuwien.ac.at/forth/objects/objects.html,
 9958: Yet Another Forth Objects Package}}, Forth Dimensions 19(2), pages
 9959: 37--43.
 9960: @c McKewan's and Zsoter's packages
 9961: 
 9962: This section assumes that you have read @ref{Structures}.
 9963: 
 9964: The techniques on which this model is based have been used to implement
 9965: the parser generator, Gray, and have also been used in Gforth for
 9966: implementing the various flavours of word lists (hashed or not,
 9967: case-sensitive or not, special-purpose word lists for locals etc.).
 9968: 
 9969: 
 9970: @menu
 9971: * Properties of the Objects model::  
 9972: * Basic Objects Usage::         
 9973: * The Objects base class::      
 9974: * Creating objects::            
 9975: * Object-Oriented Programming Style::  
 9976: * Class Binding::               
 9977: * Method conveniences::         
 9978: * Classes and Scoping::         
 9979: * Dividing classes::            
 9980: * Object Interfaces::           
 9981: * Objects Implementation::      
 9982: * Objects Glossary::            
 9983: @end menu
 9984: 
 9985: Marcel Hendrix provided helpful comments on this section.
 9986: 
 9987: @node Properties of the Objects model, Basic Objects Usage, Objects, Objects
 9988: @subsubsection Properties of the @file{objects.fs} model
 9989: @cindex @file{objects.fs} properties
 9990: 
 9991: @itemize @bullet
 9992: @item
 9993: It is straightforward to pass objects on the stack. Passing
 9994: selectors on the stack is a little less convenient, but possible.
 9995: 
 9996: @item
 9997: Objects are just data structures in memory, and are referenced by their
 9998: address. You can create words for objects with normal defining words
 9999: like @code{constant}. Likewise, there is no difference between instance
10000: variables that contain objects and those that contain other data.
10001: 
10002: @item
10003: Late binding is efficient and easy to use.
10004: 
10005: @item
10006: It avoids parsing, and thus avoids problems with state-smartness
10007: and reduced extensibility; for convenience there are a few parsing
10008: words, but they have non-parsing counterparts. There are also a few
10009: defining words that parse. This is hard to avoid, because all standard
10010: defining words parse (except @code{:noname}); however, such
10011: words are not as bad as many other parsing words, because they are not
10012: state-smart.
10013: 
10014: @item
10015: It does not try to incorporate everything. It does a few things and does
10016: them well (IMO). In particular, this model was not designed to support
10017: information hiding (although it has features that may help); you can use
10018: a separate package for achieving this.
10019: 
10020: @item
10021: It is layered; you don't have to learn and use all features to use this
10022: model. Only a few features are necessary (@pxref{Basic Objects Usage},
10023: @pxref{The Objects base class}, @pxref{Creating objects}.), the others
10024: are optional and independent of each other.
10025: 
10026: @item
10027: An implementation in ANS Forth is available.
10028: 
10029: @end itemize
10030: 
10031: 
10032: @node Basic Objects Usage, The Objects base class, Properties of the Objects model, Objects
10033: @subsubsection Basic @file{objects.fs} Usage
10034: @cindex basic objects usage
10035: @cindex objects, basic usage
10036: 
10037: You can define a class for graphical objects like this:
10038: 
10039: @cindex @code{class} usage
10040: @cindex @code{end-class} usage
10041: @cindex @code{selector} usage
10042: @example
10043: object class \ "object" is the parent class
10044:   selector draw ( x y graphical -- )
10045: end-class graphical
10046: @end example
10047: 
10048: This code defines a class @code{graphical} with an
10049: operation @code{draw}.  We can perform the operation
10050: @code{draw} on any @code{graphical} object, e.g.:
10051: 
10052: @example
10053: 100 100 t-rex draw
10054: @end example
10055: 
10056: @noindent
10057: where @code{t-rex} is a word (say, a constant) that produces a
10058: graphical object.
10059: 
10060: @comment TODO add a 2nd operation eg perimeter.. and use for
10061: @comment a concrete example
10062: 
10063: @cindex abstract class
10064: How do we create a graphical object? With the present definitions,
10065: we cannot create a useful graphical object. The class
10066: @code{graphical} describes graphical objects in general, but not
10067: any concrete graphical object type (C++ users would call it an
10068: @emph{abstract class}); e.g., there is no method for the selector
10069: @code{draw} in the class @code{graphical}.
10070: 
10071: For concrete graphical objects, we define child classes of the
10072: class @code{graphical}, e.g.:
10073: 
10074: @cindex @code{overrides} usage
10075: @cindex @code{field} usage in class definition
10076: @example
10077: graphical class \ "graphical" is the parent class
10078:   cell% field circle-radius
10079: 
10080: :noname ( x y circle -- )
10081:   circle-radius @@ draw-circle ;
10082: overrides draw
10083: 
10084: :noname ( n-radius circle -- )
10085:   circle-radius ! ;
10086: overrides construct
10087: 
10088: end-class circle
10089: @end example
10090: 
10091: Here we define a class @code{circle} as a child of @code{graphical},
10092: with field @code{circle-radius} (which behaves just like a field
10093: (@pxref{Structures}); it defines (using @code{overrides}) new methods
10094: for the selectors @code{draw} and @code{construct} (@code{construct} is
10095: defined in @code{object}, the parent class of @code{graphical}).
10096: 
10097: Now we can create a circle on the heap (i.e.,
10098: @code{allocate}d memory) with:
10099: 
10100: @cindex @code{heap-new} usage
10101: @example
10102: 50 circle heap-new constant my-circle
10103: @end example
10104: 
10105: @noindent
10106: @code{heap-new} invokes @code{construct}, thus
10107: initializing the field @code{circle-radius} with 50. We can draw
10108: this new circle at (100,100) with:
10109: 
10110: @example
10111: 100 100 my-circle draw
10112: @end example
10113: 
10114: @cindex selector invocation, restrictions
10115: @cindex class definition, restrictions
10116: Note: You can only invoke a selector if the object on the TOS
10117: (the receiving object) belongs to the class where the selector was
10118: defined or one of its descendents; e.g., you can invoke
10119: @code{draw} only for objects belonging to @code{graphical}
10120: or its descendents (e.g., @code{circle}).  Immediately before
10121: @code{end-class}, the search order has to be the same as
10122: immediately after @code{class}.
10123: 
10124: @node The Objects base class, Creating objects, Basic Objects Usage, Objects
10125: @subsubsection The @file{object.fs} base class
10126: @cindex @code{object} class
10127: 
10128: When you define a class, you have to specify a parent class.  So how do
10129: you start defining classes? There is one class available from the start:
10130: @code{object}. It is ancestor for all classes and so is the
10131: only class that has no parent. It has two selectors: @code{construct}
10132: and @code{print}.
10133: 
10134: @node Creating objects, Object-Oriented Programming Style, The Objects base class, Objects
10135: @subsubsection Creating objects
10136: @cindex creating objects
10137: @cindex object creation
10138: @cindex object allocation options
10139: 
10140: @cindex @code{heap-new} discussion
10141: @cindex @code{dict-new} discussion
10142: @cindex @code{construct} discussion
10143: You can create and initialize an object of a class on the heap with
10144: @code{heap-new} ( ... class -- object ) and in the dictionary
10145: (allocation with @code{allot}) with @code{dict-new} (
10146: ... class -- object ). Both words invoke @code{construct}, which
10147: consumes the stack items indicated by "..." above.
10148: 
10149: @cindex @code{init-object} discussion
10150: @cindex @code{class-inst-size} discussion
10151: If you want to allocate memory for an object yourself, you can get its
10152: alignment and size with @code{class-inst-size 2@@} ( class --
10153: align size ). Once you have memory for an object, you can initialize
10154: it with @code{init-object} ( ... class object -- );
10155: @code{construct} does only a part of the necessary work.
10156: 
10157: @node Object-Oriented Programming Style, Class Binding, Creating objects, Objects
10158: @subsubsection Object-Oriented Programming Style
10159: @cindex object-oriented programming style
10160: @cindex programming style, object-oriented
10161: 
10162: This section is not exhaustive.
10163: 
10164: @cindex stack effects of selectors
10165: @cindex selectors and stack effects
10166: In general, it is a good idea to ensure that all methods for the
10167: same selector have the same stack effect: when you invoke a selector,
10168: you often have no idea which method will be invoked, so, unless all
10169: methods have the same stack effect, you will not know the stack effect
10170: of the selector invocation.
10171: 
10172: One exception to this rule is methods for the selector
10173: @code{construct}. We know which method is invoked, because we
10174: specify the class to be constructed at the same place. Actually, I
10175: defined @code{construct} as a selector only to give the users a
10176: convenient way to specify initialization. The way it is used, a
10177: mechanism different from selector invocation would be more natural
10178: (but probably would take more code and more space to explain).
10179: 
10180: @node Class Binding, Method conveniences, Object-Oriented Programming Style, Objects
10181: @subsubsection Class Binding
10182: @cindex class binding
10183: @cindex early binding
10184: 
10185: @cindex late binding
10186: Normal selector invocations determine the method at run-time depending
10187: on the class of the receiving object. This run-time selection is called
10188: @i{late binding}.
10189: 
10190: Sometimes it's preferable to invoke a different method. For example,
10191: you might want to use the simple method for @code{print}ing
10192: @code{object}s instead of the possibly long-winded @code{print} method
10193: of the receiver class. You can achieve this by replacing the invocation
10194: of @code{print} with:
10195: 
10196: @cindex @code{[bind]} usage
10197: @example
10198: [bind] object print
10199: @end example
10200: 
10201: @noindent
10202: in compiled code or:
10203: 
10204: @cindex @code{bind} usage
10205: @example
10206: bind object print
10207: @end example
10208: 
10209: @cindex class binding, alternative to
10210: @noindent
10211: in interpreted code. Alternatively, you can define the method with a
10212: name (e.g., @code{print-object}), and then invoke it through the
10213: name. Class binding is just a (often more convenient) way to achieve
10214: the same effect; it avoids name clutter and allows you to invoke
10215: methods directly without naming them first.
10216: 
10217: @cindex superclass binding
10218: @cindex parent class binding
10219: A frequent use of class binding is this: When we define a method
10220: for a selector, we often want the method to do what the selector does
10221: in the parent class, and a little more. There is a special word for
10222: this purpose: @code{[parent]}; @code{[parent]
10223: @emph{selector}} is equivalent to @code{[bind] @emph{parent
10224: selector}}, where @code{@emph{parent}} is the parent
10225: class of the current class. E.g., a method definition might look like:
10226: 
10227: @cindex @code{[parent]} usage
10228: @example
10229: :noname
10230:   dup [parent] foo \ do parent's foo on the receiving object
10231:   ... \ do some more
10232: ; overrides foo
10233: @end example
10234: 
10235: @cindex class binding as optimization
10236: In @cite{Object-oriented programming in ANS Forth} (Forth Dimensions,
10237: March 1997), Andrew McKewan presents class binding as an optimization
10238: technique. I recommend not using it for this purpose unless you are in
10239: an emergency. Late binding is pretty fast with this model anyway, so the
10240: benefit of using class binding is small; the cost of using class binding
10241: where it is not appropriate is reduced maintainability.
10242: 
10243: While we are at programming style questions: You should bind
10244: selectors only to ancestor classes of the receiving object. E.g., say,
10245: you know that the receiving object is of class @code{foo} or its
10246: descendents; then you should bind only to @code{foo} and its
10247: ancestors.
10248: 
10249: @node Method conveniences, Classes and Scoping, Class Binding, Objects
10250: @subsubsection Method conveniences
10251: @cindex method conveniences
10252: 
10253: In a method you usually access the receiving object pretty often.  If
10254: you define the method as a plain colon definition (e.g., with
10255: @code{:noname}), you may have to do a lot of stack
10256: gymnastics. To avoid this, you can define the method with @code{m:
10257: ... ;m}. E.g., you could define the method for
10258: @code{draw}ing a @code{circle} with
10259: 
10260: @cindex @code{this} usage
10261: @cindex @code{m:} usage
10262: @cindex @code{;m} usage
10263: @example
10264: m: ( x y circle -- )
10265:   ( x y ) this circle-radius @@ draw-circle ;m
10266: @end example
10267: 
10268: @cindex @code{exit} in @code{m: ... ;m}
10269: @cindex @code{exitm} discussion
10270: @cindex @code{catch} in @code{m: ... ;m}
10271: When this method is executed, the receiver object is removed from the
10272: stack; you can access it with @code{this} (admittedly, in this
10273: example the use of @code{m: ... ;m} offers no advantage). Note
10274: that I specify the stack effect for the whole method (i.e. including
10275: the receiver object), not just for the code between @code{m:}
10276: and @code{;m}. You cannot use @code{exit} in
10277: @code{m:...;m}; instead, use
10278: @code{exitm}.@footnote{Moreover, for any word that calls
10279: @code{catch} and was defined before loading
10280: @code{objects.fs}, you have to redefine it like I redefined
10281: @code{catch}: @code{: catch this >r catch r> to-this ;}}
10282: 
10283: @cindex @code{inst-var} usage
10284: You will frequently use sequences of the form @code{this
10285: @emph{field}} (in the example above: @code{this
10286: circle-radius}). If you use the field only in this way, you can
10287: define it with @code{inst-var} and eliminate the
10288: @code{this} before the field name. E.g., the @code{circle}
10289: class above could also be defined with:
10290: 
10291: @example
10292: graphical class
10293:   cell% inst-var radius
10294: 
10295: m: ( x y circle -- )
10296:   radius @@ draw-circle ;m
10297: overrides draw
10298: 
10299: m: ( n-radius circle -- )
10300:   radius ! ;m
10301: overrides construct
10302: 
10303: end-class circle
10304: @end example
10305: 
10306: @code{radius} can only be used in @code{circle} and its
10307: descendent classes and inside @code{m:...;m}.
10308: 
10309: @cindex @code{inst-value} usage
10310: You can also define fields with @code{inst-value}, which is
10311: to @code{inst-var} what @code{value} is to
10312: @code{variable}.  You can change the value of such a field with
10313: @code{[to-inst]}.  E.g., we could also define the class
10314: @code{circle} like this:
10315: 
10316: @example
10317: graphical class
10318:   inst-value radius
10319: 
10320: m: ( x y circle -- )
10321:   radius draw-circle ;m
10322: overrides draw
10323: 
10324: m: ( n-radius circle -- )
10325:   [to-inst] radius ;m
10326: overrides construct
10327: 
10328: end-class circle
10329: @end example
10330: 
10331: @c !! :m is easy to confuse with m:.  Another name would be better.
10332: 
10333: @c Finally, you can define named methods with @code{:m}.  One use of this
10334: @c feature is the definition of words that occur only in one class and are
10335: @c not intended to be overridden, but which still need method context
10336: @c (e.g., for accessing @code{inst-var}s).  Another use is for methods that
10337: @c would be bound frequently, if defined anonymously.
10338: 
10339: 
10340: @node Classes and Scoping, Dividing classes, Method conveniences, Objects
10341: @subsubsection Classes and Scoping
10342: @cindex classes and scoping
10343: @cindex scoping and classes
10344: 
10345: Inheritance is frequent, unlike structure extension. This exacerbates
10346: the problem with the field name convention (@pxref{Structure Naming
10347: Convention}): One always has to remember in which class the field was
10348: originally defined; changing a part of the class structure would require
10349: changes for renaming in otherwise unaffected code.
10350: 
10351: @cindex @code{inst-var} visibility
10352: @cindex @code{inst-value} visibility
10353: To solve this problem, I added a scoping mechanism (which was not in my
10354: original charter): A field defined with @code{inst-var} (or
10355: @code{inst-value}) is visible only in the class where it is defined and in
10356: the descendent classes of this class.  Using such fields only makes
10357: sense in @code{m:}-defined methods in these classes anyway.
10358: 
10359: This scoping mechanism allows us to use the unadorned field name,
10360: because name clashes with unrelated words become much less likely.
10361: 
10362: @cindex @code{protected} discussion
10363: @cindex @code{private} discussion
10364: Once we have this mechanism, we can also use it for controlling the
10365: visibility of other words: All words defined after
10366: @code{protected} are visible only in the current class and its
10367: descendents. @code{public} restores the compilation
10368: (i.e. @code{current}) word list that was in effect before. If you
10369: have several @code{protected}s without an intervening
10370: @code{public} or @code{set-current}, @code{public}
10371: will restore the compilation word list in effect before the first of
10372: these @code{protected}s.
10373: 
10374: @node Dividing classes, Object Interfaces, Classes and Scoping, Objects
10375: @subsubsection Dividing classes
10376: @cindex Dividing classes
10377: @cindex @code{methods}...@code{end-methods}
10378: 
10379: You may want to do the definition of methods separate from the
10380: definition of the class, its selectors, fields, and instance variables,
10381: i.e., separate the implementation from the definition.  You can do this
10382: in the following way:
10383: 
10384: @example
10385: graphical class
10386:   inst-value radius
10387: end-class circle
10388: 
10389: ... \ do some other stuff
10390: 
10391: circle methods \ now we are ready
10392: 
10393: m: ( x y circle -- )
10394:   radius draw-circle ;m
10395: overrides draw
10396: 
10397: m: ( n-radius circle -- )
10398:   [to-inst] radius ;m
10399: overrides construct
10400: 
10401: end-methods
10402: @end example
10403: 
10404: You can use several @code{methods}...@code{end-methods} sections.  The
10405: only things you can do to the class in these sections are: defining
10406: methods, and overriding the class's selectors.  You must not define new
10407: selectors or fields.
10408: 
10409: Note that you often have to override a selector before using it.  In
10410: particular, you usually have to override @code{construct} with a new
10411: method before you can invoke @code{heap-new} and friends.  E.g., you
10412: must not create a circle before the @code{overrides construct} sequence
10413: in the example above.
10414: 
10415: @node Object Interfaces, Objects Implementation, Dividing classes, Objects
10416: @subsubsection Object Interfaces
10417: @cindex object interfaces
10418: @cindex interfaces for objects
10419: 
10420: In this model you can only call selectors defined in the class of the
10421: receiving objects or in one of its ancestors. If you call a selector
10422: with a receiving object that is not in one of these classes, the
10423: result is undefined; if you are lucky, the program crashes
10424: immediately.
10425: 
10426: @cindex selectors common to hardly-related classes
10427: Now consider the case when you want to have a selector (or several)
10428: available in two classes: You would have to add the selector to a
10429: common ancestor class, in the worst case to @code{object}. You
10430: may not want to do this, e.g., because someone else is responsible for
10431: this ancestor class.
10432: 
10433: The solution for this problem is interfaces. An interface is a
10434: collection of selectors. If a class implements an interface, the
10435: selectors become available to the class and its descendents. A class
10436: can implement an unlimited number of interfaces. For the problem
10437: discussed above, we would define an interface for the selector(s), and
10438: both classes would implement the interface.
10439: 
10440: As an example, consider an interface @code{storage} for
10441: writing objects to disk and getting them back, and a class
10442: @code{foo} that implements it. The code would look like this:
10443: 
10444: @cindex @code{interface} usage
10445: @cindex @code{end-interface} usage
10446: @cindex @code{implementation} usage
10447: @example
10448: interface
10449:   selector write ( file object -- )
10450:   selector read1 ( file object -- )
10451: end-interface storage
10452: 
10453: bar class
10454:   storage implementation
10455: 
10456: ... overrides write
10457: ... overrides read1
10458: ...
10459: end-class foo
10460: @end example
10461: 
10462: @noindent
10463: (I would add a word @code{read} @i{( file -- object )} that uses
10464: @code{read1} internally, but that's beyond the point illustrated
10465: here.)
10466: 
10467: Note that you cannot use @code{protected} in an interface; and
10468: of course you cannot define fields.
10469: 
10470: In the Neon model, all selectors are available for all classes;
10471: therefore it does not need interfaces. The price you pay in this model
10472: is slower late binding, and therefore, added complexity to avoid late
10473: binding.
10474: 
10475: @node Objects Implementation, Objects Glossary, Object Interfaces, Objects
10476: @subsubsection @file{objects.fs} Implementation
10477: @cindex @file{objects.fs} implementation
10478: 
10479: @cindex @code{object-map} discussion
10480: An object is a piece of memory, like one of the data structures
10481: described with @code{struct...end-struct}. It has a field
10482: @code{object-map} that points to the method map for the object's
10483: class.
10484: 
10485: @cindex method map
10486: @cindex virtual function table
10487: The @emph{method map}@footnote{This is Self terminology; in C++
10488: terminology: virtual function table.} is an array that contains the
10489: execution tokens (@i{xt}s) of the methods for the object's class. Each
10490: selector contains an offset into a method map.
10491: 
10492: @cindex @code{selector} implementation, class
10493: @code{selector} is a defining word that uses
10494: @code{CREATE} and @code{DOES>}. The body of the
10495: selector contains the offset; the @code{DOES>} action for a
10496: class selector is, basically:
10497: 
10498: @example
10499: ( object addr ) @@ over object-map @@ + @@ execute
10500: @end example
10501: 
10502: Since @code{object-map} is the first field of the object, it
10503: does not generate any code. As you can see, calling a selector has a
10504: small, constant cost.
10505: 
10506: @cindex @code{current-interface} discussion
10507: @cindex class implementation and representation
10508: A class is basically a @code{struct} combined with a method
10509: map. During the class definition the alignment and size of the class
10510: are passed on the stack, just as with @code{struct}s, so
10511: @code{field} can also be used for defining class
10512: fields. However, passing more items on the stack would be
10513: inconvenient, so @code{class} builds a data structure in memory,
10514: which is accessed through the variable
10515: @code{current-interface}. After its definition is complete, the
10516: class is represented on the stack by a pointer (e.g., as parameter for
10517: a child class definition).
10518: 
10519: A new class starts off with the alignment and size of its parent,
10520: and a copy of the parent's method map. Defining new fields extends the
10521: size and alignment; likewise, defining new selectors extends the
10522: method map. @code{overrides} just stores a new @i{xt} in the method
10523: map at the offset given by the selector.
10524: 
10525: @cindex class binding, implementation
10526: Class binding just gets the @i{xt} at the offset given by the selector
10527: from the class's method map and @code{compile,}s (in the case of
10528: @code{[bind]}) it.
10529: 
10530: @cindex @code{this} implementation
10531: @cindex @code{catch} and @code{this}
10532: @cindex @code{this} and @code{catch}
10533: I implemented @code{this} as a @code{value}. At the
10534: start of an @code{m:...;m} method the old @code{this} is
10535: stored to the return stack and restored at the end; and the object on
10536: the TOS is stored @code{TO this}. This technique has one
10537: disadvantage: If the user does not leave the method via
10538: @code{;m}, but via @code{throw} or @code{exit},
10539: @code{this} is not restored (and @code{exit} may
10540: crash). To deal with the @code{throw} problem, I have redefined
10541: @code{catch} to save and restore @code{this}; the same
10542: should be done with any word that can catch an exception. As for
10543: @code{exit}, I simply forbid it (as a replacement, there is
10544: @code{exitm}).
10545: 
10546: @cindex @code{inst-var} implementation
10547: @code{inst-var} is just the same as @code{field}, with
10548: a different @code{DOES>} action:
10549: @example
10550: @@ this +
10551: @end example
10552: Similar for @code{inst-value}.
10553: 
10554: @cindex class scoping implementation
10555: Each class also has a word list that contains the words defined with
10556: @code{inst-var} and @code{inst-value}, and its protected
10557: words. It also has a pointer to its parent. @code{class} pushes
10558: the word lists of the class and all its ancestors onto the search order stack,
10559: and @code{end-class} drops them.
10560: 
10561: @cindex interface implementation
10562: An interface is like a class without fields, parent and protected
10563: words; i.e., it just has a method map. If a class implements an
10564: interface, its method map contains a pointer to the method map of the
10565: interface. The positive offsets in the map are reserved for class
10566: methods, therefore interface map pointers have negative
10567: offsets. Interfaces have offsets that are unique throughout the
10568: system, unlike class selectors, whose offsets are only unique for the
10569: classes where the selector is available (invokable).
10570: 
10571: This structure means that interface selectors have to perform one
10572: indirection more than class selectors to find their method. Their body
10573: contains the interface map pointer offset in the class method map, and
10574: the method offset in the interface method map. The
10575: @code{does>} action for an interface selector is, basically:
10576: 
10577: @example
10578: ( object selector-body )
10579: 2dup selector-interface @@ ( object selector-body object interface-offset )
10580: swap object-map @@ + @@ ( object selector-body map )
10581: swap selector-offset @@ + @@ execute
10582: @end example
10583: 
10584: where @code{object-map} and @code{selector-offset} are
10585: first fields and generate no code.
10586: 
10587: As a concrete example, consider the following code:
10588: 
10589: @example
10590: interface
10591:   selector if1sel1
10592:   selector if1sel2
10593: end-interface if1
10594: 
10595: object class
10596:   if1 implementation
10597:   selector cl1sel1
10598:   cell% inst-var cl1iv1
10599: 
10600: ' m1 overrides construct
10601: ' m2 overrides if1sel1
10602: ' m3 overrides if1sel2
10603: ' m4 overrides cl1sel2
10604: end-class cl1
10605: 
10606: create obj1 object dict-new drop
10607: create obj2 cl1    dict-new drop
10608: @end example
10609: 
10610: The data structure created by this code (including the data structure
10611: for @code{object}) is shown in the
10612: @uref{objects-implementation.eps,figure}, assuming a cell size of 4.
10613: @comment TODO add this diagram..
10614: 
10615: @node Objects Glossary,  , Objects Implementation, Objects
10616: @subsubsection @file{objects.fs} Glossary
10617: @cindex @file{objects.fs} Glossary
10618: 
10619: 
10620: doc---objects-bind
10621: doc---objects-<bind>
10622: doc---objects-bind'
10623: doc---objects-[bind]
10624: doc---objects-class
10625: doc---objects-class->map
10626: doc---objects-class-inst-size
10627: doc---objects-class-override!
10628: doc---objects-class-previous
10629: doc---objects-class>order
10630: doc---objects-construct
10631: doc---objects-current'
10632: doc---objects-[current]
10633: doc---objects-current-interface
10634: doc---objects-dict-new
10635: doc---objects-end-class
10636: doc---objects-end-class-noname
10637: doc---objects-end-interface
10638: doc---objects-end-interface-noname
10639: doc---objects-end-methods
10640: doc---objects-exitm
10641: doc---objects-heap-new
10642: doc---objects-implementation
10643: doc---objects-init-object
10644: doc---objects-inst-value
10645: doc---objects-inst-var
10646: doc---objects-interface
10647: doc---objects-m:
10648: doc---objects-:m
10649: doc---objects-;m
10650: doc---objects-method
10651: doc---objects-methods
10652: doc---objects-object
10653: doc---objects-overrides
10654: doc---objects-[parent]
10655: doc---objects-print
10656: doc---objects-protected
10657: doc---objects-public
10658: doc---objects-selector
10659: doc---objects-this
10660: doc---objects-<to-inst>
10661: doc---objects-[to-inst]
10662: doc---objects-to-this
10663: doc---objects-xt-new
10664: 
10665: 
10666: @c -------------------------------------------------------------
10667: @node OOF, Mini-OOF, Objects, Object-oriented Forth
10668: @subsection The @file{oof.fs} model
10669: @cindex oof
10670: @cindex object-oriented programming
10671: 
10672: @cindex @file{objects.fs}
10673: @cindex @file{oof.fs}
10674: 
10675: This section describes the @file{oof.fs} package.
10676: 
10677: The package described in this section has been used in bigFORTH since 1991, and
10678: used for two large applications: a chromatographic system used to
10679: create new medicaments, and a graphic user interface library (MINOS).
10680: 
10681: You can find a description (in German) of @file{oof.fs} in @cite{Object
10682: oriented bigFORTH} by Bernd Paysan, published in @cite{Vierte Dimension}
10683: 10(2), 1994.
