File:  [gforth] / gforth / Attic / gforth.ds
Revision 1.14: download - view: text, annotated - select for diffs
Thu Apr 20 09:42:52 1995 UTC (24 years, 7 months ago) by anton
Branches: MAIN
CVS tags: HEAD
added "system documentation requirements" section to gforth.ds.
added answers for environmental queries for wordsets.
changed W/O file access mode from "w+" to "w".
S" now uses a buffer
BIN is now idempotent
added FILE-STATUS
some other minor changes and bug fixes.

    1: \input texinfo   @c -*-texinfo-*-
    2: @comment The source is gforth.ds, from which gforth.texi is generated
    3: @comment %**start of header (This is for running Texinfo on a region.)
    4: @setfilename gforth.info
    5: @settitle GNU Forth Manual
    6: @comment @setchapternewpage odd
    7: @comment %**end of header (This is for running Texinfo on a region.)
    8: 
    9: @ifinfo
   10: This file documents GNU Forth 0.0
   11: 
   12: Copyright @copyright{} 1994 GNU Forth Development Group
   13: 
   14:      Permission is granted to make and distribute verbatim copies of
   15:      this manual provided the copyright notice and this permission notice
   16:      are preserved on all copies.
   17:      
   18: @ignore
   19:      Permission is granted to process this file through TeX and print the
   20:      results, provided the printed document carries a copying permission
   21:      notice identical to this one except for the removal of this paragraph
   22:      (this paragraph not being relevant to the printed manual).
   23:      
   24: @end ignore
   25:      Permission is granted to copy and distribute modified versions of this
   26:      manual under the conditions for verbatim copying, provided also that the
   27:      sections entitled "Distribution" and "General Public License" are
   28:      included exactly as in the original, and provided that the entire
   29:      resulting derived work is distributed under the terms of a permission
   30:      notice identical to this one.
   31:      
   32:      Permission is granted to copy and distribute translations of this manual
   33:      into another language, under the above conditions for modified versions,
   34:      except that the sections entitled "Distribution" and "General Public
   35:      License" may be included in a translation approved by the author instead
   36:      of in the original English.
   37: @end ifinfo
   38: 
   39: @titlepage
   40: @sp 10
   41: @center @titlefont{GNU Forth Manual}
   42: @sp 2
   43: @center for version 0.0
   44: @sp 2
   45: @center Anton Ertl
   46: 
   47: @comment  The following two commands start the copyright page.
   48: @page
   49: @vskip 0pt plus 1filll
   50: Copyright @copyright{} 1994 GNU Forth Development Group
   51: 
   52: @comment !! Published by ... or You can get a copy of this manual ...
   53: 
   54:      Permission is granted to make and distribute verbatim copies of
   55:      this manual provided the copyright notice and this permission notice
   56:      are preserved on all copies.
   57:      
   58:      Permission is granted to copy and distribute modified versions of this
   59:      manual under the conditions for verbatim copying, provided also that the
   60:      sections entitled "Distribution" and "General Public License" are
   61:      included exactly as in the original, and provided that the entire
   62:      resulting derived work is distributed under the terms of a permission
   63:      notice identical to this one.
   64:      
   65:      Permission is granted to copy and distribute translations of this manual
   66:      into another language, under the above conditions for modified versions,
   67:      except that the sections entitled "Distribution" and "General Public
   68:      License" may be included in a translation approved by the author instead
   69:      of in the original English.
   70: @end titlepage
   71: 
   72: 
   73: @node Top, License, (dir), (dir)
   74: @ifinfo
   75: GNU Forth is a free implementation of ANS Forth available on many
   76: personal machines. This manual corresponds to version 0.0.
   77: @end ifinfo
   78: 
   79: @menu
   80: * License::                     
   81: * Goals::                       About the GNU Forth Project
   82: * Other Books::                 Things you might want to read
   83: * Invocation::                  Starting GNU Forth
   84: * Words::                       Forth words available in GNU Forth
   85: * ANS conformance::             Implementation-defined options etc.
   86: * Model::                       The abstract machine of GNU Forth
   87: * Emacs and GForth::            The GForth Mode
   88: * Internals::                   Implementation details
   89: * Bugs::                        How to report them
   90: * Pedigree::                    Ancestors of GNU Forth
   91: * Word Index::                  An item for each Forth word
   92: * Node Index::                  An item for each node
   93: @end menu
   94: 
   95: @node License, Goals, Top, Top
   96: @unnumbered License
   97: !! Insert GPL here
   98: 
   99: @iftex
  100: @unnumbered Preface
  101: This manual documents GNU Forth. The reader is expected to know
  102: Forth. This manual is primarily a reference manual. @xref{Other Books}
  103: for introductory material.
  104: @end iftex
  105: 
  106: @node    Goals, Other Books, License, Top
  107: @comment node-name,     next,           previous, up
  108: @chapter Goals of GNU Forth
  109: @cindex Goals
  110: The goal of the GNU Forth Project is to develop a standard model for
  111: ANSI Forth. This can be split into several subgoals:
  112: 
  113: @itemize @bullet
  114: @item
  115: GNU Forth should conform to the ANSI Forth standard.
  116: @item
  117: It should be a model, i.e. it should define all the
  118: implementation-dependent things.
  119: @item
  120: It should become standard, i.e. widely accepted and used. This goal
  121: is the most difficult one.
  122: @end itemize
  123: 
  124: To achieve these goals GNU Forth should be
  125: @itemize @bullet
  126: @item
  127: Similar to previous models (fig-Forth, F83)
  128: @item
  129: Powerful. It should provide for all the things that are considered
  130: necessary today and even some that are not yet considered necessary.
  131: @item
  132: Efficient. It should not get the reputation of being exceptionally
  133: slow.
  134: @item
  135: Free.
  136: @item
  137: Available on many machines/easy to port.
  138: @end itemize
  139: 
  140: Have we achieved these goals? GNU Forth conforms to the ANS Forth
  141: standard; it may be considered a model, but we have not yet documented
  142: which parts of the model are stable and which parts we are likely to
  143: change; it certainly has not yet become a de facto standard. It has some
  144: similarities and some differences to previous models; It has some
  145: powerful features, but not yet everything that we envisioned; on RISCs
  146: it is as fast as interpreters programmed in assembly, on
  147: register-starved machines it is not so fast, but still faster than any
  148: other C-based interpretive implementation; it is free and available on
  149: many machines.
  150: 
  151: @node Other Books, Invocation, Goals, Top
  152: @chapter Other books on ANS Forth
  153: 
  154: As the standard is relatively new, there are not many books out yet. It
  155: is not recommended to learn Forth by using GNU Forth and a book that is
  156: not written for ANS Forth, as you will not know your mistakes from the
  157: deviations of the book.
  158: 
  159: There is, of course, the standard, the definite reference if you want to
  160: write ANS Forth programs. It will be available in printed form from
  161: Global Engineering Documents !! somtime in spring or summer 1994. If you
  162: are lucky, you can still get dpANS6 (the draft that was approved as
  163: standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14.
  164: 
  165: @cite{Forth: The new model} by Jack Woehr (!! Publisher) is an
  166: introductory book based on a draft version of the standard. It does not
  167: cover the whole standard. It also contains interesting background
  168: information (Jack Woehr was in the ANS Forth Technical Committe). It is
  169: not appropriate for complete newbies, but programmers experienced in
  170: other languages should find it ok.
  171: 
  172: @node Invocation, Words, Other Books, Top
  173: @chapter Invocation
  174: 
  175: You will usually just say @code{gforth}. In many other cases the default
  176: GNU Forth image will be invoked like this:
  177: 
  178: @example
  179: gforth [files] [-e forth-code]
  180: @end example
  181: 
  182: executing the contents of the files and the Forth code in the order they
  183: are given.
  184: 
  185: In general, the command line looks like this:
  186: 
  187: @example
  188: gforth [initialization options] [image-specific options]
  189: @end example
  190: 
  191: The initialization options must come before the rest of the command
  192: line. They are:
  193: 
  194: @table @code
  195: @item --image-file @var{file}
  196: Loads the Forth image @var{file} instead of the default
  197: @file{gforth.fi}.
  198: 
  199: @item --path @var{path}
  200: Uses @var{path} for searching the image file and Forth source code
  201: files instead of the default in the environment variable
  202: @code{GFORTHPATH} or the path specified at installation time (typically
  203: @file{/usr/local/lib/gforth:.}). A path is given as a @code{:}-separated
  204: list.
  205: 
  206: @item --dictionary-size @var{size}
  207: @item -m @var{size}
  208: Allocate @var{size} space for the Forth dictionary space instead of
  209: using the default specified in the image (typically 256K). The
  210: @var{size} specification consists of an integer and a unit (e.g.,
  211: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
  212: size, in this case Cells), @code{k} (kilobytes), and @code{M}
  213: (Megabytes). If no unit is specified, @code{e} is used.
  214: 
  215: @item --data-stack-size @var{size}
  216: @item -d @var{size}
  217: Allocate @var{size} space for the data stack instead of using the
  218: default specified in the image (typically 16K).
  219: 
  220: @item --return-stack-size @var{size}
  221: @item -r @var{size}
  222: Allocate @var{size} space for the return stack instead of using the
  223: default specified in the image (typically 16K).
  224: 
  225: @item --fp-stack-size @var{size}
  226: @item -f @var{size}
  227: Allocate @var{size} space for the floating point stack instead of
  228: using the default specified in the image (typically 16K). In this case
  229: the unit specifier @code{e} refers to floating point numbers.
  230: 
  231: @item --locals-stack-size @var{size}
  232: @item -l @var{size}
  233: Allocate @var{size} space for the locals stack instead of using the
  234: default specified in the image (typically 16K).
  235: 
  236: @end table
  237: 
  238: As explained above, the image-specific command-line arguments for the
  239: default image @file{gforth.fi} consist of a sequence of filenames and
  240: @code{-e @var{forth-code}} options that are interpreted in the seqence
  241: in which they are given. The @code{-e @var{forth-code}} or
  242: @code{--evaluate @var{forth-code}} option evaluates the forth
  243: code. This option takes only one argument; if you want to evaluate more
  244: Forth words, you have to quote them or use several @code{-e}s. To exit
  245: after processing the command line (instead of entering interactive mode)
  246: append @code{-e bye} to the command line.
  247: 
  248: Not yet implemented:
  249: On startup the system first executes the system initialization file
  250: (unless the option @code{--no-init-file} is given; note that the system
  251: resulting from using this option may not be ANS Forth conformant). Then
  252: the user initialization file @file{.gforth.fs} is executed, unless the
  253: option @code{--no-rc} is given; this file is first searched in @file{.},
  254: then in @file{~}, then in the normal path (see above).
  255: 
  256: @node Words, ANS conformance, Invocation, Top
  257: @chapter Forth Words
  258: 
  259: @menu
  260: * Notation::                    
  261: * Arithmetic::                  
  262: * Stack Manipulation::          
  263: * Memory access::               
  264: * Control Structures::          
  265: * Locals::                      
  266: * Defining Words::              
  267: * Wordlists::                   
  268: * Files::                       
  269: * Blocks::                      
  270: * Other I/O::                   
  271: * Programming Tools::           
  272: * Threading Words::             
  273: @end menu
  274: 
  275: @node Notation, Arithmetic, Words, Words
  276: @section Notation
  277: 
  278: The Forth words are described in this section in the glossary notation
  279: that has become a de-facto standard for Forth texts, i.e.
  280: 
  281: @format
  282: @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
  283: @end format
  284: @var{Description}
  285: 
  286: @table @var
  287: @item word
  288: The name of the word. BTW, GNU Forth is case insensitive, so you can
  289: type the words in in lower case (However, @pxref{core-idef}).
  290: 
  291: @item Stack effect
  292: The stack effect is written in the notation @code{@var{before} --
  293: @var{after}}, where @var{before} and @var{after} describe the top of
  294: stack entries before and after the execution of the word. The rest of
  295: the stack is not touched by the word. The top of stack is rightmost,
  296: i.e., a stack sequence is written as it is typed in. Note that GNU Forth
  297: uses a separate floating point stack, but a unified stack
  298: notation. Also, return stack effects are not shown in @var{stack
  299: effect}, but in @var{Description}. The name of a stack item describes
  300: the type and/or the function of the item. See below for a discussion of
  301: the types.
  302: 
  303: @item pronunciation
  304: How the word is pronounced
  305: 
  306: @item wordset
  307: The ANS Forth standard is divided into several wordsets. A standard
  308: system need not support all of them. So, the fewer wordsets your program
  309: uses the more portable it will be in theory. However, we suspect that
  310: most ANS Forth systems on personal machines will feature all
  311: wordsets. Words that are not defined in the ANS standard have
  312: @code{gforth} as wordset.
  313: 
  314: @item Description
  315: A description of the behaviour of the word.
  316: @end table
  317: 
  318: The type of a stack item is specified by the character(s) the name
  319: starts with:
  320: 
  321: @table @code
  322: @item f
  323: Bool, i.e. @code{false} or @code{true}.
  324: @item c
  325: Char
  326: @item w
  327: Cell, can contain an integer or an address
  328: @item n
  329: signed integer
  330: @item u
  331: unsigned integer
  332: @item d
  333: double sized signed integer
  334: @item ud
  335: double sized unsigned integer
  336: @item r
  337: Float
  338: @item a_
  339: Cell-aligned address
  340: @item c_
  341: Char-aligned address (note that a Char is two bytes in Windows NT)
  342: @item f_
  343: Float-aligned address
  344: @item df_
  345: Address aligned for IEEE double precision float
  346: @item sf_
  347: Address aligned for IEEE single precision float
  348: @item xt
  349: Execution token, same size as Cell
  350: @item wid
  351: Wordlist ID, same size as Cell
  352: @item f83name
  353: Pointer to a name structure
  354: @end table
  355: 
  356: @node Arithmetic, Stack Manipulation, Notation, Words
  357: @section Arithmetic
  358: Forth arithmetic is not checked, i.e., you will not hear about integer
  359: overflow on addition or multiplication, you may hear about division by
  360: zero if you are lucky. The operator is written after the operands, but
  361: the operands are still in the original order. I.e., the infix @code{2-1}
  362: corresponds to @code{2 1 -}. Forth offers a variety of division
  363: operators. If you perform division with potentially negative operands,
  364: you do not want to use @code{/} or @code{/mod} with its undefined
  365: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
  366: former, @pxref{Mixed precision}).
