File:  [gforth] / gforth / Attic / gforth.ds
Revision 1.3: download - view: text, annotated - select for diffs
Wed Nov 23 16:54:39 1994 UTC (26 years, 4 months ago) by anton
Branches: MAIN
CVS tags: HEAD
added package target to
some documentation changes

    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.)
    9: @ifinfo
   10: This file documents GNU Forth 0.0
   12: Copyright @copyright{} 1994 GNU Forth Development Group
   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.
   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).
   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.
   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
   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
   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
   52: @comment !! Published by ... or You can get a copy of this manual ...
   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.
   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.
   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
   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
   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
   95: @node License, Goals, Top, Top
   96: @unnumbered License
   97: !! Insert GPL here
   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
  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:
  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
  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
  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.
  151: @node Other Books, Invocation, Goals, Top
  152: @chapter Other books on ANS Forth
  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.
  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
  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.
  172: @node Invocation, Words, Other Books, Top
  173: @chapter Invocation
  175: You will usually just say @code{gforth}. In many other cases the default
  176: GNU Forth image will be invoked like this:
  178: @example
  179: gforth [files] [-e forth-code]
  180: @end example
  182: executing the contents of the files and the Forth code in the order they
  183: are given.
  185: In general, the command line looks like this:
  187: @example
  188: gforth [initialization options] [image-specific options]
  189: @end example
  191: The initialization options must come before the rest of the command
  192: line. They are:
  194: @table @code
  195: @item --image-file @var{file}
  196: Loads the Forth image @var{file} instead of the default
  197: @file{}.
  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.
  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.
  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).
  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).
  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.
  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).
  236: @end table
  238: As explained above, the image-specific command-line arguments for the
  239: default image @file{} 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.
  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).
  256: @node Words,  , Invocation, Top
  257: @chapter Forth Words
  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
  274: @node Notation, Arithmetic, Words, Words
  275: @section Notation
  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.
  280: @quotation
  281: @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
  282: @var{Description}
  283: @end quotation
  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.
  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.
  302: @item pronunciation
  303: How the word is pronounced
  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.
  313: @item Description
  314: A description of the behaviour of the word.
  315: @end table
  317: The name of a stack item corresponds in the following way with its type:
  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
  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).
  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
  380: @subsection Bitwise operations
  381: doc-and
  382: doc-or
  383: doc-xor
  384: doc-invert
  385: doc-2*
  386: doc-2/
  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
  399: @subsection Double precision
  400: doc-d+
  401: doc-d-
  402: doc-dnegate
  403: doc-dabs
  404: doc-dmin
  405: doc-dmax
  407: @node Stack Manipulation,,,
  408: @section Stack Manipulation
  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).
  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
  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
  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
  466: @subsection Locals stack
  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!
  478: @node Memory access
  479: @section Memory access
  481: @subsection Stack-Memory transfers
  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!
  497: @subsection Address arithmetic
  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).
  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. 
  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.
  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.
  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.
  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
  545: @subsection Memory block access
  547: doc-move
  548: doc-erase
  550: While the previous words work on address units, the rest works on
  551: characters.
  553: doc-cmove
  554: doc-cmove>
  555: doc-fill
  556: doc-blank
  558: @node Control Structures
  559: @section Control Structures
  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.
  566: @subsection Selection
  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
  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
  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.]
  603: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
  604: can avoid using @code{?dup}.
  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
  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.
  620: @subsection Simple Loops
  622: @example
  623: BEGIN
  624:   @var{code1}
  625:   @var{flag}
  626: WHILE
  627:   @var{code2}
  628: REPEAT
  629: @end example
  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}.
  634: @example
  635: BEGIN
  636:   @var{code}
  637:   @var{flag}
  638: UNTIL
  639: @end example
  641: @var{code} is executed. The loop is restarted if @code{flag} is false.
  643: @example
  644: BEGIN
  645:   @var{code}
  646: AGAIN
  647: @end example
  649: This is an endless loop.
  651: @subsection Counted Loops
  653: The basic counted loop is:
  654: @example
  655: @var{limit} @var{start}
  656: ?DO
  657:   @var{body}
  658: LOOP
  659: @end example
  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}.
  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.
  684: There are several variations on the counted loop:
  686: @code{LEAVE} leaves the innermost counted loop immediately.
  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.:
  692: @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
  694: @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
  696: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
  698: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  700: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
  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:
  706: @code{-2 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  708: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0}
  710: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
  712: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
  713: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
  714: Forth standard.
  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.
  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.
  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.
  737: @subsection Arbitrary control structures
  739: ANS Forth permits and supports using control structures in a non-nested
  740: way. Information about incomplete control structures is stored on the
  741: control-flow stack. This stack may be implemented on the Forth data
  742: stack, and this is what we have done in gforth.
  744: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
  745: entry represents a backward branch target. A few words are the basis for
  746: building any control structure possible (except control structures that
  747: need storage, like calls, coroutines, and backtracking).
