Annotation of gforth/gforth.ds, revision 1.1

1.1     ! anton       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: @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,  , Invocation, Top
        !           257: @chapter Forth Words
        !           258: 
        !           259: @menu
        !           260: * Notation::
        !           261: * Arithmetic::
        !           262: * Stack Manipulation::
        !           263: * Memory access::
        !           264: * Control Structures::
        !           265: * Local Variables::
        !           266: * Defining Words::
        !           267: * Vocabularies::
        !           268: * Files::
        !           269: * Blocks::
        !           270: * Other I/O::
        !           271: * Programming Tools::
        !           272: @end menu
        !           273: 
        !           274: @node Notation, Arithmetic, Words, Words
        !           275: @section Notation
        !           276: 
        !           277: The Forth words are described in this section in the glossary notation
        !           278: that has become a de-facto standard for Forth texts, i.e.
        !           279: 
        !           280: @quotation
        !           281: @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
        !           282: @var{Description}
        !           283: @end quotation
        !           284: 
        !           285: @table @var
        !           286: @item word
        !           287: The name of the word. BTW, GNU Forth is case insensitive, so you can
        !           288: type the words in in lower case.
        !           289: 
        !           290: @item Stack effect
        !           291: The stack effect is written in the notation @code{@var{before} --
        !           292: @var{after}}, where @var{before} and @var{after} describe the top of
        !           293: stack entries before and after the execution of the word. The rest of
        !           294: the stack is not touched by the word. The top of stack is rightmost,
        !           295: i.e., a stack sequence is written as it is typed in. Note that GNU Forth
        !           296: uses a separate floating point stack, but a unified stack
        !           297: notation. Also, return stack effects are not shown in @var{stack
        !           298: effect}, but in @var{Description}. The name of a stack item describes
        !           299: the type and/or the function of the item. See below for a discussion of
        !           300: the types.
        !           301: 
        !           302: @item pronunciation
        !           303: How the word is pronounced
        !           304: 
        !           305: @item wordset
        !           306: The ANS Forth standard is divided into several wordsets. A standard
        !           307: system need not support all of them. So, the fewer wordsets your program
        !           308: uses the more portable it will be in theory. However, we suspect that
        !           309: most ANS Forth systems on personal machines will feature all
        !           310: wordsets. Words that are not defined in the ANS standard have
        !           311: @code{gforth} as wordset.
        !           312: 
        !           313: @item Description
        !           314: A description of the behaviour of the word.
        !           315: @end table
        !           316: 
        !           317: The name of a stack item corresponds in the following way with its type:
        !           318: 
        !           319: @table @code
        !           320: @item name starts with
        !           321: Type
        !           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,  , 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).
        !           367: 
        !           368: @subsection Single precision
        !           369: doc-+
        !           370: doc--
        !           371: doc-*
        !           372: doc-/
        !           373: doc-mod
        !           374: doc-/mod
        !           375: doc-negate
        !           376: doc-abs
        !           377: doc-min
        !           378: doc-max
        !           379: 
        !           380: @subsection Bitwise operations
        !           381: doc-and
        !           382: doc-or
        !           383: doc-xor
        !           384: doc-invert
        !           385: doc-2*
        !           386: doc-2/
        !           387: 
        !           388: @subsection Mixed precision
        !           389: doc-m+
        !           390: doc-*/
        !           391: doc-*/mod
        !           392: doc-m*
        !           393: doc-um*
        !           394: doc-m*/
        !           395: doc-um/mod
        !           396: doc-fm/mod
        !           397: doc-sm/rem
        !           398: 
        !           399: @subsection Double precision
        !           400: doc-d+
        !           401: doc-d-
        !           402: doc-dnegate
        !           403: doc-dabs
        !           404: doc-dmin
        !           405: doc-dmax
        !           406: 
        !           407: @node Stack Manipulation,,,
        !           408: @section Stack Manipulation
        !           409: 
        !           410: gforth has a data stack (aka parameter stack) for characters, cells,
        !           411: addresses, and double cells, a floating point stack for floating point
        !           412: numbers, a return stack for storing the return addresses of colon
        !           413: definitions and other data, and a locals stack for storing local
        !           414: variables. Note that while every sane Forth has a separate floating
        !           415: point stack, this is not strictly required; an ANS Forth system could
        !           416: theoretically keep floating point numbers on the data stack. As an
        !           417: additional difficulty, you don't know how many cells a floating point
        !           418: number takes. It is reportedly possible to write words in a way that
        !           419: they work also for a unified stack model, but we do not recommend trying
        !           420: it. Also, a Forth system is allowed to keep the local variables on the
        !           421: return stack. This is reasonable, as local variables usually eliminate
        !           422: the need to use the return stack explicitly. So, if you want to produce
        !           423: a standard complying program and if you are using local variables in a
        !           424: word, forget about return stack manipulations in that word (see the
        !           425: standard document for the exact rules).