  367: 
  368: @menu
  369: * Single precision::            
  370: * Bitwise operations::          
  371: * Mixed precision::             operations with single and double-cell integers
  372: * Double precision::            Double-cell integer arithmetic
  373: * Floating Point::              
  374: @end menu
  375: 
  376: @node Single precision, Bitwise operations, Arithmetic, Arithmetic
  377: @subsection Single precision
  378: doc-+
  379: doc--
  380: doc-*
  381: doc-/
  382: doc-mod
  383: doc-/mod
  384: doc-negate
  385: doc-abs
  386: doc-min
  387: doc-max
  388: 
  389: @node Bitwise operations, Mixed precision, Single precision, Arithmetic
  390: @subsection Bitwise operations
  391: doc-and
  392: doc-or
  393: doc-xor
  394: doc-invert
  395: doc-2*
  396: doc-2/
  397: 
  398: @node Mixed precision, Double precision, Bitwise operations, Arithmetic
  399: @subsection Mixed precision
  400: doc-m+
  401: doc-*/
  402: doc-*/mod
  403: doc-m*
  404: doc-um*
  405: doc-m*/
  406: doc-um/mod
  407: doc-fm/mod
  408: doc-sm/rem
  409: 
  410: @node Double precision, Floating Point, Mixed precision, Arithmetic
  411: @subsection Double precision
  412: doc-d+
  413: doc-d-
  414: doc-dnegate
  415: doc-dabs
  416: doc-dmin
  417: doc-dmax
  418: 
  419: @node Floating Point,  , Double precision, Arithmetic
  420: @subsection Floating Point
  421: 
  422: Angles in floating point operations are given in radians (a full circle
  423: has 2 pi radians). Note, that gforth has a separate floating point
  424: stack, but we use the unified notation.
  425: 
  426: Floating point numbers have a number of unpleasant surprises for the
  427: unwary (e.g., floating point addition is not associative) and even a few
  428: for the wary. You should not use them unless you know what you are doing
  429: or you don't care that the results you get are totally bogus. If you
  430: want to learn about the problems of floating point numbers (and how to
  431: avoid them), you might start with @cite{David Goldberg, What Every
  432: Computer Scientist Should Know About Floating-Point Arithmetic, ACM
  433: Computing Surveys 23(1):5@minus{}48, March 1991}.
  434: 
  435: doc-f+
  436: doc-f-
  437: doc-f*
  438: doc-f/
  439: doc-fnegate
  440: doc-fabs
  441: doc-fmax
  442: doc-fmin
  443: doc-floor
  444: doc-fround
  445: doc-f**
  446: doc-fsqrt
  447: doc-fexp
  448: doc-fexpm1
  449: doc-fln
  450: doc-flnp1
  451: doc-flog
  452: doc-falog
  453: doc-fsin
  454: doc-fcos
  455: doc-fsincos
  456: doc-ftan
  457: doc-fasin
  458: doc-facos
  459: doc-fatan
  460: doc-fatan2
  461: doc-fsinh
  462: doc-fcosh
  463: doc-ftanh
  464: doc-fasinh
  465: doc-facosh
  466: doc-fatanh
  467: 
  468: @node Stack Manipulation, Memory access, Arithmetic, Words
  469: @section Stack Manipulation
  470: 
  471: gforth has a data stack (aka parameter stack) for characters, cells,
  472: addresses, and double cells, a floating point stack for floating point
  473: numbers, a return stack for storing the return addresses of colon
  474: definitions and other data, and a locals stack for storing local
  475: variables. Note that while every sane Forth has a separate floating
  476: point stack, this is not strictly required; an ANS Forth system could
  477: theoretically keep floating point numbers on the data stack. As an
  478: additional difficulty, you don't know how many cells a floating point
  479: number takes. It is reportedly possible to write words in a way that
  480: they work also for a unified stack model, but we do not recommend trying
  481: it. Instead, just say that your program has an environmental dependency
  482: on a separate FP stack.
  483: 
  484: Also, a Forth system is allowed to keep the local variables on the
  485: return stack. This is reasonable, as local variables usually eliminate
  486: the need to use the return stack explicitly. So, if you want to produce
  487: a standard complying program and if you are using local variables in a
  488: word, forget about return stack manipulations in that word (see the
  489: standard document for the exact rules).
  490: 
  491: @menu
  492: * Data stack::                  
  493: * Floating point stack::        
  494: * Return stack::                
  495: * Locals stack::                
  496: * Stack pointer manipulation::  
  497: @end menu
  498: 
  499: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
  500: @subsection Data stack
  501: doc-drop
  502: doc-nip
  503: doc-dup
  504: doc-over
  505: doc-tuck
  506: doc-swap
  507: doc-rot
  508: doc--rot
  509: doc-?dup
  510: doc-pick
  511: doc-roll
  512: doc-2drop
  513: doc-2nip
  514: doc-2dup
  515: doc-2over
  516: doc-2tuck
  517: doc-2swap
  518: doc-2rot
  519: 
  520: @node Floating point stack, Return stack, Data stack, Stack Manipulation
  521: @subsection Floating point stack
  522: doc-fdrop
  523: doc-fnip
  524: doc-fdup
  525: doc-fover
  526: doc-ftuck
  527: doc-fswap
  528: doc-frot
  529: 
  530: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
  531: @subsection Return stack
  532: doc->r
  533: doc-r>
  534: doc-r@
  535: doc-rdrop
  536: doc-2>r
  537: doc-2r>
  538: doc-2r@
  539: doc-2rdrop
  540: 
  541: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
  542: @subsection Locals stack
  543: 
  544: @node Stack pointer manipulation,  , Locals stack, Stack Manipulation
  545: @subsection Stack pointer manipulation
  546: doc-sp@
  547: doc-sp!
  548: doc-fp@
  549: doc-fp!
  550: doc-rp@
  551: doc-rp!
  552: doc-lp@
  553: doc-lp!
  554: 
  555: @node Memory access, Control Structures, Stack Manipulation, Words
  556: @section Memory access
  557: 
  558: @menu
  559: * Stack-Memory transfers::      
  560: * Address arithmetic::          
  561: * Memory block access::         
  562: @end menu
  563: 
  564: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
  565: @subsection Stack-Memory transfers
  566: 
  567: doc-@
  568: doc-!
  569: doc-+!
  570: doc-c@
  571: doc-c!
  572: doc-2@
  573: doc-2!
  574: doc-f@
  575: doc-f!
  576: doc-sf@
  577: doc-sf!
  578: doc-df@
  579: doc-df!
  580: 
  581: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
  582: @subsection Address arithmetic
  583: 
  584: ANS Forth does not specify the sizes of the data types. Instead, it
  585: offers a number of words for computing sizes and doing address
  586: arithmetic. Basically, address arithmetic is performed in terms of
  587: address units (aus); on most systems the address unit is one byte. Note
  588: that a character may have more than one au, so @code{chars} is no noop
  589: (on systems where it is a noop, it compiles to nothing).
  590: 
  591: ANS Forth also defines words for aligning addresses for specific
  592: addresses. Many computers require that accesses to specific data types
  593: must only occur at specific addresses; e.g., that cells may only be
  594: accessed at addresses divisible by 4. Even if a machine allows unaligned
  595: accesses, it can usually perform aligned accesses faster. 
  596: 
  597: For the performance-concious: alignment operations are usually only
  598: necessary during the definition of a data structure, not during the
  599: (more frequent) accesses to it.
  600: 
  601: ANS Forth defines no words for character-aligning addresses. This is not
  602: an oversight, but reflects the fact that addresses that are not
  603: char-aligned have no use in the standard and therefore will not be
  604: created.
  605: 
  606: The standard guarantees that addresses returned by @code{CREATE}d words
  607: are cell-aligned; in addition, gforth guarantees that these addresses
  608: are aligned for all purposes.
  609: 
  610: Note that the standard defines a word @code{char}, which has nothing to
  611: do with address arithmetic.
  612: 
  613: doc-chars
  614: doc-char+
  615: doc-cells
  616: doc-cell+
  617: doc-align
  618: doc-aligned
  619: doc-floats
  620: doc-float+
  621: doc-falign
  622: doc-faligned
  623: doc-sfloats
  624: doc-sfloat+
  625: doc-sfalign
  626: doc-sfaligned
  627: doc-dfloats
  628: doc-dfloat+
  629: doc-dfalign
  630: doc-dfaligned
  631: doc-maxalign
  632: doc-maxaligned
  633: doc-cfalign
  634: doc-cfaligned
  635: doc-address-unit-bits
  636: 
  637: @node Memory block access,  , Address arithmetic, Memory access
  638: @subsection Memory block access
  639: 
  640: doc-move
  641: doc-erase
  642: 
  643: While the previous words work on address units, the rest works on
  644: characters.
  645: 
  646: doc-cmove
  647: doc-cmove>
  648: doc-fill
  649: doc-blank
  650: 
  651: @node Control Structures, Locals, Memory access, Words
  652: @section Control Structures
  653: 
  654: Control structures in Forth cannot be used in interpret state, only in
  655: compile state, i.e., in a colon definition. We do not like this
  656: limitation, but have not seen a satisfying way around it yet, although
  657: many schemes have been proposed.
  658: 
  659: @menu
  660: * Selection::                   
  661: * Simple Loops::                
  662: * Counted Loops::               
  663: * Arbitrary control structures::  
  664: * Calls and returns::           
  665: * Exception Handling::          
  666: @end menu
  667: 
  668: @node Selection, Simple Loops, Control Structures, Control Structures
  669: @subsection Selection
  670: 
  671: @example
  672: @var{flag}
  673: IF
  674:   @var{code}
  675: ENDIF
  676: @end example
  677: or
  678: @example
  679: @var{flag}
  680: IF
  681:   @var{code1}
  682: ELSE
  683:   @var{code2}
  684: ENDIF
  685: @end example
  686: 
  687: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
  688: standard, and @code{ENDIF} is not, although it is quite popular. We
  689: recommend using @code{ENDIF}, because it is less confusing for people
  690: who also know other languages (and is not prone to reinforcing negative
  691: prejudices against Forth in these people). Adding @code{ENDIF} to a
  692: system that only supplies @code{THEN} is simple:
  693: @example
  694: : endif   POSTPONE then ; immediate
  695: @end example
  696: 
  697: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
  698: (adv.)}  has the following meanings:
  699: @quotation
  700: ... 2b: following next after in order ... 3d: as a necessary consequence
  701: (if you were there, then you saw them).
  702: @end quotation
  703: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
  704: and many other programming languages has the meaning 3d.]
  705: 
  706: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
  707: can avoid using @code{?dup}.
  708: 
  709: @example
  710: @var{n}
  711: CASE
  712:   @var{n1} OF @var{code1} ENDOF
  713:   @var{n2} OF @var{code2} ENDOF
  714:   @dots{}
  715: ENDCASE
  716: @end example
  717: 
  718: Executes the first @var{codei}, where the @var{ni} is equal to
  719: @var{n}. A default case can be added by simply writing the code after
  720: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
  721: but must not consume it.
  722: 
  723: @node Simple Loops, Counted Loops, Selection, Control Structures
  724: @subsection Simple Loops
  725: 
  726: @example
  727: BEGIN
  728:   @var{code1}
  729:   @var{flag}
  730: WHILE
  731:   @var{code2}
  732: REPEAT
  733: @end example
  734: 
  735: @var{code1} is executed and @var{flag} is computed. If it is true,
  736: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
  737: 
  738: @example
  739: BEGIN
  740:   @var{code}
  741:   @var{flag}
  742: UNTIL
  743: @end example
  744: 
  745: @var{code} is executed. The loop is restarted if @code{flag} is false.
  746: 
  747: @example
  748: BEGIN
  749:   @var{code}
  750: AGAIN
  751: @end example
  752: 
  753: This is an endless loop.
  754: 
  755: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
  756: @subsection Counted Loops
  757: 
  758: The basic counted loop is:
  759: @example
  760: @var{limit} @var{start}
  761: ?DO
  762:   @var{body}
  763: LOOP
  764: @end example
  765: 
  766: This performs one iteration for every integer, starting from @var{start}
  767: and up to, but excluding @var{limit}. The counter, aka index, can be
  768: accessed with @code{i}. E.g., the loop
  769: @example
  770: 10 0 ?DO
  771:   i .
  772: LOOP
  773: @end example
  774: prints
  775: @example
  776: 0 1 2 3 4 5 6 7 8 9
  777: @end example
  778: The index of the innermost loop can be accessed with @code{i}, the index
  779: of the next loop with @code{j}, and the index of the third loop with
  780: @code{k}.
  781: 
  782: The loop control data are kept on the return stack, so there are some
  783: restrictions on mixing return stack accesses and counted loop
  784: words. E.g., if you put values on the return stack outside the loop, you
  785: cannot read them inside the loop. If you put values on the return stack
  786: within a loop, you have to remove them before the end of the loop and
  787: before accessing the index of the loop.
  788: 
  789: There are several variations on the counted loop:
  790: 
  791: @code{LEAVE} leaves the innermost counted loop immediately.
  792: 
  793: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
  794: index by @var{n} instead of by 1. The loop is terminated when the border
  795: between @var{limit-1} and @var{limit} is crossed. E.g.:
  796: 
  797: @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
  798: 
  799: @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
  800: 
  801: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
  802: 
  803: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  804: 
  805: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
  806: 
  807: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
  808: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
  809: case behaves symmetrical to the positive case:
  810: 
  811: @code{-2 0 ?DO  i .  -1 S+LOOP}  prints @code{0 -1}
  812: 
  813: @code{-1 0 ?DO  i .  -1 S+LOOP}  prints @code{0}
  814: 
  815: @code{ 0 0 ?DO  i .  -1 S+LOOP}  prints nothing
  816: 
  817: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
  818: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
  819: Forth standard.
  820: 
  821: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
  822: when the start and the limit value are equal. We do not recommend using
  823: @code{DO}. It will just give you maintenance troubles.
  824: 
  825: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
  826: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
  827: return stack so @code{EXIT} can get to its return address.
  828: 
  829: Another counted loop is
  830: @example
  831: @var{n}
  832: FOR
  833:   @var{body}
  834: NEXT
  835: @end example
  836: This is the preferred loop of native code compiler writers who are too
  837: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
  838: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
  839: and ending with 0. Other Forth systems may behave differently, even if
  840: they support @code{FOR} loops.
  841: 
  842: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
  843: @subsection Arbitrary control structures
  844: 
  845: ANS Forth permits and supports using control structures in a non-nested
  846: way. Information about incomplete control structures is stored on the
  847: control-flow stack. This stack may be implemented on the Forth data
  848: stack, and this is what we have done in gforth.
  849: 
  850: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
  851: entry represents a backward branch target. A few words are the basis for
  852: building any control structure possible (except control structures that
  853: need storage, like calls, coroutines, and backtracking).
