File:  [gforth] / gforth / Attic / gforth.ds
Revision 1.1: download - view: text, annotated - select for diffs
Mon Oct 24 19:15:57 1994 UTC (25 years, 1 month ago) by anton
Branches: MAIN
CVS tags: HEAD
Added automatic glossary entry transfer from primitives to the texi file.
renamed gfoprth.texi to gforth.ds.
fixed a few minor bugs.
changed the behaviour of locals scoping when encountering an unreachable BEGIN.
made UNREACHABLE immediate

    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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