        !           426: 
        !           427: @subsection Data stack
        !           428: doc-drop
        !           429: doc-nip
        !           430: doc-dup
        !           431: doc-over
        !           432: doc-tuck
        !           433: doc-swap
        !           434: doc-rot
        !           435: doc--rot
        !           436: doc-?dup
        !           437: doc-pick
        !           438: doc-roll
        !           439: doc-2drop
        !           440: doc-2nip
        !           441: doc-2dup
        !           442: doc-2over
        !           443: doc-2tuck
        !           444: doc-2swap
        !           445: doc-2rot
        !           446: 
        !           447: @subsection Floating point stack
        !           448: doc-fdrop
        !           449: doc-fnip
        !           450: doc-fdup
        !           451: doc-fover
        !           452: doc-ftuck
        !           453: doc-fswap
        !           454: doc-frot
        !           455: 
        !           456: @subsection Return stack
        !           457: doc->r
        !           458: doc-r>
        !           459: doc-r@
        !           460: doc-rdrop
        !           461: doc-2>r
        !           462: doc-2r>
        !           463: doc-2r@
        !           464: doc-2rdrop
        !           465: 
        !           466: @subsection Locals stack
        !           467: 
        !           468: @subsection Stack pointer manipulation
        !           469: doc-sp@
        !           470: doc-sp!
        !           471: doc-fp@
        !           472: doc-fp!
        !           473: doc-rp@
        !           474: doc-rp!
        !           475: doc-lp@
        !           476: doc-lp!
        !           477: 
        !           478: @node Memory access
        !           479: @section Memory access
        !           480: 
        !           481: @subsection Stack-Memory transfers
        !           482: 
        !           483: doc-@
        !           484: doc-!
        !           485: doc-+!
        !           486: doc-c@
        !           487: doc-c!
        !           488: doc-2@
        !           489: doc-2!
        !           490: doc-f@
        !           491: doc-f!
        !           492: doc-sf@
        !           493: doc-sf!
        !           494: doc-df@
        !           495: doc-df!
        !           496: 
        !           497: @subsection Address arithmetic
        !           498: 
        !           499: ANS Forth does not specify the sizes of the data types. Instead, it
        !           500: offers a number of words for computing sizes and doing address
        !           501: arithmetic. Basically, address arithmetic is performed in terms of
        !           502: address units (aus); on most systems the address unit is one byte. Note
        !           503: that a character may have more than one au, so @code{chars} is no noop
        !           504: (on systems where it is a noop, it compiles to nothing).
        !           505: 
        !           506: ANS Forth also defines words for aligning addresses for specific
        !           507: addresses. Many computers require that accesses to specific data types
        !           508: must only occur at specific addresses; e.g., that cells may only be
        !           509: accessed at addresses divisible by 4. Even if a machine allows unaligned
        !           510: accesses, it can usually perform aligned accesses faster. 
        !           511: 
        !           512: For the performance-concious: alignment operations are usually only
        !           513: necessary during the definition of a data structure, not during the
        !           514: (more frequent) accesses to it.