  854: 
  855: doc-if
  856: doc-ahead
  857: doc-then
  858: doc-begin
  859: doc-until
  860: doc-again
  861: doc-cs-pick
  862: doc-cs-roll
  863: 
  864: On many systems control-flow stack items take one word, in gforth they
  865: currently take three (this may change in the future). Therefore it is a
  866: really good idea to manipulate the control flow stack with
  867: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
  868: words.
  869: 
  870: Some standard control structure words are built from these words:
  871: 
  872: doc-else
  873: doc-while
  874: doc-repeat
  875: 
  876: Counted loop words constitute a separate group of words:
  877: 
  878: doc-?do
  879: doc-do
  880: doc-for
  881: doc-loop
  882: doc-s+loop
  883: doc-+loop
  884: doc-next
  885: doc-leave
  886: doc-?leave
  887: doc-unloop
  888: doc-done
  889: 
  890: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
  891: @i{do-sys}. Our system allows it, but it's your job to ensure that for
  892: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
  893: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
  894: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
  895: resolved (by using one of the loop-ending words or @code{DONE}).
  896: 
  897: Another group of control structure words are
  898: 
  899: doc-case
  900: doc-endcase
  901: doc-of
  902: doc-endof
  903: 
  904: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
  905: @code{cs-roll}.
  906: 
  907: @subsubsection Programming Style
  908: 
  909: In order to ensure readability we recommend that you do not create
  910: arbitrary control structures directly, but define new control structure
  911: words for the control structure you want and use these words in your
  912: program.
  913: 
  914: E.g., instead of writing
  915: 
  916: @example
  917: begin
  918:   ...
  919: if [ 1 cs-roll ]
  920:   ...
  921: again then
  922: @end example
  923: 
  924: we recommend defining control structure words, e.g.,
  925: 
  926: @example
  927: : while ( dest -- orig dest )
  928:  POSTPONE if
  929:  1 cs-roll ; immediate
  930: 
  931: : repeat ( orig dest -- )
  932:  POSTPONE again
  933:  POSTPONE then ; immediate
  934: @end example
  935: 
  936: and then using these to create the control structure:
  937: 
  938: @example
  939: begin
  940:   ...
  941: while
  942:   ...
  943: repeat
  944: @end example
  945: 
  946: That's much easier to read, isn't it? Of course, @code{BEGIN} and
  947: @code{WHILE} are predefined, so in this example it would not be
  948: necessary to define them.
  949: 
  950: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
  951: @subsection Calls and returns
  952: 
  953: A definition can be called simply be writing the name of the
  954: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
  955: 
  956: doc-exit
  957: 
  958: Don't forget to clean up the return stack and @code{UNLOOP} any
  959: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
  960: primitive compiled by @code{EXIT} is
  961: 
  962: doc-;s
  963: 
  964: @node Exception Handling,  , Calls and returns, Control Structures
  965: @subsection Exception Handling
  966: 
  967: doc-catch
  968: doc-throw
  969: 
  970: @node Locals, Defining Words, Control Structures, Words
  971: @section Locals
  972: 
  973: Local variables can make Forth programming more enjoyable and Forth
  974: programs easier to read. Unfortunately, the locals of ANS Forth are
  975: laden with restrictions. Therefore, we provide not only the ANS Forth
  976: locals wordset, but also our own, more powerful locals wordset (we
  977: implemented the ANS Forth locals wordset through our locals wordset).
  978: 
  979: @menu
  980: * gforth locals::               
  981: * ANS Forth locals::            
  982: @end menu
  983: 
  984: @node gforth locals, ANS Forth locals, Locals, Locals
  985: @subsection gforth locals
  986: 
  987: Locals can be defined with
  988: 
  989: @example
  990: @{ local1 local2 ... -- comment @}
  991: @end example
  992: or
  993: @example
  994: @{ local1 local2 ... @}
  995: @end example
  996: 
  997: E.g.,
  998: @example
  999: : max @{ n1 n2 -- n3 @}
 1000:  n1 n2 > if
 1001:    n1
 1002:  else
 1003:    n2
 1004:  endif ;
 1005: @end example
 1006: 
 1007: The similarity of locals definitions with stack comments is intended. A
 1008: locals definition often replaces the stack comment of a word. The order
 1009: of the locals corresponds to the order in a stack comment and everything
 1010: after the @code{--} is really a comment.
 1011: 
 1012: This similarity has one disadvantage: It is too easy to confuse locals
 1013: declarations with stack comments, causing bugs and making them hard to
 1014: find. However, this problem can be avoided by appropriate coding
 1015: conventions: Do not use both notations in the same program. If you do,
 1016: they should be distinguished using additional means, e.g. by position.
 1017: 
 1018: The name of the local may be preceded by a type specifier, e.g.,
 1019: @code{F:} for a floating point value:
 1020: 
 1021: @example
 1022: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
 1023: \ complex multiplication
 1024:  Ar Br f* Ai Bi f* f-
 1025:  Ar Bi f* Ai Br f* f+ ;
 1026: @end example
 1027: 
 1028: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
 1029: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
 1030: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
 1031: with @code{W:}, @code{D:} etc.) produces its value and can be changed
 1032: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
 1033: produces its address (which becomes invalid when the variable's scope is
 1034: left). E.g., the standard word @code{emit} can be defined in therms of
 1035: @code{type} like this:
 1036: 
 1037: @example
 1038: : emit @{ C^ char* -- @}
 1039:     char* 1 type ;
 1040: @end example
 1041: 
 1042: A local without type specifier is a @code{W:} local. Both flavours of
 1043: locals are initialized with values from the data or FP stack.
 1044: 
 1045: Currently there is no way to define locals with user-defined data
 1046: structures, but we are working on it.
 1047: 
 1048: GNU Forth allows defining locals everywhere in a colon definition. This
 1049: poses the following questions:
 1050: 
 1051: @menu
 1052: * Where are locals visible by name?::  
 1053: * How long do locals live?::    
 1054: * Programming Style::           
 1055: * Implementation::              
 1056: @end menu
 1057: 
 1058: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
 1059: @subsubsection Where are locals visible by name?
 1060: 
 1061: Basically, the answer is that locals are visible where you would expect
 1062: it in block-structured languages, and sometimes a little longer. If you
 1063: want to restrict the scope of a local, enclose its definition in
 1064: @code{SCOPE}...@code{ENDSCOPE}.
 1065: 
 1066: doc-scope
 1067: doc-endscope
 1068: 
 1069: These words behave like control structure words, so you can use them
 1070: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
 1071: arbitrary ways.
 1072: 
 1073: If you want a more exact answer to the visibility question, here's the
 1074: basic principle: A local is visible in all places that can only be
 1075: reached through the definition of the local@footnote{In compiler
 1076: construction terminology, all places dominated by the definition of the
 1077: local.}. In other words, it is not visible in places that can be reached
 1078: without going through the definition of the local. E.g., locals defined
 1079: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
 1080: defined in @code{BEGIN}...@code{UNTIL} are visible after the
 1081: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
 1082: 
 1083: The reasoning behind this solution is: We want to have the locals
 1084: visible as long as it is meaningful. The user can always make the
 1085: visibility shorter by using explicit scoping. In a place that can
 1086: only be reached through the definition of a local, the meaning of a
 1087: local name is clear. In other places it is not: How is the local
 1088: initialized at the control flow path that does not contain the
 1089: definition? Which local is meant, if the same name is defined twice in
 1090: two independent control flow paths?
 1091: 
 1092: This should be enough detail for nearly all users, so you can skip the
 1093: rest of this section. If you relly must know all the gory details and
 1094: options, read on.
 1095: 
 1096: In order to implement this rule, the compiler has to know which places
 1097: are unreachable. It knows this automatically after @code{AHEAD},
 1098: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
 1099: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
 1100: compiler that the control flow never reaches that place. If
 1101: @code{UNREACHABLE} is not used where it could, the only consequence is
 1102: that the visibility of some locals is more limited than the rule above
 1103: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
 1104: lie to the compiler), buggy code will be produced.
 1105: 
 1106: Another problem with this rule is that at @code{BEGIN}, the compiler
 1107: does not know which locals will be visible on the incoming
 1108: back-edge. All problems discussed in the following are due to this
 1109: ignorance of the compiler (we discuss the problems using @code{BEGIN}
 1110: loops as examples; the discussion also applies to @code{?DO} and other
 1111: loops). Perhaps the most insidious example is:
 1112: @example
 1113: AHEAD
 1114: BEGIN
 1115:   x
 1116: [ 1 CS-ROLL ] THEN
 1117:   @{ x @}
 1118:   ...
 1119: UNTIL
 1120: @end example
 1121: 
 1122: This should be legal according to the visibility rule. The use of
 1123: @code{x} can only be reached through the definition; but that appears
 1124: textually below the use.
 1125: 
 1126: From this example it is clear that the visibility rules cannot be fully
 1127: implemented without major headaches. Our implementation treats common
 1128: cases as advertised and the exceptions are treated in a safe way: The
 1129: compiler makes a reasonable guess about the locals visible after a
 1130: @code{BEGIN}; if it is too pessimistic, the
 1131: user will get a spurious error about the local not being defined; if the
 1132: compiler is too optimistic, it will notice this later and issue a
 1133: warning. In the case above the compiler would complain about @code{x}
 1134: being undefined at its use. You can see from the obscure examples in
 1135: this section that it takes quite unusual control structures to get the
 1136: compiler into trouble, and even then it will often do fine.
 1137: 
 1138: If the @code{BEGIN} is reachable from above, the most optimistic guess
 1139: is that all locals visible before the @code{BEGIN} will also be
 1140: visible after the @code{BEGIN}. This guess is valid for all loops that
 1141: are entered only through the @code{BEGIN}, in particular, for normal
 1142: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
 1143: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
 1144: compiler. When the branch to the @code{BEGIN} is finally generated by
 1145: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
 1146: warns the user if it was too optimisitic:
 1147: @example
 1148: IF
 1149:   @{ x @}
 1150: BEGIN
 1151:   \ x ? 
 1152: [ 1 cs-roll ] THEN
 1153:   ...
 1154: UNTIL
 1155: @end example
 1156: 
 1157: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
 1158: optimistically assumes that it lives until the @code{THEN}. It notices
 1159: this difference when it compiles the @code{UNTIL} and issues a
 1160: warning. The user can avoid the warning, and make sure that @code{x}
 1161: is not used in the wrong area by using explicit scoping:
 1162: @example
 1163: IF
 1164:   SCOPE
 1165:   @{ x @}
 1166:   ENDSCOPE
 1167: BEGIN
 1168: [ 1 cs-roll ] THEN
 1169:   ...
 1170: UNTIL
 1171: @end example
 1172: 
 1173: Since the guess is optimistic, there will be no spurious error messages
 1174: about undefined locals.
 1175: 
 1176: If the @code{BEGIN} is not reachable from above (e.g., after
 1177: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
 1178: optimistic guess, as the locals visible after the @code{BEGIN} may be
 1179: defined later. Therefore, the compiler assumes that no locals are
 1180: visible after the @code{BEGIN}. However, the useer can use
 1181: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
 1182: visible at the BEGIN as at the point where the item was created.
 1183: 
 1184: doc-assume-live
 1185: 
 1186: E.g.,
 1187: @example
 1188: @{ x @}
 1189: AHEAD
 1190: ASSUME-LIVE
 1191: BEGIN
 1192:   x
 1193: [ 1 CS-ROLL ] THEN
 1194:   ...
 1195: UNTIL
 1196: @end example
 1197: 
 1198: Other cases where the locals are defined before the @code{BEGIN} can be
 1199: handled by inserting an appropriate @code{CS-ROLL} before the
 1200: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
 1201: behind the @code{ASSUME-LIVE}).
 1202: 
 1203: Cases where locals are defined after the @code{BEGIN} (but should be
 1204: visible immediately after the @code{BEGIN}) can only be handled by
 1205: rearranging the loop. E.g., the ``most insidious'' example above can be
 1206: arranged into:
 1207: @example
 1208: BEGIN
 1209:   @{ x @}
 1210:   ... 0=
 1211: WHILE
 1212:   x
 1213: REPEAT
 1214: @end example
 1215: 
 1216: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
 1217: @subsubsection How long do locals live?
 1218: 
 1219: The right answer for the lifetime question would be: A local lives at
 1220: least as long as it can be accessed. For a value-flavoured local this
 1221: means: until the end of its visibility. However, a variable-flavoured
 1222: local could be accessed through its address far beyond its visibility
 1223: scope. Ultimately, this would mean that such locals would have to be
 1224: garbage collected. Since this entails un-Forth-like implementation
 1225: complexities, I adopted the same cowardly solution as some other
 1226: languages (e.g., C): The local lives only as long as it is visible;
 1227: afterwards its address is invalid (and programs that access it
 1228: afterwards are erroneous).
 1229: 
 1230: @node Programming Style, Implementation, How long do locals live?, gforth locals
 1231: @subsubsection Programming Style
 1232: 
 1233: The freedom to define locals anywhere has the potential to change
 1234: programming styles dramatically. In particular, the need to use the
 1235: return stack for intermediate storage vanishes. Moreover, all stack
 1236: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
 1237: determined arguments) can be eliminated: If the stack items are in the
 1238: wrong order, just write a locals definition for all of them; then
 1239: write the items in the order you want.
 1240: 
 1241: This seems a little far-fetched and eliminating stack manipulations is
 1242: unlikely to become a conscious programming objective. Still, the number
 1243: of stack manipulations will be reduced dramatically if local variables
 1244: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
 1245: a traditional implementation of @code{max}).
 1246: 
 1247: This shows one potential benefit of locals: making Forth programs more
 1248: readable. Of course, this benefit will only be realized if the
 1249: programmers continue to honour the principle of factoring instead of
 1250: using the added latitude to make the words longer.
 1251: 
 1252: Using @code{TO} can and should be avoided.  Without @code{TO},
 1253: every value-flavoured local has only a single assignment and many
 1254: advantages of functional languages apply to Forth. I.e., programs are
 1255: easier to analyse, to optimize and to read: It is clear from the
 1256: definition what the local stands for, it does not turn into something
 1257: different later.
 1258: 
 1259: E.g., a definition using @code{TO} might look like this:
 1260: @example
 1261: : strcmp @{ addr1 u1 addr2 u2 -- n @}
 1262:  u1 u2 min 0
 1263:  ?do
 1264:    addr1 c@ addr2 c@ - ?dup
 1265:    if
 1266:      unloop exit
 1267:    then
 1268:    addr1 char+ TO addr1
 1269:    addr2 char+ TO addr2
 1270:  loop
 1271:  u1 u2 - ;
 1272: @end example
 1273: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
 1274: every loop iteration. @code{strcmp} is a typical example of the
 1275: readability problems of using @code{TO}. When you start reading
 1276: @code{strcmp}, you think that @code{addr1} refers to the start of the
 1277: string. Only near the end of the loop you realize that it is something
 1278: else.