10684: 
10685: @menu
10686: * Properties of the OOF model::  
10687: * Basic OOF Usage::             
10688: * The OOF base class::          
10689: * Class Declaration::           
10690: * Class Implementation::        
10691: @end menu
10692: 
10693: @node Properties of the OOF model, Basic OOF Usage, OOF, OOF
10694: @subsubsection Properties of the @file{oof.fs} model
10695: @cindex @file{oof.fs} properties
10696: 
10697: @itemize @bullet
10698: @item
10699: This model combines object oriented programming with information
10700: hiding. It helps you writing large application, where scoping is
10701: necessary, because it provides class-oriented scoping.
10702: 
10703: @item
10704: Named objects, object pointers, and object arrays can be created,
10705: selector invocation uses the ``object selector'' syntax. Selector invocation
10706: to objects and/or selectors on the stack is a bit less convenient, but
10707: possible.
10708: 
10709: @item
10710: Selector invocation and instance variable usage of the active object is
10711: straightforward, since both make use of the active object.
10712: 
10713: @item
10714: Late binding is efficient and easy to use.
10715: 
10716: @item
10717: State-smart objects parse selectors. However, extensibility is provided
10718: using a (parsing) selector @code{postpone} and a selector @code{'}.
10719: 
10720: @item
10721: An implementation in ANS Forth is available.
10722: 
10723: @end itemize
10724: 
10725: 
10726: @node Basic OOF Usage, The OOF base class, Properties of the OOF model, OOF
10727: @subsubsection Basic @file{oof.fs} Usage
10728: @cindex @file{oof.fs} usage
10729: 
10730: This section uses the same example as for @code{objects} (@pxref{Basic Objects Usage}).
10731: 
10732: You can define a class for graphical objects like this:
10733: 
10734: @cindex @code{class} usage
10735: @cindex @code{class;} usage
10736: @cindex @code{method} usage
10737: @example
10738: object class graphical \ "object" is the parent class
10739:   method draw ( x y graphical -- )
10740: class;
10741: @end example
10742: 
10743: This code defines a class @code{graphical} with an
10744: operation @code{draw}.  We can perform the operation
10745: @code{draw} on any @code{graphical} object, e.g.:
10746: 
10747: @example
10748: 100 100 t-rex draw
10749: @end example
10750: 
10751: @noindent
10752: where @code{t-rex} is an object or object pointer, created with e.g.
10753: @code{graphical : t-rex}.
10754: 
10755: @cindex abstract class
10756: How do we create a graphical object? With the present definitions,
10757: we cannot create a useful graphical object. The class
10758: @code{graphical} describes graphical objects in general, but not
10759: any concrete graphical object type (C++ users would call it an
10760: @emph{abstract class}); e.g., there is no method for the selector
10761: @code{draw} in the class @code{graphical}.
10762: 
10763: For concrete graphical objects, we define child classes of the
10764: class @code{graphical}, e.g.:
10765: 
10766: @example
10767: graphical class circle \ "graphical" is the parent class
10768:   cell var circle-radius
10769: how:
10770:   : draw ( x y -- )
10771:     circle-radius @@ draw-circle ;
10772: 
10773:   : init ( n-radius -- (
10774:     circle-radius ! ;
10775: class;
10776: @end example
10777: 
10778: Here we define a class @code{circle} as a child of @code{graphical},
10779: with a field @code{circle-radius}; it defines new methods for the
10780: selectors @code{draw} and @code{init} (@code{init} is defined in
10781: @code{object}, the parent class of @code{graphical}).
10782: 
10783: Now we can create a circle in the dictionary with:
10784: 
10785: @example
10786: 50 circle : my-circle
10787: @end example
10788: 
10789: @noindent
10790: @code{:} invokes @code{init}, thus initializing the field
10791: @code{circle-radius} with 50. We can draw this new circle at (100,100)
10792: with:
10793: 
10794: @example
10795: 100 100 my-circle draw
10796: @end example
10797: 
10798: @cindex selector invocation, restrictions
10799: @cindex class definition, restrictions
10800: Note: You can only invoke a selector if the receiving object belongs to
10801: the class where the selector was defined or one of its descendents;
10802: e.g., you can invoke @code{draw} only for objects belonging to
10803: @code{graphical} or its descendents (e.g., @code{circle}). The scoping
10804: mechanism will check if you try to invoke a selector that is not
10805: defined in this class hierarchy, so you'll get an error at compilation
10806: time.
10807: 
10808: 
10809: @node The OOF base class, Class Declaration, Basic OOF Usage, OOF
10810: @subsubsection The @file{oof.fs} base class
10811: @cindex @file{oof.fs} base class
10812: 
10813: When you define a class, you have to specify a parent class.  So how do
10814: you start defining classes? There is one class available from the start:
10815: @code{object}. You have to use it as ancestor for all classes. It is the
10816: only class that has no parent. Classes are also objects, except that
10817: they don't have instance variables; class manipulation such as
10818: inheritance or changing definitions of a class is handled through
10819: selectors of the class @code{object}.
10820: 
10821: @code{object} provides a number of selectors:
10822: 
10823: @itemize @bullet
10824: @item
10825: @code{class} for subclassing, @code{definitions} to add definitions
10826: later on, and @code{class?} to get type informations (is the class a
10827: subclass of the class passed on the stack?).
10828: 
10829: doc---object-class
10830: doc---object-definitions
10831: doc---object-class?
10832: 
10833: 
10834: @item
10835: @code{init} and @code{dispose} as constructor and destructor of the
10836: object. @code{init} is invocated after the object's memory is allocated,
10837: while @code{dispose} also handles deallocation. Thus if you redefine
10838: @code{dispose}, you have to call the parent's dispose with @code{super
10839: dispose}, too.
10840: 
10841: doc---object-init
10842: doc---object-dispose
10843: 
10844: 
10845: @item
10846: @code{new}, @code{new[]}, @code{:}, @code{ptr}, @code{asptr}, and
10847: @code{[]} to create named and unnamed objects and object arrays or
10848: object pointers.
10849: 
10850: doc---object-new
10851: doc---object-new[]
10852: doc---object-:
10853: doc---object-ptr
10854: doc---object-asptr
10855: doc---object-[]
10856: 
10857: 
10858: @item
10859: @code{::} and @code{super} for explicit scoping. You should use explicit
10860: scoping only for super classes or classes with the same set of instance
10861: variables. Explicitly-scoped selectors use early binding.
10862: 
10863: doc---object-::
10864: doc---object-super
10865: 
10866: 
10867: @item
10868: @code{self} to get the address of the object
10869: 
10870: doc---object-self
10871: 
10872: 
10873: @item
10874: @code{bind}, @code{bound}, @code{link}, and @code{is} to assign object
10875: pointers and instance defers.
10876: 
10877: doc---object-bind
10878: doc---object-bound
10879: doc---object-link
10880: doc---object-is
10881: 
10882: 
10883: @item
10884: @code{'} to obtain selector tokens, @code{send} to invocate selectors
10885: form the stack, and @code{postpone} to generate selector invocation code.
10886: 
10887: doc---object-'
10888: doc---object-postpone
10889: 
10890: 
10891: @item
10892: @code{with} and @code{endwith} to select the active object from the
10893: stack, and enable its scope. Using @code{with} and @code{endwith}
10894: also allows you to create code using selector @code{postpone} without being
10895: trapped by the state-smart objects.
10896: 
10897: doc---object-with
10898: doc---object-endwith
10899: 
10900: 
10901: @end itemize
10902: 
10903: @node Class Declaration, Class Implementation, The OOF base class, OOF
10904: @subsubsection Class Declaration
10905: @cindex class declaration
10906: 
10907: @itemize @bullet
10908: @item
10909: Instance variables
10910: 
10911: doc---oof-var
10912: 
10913: 
10914: @item
10915: Object pointers
10916: 
10917: doc---oof-ptr
10918: doc---oof-asptr
10919: 
10920: 
10921: @item
10922: Instance defers
10923: 
10924: doc---oof-defer
10925: 
10926: 
10927: @item
10928: Method selectors
10929: 
10930: doc---oof-early
10931: doc---oof-method
10932: 
10933: 
10934: @item
10935: Class-wide variables
10936: 
10937: doc---oof-static
10938: 
10939: 
10940: @item
10941: End declaration
10942: 
10943: doc---oof-how:
10944: doc---oof-class;
10945: 
10946: 
10947: @end itemize
10948: 
10949: @c -------------------------------------------------------------
10950: @node Class Implementation,  , Class Declaration, OOF
10951: @subsubsection Class Implementation
10952: @cindex class implementation
10953: 
10954: @c -------------------------------------------------------------
10955: @node Mini-OOF, Comparison with other object models, OOF, Object-oriented Forth
10956: @subsection The @file{mini-oof.fs} model
10957: @cindex mini-oof
10958: 
10959: Gforth's third object oriented Forth package is a 12-liner. It uses a
10960: mixture of the @file{objects.fs} and the @file{oof.fs} syntax,
10961: and reduces to the bare minimum of features. This is based on a posting
10962: of Bernd Paysan in comp.lang.forth.
10963: 
10964: @menu
10965: * Basic Mini-OOF Usage::        
10966: * Mini-OOF Example::            
10967: * Mini-OOF Implementation::     
10968: @end menu
10969: 
10970: @c -------------------------------------------------------------
10971: @node Basic Mini-OOF Usage, Mini-OOF Example, Mini-OOF, Mini-OOF
10972: @subsubsection Basic @file{mini-oof.fs} Usage
10973: @cindex mini-oof usage
10974: 
10975: There is a base class (@code{class}, which allocates one cell for the
10976: object pointer) plus seven other words: to define a method, a variable,
10977: a class; to end a class, to resolve binding, to allocate an object and
10978: to compile a class method.
10979: @comment TODO better description of the last one
10980: 
10981: 
10982: doc-object
10983: doc-method
10984: doc-var
10985: doc-class
10986: doc-end-class
10987: doc-defines
10988: doc-new
10989: doc-::
10990: 
10991: 
10992: 
10993: @c -------------------------------------------------------------
10994: @node Mini-OOF Example, Mini-OOF Implementation, Basic Mini-OOF Usage, Mini-OOF
10995: @subsubsection Mini-OOF Example
10996: @cindex mini-oof example
10997: 
10998: A short example shows how to use this package. This example, in slightly
10999: extended form, is supplied as @file{moof-exm.fs}
11000: @comment TODO could flesh this out with some comments from the Forthwrite article
11001: 
11002: @example
11003: object class
11004:   method init
11005:   method draw
11006: end-class graphical
11007: @end example
11008: 
11009: This code defines a class @code{graphical} with an
11010: operation @code{draw}.  We can perform the operation
11011: @code{draw} on any @code{graphical} object, e.g.:
11012: 
11013: @example
11014: 100 100 t-rex draw
11015: @end example
11016: 
11017: where @code{t-rex} is an object or object pointer, created with e.g.
11018: @code{graphical new Constant t-rex}.
11019: 
11020: For concrete graphical objects, we define child classes of the
11021: class @code{graphical}, e.g.:
11022: 
11023: @example
11024: graphical class
11025:   cell var circle-radius
11026: end-class circle \ "graphical" is the parent class
11027: 
11028: :noname ( x y -- )
11029:   circle-radius @@ draw-circle ; circle defines draw
11030: :noname ( r -- )
11031:   circle-radius ! ; circle defines init
11032: @end example
11033: 
11034: There is no implicit init method, so we have to define one. The creation
11035: code of the object now has to call init explicitely.
11036: 
11037: @example
11038: circle new Constant my-circle
11039: 50 my-circle init
11040: @end example
11041: 
11042: It is also possible to add a function to create named objects with
11043: automatic call of @code{init}, given that all objects have @code{init}
11044: on the same place:
11045: 
11046: @example
11047: : new: ( .. o "name" -- )
11048:     new dup Constant init ;
11049: 80 circle new: large-circle
11050: @end example
11051: 
11052: We can draw this new circle at (100,100) with:
11053: 
11054: @example
11055: 100 100 my-circle draw
11056: @end example
11057: 
11058: @node Mini-OOF Implementation,  , Mini-OOF Example, Mini-OOF
11059: @subsubsection @file{mini-oof.fs} Implementation
11060: 
11061: Object-oriented systems with late binding typically use a
11062: ``vtable''-approach: the first variable in each object is a pointer to a
11063: table, which contains the methods as function pointers. The vtable
11064: may also contain other information.
11065: 
11066: So first, let's declare selectors:
11067: 
11068: @example
11069: : method ( m v "name" -- m' v ) Create  over , swap cell+ swap
11070:   DOES> ( ... o -- ... ) @@ over @@ + @@ execute ;
11071: @end example
11072: 
11073: During selector declaration, the number of selectors and instance
11074: variables is on the stack (in address units). @code{method} creates one
11075: selector and increments the selector number. To execute a selector, it
11076: takes the object, fetches the vtable pointer, adds the offset, and
11077: executes the method @i{xt} stored there. Each selector takes the object
11078: it is invoked with as top of stack parameter; it passes the parameters
11079: (including the object) unchanged to the appropriate method which should
11080: consume that object.
11081: 
11082: Now, we also have to declare instance variables
11083: 
11084: @example
11085: : var ( m v size "name" -- m v' ) Create  over , +
11086:   DOES> ( o -- addr ) @@ + ;
11087: @end example
11088: 
11089: As before, a word is created with the current offset. Instance
11090: variables can have different sizes (cells, floats, doubles, chars), so
11091: all we do is take the size and add it to the offset. If your machine
11092: has alignment restrictions, put the proper @code{aligned} or
11093: @code{faligned} before the variable, to adjust the variable
11094: offset. That's why it is on the top of stack.
11095: 
11096: We need a starting point (the base object) and some syntactic sugar:
11097: 
11098: @example
11099: Create object  1 cells , 2 cells ,
11100: : class ( class -- class selectors vars ) dup 2@@ ;
11101: @end example
11102: 
11103: For inheritance, the vtable of the parent object has to be
11104: copied when a new, derived class is declared. This gives all the
11105: methods of the parent class, which can be overridden, though.
11106: 
11107: @example
11108: : end-class  ( class selectors vars "name" -- )
11109:   Create  here >r , dup , 2 cells ?DO ['] noop , 1 cells +LOOP
11110:   cell+ dup cell+ r> rot @@ 2 cells /string move ;
11111: @end example
11112: 
11113: The first line creates the vtable, initialized with
11114: @code{noop}s. The second line is the inheritance mechanism, it
11115: copies the xts from the parent vtable.
11116: 
11117: We still have no way to define new methods, let's do that now:
11118: 
11119: @example
11120: : defines ( xt class "name" -- ) ' >body @@ + ! ;
11121: @end example
11122: 
11123: To allocate a new object, we need a word, too:
11124: 
11125: @example
11126: : new ( class -- o )  here over @@ allot swap over ! ;
11127: @end example
11128: 
11129: Sometimes derived classes want to access the method of the
11130: parent object. There are two ways to achieve this with Mini-OOF:
11131: first, you could use named words, and second, you could look up the
11132: vtable of the parent object.
11133: 
11134: @example
11135: : :: ( class "name" -- ) ' >body @@ + @@ compile, ;
11136: @end example
11137: 
11138: 
11139: Nothing can be more confusing than a good example, so here is
11140: one. First let's declare a text object (called
11141: @code{button}), that stores text and position:
11142: 
11143: @example
11144: object class
11145:   cell var text
11146:   cell var len
11147:   cell var x
11148:   cell var y
11149:   method init
11150:   method draw
11151: end-class button
11152: @end example
11153: 
11154: @noindent
11155: Now, implement the two methods, @code{draw} and @code{init}:
11156: 
11157: @example
11158: :noname ( o -- )
11159:  >r r@@ x @@ r@@ y @@ at-xy  r@@ text @@ r> len @@ type ;
11160:  button defines draw
11161: :noname ( addr u o -- )
11162:  >r 0 r@@ x ! 0 r@@ y ! r@@ len ! r> text ! ;
11163:  button defines init
11164: @end example
11165: 
11166: @noindent
11167: To demonstrate inheritance, we define a class @code{bold-button}, with no
11168: new data and no new selectors:
11169: 
11170: @example
11171: button class
11172: end-class bold-button
11173: 
11174: : bold   27 emit ." [1m" ;
11175: : normal 27 emit ." [0m" ;
11176: @end example
11177: 
11178: @noindent
11179: The class @code{bold-button} has a different draw method to
11180: @code{button}, but the new method is defined in terms of the draw method
11181: for @code{button}:
11182: 
11183: @example
11184: :noname bold [ button :: draw ] normal ; bold-button defines draw
11185: @end example
11186: 
11187: @noindent
11188: Finally, create two objects and apply selectors:
11189: 
11190: @example
11191: button new Constant foo
11192: s" thin foo" foo init
11193: page
11194: foo draw
11195: bold-button new Constant bar
11196: s" fat bar" bar init
11197: 1 bar y !
11198: bar draw
11199: @end example
11200: 
11201: 
11202: @node Comparison with other object models,  , Mini-OOF, Object-oriented Forth
11203: @subsection Comparison with other object models
11204: @cindex comparison of object models
11205: @cindex object models, comparison
11206: 
11207: Many object-oriented Forth extensions have been proposed (@cite{A survey
11208: of object-oriented Forths} (SIGPLAN Notices, April 1996) by Bradford
11209: J. Rodriguez and W. F. S. Poehlman lists 17). This section discusses the
11210: relation of the object models described here to two well-known and two
11211: closely-related (by the use of method maps) models.  Andras Zsoter
11212: helped us with this section.
11213: 
11214: @cindex Neon model
11215: The most popular model currently seems to be the Neon model (see
11216: @cite{Object-oriented programming in ANS Forth} (Forth Dimensions, March
11217: 1997) by Andrew McKewan) but this model has a number of limitations
11218: @footnote{A longer version of this critique can be
11219: found in @cite{On Standardizing Object-Oriented Forth Extensions} (Forth
11220: Dimensions, May 1997) by Anton Ertl.}:
11221: 
11222: @itemize @bullet
11223: @item
11224: It uses a @code{@emph{selector object}} syntax, which makes it unnatural
11225: to pass objects on the stack.
11226: 
11227: @item
11228: It requires that the selector parses the input stream (at
11229: compile time); this leads to reduced extensibility and to bugs that are
11230: hard to find.
11231: 
11232: @item
11233: It allows using every selector on every object; this eliminates the
11234: need for interfaces, but makes it harder to create efficient
11235: implementations.
11236: @end itemize
11237: 
11238: @cindex Pountain's object-oriented model
11239: Another well-known publication is @cite{Object-Oriented Forth} (Academic
11240: Press, London, 1987) by Dick Pountain. However, it is not really about
11241: object-oriented programming, because it hardly deals with late
11242: binding. Instead, it focuses on features like information hiding and
11243: overloading that are characteristic of modular languages like Ada (83).
11244: 
11245: @cindex Zsoter's object-oriented model
11246: In @uref{http://www.forth.org/oopf.html, Does late binding have to be
11247: slow?} (Forth Dimensions 18(1) 1996, pages 31-35) Andras Zsoter
11248: describes a model that makes heavy use of an active object (like
11249: @code{this} in @file{objects.fs}): The active object is not only used
11250: for accessing all fields, but also specifies the receiving object of
11251: every selector invocation; you have to change the active object
11252: explicitly with @code{@{ ... @}}, whereas in @file{objects.fs} it
11253: changes more or less implicitly at @code{m: ... ;m}. Such a change at
11254: the method entry point is unnecessary with Zsoter's model, because the
11255: receiving object is the active object already. On the other hand, the
11256: explicit change is absolutely necessary in that model, because otherwise
11257: no one could ever change the active object. An ANS Forth implementation
11258: of this model is available through
11259: @uref{http://www.forth.org/oopf.html}.
11260: 
11261: @cindex @file{oof.fs}, differences to other models
11262: The @file{oof.fs} model combines information hiding and overloading
11263: resolution (by keeping names in various word lists) with object-oriented
11264: programming. It sets the active object implicitly on method entry, but
11265: also allows explicit changing (with @code{>o...o>} or with
11266: @code{with...endwith}). It uses parsing and state-smart objects and
11267: classes for resolving overloading and for early binding: the object or
11268: class parses the selector and determines the method from this. If the
11269: selector is not parsed by an object or class, it performs a call to the
11270: selector for the active object (late binding), like Zsoter's model.
11271: Fields are always accessed through the active object. The big
11272: disadvantage of this model is the parsing and the state-smartness, which
11273: reduces extensibility and increases the opportunities for subtle bugs;
11274: essentially, you are only safe if you never tick or @code{postpone} an
11275: object or class (Bernd disagrees, but I (Anton) am not convinced).
11276: 
11277: @cindex @file{mini-oof.fs}, differences to other models
11278: The @file{mini-oof.fs} model is quite similar to a very stripped-down
11279: version of the @file{objects.fs} model, but syntactically it is a
11280: mixture of the @file{objects.fs} and @file{oof.fs} models.
11281: 
11282: 
11283: @c -------------------------------------------------------------
11284: @node Programming Tools, Assembler and Code Words, Object-oriented Forth, Words
11285: @section Programming Tools
11286: @cindex programming tools
11287: 
11288: @c !! move this and assembler down below OO stuff.
11289: 
11290: @menu
11291: * Examining::                   
11292: * Forgetting words::            
11293: * Debugging::                   Simple and quick.
11294: * Assertions::                  Making your programs self-checking.
11295: * Singlestep Debugger::         Executing your program word by word.
11296: @end menu
11297: 
11298: @node Examining, Forgetting words, Programming Tools, Programming Tools
11299: @subsection Examining data and code
11300: @cindex examining data and code
11301: @cindex data examination
11302: @cindex code examination
11303: 
11304: The following words inspect the stack non-destructively:
11305: 
11306: doc-.s
11307: doc-f.s
11308: 
11309: There is a word @code{.r} but it does @i{not} display the return stack!
11310: It is used for formatted numeric output (@pxref{Simple numeric output}).
11311: 
11312: doc-depth
11313: doc-fdepth
11314: doc-clearstack
11315: doc-clearstacks
11316: 
11317: The following words inspect memory.
11318: 
11319: doc-?
11320: doc-dump
11321: 
11322: And finally, @code{see} allows to inspect code:
11323: 
11324: doc-see
11325: doc-xt-see
11326: doc-simple-see
11327: doc-simple-see-range
11328: 
11329: @node Forgetting words, Debugging, Examining, Programming Tools
11330: @subsection Forgetting words
11331: @cindex words, forgetting
11332: @cindex forgeting words
11333: 
11334: @c  anton: other, maybe better places for this subsection: Defining Words;
11335: @c  Dictionary allocation.  At least a reference should be there.
11336: 
11337: Forth allows you to forget words (and everything that was alloted in the
11338: dictonary after them) in a LIFO manner.
11339: 
11340: doc-marker
11341: 
11342: The most common use of this feature is during progam development: when
11343: you change a source file, forget all the words it defined and load it
11344: again (since you also forget everything defined after the source file
11345: was loaded, you have to reload that, too).  Note that effects like
11346: storing to variables and destroyed system words are not undone when you
11347: forget words.  With a system like Gforth, that is fast enough at
11348: starting up and compiling, I find it more convenient to exit and restart
11349: Gforth, as this gives me a clean slate.
11350: 
11351: Here's an example of using @code{marker} at the start of a source file
11352: that you are debugging; it ensures that you only ever have one copy of
11353: the file's definitions compiled at any time:
11354: 
11355: @example
11356: [IFDEF] my-code
11357:     my-code
11358: [ENDIF]
11359: 
11360: marker my-code
11361: init-included-files
11362: 
11363: \ .. definitions start here
11364: \ .
11365: \ .
11366: \ end
11367: @end example
11368: 
11369: 
11370: @node Debugging, Assertions, Forgetting words, Programming Tools
11371: @subsection Debugging
11372: @cindex debugging
11373: 
11374: Languages with a slow edit/compile/link/test development loop tend to
11375: require sophisticated tracing/stepping debuggers to facilate debugging.
11376: 
11377: A much better (faster) way in fast-compiling languages is to add
11378: printing code at well-selected places, let the program run, look at
11379: the output, see where things went wrong, add more printing code, etc.,
11380: until the bug is found.
11381: 
11382: The simple debugging aids provided in @file{debugs.fs}
11383: are meant to support this style of debugging.
11384: 
11385: The word @code{~~} prints debugging information (by default the source
11386: location and the stack contents). It is easy to insert. If you use Emacs
11387: it is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
11388: query-replace them with nothing). The deferred words
11389: @code{printdebugdata} and @code{.debugline} control the output of
11390: @code{~~}. The default source location output format works well with
11391: Emacs' compilation mode, so you can step through the program at the
11392: source level using @kbd{C-x `} (the advantage over a stepping debugger
11393: is that you can step in any direction and you know where the crash has
11394: happened or where the strange data has occurred).
11395: 
11396: doc-~~
11397: doc-printdebugdata
11398: doc-.debugline
11399: 
11400: @cindex filenames in @code{~~} output
11401: @code{~~} (and assertions) will usually print the wrong file name if a
11402: marker is executed in the same file after their occurance.  They will
11403: print @samp{*somewhere*} as file name if a marker is executed in the
11404: same file before their occurance.
11405: 
11406: 
11407: @node Assertions, Singlestep Debugger, Debugging, Programming Tools
11408: @subsection Assertions
11409: @cindex assertions
11410: 
11411: It is a good idea to make your programs self-checking, especially if you
11412: make an assumption that may become invalid during maintenance (for
11413: example, that a certain field of a data structure is never zero). Gforth
11414: supports @dfn{assertions} for this purpose. They are used like this:
11415: 
11416: @example
11417: assert( @i{flag} )
11418: @end example
11419: 
11420: The code between @code{assert(} and @code{)} should compute a flag, that
11421: should be true if everything is alright and false otherwise. It should
11422: not change anything else on the stack. The overall stack effect of the
11423: assertion is @code{( -- )}. E.g.
11424: 
11425: @example
11426: assert( 1 1 + 2 = ) \ what we learn in school
11427: assert( dup 0<> ) \ assert that the top of stack is not zero
11428: assert( false ) \ this code should not be reached
11429: @end example
11430: 
11431: The need for assertions is different at different times. During
11432: debugging, we want more checking, in production we sometimes care more
11433: for speed. Therefore, assertions can be turned off, i.e., the assertion
11434: becomes a comment. Depending on the importance of an assertion and the
11435: time it takes to check it, you may want to turn off some assertions and
11436: keep others turned on. Gforth provides several levels of assertions for
11437: this purpose:
11438: 
11439: 
11440: doc-assert0(
11441: doc-assert1(
11442: doc-assert2(
11443: doc-assert3(
11444: doc-assert(
11445: doc-)
11446: 
11447: 
11448: The variable @code{assert-level} specifies the highest assertions that
11449: are turned on. I.e., at the default @code{assert-level} of one,
11450: @code{assert0(} and @code{assert1(} assertions perform checking, while
11451: @code{assert2(} and @code{assert3(} assertions are treated as comments.
11452: 
11453: The value of @code{assert-level} is evaluated at compile-time, not at
11454: run-time. Therefore you cannot turn assertions on or off at run-time;
11455: you have to set the @code{assert-level} appropriately before compiling a
11456: piece of code. You can compile different pieces of code at different
11457: @code{assert-level}s (e.g., a trusted library at level 1 and
11458: newly-written code at level 3).
11459: 
11460: 
11461: doc-assert-level
11462: 
11463: 
11464: If an assertion fails, a message compatible with Emacs' compilation mode
11465: is produced and the execution is aborted (currently with @code{ABORT"}.
11466: If there is interest, we will introduce a special throw code. But if you
11467: intend to @code{catch} a specific condition, using @code{throw} is
11468: probably more appropriate than an assertion).