  749: doc-if
  750: doc-ahead
  751: doc-then
  752: doc-begin
  753: doc-until
  754: doc-again
  755: doc-cs-pick
  756: doc-cs-roll
  758: On many systems control-flow stack items take one word, in gforth they
  759: currently take three (this may change in the future). Therefore it is a
  760: really good idea to manipulate the control flow stack with
  761: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
  762: words.
  764: Some standard control structure words are built from these words:
  766: doc-else
  767: doc-while
  768: doc-repeat
  770: Counted loop words constitute a separate group of words:
  772: doc-?do
  773: doc-do
  774: doc-for
  775: doc-loop
  776: doc-s+loop
  777: doc-+loop
  778: doc-next
  779: doc-leave
  780: doc-?leave
  781: doc-unloop
  782: doc-undo
  784: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
  785: @i{do-sys}. Our system allows it, but it's your job to ensure that for
  786: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
  787: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
  788: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
  789: resolved (by using one of the loop-ending words or @code{UNDO}).
  791: Another group of control structure words are
  793: doc-case
  794: doc-endcase
  795: doc-of
  796: doc-endof
  798: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
  799: @code{cs-roll}.
  801: @subsubsection Programming Style
  803: In order to ensure readability we recommend that you do not create
  804: arbitrary control structures directly, but define new control structure
  805: words for the control structure you want and use these words in your
  806: program.
  808: E.g., instead of writing
  810: @example
  811: begin
  812:   ...
  813: if [ 1 cs-roll ]
  814:   ...
  815: again then
  816: @end example
  818: we recommend defining control structure words, e.g.,
  820: @example
  821: : while ( dest -- orig dest )
  822:  POSTPONE if
  823:  1 cs-roll ; immediate
  825: : repeat ( orig dest -- )
  826:  POSTPONE again
  827:  POSTPONE then ; immediate
  828: @end example
  830: and then using these to create the control structure:
  832: @example
  833: begin
  834:   ...
  835: while
  836:   ...
  837: repeat
  838: @end example
  840: That's much easier to read, isn't it? Of course, @code{BEGIN} and
  841: @code{WHILE} are predefined, so in this example it would not be
  842: necessary to define them.
  844: @subsection Calls and returns
  846: A definition can be called simply be writing the name of the
  847: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
  849: doc-exit
  851: Don't forget to clean up the return stack and @code{UNLOOP} any
  852: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
  853: primitive compiled by @code{EXIT} is
  855: doc-;s
  857: @subsection Exception Handling
  859: doc-catch
  860: doc-throw
  862: @node Locals
  863: @section Locals
  865: Local variables can make Forth programming more enjoyable and Forth
  866: programs easier to read. Unfortunately, the locals of ANS Forth are
  867: laden with restrictions. Therefore, we provide not only the ANS Forth
  868: locals wordset, but also our own, more powerful locals wordset (we
  869: implemented the ANS Forth locals wordset through our locals wordset).
  871: @menu
  872: @end menu
  874: @subsection gforth locals
  876: Locals can be defined with
  878: @example
  879: @{ local1 local2 ... -- comment @}
  880: @end example
  881: or
  882: @example
  883: @{ local1 local2 ... @}
  884: @end example
  886: E.g.,
  887: @example
  888: : max @{ n1 n2 -- n3 @}
  889:  n1 n2 > if
  890:    n1
  891:  else
  892:    n2
  893:  endif ;
  894: @end example
  896: The similarity of locals definitions with stack comments is intended. A
  897: locals definition often replaces the stack comment of a word. The order
  898: of the locals corresponds to the order in a stack comment and everything
  899: after the @code{--} is really a comment.
  901: This similarity has one disadvantage: It is too easy to confuse locals
  902: declarations with stack comments, causing bugs and making them hard to
  903: find. However, this problem can be avoided by appropriate coding
  904: conventions: Do not use both notations in the same program. If you do,
  905: they should be distinguished using additional means, e.g. by position.
  907: The name of the local may be preceded by a type specifier, e.g.,
  908: @code{F:} for a floating point value:
  910: @example
  911: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
  912: \ complex multiplication
  913:  Ar Br f* Ai Bi f* f-
  914:  Ar Bi f* Ai Br f* f+ ;
  915: @end example
  917: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
  918: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
  919: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
  920: with @code{W:}, @code{D:} etc.) produces its value and can be changed
  921: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
  922: produces its address (which becomes invalid when the variable's scope is
  923: left). E.g., the standard word @code{emit} can be defined in therms of
  924: @code{type} like this:
  926: @example
  927: : emit @{ C^ char* -- @}
  928:     char* 1 type ;
  929: @end example
  931: A local without type specifier is a @code{W:} local. Both flavours of
  932: locals are initialized with values from the data or FP stack.
  934: Currently there is no way to define locals with user-defined data
  935: structures, but we are working on it.
  937: GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:
  939: @subsubsection Where are locals visible by name?
  941: Basically, the answer is that locals are visible where you would expect
  942: it in block-structured languages, and sometimes a little longer. If you
  943: want to restrict the scope of a local, enclose its definition in
  944: @code{SCOPE}...@code{ENDSCOPE}.