 1279: 
 1280: This can be avoided by defining two locals at the start of the loop that
 1281: are initialized with the right value for the current iteration.
 1282: @example
 1283: : strcmp @{ addr1 u1 addr2 u2 -- n @}
 1284:  addr1 addr2
 1285:  u1 u2 min 0 
 1286:  ?do @{ s1 s2 @}
 1287:    s1 c@ s2 c@ - ?dup 
 1288:    if
 1289:      unloop exit
 1290:    then
 1291:    s1 char+ s2 char+
 1292:  loop
 1293:  2drop
 1294:  u1 u2 - ;
 1295: @end example
 1296: Here it is clear from the start that @code{s1} has a different value
 1297: in every loop iteration.
 1298: 
 1299: @node Implementation,  , Programming Style, gforth locals
 1300: @subsubsection Implementation
 1301: 
 1302: GNU Forth uses an extra locals stack. The most compelling reason for
 1303: this is that the return stack is not float-aligned; using an extra stack
 1304: also eliminates the problems and restrictions of using the return stack
 1305: as locals stack. Like the other stacks, the locals stack grows toward
 1306: lower addresses. A few primitives allow an efficient implementation:
 1307: 
 1308: doc-@local#
 1309: doc-f@local#
 1310: doc-laddr#
 1311: doc-lp+!#
 1312: doc-lp!
 1313: doc->l
 1314: doc-f>l
 1315: 
 1316: In addition to these primitives, some specializations of these
 1317: primitives for commonly occurring inline arguments are provided for
 1318: efficiency reasons, e.g., @code{@@local0} as specialization of
 1319: @code{@@local#} for the inline argument 0. The following compiling words
 1320: compile the right specialized version, or the general version, as
 1321: appropriate:
 1322: 
 1323: doc-compile-@local
 1324: doc-compile-f@local
 1325: doc-compile-lp+!
 1326: 
 1327: Combinations of conditional branches and @code{lp+!#} like
 1328: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
 1329: is taken) are provided for efficiency and correctness in loops.
 1330: 
 1331: A special area in the dictionary space is reserved for keeping the
 1332: local variable names. @code{@{} switches the dictionary pointer to this
 1333: area and @code{@}} switches it back and generates the locals
 1334: initializing code. @code{W:} etc.@ are normal defining words. This
 1335: special area is cleared at the start of every colon definition.
 1336: 
 1337: A special feature of GNU Forths dictionary is used to implement the
 1338: definition of locals without type specifiers: every wordlist (aka
 1339: vocabulary) has its own methods for searching
 1340: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
 1341: with a special search method: When it is searched for a word, it
 1342: actually creates that word using @code{W:}. @code{@{} changes the search
 1343: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
 1344: and then the wordlist for defining locals without type specifiers.
 1345: 
 1346: The lifetime rules support a stack discipline within a colon
 1347: definition: The lifetime of a local is either nested with other locals
 1348: lifetimes or it does not overlap them.
 1349: 
 1350: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
 1351: pointer manipulation is generated. Between control structure words
 1352: locals definitions can push locals onto the locals stack. @code{AGAIN}
 1353: is the simplest of the other three control flow words. It has to
 1354: restore the locals stack depth of the corresponding @code{BEGIN}
 1355: before branching. The code looks like this:
 1356: @format
 1357: @code{lp+!#} current-locals-size @minus{} dest-locals-size
 1358: @code{branch} <begin>
 1359: @end format
 1360: 
 1361: @code{UNTIL} is a little more complicated: If it branches back, it
 1362: must adjust the stack just like @code{AGAIN}. But if it falls through,
 1363: the locals stack must not be changed. The compiler generates the
 1364: following code:
 1365: @format
 1366: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
 1367: @end format
 1368: The locals stack pointer is only adjusted if the branch is taken.
 1369: 
 1370: @code{THEN} can produce somewhat inefficient code:
 1371: @format
 1372: @code{lp+!#} current-locals-size @minus{} orig-locals-size
 1373: <orig target>:
 1374: @code{lp+!#} orig-locals-size @minus{} new-locals-size
 1375: @end format
 1376: The second @code{lp+!#} adjusts the locals stack pointer from the
 1377: level at the @var{orig} point to the level after the @code{THEN}. The
 1378: first @code{lp+!#} adjusts the locals stack pointer from the current
 1379: level to the level at the orig point, so the complete effect is an
 1380: adjustment from the current level to the right level after the
 1381: @code{THEN}.
 1382: 
 1383: In a conventional Forth implementation a dest control-flow stack entry
 1384: is just the target address and an orig entry is just the address to be
 1385: patched. Our locals implementation adds a wordlist to every orig or dest
 1386: item. It is the list of locals visible (or assumed visible) at the point
 1387: described by the entry. Our implementation also adds a tag to identify
 1388: the kind of entry, in particular to differentiate between live and dead
 1389: (reachable and unreachable) orig entries.
 1390: 
 1391: A few unusual operations have to be performed on locals wordlists:
 1392: 
 1393: doc-common-list
 1394: doc-sub-list?
 1395: doc-list-size
 1396: 
 1397: Several features of our locals wordlist implementation make these
 1398: operations easy to implement: The locals wordlists are organised as
 1399: linked lists; the tails of these lists are shared, if the lists
 1400: contain some of the same locals; and the address of a name is greater
 1401: than the address of the names behind it in the list.
 1402: 
 1403: Another important implementation detail is the variable
 1404: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
 1405: determine if they can be reached directly or only through the branch
 1406: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
 1407: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
 1408: definition, by @code{BEGIN} and usually by @code{THEN}.
 1409: 
 1410: Counted loops are similar to other loops in most respects, but
 1411: @code{LEAVE} requires special attention: It performs basically the same
 1412: service as @code{AHEAD}, but it does not create a control-flow stack
 1413: entry. Therefore the information has to be stored elsewhere;
 1414: traditionally, the information was stored in the target fields of the
 1415: branches created by the @code{LEAVE}s, by organizing these fields into a
 1416: linked list. Unfortunately, this clever trick does not provide enough
 1417: space for storing our extended control flow information. Therefore, we
 1418: introduce another stack, the leave stack. It contains the control-flow
 1419: stack entries for all unresolved @code{LEAVE}s.
 1420: 
 1421: Local names are kept until the end of the colon definition, even if
 1422: they are no longer visible in any control-flow path. In a few cases
 1423: this may lead to increased space needs for the locals name area, but
 1424: usually less than reclaiming this space would cost in code size.
 1425: 
 1426: 
 1427: @node ANS Forth locals,  , gforth locals, Locals
 1428: @subsection ANS Forth locals
 1429: 
 1430: The ANS Forth locals wordset does not define a syntax for locals, but
 1431: words that make it possible to define various syntaxes. One of the
 1432: possible syntaxes is a subset of the syntax we used in the gforth locals
 1433: wordset, i.e.:
 1434: 
 1435: @example
 1436: @{ local1 local2 ... -- comment @}
 1437: @end example
 1438: or
 1439: @example
 1440: @{ local1 local2 ... @}
 1441: @end example
 1442: 
 1443: The order of the locals corresponds to the order in a stack comment. The
 1444: restrictions are:
 1445: 
 1446: @itemize @bullet
 1447: @item
 1448: Locals can only be cell-sized values (no type specifers are allowed).
 1449: @item
 1450: Locals can be defined only outside control structures.
 1451: @item
 1452: Locals can interfere with explicit usage of the return stack. For the
 1453: exact (and long) rules, see the standard. If you don't use return stack
 1454: accessing words in a definition using locals, you will we all right. The
 1455: purpose of this rule is to make locals implementation on the return
 1456: stack easier.
 1457: @item
 1458: The whole definition must be in one line.
 1459: @end itemize
 1460: 
 1461: Locals defined in this way behave like @code{VALUE}s
 1462: (@xref{Values}). I.e., they are initialized from the stack. Using their
 1463: name produces their value. Their value can be changed using @code{TO}.
 1464: 
 1465: Since this syntax is supported by gforth directly, you need not do
 1466: anything to use it. If you want to port a program using this syntax to
 1467: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
 1468: on the other system.
 1469: 
 1470: Note that a syntax shown in the standard, section A.13 looks
 1471: similar, but is quite different in having the order of locals
 1472: reversed. Beware!
 1473: 
 1474: The ANS Forth locals wordset itself consists of the following word
 1475: 
 1476: doc-(local)
 1477: 
 1478: The ANS Forth locals extension wordset defines a syntax, but it is so
 1479: awful that we strongly recommend not to use it. We have implemented this
 1480: syntax to make porting to gforth easy, but do not document it here. The
 1481: problem with this syntax is that the locals are defined in an order
 1482: reversed with respect to the standard stack comment notation, making
 1483: programs harder to read, and easier to misread and miswrite. The only
 1484: merit of this syntax is that it is easy to implement using the ANS Forth
 1485: locals wordset.
 1486: 
 1487: @node Defining Words, Wordlists, Locals, Words
 1488: @section Defining Words
 1489: 
 1490: @menu
 1491: * Values::                      
 1492: @end menu
 1493: 
 1494: @node Values,  , Defining Words, Defining Words
 1495: @subsection Values
 1496: 
 1497: @node Wordlists, Files, Defining Words, Words
 1498: @section Wordlists
 1499: 
 1500: @node Files, Blocks, Wordlists, Words
 1501: @section Files
 1502: 
 1503: @node Blocks, Other I/O, Files, Words
 1504: @section Blocks
 1505: 
 1506: @node Other I/O, Programming Tools, Blocks, Words
 1507: @section Other I/O
 1508: 
 1509: @node Programming Tools, Threading Words, Other I/O, Words
 1510: @section Programming Tools
 1511: 
 1512: @menu
 1513: * Debugging::                   Simple and quick.
 1514: * Assertions::                  Making your programs self-checking.
 1515: @end menu
 1516: 
 1517: @node Debugging, Assertions, Programming Tools, Programming Tools
 1518: @subsection Debugging
 1519: 
 1520: The simple debugging aids provided in @file{debugging.fs}
 1521: are meant to support a different style of debugging than the
 1522: tracing/stepping debuggers used in languages with long turn-around
 1523: times.
 1524: 
 1525: A much better (faster) way in fast-compilig languages is to add
 1526: printing code at well-selected places, let the program run, look at
 1527: the output, see where things went wrong, add more printing code, etc.,
 1528: until the bug is found.
 1529: 
 1530: The word @code{~~} is easy to insert. It just prints debugging
 1531: information (by default the source location and the stack contents). It
 1532: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
 1533: query-replace them with nothing). The deferred words
 1534: @code{printdebugdata} and @code{printdebugline} control the output of
 1535: @code{~~}. The default source location output format works well with
 1536: Emacs' compilation mode, so you can step through the program at the
 1537: source level using @kbd{C-x `} (the advantage over a stepping debugger
 1538: is that you can step in any direction and you know where the crash has
 1539: happened or where the strange data has occurred).
 1540: 
 1541: Note that the default actions clobber the contents of the pictured
 1542: numeric output string, so you should not use @code{~~}, e.g., between
 1543: @code{<#} and @code{#>}.
 1544: 
 1545: doc-~~
 1546: doc-printdebugdata
 1547: doc-printdebugline
 1548: 
 1549: @node Assertions,  , Debugging, Programming Tools
 1550: @subsection Assertions
 1551: 
 1552: It is a good idea to make your programs self-checking, in particular, if
 1553: you use an assumption (e.g., that a certain field of a data structure is
 1554: never zero) that may become wrong during maintenance. GForth supports
 1555: assertions for this purpose. They are used like this:
 1556: 
 1557: @example
 1558: assert( @var{flag} )
 1559: @end example
 1560: 
 1561: The code between @code{assert(} and @code{)} should compute a flag, that
 1562: should be true if everything is alright and false otherwise. It should
 1563: not change anything else on the stack. The overall stack effect of the
 1564: assertion is @code{( -- )}. E.g.
 1565: 
 1566: @example
 1567: assert( 1 1 + 2 = ) \ what we learn in school
 1568: assert( dup 0<> ) \ assert that the top of stack is not zero
 1569: assert( false ) \ this code should not be reached
 1570: @end example
 1571: 
 1572: The need for assertions is different at different times. During
 1573: debugging, we want more checking, in production we sometimes care more
 1574: for speed. Therefore, assertions can be turned off, i.e., the assertion
 1575: becomes a comment. Depending on the importance of an assertion and the
 1576: time it takes to check it, you may want to turn off some assertions and
 1577: keep others turned on. GForth provides several levels of assertions for
 1578: this purpose:
 1579: 
 1580: doc-assert0(
 1581: doc-assert1(
 1582: doc-assert2(
 1583: doc-assert3(
 1584: doc-assert(
 1585: doc-)
 1586: 
 1587: @code{Assert(} is the same as @code{assert1(}. The variable
 1588: @code{assert-level} specifies the highest assertions that are turned
 1589: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
 1590: @code{assert1(} assertions perform checking, while @code{assert2(} and
 1591: @code{assert3(} assertions are treated as comments.
 1592: 
 1593: Note that the @code{assert-level} is evaluated at compile-time, not at
 1594: run-time. I.e., you cannot turn assertions on or off at run-time, you
 1595: have to set the @code{assert-level} appropriately before compiling a
 1596: piece of code. You can compile several pieces of code at several
 1597: @code{assert-level}s (e.g., a trusted library at level 1 and newly
 1598: written code at level 3).
 1599: 
 1600: doc-assert-level
 1601: 
 1602: If an assertion fails, a message compatible with Emacs' compilation mode
 1603: is produced and the execution is aborted (currently with @code{ABORT"}.
 1604: If there is interest, we will introduce a special throw code. But if you
 1605: intend to @code{catch} a specific condition, using @code{throw} is
 1606: probably more appropriate than an assertion).
 1607: 
 1608: @node Threading Words,  , Programming Tools, Words
 1609: @section Threading Words
 1610: 
 1611: These words provide access to code addresses and other threading stuff
 1612: in gforth (and, possibly, other interpretive Forths). It more or less
 1613: abstracts away the differences between direct and indirect threading
 1614: (and, for direct threading, the machine dependences). However, at
 1615: present this wordset is still inclomplete. It is also pretty low-level;
 1616: some day it will hopefully be made unnecessary by an internals words set
 1617: that abstracts implementation details away completely.
 1618: 
 1619: doc->code-address
 1620: doc->does-code
 1621: doc-code-address!
 1622: doc-does-code!
 1623: doc-does-handler!