11469: 
11470: @cindex filenames in assertion output
11471: Assertions (and @code{~~}) will usually print the wrong file name if a
11472: marker is executed in the same file after their occurance.  They will
11473: print @samp{*somewhere*} as file name if a marker is executed in the
11474: same file before their occurance.
11475: 
11476: Definitions in ANS Forth for these assertion words are provided
11477: in @file{compat/assert.fs}.
11478: 
11479: 
11480: @node Singlestep Debugger,  , Assertions, Programming Tools
11481: @subsection Singlestep Debugger
11482: @cindex singlestep Debugger
11483: @cindex debugging Singlestep
11484: 
11485: The singlestep debugger does not work in this release.
11486: 
11487: When you create a new word there's often the need to check whether it
11488: behaves correctly or not. You can do this by typing @code{dbg
11489: badword}. A debug session might look like this:
11490: 
11491: @example
11492: : badword 0 DO i . LOOP ;  ok
11493: 2 dbg badword 
11494: : badword  
11495: Scanning code...
11496: 
11497: Nesting debugger ready!
11498: 
11499: 400D4738  8049BC4 0              -> [ 2 ] 00002 00000 
11500: 400D4740  8049F68 DO             -> [ 0 ] 
11501: 400D4744  804A0C8 i              -> [ 1 ] 00000 
11502: 400D4748 400C5E60 .              -> 0 [ 0 ] 
11503: 400D474C  8049D0C LOOP           -> [ 0 ] 
11504: 400D4744  804A0C8 i              -> [ 1 ] 00001 
11505: 400D4748 400C5E60 .              -> 1 [ 0 ] 
11506: 400D474C  8049D0C LOOP           -> [ 0 ] 
11507: 400D4758  804B384 ;              ->  ok
11508: @end example
11509: 
11510: Each line displayed is one step. You always have to hit return to
11511: execute the next word that is displayed. If you don't want to execute
11512: the next word in a whole, you have to type @kbd{n} for @code{nest}. Here is
11513: an overview what keys are available:
11514: 
11515: @table @i
11516: 
11517: @item @key{RET}
11518: Next; Execute the next word.
11519: 
11520: @item n
11521: Nest; Single step through next word.
11522: 
11523: @item u
11524: Unnest; Stop debugging and execute rest of word. If we got to this word
11525: with nest, continue debugging with the calling word.
11526: 
11527: @item d
11528: Done; Stop debugging and execute rest.
11529: 
11530: @item s
11531: Stop; Abort immediately.
11532: 
11533: @end table
11534: 
11535: Debugging large application with this mechanism is very difficult, because
11536: you have to nest very deeply into the program before the interesting part
11537: begins. This takes a lot of time. 
11538: 
11539: To do it more directly put a @code{BREAK:} command into your source code.
11540: When program execution reaches @code{BREAK:} the single step debugger is
11541: invoked and you have all the features described above.
11542: 
11543: If you have more than one part to debug it is useful to know where the
11544: program has stopped at the moment. You can do this by the 
11545: @code{BREAK" string"} command. This behaves like @code{BREAK:} except that
11546: string is typed out when the ``breakpoint'' is reached.
11547: 
11548: 
11549: doc-dbg
11550: doc-break:
11551: doc-break"
11552: 
11553: 
11554: 
11555: @c -------------------------------------------------------------
11556: @node Assembler and Code Words, Threading Words, Programming Tools, Words
11557: @section Assembler and Code Words
11558: @cindex assembler
11559: @cindex code words
11560: 
11561: @menu
11562: * Code and ;code::              
11563: * Common Assembler::            Assembler Syntax
11564: * Common Disassembler::         
11565: * 386 Assembler::               Deviations and special cases
11566: * Alpha Assembler::             Deviations and special cases
11567: * MIPS assembler::              Deviations and special cases
11568: * Other assemblers::            How to write them
11569: @end menu
11570: 
11571: @node Code and ;code, Common Assembler, Assembler and Code Words, Assembler and Code Words
11572: @subsection @code{Code} and @code{;code}
11573: 
11574: Gforth provides some words for defining primitives (words written in
11575: machine code), and for defining the machine-code equivalent of
11576: @code{DOES>}-based defining words. However, the machine-independent
11577: nature of Gforth poses a few problems: First of all, Gforth runs on
11578: several architectures, so it can provide no standard assembler. What's
11579: worse is that the register allocation not only depends on the processor,
11580: but also on the @code{gcc} version and options used.
11581: 
11582: The words that Gforth offers encapsulate some system dependences (e.g.,
11583: the header structure), so a system-independent assembler may be used in
11584: Gforth. If you do not have an assembler, you can compile machine code
11585: directly with @code{,} and @code{c,}@footnote{This isn't portable,
11586: because these words emit stuff in @i{data} space; it works because
11587: Gforth has unified code/data spaces. Assembler isn't likely to be
11588: portable anyway.}.
11589: 
11590: 
11591: doc-assembler
11592: doc-init-asm
11593: doc-code
11594: doc-end-code
11595: doc-;code
11596: doc-flush-icache
11597: 
11598: 
11599: If @code{flush-icache} does not work correctly, @code{code} words
11600: etc. will not work (reliably), either.
11601: 
11602: The typical usage of these @code{code} words can be shown most easily by
11603: analogy to the equivalent high-level defining words:
11604: 
11605: @example
11606: : foo                              code foo
11607:    <high-level Forth words>              <assembler>
11608: ;                                  end-code
11609:                                 
11610: : bar                              : bar
11611:    <high-level Forth words>           <high-level Forth words>
11612:    CREATE                             CREATE
11613:       <high-level Forth words>           <high-level Forth words>
11614:    DOES>                              ;code
11615:       <high-level Forth words>           <assembler>
11616: ;                                  end-code
11617: @end example
11618: 
11619: @c anton: the following stuff is also in "Common Assembler", in less detail.
11620: 
11621: @cindex registers of the inner interpreter
11622: In the assembly code you will want to refer to the inner interpreter's
11623: registers (e.g., the data stack pointer) and you may want to use other
11624: registers for temporary storage. Unfortunately, the register allocation
11625: is installation-dependent.
11626: 
11627: In particular, @code{ip} (Forth instruction pointer) and @code{rp}
11628: (return stack pointer) may be in different places in @code{gforth} and
11629: @code{gforth-fast}, or different installations.  This means that you
11630: cannot write a @code{NEXT} routine that works reliably on both versions
11631: or different installations; so for doing @code{NEXT}, I recommend
11632: jumping to @code{' noop >code-address}, which contains nothing but a
11633: @code{NEXT}.
11634: 
11635: For general accesses to the inner interpreter's registers, the easiest
11636: solution is to use explicit register declarations (@pxref{Explicit Reg
11637: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) for
11638: all of the inner interpreter's registers: You have to compile Gforth
11639: with @code{-DFORCE_REG} (configure option @code{--enable-force-reg}) and
11640: the appropriate declarations must be present in the @code{machine.h}
11641: file (see @code{mips.h} for an example; you can find a full list of all
11642: declarable register symbols with @code{grep register engine.c}). If you
11643: give explicit registers to all variables that are declared at the
11644: beginning of @code{engine()}, you should be able to use the other
11645: caller-saved registers for temporary storage. Alternatively, you can use
11646: the @code{gcc} option @code{-ffixed-REG} (@pxref{Code Gen Options, ,
11647: Options for Code Generation Conventions, gcc.info, GNU C Manual}) to
11648: reserve a register (however, this restriction on register allocation may
11649: slow Gforth significantly).
11650: 
11651: If this solution is not viable (e.g., because @code{gcc} does not allow
11652: you to explicitly declare all the registers you need), you have to find
11653: out by looking at the code where the inner interpreter's registers
11654: reside and which registers can be used for temporary storage. You can
11655: get an assembly listing of the engine's code with @code{make engine.s}.
11656: 
11657: In any case, it is good practice to abstract your assembly code from the
11658: actual register allocation. E.g., if the data stack pointer resides in
11659: register @code{$17}, create an alias for this register called @code{sp},
11660: and use that in your assembly code.
11661: 
11662: @cindex code words, portable
11663: Another option for implementing normal and defining words efficiently
11664: is to add the desired functionality to the source of Gforth. For normal
11665: words you just have to edit @file{primitives} (@pxref{Automatic
11666: Generation}). Defining words (equivalent to @code{;CODE} words, for fast
11667: defined words) may require changes in @file{engine.c}, @file{kernel.fs},
11668: @file{prims2x.fs}, and possibly @file{cross.fs}.
11669: 
11670: @node Common Assembler, Common Disassembler, Code and ;code, Assembler and Code Words
11671: @subsection Common Assembler
11672: 
11673: The assemblers in Gforth generally use a postfix syntax, i.e., the
11674: instruction name follows the operands.
11675: 
11676: The operands are passed in the usual order (the same that is used in the
11677: manual of the architecture).  Since they all are Forth words, they have
11678: to be separated by spaces; you can also use Forth words to compute the
11679: operands.
11680: 
11681: The instruction names usually end with a @code{,}.  This makes it easier
11682: to visually separate instructions if you put several of them on one
11683: line; it also avoids shadowing other Forth words (e.g., @code{and}).
11684: 
11685: Registers are usually specified by number; e.g., (decimal) @code{11}
11686: specifies registers R11 and F11 on the Alpha architecture (which one,
11687: depends on the instruction).  The usual names are also available, e.g.,
11688: @code{s2} for R11 on Alpha.
11689: 
11690: Control flow is specified similar to normal Forth code (@pxref{Arbitrary
11691: control structures}), with @code{if,}, @code{ahead,}, @code{then,},
11692: @code{begin,}, @code{until,}, @code{again,}, @code{cs-roll},
11693: @code{cs-pick}, @code{else,}, @code{while,}, and @code{repeat,}.  The
11694: conditions are specified in a way specific to each assembler.
11695: 
11696: Note that the register assignments of the Gforth engine can change
11697: between Gforth versions, or even between different compilations of the
11698: same Gforth version (e.g., if you use a different GCC version).  So if
11699: you want to refer to Gforth's registers (e.g., the stack pointer or
11700: TOS), I recommend defining your own words for refering to these
11701: registers, and using them later on; then you can easily adapt to a
11702: changed register assignment.  The stability of the register assignment
11703: is usually better if you build Gforth with @code{--enable-force-reg}.
11704: 
11705: The most common use of these registers is to dispatch to the next word
11706: (the @code{next} routine).  A portable way to do this is to jump to
11707: @code{' noop >code-address} (of course, this is less efficient than
11708: integrating the @code{next} code and scheduling it well).
11709: 
11710: Another difference between Gforth version is that the top of stack is
11711: kept in memory in @code{gforth} and, on most platforms, in a register in
11712: @code{gforth-fast}.
11713: 
11714: @node  Common Disassembler, 386 Assembler, Common Assembler, Assembler and Code Words
11715: @subsection Common Disassembler
11716: 
11717: You can disassemble a @code{code} word with @code{see}
11718: (@pxref{Debugging}).  You can disassemble a section of memory with
11719: 
11720: doc-disasm
11721: 
11722: The disassembler generally produces output that can be fed into the
11723: assembler (i.e., same syntax, etc.).  It also includes additional
11724: information in comments.  In particular, the address of the instruction
11725: is given in a comment before the instruction.
11726: 
11727: @code{See} may display more or less than the actual code of the word,
11728: because the recognition of the end of the code is unreliable.  You can
11729: use @code{disasm} if it did not display enough.  It may display more, if
11730: the code word is not immediately followed by a named word.  If you have
11731: something else there, you can follow the word with @code{align latest ,}
11732: to ensure that the end is recognized.
11733: 
11734: @node 386 Assembler, Alpha Assembler, Common Disassembler, Assembler and Code Words
11735: @subsection 386 Assembler
11736: 
11737: The 386 assembler included in Gforth was written by Bernd Paysan, it's
11738: available under GPL, and originally part of bigFORTH.
11739: 
11740: The 386 disassembler included in Gforth was written by Andrew McKewan
11741: and is in the public domain.
11742: 
11743: The disassembler displays code in an Intel-like prefix syntax.
11744: 
11745: The assembler uses a postfix syntax with reversed parameters.
11746: 
11747: The assembler includes all instruction of the Athlon, i.e. 486 core
11748: instructions, Pentium and PPro extensions, floating point, MMX, 3Dnow!,
11749: but not ISSE. It's an integrated 16- and 32-bit assembler. Default is 32
11750: bit, you can switch to 16 bit with .86 and back to 32 bit with .386.
11751: 
11752: There are several prefixes to switch between different operation sizes,
11753: @code{.b} for byte accesses, @code{.w} for word accesses, @code{.d} for
11754: double-word accesses. Addressing modes can be switched with @code{.wa}
11755: for 16 bit addresses, and @code{.da} for 32 bit addresses. You don't
11756: need a prefix for byte register names (@code{AL} et al).
11757: 
11758: For floating point operations, the prefixes are @code{.fs} (IEEE
11759: single), @code{.fl} (IEEE double), @code{.fx} (extended), @code{.fw}
11760: (word), @code{.fd} (double-word), and @code{.fq} (quad-word).
11761: 
11762: The MMX opcodes don't have size prefixes, they are spelled out like in
11763: the Intel assembler. Instead of move from and to memory, there are
11764: PLDQ/PLDD and PSTQ/PSTD.
11765: 
11766: The registers lack the 'e' prefix; even in 32 bit mode, eax is called
11767: ax.  Immediate values are indicated by postfixing them with @code{#},
11768: e.g., @code{3 #}.  Here are some examples of addressing modes in various
11769: syntaxes:
11770: 
11771: @example
11772: Gforth          Intel (NASM)   AT&T (gas)      Name
11773: .w ax           ax             %ax             register (16 bit)
11774: ax              eax            %eax            register (32 bit)
11775: 3 #             offset 3       $3              immediate
11776: 1000 #)         byte ptr 1000  1000            displacement
11777: bx )            [ebx]          (%ebx)          base
11778: 100 di d)       100[edi]       100(%edi)       base+displacement
11779: 20 ax *4 i#)    20[eax*4]      20(,%eax,4)     (index*scale)+displacement
11780: di ax *4 i)     [edi][eax*4]   (%edi,%eax,4)   base+(index*scale)
11781: 4 bx cx di)     4[ebx][ecx]    4(%ebx,%ecx)    base+index+displacement
11782: 12 sp ax *2 di) 12[esp][eax*2] 12(%esp,%eax,2) base+(index*scale)+displacement
11783: @end example
11784: 
11785: You can use @code{L)} and @code{LI)} instead of @code{D)} and
11786: @code{DI)} to enforce 32-bit displacement fields (useful for
11787: later patching).
11788: 
11789: Some example of instructions are:
11790: 
11791: @example
11792: ax bx mov             \ move ebx,eax
11793: 3 # ax mov            \ mov eax,3
11794: 100 di ) ax mov       \ mov eax,100[edi]
11795: 4 bx cx di) ax mov    \ mov eax,4[ebx][ecx]
11796: .w ax bx mov          \ mov bx,ax
11797: @end example
11798: 
11799: The following forms are supported for binary instructions:
11800: 
11801: @example
11802: <reg> <reg> <inst>
11803: <n> # <reg> <inst>
11804: <mem> <reg> <inst>
11805: <reg> <mem> <inst>
11806: @end example
11807: 
11808: Immediate to memory is not supported.  The shift/rotate syntax is:
11809: 
11810: @example
11811: <reg/mem> 1 # shl \ shortens to shift without immediate
11812: <reg/mem> 4 # shl
11813: <reg/mem> cl shl
11814: @end example
11815: 
11816: Precede string instructions (@code{movs} etc.) with @code{.b} to get
11817: the byte version.
11818: 
11819: The control structure words @code{IF} @code{UNTIL} etc. must be preceded
11820: by one of these conditions: @code{vs vc u< u>= 0= 0<> u<= u> 0< 0>= ps
11821: pc < >= <= >}. (Note that most of these words shadow some Forth words
11822: when @code{assembler} is in front of @code{forth} in the search path,
11823: e.g., in @code{code} words).  Currently the control structure words use
11824: one stack item, so you have to use @code{roll} instead of @code{cs-roll}
11825: to shuffle them (you can also use @code{swap} etc.).
11826: 
11827: Here is an example of a @code{code} word (assumes that the stack pointer
11828: is in esi and the TOS is in ebx):
11829: 
11830: @example
11831: code my+ ( n1 n2 -- n )
11832:     4 si D) bx add
11833:     4 # si add
11834:     Next
11835: end-code
11836: @end example
11837: 
11838: @node Alpha Assembler, MIPS assembler, 386 Assembler, Assembler and Code Words
11839: @subsection Alpha Assembler
11840: 
11841: The Alpha assembler and disassembler were originally written by Bernd
11842: Thallner.
11843: 
11844: The register names @code{a0}--@code{a5} are not available to avoid
11845: shadowing hex numbers.
11846: 
11847: Immediate forms of arithmetic instructions are distinguished by a
11848: @code{#} just before the @code{,}, e.g., @code{and#,} (note: @code{lda,}
11849: does not count as arithmetic instruction).
11850: 
11851: You have to specify all operands to an instruction, even those that
11852: other assemblers consider optional, e.g., the destination register for
11853: @code{br,}, or the destination register and hint for @code{jmp,}.
11854: 
11855: You can specify conditions for @code{if,} by removing the first @code{b}
11856: and the trailing @code{,} from a branch with a corresponding name; e.g.,
11857: 
11858: @example
11859: 11 fgt if, \ if F11>0e
11860:   ...
11861: endif,
11862: @end example
11863: 
11864: @code{fbgt,} gives @code{fgt}.  
11865: 
11866: @node MIPS assembler, Other assemblers, Alpha Assembler, Assembler and Code Words
11867: @subsection MIPS assembler
11868: 
11869: The MIPS assembler was originally written by Christian Pirker.
11870: 
11871: Currently the assembler and disassembler only cover the MIPS-I
11872: architecture (R3000), and don't support FP instructions.
11873: 
11874: The register names @code{$a0}--@code{$a3} are not available to avoid
11875: shadowing hex numbers.
11876: 
11877: Because there is no way to distinguish registers from immediate values,
11878: you have to explicitly use the immediate forms of instructions, i.e.,
11879: @code{addiu,}, not just @code{addu,} (@command{as} does this
11880: implicitly).
11881: 
11882: If the architecture manual specifies several formats for the instruction
11883: (e.g., for @code{jalr,}), you usually have to use the one with more
11884: arguments (i.e., two for @code{jalr,}).  When in doubt, see
11885: @code{arch/mips/testasm.fs} for an example of correct use.
11886: 
11887: Branches and jumps in the MIPS architecture have a delay slot.  You have
11888: to fill it yourself (the simplest way is to use @code{nop,}), the
11889: assembler does not do it for you (unlike @command{as}).  Even
11890: @code{if,}, @code{ahead,}, @code{until,}, @code{again,}, @code{while,},
11891: @code{else,} and @code{repeat,} need a delay slot.  Since @code{begin,}
11892: and @code{then,} just specify branch targets, they are not affected.
11893: 
11894: Note that you must not put branches, jumps, or @code{li,} into the delay
11895: slot: @code{li,} may expand to several instructions, and control flow
11896: instructions may not be put into the branch delay slot in any case.
11897: 
11898: For branches the argument specifying the target is a relative address;
11899: You have to add the address of the delay slot to get the absolute
11900: address.
11901: 
11902: The MIPS architecture also has load delay slots and restrictions on
11903: using @code{mfhi,} and @code{mflo,}; you have to order the instructions
11904: yourself to satisfy these restrictions, the assembler does not do it for
11905: you.
11906: 
11907: You can specify the conditions for @code{if,} etc. by taking a
11908: conditional branch and leaving away the @code{b} at the start and the
11909: @code{,} at the end.  E.g.,
11910: 
11911: @example
11912: 4 5 eq if,
11913:   ... \ do something if $4 equals $5
11914: then,
11915: @end example
11916: 
11917: @node Other assemblers,  , MIPS assembler, Assembler and Code Words
11918: @subsection Other assemblers
11919: 
11920: If you want to contribute another assembler/disassembler, please contact
11921: us (@email{anton@@mips.complang.tuwien.ac.at}) to check if we have such
11922: an assembler already.  If you are writing them from scratch, please use
11923: a similar syntax style as the one we use (i.e., postfix, commas at the
11924: end of the instruction names, @pxref{Common Assembler}); make the output
11925: of the disassembler be valid input for the assembler, and keep the style
11926: similar to the style we used.
11927: 
11928: Hints on implementation: The most important part is to have a good test
11929: suite that contains all instructions.  Once you have that, the rest is
11930: easy.  For actual coding you can take a look at
11931: @file{arch/mips/disasm.fs} to get some ideas on how to use data for both
11932: the assembler and disassembler, avoiding redundancy and some potential
11933: bugs.  You can also look at that file (and @pxref{Advanced does> usage
11934: example}) to get ideas how to factor a disassembler.
11935: 
11936: Start with the disassembler, because it's easier to reuse data from the
11937: disassembler for the assembler than the other way round.
11938: 
11939: For the assembler, take a look at @file{arch/alpha/asm.fs}, which shows
11940: how simple it can be.
11941: 
11942: @c -------------------------------------------------------------
11943: @node Threading Words, Passing Commands to the OS, Assembler and Code Words, Words
11944: @section Threading Words
11945: @cindex threading words
11946: 
11947: @cindex code address
11948: These words provide access to code addresses and other threading stuff
11949: in Gforth (and, possibly, other interpretive Forths). It more or less
11950: abstracts away the differences between direct and indirect threading
11951: (and, for direct threading, the machine dependences). However, at
11952: present this wordset is still incomplete. It is also pretty low-level;
11953: some day it will hopefully be made unnecessary by an internals wordset
11954: that abstracts implementation details away completely.
11955: 
11956: The terminology used here stems from indirect threaded Forth systems; in
11957: such a system, the XT of a word is represented by the CFA (code field
11958: address) of a word; the CFA points to a cell that contains the code
11959: address.  The code address is the address of some machine code that
11960: performs the run-time action of invoking the word (e.g., the
11961: @code{dovar:} routine pushes the address of the body of the word (a
11962: variable) on the stack
11963: ).
11964: 
11965: @cindex code address
11966: @cindex code field address
11967: In an indirect threaded Forth, you can get the code address of @i{name}
11968: with @code{' @i{name} @@}; in Gforth you can get it with @code{' @i{name}
11969: >code-address}, independent of the threading method.
11970: 
11971: doc-threading-method
11972: doc->code-address
11973: doc-code-address!
11974: 
11975: @cindex @code{does>}-handler
11976: @cindex @code{does>}-code
11977: For a word defined with @code{DOES>}, the code address usually points to
11978: a jump instruction (the @dfn{does-handler}) that jumps to the dodoes
11979: routine (in Gforth on some platforms, it can also point to the dodoes
11980: routine itself).  What you are typically interested in, though, is
11981: whether a word is a @code{DOES>}-defined word, and what Forth code it
11982: executes; @code{>does-code} tells you that.
11983: 
11984: doc->does-code
11985: 
11986: To create a @code{DOES>}-defined word with the following basic words,
11987: you have to set up a @code{DOES>}-handler with @code{does-handler!};
11988: @code{/does-handler} aus behind you have to place your executable Forth
11989: code.  Finally you have to create a word and modify its behaviour with
11990: @code{does-handler!}.
11991: 
11992: doc-does-code!
11993: doc-does-handler!
11994: doc-/does-handler
11995: 
11996: The code addresses produced by various defining words are produced by
11997: the following words:
11998: 
11999: doc-docol:
12000: doc-docon:
12001: doc-dovar:
12002: doc-douser:
12003: doc-dodefer:
12004: doc-dofield:
12005: 
12006: @cindex definer
12007: The following two words generalize @code{>code-address},
12008: @code{>does-code}, @code{code-address!}, and @code{does-code!}:
12009: 
12010: doc->definer
12011: doc-definer!
12012: 
12013: @c -------------------------------------------------------------
12014: @node Passing Commands to the OS, Keeping track of Time, Threading Words, Words
12015: @section Passing Commands to the Operating System
12016: @cindex operating system - passing commands
12017: @cindex shell commands
12018: 
12019: Gforth allows you to pass an arbitrary string to the host operating
12020: system shell (if such a thing exists) for execution.
12021: 
12022: 
12023: doc-sh
12024: doc-system
12025: doc-$?
12026: doc-getenv
12027: 
12028: 
12029: @c -------------------------------------------------------------
12030: @node Keeping track of Time, Miscellaneous Words, Passing Commands to the OS, Words
12031: @section Keeping track of Time
12032: @cindex time-related words
12033: 
12034: doc-ms
12035: doc-time&date
12036: doc-utime
12037: doc-cputime
12038: 
12039: 
12040: @c -------------------------------------------------------------
12041: @node Miscellaneous Words,  , Keeping track of Time, Words
12042: @section Miscellaneous Words
12043: @cindex miscellaneous words
12044: 
12045: @comment TODO find homes for these
12046: 
12047: These section lists the ANS Forth words that are not documented
12048: elsewhere in this manual. Ultimately, they all need proper homes.
12049: 
12050: doc-quit
12051: 
12052: The following ANS Forth words are not currently supported by Gforth 
12053: (@pxref{ANS conformance}):
12054: 
12055: @code{EDITOR} 
12056: @code{EMIT?} 
12057: @code{FORGET} 
12058: 
12059: @c ******************************************************************
12060: @node Error messages, Tools, Words, Top
12061: @chapter Error messages
12062: @cindex error messages
12063: @cindex backtrace
12064: 
12065: A typical Gforth error message looks like this:
12066: 
12067: @example
12068: in file included from \evaluated string/:-1
12069: in file included from ./yyy.fs:1
12070: ./xxx.fs:4: Invalid memory address
12071: bar
12072: ^^^
12073: Backtrace:
12074: $400E664C @@
12075: $400E6664 foo
12076: @end example
12077: 
12078: The message identifying the error is @code{Invalid memory address}.  The
12079: error happened when text-interpreting line 4 of the file
12080: @file{./xxx.fs}. This line is given (it contains @code{bar}), and the
12081: word on the line where the error happened, is pointed out (with
12082: @code{^^^}).
12083: 
12084: The file containing the error was included in line 1 of @file{./yyy.fs},
12085: and @file{yyy.fs} was included from a non-file (in this case, by giving
12086: @file{yyy.fs} as command-line parameter to Gforth).
12087: 
12088: At the end of the error message you find a return stack dump that can be
12089: interpreted as a backtrace (possibly empty). On top you find the top of
12090: the return stack when the @code{throw} happened, and at the bottom you
12091: find the return stack entry just above the return stack of the topmost
12092: text interpreter.
12093: 
12094: To the right of most return stack entries you see a guess for the word
12095: that pushed that return stack entry as its return address. This gives a
12096: backtrace. In our case we see that @code{bar} called @code{foo}, and
12097: @code{foo} called @code{@@} (and @code{@@} had an @emph{Invalid memory
12098: address} exception).
12099: 
12100: Note that the backtrace is not perfect: We don't know which return stack
12101: entries are return addresses (so we may get false positives); and in
12102: some cases (e.g., for @code{abort"}) we cannot determine from the return
12103: address the word that pushed the return address, so for some return
12104: addresses you see no names in the return stack dump.
12105: 
12106: @cindex @code{catch} and backtraces
12107: The return stack dump represents the return stack at the time when a
12108: specific @code{throw} was executed.  In programs that make use of
12109: @code{catch}, it is not necessarily clear which @code{throw} should be
12110: used for the return stack dump (e.g., consider one @code{throw} that
12111: indicates an error, which is caught, and during recovery another error
12112: happens; which @code{throw} should be used for the stack dump?).  Gforth
12113: presents the return stack dump for the first @code{throw} after the last
12114: executed (not returned-to) @code{catch}; this works well in the usual
12115: case.