        !           515: 
        !           516: ANS Forth defines no words for character-aligning addresses. This is not
        !           517: an oversight, but reflects the fact that addresses that are not
        !           518: char-aligned have no use in the standard and therefore will not be
        !           519: created.
        !           520: 
        !           521: The standard guarantees that addresses returned by @code{CREATE}d words
        !           522: are cell-aligned; in addition, gforth guarantees that these addresses
        !           523: are aligned for all purposes.
        !           524: 
        !           525: doc-chars
        !           526: doc-char+
        !           527: doc-cells
        !           528: doc-cell+
        !           529: doc-align
        !           530: doc-aligned
        !           531: doc-floats
        !           532: doc-float+
        !           533: doc-falign
        !           534: doc-faligned
        !           535: doc-sfloats
        !           536: doc-sfloat+
        !           537: doc-sfalign
        !           538: doc-sfaligned
        !           539: doc-dfloats
        !           540: doc-dfloat+
        !           541: doc-dfalign
        !           542: doc-dfaligned
        !           543: doc-address-unit-bits
        !           544: 
        !           545: @subsection Memory block access
        !           546: 
        !           547: doc-move
        !           548: doc-erase
        !           549: 
        !           550: While the previous words work on address units, the rest works on
        !           551: characters.
        !           552: 
        !           553: doc-cmove
        !           554: doc-cmove>
        !           555: doc-fill
        !           556: doc-blank
        !           557: 
        !           558: @node Control Structures
        !           559: @section Control Structures
        !           560: 
        !           561: Control structures in Forth cannot be used in interpret state, only in
        !           562: compile state, i.e., in a colon definition. We do not like this
        !           563: limitation, but have not seen a satisfying way around it yet, although
        !           564: many schemes have been proposed.
        !           565: 
        !           566: @subsection Selection
        !           567: 
        !           568: @example
        !           569: @var{flag}
        !           570: IF
        !           571:   @var{code}
        !           572: ENDIF
        !           573: @end example
        !           574: or
        !           575: @example
        !           576: @var{flag}
        !           577: IF
        !           578:   @var{code1}
        !           579: ELSE
        !           580:   @var{code2}
        !           581: ENDIF
        !           582: @end example
        !           583: 
        !           584: You can use @code{THEN} instead of {ENDIF}. Indeed, @code{THEN} is
        !           585: standard, and @code{ENDIF} is not, although it is quite popular. We
        !           586: recommend using @code{ENDIF}, because it is less confusing for people
        !           587: who also know other languages (and is not prone to reinforcing negative
        !           588: prejudices against Forth in these people). Adding @code{ENDIF} to a
        !           589: system that only supplies @code{THEN} is simple:
        !           590: @example
        !           591: : endif   POSTPONE then ; immediate
        !           592: @end example
        !           593: 
        !           594: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
        !           595: (adv.)}  has the following meanings:
        !           596: @quotation
        !           597: ... 2b: following next after in order ... 3d: as a necessary consequence
        !           598: (if you were there, then you saw them).
        !           599: @end quotation
        !           600: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
        !           601: and many other programming languages has the meaning 3d.]
        !           602: 
        !           603: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
        !           604: can avoid using @code{?dup}.
        !           605: 
        !           606: @example
        !           607: @var{n}
        !           608: CASE
        !           609:   @var{n1} OF @var{code1} ENDOF
        !           610:   @var{n2} OF @var{code2} ENDOF
        !           611:   @dots
        !           612: ENDCASE
        !           613: @end example
        !           614: 
        !           615: Executes the first @var{codei}, where the @var{ni} is equal to
        !           616: @var{n}. A default case can be added by simply writing the code after
        !           617: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
        !           618: but must not consume it.
        !           619: 
        !           620: @subsection Simple Loops
        !           621: 
        !           622: @example
        !           623: BEGIN
        !           624:   @var{code1}
        !           625:   @var{flag}
        !           626: WHILE
        !           627:   @var{code2}
        !           628: REPEAT
        !           629: @end example
        !           630: 
        !           631: @var{code1} is executed and @var{flag} is computed. If it is true,
        !           632: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
        !           633: 
        !           634: @example
        !           635: BEGIN
        !           636:   @var{code}
        !           637:   @var{flag}
        !           638: UNTIL
        !           639: @end example
        !           640: 
        !           641: @var{code} is executed. The loop is restarted if @code{flag} is false.