 1624: doc-/does-handler
 1625: 
 1626: 
 1627: 
 1628: @node ANS conformance, Model, Words, Top
 1629: @chapter ANS conformance
 1630: 
 1631: ANS Forth systems are required to document certain implementation
 1632: choices. This chapter tries to meet these requirements. In many cases it
 1633: gives a way to ask the system for the information instead of providing
 1634: the information directly, in particular, if the information depends on
 1635: the processor, the operating system or the installation options chosen,
 1636: or if they are likely to change during the maintenance of gforth.
 1637: 
 1638: @comment The framework for the rest has been taken from pfe.
 1639: 
 1640: @menu
 1641: * The Core Words::              
 1642: * The optional Block word set::  
 1643: * The optional Double Number word set::  
 1644: * The optional Exception word set::  
 1645: * The optional Facility word set::  
 1646: * The optional File-Access word set::  
 1647: * The optional Floating-Point word set::  
 1648: * The optional Locals word set::  
 1649: * The optional Memory-Allocation word set::  
 1650: * The optional Programming-Tools word set::  
 1651: * The optional Search-Order word set::  
 1652: * The optional String word set::  
 1653: @end menu
 1654: 
 1655: 
 1656: @c =====================================================================
 1657: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
 1658: @comment  node-name,  next,  previous,  up
 1659: @section The Core Words
 1660: @c =====================================================================
 1661: 
 1662: @menu
 1663: * core-idef::                   
 1664: * core-ambcond::                
 1665: * core-other::                  
 1666: @end menu
 1667: 
 1668: @c ---------------------------------------------------------------------
 1669: @node core-idef, core-ambcond, The Core Words, The Core Words
 1670: @comment  node-name,  next,  previous,  up
 1671: @subsection Implementation Defined Options
 1672: @c ---------------------------------------------------------------------
 1673: 
 1674: @table @i
 1675: 
 1676: @item (Cell) aligned addresses:
 1677: processor-dependent. Gforths alignment words perform natural alignment
 1678: (e.g., an address aligned for a datum of size 8 is divisible by
 1679: 8). Unaligned accesses usually result in a @code{-23 THROW}.
 1680: 
 1681: @item @code{EMIT} and non-graphic characters:
 1682: The character is output using the C library function (actually, macro)
 1683: @code{putchar}.
 1684: 
 1685: @item character editing of @code{ACCEPT} and @code{EXPECT}:
 1686: This is modeled on the GNU readline library (@pxref{Readline
 1687: Interaction, , Command Line Editing, readline, The GNU Readline
 1688: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
 1689: producing a full word completion every time you type it (instead of
 1690: producing the common prefix of all completions).
 1691: 
 1692: @item character set:
 1693: The character set of your computer and display device. Gforth is
 1694: 8-bit-clean (but some other component in your system may make trouble).
 1695: 
 1696: @item Character-aligned address requirements:
 1697: installation-dependent. Currently a character is represented by a C
 1698: @code{unsigned char}; in the future we might switch to @code{wchar_t}
 1699: (Comments on that requested).
 1700: 
 1701: @item character-set extensions and matching of names:
 1702: Any character except 0 can be used in a name. Matching is
 1703: case-insensitive. The matching is performed using the C function
 1704: @code{strncasecmp}, whose function is probably influenced by the
 1705: locale. E.g., the @code{C} locale does not know about accents and
 1706: umlauts, so they are matched case-sensitively in that locale. For
 1707: portability reasons it is best to write programs such that they work in
 1708: the @code{C} locale. Then one can use libraries written by a Polish
 1709: programmer (who might use words containing ISO Latin-2 encoded
 1710: characters) and by a French programmer (ISO Latin-1) in the same program
 1711: (of course, @code{WORDS} will produce funny results for some of the
 1712: words (which ones, depends on the font you are using)). Also, the locale
 1713: you prefer may not be available in other operating systems. Hopefully,
 1714: Unicode will solve these problems one day.
 1715: 
 1716: @item conditions under which control characters match a space delimiter:
 1717: If @code{WORD} is called with the space character as a delimiter, all
 1718: white-space characters (as identified by the C macro @code{isspace()})
 1719: are delimiters. @code{PARSE}, on the other hand, treats space like other
 1720: delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves
 1721: like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer
 1722: interpreter (aka text interpreter) by default, treats all white-space
 1723: characters as delimiters.
 1724: 
 1725: @item format of the control flow stack:
 1726: The data stack is used as control flow stack. The size of a control flow
 1727: stack item in cells is given by the constant @code{cs-item-size}. At the
 1728: time of this writing, an item consists of a (pointer to a) locals list
 1729: (third), an address in the code (second), and a tag for identifying the
 1730: item (TOS). The following tags are used: @code{defstart},
 1731: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
 1732: @code{scopestart}.
 1733: 
 1734: @item conversion of digits > 35
 1735: The characters @code{[\]^_'} are the digits with the decimal value
 1736: 36@minus{}41. There is no way to input many of the larger digits.
 1737: 
 1738: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
 1739: The cursor is moved to the end of the entered string. If the input is
 1740: terminated using the @kbd{Return} key, a space is typed.
 1741: 
 1742: @item exception abort sequence of @code{ABORT"}:
 1743: The error string is stored into the variable @code{"error} and a
 1744: @code{-2 throw} is performed.
 1745: 
 1746: @item input line terminator:
 1747: For interactive input, @kbd{C-m} and @kbd{C-j} terminate lines. One of
 1748: these characters is typically produced when you type the @kbd{Enter} or
 1749: @kbd{Return} key.
 1750: 
 1751: @item maximum size of a counted string:
 1752: @code{s" /counted-string" environment? drop .}. Currently 255 characters
 1753: on all ports, but this may change.
 1754: 
 1755: @item maximum size of a parsed string:
 1756: Given by the constant @code{/line}. Currently 255 characters.
 1757: 
 1758: @item maximum size of a definition name, in characters:
 1759: 31
 1760: 
 1761: @item maximum string length for @code{ENVIRONMENT?}, in characters:
 1762: 31
 1763: 
 1764: @item method of selecting the user input device:
 1765: The user input device is the standard input. There is current no way to
 1766: change it from within gforth. However, the input can typically be
 1767: redirected in the command line that starts gforth.
 1768: 
 1769: @item method of selecting the user output device:
 1770: The user output device is the standard output. It cannot be redirected
 1771: from within gforth, but typically from the command line that starts
 1772: gforth. Gforth uses buffered output, so output on a terminal does not
 1773: become visible before the next newline or buffer overflow. Output on
 1774: non-terminals is invisible until the buffer overflows.
 1775: 
 1776: @item methods of dictionary compilation:
 1777: Waht are we expected to document here?
 1778: 
 1779: @item number of bits in one address unit:
 1780: @code{s" address-units-bits" environment? drop .}. 8 in all current
 1781: ports.
 1782: 
 1783: @item number representation and arithmetic:
 1784: Processor-dependent. Binary two's complement on all current ports.
 1785: 
 1786: @item ranges for integer types:
 1787: Installation-dependent. Make environmental queries for @code{MAX-N},
 1788: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
 1789: unsigned (and positive) types is 0. The lower bound for signed types on
 1790: two's complement and one's complement machines machines can be computed
 1791: by adding 1 to the upper bound.
 1792: 
 1793: @item read-only data space regions:
 1794: The whole Forth data space is writable.
 1795: 
 1796: @item size of buffer at @code{WORD}:
 1797: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
 1798: shared with the pictured numeric output string. If overwriting
 1799: @code{PAD} is acceptable, it is as large as the remaining dictionary
 1800: space, although only as much can be sensibly used as fits in a counted
 1801: string.
 1802: 
 1803: @item size of one cell in address units:
 1804: @code{1 cells .}.
 1805: 
 1806: @item size of one character in address units:
 1807: @code{1 chars .}. 1 on all current ports.
 1808: 
 1809: @item size of the keyboard terminal buffer:
 1810: Varies. You can determine the size at a specific time using @code{lp@
 1811: tib - .}. It is shared with the locals stack and TIBs of files that
 1812: include the current file. You can change the amount of space for TIBs
 1813: and locals stack at gforth startup with the command line option
 1814: @code{-l}.
 1815: 
 1816: @item size of the pictured numeric output buffer:
 1817: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
 1818: shared with @code{WORD}.
 1819: 
 1820: @item size of the scratch area returned by @code{PAD}:
 1821: The remainder of dictionary space. You can even use the unused part of
 1822: the data stack space. The current size can be computed with @code{sp@
 1823: pad - .}.
 1824: 
 1825: @item system case-sensitivity characteristics:
 1826: Dictionary searches are case insensitive. However, as explained above
 1827: under @i{character-set extensions}, the matching for non-ASCII
 1828: characters is determined by the locale you are using. In the default
 1829: @code{C} locale all non-ASCII characters are matched case-sensitively.
 1830: 
 1831: @item system prompt:
 1832: @code{ ok} in interpret state, @code{ compiled} in compile state.
 1833: 
 1834: @item division rounding:
 1835: installation dependent. @code{s" floored" environment? drop .}. We leave
 1836: the choice to gcc (what to use for @code{/}) and to you (whether to use
 1837: @code{fm/mod}, @code{sm/rem} or simply @code{/}).
 1838: 
 1839: @item values of @code{STATE} when true:
 1840: -1.
 1841: 
 1842: @item values returned after arithmetic overflow:
 1843: On two's complement machines, arithmetic is performed modulo
 1844: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
 1845: arithmetic (with appropriate mapping for signed types). Division by zero
 1846: typically results in a @code{-55 throw} (floatingpoint unidentified
 1847: fault), although a @code{-10 throw} (divide by zero) would be more
 1848: appropriate.
 1849: 
 1850: @item whether the current definition can be found after @t{DOES>}:
 1851: No.
 1852: 
 1853: @end table
 1854: 
 1855: @c ---------------------------------------------------------------------
 1856: @node core-ambcond, core-other, core-idef, The Core Words
 1857: @comment  node-name,  next,  previous,  up
 1858: @subsection Ambiguous conditions
 1859: @c ---------------------------------------------------------------------
 1860: 
 1861: @table @i
 1862: 
 1863: @item a name is neither a word nor a number:
 1864: @code{-13 throw} (Undefined word)
 1865: 
 1866: @item a definition name exceeds the maximum length allowed:
 1867: @code{-19 throw} (Word name too long)
 1868: 
 1869: @item addressing a region not inside the various data spaces of the forth system:
 1870: The stacks, code space and name space are accessible. Machine code space is
 1871: typically readable. Accessing other addresses gives results dependent on
 1872: the operating system. On decent systems: @code{-9 throw} (Invalid memory
 1873: address).
 1874: 
 1875: @item argument type incompatible with parameter:
 1876: This is usually not caught. Some words perform checks, e.g., the control
 1877: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
 1878: mismatch).
 1879: 
 1880: @item attempting to obtain the execution token of a word with undefined execution semantics:
 1881: You get an execution token representing the compilation semantics
 1882: instead.
 1883: 
 1884: @item dividing by zero:
 1885: typically results in a @code{-55 throw} (floating point unidentified
 1886: fault), although a @code{-10 throw} (divide by zero) would be more
 1887: appropriate.
 1888: 
 1889: @item insufficient data stack or return stack space:
 1890: Not checked. This typically results in mysterious illegal memory
 1891: accesses, producing @code{-9 throw} (Invalid memory address) or
 1892: @code{-23 throw} (Address alignment exception).
 1893: 
 1894: @item insufficient space for loop control parameters:
 1895: like other return stack overflows.
 1896: 
 1897: @item insufficient space in the dictionary:
 1898: Not checked. Similar results as stack overflows. However, typically the
 1899: error appears at a different place when one inserts or removes code.
 1900: 
 1901: @item interpreting a word with undefined interpretation semantics:
 1902: For some words, we defined interpretation semantics. For the others:
 1903: @code{-14 throw} (Interpreting a compile-only word). Note that this is
 1904: checked only by the outer (aka text) interpreter; if the word is
 1905: @code{execute}d in some other way, it will typically perform it's
 1906: compilation semantics even in interpret state. (We could change @code{'}
 1907: and relatives not to give the xt of such words, but we think that would
 1908: be too restrictive).
 1909: 
 1910: @item modifying the contents of the input buffer or a string literal:
 1911: These are located in writable memory and can be modified.
 1912: 
 1913: @item overflow of the pictured numeric output string:
 1914: Not checked.
 1915: 
 1916: @item parsed string overflow:
 1917: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
 1918: 
 1919: @item producing a result out of range:
 1920: On two's complement machines, arithmetic is performed modulo
 1921: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
 1922: arithmetic (with appropriate mapping for signed types). Division by zero
 1923: typically results in a @code{-55 throw} (floatingpoint unidentified
 1924: fault), although a @code{-10 throw} (divide by zero) would be more
 1925: appropriate. @code{convert} and @code{>number} currently overflow
 1926: silently.
 1927: 
 1928: @item reading from an empty data or return stack:
 1929: The data stack is checked by the outer (aka text) interpreter after
 1930: every word executed. If it has underflowed, a @code{-4 throw} (Stack
 1931: underflow) is performed. Apart from that, the stacks are not checked and
 1932: underflows can result in similar behaviour as overflows (of adjacent
 1933: stacks).
 1934: 
 1935: @item unexepected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
 1936: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
 1937: use zero-length string as a name). Words like @code{'} probably will not
 1938: find what they search. Note that it is possible to create zero-length
 1939: names with @code{nextname} (should it not?).
 1940: 
 1941: @item @code{>IN} greater than input buffer:
 1942: The next invocation of a parsing word returns a string wih length 0.
 1943: 
 1944: @item @code{RECURSE} appears after @code{DOES>}:
 1945: Compiles a recursive call to the defining word not to the defined word.
 1946: 
 1947: @item argument input source different than current input source for @code{RESTORE-INPUT}:
 1948: !!???If the argument input source is a valid input source then it gets
 1949: restored. Otherwise causes @code{-12 THROW} which unless caught issues
 1950: the message "argument type mismatch" and aborts.
 1951: 
 1952: @item data space containing definitions gets de-allocated:
 1953: Deallocation with @code{allot} is not checked. This typically resuls in
 1954: memory access faults or execution of illegal instructions.
 1955: 
 1956: @item data space read/write with incorrect alignment:
 1957: Processor-dependent. Typically results in a @code{-23 throw} (Address
 1958: alignment exception). Under Linux on a 486 or later processor with
 1959: alignment turned on, incorrect alignment results in a @code{-9 throw}
 1960: (Invalid memory address). There are reportedly some processors with
 1961: alignment restrictions that do not report them.
 1962: 
 1963: @item data space pointer not properly aligned, @code{,}, @code{C,}:
 1964: Like other alignment errors.
 1965: 
 1966: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
 1967: Not checked. May cause an illegal memory access.