12116: 
12117: @cindex @code{gforth-fast} and backtraces
12118: @cindex @code{gforth-fast}, difference from @code{gforth}
12119: @cindex backtraces with @code{gforth-fast}
12120: @cindex return stack dump with @code{gforth-fast}
12121: @code{Gforth} is able to do a return stack dump for throws generated
12122: from primitives (e.g., invalid memory address, stack empty etc.);
12123: @code{gforth-fast} is only able to do a return stack dump from a
12124: directly called @code{throw} (including @code{abort} etc.).  Given an
12125: exception caused by a primitive in @code{gforth-fast}, you will
12126: typically see no return stack dump at all; however, if the exception is
12127: caught by @code{catch} (e.g., for restoring some state), and then
12128: @code{throw}n again, the return stack dump will be for the first such
12129: @code{throw}.
12130: 
12131: @c ******************************************************************
12132: @node Tools, ANS conformance, Error messages, Top
12133: @chapter Tools
12134: 
12135: @menu
12136: * ANS Report::                  Report the words used, sorted by wordset.
12137: @end menu
12138: 
12139: See also @ref{Emacs and Gforth}.
12140: 
12141: @node ANS Report,  , Tools, Tools
12142: @section @file{ans-report.fs}: Report the words used, sorted by wordset
12143: @cindex @file{ans-report.fs}
12144: @cindex report the words used in your program
12145: @cindex words used in your program
12146: 
12147: If you want to label a Forth program as ANS Forth Program, you must
12148: document which wordsets the program uses; for extension wordsets, it is
12149: helpful to list the words the program requires from these wordsets
12150: (because Forth systems are allowed to provide only some words of them).
12151: 
12152: The @file{ans-report.fs} tool makes it easy for you to determine which
12153: words from which wordset and which non-ANS words your application
12154: uses. You simply have to include @file{ans-report.fs} before loading the
12155: program you want to check. After loading your program, you can get the
12156: report with @code{print-ans-report}. A typical use is to run this as
12157: batch job like this:
12158: @example
12159: gforth ans-report.fs myprog.fs -e "print-ans-report bye"
12160: @end example
12161: 
12162: The output looks like this (for @file{compat/control.fs}):
12163: @example
12164: The program uses the following words
12165: from CORE :
12166: : POSTPONE THEN ; immediate ?dup IF 0= 
12167: from BLOCK-EXT :
12168: \ 
12169: from FILE :
12170: ( 
12171: @end example
12172: 
12173: @subsection Caveats
12174: 
12175: Note that @file{ans-report.fs} just checks which words are used, not whether
12176: they are used in an ANS Forth conforming way!
12177: 
12178: Some words are defined in several wordsets in the
12179: standard. @file{ans-report.fs} reports them for only one of the
12180: wordsets, and not necessarily the one you expect. It depends on usage
12181: which wordset is the right one to specify. E.g., if you only use the
12182: compilation semantics of @code{S"}, it is a Core word; if you also use
12183: its interpretation semantics, it is a File word.
12184: 
12185: 
12186: @node Stack depth changes
12187: @section Stack depth changes during interpretation
12188: @cindex @file{depth-changes.fs}
12189: @cindex depth changes during interpretation
12190: @cindex stack depth changes during interpretation
12191: @cindex items on the stack after interpretation
12192: 
12193: Sometimes you notice that, after loading a file, there are items left
12194: on the stack.  The tool @file{depth-changes.fs} helps you find out
12195: quickly where in the file these stack items are coming from.
12196: 
12197: The simplest way of using @file{depth-changes.fs} is to include it
12198: before the file(s) you want to check, e.g.:
12199: 
12200: @example
12201: gforth depth-changes.fs my-file.fs
12202: @end example
12203: 
12204: This will compare the stack depths of the data and FP stack at every
12205: empty line (in interpretation state) against these depths at the last
12206: empty line (in interpretation state).  If the depths are not equal,
12207: the position in the file and the stack contents are printed with
12208: @code{~~} (@pxref{Debugging}).  This indicates that a stack depth
12209: change has occured in the paragraph of non-empty lines before the
12210: indicated line.  It is a good idea to leave an empty line at the end
12211: of the file, so the last paragraph is checked, too.
12212: 
12213: Checking only at empty lines usually works well, but sometimes you
12214: have big blocks of non-empty lines (e.g., when building a big table),
12215: and you want to know where in this block the stack depth changed.  You
12216: can check all interpreted lines with
12217: 
12218: @example
12219: gforth depth-changes.fs -e "' all-lines is depth-changes-filter" my-file.fs
12220: @end example
12221: 
12222: This checks the stack depth at every end-of-line.  So the depth change
12223: occured in the line reported by the @code{~~} (not in the line
12224: before).
12225: 
12226: Note that, while this offers better accuracy in indicating where the
12227: stack depth changes, it will often report many intentional stack depth
12228: changes (e.g., when an interpreted computation stretches across
12229: several lines).  You can suppress the checking of some lines by
12230: putting backslashes at the end of these lines (not followed by white
12231: space), and using
12232: 
12233: @example
12234: gforth depth-changes.fs -e "' most-lines is depth-changes-filter" my-file.fs
12235: @end example
12236: 
12237: @c ******************************************************************
12238: @node ANS conformance, Standard vs Extensions, Tools, Top
12239: @chapter ANS conformance
12240: @cindex ANS conformance of Gforth
12241: 
12242: To the best of our knowledge, Gforth is an
12243: 
12244: ANS Forth System
12245: @itemize @bullet
12246: @item providing the Core Extensions word set
12247: @item providing the Block word set
12248: @item providing the Block Extensions word set
12249: @item providing the Double-Number word set
12250: @item providing the Double-Number Extensions word set
12251: @item providing the Exception word set
12252: @item providing the Exception Extensions word set
12253: @item providing the Facility word set
12254: @item providing @code{EKEY}, @code{EKEY>CHAR}, @code{EKEY?}, @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
12255: @item providing the File Access word set
12256: @item providing the File Access Extensions word set
12257: @item providing the Floating-Point word set
12258: @item providing the Floating-Point Extensions word set
12259: @item providing the Locals word set
12260: @item providing the Locals Extensions word set
12261: @item providing the Memory-Allocation word set
12262: @item providing the Memory-Allocation Extensions word set (that one's easy)
12263: @item providing the Programming-Tools word set
12264: @item providing @code{;CODE}, @code{AHEAD}, @code{ASSEMBLER}, @code{BYE}, @code{CODE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set
12265: @item providing the Search-Order word set
12266: @item providing the Search-Order Extensions word set
12267: @item providing the String word set
12268: @item providing the String Extensions word set (another easy one)
12269: @end itemize
12270: 
12271: Gforth has the following environmental restrictions:
12272: 
12273: @cindex environmental restrictions
12274: @itemize @bullet
12275: @item
12276: While processing the OS command line, if an exception is not caught,
12277: Gforth exits with a non-zero exit code instyead of performing QUIT.
12278: 
12279: @item
12280: When an @code{throw} is performed after a @code{query}, Gforth does not
12281: allways restore the input source specification in effect at the
12282: corresponding catch.
12283: 
12284: @end itemize
12285: 
12286: 
12287: @cindex system documentation
12288: In addition, ANS Forth systems are required to document certain
12289: implementation choices. This chapter tries to meet these
12290: requirements. In many cases it gives a way to ask the system for the
12291: information instead of providing the information directly, in
12292: particular, if the information depends on the processor, the operating
12293: system or the installation options chosen, or if they are likely to
12294: change during the maintenance of Gforth.
12295: 
12296: @comment The framework for the rest has been taken from pfe.
12297: 
12298: @menu
12299: * The Core Words::              
12300: * The optional Block word set::  
12301: * The optional Double Number word set::  
12302: * The optional Exception word set::  
12303: * The optional Facility word set::  
12304: * The optional File-Access word set::  
12305: * The optional Floating-Point word set::  
12306: * The optional Locals word set::  
12307: * The optional Memory-Allocation word set::  
12308: * The optional Programming-Tools word set::  
12309: * The optional Search-Order word set::  
12310: @end menu
12311: 
12312: 
12313: @c =====================================================================
12314: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
12315: @comment  node-name,  next,  previous,  up
12316: @section The Core Words
12317: @c =====================================================================
12318: @cindex core words, system documentation
12319: @cindex system documentation, core words
12320: 
12321: @menu
12322: * core-idef::                   Implementation Defined Options                   
12323: * core-ambcond::                Ambiguous Conditions                
12324: * core-other::                  Other System Documentation                  
12325: @end menu
12326: 
12327: @c ---------------------------------------------------------------------
12328: @node core-idef, core-ambcond, The Core Words, The Core Words
12329: @subsection Implementation Defined Options
12330: @c ---------------------------------------------------------------------
12331: @cindex core words, implementation-defined options
12332: @cindex implementation-defined options, core words
12333: 
12334: 
12335: @table @i
12336: @item (Cell) aligned addresses:
12337: @cindex cell-aligned addresses
12338: @cindex aligned addresses
12339: processor-dependent. Gforth's alignment words perform natural alignment
12340: (e.g., an address aligned for a datum of size 8 is divisible by
12341: 8). Unaligned accesses usually result in a @code{-23 THROW}.
12342: 
12343: @item @code{EMIT} and non-graphic characters:
12344: @cindex @code{EMIT} and non-graphic characters
12345: @cindex non-graphic characters and @code{EMIT}
12346: The character is output using the C library function (actually, macro)
12347: @code{putc}.
12348: 
12349: @item character editing of @code{ACCEPT} and @code{EXPECT}:
12350: @cindex character editing of @code{ACCEPT} and @code{EXPECT}
12351: @cindex editing in @code{ACCEPT} and @code{EXPECT}
12352: @cindex @code{ACCEPT}, editing
12353: @cindex @code{EXPECT}, editing
12354: This is modeled on the GNU readline library (@pxref{Readline
12355: Interaction, , Command Line Editing, readline, The GNU Readline
12356: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
12357: producing a full word completion every time you type it (instead of
12358: producing the common prefix of all completions). @xref{Command-line editing}.
12359: 
12360: @item character set:
12361: @cindex character set
12362: The character set of your computer and display device. Gforth is
12363: 8-bit-clean (but some other component in your system may make trouble).
12364: 
12365: @item Character-aligned address requirements:
12366: @cindex character-aligned address requirements
12367: installation-dependent. Currently a character is represented by a C
12368: @code{unsigned char}; in the future we might switch to @code{wchar_t}
12369: (Comments on that requested).
12370: 
12371: @item character-set extensions and matching of names:
12372: @cindex character-set extensions and matching of names
12373: @cindex case-sensitivity for name lookup
12374: @cindex name lookup, case-sensitivity
12375: @cindex locale and case-sensitivity
12376: Any character except the ASCII NUL character can be used in a
12377: name. Matching is case-insensitive (except in @code{TABLE}s). The
12378: matching is performed using the C library function @code{strncasecmp}, whose
12379: function is probably influenced by the locale. E.g., the @code{C} locale
12380: does not know about accents and umlauts, so they are matched
12381: case-sensitively in that locale. For portability reasons it is best to
12382: write programs such that they work in the @code{C} locale. Then one can
12383: use libraries written by a Polish programmer (who might use words
12384: containing ISO Latin-2 encoded characters) and by a French programmer
12385: (ISO Latin-1) in the same program (of course, @code{WORDS} will produce
12386: funny results for some of the words (which ones, depends on the font you
12387: are using)). Also, the locale you prefer may not be available in other
12388: operating systems. Hopefully, Unicode will solve these problems one day.
12389: 
12390: @item conditions under which control characters match a space delimiter:
12391: @cindex space delimiters
12392: @cindex control characters as delimiters
12393: If @code{word} is called with the space character as a delimiter, all
12394: white-space characters (as identified by the C macro @code{isspace()})
12395: are delimiters. @code{Parse}, on the other hand, treats space like other
12396: delimiters.  @code{Parse-word}, which is used by the outer
12397: interpreter (aka text interpreter) by default, treats all white-space
12398: characters as delimiters.
12399: 
12400: @item format of the control-flow stack:
12401: @cindex control-flow stack, format
12402: The data stack is used as control-flow stack. The size of a control-flow
12403: stack item in cells is given by the constant @code{cs-item-size}. At the
12404: time of this writing, an item consists of a (pointer to a) locals list
12405: (third), an address in the code (second), and a tag for identifying the
12406: item (TOS). The following tags are used: @code{defstart},
12407: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
12408: @code{scopestart}.
12409: 
12410: @item conversion of digits > 35
12411: @cindex digits > 35
12412: The characters @code{[\]^_'} are the digits with the decimal value
12413: 36@minus{}41. There is no way to input many of the larger digits.
12414: 
12415: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
12416: @cindex @code{EXPECT}, display after end of input
12417: @cindex @code{ACCEPT}, display after end of input
12418: The cursor is moved to the end of the entered string. If the input is
12419: terminated using the @kbd{Return} key, a space is typed.
12420: 
12421: @item exception abort sequence of @code{ABORT"}:
12422: @cindex exception abort sequence of @code{ABORT"}
12423: @cindex @code{ABORT"}, exception abort sequence
12424: The error string is stored into the variable @code{"error} and a
12425: @code{-2 throw} is performed.
12426: 
12427: @item input line terminator:
12428: @cindex input line terminator
12429: @cindex line terminator on input
12430: @cindex newline character on input
12431: For interactive input, @kbd{C-m} (CR) and @kbd{C-j} (LF) terminate
12432: lines. One of these characters is typically produced when you type the
12433: @kbd{Enter} or @kbd{Return} key.
12434: 
12435: @item maximum size of a counted string:
12436: @cindex maximum size of a counted string
12437: @cindex counted string, maximum size
12438: @code{s" /counted-string" environment? drop .}. Currently 255 characters
12439: on all platforms, but this may change.
12440: 
12441: @item maximum size of a parsed string:
12442: @cindex maximum size of a parsed string
12443: @cindex parsed string, maximum size
12444: Given by the constant @code{/line}. Currently 255 characters.
12445: 
12446: @item maximum size of a definition name, in characters:
12447: @cindex maximum size of a definition name, in characters
12448: @cindex name, maximum length
12449: MAXU/8
12450: 
12451: @item maximum string length for @code{ENVIRONMENT?}, in characters:
12452: @cindex maximum string length for @code{ENVIRONMENT?}, in characters
12453: @cindex @code{ENVIRONMENT?} string length, maximum
12454: MAXU/8
12455: 
12456: @item method of selecting the user input device:
12457: @cindex user input device, method of selecting
12458: The user input device is the standard input. There is currently no way to
12459: change it from within Gforth. However, the input can typically be
12460: redirected in the command line that starts Gforth.
12461: 
12462: @item method of selecting the user output device:
12463: @cindex user output device, method of selecting
12464: @code{EMIT} and @code{TYPE} output to the file-id stored in the value
12465: @code{outfile-id} (@code{stdout} by default). Gforth uses unbuffered
12466: output when the user output device is a terminal, otherwise the output
12467: is buffered.
12468: 
12469: @item methods of dictionary compilation:
12470: What are we expected to document here?
12471: 
12472: @item number of bits in one address unit:
12473: @cindex number of bits in one address unit
12474: @cindex address unit, size in bits
12475: @code{s" address-units-bits" environment? drop .}. 8 in all current
12476: platforms.
12477: 
12478: @item number representation and arithmetic:
12479: @cindex number representation and arithmetic
12480: Processor-dependent. Binary two's complement on all current platforms.
12481: 
12482: @item ranges for integer types:
12483: @cindex ranges for integer types
12484: @cindex integer types, ranges
12485: Installation-dependent. Make environmental queries for @code{MAX-N},
12486: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
12487: unsigned (and positive) types is 0. The lower bound for signed types on
12488: two's complement and one's complement machines machines can be computed
12489: by adding 1 to the upper bound.
12490: 
12491: @item read-only data space regions:
12492: @cindex read-only data space regions
12493: @cindex data-space, read-only regions
12494: The whole Forth data space is writable.
12495: 
12496: @item size of buffer at @code{WORD}:
12497: @cindex size of buffer at @code{WORD}
12498: @cindex @code{WORD} buffer size
12499: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
12500: shared with the pictured numeric output string. If overwriting
12501: @code{PAD} is acceptable, it is as large as the remaining dictionary
12502: space, although only as much can be sensibly used as fits in a counted
12503: string.
12504: 
12505: @item size of one cell in address units:
12506: @cindex cell size
12507: @code{1 cells .}.
12508: 
12509: @item size of one character in address units:
12510: @cindex char size
12511: @code{1 chars .}. 1 on all current platforms.
12512: 
12513: @item size of the keyboard terminal buffer:
12514: @cindex size of the keyboard terminal buffer
12515: @cindex terminal buffer, size
12516: Varies. You can determine the size at a specific time using @code{lp@@
12517: tib - .}. It is shared with the locals stack and TIBs of files that
12518: include the current file. You can change the amount of space for TIBs
12519: and locals stack at Gforth startup with the command line option
12520: @code{-l}.
12521: 
12522: @item size of the pictured numeric output buffer:
12523: @cindex size of the pictured numeric output buffer
12524: @cindex pictured numeric output buffer, size
12525: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
12526: shared with @code{WORD}.
12527: 
12528: @item size of the scratch area returned by @code{PAD}:
12529: @cindex size of the scratch area returned by @code{PAD}
12530: @cindex @code{PAD} size
12531: The remainder of dictionary space. @code{unused pad here - - .}.
12532: 
12533: @item system case-sensitivity characteristics:
12534: @cindex case-sensitivity characteristics
12535: Dictionary searches are case-insensitive (except in
12536: @code{TABLE}s). However, as explained above under @i{character-set
12537: extensions}, the matching for non-ASCII characters is determined by the
12538: locale you are using. In the default @code{C} locale all non-ASCII
12539: characters are matched case-sensitively.
12540: 
12541: @item system prompt:
12542: @cindex system prompt
12543: @cindex prompt
12544: @code{ ok} in interpret state, @code{ compiled} in compile state.
12545: 
12546: @item division rounding:
12547: @cindex division rounding
12548: installation dependent. @code{s" floored" environment? drop .}. We leave
12549: the choice to @code{gcc} (what to use for @code{/}) and to you (whether
12550: to use @code{fm/mod}, @code{sm/rem} or simply @code{/}).
12551: 
12552: @item values of @code{STATE} when true:
12553: @cindex @code{STATE} values
12554: -1.
12555: 
12556: @item values returned after arithmetic overflow:
12557: On two's complement machines, arithmetic is performed modulo
12558: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
12559: arithmetic (with appropriate mapping for signed types). Division by zero
12560: typically results in a @code{-55 throw} (Floating-point unidentified
12561: fault) or @code{-10 throw} (divide by zero).
12562: 
12563: @item whether the current definition can be found after @t{DOES>}:
12564: @cindex @t{DOES>}, visibility of current definition
12565: No.
12566: 
12567: @end table
12568: 
12569: @c ---------------------------------------------------------------------
12570: @node core-ambcond, core-other, core-idef, The Core Words
12571: @subsection Ambiguous conditions
12572: @c ---------------------------------------------------------------------
12573: @cindex core words, ambiguous conditions
12574: @cindex ambiguous conditions, core words
12575: 
12576: @table @i
12577: 
12578: @item a name is neither a word nor a number:
12579: @cindex name not found
12580: @cindex undefined word
12581: @code{-13 throw} (Undefined word).
12582: 
12583: @item a definition name exceeds the maximum length allowed:
12584: @cindex word name too long
12585: @code{-19 throw} (Word name too long)
12586: 
12587: @item addressing a region not inside the various data spaces of the forth system:
12588: @cindex Invalid memory address
12589: The stacks, code space and header space are accessible. Machine code space is
12590: typically readable. Accessing other addresses gives results dependent on
12591: the operating system. On decent systems: @code{-9 throw} (Invalid memory
12592: address).
12593: 
12594: @item argument type incompatible with parameter:
12595: @cindex argument type mismatch
12596: This is usually not caught. Some words perform checks, e.g., the control
12597: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
12598: mismatch).
12599: 
12600: @item attempting to obtain the execution token of a word with undefined execution semantics:
12601: @cindex Interpreting a compile-only word, for @code{'} etc.
12602: @cindex execution token of words with undefined execution semantics
12603: @code{-14 throw} (Interpreting a compile-only word). In some cases, you
12604: get an execution token for @code{compile-only-error} (which performs a
12605: @code{-14 throw} when executed).
12606: 
12607: @item dividing by zero:
12608: @cindex dividing by zero
12609: @cindex floating point unidentified fault, integer division
12610: On some platforms, this produces a @code{-10 throw} (Division by
12611: zero); on other systems, this typically results in a @code{-55 throw}
12612: (Floating-point unidentified fault).
12613: 
12614: @item insufficient data stack or return stack space:
12615: @cindex insufficient data stack or return stack space
12616: @cindex stack overflow
12617: @cindex address alignment exception, stack overflow
12618: @cindex Invalid memory address, stack overflow
12619: Depending on the operating system, the installation, and the invocation
12620: of Gforth, this is either checked by the memory management hardware, or
12621: it is not checked. If it is checked, you typically get a @code{-3 throw}
12622: (Stack overflow), @code{-5 throw} (Return stack overflow), or @code{-9
12623: throw} (Invalid memory address) (depending on the platform and how you
12624: achieved the overflow) as soon as the overflow happens. If it is not
12625: checked, overflows typically result in mysterious illegal memory
12626: accesses, producing @code{-9 throw} (Invalid memory address) or
12627: @code{-23 throw} (Address alignment exception); they might also destroy
12628: the internal data structure of @code{ALLOCATE} and friends, resulting in
12629: various errors in these words.
12630: 
12631: @item insufficient space for loop control parameters:
12632: @cindex insufficient space for loop control parameters
12633: Like other return stack overflows.
12634: 
12635: @item insufficient space in the dictionary:
12636: @cindex insufficient space in the dictionary
12637: @cindex dictionary overflow
12638: If you try to allot (either directly with @code{allot}, or indirectly
12639: with @code{,}, @code{create} etc.) more memory than available in the
12640: dictionary, you get a @code{-8 throw} (Dictionary overflow). If you try
12641: to access memory beyond the end of the dictionary, the results are
12642: similar to stack overflows.
12643: 
12644: @item interpreting a word with undefined interpretation semantics:
12645: @cindex interpreting a word with undefined interpretation semantics
12646: @cindex Interpreting a compile-only word
12647: For some words, we have defined interpretation semantics. For the
12648: others: @code{-14 throw} (Interpreting a compile-only word).
12649: 
12650: @item modifying the contents of the input buffer or a string literal:
12651: @cindex modifying the contents of the input buffer or a string literal
12652: These are located in writable memory and can be modified.
12653: 
12654: @item overflow of the pictured numeric output string:
12655: @cindex overflow of the pictured numeric output string
12656: @cindex pictured numeric output string, overflow
12657: @code{-17 throw} (Pictured numeric ouput string overflow).
12658: 
12659: @item parsed string overflow:
12660: @cindex parsed string overflow
12661: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
12662: 
12663: @item producing a result out of range:
12664: @cindex result out of range
12665: On two's complement machines, arithmetic is performed modulo
12666: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
12667: arithmetic (with appropriate mapping for signed types). Division by zero
12668: typically results in a @code{-10 throw} (divide by zero) or @code{-55
12669: throw} (floating point unidentified fault). @code{convert} and
12670: @code{>number} currently overflow silently.
12671: 
12672: @item reading from an empty data or return stack:
12673: @cindex stack empty
12674: @cindex stack underflow
12675: @cindex return stack underflow
12676: The data stack is checked by the outer (aka text) interpreter after
12677: every word executed. If it has underflowed, a @code{-4 throw} (Stack
12678: underflow) is performed. Apart from that, stacks may be checked or not,
12679: depending on operating system, installation, and invocation. If they are
12680: caught by a check, they typically result in @code{-4 throw} (Stack
12681: underflow), @code{-6 throw} (Return stack underflow) or @code{-9 throw}
12682: (Invalid memory address), depending on the platform and which stack
12683: underflows and by how much. Note that even if the system uses checking
12684: (through the MMU), your program may have to underflow by a significant
12685: number of stack items to trigger the reaction (the reason for this is
12686: that the MMU, and therefore the checking, works with a page-size
12687: granularity).  If there is no checking, the symptoms resulting from an
12688: underflow are similar to those from an overflow.  Unbalanced return
12689: stack errors can result in a variety of symptoms, including @code{-9 throw}
12690: (Invalid memory address) and Illegal Instruction (typically @code{-260
12691: throw}).
12692: 
12693: @item unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
12694: @cindex unexpected end of the input buffer
12695: @cindex zero-length string as a name
12696: @cindex Attempt to use zero-length string as a name
12697: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
12698: use zero-length string as a name). Words like @code{'} probably will not
12699: find what they search. Note that it is possible to create zero-length
12700: names with @code{nextname} (should it not?).
12701: 
12702: @item @code{>IN} greater than input buffer:
12703: @cindex @code{>IN} greater than input buffer
12704: The next invocation of a parsing word returns a string with length 0.
12705: 
12706: @item @code{RECURSE} appears after @code{DOES>}:
12707: @cindex @code{RECURSE} appears after @code{DOES>}
12708: Compiles a recursive call to the defining word, not to the defined word.
12709: 
12710: @item argument input source different than current input source for @code{RESTORE-INPUT}:
12711: @cindex argument input source different than current input source for @code{RESTORE-INPUT}
12712: @cindex argument type mismatch, @code{RESTORE-INPUT}
12713: @cindex @code{RESTORE-INPUT}, Argument type mismatch
12714: @code{-12 THROW}. Note that, once an input file is closed (e.g., because
12715: the end of the file was reached), its source-id may be
12716: reused. Therefore, restoring an input source specification referencing a
12717: closed file may lead to unpredictable results instead of a @code{-12
12718: THROW}.
12719: 
12720: In the future, Gforth may be able to restore input source specifications
12721: from other than the current input source.
12722: 
12723: @item data space containing definitions gets de-allocated:
12724: @cindex data space containing definitions gets de-allocated
12725: Deallocation with @code{allot} is not checked. This typically results in
12726: memory access faults or execution of illegal instructions.
12727: 
12728: @item data space read/write with incorrect alignment:
12729: @cindex data space read/write with incorrect alignment
12730: @cindex alignment faults
12731: @cindex address alignment exception
12732: Processor-dependent. Typically results in a @code{-23 throw} (Address
12733: alignment exception). Under Linux-Intel on a 486 or later processor with
12734: alignment turned on, incorrect alignment results in a @code{-9 throw}
12735: (Invalid memory address). There are reportedly some processors with
12736: alignment restrictions that do not report violations.
12737: 
12738: @item data space pointer not properly aligned, @code{,}, @code{C,}:
12739: @cindex data space pointer not properly aligned, @code{,}, @code{C,}
12740: Like other alignment errors.
12741: 
12742: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
12743: Like other stack underflows.
12744: 
12745: @item loop control parameters not available:
12746: @cindex loop control parameters not available
12747: Not checked. The counted loop words simply assume that the top of return
12748: stack items are loop control parameters and behave accordingly.
12749: 
12750: @item most recent definition does not have a name (@code{IMMEDIATE}):
12751: @cindex most recent definition does not have a name (@code{IMMEDIATE})
12752: @cindex last word was headerless
12753: @code{abort" last word was headerless"}.
12754: 
12755: @item name not defined by @code{VALUE} used by @code{TO}:
12756: @cindex name not defined by @code{VALUE} used by @code{TO}
12757: @cindex @code{TO} on non-@code{VALUE}s
12758: @cindex Invalid name argument, @code{TO}
12759: @code{-32 throw} (Invalid name argument) (unless name is a local or was
12760: defined by @code{CONSTANT}; in the latter case it just changes the constant).