        !           642: 
        !           643: @example
        !           644: BEGIN
        !           645:   @var{code}
        !           646: AGAIN
        !           647: @end example
        !           648: 
        !           649: This is an endless loop.
        !           650: 
        !           651: @subsection Counted Loops
        !           652: 
        !           653: The basic counted loop is:
        !           654: @example
        !           655: @var{limit} @var{start}
        !           656: ?DO
        !           657:   @var{body}
        !           658: LOOP
        !           659: @end example
        !           660: 
        !           661: This performs one iteration for every integer, starting from @var{start}
        !           662: and up to, but excluding @var{limit}. The counter, aka index, can be
        !           663: accessed with @code{i}. E.g., the loop
        !           664: @example
        !           665: 10 0 ?DO
        !           666:   i .
        !           667: LOOP
        !           668: @end example
        !           669: prints
        !           670: @example
        !           671: 0 1 2 3 4 5 6 7 8 9
        !           672: @end example
        !           673: The index of the innermost loop can be accessed with @code{i}, the index
        !           674: of the next loop with @code{j}, and the index of the third loop with
        !           675: @code{k}.
        !           676: 
        !           677: The loop control data are kept on the return stack, so there are some
        !           678: restrictions on mixing return stack accesses and counted loop
        !           679: words. E.g., if you put values on the return stack outside the loop, you
        !           680: cannot read them inside the loop. If you put values on the return stack
        !           681: within a loop, you have to remove them before the end of the loop and
        !           682: before accessing the index of the loop.
        !           683: 
        !           684: There are several variations on the counted loop:
        !           685: 
        !           686: @code{LEAVE} leaves the innermost counted loop immediately.
        !           687: 
        !           688: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
        !           689: index by @var{n} instead of by 1. The loop is terminated when the border
        !           690: between @var{limit-1} and @var{limit} is crossed. E.g.:
        !           691: 
        !           692: 4 0 ?DO  i .  2 +LOOP   prints 0 2
        !           693: 
        !           694: 4 1 ?DO  i .  2 +LOOP   prints 1 3
        !           695: 
        !           696: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
        !           697: 
        !           698: -1 0 ?DO  i .  -1 +LOOP  prints 0 -1
        !           699: 
        !           700:  0 0 ?DO  i .  -1 +LOOP  prints nothing
        !           701: 
        !           702: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
        !           703: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
        !           704: case behaves symmetrical to the positive case:
        !           705: 
        !           706: -2 0 ?DO  i .  -1 +LOOP  prints 0 -1
        !           707: 
        !           708: -1 0 ?DO  i .  -1 +LOOP  prints 0
        !           709: 
        !           710:  0 0 ?DO  i .  -1 +LOOP  prints nothing
        !           711: 
        !           712: The loop is terminated when the border between @var{limit-sgn(n)} and
        !           713: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
        !           714: Forth standard.
        !           715: 
        !           716: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
        !           717: when the start and the limit value are equal. We do not recommend using
        !           718: @code{DO}. It will just give you maintenance troubles.
        !           719: 
        !           720: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
        !           721: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
        !           722: return stack so @code{EXIT} can get to its return address.
        !           723: 
        !           724: Another counted loop is
        !           725: @example
        !           726: @var{n}
        !           727: FOR
        !           728:   @var{body}
        !           729: NEXT
        !           730: @end example
        !           731: This is the preferred loop of native code compiler writers who are too
        !           732: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
        !           733: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
        !           734: and ending with 0. Other Forth systems may behave differently, even if
        !           735: they support @code{FOR} loops.
        !           736: 
        !           737: @node Locals
        !           738: @section Locals
        !           739: 
        !           740: 
        !           741: 
        !           742: @contents
        !           743: @bye
        !           744: 

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