 1968: 
 1969: @item loop control parameters not available:
 1970: Not checked. The counted loop words simply assume that the top of return
 1971: stack items are loop control parameters and behave accordingly.
 1972: 
 1973: @item most recent definition does not have a name (@code{IMMEDIATE}):
 1974: @code{abort" last word was headerless"}.
 1975: 
 1976: @item name not defined by @code{VALUE} used by @code{TO}:
 1977: @code{-32 throw} (Invalid name argument)
 1978: 
 1979: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}:
 1980: @code{-13 throw} (Undefined word)
 1981: 
 1982: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
 1983: Gforth behaves as if they were of the same type. I.e., you can predict
 1984: the behaviour by interpreting all parameters as, e.g., signed.
 1985: 
 1986: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
 1987: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} is equivalent to
 1988: @code{TO}.
 1989: 
 1990: @item String longer than a counted string returned by @code{WORD}:
 1991: Not checked. The string will be ok, but the count will, of course,
 1992: contain only the least significant bits of the length.
 1993: 
 1994: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}:
 1995: Processor-dependent. Typical behaviours are returning 0 and using only
 1996: the low bits of the shift count.
 1997: 
 1998: @item word not defined via @code{CREATE}:
 1999: @code{>BODY} produces the PFA of the word no matter how it was defined.
 2000: 
 2001: @code{DOES>} changes the execution semantics of the last defined word no
 2002: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
 2003: @code{CREATE , DOES>}.
 2004: 
 2005: @item words improperly used outside @code{<#} and @code{#>}:
 2006: Not checked. As usual, you can expect memory faults.
 2007: 
 2008: @end table
 2009: 
 2010: 
 2011: @c ---------------------------------------------------------------------
 2012: @node core-other,  , core-ambcond, The Core Words
 2013: @comment  node-name,  next,  previous,  up
 2014: @subsection Other system documentation
 2015: @c ---------------------------------------------------------------------
 2016: 
 2017: @table @i
 2018: 
 2019: @item nonstandard words using @code{PAD}:
 2020: None.
 2021: 
 2022: @item operator's terminal facilities available:
 2023: !!??
 2024: 
 2025: @item program data space available:
 2026: @code{sp@ here - .} gives the space remaining for dictionary and data
 2027: stack together.
 2028: 
 2029: @item return stack space available:
 2030: !!??
 2031: 
 2032: @item stack space available:
 2033: @code{sp@ here - .} gives the space remaining for dictionary and data
 2034: stack together.
 2035: 
 2036: @item system dictionary space required, in address units:
 2037: Type @code{here forthstart - .} after startup. At the time of this
 2038: writing, this gives 70108 (bytes) on a 32-bit system.
 2039: @end table
 2040: 
 2041: 
 2042: @c =====================================================================
 2043: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
 2044: @comment  node-name,  next,  previous,  up
 2045: @section The optional Block word set
 2046: @c =====================================================================
 2047: 
 2048: @menu
 2049: * block-idef::                  
 2050: * block-ambcond::               
 2051: * block-other::                 
 2052: @end menu
 2053: 
 2054: 
 2055: @c ---------------------------------------------------------------------
 2056: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
 2057: @comment  node-name,  next,  previous,  up
 2058: @subsection Implementation Defined Options
 2059: @c ---------------------------------------------------------------------
 2060: 
 2061: @table @i
 2062: 
 2063: @item the format for display by @code{LIST}:
 2064: First the screen number is displayed, then 16 lines of 64 characters,
 2065: each line preceded by the line number.
 2066: 
 2067: @item the length of a line affected by @code{\}:
 2068: 64 characters.
 2069: @end table
 2070: 
 2071: 
 2072: @c ---------------------------------------------------------------------
 2073: @node block-ambcond, block-other, block-idef, The optional Block word set
 2074: @comment  node-name,  next,  previous,  up
 2075: @subsection Ambiguous conditions
 2076: @c ---------------------------------------------------------------------
 2077: 
 2078: @table @i
 2079: 
 2080: @item correct block read was not possible:
 2081: Typically results in a @code{throw} of some OS-derived value (between
 2082: -512 and -2048). If the blocks file was just not long enough, blanks are
 2083: supplied for the missing portion.
 2084: 
 2085: @item I/O exception in block transfer:
 2086: Typically results in a @code{throw} of some OS-derived value (between
 2087: -512 and -2048).
 2088: 
 2089: @item invalid block number:
 2090: @code{-35 throw} (Invalid block number)
 2091: 
 2092: @item a program directly alters the contents of @code{BLK}:
 2093: The input stream is switched to that other block, at the same
 2094: position. If the storing to @code{BLK} happens when interpreting
 2095: non-block input, the system will get quite confused when the block ends.
 2096: 
 2097: @item no current block buffer for @code{UPDATE}:
 2098: @code{UPDATE} has no effect.
 2099: 
 2100: @end table
 2101: 
 2102: 
 2103: @c ---------------------------------------------------------------------
 2104: @node block-other,  , block-ambcond, The optional Block word set
 2105: @comment  node-name,  next,  previous,  up
 2106: @subsection Other system documentation
 2107: @c ---------------------------------------------------------------------
 2108: 
 2109: @table @i
 2110: 
 2111: @item any restrictions a multiprogramming system places on the use of buffer addresses:
 2112: No restrictions (yet).
 2113: 
 2114: @item the number of blocks available for source and data:
 2115: depends on your disk space.
 2116: 
 2117: @end table
 2118: 
 2119: 
 2120: @c =====================================================================
 2121: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
 2122: @comment  node-name,  next,  previous,  up
 2123: @section The optional Double Number word set
 2124: @c =====================================================================
 2125: 
 2126: @menu
 2127: * double-idef::                 
 2128: * double-ambcond::              
 2129: * double-other::                
 2130: @end menu
 2131: 
 2132: 
 2133: @c ---------------------------------------------------------------------
 2134: @node double-idef, double-ambcond, The optional Double Number word set, The optional Double Number word set
 2135: @comment  node-name,  next,  previous,  up
 2136: @subsection Implementation Defined Options
 2137: @c ---------------------------------------------------------------------
 2138: 
 2139: No additional documentation requirements.
 2140: 
 2141: 
 2142: @c ---------------------------------------------------------------------
 2143: @node double-ambcond, double-other, double-idef, The optional Double Number word set
 2144: @comment  node-name,  next,  previous,  up
 2145: @subsection Ambiguous conditions
 2146: @c ---------------------------------------------------------------------
 2147: 
 2148: @table @i
 2149: 
 2150: @item @var{d} outside of range on @var{n} in @code{D>S}:
 2151: The least significant cell of @var{d} is produced.
 2152: 
 2153: @end table
 2154: 
 2155: 
 2156: @c ---------------------------------------------------------------------
 2157: @node double-other,  , double-ambcond, The optional Double Number word set
 2158: @comment  node-name,  next,  previous,  up
 2159: @subsection Other system documentation
 2160: @c ---------------------------------------------------------------------
 2161: 
 2162: No additional documentation requirements.
 2163: 
 2164: 
 2165: @c =====================================================================
 2166: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
 2167: @comment  node-name,  next,  previous,  up
 2168: @section The optional Exception word set
 2169: @c =====================================================================
 2170: 
 2171: @menu
 2172: * exception-idef::              
 2173: * exception-ambcond::           
 2174: * exception-other::             
 2175: @end menu
 2176: 
 2177: 
 2178: @c ---------------------------------------------------------------------
 2179: @node exception-idef, exception-ambcond, The optional Exception word set, The optional Exception word set
 2180: @comment  node-name,  next,  previous,  up
 2181: @subsection Implementation Defined Options
 2182: @c ---------------------------------------------------------------------
 2183: 
 2184: @table @i
 2185: @item @code{THROW}-codes used in the system:
 2186: The codes -256@minus{}-511 are used for reporting signals (see
 2187: @file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors
 2188: (for file and memory allocation operations). The mapping from OS error
 2189: numbers to throw code is -512@minus{}@var{errno}. One side effect of
 2190: this mapping is that undefined OS errors produce a message with a
 2191: strange number; e.g., @code{-1000 THROW} results in @code{Unknown error
 2192: 488} on my system.
 2193: @end table
 2194: 
 2195: 
 2196: @c ---------------------------------------------------------------------
 2197: @node exception-ambcond, exception-other, exception-idef, The optional Exception word set
 2198: @comment  node-name,  next,  previous,  up
 2199: @subsection Ambiguous conditions
 2200: @c ---------------------------------------------------------------------
 2201: 
 2202: No additional documentation requirements.
 2203: 
 2204: 
 2205: @c ---------------------------------------------------------------------
 2206: @node exception-other,  , exception-ambcond, The optional Exception word set
 2207: @comment  node-name,  next,  previous,  up
 2208: @subsection Other system documentation
 2209: @c ---------------------------------------------------------------------
 2210: 
 2211: No additional documentation requirements.
 2212: 
 2213: 
 2214: @c =====================================================================
 2215: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
 2216: @comment  node-name,  next,  previous,  up
 2217: @section The optional Facility word set
 2218: @c =====================================================================
 2219: 
 2220: @menu
 2221: * facility-idef::               
 2222: * facility-ambcond::            
 2223: * facility-other::              
 2224: @end menu
 2225: 
 2226: 
 2227: @c ---------------------------------------------------------------------
 2228: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
 2229: @comment  node-name,  next,  previous,  up
 2230: @subsection Implementation Defined Options
 2231: @c ---------------------------------------------------------------------
 2232: 
 2233: @table @i
 2234: 
 2235: @item encoding of keyboard events (@code{EKEY}):
 2236: Not yet implemeted.
 2237: 
 2238: @item duration of a system clock tick
 2239: System dependent. With respect to @code{MS}, the time is specified in
 2240: microseconds. How well the OS and the hardware implement this, is
 2241: another question.
 2242: 
 2243: @item repeatability to be expected from the execution of @code{MS}:
 2244: System dependent. On Unix, a lot depends on load. If the system is
 2245: lightly loaded, and the delay is short enough that gforth does not get
 2246: swapped out, the performance should be acceptable. Under MS-DOS and
 2247: other single-tasking systems, it should be good.
 2248: 
 2249: @end table
 2250: 
 2251: 
 2252: @c ---------------------------------------------------------------------
 2253: @node facility-ambcond, facility-other, facility-idef, The optional Facility word set
 2254: @comment  node-name,  next,  previous,  up
 2255: @subsection Ambiguous conditions
 2256: @c ---------------------------------------------------------------------
 2257: 
 2258: @table @i
 2259: 
 2260: @item @code{AT-XY} can't be performed on user output device:
 2261: Largely terminal dependant. No range checks are done on the arguments.
 2262: No errors are reported. You may see some garbage appearing, you may see
 2263: simply nothing happen.
 2264: 
 2265: @end table
 2266: 
 2267: 
 2268: @c ---------------------------------------------------------------------
 2269: @node facility-other,  , facility-ambcond, The optional Facility word set
 2270: @comment  node-name,  next,  previous,  up
 2271: @subsection Other system documentation
 2272: @c ---------------------------------------------------------------------
 2273: 
 2274: No additional documentation requirements.
 2275: 
 2276: 
 2277: @c =====================================================================
 2278: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
 2279: @comment  node-name,  next,  previous,  up
 2280: @section The optional File-Access word set
 2281: @c =====================================================================
 2282: 
 2283: @menu
 2284: * file-idef::                   
 2285: * file-ambcond::                
 2286: * file-other::                  
 2287: @end menu
 2288: 
 2289: 
 2290: @c ---------------------------------------------------------------------
 2291: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
 2292: @comment  node-name,  next,  previous,  up
 2293: @subsection Implementation Defined Options
 2294: @c ---------------------------------------------------------------------
 2295: 
 2296: @table @i
 2297: 
 2298: @item File access methods used:
 2299: @code{R/O}, @code{R/W} and @code{BIN} work as you would
 2300: expect. @code{W/O} translates into the C file opening mode @code{w} (or
 2301: @code{wb}): The file is cleared, if it exists, and created, if it does
 2302: not (both with @code{open-file} and @code{create-file}.  Under Unix
 2303: @code{create-file} creates a file with 666 permissions modified by your
 2304: umask.
 2305: 
 2306: @item file exceptions:
 2307: The file words do not raise exceptions (except, perhaps, memory access
 2308: faults when you pass illegal addresses or file-ids).
 2309: 
 2310: @item file line terminator:
 2311: System-dependent. Gforth uses C's newline character as line
 2312: terminator. What the actual character code(s) of this are is
 2313: system-dependent.
 2314: 
 2315: @item file name format
 2316: System dependent. Gforth just uses the file name format of your OS.
 2317: 
 2318: @item information returned by @code{FILE-STATUS}:
 2319: @code{FILE-STATUS} returns the most powerful file access mode allowed
 2320: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
 2321: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
 2322: along with the retured mode.
 2323: 
 2324: @item input file state after an exception when including source:
 2325: All files that are left via the exception are closed.
 2326: 
 2327: @item @var{ior} values and meaning:
 2328: The @var{ior}s returned by the file words are intended as throw
 2329: codes. They typically are in the range -512@minus{}-2047 of OS errors.
 2330: The mapping from OS error numbers to @var{ior}s is
 2331: -512@minus{}@var{errno}.
 2332: 
 2333: @item maximum depth of file input nesting:
 2334: limited by the amount of return stack, locals/TIB stack, and the number
 2335: of open files available. This should not give you troubles.
 2336: 
 2337: @item maximum size of input line:
 2338: @code{/line}. Currently 255.
 2339: 
 2340: @item methods of mapping block ranges to files:
 2341: Currently, the block words automatically access the file
 2342: @file{blocks.fb} in the currend working directory. More sophisticated
 2343: methods could be implemented if there is demand (and a volunteer).
 2344: 
 2345: @item number of string buffers provided by @code{S"}:
 2346: 1
 2347: 
 2348: @item size of string buffer used by @code{S"}:
 2349: @code{/line}. currently 255.
 2350: 
 2351: @end table
 2352: 
 2353: @c ---------------------------------------------------------------------
 2354: @node file-ambcond, file-other, file-idef, The optional File-Access word set
 2355: @comment  node-name,  next,  previous,  up
 2356: @subsection Ambiguous conditions
 2357: @c ---------------------------------------------------------------------
 2358: 
 2359: @table @i
 2360: 
 2361: @item attempting to position a file outside it's boundaries:
 2362: @code{REPOSITION-FILE} is performed as usual: Afterwards,
 2363: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
 2364: 
 2365: @item attempting to read from file positions not yet written:
 2366: End-of-file, i.e., zero characters are read and no error is reported.
 2367: 
 2368: @item @var{file-id} is invalid (@code{INCLUDE-FILE}):
 2369: An appropriate exception may be thrown, but a memory fault or other
 2370: problem is more probable.