12761: 
12762: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
12763: @cindex name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]})
12764: @cindex undefined word, @code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}
12765: @code{-13 throw} (Undefined word)
12766: 
12767: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
12768: @cindex parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN})
12769: Gforth behaves as if they were of the same type. I.e., you can predict
12770: the behaviour by interpreting all parameters as, e.g., signed.
12771: 
12772: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
12773: @cindex @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}
12774: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} performs the
12775: compilation semantics of @code{TO}.
12776: 
12777: @item String longer than a counted string returned by @code{WORD}:
12778: @cindex string longer than a counted string returned by @code{WORD}
12779: @cindex @code{WORD}, string overflow
12780: Not checked. The string will be ok, but the count will, of course,
12781: contain only the least significant bits of the length.
12782: 
12783: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
12784: @cindex @code{LSHIFT}, large shift counts
12785: @cindex @code{RSHIFT}, large shift counts
12786: Processor-dependent. Typical behaviours are returning 0 and using only
12787: the low bits of the shift count.
12788: 
12789: @item word not defined via @code{CREATE}:
12790: @cindex @code{>BODY} of non-@code{CREATE}d words
12791: @code{>BODY} produces the PFA of the word no matter how it was defined.
12792: 
12793: @cindex @code{DOES>} of non-@code{CREATE}d words
12794: @code{DOES>} changes the execution semantics of the last defined word no
12795: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
12796: @code{CREATE , DOES>}.
12797: 
12798: @item words improperly used outside @code{<#} and @code{#>}:
12799: Not checked. As usual, you can expect memory faults.
12800: 
12801: @end table
12802: 
12803: 
12804: @c ---------------------------------------------------------------------
12805: @node core-other,  , core-ambcond, The Core Words
12806: @subsection Other system documentation
12807: @c ---------------------------------------------------------------------
12808: @cindex other system documentation, core words
12809: @cindex core words, other system documentation
12810: 
12811: @table @i
12812: @item nonstandard words using @code{PAD}:
12813: @cindex @code{PAD} use by nonstandard words
12814: None.
12815: 
12816: @item operator's terminal facilities available:
12817: @cindex operator's terminal facilities available
12818: After processing the OS's command line, Gforth goes into interactive mode,
12819: and you can give commands to Gforth interactively. The actual facilities
12820: available depend on how you invoke Gforth.
12821: 
12822: @item program data space available:
12823: @cindex program data space available
12824: @cindex data space available
12825: @code{UNUSED .} gives the remaining dictionary space. The total
12826: dictionary space can be specified with the @code{-m} switch
12827: (@pxref{Invoking Gforth}) when Gforth starts up.
12828: 
12829: @item return stack space available:
12830: @cindex return stack space available
12831: You can compute the total return stack space in cells with
12832: @code{s" RETURN-STACK-CELLS" environment? drop .}. You can specify it at
12833: startup time with the @code{-r} switch (@pxref{Invoking Gforth}).
12834: 
12835: @item stack space available:
12836: @cindex stack space available
12837: You can compute the total data stack space in cells with
12838: @code{s" STACK-CELLS" environment? drop .}. You can specify it at
12839: startup time with the @code{-d} switch (@pxref{Invoking Gforth}).
12840: 
12841: @item system dictionary space required, in address units:
12842: @cindex system dictionary space required, in address units
12843: Type @code{here forthstart - .} after startup. At the time of this
12844: writing, this gives 80080 (bytes) on a 32-bit system.
12845: @end table
12846: 
12847: 
12848: @c =====================================================================
12849: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
12850: @section The optional Block word set
12851: @c =====================================================================
12852: @cindex system documentation, block words
12853: @cindex block words, system documentation
12854: 
12855: @menu
12856: * block-idef::                  Implementation Defined Options
12857: * block-ambcond::               Ambiguous Conditions               
12858: * block-other::                 Other System Documentation                 
12859: @end menu
12860: 
12861: 
12862: @c ---------------------------------------------------------------------
12863: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
12864: @subsection Implementation Defined Options
12865: @c ---------------------------------------------------------------------
12866: @cindex implementation-defined options, block words
12867: @cindex block words, implementation-defined options
12868: 
12869: @table @i
12870: @item the format for display by @code{LIST}:
12871: @cindex @code{LIST} display format
12872: First the screen number is displayed, then 16 lines of 64 characters,
12873: each line preceded by the line number.
12874: 
12875: @item the length of a line affected by @code{\}:
12876: @cindex length of a line affected by @code{\}
12877: @cindex @code{\}, line length in blocks
12878: 64 characters.
12879: @end table
12880: 
12881: 
12882: @c ---------------------------------------------------------------------
12883: @node block-ambcond, block-other, block-idef, The optional Block word set
12884: @subsection Ambiguous conditions
12885: @c ---------------------------------------------------------------------
12886: @cindex block words, ambiguous conditions
12887: @cindex ambiguous conditions, block words
12888: 
12889: @table @i
12890: @item correct block read was not possible:
12891: @cindex block read not possible
12892: Typically results in a @code{throw} of some OS-derived value (between
12893: -512 and -2048). If the blocks file was just not long enough, blanks are
12894: supplied for the missing portion.
12895: 
12896: @item I/O exception in block transfer:
12897: @cindex I/O exception in block transfer
12898: @cindex block transfer, I/O exception
12899: Typically results in a @code{throw} of some OS-derived value (between
12900: -512 and -2048).
12901: 
12902: @item invalid block number:
12903: @cindex invalid block number
12904: @cindex block number invalid
12905: @code{-35 throw} (Invalid block number)
12906: 
12907: @item a program directly alters the contents of @code{BLK}:
12908: @cindex @code{BLK}, altering @code{BLK}
12909: The input stream is switched to that other block, at the same
12910: position. If the storing to @code{BLK} happens when interpreting
12911: non-block input, the system will get quite confused when the block ends.
12912: 
12913: @item no current block buffer for @code{UPDATE}:
12914: @cindex @code{UPDATE}, no current block buffer
12915: @code{UPDATE} has no effect.
12916: 
12917: @end table
12918: 
12919: @c ---------------------------------------------------------------------
12920: @node block-other,  , block-ambcond, The optional Block word set
12921: @subsection Other system documentation
12922: @c ---------------------------------------------------------------------
12923: @cindex other system documentation, block words
12924: @cindex block words, other system documentation
12925: 
12926: @table @i
12927: @item any restrictions a multiprogramming system places on the use of buffer addresses:
12928: No restrictions (yet).
12929: 
12930: @item the number of blocks available for source and data:
12931: depends on your disk space.
12932: 
12933: @end table
12934: 
12935: 
12936: @c =====================================================================
12937: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
12938: @section The optional Double Number word set
12939: @c =====================================================================
12940: @cindex system documentation, double words
12941: @cindex double words, system documentation
12942: 
12943: @menu
12944: * double-ambcond::              Ambiguous Conditions              
12945: @end menu
12946: 
12947: 
12948: @c ---------------------------------------------------------------------
12949: @node double-ambcond,  , The optional Double Number word set, The optional Double Number word set
12950: @subsection Ambiguous conditions
12951: @c ---------------------------------------------------------------------
12952: @cindex double words, ambiguous conditions
12953: @cindex ambiguous conditions, double words
12954: 
12955: @table @i
12956: @item @i{d} outside of range of @i{n} in @code{D>S}:
12957: @cindex @code{D>S}, @i{d} out of range of @i{n} 
12958: The least significant cell of @i{d} is produced.
12959: 
12960: @end table
12961: 
12962: 
12963: @c =====================================================================
12964: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
12965: @section The optional Exception word set
12966: @c =====================================================================
12967: @cindex system documentation, exception words
12968: @cindex exception words, system documentation
12969: 
12970: @menu
12971: * exception-idef::              Implementation Defined Options              
12972: @end menu
12973: 
12974: 
12975: @c ---------------------------------------------------------------------
12976: @node exception-idef,  , The optional Exception word set, The optional Exception word set
12977: @subsection Implementation Defined Options
12978: @c ---------------------------------------------------------------------
12979: @cindex implementation-defined options, exception words
12980: @cindex exception words, implementation-defined options
12981: 
12982: @table @i
12983: @item @code{THROW}-codes used in the system:
12984: @cindex @code{THROW}-codes used in the system
12985: The codes -256@minus{}-511 are used for reporting signals. The mapping
12986: from OS signal numbers to throw codes is -256@minus{}@i{signal}. The
12987: codes -512@minus{}-2047 are used for OS errors (for file and memory
12988: allocation operations). The mapping from OS error numbers to throw codes
12989: is -512@minus{}@code{errno}. One side effect of this mapping is that
12990: undefined OS errors produce a message with a strange number; e.g.,
12991: @code{-1000 THROW} results in @code{Unknown error 488} on my system.
12992: @end table
12993: 
12994: @c =====================================================================
12995: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
12996: @section The optional Facility word set
12997: @c =====================================================================
12998: @cindex system documentation, facility words
12999: @cindex facility words, system documentation
13000: 
13001: @menu
13002: * facility-idef::               Implementation Defined Options               
13003: * facility-ambcond::            Ambiguous Conditions            
13004: @end menu
13005: 
13006: 
13007: @c ---------------------------------------------------------------------
13008: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
13009: @subsection Implementation Defined Options
13010: @c ---------------------------------------------------------------------
13011: @cindex implementation-defined options, facility words
13012: @cindex facility words, implementation-defined options
13013: 
13014: @table @i
13015: @item encoding of keyboard events (@code{EKEY}):
13016: @cindex keyboard events, encoding in @code{EKEY}
13017: @cindex @code{EKEY}, encoding of keyboard events
13018: Keys corresponding to ASCII characters are encoded as ASCII characters.
13019: Other keys are encoded with the constants @code{k-left}, @code{k-right},
13020: @code{k-up}, @code{k-down}, @code{k-home}, @code{k-end}, @code{k1},
13021: @code{k2}, @code{k3}, @code{k4}, @code{k5}, @code{k6}, @code{k7},
13022: @code{k8}, @code{k9}, @code{k10}, @code{k11}, @code{k12}.
13023: 
13024: 
13025: @item duration of a system clock tick:
13026: @cindex duration of a system clock tick
13027: @cindex clock tick duration
13028: System dependent. With respect to @code{MS}, the time is specified in
13029: microseconds. How well the OS and the hardware implement this, is
13030: another question.
13031: 
13032: @item repeatability to be expected from the execution of @code{MS}:
13033: @cindex repeatability to be expected from the execution of @code{MS}
13034: @cindex @code{MS}, repeatability to be expected
13035: System dependent. On Unix, a lot depends on load. If the system is
13036: lightly loaded, and the delay is short enough that Gforth does not get
13037: swapped out, the performance should be acceptable. Under MS-DOS and
13038: other single-tasking systems, it should be good.
13039: 
13040: @end table
13041: 
13042: 
13043: @c ---------------------------------------------------------------------
13044: @node facility-ambcond,  , facility-idef, The optional Facility word set
13045: @subsection Ambiguous conditions
13046: @c ---------------------------------------------------------------------
13047: @cindex facility words, ambiguous conditions
13048: @cindex ambiguous conditions, facility words
13049: 
13050: @table @i
13051: @item @code{AT-XY} can't be performed on user output device:
13052: @cindex @code{AT-XY} can't be performed on user output device
13053: Largely terminal dependent. No range checks are done on the arguments.
13054: No errors are reported. You may see some garbage appearing, you may see
13055: simply nothing happen.
13056: 
13057: @end table
13058: 
13059: 
13060: @c =====================================================================
13061: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
13062: @section The optional File-Access word set
13063: @c =====================================================================
13064: @cindex system documentation, file words
13065: @cindex file words, system documentation
13066: 
13067: @menu
13068: * file-idef::                   Implementation Defined Options
13069: * file-ambcond::                Ambiguous Conditions                
13070: @end menu
13071: 
13072: @c ---------------------------------------------------------------------
13073: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
13074: @subsection Implementation Defined Options
13075: @c ---------------------------------------------------------------------
13076: @cindex implementation-defined options, file words
13077: @cindex file words, implementation-defined options
13078: 
13079: @table @i
13080: @item file access methods used:
13081: @cindex file access methods used
13082: @code{R/O}, @code{R/W} and @code{BIN} work as you would
13083: expect. @code{W/O} translates into the C file opening mode @code{w} (or
13084: @code{wb}): The file is cleared, if it exists, and created, if it does
13085: not (with both @code{open-file} and @code{create-file}).  Under Unix
13086: @code{create-file} creates a file with 666 permissions modified by your
13087: umask.
13088: 
13089: @item file exceptions:
13090: @cindex file exceptions
13091: The file words do not raise exceptions (except, perhaps, memory access
13092: faults when you pass illegal addresses or file-ids).
13093: 
13094: @item file line terminator:
13095: @cindex file line terminator
13096: System-dependent. Gforth uses C's newline character as line
13097: terminator. What the actual character code(s) of this are is
13098: system-dependent.
13099: 
13100: @item file name format:
13101: @cindex file name format
13102: System dependent. Gforth just uses the file name format of your OS.
13103: 
13104: @item information returned by @code{FILE-STATUS}:
13105: @cindex @code{FILE-STATUS}, returned information
13106: @code{FILE-STATUS} returns the most powerful file access mode allowed
13107: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
13108: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
13109: along with the returned mode.
13110: 
13111: @item input file state after an exception when including source:
13112: @cindex exception when including source
13113: All files that are left via the exception are closed.
13114: 
13115: @item @i{ior} values and meaning:
13116: @cindex @i{ior} values and meaning
13117: @cindex @i{wior} values and meaning
13118: The @i{ior}s returned by the file and memory allocation words are
13119: intended as throw codes. They typically are in the range
13120: -512@minus{}-2047 of OS errors.  The mapping from OS error numbers to
13121: @i{ior}s is -512@minus{}@i{errno}.
13122: 
13123: @item maximum depth of file input nesting:
13124: @cindex maximum depth of file input nesting
13125: @cindex file input nesting, maximum depth
13126: limited by the amount of return stack, locals/TIB stack, and the number
13127: of open files available. This should not give you troubles.
13128: 
13129: @item maximum size of input line:
13130: @cindex maximum size of input line
13131: @cindex input line size, maximum
13132: @code{/line}. Currently 255.
13133: 
13134: @item methods of mapping block ranges to files:
13135: @cindex mapping block ranges to files
13136: @cindex files containing blocks
13137: @cindex blocks in files
13138: By default, blocks are accessed in the file @file{blocks.fb} in the
13139: current working directory. The file can be switched with @code{USE}.
13140: 
13141: @item number of string buffers provided by @code{S"}:
13142: @cindex @code{S"}, number of string buffers
13143: 1
13144: 
13145: @item size of string buffer used by @code{S"}:
13146: @cindex @code{S"}, size of string buffer
13147: @code{/line}. currently 255.
13148: 
13149: @end table
13150: 
13151: @c ---------------------------------------------------------------------
13152: @node file-ambcond,  , file-idef, The optional File-Access word set
13153: @subsection Ambiguous conditions
13154: @c ---------------------------------------------------------------------
13155: @cindex file words, ambiguous conditions
13156: @cindex ambiguous conditions, file words
13157: 
13158: @table @i
13159: @item attempting to position a file outside its boundaries:
13160: @cindex @code{REPOSITION-FILE}, outside the file's boundaries
13161: @code{REPOSITION-FILE} is performed as usual: Afterwards,
13162: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
13163: 
13164: @item attempting to read from file positions not yet written:
13165: @cindex reading from file positions not yet written
13166: End-of-file, i.e., zero characters are read and no error is reported.
13167: 
13168: @item @i{file-id} is invalid (@code{INCLUDE-FILE}):
13169: @cindex @code{INCLUDE-FILE}, @i{file-id} is invalid 
13170: An appropriate exception may be thrown, but a memory fault or other
13171: problem is more probable.
13172: 
13173: @item I/O exception reading or closing @i{file-id} (@code{INCLUDE-FILE}, @code{INCLUDED}):
13174: @cindex @code{INCLUDE-FILE}, I/O exception reading or closing @i{file-id}
13175: @cindex @code{INCLUDED}, I/O exception reading or closing @i{file-id}
13176: The @i{ior} produced by the operation, that discovered the problem, is
13177: thrown.
13178: 
13179: @item named file cannot be opened (@code{INCLUDED}):
13180: @cindex @code{INCLUDED}, named file cannot be opened
13181: The @i{ior} produced by @code{open-file} is thrown.
13182: 
13183: @item requesting an unmapped block number:
13184: @cindex unmapped block numbers
13185: There are no unmapped legal block numbers. On some operating systems,
13186: writing a block with a large number may overflow the file system and
13187: have an error message as consequence.
13188: 
13189: @item using @code{source-id} when @code{blk} is non-zero:
13190: @cindex @code{SOURCE-ID}, behaviour when @code{BLK} is non-zero
13191: @code{source-id} performs its function. Typically it will give the id of
13192: the source which loaded the block. (Better ideas?)
13193: 
13194: @end table
13195: 
13196: 
13197: @c =====================================================================
13198: @node  The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
13199: @section The optional Floating-Point word set
13200: @c =====================================================================
13201: @cindex system documentation, floating-point words
13202: @cindex floating-point words, system documentation
13203: 
13204: @menu
13205: * floating-idef::               Implementation Defined Options
13206: * floating-ambcond::            Ambiguous Conditions            
13207: @end menu
13208: 
13209: 
13210: @c ---------------------------------------------------------------------
13211: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
13212: @subsection Implementation Defined Options
13213: @c ---------------------------------------------------------------------
13214: @cindex implementation-defined options, floating-point words
13215: @cindex floating-point words, implementation-defined options
13216: 
13217: @table @i
13218: @item format and range of floating point numbers:
13219: @cindex format and range of floating point numbers
13220: @cindex floating point numbers, format and range
13221: System-dependent; the @code{double} type of C.
13222: 
13223: @item results of @code{REPRESENT} when @i{float} is out of range:
13224: @cindex  @code{REPRESENT}, results when @i{float} is out of range
13225: System dependent; @code{REPRESENT} is implemented using the C library
13226: function @code{ecvt()} and inherits its behaviour in this respect.
13227: 
13228: @item rounding or truncation of floating-point numbers:
13229: @cindex rounding of floating-point numbers
13230: @cindex truncation of floating-point numbers
13231: @cindex floating-point numbers, rounding or truncation
13232: System dependent; the rounding behaviour is inherited from the hosting C
13233: compiler. IEEE-FP-based (i.e., most) systems by default round to
13234: nearest, and break ties by rounding to even (i.e., such that the last
13235: bit of the mantissa is 0).
13236: 
13237: @item size of floating-point stack:
13238: @cindex floating-point stack size
13239: @code{s" FLOATING-STACK" environment? drop .} gives the total size of
13240: the floating-point stack (in floats). You can specify this on startup
13241: with the command-line option @code{-f} (@pxref{Invoking Gforth}).
13242: 
13243: @item width of floating-point stack:
13244: @cindex floating-point stack width 
13245: @code{1 floats}.
13246: 
13247: @end table
13248: 
13249: 
13250: @c ---------------------------------------------------------------------
13251: @node floating-ambcond,  , floating-idef, The optional Floating-Point word set
13252: @subsection Ambiguous conditions
13253: @c ---------------------------------------------------------------------
13254: @cindex floating-point words, ambiguous conditions
13255: @cindex ambiguous conditions, floating-point words
13256: 
13257: @table @i
13258: @item @code{df@@} or @code{df!} used with an address that is not double-float  aligned:
13259: @cindex @code{df@@} or @code{df!} used with an address that is not double-float  aligned
13260: System-dependent. Typically results in a @code{-23 THROW} like other
13261: alignment violations.
13262: 
13263: @item @code{f@@} or @code{f!} used with an address that is not float  aligned:
13264: @cindex @code{f@@} used with an address that is not float aligned
13265: @cindex @code{f!} used with an address that is not float aligned
13266: System-dependent. Typically results in a @code{-23 THROW} like other
13267: alignment violations.
13268: 
13269: @item floating-point result out of range:
13270: @cindex floating-point result out of range
13271: System-dependent. Can result in a @code{-43 throw} (floating point
13272: overflow), @code{-54 throw} (floating point underflow), @code{-41 throw}
13273: (floating point inexact result), @code{-55 THROW} (Floating-point
13274: unidentified fault), or can produce a special value representing, e.g.,
13275: Infinity.
13276: 
13277: @item @code{sf@@} or @code{sf!} used with an address that is not single-float  aligned:
13278: @cindex @code{sf@@} or @code{sf!} used with an address that is not single-float  aligned
13279: System-dependent. Typically results in an alignment fault like other
13280: alignment violations.
13281: 
13282: @item @code{base} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
13283: @cindex @code{base} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.})
13284: The floating-point number is converted into decimal nonetheless.
13285: 
13286: @item Both arguments are equal to zero (@code{FATAN2}):
13287: @cindex @code{FATAN2}, both arguments are equal to zero
13288: System-dependent. @code{FATAN2} is implemented using the C library
13289: function @code{atan2()}.
13290: 
13291: @item Using @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero:
13292: @cindex @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero
13293: System-dependent. Anyway, typically the cos of @i{r1} will not be zero
13294: because of small errors and the tan will be a very large (or very small)
13295: but finite number.
13296: 
13297: @item @i{d} cannot be presented precisely as a float in @code{D>F}:
13298: @cindex @code{D>F}, @i{d} cannot be presented precisely as a float
13299: The result is rounded to the nearest float.
13300: 
13301: @item dividing by zero:
13302: @cindex dividing by zero, floating-point
13303: @cindex floating-point dividing by zero
13304: @cindex floating-point unidentified fault, FP divide-by-zero
13305: Platform-dependent; can produce an Infinity, NaN, @code{-42 throw}
13306: (floating point divide by zero) or @code{-55 throw} (Floating-point
13307: unidentified fault).
13308: 
13309: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
13310: @cindex exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@})
13311: System dependent. On IEEE-FP based systems the number is converted into
13312: an infinity.
13313: 
13314: @item @i{float}<1 (@code{FACOSH}):
13315: @cindex @code{FACOSH}, @i{float}<1
13316: @cindex floating-point unidentified fault, @code{FACOSH}
13317: Platform-dependent; on IEEE-FP systems typically produces a NaN.
13318: 
13319: @item @i{float}=<-1 (@code{FLNP1}):
13320: @cindex @code{FLNP1}, @i{float}=<-1
13321: @cindex floating-point unidentified fault, @code{FLNP1}
13322: Platform-dependent; on IEEE-FP systems typically produces a NaN (or a
13323: negative infinity for @i{float}=-1).
13324: 
13325: @item @i{float}=<0 (@code{FLN}, @code{FLOG}):
13326: @cindex @code{FLN}, @i{float}=<0
13327: @cindex @code{FLOG}, @i{float}=<0
13328: @cindex floating-point unidentified fault, @code{FLN} or @code{FLOG}
13329: Platform-dependent; on IEEE-FP systems typically produces a NaN (or a
13330: negative infinity for @i{float}=0).
13331: 
13332: @item @i{float}<0 (@code{FASINH}, @code{FSQRT}):
13333: @cindex @code{FASINH}, @i{float}<0
13334: @cindex @code{FSQRT}, @i{float}<0
13335: @cindex floating-point unidentified fault, @code{FASINH} or @code{FSQRT}
13336: Platform-dependent; for @code{fsqrt} this typically gives a NaN, for
13337: @code{fasinh} some platforms produce a NaN, others a number (bug in the
13338: C library?).
13339: 
13340: @item |@i{float}|>1 (@code{FACOS}, @code{FASIN}, @code{FATANH}):
13341: @cindex @code{FACOS}, |@i{float}|>1
13342: @cindex @code{FASIN}, |@i{float}|>1
13343: @cindex @code{FATANH}, |@i{float}|>1
13344: @cindex floating-point unidentified fault, @code{FACOS}, @code{FASIN} or @code{FATANH}
13345: Platform-dependent; IEEE-FP systems typically produce a NaN.
13346: 
13347: @item integer part of float cannot be represented by @i{d} in @code{F>D}:
13348: @cindex @code{F>D}, integer part of float cannot be represented by @i{d}
13349: @cindex floating-point unidentified fault, @code{F>D}
13350: Platform-dependent; typically, some double number is produced and no
13351: error is reported.
13352: 
13353: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
13354: @cindex string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.})
13355: @code{Precision} characters of the numeric output area are used.  If
13356: @code{precision} is too high, these words will smash the data or code
13357: close to @code{here}.
13358: @end table
13359: 
13360: @c =====================================================================
13361: @node  The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
13362: @section The optional Locals word set
13363: @c =====================================================================
13364: @cindex system documentation, locals words
13365: @cindex locals words, system documentation
13366: 
13367: @menu
13368: * locals-idef::                 Implementation Defined Options                 
13369: * locals-ambcond::              Ambiguous Conditions              
13370: @end menu
13371: 
13372: 
13373: @c ---------------------------------------------------------------------
13374: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
13375: @subsection Implementation Defined Options
13376: @c ---------------------------------------------------------------------
13377: @cindex implementation-defined options, locals words
13378: @cindex locals words, implementation-defined options
13379: 
13380: @table @i
13381: @item maximum number of locals in a definition:
13382: @cindex maximum number of locals in a definition
13383: @cindex locals, maximum number in a definition
13384: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
13385: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
13386: characters. The number of locals in a definition is bounded by the size
13387: of locals-buffer, which contains the names of the locals.
13388: 
13389: @end table
13390: 
13391: 
13392: @c ---------------------------------------------------------------------
13393: @node locals-ambcond,  , locals-idef, The optional Locals word set
13394: @subsection Ambiguous conditions
13395: @c ---------------------------------------------------------------------
13396: @cindex locals words, ambiguous conditions
13397: @cindex ambiguous conditions, locals words
13398: 
13399: @table @i
13400: @item executing a named local in interpretation state:
13401: @cindex local in interpretation state
13402: @cindex Interpreting a compile-only word, for a local
13403: Locals have no interpretation semantics. If you try to perform the
13404: interpretation semantics, you will get a @code{-14 throw} somewhere
13405: (Interpreting a compile-only word). If you perform the compilation
13406: semantics, the locals access will be compiled (irrespective of state).
13407: 
13408: @item @i{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
13409: @cindex name not defined by @code{VALUE} or @code{(LOCAL)} used by @code{TO}
13410: @cindex @code{TO} on non-@code{VALUE}s and non-locals
13411: @cindex Invalid name argument, @code{TO}
13412: @code{-32 throw} (Invalid name argument)
13413: 
13414: @end table
13415: 
13416: 
13417: @c =====================================================================
13418: @node  The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
13419: @section The optional Memory-Allocation word set
13420: @c =====================================================================
13421: @cindex system documentation, memory-allocation words
13422: @cindex memory-allocation words, system documentation
13423: 
13424: @menu
13425: * memory-idef::                 Implementation Defined Options                 
13426: @end menu
13427: 
13428: 
13429: @c ---------------------------------------------------------------------
13430: @node memory-idef,  , The optional Memory-Allocation word set, The optional Memory-Allocation word set
13431: @subsection Implementation Defined Options
13432: @c ---------------------------------------------------------------------
13433: @cindex implementation-defined options, memory-allocation words
13434: @cindex memory-allocation words, implementation-defined options
13435: 
13436: @table @i
13437: @item values and meaning of @i{ior}:
13438: @cindex  @i{ior} values and meaning
13439: The @i{ior}s returned by the file and memory allocation words are
13440: intended as throw codes. They typically are in the range
13441: -512@minus{}-2047 of OS errors.  The mapping from OS error numbers to
13442: @i{ior}s is -512@minus{}@i{errno}.
13443: 
13444: @end table
13445: 
13446: @c =====================================================================
13447: @node  The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
13448: @section The optional Programming-Tools word set
13449: @c =====================================================================
13450: @cindex system documentation, programming-tools words
13451: @cindex programming-tools words, system documentation
13452: 
13453: @menu
13454: * programming-idef::            Implementation Defined Options            
13455: * programming-ambcond::         Ambiguous Conditions         
13456: @end menu
13457: 
13458: 
13459: @c ---------------------------------------------------------------------
13460: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
13461: @subsection Implementation Defined Options
13462: @c ---------------------------------------------------------------------
13463: @cindex implementation-defined options, programming-tools words
13464: @cindex programming-tools words, implementation-defined options
13465: 
13466: @table @i
13467: @item ending sequence for input following @code{;CODE} and @code{CODE}:
13468: @cindex @code{;CODE} ending sequence
13469: @cindex @code{CODE} ending sequence
13470: @code{END-CODE}
13471: 
13472: @item manner of processing input following @code{;CODE} and @code{CODE}:
13473: @cindex @code{;CODE}, processing input
13474: @cindex @code{CODE}, processing input
13475: The @code{ASSEMBLER} vocabulary is pushed on the search order stack, and
13476: the input is processed by the text interpreter, (starting) in interpret
13477: state.
13478: 
13479: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
13480: @cindex @code{ASSEMBLER}, search order capability
13481: The ANS Forth search order word set.
13482: 
13483: @item source and format of display by @code{SEE}:
13484: @cindex @code{SEE}, source and format of output
13485: The source for @code{see} is the executable code used by the inner
13486: interpreter.  The current @code{see} tries to output Forth source code
13487: (and on some platforms, assembly code for primitives) as well as
13488: possible.
13489: 
13490: @end table
13491: 
13492: @c ---------------------------------------------------------------------
13493: @node programming-ambcond,  , programming-idef, The optional Programming-Tools word set
13494: @subsection Ambiguous conditions
13495: @c ---------------------------------------------------------------------
13496: @cindex programming-tools words, ambiguous conditions
13497: @cindex ambiguous conditions, programming-tools words
13498: 
13499: @table @i
13500: 
13501: @item deleting the compilation word list (@code{FORGET}):
13502: @cindex @code{FORGET}, deleting the compilation word list
13503: Not implemented (yet).
13504: 
13505: @item fewer than @i{u}+1 items on the control-flow stack (@code{CS-PICK}, @code{CS-ROLL}):
13506: @cindex @code{CS-PICK}, fewer than @i{u}+1 items on the control flow-stack
13507: @cindex @code{CS-ROLL}, fewer than @i{u}+1 items on the control flow-stack
13508: @cindex control-flow stack underflow
13509: This typically results in an @code{abort"} with a descriptive error
13510: message (may change into a @code{-22 throw} (Control structure mismatch)
13511: in the future). You may also get a memory access error. If you are
13512: unlucky, this ambiguous condition is not caught.
13513: 
13514: @item @i{name} can't be found (@code{FORGET}):
13515: @cindex @code{FORGET}, @i{name} can't be found
13516: Not implemented (yet).
13517: 
13518: @item @i{name} not defined via @code{CREATE}:
13519: @cindex @code{;CODE}, @i{name} not defined via @code{CREATE}
13520: @code{;CODE} behaves like @code{DOES>} in this respect, i.e., it changes
13521: the execution semantics of the last defined word no matter how it was
13522: defined.
13523: 
13524: @item @code{POSTPONE} applied to @code{[IF]}:
13525: @cindex @code{POSTPONE} applied to @code{[IF]}
13526: @cindex @code{[IF]} and @code{POSTPONE}
13527: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
13528: equivalent to @code{[IF]}.
13529: 
13530: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
13531: @cindex @code{[IF]}, end of the input source before matching @code{[ELSE]} or @code{[THEN]}
13532: Continue in the same state of conditional compilation in the next outer
13533: input source. Currently there is no warning to the user about this.
13534: 
13535: @item removing a needed definition (@code{FORGET}):
13536: @cindex @code{FORGET}, removing a needed definition
13537: Not implemented (yet).
13538: 
13539: @end table
13540: 
13541: 
13542: @c =====================================================================
13543: @node  The optional Search-Order word set,  , The optional Programming-Tools word set, ANS conformance
13544: @section The optional Search-Order word set
13545: @c =====================================================================
13546: @cindex system documentation, search-order words
13547: @cindex search-order words, system documentation
13548: 
13549: @menu
13550: * search-idef::                 Implementation Defined Options                 
13551: * search-ambcond::              Ambiguous Conditions              
13552: @end menu
13553: 
13554: 
13555: @c ---------------------------------------------------------------------
13556: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
13557: @subsection Implementation Defined Options
13558: @c ---------------------------------------------------------------------
13559: @cindex implementation-defined options, search-order words
13560: @cindex search-order words, implementation-defined options
13561: 
13562: @table @i
13563: @item maximum number of word lists in search order:
13564: @cindex maximum number of word lists in search order
13565: @cindex search order, maximum depth
13566: @code{s" wordlists" environment? drop .}. Currently 16.
13567: 
13568: @item minimum search order:
13569: @cindex minimum search order
13570: @cindex search order, minimum
13571: @code{root root}.
13572: 
13573: @end table
13574: 
13575: @c ---------------------------------------------------------------------
13576: @node search-ambcond,  , search-idef, The optional Search-Order word set
13577: @subsection Ambiguous conditions
13578: @c ---------------------------------------------------------------------
13579: @cindex search-order words, ambiguous conditions
13580: @cindex ambiguous conditions, search-order words
13581: 
13582: @table @i
13583: @item changing the compilation word list (during compilation):
13584: @cindex changing the compilation word list (during compilation)
13585: @cindex compilation word list, change before definition ends
13586: The word is entered into the word list that was the compilation word list
13587: at the start of the definition. Any changes to the name field (e.g.,
13588: @code{immediate}) or the code field (e.g., when executing @code{DOES>})
13589: are applied to the latest defined word (as reported by @code{latest} or
13590: @code{latestxt}), if possible, irrespective of the compilation word list.
13591: 
13592: @item search order empty (@code{previous}):
13593: @cindex @code{previous}, search order empty
13594: @cindex vocstack empty, @code{previous}
13595: @code{abort" Vocstack empty"}.
13596: 
13597: @item too many word lists in search order (@code{also}):
13598: @cindex @code{also}, too many word lists in search order
13599: @cindex vocstack full, @code{also}
13600: @code{abort" Vocstack full"}.
13601: 
13602: @end table
13603: 
13604: @c ***************************************************************
13605: @node Standard vs Extensions, Model, ANS conformance, Top
13606: @chapter Should I use Gforth extensions?
13607: @cindex Gforth extensions
13608: 
13609: As you read through the rest of this manual, you will see documentation
13610: for @i{Standard} words, and documentation for some appealing Gforth
13611: @i{extensions}. You might ask yourself the question: @i{``Should I
13612: restrict myself to the standard, or should I use the extensions?''}
13613: 
13614: The answer depends on the goals you have for the program you are working
13615: on:
13616: 
13617: @itemize @bullet
13618: 
13619: @item Is it just for yourself or do you want to share it with others?
13620: 
13621: @item
13622: If you want to share it, do the others all use Gforth?
13623: 
13624: @item
13625: If it is just for yourself, do you want to restrict yourself to Gforth?
13626: 
13627: @end itemize
13628: 
13629: If restricting the program to Gforth is ok, then there is no reason not
13630: to use extensions.  It is still a good idea to keep to the standard
13631: where it is easy, in case you want to reuse these parts in another
13632: program that you want to be portable.
13633: 
13634: If you want to be able to port the program to other Forth systems, there
13635: are the following points to consider:
13636: 
13637: @itemize @bullet
13638: 
13639: @item
13640: Most Forth systems that are being maintained support the ANS Forth
13641: standard.  So if your program complies with the standard, it will be
13642: portable among many systems.
13643: 
13644: @item
13645: A number of the Gforth extensions can be implemented in ANS Forth using
13646: public-domain files provided in the @file{compat/} directory. These are
13647: mentioned in the text in passing.  There is no reason not to use these
13648: extensions, your program will still be ANS Forth compliant; just include
13649: the appropriate compat files with your program.
13650: 
13651: @item
13652: The tool @file{ans-report.fs} (@pxref{ANS Report}) makes it easy to
13653: analyse your program and determine what non-Standard words it relies
13654: upon.  However, it does not check whether you use standard words in a
13655: non-standard way.
13656: 
13657: @item
13658: Some techniques are not standardized by ANS Forth, and are hard or
13659: impossible to implement in a standard way, but can be implemented in
13660: most Forth systems easily, and usually in similar ways (e.g., accessing
13661: word headers).  Forth has a rich historical precedent for programmers
13662: taking advantage of implementation-dependent features of their tools
13663: (for example, relying on a knowledge of the dictionary
13664: structure). Sometimes these techniques are necessary to extract every
13665: last bit of performance from the hardware, sometimes they are just a
13666: programming shorthand.
13667: 
13668: @item
13669: Does using a Gforth extension save more work than the porting this part
13670: to other Forth systems (if any) will cost?
13671: 
13672: @item
13673: Is the additional functionality worth the reduction in portability and
13674: the additional porting problems?
13675: 
13676: @end itemize
13677: 
13678: In order to perform these consideratios, you need to know what's
13679: standard and what's not.  This manual generally states if something is
13680: non-standard, but the authoritative source is the
13681: @uref{http://www.taygeta.com/forth/dpans.html,standard document}.
13682: Appendix A of the Standard (@var{Rationale}) provides a valuable insight
13683: into the thought processes of the technical committee.
13684: 
13685: Note also that portability between Forth systems is not the only
13686: portability issue; there is also the issue of portability between
13687: different platforms (processor/OS combinations).
13688: 
13689: @c ***************************************************************
13690: @node Model, Integrating Gforth, Standard vs Extensions, Top
13691: @chapter Model
13692: 
13693: This chapter has yet to be written. It will contain information, on
13694: which internal structures you can rely.
13695: 
13696: @c ***************************************************************
13697: @node Integrating Gforth, Emacs and Gforth, Model, Top
13698: @chapter Integrating Gforth into C programs
13699: 
13700: This is not yet implemented.
13701: 
13702: Several people like to use Forth as scripting language for applications
13703: that are otherwise written in C, C++, or some other language.
13704: 
13705: The Forth system ATLAST provides facilities for embedding it into
13706: applications; unfortunately it has several disadvantages: most
13707: importantly, it is not based on ANS Forth, and it is apparently dead
13708: (i.e., not developed further and not supported). The facilities
13709: provided by Gforth in this area are inspired by ATLAST's facilities, so
13710: making the switch should not be hard.
13711: 
13712: We also tried to design the interface such that it can easily be
13713: implemented by other Forth systems, so that we may one day arrive at a
13714: standardized interface. Such a standard interface would allow you to
13715: replace the Forth system without having to rewrite C code.
13716: 
13717: You embed the Gforth interpreter by linking with the library
13718: @code{libgforth.a} (give the compiler the option @code{-lgforth}).  All
13719: global symbols in this library that belong to the interface, have the
13720: prefix @code{forth_}. (Global symbols that are used internally have the
13721: prefix @code{gforth_}).
13722: 
13723: You can include the declarations of Forth types and the functions and
13724: variables of the interface with @code{#include <forth.h>}.
13725: 
13726: Types.
13727: 
13728: Variables.
13729: 
13730: Data and FP Stack pointer. Area sizes.
13731: 
13732: functions.
13733: 
13734: forth_init(imagefile)
13735: forth_evaluate(string) exceptions?
13736: forth_goto(address) (or forth_execute(xt)?)
13737: forth_continue() (a corountining mechanism)
13738: 
13739: Adding primitives.
13740: 
13741: No checking.
13742: 
13743: Signals?
13744: 
13745: Accessing the Stacks
13746: 
13747: @c ******************************************************************
13748: @node Emacs and Gforth, Image Files, Integrating Gforth, Top
13749: @chapter Emacs and Gforth
13750: @cindex Emacs and Gforth
13751: 
13752: @cindex @file{gforth.el}
13753: @cindex @file{forth.el}
13754: @cindex Rydqvist, Goran
13755: @cindex Kuehling, David
13756: @cindex comment editing commands
13757: @cindex @code{\}, editing with Emacs
13758: @cindex debug tracer editing commands
13759: @cindex @code{~~}, removal with Emacs
13760: @cindex Forth mode in Emacs
13761: 
13762: Gforth comes with @file{gforth.el}, an improved version of
13763: @file{forth.el} by Goran Rydqvist (included in the TILE package). The
13764: improvements are:
13765: 
13766: @itemize @bullet
13767: @item
13768: A better handling of indentation.
13769: @item
13770: A custom hilighting engine for Forth-code.
13771: @item
13772: Comment paragraph filling (@kbd{M-q})
13773: @item
13774: Commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) of regions
13775: @item
13776: Removal of debugging tracers (@kbd{C-x ~}, @pxref{Debugging}).
13777: @item
13778: Support of the @code{info-lookup} feature for looking up the
13779: documentation of a word.
13780: @item
13781: Support for reading and writing blocks files.
13782: @end itemize
13783: 
13784: To get a basic description of these features, enter Forth mode and
13785: type @kbd{C-h m}.
13786: 
13787: @cindex source location of error or debugging output in Emacs
13788: @cindex error output, finding the source location in Emacs
13789: @cindex debugging output, finding the source location in Emacs
13790: In addition, Gforth supports Emacs quite well: The source code locations
13791: given in error messages, debugging output (from @code{~~}) and failed
13792: assertion messages are in the right format for Emacs' compilation mode
13793: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
13794: Manual}) so the source location corresponding to an error or other
13795: message is only a few keystrokes away (@kbd{C-x `} for the next error,
13796: @kbd{C-c C-c} for the error under the cursor).
13797: 
13798: @cindex viewing the documentation of a word in Emacs
13799: @cindex context-sensitive help
13800: Moreover, for words documented in this manual, you can look up the
13801: glossary entry quickly by using @kbd{C-h TAB}
13802: (@code{info-lookup-symbol}, @pxref{Documentation, ,Documentation
13803: Commands, emacs, Emacs Manual}).  This feature requires Emacs 20.3 or
13804: later and does not work for words containing @code{:}.
13805: 
13806: @menu
13807: * Installing gforth.el::        Making Emacs aware of Forth.
13808: * Emacs Tags::                  Viewing the source of a word in Emacs.
13809: * Hilighting::                  Making Forth code look prettier.
13810: * Auto-Indentation::            Customizing auto-indentation.
13811: * Blocks Files::                Reading and writing blocks files.
13812: @end menu
13813: 
13814: @c ----------------------------------
13815: @node Installing gforth.el, Emacs Tags, Emacs and Gforth, Emacs and Gforth
13816: @section Installing gforth.el
13817: @cindex @file{.emacs}
13818: @cindex @file{gforth.el}, installation
13819: To make the features from @file{gforth.el} available in Emacs, add
13820: the following lines to your @file{.emacs} file:
13821: 
13822: @example
13823: (autoload 'forth-mode "gforth.el")
13824: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) 
13825: 			    auto-mode-alist))
13826: (autoload 'forth-block-mode "gforth.el")
13827: (setq auto-mode-alist (cons '("\\.fb\\'" . forth-block-mode) 
13828: 			    auto-mode-alist))
13829: (add-hook 'forth-mode-hook (function (lambda ()
13830:    ;; customize variables here:
13831:    (setq forth-indent-level 4)
13832:    (setq forth-minor-indent-level 2)
13833:    (setq forth-hilight-level 3)
13834:    ;;; ...
13835: )))
13836: @end example
13837: 
13838: @c ----------------------------------
13839: @node Emacs Tags, Hilighting, Installing gforth.el, Emacs and Gforth
13840: @section Emacs Tags
13841: @cindex @file{TAGS} file
13842: @cindex @file{etags.fs}
13843: @cindex viewing the source of a word in Emacs
13844: @cindex @code{require}, placement in files
13845: @cindex @code{include}, placement in files
13846: If you @code{require} @file{etags.fs}, a new @file{TAGS} file will be
13847: produced (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) that
13848: contains the definitions of all words defined afterwards. You can then
13849: find the source for a word using @kbd{M-.}. Note that Emacs can use
13850: several tags files at the same time (e.g., one for the Gforth sources
13851: and one for your program, @pxref{Select Tags Table,,Selecting a Tags
13852: Table,emacs, Emacs Manual}). The TAGS file for the preloaded words is
13853: @file{$(datadir)/gforth/$(VERSION)/TAGS} (e.g.,
13854: @file{/usr/local/share/gforth/0.2.0/TAGS}).  To get the best behaviour
13855: with @file{etags.fs}, you should avoid putting definitions both before
13856: and after @code{require} etc., otherwise you will see the same file
13857: visited several times by commands like @code{tags-search}.
13858: 
13859: @c ----------------------------------
13860: @node Hilighting, Auto-Indentation, Emacs Tags, Emacs and Gforth
13861: @section Hilighting
13862: @cindex hilighting Forth code in Emacs
13863: @cindex highlighting Forth code in Emacs
13864: @file{gforth.el} comes with a custom source hilighting engine.  When
13865: you open a file in @code{forth-mode}, it will be completely parsed,
13866: assigning faces to keywords, comments, strings etc.  While you edit
13867: the file, modified regions get parsed and updated on-the-fly. 
13868: 
13869: Use the variable `forth-hilight-level' to change the level of
13870: decoration from 0 (no hilighting at all) to 3 (the default).  Even if
13871: you set the hilighting level to 0, the parser will still work in the
13872: background, collecting information about whether regions of text are
13873: ``compiled'' or ``interpreted''.  Those information are required for
13874: auto-indentation to work properly.  Set `forth-disable-parser' to
13875: non-nil if your computer is too slow to handle parsing.  This will
13876: have an impact on the smartness of the auto-indentation engine,
13877: though.
13878: 
13879: Sometimes Forth sources define new features that should be hilighted,
13880: new control structures, defining-words etc.  You can use the variable
13881: `forth-custom-words' to make @code{forth-mode} hilight additional
13882: words and constructs.  See the docstring of `forth-words' for details
13883: (in Emacs, type @kbd{C-h v forth-words}).
13884: 
13885: `forth-custom-words' is meant to be customized in your
13886: @file{.emacs} file.  To customize hilighing in a file-specific manner,
13887: set `forth-local-words' in a local-variables section at the end of
13888: your source file (@pxref{Local Variables in Files,, Variables, emacs, Emacs Manual}).
13889: 
13890: Example:
13891: @example
13892: 0 [IF]
13893:    Local Variables:
13894:    forth-local-words:
13895:       ((("t:") definition-starter (font-lock-keyword-face . 1)
13896:         "[ \t\n]" t name (font-lock-function-name-face . 3))
13897:        ((";t") definition-ender (font-lock-keyword-face . 1)))
13898:    End:
13899: [THEN]
13900: @end example
13901: 
13902: @c ----------------------------------
13903: @node Auto-Indentation, Blocks Files, Hilighting, Emacs and Gforth
13904: @section Auto-Indentation
13905: @cindex auto-indentation of Forth code in Emacs
13906: @cindex indentation of Forth code in Emacs
13907: @code{forth-mode} automatically tries to indent lines in a smart way,
13908: whenever you type @key{TAB} or break a line with @kbd{C-m}.
13909: 
13910: Simple customization can be achieved by setting
13911: `forth-indent-level' and `forth-minor-indent-level' in your
13912: @file{.emacs} file. For historical reasons @file{gforth.el} indents
13913: per default by multiples of 4 columns.  To use the more traditional
13914: 3-column indentation, add the following lines to your @file{.emacs}:
13915: 
13916: @example
13917: (add-hook 'forth-mode-hook (function (lambda ()
13918:    ;; customize variables here:
13919:    (setq forth-indent-level 3)
13920:    (setq forth-minor-indent-level 1)
13921: )))
13922: @end example
13923: 
13924: If you want indentation to recognize non-default words, customize it
13925: by setting `forth-custom-indent-words' in your @file{.emacs}.  See the
13926: docstring of `forth-indent-words' for details (in Emacs, type @kbd{C-h
13927: v forth-indent-words}).
13928: 
13929: To customize indentation in a file-specific manner, set
13930: `forth-local-indent-words' in a local-variables section at the end of
13931: your source file (@pxref{Local Variables in Files, Variables,,emacs,
13932: Emacs Manual}).
13933: 
13934: Example:
13935: @example
13936: 0 [IF]
13937:    Local Variables:
13938:    forth-local-indent-words:
13939:       ((("t:") (0 . 2) (0 . 2))
13940:        ((";t") (-2 . 0) (0 . -2)))
13941:    End:
13942: [THEN]
13943: @end example
13944: 
13945: @c ----------------------------------
13946: @node Blocks Files,  , Auto-Indentation, Emacs and Gforth
13947: @section Blocks Files
13948: @cindex blocks files, use with Emacs
13949: @code{forth-mode} Autodetects blocks files by checking whether the
13950: length of the first line exceeds 1023 characters.  It then tries to
13951: convert the file into normal text format.  When you save the file, it
13952: will be written to disk as normal stream-source file.
13953: 
13954: If you want to write blocks files, use @code{forth-blocks-mode}.  It
13955: inherits all the features from @code{forth-mode}, plus some additions:
13956: 
13957: @itemize @bullet
13958: @item
13959: Files are written to disk in blocks file format.
13960: @item
13961: Screen numbers are displayed in the mode line (enumerated beginning
13962: with the value of `forth-block-base')
13963: @item
13964: Warnings are displayed when lines exceed 64 characters.
13965: @item
13966: The beginning of the currently edited block is marked with an
13967: overlay-arrow. 
13968: @end itemize
13969: 
13970: There are some restrictions you should be aware of.  When you open a
13971: blocks file that contains tabulator or newline characters, these
13972: characters will be translated into spaces when the file is written
13973: back to disk.  If tabs or newlines are encountered during blocks file
13974: reading, an error is output to the echo area. So have a look at the
13975: `*Messages*' buffer, when Emacs' bell rings during reading.
13976: 
13977: Please consult the docstring of @code{forth-blocks-mode} for more
13978: information by typing @kbd{C-h v forth-blocks-mode}).
13979: 
13980: @c ******************************************************************
13981: @node Image Files, Engine, Emacs and Gforth, Top
13982: @chapter Image Files
13983: @cindex image file
13984: @cindex @file{.fi} files
13985: @cindex precompiled Forth code
13986: @cindex dictionary in persistent form
13987: @cindex persistent form of dictionary
13988: 
13989: An image file is a file containing an image of the Forth dictionary,
13990: i.e., compiled Forth code and data residing in the dictionary.  By
13991: convention, we use the extension @code{.fi} for image files.
13992: 
13993: @menu
13994: * Image Licensing Issues::      Distribution terms for images.
13995: * Image File Background::       Why have image files?
13996: * Non-Relocatable Image Files::  don't always work.
13997: * Data-Relocatable Image Files::  are better.
13998: * Fully Relocatable Image Files::  better yet.
13999: * Stack and Dictionary Sizes::  Setting the default sizes for an image.
14000: * Running Image Files::         @code{gforth -i @i{file}} or @i{file}.
14001: * Modifying the Startup Sequence::  and turnkey applications.
14002: @end menu
14003: 
14004: @node Image Licensing Issues, Image File Background, Image Files, Image Files
14005: @section Image Licensing Issues
14006: @cindex license for images
14007: @cindex image license
14008: 
14009: An image created with @code{gforthmi} (@pxref{gforthmi}) or
14010: @code{savesystem} (@pxref{Non-Relocatable Image Files}) includes the
14011: original image; i.e., according to copyright law it is a derived work of
14012: the original image.
14013: 
14014: Since Gforth is distributed under the GNU GPL, the newly created image
14015: falls under the GNU GPL, too. In particular, this means that if you
14016: distribute the image, you have to make all of the sources for the image
14017: available, including those you wrote.  For details see @ref{Copying, ,
14018: GNU General Public License (Section 3)}.
14019: 
14020: If you create an image with @code{cross} (@pxref{cross.fs}), the image
14021: contains only code compiled from the sources you gave it; if none of
14022: these sources is under the GPL, the terms discussed above do not apply
14023: to the image. However, if your image needs an engine (a gforth binary)
14024: that is under the GPL, you should make sure that you distribute both in
14025: a way that is at most a @emph{mere aggregation}, if you don't want the
14026: terms of the GPL to apply to the image.
14027: 
14028: @node Image File Background, Non-Relocatable Image Files, Image Licensing Issues, Image Files
14029: @section Image File Background
14030: @cindex image file background
14031: 
14032: Gforth consists not only of primitives (in the engine), but also of
14033: definitions written in Forth. Since the Forth compiler itself belongs to
14034: those definitions, it is not possible to start the system with the
14035: engine and the Forth source alone. Therefore we provide the Forth
14036: code as an image file in nearly executable form. When Gforth starts up,
14037: a C routine loads the image file into memory, optionally relocates the
14038: addresses, then sets up the memory (stacks etc.) according to
14039: information in the image file, and (finally) starts executing Forth
14040: code.
14041: 
14042: The image file variants represent different compromises between the
14043: goals of making it easy to generate image files and making them
14044: portable.
14045: 
14046: @cindex relocation at run-time
14047: Win32Forth 3.4 and Mitch Bradley's @code{cforth} use relocation at
14048: run-time. This avoids many of the complications discussed below (image
14049: files are data relocatable without further ado), but costs performance
14050: (one addition per memory access).
14051: 
14052: @cindex relocation at load-time
14053: By contrast, the Gforth loader performs relocation at image load time. The
14054: loader also has to replace tokens that represent primitive calls with the
14055: appropriate code-field addresses (or code addresses in the case of
14056: direct threading).
14057: 
14058: There are three kinds of image files, with different degrees of
14059: relocatability: non-relocatable, data-relocatable, and fully relocatable
14060: image files.
14061: 
14062: @cindex image file loader
14063: @cindex relocating loader
14064: @cindex loader for image files
14065: These image file variants have several restrictions in common; they are
14066: caused by the design of the image file loader:
14067: 
14068: @itemize @bullet
14069: @item
14070: There is only one segment; in particular, this means, that an image file
14071: cannot represent @code{ALLOCATE}d memory chunks (and pointers to
14072: them). The contents of the stacks are not represented, either.
14073: 
14074: @item
14075: The only kinds of relocation supported are: adding the same offset to
14076: all cells that represent data addresses; and replacing special tokens
14077: with code addresses or with pieces of machine code.
14078: 
14079: If any complex computations involving addresses are performed, the
14080: results cannot be represented in the image file. Several applications that
14081: use such computations come to mind:
14082: @itemize @minus
14083: @item
14084: Hashing addresses (or data structures which contain addresses) for table
14085: lookup. If you use Gforth's @code{table}s or @code{wordlist}s for this
14086: purpose, you will have no problem, because the hash tables are
14087: recomputed automatically when the system is started. If you use your own
14088: hash tables, you will have to do something similar.
14089: 
14090: @item
14091: There's a cute implementation of doubly-linked lists that uses
14092: @code{XOR}ed addresses. You could represent such lists as singly-linked
14093: in the image file, and restore the doubly-linked representation on
14094: startup.@footnote{In my opinion, though, you should think thrice before
14095: using a doubly-linked list (whatever implementation).}
14096: 
14097: @item
14098: The code addresses of run-time routines like @code{docol:} cannot be
14099: represented in the image file (because their tokens would be replaced by
14100: machine code in direct threaded implementations). As a workaround,
14101: compute these addresses at run-time with @code{>code-address} from the
14102: executions tokens of appropriate words (see the definitions of
14103: @code{docol:} and friends in @file{kernel/getdoers.fs}).