 2371: 
 2372: @item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}):
 2373: The @var{ior} produced by the operation, that discovered the problem, is
 2374: thrown.
 2375: 
 2376: @item named file cannot be opened (@code{included}):
 2377: The @var{ior} produced by @code{open-file} is thrown.
 2378: 
 2379: @item requesting an unmapped block number:
 2380: There are no unmapped legal block numbers. On some operating systems,
 2381: writing a block with a large number may overflow the file system and
 2382: have an error message as consequence.
 2383: 
 2384: @item using @code{source-id} when @code{blk} is non-zero:
 2385: @code{source-id} performs its function. Typically it will give the id of
 2386: the source which loaded the block. (Better ideas?)
 2387: 
 2388: @end table
 2389: 
 2390: 
 2391: @c ---------------------------------------------------------------------
 2392: @node file-other,  , file-ambcond, The optional File-Access word set
 2393: @comment  node-name,  next,  previous,  up
 2394: @subsection Other system documentation
 2395: @c ---------------------------------------------------------------------
 2396: 
 2397: @table @i
 2398: 
 2399: @item
 2400: 
 2401: @end table
 2402: 
 2403: 
 2404: @c =====================================================================
 2405: @node  The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
 2406: @comment  node-name,  next,  previous,  up
 2407: @section
 2408: @c =====================================================================
 2409: 
 2410: @menu
 2411: * floating-idef::               floating-ambcond:: floating-other::
 2412: * floating-ambcond::            
 2413: * floating-other::              
 2414: @end menu
 2415: 
 2416: 
 2417: @c ---------------------------------------------------------------------
 2418: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
 2419: @comment  node-name,  next,  previous,  up
 2420: @subsection Implementation Defined Options
 2421: @c ---------------------------------------------------------------------
 2422: 
 2423: @table @i
 2424: 
 2425: @item
 2426: 
 2427: @end table
 2428: 
 2429: 
 2430: @c ---------------------------------------------------------------------
 2431: @node floating-ambcond, floating-other, floating-idef, The optional Floating-Point word set
 2432: @comment  node-name,  next,  previous,  up
 2433: @subsection Ambiguous conditions
 2434: @c ---------------------------------------------------------------------
 2435: 
 2436: @table @i
 2437: 
 2438: @item
 2439: 
 2440: @end table
 2441: 
 2442: 
 2443: @c ---------------------------------------------------------------------
 2444: @node floating-other,  , floating-ambcond, The optional Floating-Point word set
 2445: @comment  node-name,  next,  previous,  up
 2446: @subsection Other system documentation
 2447: @c ---------------------------------------------------------------------
 2448: 
 2449: @table @i
 2450: 
 2451: @item
 2452: 
 2453: @end table
 2454: 
 2455: 
 2456: @c =====================================================================
 2457: @node  The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
 2458: @comment  node-name,  next,  previous,  up
 2459: @section
 2460: @c =====================================================================
 2461: 
 2462: @menu
 2463: * locals-idef::                 
 2464: * locals-ambcond::              
 2465: * locals-other::                
 2466: @end menu
 2467: 
 2468: 
 2469: @c ---------------------------------------------------------------------
 2470: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
 2471: @comment  node-name,  next,  previous,  up
 2472: @subsection Implementation Defined Options
 2473: @c ---------------------------------------------------------------------
 2474: 
 2475: @table @i
 2476: 
 2477: @item
 2478: 
 2479: @end table
 2480: 
 2481: 
 2482: @c ---------------------------------------------------------------------
 2483: @node locals-ambcond, locals-other, locals-idef, The optional Locals word set
 2484: @comment  node-name,  next,  previous,  up
 2485: @subsection Ambiguous conditions
 2486: @c ---------------------------------------------------------------------
 2487: 
 2488: @table @i
 2489: 
 2490: @item
 2491: 
 2492: @end table
 2493: 
 2494: 
 2495: @c ---------------------------------------------------------------------
 2496: @node locals-other,  , locals-ambcond, The optional Locals word set
 2497: @comment  node-name,  next,  previous,  up
 2498: @subsection Other system documentation
 2499: @c ---------------------------------------------------------------------
 2500: 
 2501: @table @i
 2502: 
 2503: @item
 2504: 
 2505: @end table
 2506: 
 2507: 
 2508: @c =====================================================================
 2509: @node  The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
 2510: @comment  node-name,  next,  previous,  up
 2511: @section
 2512: @c =====================================================================
 2513: 
 2514: @menu
 2515: * memory-idef::                 
 2516: * memory-ambcond::              
 2517: * memory-other::                
 2518: @end menu
 2519: 
 2520: 
 2521: @c ---------------------------------------------------------------------
 2522: @node memory-idef, memory-ambcond, The optional Memory-Allocation word set, The optional Memory-Allocation word set
 2523: @comment  node-name,  next,  previous,  up
 2524: @subsection Implementation Defined Options
 2525: @c ---------------------------------------------------------------------
 2526: 
 2527: @table @i
 2528: 
 2529: @item
 2530: 
 2531: @end table
 2532: 
 2533: 
 2534: @c ---------------------------------------------------------------------
 2535: @node memory-ambcond, memory-other, memory-idef, The optional Memory-Allocation word set
 2536: @comment  node-name,  next,  previous,  up
 2537: @subsection Ambiguous conditions
 2538: @c ---------------------------------------------------------------------
 2539: 
 2540: @table @i
 2541: 
 2542: @item
 2543: 
 2544: @end table
 2545: 
 2546: 
 2547: @c ---------------------------------------------------------------------
 2548: @node memory-other,  , memory-ambcond, The optional Memory-Allocation word set
 2549: @comment  node-name,  next,  previous,  up
 2550: @subsection Other system documentation
 2551: @c ---------------------------------------------------------------------
 2552: 
 2553: @table @i
 2554: 
 2555: @item
 2556: 
 2557: @end table
 2558: 
 2559: 
 2560: @c =====================================================================
 2561: @node  The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
 2562: @comment  node-name,  next,  previous,  up
 2563: @section
 2564: @c =====================================================================
 2565: 
 2566: @menu
 2567: * programming-idef::            
 2568: * programming-ambcond::         
 2569: * programming-other::           
 2570: @end menu
 2571: 
 2572: 
 2573: @c ---------------------------------------------------------------------
 2574: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
 2575: @comment  node-name,  next,  previous,  up
 2576: @subsection Implementation Defined Options
 2577: @c ---------------------------------------------------------------------
 2578: 
 2579: @table @i
 2580: 
 2581: @item
 2582: 
 2583: @end table
 2584: 
 2585: 
 2586: @c ---------------------------------------------------------------------
 2587: @node programming-ambcond, programming-other, programming-idef, The optional Programming-Tools word set
 2588: @comment  node-name,  next,  previous,  up
 2589: @subsection Ambiguous conditions
 2590: @c ---------------------------------------------------------------------
 2591: 
 2592: @table @i
 2593: 
 2594: @item
 2595: 
 2596: @end table
 2597: 
 2598: 
 2599: @c ---------------------------------------------------------------------
 2600: @node programming-other,  , programming-ambcond, The optional Programming-Tools word set
 2601: @comment  node-name,  next,  previous,  up
 2602: @subsection Other system documentation
 2603: @c ---------------------------------------------------------------------
 2604: 
 2605: @table @i
 2606: 
 2607: @item
 2608: 
 2609: @end table
 2610: 
 2611: 
 2612: @c =====================================================================
 2613: @node  The optional Search-Order word set, The optional String word set, The optional Programming-Tools word set, ANS conformance
 2614: @comment  node-name,  next,  previous,  up
 2615: @section
 2616: @c =====================================================================
 2617: 
 2618: @menu
 2619: * search-idef::                 
 2620: * search-ambcond::              
 2621: * search-other::                
 2622: @end menu
 2623: 
 2624: 
 2625: @c ---------------------------------------------------------------------
 2626: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
 2627: @comment  node-name,  next,  previous,  up
 2628: @subsection Implementation Defined Options
 2629: @c ---------------------------------------------------------------------
 2630: 
 2631: @table @i
 2632: 
 2633: @item
 2634: 
 2635: @end table
 2636: 
 2637: 
 2638: @c ---------------------------------------------------------------------
 2639: @node search-ambcond, search-other, search-idef, The optional Search-Order word set
 2640: @comment  node-name,  next,  previous,  up
 2641: @subsection Ambiguous conditions
 2642: @c ---------------------------------------------------------------------
 2643: 
 2644: @table @i
 2645: 
 2646: @item
 2647: 
 2648: @end table
 2649: 
 2650: 
 2651: @c ---------------------------------------------------------------------
 2652: @node search-other,  , search-ambcond, The optional Search-Order word set
 2653: @comment  node-name,  next,  previous,  up
 2654: @subsection Other system documentation
 2655: @c ---------------------------------------------------------------------
 2656: 
 2657: @table @i
 2658: 
 2659: @item
 2660: 
 2661: @end table
 2662: 
 2663: 
 2664: @c =====================================================================
 2665: @node  The optional String word set,  , The optional Search-Order word set, ANS conformance
 2666: @comment  node-name,  next,  previous,  up
 2667: @section
 2668: @c =====================================================================
 2669: 
 2670: @menu
 2671: * string-idef::                 
 2672: * string-ambcond::              
 2673: * string-other::                
 2674: @end menu
 2675: 
 2676: 
 2677: @c ---------------------------------------------------------------------
 2678: @node string-idef, string-ambcond, The optional String word set, The optional String word set
 2679: @comment  node-name,  next,  previous,  up
 2680: @subsection Implementation Defined Options
 2681: @c ---------------------------------------------------------------------
 2682: 
 2683: @table @i
 2684: 
 2685: @item
 2686: 
 2687: @end table
 2688: 
 2689: 
 2690: @c ---------------------------------------------------------------------
 2691: @node string-ambcond, string-other, string-idef, The optional String word set
 2692: @comment  node-name,  next,  previous,  up
 2693: @subsection Ambiguous conditions
 2694: @c ---------------------------------------------------------------------
 2695: 
 2696: @table @i
 2697: 
 2698: @item
 2699: 
 2700: @end table
 2701: 
 2702: 
 2703: @c ---------------------------------------------------------------------
 2704: @node string-other,  , string-ambcond, The optional String word set
 2705: @comment  node-name,  next,  previous,  up
 2706: @subsection Other system documentation
 2707: @c ---------------------------------------------------------------------
 2708: 
 2709: @table @i
 2710: 
 2711: @item
 2712: 
 2713: @end table
 2714: 
 2715: 
 2716: 
 2717: @node Model, Emacs and GForth, ANS conformance, Top
 2718: @chapter Model
 2719: 
 2720: @node Emacs and GForth, Internals, Model, Top
 2721: @chapter Emacs and GForth
 2722: 
 2723: GForth comes with @file{gforth.el}, an improved version of
 2724: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
 2725: improvements are a better (but still not perfect) handling of
 2726: indentation. I have also added comment paragraph filling (@kbd{M-q}),
 2727: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
 2728: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
 2729: stuff I do not use alone, even though some of it only makes sense for
 2730: TILE. To get a description of these features, enter Forth mode and type
 2731: @kbd{C-h m}.
 2732: 
 2733: In addition, GForth supports Emacs quite well: The source code locations
 2734: given in error messages, debugging output (from @code{~~}) and failed
 2735: assertion messages are in the right format for Emacs' compilation mode
 2736: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
 2737: Manual}) so the source location corresponding to an error or other
 2738: message is only a few keystrokes away (@kbd{C-x `} for the next error,
 2739: @kbd{C-c C-c} for the error under the cursor).
 2740: 
 2741: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
 2742: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
 2743: contains the definitions of all words defined afterwards. You can then
 2744: find the source for a word using @kbd{M-.}. Note that emacs can use
 2745: several tags files at the same time (e.g., one for the gforth sources
 2746: and one for your program).
 2747: 
 2748: To get all these benefits, add the following lines to your @file{.emacs}
 2749: file:
 2750: 
 2751: @example
 2752: (autoload 'forth-mode "gforth.el")
 2753: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
 2754: @end example
 2755: 
 2756: @node Internals, Bugs, Emacs and GForth, Top
 2757: @chapter Internals
 2758: 
 2759: Reading this section is not necessary for programming with gforth. It
 2760: should be helpful for finding your way in the gforth sources.
 2761: 
 2762: @menu
 2763: * Portability::                 
 2764: * Threading::                   
 2765: * Primitives::                  
 2766: * System Architecture::         
 2767: @end menu
 2768: 
 2769: @node Portability, Threading, Internals, Internals
 2770: @section Portability
 2771: 
 2772: One of the main goals of the effort is availability across a wide range
 2773: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
 2774: this goal by manually coding the engine in assembly language for several
 2775: then-popular processors. This approach is very labor-intensive and the
 2776: results are short-lived due to progress in computer architecture.
 2777: 
 2778: Others have avoided this problem by coding in C, e.g., Mitch Bradley
 2779: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
 2780: particularly popular for UNIX-based Forths due to the large variety of
 2781: architectures of UNIX machines. Unfortunately an implementation in C
 2782: does not mix well with the goals of efficiency and with using
 2783: traditional techniques: Indirect or direct threading cannot be expressed
 2784: in C, and switch threading, the fastest technique available in C, is
 2785: significantly slower. Another problem with C is that it's very
 2786: cumbersome to express double integer arithmetic.
 2787: 
 2788: Fortunately, there is a portable language that does not have these
 2789: limitations: GNU C, the version of C processed by the GNU C compiler
 2790: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
 2791: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
 2792: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
 2793: threading possible, its @code{long long} type (@pxref{Long Long, ,
 2794: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
 2795: double numbers. GNU C is available for free on all important (and many
 2796: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
 2797: the Atari ST, so a Forth written in GNU C can run on all these
 2798: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
 2799: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
 2800: ``Look And Feel'', gcc.info, GNU C Manual}).}.
 2801: 
 2802: Writing in a portable language has the reputation of producing code that
 2803: is slower than assembly. For our Forth engine we repeatedly looked at
 2804: the code produced by the compiler and eliminated most compiler-induced
 2805: inefficiencies by appropriate changes in the source-code.
 2806: 
 2807: However, register allocation cannot be portably influenced by the
 2808: programmer, leading to some inefficiencies on register-starved
 2809: machines. We use explicit register declarations (@pxref{Explicit Reg
 2810: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
 2811: improve the speed on some machines. They are turned on by using the
 2812: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
 2813: only depends on the machine, but also on the compiler version: On some
 2814: machines some compiler versions produce incorrect code when certain
 2815: explicit register declarations are used. So by default
 2816: @code{-DFORCE_REG} is not used.