14104: 
14105: @item
14106: On many architectures addresses are represented in machine code in some
14107: shifted or mangled form. You cannot put @code{CODE} words that contain
14108: absolute addresses in this form in a relocatable image file. Workarounds
14109: are representing the address in some relative form (e.g., relative to
14110: the CFA, which is present in some register), or loading the address from
14111: a place where it is stored in a non-mangled form.
14112: @end itemize
14113: @end itemize
14114: 
14115: @node  Non-Relocatable Image Files, Data-Relocatable Image Files, Image File Background, Image Files
14116: @section Non-Relocatable Image Files
14117: @cindex non-relocatable image files
14118: @cindex image file, non-relocatable
14119: 
14120: These files are simple memory dumps of the dictionary. They are specific
14121: to the executable (i.e., @file{gforth} file) they were created
14122: with. What's worse, they are specific to the place on which the
14123: dictionary resided when the image was created. Now, there is no
14124: guarantee that the dictionary will reside at the same place the next
14125: time you start Gforth, so there's no guarantee that a non-relocatable
14126: image will work the next time (Gforth will complain instead of crashing,
14127: though).
14128: 
14129: You can create a non-relocatable image file with
14130: 
14131: 
14132: doc-savesystem
14133: 
14134: 
14135: @node Data-Relocatable Image Files, Fully Relocatable Image Files, Non-Relocatable Image Files, Image Files
14136: @section Data-Relocatable Image Files
14137: @cindex data-relocatable image files
14138: @cindex image file, data-relocatable
14139: 
14140: These files contain relocatable data addresses, but fixed code addresses
14141: (instead of tokens). They are specific to the executable (i.e.,
14142: @file{gforth} file) they were created with. For direct threading on some
14143: architectures (e.g., the i386), data-relocatable images do not work. You
14144: get a data-relocatable image, if you use @file{gforthmi} with a
14145: Gforth binary that is not doubly indirect threaded (@pxref{Fully
14146: Relocatable Image Files}).
14147: 
14148: @node Fully Relocatable Image Files, Stack and Dictionary Sizes, Data-Relocatable Image Files, Image Files
14149: @section Fully Relocatable Image Files
14150: @cindex fully relocatable image files
14151: @cindex image file, fully relocatable
14152: 
14153: @cindex @file{kern*.fi}, relocatability
14154: @cindex @file{gforth.fi}, relocatability
14155: These image files have relocatable data addresses, and tokens for code
14156: addresses. They can be used with different binaries (e.g., with and
14157: without debugging) on the same machine, and even across machines with
14158: the same data formats (byte order, cell size, floating point
14159: format). However, they are usually specific to the version of Gforth
14160: they were created with. The files @file{gforth.fi} and @file{kernl*.fi}
14161: are fully relocatable.
14162: 
14163: There are two ways to create a fully relocatable image file:
14164: 
14165: @menu
14166: * gforthmi::                    The normal way
14167: * cross.fs::                    The hard way
14168: @end menu
14169: 
14170: @node gforthmi, cross.fs, Fully Relocatable Image Files, Fully Relocatable Image Files
14171: @subsection @file{gforthmi}
14172: @cindex @file{comp-i.fs}
14173: @cindex @file{gforthmi}
14174: 
14175: You will usually use @file{gforthmi}. If you want to create an
14176: image @i{file} that contains everything you would load by invoking
14177: Gforth with @code{gforth @i{options}}, you simply say:
14178: @example
14179: gforthmi @i{file} @i{options}
14180: @end example
14181: 
14182: E.g., if you want to create an image @file{asm.fi} that has the file
14183: @file{asm.fs} loaded in addition to the usual stuff, you could do it
14184: like this:
14185: 
14186: @example
14187: gforthmi asm.fi asm.fs
14188: @end example
14189: 
14190: @file{gforthmi} is implemented as a sh script and works like this: It
14191: produces two non-relocatable images for different addresses and then
14192: compares them. Its output reflects this: first you see the output (if
14193: any) of the two Gforth invocations that produce the non-relocatable image
14194: files, then you see the output of the comparing program: It displays the
14195: offset used for data addresses and the offset used for code addresses;
14196: moreover, for each cell that cannot be represented correctly in the
14197: image files, it displays a line like this:
14198: 
14199: @example
14200:      78DC         BFFFFA50         BFFFFA40
14201: @end example
14202: 
14203: This means that at offset $78dc from @code{forthstart}, one input image
14204: contains $bffffa50, and the other contains $bffffa40. Since these cells
14205: cannot be represented correctly in the output image, you should examine
14206: these places in the dictionary and verify that these cells are dead
14207: (i.e., not read before they are written).
14208: 
14209: @cindex --application, @code{gforthmi} option
14210: If you insert the option @code{--application} in front of the image file
14211: name, you will get an image that uses the @code{--appl-image} option
14212: instead of the @code{--image-file} option (@pxref{Invoking
14213: Gforth}). When you execute such an image on Unix (by typing the image
14214: name as command), the Gforth engine will pass all options to the image
14215: instead of trying to interpret them as engine options.
14216: 
14217: If you type @file{gforthmi} with no arguments, it prints some usage
14218: instructions.
14219: 
14220: @cindex @code{savesystem} during @file{gforthmi}
14221: @cindex @code{bye} during @file{gforthmi}
14222: @cindex doubly indirect threaded code
14223: @cindex environment variables
14224: @cindex @code{GFORTHD} -- environment variable
14225: @cindex @code{GFORTH} -- environment variable
14226: @cindex @code{gforth-ditc}
14227: There are a few wrinkles: After processing the passed @i{options}, the
14228: words @code{savesystem} and @code{bye} must be visible. A special doubly
14229: indirect threaded version of the @file{gforth} executable is used for
14230: creating the non-relocatable images; you can pass the exact filename of
14231: this executable through the environment variable @code{GFORTHD}
14232: (default: @file{gforth-ditc}); if you pass a version that is not doubly
14233: indirect threaded, you will not get a fully relocatable image, but a
14234: data-relocatable image (because there is no code address offset). The
14235: normal @file{gforth} executable is used for creating the relocatable
14236: image; you can pass the exact filename of this executable through the
14237: environment variable @code{GFORTH}.
14238: 
14239: @node cross.fs,  , gforthmi, Fully Relocatable Image Files
14240: @subsection @file{cross.fs}
14241: @cindex @file{cross.fs}
14242: @cindex cross-compiler
14243: @cindex metacompiler
14244: @cindex target compiler
14245: 
14246: You can also use @code{cross}, a batch compiler that accepts a Forth-like
14247: programming language (@pxref{Cross Compiler}).
14248: 
14249: @code{cross} allows you to create image files for machines with
14250: different data sizes and data formats than the one used for generating
14251: the image file. You can also use it to create an application image that
14252: does not contain a Forth compiler. These features are bought with
14253: restrictions and inconveniences in programming. E.g., addresses have to
14254: be stored in memory with special words (@code{A!}, @code{A,}, etc.) in
14255: order to make the code relocatable.
14256: 
14257: 
14258: @node Stack and Dictionary Sizes, Running Image Files, Fully Relocatable Image Files, Image Files
14259: @section Stack and Dictionary Sizes
14260: @cindex image file, stack and dictionary sizes
14261: @cindex dictionary size default
14262: @cindex stack size default
14263: 
14264: If you invoke Gforth with a command line flag for the size
14265: (@pxref{Invoking Gforth}), the size you specify is stored in the
14266: dictionary. If you save the dictionary with @code{savesystem} or create
14267: an image with @file{gforthmi}, this size will become the default
14268: for the resulting image file. E.g., the following will create a
14269: fully relocatable version of @file{gforth.fi} with a 1MB dictionary:
14270: 
14271: @example
14272: gforthmi gforth.fi -m 1M
14273: @end example
14274: 
14275: In other words, if you want to set the default size for the dictionary
14276: and the stacks of an image, just invoke @file{gforthmi} with the
14277: appropriate options when creating the image.
14278: 
14279: @cindex stack size, cache-friendly
14280: Note: For cache-friendly behaviour (i.e., good performance), you should
14281: make the sizes of the stacks modulo, say, 2K, somewhat different. E.g.,
14282: the default stack sizes are: data: 16k (mod 2k=0); fp: 15.5k (mod
14283: 2k=1.5k); return: 15k(mod 2k=1k); locals: 14.5k (mod 2k=0.5k).
14284: 
14285: @node Running Image Files, Modifying the Startup Sequence, Stack and Dictionary Sizes, Image Files
14286: @section Running Image Files
14287: @cindex running image files
14288: @cindex invoking image files
14289: @cindex image file invocation
14290: 
14291: @cindex -i, invoke image file
14292: @cindex --image file, invoke image file
14293: You can invoke Gforth with an image file @i{image} instead of the
14294: default @file{gforth.fi} with the @code{-i} flag (@pxref{Invoking Gforth}):
14295: @example
14296: gforth -i @i{image}
14297: @end example
14298: 
14299: @cindex executable image file
14300: @cindex image file, executable
14301: If your operating system supports starting scripts with a line of the
14302: form @code{#! ...}, you just have to type the image file name to start
14303: Gforth with this image file (note that the file extension @code{.fi} is
14304: just a convention). I.e., to run Gforth with the image file @i{image},
14305: you can just type @i{image} instead of @code{gforth -i @i{image}}.
14306: This works because every @code{.fi} file starts with a line of this
14307: format:
14308: 
14309: @example
14310: #! /usr/local/bin/gforth-0.4.0 -i
14311: @end example
14312: 
14313: The file and pathname for the Gforth engine specified on this line is
14314: the specific Gforth executable that it was built against; i.e. the value
14315: of the environment variable @code{GFORTH} at the time that
14316: @file{gforthmi} was executed.
14317: 
14318: You can make use of the same shell capability to make a Forth source
14319: file into an executable. For example, if you place this text in a file:
14320: 
14321: @example
14322: #! /usr/local/bin/gforth
14323: 
14324: ." Hello, world" CR
14325: bye
14326: @end example
14327: 
14328: @noindent
14329: and then make the file executable (chmod +x in Unix), you can run it
14330: directly from the command line. The sequence @code{#!} is used in two
14331: ways; firstly, it is recognised as a ``magic sequence'' by the operating
14332: system@footnote{The Unix kernel actually recognises two types of files:
14333: executable files and files of data, where the data is processed by an
14334: interpreter that is specified on the ``interpreter line'' -- the first
14335: line of the file, starting with the sequence #!. There may be a small
14336: limit (e.g., 32) on the number of characters that may be specified on
14337: the interpreter line.} secondly it is treated as a comment character by
14338: Gforth. Because of the second usage, a space is required between
14339: @code{#!} and the path to the executable (moreover, some Unixes
14340: require the sequence @code{#! /}).
14341: 
14342: The disadvantage of this latter technique, compared with using
14343: @file{gforthmi}, is that it is slightly slower; the Forth source code is
14344: compiled on-the-fly, each time the program is invoked.
14345: 
14346: doc-#!
14347: 
14348: 
14349: @node Modifying the Startup Sequence,  , Running Image Files, Image Files
14350: @section Modifying the Startup Sequence
14351: @cindex startup sequence for image file
14352: @cindex image file initialization sequence
14353: @cindex initialization sequence of image file
14354: 
14355: You can add your own initialization to the startup sequence of an image
14356: through the deferred word @code{'cold}. @code{'cold} is invoked just
14357: before the image-specific command line processing (i.e., loading files
14358: and evaluating (@code{-e}) strings) starts.
14359: 
14360: A sequence for adding your initialization usually looks like this:
14361: 
14362: @example
14363: :noname
14364:     Defers 'cold \ do other initialization stuff (e.g., rehashing wordlists)
14365:     ... \ your stuff
14366: ; IS 'cold
14367: @end example
14368: 
14369: @cindex turnkey image files
14370: @cindex image file, turnkey applications
14371: You can make a turnkey image by letting @code{'cold} execute a word
14372: (your turnkey application) that never returns; instead, it exits Gforth
14373: via @code{bye} or @code{throw}.
14374: 
14375: You can access the (image-specific) command-line arguments through
14376: @code{argc}, @code{argv} and @code{arg} (@pxref{OS command line
14377: arguments}).
14378: 
14379: If @code{'cold} exits normally, Gforth processes the command-line
14380: arguments as files to be loaded and strings to be evaluated.  Therefore,
14381: @code{'cold} should remove the arguments it has used in this case.
14382: 
14383: doc-'cold
14384: 
14385: @c ******************************************************************
14386: @node Engine, Cross Compiler, Image Files, Top
14387: @chapter Engine
14388: @cindex engine
14389: @cindex virtual machine
14390: 
14391: Reading this chapter is not necessary for programming with Gforth. It
14392: may be helpful for finding your way in the Gforth sources.
14393: 
14394: The ideas in this section have also been published in the following
14395: papers: Bernd Paysan, @cite{ANS fig/GNU/??? Forth} (in German),
14396: Forth-Tagung '93; M. Anton Ertl,
14397: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z, A
14398: Portable Forth Engine}}, EuroForth '93; M. Anton Ertl,
14399: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl02.ps.gz,
14400: Threaded code variations and optimizations (extended version)}},
14401: Forth-Tagung '02.
14402: 
14403: @menu
14404: * Portability::                 
14405: * Threading::                   
14406: * Primitives::                  
14407: * Performance::                 
14408: @end menu
14409: 
14410: @node Portability, Threading, Engine, Engine
14411: @section Portability
14412: @cindex engine portability
14413: 
14414: An important goal of the Gforth Project is availability across a wide
14415: range of personal machines. fig-Forth, and, to a lesser extent, F83,
14416: achieved this goal by manually coding the engine in assembly language
14417: for several then-popular processors. This approach is very
14418: labor-intensive and the results are short-lived due to progress in
14419: computer architecture.
14420: 
14421: @cindex C, using C for the engine
14422: Others have avoided this problem by coding in C, e.g., Mitch Bradley
14423: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
14424: particularly popular for UNIX-based Forths due to the large variety of
14425: architectures of UNIX machines. Unfortunately an implementation in C
14426: does not mix well with the goals of efficiency and with using
14427: traditional techniques: Indirect or direct threading cannot be expressed
14428: in C, and switch threading, the fastest technique available in C, is
14429: significantly slower. Another problem with C is that it is very
14430: cumbersome to express double integer arithmetic.
14431: 
14432: @cindex GNU C for the engine
14433: @cindex long long
14434: Fortunately, there is a portable language that does not have these
14435: limitations: GNU C, the version of C processed by the GNU C compiler
14436: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
14437: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
14438: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
14439: threading possible, its @code{long long} type (@pxref{Long Long, ,
14440: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forth's
14441: double numbers on many systems.  GNU C is freely available on all
14442: important (and many unimportant) UNIX machines, VMS, 80386s running
14443: MS-DOS, the Amiga, and the Atari ST, so a Forth written in GNU C can run
14444: on all these machines.
14445: 
14446: Writing in a portable language has the reputation of producing code that
14447: is slower than assembly. For our Forth engine we repeatedly looked at
14448: the code produced by the compiler and eliminated most compiler-induced
14449: inefficiencies by appropriate changes in the source code.
14450: 
14451: @cindex explicit register declarations
14452: @cindex --enable-force-reg, configuration flag
14453: @cindex -DFORCE_REG
14454: However, register allocation cannot be portably influenced by the
14455: programmer, leading to some inefficiencies on register-starved
14456: machines. We use explicit register declarations (@pxref{Explicit Reg
14457: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
14458: improve the speed on some machines. They are turned on by using the
14459: configuration flag @code{--enable-force-reg} (@code{gcc} switch
14460: @code{-DFORCE_REG}). Unfortunately, this feature not only depends on the
14461: machine, but also on the compiler version: On some machines some
14462: compiler versions produce incorrect code when certain explicit register
14463: declarations are used. So by default @code{-DFORCE_REG} is not used.
14464: 
14465: @node Threading, Primitives, Portability, Engine
14466: @section Threading
14467: @cindex inner interpreter implementation
14468: @cindex threaded code implementation
14469: 
14470: @cindex labels as values
14471: GNU C's labels as values extension (available since @code{gcc-2.0},
14472: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
14473: makes it possible to take the address of @i{label} by writing
14474: @code{&&@i{label}}.  This address can then be used in a statement like
14475: @code{goto *@i{address}}. I.e., @code{goto *&&x} is the same as
14476: @code{goto x}.
14477: 
14478: @cindex @code{NEXT}, indirect threaded
14479: @cindex indirect threaded inner interpreter
14480: @cindex inner interpreter, indirect threaded
14481: With this feature an indirect threaded @code{NEXT} looks like:
14482: @example
14483: cfa = *ip++;
14484: ca = *cfa;
14485: goto *ca;
14486: @end example
14487: @cindex instruction pointer
14488: For those unfamiliar with the names: @code{ip} is the Forth instruction
14489: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
14490: execution token and points to the code field of the next word to be
14491: executed; The @code{ca} (code address) fetched from there points to some
14492: executable code, e.g., a primitive or the colon definition handler
14493: @code{docol}.
14494: 
14495: @cindex @code{NEXT}, direct threaded
14496: @cindex direct threaded inner interpreter
14497: @cindex inner interpreter, direct threaded
14498: Direct threading is even simpler:
14499: @example
14500: ca = *ip++;
14501: goto *ca;
14502: @end example
14503: 
14504: Of course we have packaged the whole thing neatly in macros called
14505: @code{NEXT} and @code{NEXT1} (the part of @code{NEXT} after fetching the cfa).
14506: 
14507: @menu
14508: * Scheduling::                  
14509: * Direct or Indirect Threaded?::  
14510: * Dynamic Superinstructions::   
14511: * DOES>::                       
14512: @end menu
14513: 
14514: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
14515: @subsection Scheduling
14516: @cindex inner interpreter optimization
14517: 
14518: There is a little complication: Pipelined and superscalar processors,
14519: i.e., RISC and some modern CISC machines can process independent
14520: instructions while waiting for the results of an instruction. The
14521: compiler usually reorders (schedules) the instructions in a way that
14522: achieves good usage of these delay slots. However, on our first tries
14523: the compiler did not do well on scheduling primitives. E.g., for
14524: @code{+} implemented as
14525: @example
14526: n=sp[0]+sp[1];
14527: sp++;
14528: sp[0]=n;
14529: NEXT;
14530: @end example
14531: the @code{NEXT} comes strictly after the other code, i.e., there is
14532: nearly no scheduling. After a little thought the problem becomes clear:
14533: The compiler cannot know that @code{sp} and @code{ip} point to different
14534: addresses (and the version of @code{gcc} we used would not know it even
14535: if it was possible), so it could not move the load of the cfa above the
14536: store to the TOS. Indeed the pointers could be the same, if code on or
14537: very near the top of stack were executed. In the interest of speed we
14538: chose to forbid this probably unused ``feature'' and helped the compiler
14539: in scheduling: @code{NEXT} is divided into several parts:
14540: @code{NEXT_P0}, @code{NEXT_P1} and @code{NEXT_P2}). @code{+} now looks
14541: like:
14542: @example
14543: NEXT_P0;
14544: n=sp[0]+sp[1];
14545: sp++;
14546: NEXT_P1;
14547: sp[0]=n;
14548: NEXT_P2;
14549: @end example
14550: 
14551: There are various schemes that distribute the different operations of
14552: NEXT between these parts in several ways; in general, different schemes
14553: perform best on different processors.  We use a scheme for most
14554: architectures that performs well for most processors of this
14555: architecture; in the future we may switch to benchmarking and chosing
14556: the scheme on installation time.
14557: 
14558: 
14559: @node Direct or Indirect Threaded?, Dynamic Superinstructions, Scheduling, Threading
14560: @subsection Direct or Indirect Threaded?
14561: @cindex threading, direct or indirect?
14562: 
14563: Threaded forth code consists of references to primitives (simple machine
14564: code routines like @code{+}) and to non-primitives (e.g., colon
14565: definitions, variables, constants); for a specific class of
14566: non-primitives (e.g., variables) there is one code routine (e.g.,
14567: @code{dovar}), but each variable needs a separate reference to its data.
14568: 
14569: Traditionally Forth has been implemented as indirect threaded code,
14570: because this allows to use only one cell to reference a non-primitive
14571: (basically you point to the data, and find the code address there).
14572: 
14573: @cindex primitive-centric threaded code
14574: However, threaded code in Gforth (since 0.6.0) uses two cells for
14575: non-primitives, one for the code address, and one for the data address;
14576: the data pointer is an immediate argument for the virtual machine
14577: instruction represented by the code address.  We call this
14578: @emph{primitive-centric} threaded code, because all code addresses point
14579: to simple primitives.  E.g., for a variable, the code address is for
14580: @code{lit} (also used for integer literals like @code{99}).
14581: 
14582: Primitive-centric threaded code allows us to use (faster) direct
14583: threading as dispatch method, completely portably (direct threaded code
14584: in Gforth before 0.6.0 required architecture-specific code).  It also
14585: eliminates the performance problems related to I-cache consistency that
14586: 386 implementations have with direct threaded code, and allows
14587: additional optimizations.
14588: 
14589: @cindex hybrid direct/indirect threaded code
14590: There is a catch, however: the @var{xt} parameter of @code{execute} can
14591: occupy only one cell, so how do we pass non-primitives with their code
14592: @emph{and} data addresses to them?  Our answer is to use indirect
14593: threaded dispatch for @code{execute} and other words that use a
14594: single-cell xt.  So, normal threaded code in colon definitions uses
14595: direct threading, and @code{execute} and similar words, which dispatch
14596: to xts on the data stack, use indirect threaded code.  We call this
14597: @emph{hybrid direct/indirect} threaded code.
14598: 
14599: @cindex engines, gforth vs. gforth-fast vs. gforth-itc
14600: @cindex gforth engine
14601: @cindex gforth-fast engine
14602: The engines @command{gforth} and @command{gforth-fast} use hybrid
14603: direct/indirect threaded code.  This means that with these engines you
14604: cannot use @code{,} to compile an xt.  Instead, you have to use
14605: @code{compile,}.
14606: 
14607: @cindex gforth-itc engine
14608: If you want to compile xts with @code{,}, use @command{gforth-itc}.
14609: This engine uses plain old indirect threaded code.  It still compiles in
14610: a primitive-centric style, so you cannot use @code{compile,} instead of
14611: @code{,} (e.g., for producing tables of xts with @code{] word1 word2
14612: ... [}).  If you want to do that, you have to use @command{gforth-itc}
14613: and execute @code{' , is compile,}.  Your program can check if it is
14614: running on a hybrid direct/indirect threaded engine or a pure indirect
14615: threaded engine with @code{threading-method} (@pxref{Threading Words}).
14616: 
14617: 
14618: @node Dynamic Superinstructions, DOES>, Direct or Indirect Threaded?, Threading
14619: @subsection Dynamic Superinstructions
14620: @cindex Dynamic superinstructions with replication
14621: @cindex Superinstructions
14622: @cindex Replication
14623: 
14624: The engines @command{gforth} and @command{gforth-fast} use another
14625: optimization: Dynamic superinstructions with replication.  As an
14626: example, consider the following colon definition:
14627: 
14628: @example
14629: : squared ( n1 -- n2 )
14630:   dup * ;
14631: @end example
14632: 
14633: Gforth compiles this into the threaded code sequence
14634: 
14635: @example
14636: dup
14637: *
14638: ;s
14639: @end example
14640: 
14641: In normal direct threaded code there is a code address occupying one
14642: cell for each of these primitives.  Each code address points to a
14643: machine code routine, and the interpreter jumps to this machine code in
14644: order to execute the primitive.  The routines for these three
14645: primitives are (in @command{gforth-fast} on the 386):
14646: 
14647: @example
14648: Code dup  
14649: ( $804B950 )  add     esi , # -4  \ $83 $C6 $FC 
14650: ( $804B953 )  add     ebx , # 4  \ $83 $C3 $4 
14651: ( $804B956 )  mov     dword ptr 4 [esi] , ecx  \ $89 $4E $4 
14652: ( $804B959 )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC 
14653: end-code
14654: Code *  
14655: ( $804ACC4 )  mov     eax , dword ptr 4 [esi]  \ $8B $46 $4 
14656: ( $804ACC7 )  add     esi , # 4  \ $83 $C6 $4 
14657: ( $804ACCA )  add     ebx , # 4  \ $83 $C3 $4 
14658: ( $804ACCD )  imul    ecx , eax  \ $F $AF $C8 
14659: ( $804ACD0 )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC 
14660: end-code
14661: Code ;s  
14662: ( $804A693 )  mov     eax , dword ptr [edi]  \ $8B $7 
14663: ( $804A695 )  add     edi , # 4  \ $83 $C7 $4 
14664: ( $804A698 )  lea     ebx , dword ptr 4 [eax]  \ $8D $58 $4 
14665: ( $804A69B )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC 
14666: end-code
14667: @end example
14668: 
14669: With dynamic superinstructions and replication the compiler does not
14670: just lay down the threaded code, but also copies the machine code
14671: fragments, usually without the jump at the end.
14672: 
14673: @example
14674: ( $4057D27D )  add     esi , # -4  \ $83 $C6 $FC 
14675: ( $4057D280 )  add     ebx , # 4  \ $83 $C3 $4 
14676: ( $4057D283 )  mov     dword ptr 4 [esi] , ecx  \ $89 $4E $4 
14677: ( $4057D286 )  mov     eax , dword ptr 4 [esi]  \ $8B $46 $4 
14678: ( $4057D289 )  add     esi , # 4  \ $83 $C6 $4 
14679: ( $4057D28C )  add     ebx , # 4  \ $83 $C3 $4 
14680: ( $4057D28F )  imul    ecx , eax  \ $F $AF $C8 
14681: ( $4057D292 )  mov     eax , dword ptr [edi]  \ $8B $7 
14682: ( $4057D294 )  add     edi , # 4  \ $83 $C7 $4 
14683: ( $4057D297 )  lea     ebx , dword ptr 4 [eax]  \ $8D $58 $4 
14684: ( $4057D29A )  jmp     dword ptr FC [ebx]  \ $FF $63 $FC 
14685: @end example
14686: 
14687: Only when a threaded-code control-flow change happens (e.g., in
14688: @code{;s}), the jump is appended.  This optimization eliminates many of
14689: these jumps and makes the rest much more predictable.  The speedup
14690: depends on the processor and the application; on the Athlon and Pentium
14691: III this optimization typically produces a speedup by a factor of 2.
14692: 
14693: The code addresses in the direct-threaded code are set to point to the
14694: appropriate points in the copied machine code, in this example like
14695: this:
14696: 
14697: @example
14698: primitive  code address
14699:    dup       $4057D27D
14700:    *         $4057D286
14701:    ;s        $4057D292
14702: @end example
14703: 
14704: Thus there can be threaded-code jumps to any place in this piece of
14705: code.  This also simplifies decompilation quite a bit.
14706: 
14707: @cindex --no-dynamic command-line option
14708: @cindex --no-super command-line option
14709: You can disable this optimization with @option{--no-dynamic}.  You can
14710: use the copying without eliminating the jumps (i.e., dynamic
14711: replication, but without superinstructions) with @option{--no-super};
14712: this gives the branch prediction benefit alone; the effect on
14713: performance depends on the CPU; on the Athlon and Pentium III the
14714: speedup is a little less than for dynamic superinstructions with
14715: replication.
14716: 
14717: @cindex patching threaded code
14718: One use of these options is if you want to patch the threaded code.
14719: With superinstructions, many of the dispatch jumps are eliminated, so
14720: patching often has no effect.  These options preserve all the dispatch
14721: jumps.
14722: 
14723: @cindex --dynamic command-line option
14724: On some machines dynamic superinstructions are disabled by default,
14725: because it is unsafe on these machines.  However, if you feel
14726: adventurous, you can enable it with @option{--dynamic}.