 2817: 
 2818: @node Threading, Primitives, Portability, Internals
 2819: @section Threading
 2820: 
 2821: GNU C's labels as values extension (available since @code{gcc-2.0},
 2822: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
 2823: makes it possible to take the address of @var{label} by writing
 2824: @code{&&@var{label}}.  This address can then be used in a statement like
 2825: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
 2826: @code{goto x}.
 2827: 
 2828: With this feature an indirect threaded NEXT looks like:
 2829: @example
 2830: cfa = *ip++;
 2831: ca = *cfa;
 2832: goto *ca;
 2833: @end example
 2834: For those unfamiliar with the names: @code{ip} is the Forth instruction
 2835: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
 2836: execution token and points to the code field of the next word to be
 2837: executed; The @code{ca} (code address) fetched from there points to some
 2838: executable code, e.g., a primitive or the colon definition handler
 2839: @code{docol}.
 2840: 
 2841: Direct threading is even simpler:
 2842: @example
 2843: ca = *ip++;
 2844: goto *ca;
 2845: @end example
 2846: 
 2847: Of course we have packaged the whole thing neatly in macros called
 2848: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
 2849: 
 2850: @menu
 2851: * Scheduling::                  
 2852: * Direct or Indirect Threaded?::  
 2853: * DOES>::                       
 2854: @end menu
 2855: 
 2856: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
 2857: @subsection Scheduling
 2858: 
 2859: There is a little complication: Pipelined and superscalar processors,
 2860: i.e., RISC and some modern CISC machines can process independent
 2861: instructions while waiting for the results of an instruction. The
 2862: compiler usually reorders (schedules) the instructions in a way that
 2863: achieves good usage of these delay slots. However, on our first tries
 2864: the compiler did not do well on scheduling primitives. E.g., for
 2865: @code{+} implemented as
 2866: @example
 2867: n=sp[0]+sp[1];
 2868: sp++;
 2869: sp[0]=n;
 2870: NEXT;
 2871: @end example
 2872: the NEXT comes strictly after the other code, i.e., there is nearly no
 2873: scheduling. After a little thought the problem becomes clear: The
 2874: compiler cannot know that sp and ip point to different addresses (and
 2875: the version of @code{gcc} we used would not know it even if it was
 2876: possible), so it could not move the load of the cfa above the store to
 2877: the TOS. Indeed the pointers could be the same, if code on or very near
 2878: the top of stack were executed. In the interest of speed we chose to
 2879: forbid this probably unused ``feature'' and helped the compiler in
 2880: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
 2881: the goto part (@code{NEXT_P2}). @code{+} now looks like:
 2882: @example
 2883: n=sp[0]+sp[1];
 2884: sp++;
 2885: NEXT_P1;
 2886: sp[0]=n;
 2887: NEXT_P2;
 2888: @end example
 2889: This can be scheduled optimally by the compiler.
 2890: 
 2891: This division can be turned off with the switch @code{-DCISC_NEXT}. This
 2892: switch is on by default on machines that do not profit from scheduling
 2893: (e.g., the 80386), in order to preserve registers.
 2894: 
 2895: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
 2896: @subsection Direct or Indirect Threaded?
 2897: 
 2898: Both! After packaging the nasty details in macro definitions we
 2899: realized that we could switch between direct and indirect threading by
 2900: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
 2901: defining a few machine-specific macros for the direct-threading case.
 2902: On the Forth level we also offer access words that hide the
 2903: differences between the threading methods (@pxref{Threading Words}).
 2904: 
 2905: Indirect threading is implemented completely
 2906: machine-independently. Direct threading needs routines for creating
 2907: jumps to the executable code (e.g. to docol or dodoes). These routines
 2908: are inherently machine-dependent, but they do not amount to many source
 2909: lines. I.e., even porting direct threading to a new machine is a small
 2910: effort.
 2911: 
 2912: @node DOES>,  , Direct or Indirect Threaded?, Threading
 2913: @subsection DOES>
 2914: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
 2915: the chunk of code executed by every word defined by a
 2916: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
 2917: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
 2918: DOES-code)? There are two solutions:
 2919: 
 2920: In fig-Forth the code field points directly to the dodoes and the
 2921: DOES-code address is stored in the cell after the code address
 2922: (i.e. at cfa cell+). It may seem that this solution is illegal in the
 2923: Forth-79 and all later standards, because in fig-Forth this address
 2924: lies in the body (which is illegal in these standards). However, by
 2925: making the code field larger for all words this solution becomes legal
 2926: again. We use this approach for the indirect threaded version. Leaving
 2927: a cell unused in most words is a bit wasteful, but on the machines we
 2928: are targetting this is hardly a problem. The other reason for having a
 2929: code field size of two cells is to avoid having different image files
 2930: for direct and indirect threaded systems (@pxref{System Architecture}).
 2931: 
 2932: The other approach is that the code field points or jumps to the cell
 2933: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
 2934: this address. @code{dodoes} can then get the DOES-code address by
 2935: computing the code address, i.e., the address of the jump to dodoes,
 2936: and add the length of that jump field. A variant of this is to have a
 2937: call to @code{dodoes} after the @code{DOES>}; then the return address
 2938: (which can be found in the return register on RISCs) is the DOES-code
 2939: address. Since the two cells available in the code field are usually
 2940: used up by the jump to the code address in direct threading, we use
 2941: this approach for direct threading. We did not want to add another
 2942: cell to the code field.
 2943: 
 2944: @node Primitives, System Architecture, Threading, Internals
 2945: @section Primitives
 2946: 
 2947: @menu
 2948: * Automatic Generation::        
 2949: * TOS Optimization::            
 2950: * Produced code::               
 2951: @end menu
 2952: 
 2953: @node Automatic Generation, TOS Optimization, Primitives, Primitives
 2954: @subsection Automatic Generation
 2955: 
 2956: Since the primitives are implemented in a portable language, there is no
 2957: longer any need to minimize the number of primitives. On the contrary,
 2958: having many primitives is an advantage: speed. In order to reduce the
 2959: number of errors in primitives and to make programming them easier, we
 2960: provide a tool, the primitive generator (@file{prims2x.fs}), that
 2961: automatically generates most (and sometimes all) of the C code for a
 2962: primitive from the stack effect notation.  The source for a primitive
 2963: has the following form:
 2964: 
 2965: @format
 2966: @var{Forth-name}	@var{stack-effect}	@var{category}	[@var{pronounc.}]
 2967: [@code{""}@var{glossary entry}@code{""}]
 2968: @var{C code}
 2969: [@code{:}
 2970: @var{Forth code}]
 2971: @end format
 2972: 
 2973: The items in brackets are optional. The category and glossary fields
 2974: are there for generating the documentation, the Forth code is there
 2975: for manual implementations on machines without GNU C. E.g., the source
 2976: for the primitive @code{+} is:
 2977: @example
 2978: +    n1 n2 -- n    core    plus
 2979: n = n1+n2;
 2980: @end example
 2981: 
 2982: This looks like a specification, but in fact @code{n = n1+n2} is C
 2983: code. Our primitive generation tool extracts a lot of information from
 2984: the stack effect notations@footnote{We use a one-stack notation, even
 2985: though we have separate data and floating-point stacks; The separate
 2986: notation can be generated easily from the unified notation.}: The number
 2987: of items popped from and pushed on the stack, their type, and by what
 2988: name they are referred to in the C code. It then generates a C code
 2989: prelude and postlude for each primitive. The final C code for @code{+}
 2990: looks like this:
 2991: 
 2992: @example
 2993: I_plus:	/* + ( n1 n2 -- n ) */  /* label, stack effect */
 2994: /*  */                          /* documentation */
 2995: @{
 2996: DEF_CA                          /* definition of variable ca (indirect threading) */
 2997: Cell n1;                        /* definitions of variables */
 2998: Cell n2;
 2999: Cell n;
 3000: n1 = (Cell) sp[1];              /* input */
 3001: n2 = (Cell) TOS;
 3002: sp += 1;                        /* stack adjustment */
 3003: NAME("+")                       /* debugging output (with -DDEBUG) */
 3004: @{
 3005: n = n1+n2;                      /* C code taken from the source */
 3006: @}
 3007: NEXT_P1;                        /* NEXT part 1 */
 3008: TOS = (Cell)n;                  /* output */
 3009: NEXT_P2;                        /* NEXT part 2 */
 3010: @}
 3011: @end example
 3012: 
 3013: This looks long and inefficient, but the GNU C compiler optimizes quite
 3014: well and produces optimal code for @code{+} on, e.g., the R3000 and the
 3015: HP RISC machines: Defining the @code{n}s does not produce any code, and
 3016: using them as intermediate storage also adds no cost.
 3017: 
 3018: There are also other optimizations, that are not illustrated by this
 3019: example: Assignments between simple variables are usually for free (copy
 3020: propagation). If one of the stack items is not used by the primitive
 3021: (e.g.  in @code{drop}), the compiler eliminates the load from the stack
 3022: (dead code elimination). On the other hand, there are some things that
 3023: the compiler does not do, therefore they are performed by
 3024: @file{prims2x.fs}: The compiler does not optimize code away that stores
 3025: a stack item to the place where it just came from (e.g., @code{over}).
 3026: 
 3027: While programming a primitive is usually easy, there are a few cases
 3028: where the programmer has to take the actions of the generator into
 3029: account, most notably @code{?dup}, but also words that do not (always)
 3030: fall through to NEXT.
 3031: 
 3032: @node TOS Optimization, Produced code, Automatic Generation, Primitives
 3033: @subsection TOS Optimization
 3034: 
 3035: An important optimization for stack machine emulators, e.g., Forth
 3036: engines, is keeping  one or more of the top stack items in
 3037: registers.  If a word has the stack effect @var{in1}...@var{inx} @code{--}
 3038: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
 3039: @itemize
 3040: @item
 3041: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
 3042: due to fewer loads from and stores to the stack.
 3043: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
 3044: @var{y<n}, due to additional moves between registers.
 3045: @end itemize
 3046: 
 3047: In particular, keeping one item in a register is never a disadvantage,
 3048: if there are enough registers. Keeping two items in registers is a
 3049: disadvantage for frequent words like @code{?branch}, constants,
 3050: variables, literals and @code{i}. Therefore our generator only produces
 3051: code that keeps zero or one items in registers. The generated C code
 3052: covers both cases; the selection between these alternatives is made at
 3053: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
 3054: code for @code{+} is just a simple variable name in the one-item case,
 3055: otherwise it is a macro that expands into @code{sp[0]}. Note that the
 3056: GNU C compiler tries to keep simple variables like @code{TOS} in
 3057: registers, and it usually succeeds, if there are enough registers.
 3058: 
 3059: The primitive generator performs the TOS optimization for the
 3060: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
 3061: operations the benefit of this optimization is even larger:
 3062: floating-point operations take quite long on most processors, but can be
 3063: performed in parallel with other operations as long as their results are
 3064: not used. If the FP-TOS is kept in a register, this works. If
 3065: it is kept on the stack, i.e., in memory, the store into memory has to
 3066: wait for the result of the floating-point operation, lengthening the
 3067: execution time of the primitive considerably.
 3068: 
 3069: The TOS optimization makes the automatic generation of primitives a
 3070: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
 3071: @code{TOS} is not sufficient. There are some special cases to
 3072: consider:
 3073: @itemize
 3074: @item In the case of @code{dup ( w -- w w )} the generator must not
 3075: eliminate the store to the original location of the item on the stack,
 3076: if the TOS optimization is turned on.
 3077: @item Primitives with stack effects of the form @code{--}
 3078: @var{out1}...@var{outy} must store the TOS to the stack at the start.
 3079: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
 3080: must load the TOS from the stack at the end. But for the null stack
 3081: effect @code{--} no stores or loads should be generated.
 3082: @end itemize
 3083: 
 3084: @node Produced code,  , TOS Optimization, Primitives
 3085: @subsection Produced code
 3086: 
 3087: To see what assembly code is produced for the primitives on your machine
 3088: with your compiler and your flag settings, type @code{make engine.s} and
 3089: look at the resulting file @file{engine.s}.
 3090: 
 3091: @node System Architecture,  , Primitives, Internals
 3092: @section System Architecture
 3093: 
 3094: Our Forth system consists not only of primitives, but also of
 3095: definitions written in Forth. Since the Forth compiler itself belongs
 3096: to those definitions, it is not possible to start the system with the
 3097: primitives and the Forth source alone. Therefore we provide the Forth
 3098: code as an image file in nearly executable form. At the start of the
 3099: system a C routine loads the image file into memory, sets up the
 3100: memory (stacks etc.) according to information in the image file, and
 3101: starts executing Forth code.
 3102: 
 3103: The image file format is a compromise between the goals of making it
 3104: easy to generate image files and making them portable. The easiest way
 3105: to generate an image file is to just generate a memory dump. However,
 3106: this kind of image file cannot be used on a different machine, or on
 3107: the next version of the engine on the same machine, it even might not
 3108: work with the same engine compiled by a different version of the C
 3109: compiler. We would like to have as few versions of the image file as
 3110: possible, because we do not want to distribute many versions of the
 3111: same image file, and to make it easy for the users to use their image
 3112: files on many machines. We currently need to create a different image
 3113: file for machines with different cell sizes and different byte order
 3114: (little- or big-endian)@footnote{We consider adding information to the
 3115: image file that enables the loader to change the byte order.}.
 3116: 
 3117: Forth code that is going to end up in a portable image file has to
 3118: comply to some restrictions: addresses have to be stored in memory with
 3119: special words (@code{A!}, @code{A,}, etc.) in order to make the code
 3120: relocatable. Cells, floats, etc., have to be stored at the natural
 3121: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
 3122: dividable by~8. This happens automatically in our system when you use
 3123: the ANS Forth alignment words.}, in order to avoid alignment faults on
 3124: machines with stricter alignment. The image file is produced by a
 3125: metacompiler (@file{cross.fs}).
 3126: 
 3127: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
 3128: file is not directly executable, but has to undergo some manipulations
 3129: during loading. Address relocation is performed at image load-time, not
 3130: at run-time. The loader also has to replace tokens standing for
 3131: primitive calls with the appropriate code-field addresses (or code
 3132: addresses in the case of direct threading).
 3133: 
 3134: @node Bugs, Pedigree, Internals, Top
 3135: @chapter Bugs
 3136: 
 3137: @node Pedigree, Word Index, Bugs, Top
 3138: @chapter Pedigree
 3139: 
 3140: @node Word Index, Node Index, Pedigree, Top
 3141: @chapter Word Index
 3142: 
 3143: @node Node Index,  , Word Index, Top
 3144: @chapter Node Index
 3145: 
 3146: @contents
 3147: @bye
 3148: 

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