14727: 
14728: @node DOES>,  , Dynamic Superinstructions, Threading
14729: @subsection DOES>
14730: @cindex @code{DOES>} implementation
14731: 
14732: @cindex @code{dodoes} routine
14733: @cindex @code{DOES>}-code
14734: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
14735: the chunk of code executed by every word defined by a
14736: @code{CREATE}...@code{DOES>} pair; actually with primitive-centric code,
14737: this is only needed if the xt of the word is @code{execute}d. The main
14738: problem here is: How to find the Forth code to be executed, i.e. the
14739: code after the @code{DOES>} (the @code{DOES>}-code)? There are two
14740: solutions:
14741: 
14742: In fig-Forth the code field points directly to the @code{dodoes} and the
14743: @code{DOES>}-code address is stored in the cell after the code address
14744: (i.e. at @code{@i{CFA} cell+}). It may seem that this solution is
14745: illegal in the Forth-79 and all later standards, because in fig-Forth
14746: this address lies in the body (which is illegal in these
14747: standards). However, by making the code field larger for all words this
14748: solution becomes legal again.  We use this approach.  Leaving a cell
14749: unused in most words is a bit wasteful, but on the machines we are
14750: targeting this is hardly a problem.
14751: 
14752: 
14753: @node Primitives, Performance, Threading, Engine
14754: @section Primitives
14755: @cindex primitives, implementation
14756: @cindex virtual machine instructions, implementation
14757: 
14758: @menu
14759: * Automatic Generation::        
14760: * TOS Optimization::            
14761: * Produced code::               
14762: @end menu
14763: 
14764: @node Automatic Generation, TOS Optimization, Primitives, Primitives
14765: @subsection Automatic Generation
14766: @cindex primitives, automatic generation
14767: 
14768: @cindex @file{prims2x.fs}
14769: 
14770: Since the primitives are implemented in a portable language, there is no
14771: longer any need to minimize the number of primitives. On the contrary,
14772: having many primitives has an advantage: speed. In order to reduce the
14773: number of errors in primitives and to make programming them easier, we
14774: provide a tool, the primitive generator (@file{prims2x.fs} aka Vmgen,
14775: @pxref{Top, Vmgen, Introduction, vmgen, Vmgen}), that automatically
14776: generates most (and sometimes all) of the C code for a primitive from
14777: the stack effect notation.  The source for a primitive has the following
14778: form:
14779: 
14780: @cindex primitive source format
14781: @format
14782: @i{Forth-name}  ( @i{stack-effect} )        @i{category}    [@i{pronounc.}]
14783: [@code{""}@i{glossary entry}@code{""}]
14784: @i{C code}
14785: [@code{:}
14786: @i{Forth code}]
14787: @end format
14788: 
14789: The items in brackets are optional. The category and glossary fields
14790: are there for generating the documentation, the Forth code is there
14791: for manual implementations on machines without GNU C. E.g., the source
14792: for the primitive @code{+} is:
14793: @example
14794: +    ( n1 n2 -- n )   core    plus
14795: n = n1+n2;
14796: @end example
14797: 
14798: This looks like a specification, but in fact @code{n = n1+n2} is C
14799: code. Our primitive generation tool extracts a lot of information from
14800: the stack effect notations@footnote{We use a one-stack notation, even
14801: though we have separate data and floating-point stacks; The separate
14802: notation can be generated easily from the unified notation.}: The number
14803: of items popped from and pushed on the stack, their type, and by what
14804: name they are referred to in the C code. It then generates a C code
14805: prelude and postlude for each primitive. The final C code for @code{+}
14806: looks like this:
14807: 
14808: @example
14809: I_plus: /* + ( n1 n2 -- n ) */  /* label, stack effect */
14810: /*  */                          /* documentation */
14811: NAME("+")                       /* debugging output (with -DDEBUG) */
14812: @{
14813: DEF_CA                          /* definition of variable ca (indirect threading) */
14814: Cell n1;                        /* definitions of variables */
14815: Cell n2;
14816: Cell n;
14817: NEXT_P0;                        /* NEXT part 0 */
14818: n1 = (Cell) sp[1];              /* input */
14819: n2 = (Cell) TOS;
14820: sp += 1;                        /* stack adjustment */
14821: @{
14822: n = n1+n2;                      /* C code taken from the source */
14823: @}
14824: NEXT_P1;                        /* NEXT part 1 */
14825: TOS = (Cell)n;                  /* output */
14826: NEXT_P2;                        /* NEXT part 2 */
14827: @}
14828: @end example
14829: 
14830: This looks long and inefficient, but the GNU C compiler optimizes quite
14831: well and produces optimal code for @code{+} on, e.g., the R3000 and the
14832: HP RISC machines: Defining the @code{n}s does not produce any code, and
14833: using them as intermediate storage also adds no cost.
14834: 
14835: There are also other optimizations that are not illustrated by this
14836: example: assignments between simple variables are usually for free (copy
14837: propagation). If one of the stack items is not used by the primitive
14838: (e.g.  in @code{drop}), the compiler eliminates the load from the stack
14839: (dead code elimination). On the other hand, there are some things that
14840: the compiler does not do, therefore they are performed by
14841: @file{prims2x.fs}: The compiler does not optimize code away that stores
14842: a stack item to the place where it just came from (e.g., @code{over}).
14843: 
14844: While programming a primitive is usually easy, there are a few cases
14845: where the programmer has to take the actions of the generator into
14846: account, most notably @code{?dup}, but also words that do not (always)
14847: fall through to @code{NEXT}.
14848: 
14849: For more information
14850: 
14851: @node TOS Optimization, Produced code, Automatic Generation, Primitives
14852: @subsection TOS Optimization
14853: @cindex TOS optimization for primitives
14854: @cindex primitives, keeping the TOS in a register
14855: 
14856: An important optimization for stack machine emulators, e.g., Forth
14857: engines, is keeping  one or more of the top stack items in
14858: registers.  If a word has the stack effect @i{in1}...@i{inx} @code{--}
14859: @i{out1}...@i{outy}, keeping the top @i{n} items in registers
14860: @itemize @bullet
14861: @item
14862: is better than keeping @i{n-1} items, if @i{x>=n} and @i{y>=n},
14863: due to fewer loads from and stores to the stack.
14864: @item is slower than keeping @i{n-1} items, if @i{x<>y} and @i{x<n} and
14865: @i{y<n}, due to additional moves between registers.
14866: @end itemize
14867: 
14868: @cindex -DUSE_TOS
14869: @cindex -DUSE_NO_TOS
14870: In particular, keeping one item in a register is never a disadvantage,
14871: if there are enough registers. Keeping two items in registers is a
14872: disadvantage for frequent words like @code{?branch}, constants,
14873: variables, literals and @code{i}. Therefore our generator only produces
14874: code that keeps zero or one items in registers. The generated C code
14875: covers both cases; the selection between these alternatives is made at
14876: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
14877: code for @code{+} is just a simple variable name in the one-item case,
14878: otherwise it is a macro that expands into @code{sp[0]}. Note that the
14879: GNU C compiler tries to keep simple variables like @code{TOS} in
14880: registers, and it usually succeeds, if there are enough registers.
14881: 
14882: @cindex -DUSE_FTOS
14883: @cindex -DUSE_NO_FTOS
14884: The primitive generator performs the TOS optimization for the
14885: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
14886: operations the benefit of this optimization is even larger:
14887: floating-point operations take quite long on most processors, but can be
14888: performed in parallel with other operations as long as their results are
14889: not used. If the FP-TOS is kept in a register, this works. If
14890: it is kept on the stack, i.e., in memory, the store into memory has to
14891: wait for the result of the floating-point operation, lengthening the
14892: execution time of the primitive considerably.
14893: 
14894: The TOS optimization makes the automatic generation of primitives a
14895: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
14896: @code{TOS} is not sufficient. There are some special cases to
14897: consider:
14898: @itemize @bullet
14899: @item In the case of @code{dup ( w -- w w )} the generator must not
14900: eliminate the store to the original location of the item on the stack,
14901: if the TOS optimization is turned on.
14902: @item Primitives with stack effects of the form @code{--}
14903: @i{out1}...@i{outy} must store the TOS to the stack at the start.
14904: Likewise, primitives with the stack effect @i{in1}...@i{inx} @code{--}
14905: must load the TOS from the stack at the end. But for the null stack
14906: effect @code{--} no stores or loads should be generated.
14907: @end itemize
14908: 
14909: @node Produced code,  , TOS Optimization, Primitives
14910: @subsection Produced code
14911: @cindex primitives, assembly code listing
14912: 
14913: @cindex @file{engine.s}
14914: To see what assembly code is produced for the primitives on your machine
14915: with your compiler and your flag settings, type @code{make engine.s} and
14916: look at the resulting file @file{engine.s}.  Alternatively, you can also
14917: disassemble the code of primitives with @code{see} on some architectures.
14918: 
14919: @node  Performance,  , Primitives, Engine
14920: @section Performance
14921: @cindex performance of some Forth interpreters
14922: @cindex engine performance
14923: @cindex benchmarking Forth systems
14924: @cindex Gforth performance
14925: 
14926: On RISCs the Gforth engine is very close to optimal; i.e., it is usually
14927: impossible to write a significantly faster threaded-code engine.
14928: 
14929: On register-starved machines like the 386 architecture processors
14930: improvements are possible, because @code{gcc} does not utilize the
14931: registers as well as a human, even with explicit register declarations;
14932: e.g., Bernd Beuster wrote a Forth system fragment in assembly language
14933: and hand-tuned it for the 486; this system is 1.19 times faster on the
14934: Sieve benchmark on a 486DX2/66 than Gforth compiled with
14935: @code{gcc-2.6.3} with @code{-DFORCE_REG}.  The situation has improved
14936: with gcc-2.95 and gforth-0.4.9; now the most important virtual machine
14937: registers fit in real registers (and we can even afford to use the TOS
14938: optimization), resulting in a speedup of 1.14 on the sieve over the
14939: earlier results.  And dynamic superinstructions provide another speedup
14940: (but only around a factor 1.2 on the 486).
14941: 
14942: @cindex Win32Forth performance
14943: @cindex NT Forth performance
14944: @cindex eforth performance
14945: @cindex ThisForth performance
14946: @cindex PFE performance
14947: @cindex TILE performance
14948: The potential advantage of assembly language implementations is not
14949: necessarily realized in complete Forth systems: We compared Gforth-0.5.9
14950: (direct threaded, compiled with @code{gcc-2.95.1} and
14951: @code{-DFORCE_REG}) with Win32Forth 1.2093 (newer versions are
14952: reportedly much faster), LMI's NT Forth (Beta, May 1994) and Eforth
14953: (with and without peephole (aka pinhole) optimization of the threaded
14954: code); all these systems were written in assembly language. We also
14955: compared Gforth with three systems written in C: PFE-0.9.14 (compiled
14956: with @code{gcc-2.6.3} with the default configuration for Linux:
14957: @code{-O2 -fomit-frame-pointer -DUSE_REGS -DUNROLL_NEXT}), ThisForth
14958: Beta (compiled with @code{gcc-2.6.3 -O3 -fomit-frame-pointer}; ThisForth
14959: employs peephole optimization of the threaded code) and TILE (compiled
14960: with @code{make opt}). We benchmarked Gforth, PFE, ThisForth and TILE on
14961: a 486DX2/66 under Linux. Kenneth O'Heskin kindly provided the results
14962: for Win32Forth and NT Forth on a 486DX2/66 with similar memory
14963: performance under Windows NT. Marcel Hendrix ported Eforth to Linux,
14964: then extended it to run the benchmarks, added the peephole optimizer,
14965: ran the benchmarks and reported the results.
14966: 
14967: We used four small benchmarks: the ubiquitous Sieve; bubble-sorting and
14968: matrix multiplication come from the Stanford integer benchmarks and have
14969: been translated into Forth by Martin Fraeman; we used the versions
14970: included in the TILE Forth package, but with bigger data set sizes; and
14971: a recursive Fibonacci number computation for benchmarking calling
14972: performance. The following table shows the time taken for the benchmarks
14973: scaled by the time taken by Gforth (in other words, it shows the speedup
14974: factor that Gforth achieved over the other systems).
14975: 
14976: @example
14977: relative       Win32-    NT       eforth       This-      
14978: time     Gforth Forth Forth eforth  +opt   PFE Forth  TILE
14979: sieve      1.00  2.16  1.78   2.16  1.32  2.46  4.96 13.37
14980: bubble     1.00  1.93  2.07   2.18  1.29  2.21        5.70
14981: matmul     1.00  1.92  1.76   1.90  0.96  2.06        5.32
14982: fib        1.00  2.32  2.03   1.86  1.31  2.64  4.55  6.54
14983: @end example
14984: 
14985: You may be quite surprised by the good performance of Gforth when
14986: compared with systems written in assembly language. One important reason
14987: for the disappointing performance of these other systems is probably
14988: that they are not written optimally for the 486 (e.g., they use the
14989: @code{lods} instruction). In addition, Win32Forth uses a comfortable,
14990: but costly method for relocating the Forth image: like @code{cforth}, it
14991: computes the actual addresses at run time, resulting in two address
14992: computations per @code{NEXT} (@pxref{Image File Background}).
14993: 
14994: The speedup of Gforth over PFE, ThisForth and TILE can be easily
14995: explained with the self-imposed restriction of the latter systems to
14996: standard C, which makes efficient threading impossible (however, the
14997: measured implementation of PFE uses a GNU C extension: @pxref{Global Reg
14998: Vars, , Defining Global Register Variables, gcc.info, GNU C Manual}).
14999: Moreover, current C compilers have a hard time optimizing other aspects
15000: of the ThisForth and the TILE source.
15001: 
15002: The performance of Gforth on 386 architecture processors varies widely
15003: with the version of @code{gcc} used. E.g., @code{gcc-2.5.8} failed to
15004: allocate any of the virtual machine registers into real machine
15005: registers by itself and would not work correctly with explicit register
15006: declarations, giving a significantly slower engine (on a 486DX2/66
15007: running the Sieve) than the one measured above.
15008: 
15009: Note that there have been several releases of Win32Forth since the
15010: release presented here, so the results presented above may have little
15011: predictive value for the performance of Win32Forth today (results for
15012: the current release on an i486DX2/66 are welcome).
15013: 
15014: @cindex @file{Benchres}
15015: In
15016: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl&maierhofer95.ps.gz,
15017: Translating Forth to Efficient C}} by M. Anton Ertl and Martin
15018: Maierhofer (presented at EuroForth '95), an indirect threaded version of
15019: Gforth is compared with Win32Forth, NT Forth, PFE, ThisForth, and
15020: several native code systems; that version of Gforth is slower on a 486
15021: than the version used here. You can find a newer version of these
15022: measurements at
15023: @uref{http://www.complang.tuwien.ac.at/forth/performance.html}. You can
15024: find numbers for Gforth on various machines in @file{Benchres}.
15025: 
15026: @c ******************************************************************
15027: @c @node Binding to System Library, Cross Compiler, Engine, Top
15028: @c @chapter Binding to System Library
15029: 
15030: @c ****************************************************************
15031: @node Cross Compiler, Bugs, Engine, Top
15032: @chapter Cross Compiler
15033: @cindex @file{cross.fs}
15034: @cindex cross-compiler
15035: @cindex metacompiler
15036: @cindex target compiler
15037: 
15038: The cross compiler is used to bootstrap a Forth kernel. Since Gforth is
15039: mostly written in Forth, including crucial parts like the outer
15040: interpreter and compiler, it needs compiled Forth code to get
15041: started. The cross compiler allows to create new images for other
15042: architectures, even running under another Forth system.
15043: 
15044: @menu
15045: * Using the Cross Compiler::    
15046: * How the Cross Compiler Works::  
15047: @end menu
15048: 
15049: @node Using the Cross Compiler, How the Cross Compiler Works, Cross Compiler, Cross Compiler
15050: @section Using the Cross Compiler
15051: 
15052: The cross compiler uses a language that resembles Forth, but isn't. The
15053: main difference is that you can execute Forth code after definition,
15054: while you usually can't execute the code compiled by cross, because the
15055: code you are compiling is typically for a different computer than the
15056: one you are compiling on.
15057: 
15058: @c anton: This chapter is somewhat different from waht I would expect: I
15059: @c would expect an explanation of the cross language and how to create an
15060: @c application image with it.  The section explains some aspects of
15061: @c creating a Gforth kernel.
15062: 
15063: The Makefile is already set up to allow you to create kernels for new
15064: architectures with a simple make command. The generic kernels using the
15065: GCC compiled virtual machine are created in the normal build process
15066: with @code{make}. To create a embedded Gforth executable for e.g. the
15067: 8086 processor (running on a DOS machine), type
15068: 
15069: @example
15070: make kernl-8086.fi
15071: @end example
15072: 
15073: This will use the machine description from the @file{arch/8086}
15074: directory to create a new kernel. A machine file may look like that:
15075: 
15076: @example
15077: \ Parameter for target systems                         06oct92py
15078: 
15079:     4 Constant cell             \ cell size in bytes
15080:     2 Constant cell<<           \ cell shift to bytes
15081:     5 Constant cell>bit         \ cell shift to bits
15082:     8 Constant bits/char        \ bits per character
15083:     8 Constant bits/byte        \ bits per byte [default: 8]
15084:     8 Constant float            \ bytes per float
15085:     8 Constant /maxalign        \ maximum alignment in bytes
15086: false Constant bigendian        \ byte order
15087: ( true=big, false=little )
15088: 
15089: include machpc.fs               \ feature list
15090: @end example
15091: 
15092: This part is obligatory for the cross compiler itself, the feature list
15093: is used by the kernel to conditionally compile some features in and out,
15094: depending on whether the target supports these features.
15095: 
15096: There are some optional features, if you define your own primitives,
15097: have an assembler, or need special, nonstandard preparation to make the
15098: boot process work. @code{asm-include} includes an assembler,
15099: @code{prims-include} includes primitives, and @code{>boot} prepares for
15100: booting.
15101: 
15102: @example
15103: : asm-include    ." Include assembler" cr
15104:   s" arch/8086/asm.fs" included ;
15105: 
15106: : prims-include  ." Include primitives" cr
15107:   s" arch/8086/prim.fs" included ;
15108: 
15109: : >boot          ." Prepare booting" cr
15110:   s" ' boot >body into-forth 1+ !" evaluate ;
15111: @end example
15112: 
15113: These words are used as sort of macro during the cross compilation in
15114: the file @file{kernel/main.fs}. Instead of using these macros, it would
15115: be possible --- but more complicated --- to write a new kernel project
15116: file, too.
15117: 
15118: @file{kernel/main.fs} expects the machine description file name on the
15119: stack; the cross compiler itself (@file{cross.fs}) assumes that either
15120: @code{mach-file} leaves a counted string on the stack, or
15121: @code{machine-file} leaves an address, count pair of the filename on the
15122: stack.
15123: 
15124: The feature list is typically controlled using @code{SetValue}, generic
15125: files that are used by several projects can use @code{DefaultValue}
15126: instead. Both functions work like @code{Value}, when the value isn't
15127: defined, but @code{SetValue} works like @code{to} if the value is
15128: defined, and @code{DefaultValue} doesn't set anything, if the value is
15129: defined.
15130: 
15131: @example
15132: \ generic mach file for pc gforth                       03sep97jaw
15133: 
15134: true DefaultValue NIL  \ relocating
15135: 
15136: >ENVIRON
15137: 
15138: true DefaultValue file          \ controls the presence of the
15139:                                 \ file access wordset
15140: true DefaultValue OS            \ flag to indicate a operating system
15141: 
15142: true DefaultValue prims         \ true: primitives are c-code
15143: 
15144: true DefaultValue floating      \ floating point wordset is present
15145: 
15146: true DefaultValue glocals       \ gforth locals are present
15147:                                 \ will be loaded
15148: true DefaultValue dcomps        \ double number comparisons
15149: 
15150: true DefaultValue hash          \ hashing primitives are loaded/present
15151: 
15152: true DefaultValue xconds        \ used together with glocals,
15153:                                 \ special conditionals supporting gforths'
15154:                                 \ local variables
15155: true DefaultValue header        \ save a header information
15156: 
15157: true DefaultValue backtrace     \ enables backtrace code
15158: 
15159: false DefaultValue ec
15160: false DefaultValue crlf
15161: 
15162: cell 2 = [IF] &32 [ELSE] &256 [THEN] KB DefaultValue kernel-size
15163: 
15164: &16 KB          DefaultValue stack-size
15165: &15 KB &512 +   DefaultValue fstack-size
15166: &15 KB          DefaultValue rstack-size
15167: &14 KB &512 +   DefaultValue lstack-size
15168: @end example
15169: 
15170: @node How the Cross Compiler Works,  , Using the Cross Compiler, Cross Compiler
15171: @section How the Cross Compiler Works
15172: 
15173: @node Bugs, Origin, Cross Compiler, Top
15174: @appendix Bugs
15175: @cindex bug reporting
15176: 
15177: Known bugs are described in the file @file{BUGS} in the Gforth distribution.
15178: 
15179: If you find a bug, please submit a bug report through
15180: @uref{https://savannah.gnu.org/bugs/?func=addbug&group=gforth}.
15181: 
15182: @itemize @bullet
15183: @item
15184: A program (or a sequence of keyboard commands) that reproduces the bug.
15185: @item
15186: A description of what you think constitutes the buggy behaviour.
15187: @item
15188: The Gforth version used (it is announced at the start of an
15189: interactive Gforth session).
15190: @item
15191: The machine and operating system (on Unix
15192: systems @code{uname -a} will report this information).
15193: @item
15194: The installation options (you can find the configure options at the
15195: start of @file{config.status}) and configuration (@code{configure}
15196: output or @file{config.cache}).
15197: @item
15198: A complete list of changes (if any) you (or your installer) have made to the
15199: Gforth sources.
15200: @end itemize
15201: 
15202: For a thorough guide on reporting bugs read @ref{Bug Reporting, , How
15203: to Report Bugs, gcc.info, GNU C Manual}.
15204: 
15205: 
15206: @node Origin, Forth-related information, Bugs, Top
15207: @appendix Authors and Ancestors of Gforth
15208: 
15209: @section Authors and Contributors
15210: @cindex authors of Gforth
15211: @cindex contributors to Gforth
15212: 
15213: The Gforth project was started in mid-1992 by Bernd Paysan and Anton
15214: Ertl. The third major author was Jens Wilke.  Neal Crook contributed a
15215: lot to the manual.  Assemblers and disassemblers were contributed by
15216: Andrew McKewan, Christian Pirker, and Bernd Thallner.  Lennart Benschop
15217: (who was one of Gforth's first users, in mid-1993) and Stuart Ramsden
15218: inspired us with their continuous feedback. Lennart Benshop contributed
15219: @file{glosgen.fs}, while Stuart Ramsden has been working on automatic
15220: support for calling C libraries. Helpful comments also came from Paul
15221: Kleinrubatscher, Christian Pirker, Dirk Zoller, Marcel Hendrix, John
15222: Wavrik, Barrie Stott, Marc de Groot, Jorge Acerada, Bruce Hoyt, Robert
15223: Epprecht, Dennis Ruffer and David N. Williams. Since the release of
15224: Gforth-0.2.1 there were also helpful comments from many others; thank
15225: you all, sorry for not listing you here (but digging through my mailbox
15226: to extract your names is on my to-do list).
15227: 
15228: Gforth also owes a lot to the authors of the tools we used (GCC, CVS,
15229: and autoconf, among others), and to the creators of the Internet: Gforth
15230: was developed across the Internet, and its authors did not meet
15231: physically for the first 4 years of development.
15232: 
15233: @section Pedigree
15234: @cindex pedigree of Gforth
15235: 
15236: Gforth descends from bigFORTH (1993) and fig-Forth.  Of course, a
15237: significant part of the design of Gforth was prescribed by ANS Forth.
15238: 
15239: Bernd Paysan wrote bigFORTH, a descendent from TurboForth, an unreleased
15240: 32 bit native code version of VolksForth for the Atari ST, written
15241: mostly by Dietrich Weineck.
15242: 
15243: VolksForth was written by Klaus Schleisiek, Bernd Pennemann, Georg
15244: Rehfeld and Dietrich Weineck for the C64 (called UltraForth there) in
15245: the mid-80s and ported to the Atari ST in 1986.  It descends from F83.
15246: 
15247: Henry Laxen and Mike Perry wrote F83 as a model implementation of the
15248: Forth-83 standard. !! Pedigree? When?
15249: 
15250: A team led by Bill Ragsdale implemented fig-Forth on many processors in
15251: 1979. Robert Selzer and Bill Ragsdale developed the original
15252: implementation of fig-Forth for the 6502 based on microForth.
15253: 
15254: The principal architect of microForth was Dean Sanderson. microForth was
15255: FORTH, Inc.'s first off-the-shelf product. It was developed in 1976 for
15256: the 1802, and subsequently implemented on the 8080, the 6800 and the
15257: Z80.
15258: 
15259: All earlier Forth systems were custom-made, usually by Charles Moore,
15260: who discovered (as he puts it) Forth during the late 60s. The first full
15261: Forth existed in 1971.
15262: 
15263: A part of the information in this section comes from
15264: @cite{@uref{http://www.forth.com/Content/History/History1.htm,The
15265: Evolution of Forth}} by Elizabeth D. Rather, Donald R. Colburn and
15266: Charles H. Moore, presented at the HOPL-II conference and preprinted in
15267: SIGPLAN Notices 28(3), 1993.  You can find more historical and
15268: genealogical information about Forth there.
15269: 
15270: @c ------------------------------------------------------------------
15271: @node Forth-related information, Licenses, Origin, Top
15272: @appendix Other Forth-related information
15273: @cindex Forth-related information
15274: 
15275: @c anton: I threw most of this stuff out, because it can be found through
15276: @c the FAQ and the FAQ is more likely to be up-to-date.
15277: 
15278: @cindex comp.lang.forth
15279: @cindex frequently asked questions
15280: There is an active news group (comp.lang.forth) discussing Forth
15281: (including Gforth) and Forth-related issues. Its
15282: @uref{http://www.complang.tuwien.ac.at/forth/faq/faq-general-2.html,FAQs}
15283: (frequently asked questions and their answers) contains a lot of
15284: information on Forth.  You should read it before posting to
15285: comp.lang.forth.
15286: 
15287: The ANS Forth standard is most usable in its
15288: @uref{http://www.taygeta.com/forth/dpans.html, HTML form}.
15289: 
15290: @c ---------------------------------------------------
15291: @node  Licenses, Word Index, Forth-related information, Top
15292: @appendix Licenses
15293: 
15294: @menu
15295: * GNU Free Documentation License::  License for copying this manual.
15296: * Copying::                         GPL (for copying this software).
15297: @end menu
15298: 
15299: @include fdl.texi
15300: 
15301: @include gpl.texi
15302: 
15303: 
15304: 
15305: @c ------------------------------------------------------------------
15306: @node Word Index, Concept Index, Licenses, Top
15307: @unnumbered Word Index
15308: 
15309: This index is a list of Forth words that have ``glossary'' entries
15310: within this manual. Each word is listed with its stack effect and
15311: wordset.
15312: 
15313: @printindex fn
15314: 
15315: @c anton: the name index seems superfluous given the word and concept indices.
15316: 
15317: @c @node Name Index, Concept Index, Word Index, Top
15318: @c @unnumbered Name Index
15319: 
15320: @c This index is a list of Forth words that have ``glossary'' entries
15321: @c within this manual.
15322: 
15323: @c @printindex ky
15324: 
15325: @c -------------------------------------------------------
15326: @node Concept Index,  , Word Index, Top
15327: @unnumbered Concept and Word Index
15328: 
15329: Not all entries listed in this index are present verbatim in the
15330: text. This index also duplicates, in abbreviated form, all of the words
15331: listed in the Word Index (only the names are listed for the words here).
15332: 
15333: @printindex cp
15334: 
15335: @bye
15336: 
15337: 
15338: 

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