  946: doc-scope
  947: doc-endscope
  949: These words behave like control structure words, so you can use them
  950: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
  951: arbitrary ways.
  953: If you want a more exact answer to the visibility question, here's the
  954: basic principle: A local is visible in all places that can only be
  955: reached through the definition of the local@footnote{In compiler
  956: construction terminology, all places dominated by the definition of the
  957: local.}. In other words, it is not visible in places that can be reached
  958: without going through the definition of the local. E.g., locals defined
  959: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
  960: defined in @code{BEGIN}...@code{UNTIL} are visible after the
  961: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
  963: The reasoning behind this solution is: We want to have the locals
  964: visible as long as it is meaningful. The user can always make the
  965: visibility shorter by using explicit scoping. In a place that can
  966: only be reached through the definition of a local, the meaning of a
  967: local name is clear. In other places it is not: How is the local
  968: initialized at the control flow path that does not contain the
  969: definition? Which local is meant, if the same name is defined twice in
  970: two independent control flow paths?
  972: This should be enough detail for nearly all users, so you can skip the
  973: rest of this section. If you relly must know all the gory details and
  974: options, read on.
  976: In order to implement this rule, the compiler has to know which places
  977: are unreachable. It knows this automatically after @code{AHEAD},
  978: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
  979: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
  980: compiler that the control flow never reaches that place. If
  981: @code{UNREACHABLE} is not used where it could, the only consequence is
  982: that the visibility of some locals is more limited than the rule above
  983: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
  984: lie to the compiler), buggy code will be produced.
  986: Another problem with this rule is that at @code{BEGIN}, the compiler
  987: does not know which locals will be visible on the incoming
  988: back-edge. All problems discussed in the following are due to this
  989: ignorance of the compiler (we discuss the problems using @code{BEGIN}
  990: loops as examples; the discussion also applies to @code{?DO} and other
  991: loops). Perhaps the most insidious example is:
  992: @example
  993: AHEAD
  994: BEGIN
  995:   x
  996: [ 1 CS-ROLL ] THEN
  997:   { x }
  998:   ...
  999: UNTIL
 1000: @end example
 1002: This should be legal according to the visibility rule. The use of
 1003: @code{x} can only be reached through the definition; but that appears
 1004: textually below the use.
 1006: From this example it is clear that the visibility rules cannot be fully
 1007: implemented without major headaches. Our implementation treats common
 1008: cases as advertised and the exceptions are treated in a safe way: The
 1009: compiler makes a reasonable guess about the locals visible after a
 1010: @code{BEGIN}; if it is too pessimistic, the
 1011: user will get a spurious error about the local not being defined; if the
 1012: compiler is too optimistic, it will notice this later and issue a
 1013: warning. In the case above the compiler would complain about @code{x}
 1014: being undefined at its use. You can see from the obscure examples in
 1015: this section that it takes quite unusual control structures to get the
 1016: compiler into trouble, and even then it will often do fine.
 1018: If the @code{BEGIN} is reachable from above, the most optimistic guess
 1019: is that all locals visible before the @code{BEGIN} will also be
 1020: visible after the @code{BEGIN}. This guess is valid for all loops that
 1021: are entered only through the @code{BEGIN}, in particular, for normal
 1022: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
 1023: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
 1024: compiler. When the branch to the @code{BEGIN} is finally generated by
 1025: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
 1026: warns the user if it was too optimisitic:
 1027: @example
 1028: IF
 1029:   { x }
 1030: BEGIN
 1031:   \ x ? 
 1032: [ 1 cs-roll ] THEN
 1033:   ...
 1034: UNTIL
 1035: @end example
 1037: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
 1038: optimistically assumes that it lives until the @code{THEN}. It notices
 1039: this difference when it compiles the @code{UNTIL} and issues a
 1040: warning. The user can avoid the warning, and make sure that @code{x}
 1041: is not used in the wrong area by using explicit scoping:
 1042: @example
 1043: IF
 1044:   SCOPE
 1045:   { x }
 1046:   ENDSCOPE
 1047: BEGIN
 1048: [ 1 cs-roll ] THEN
 1049:   ...
 1050: UNTIL
 1051: @end example
 1053: Since the guess is optimistic, there will be no spurious error messages
 1054: about undefined locals.
 1056: If the @code{BEGIN} is not reachable from above (e.g., after
 1057: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
 1058: optimistic guess, as the locals visible after the @code{BEGIN} may be
 1059: defined later. Therefore, the compiler assumes that no locals are
 1060: visible after the @code{BEGIN}. However, the useer can use
 1061: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
 1062: visible at the BEGIN as at the point where the item was created.
 1064: doc-assume-live
 1066: E.g.,
 1067: @example
 1068: { x }
 1069: AHEAD
 1071: BEGIN
 1072:   x
 1073: [ 1 CS-ROLL ] THEN
 1074:   ...
 1075: UNTIL
 1076: @end example
 1078: Other cases where the locals are defined before the @code{BEGIN} can be
 1079: handled by inserting an appropriate @code{CS-ROLL} before the
 1080: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
 1081: behind the @code{ASSUME-LIVE}).
 1083: Cases where locals are defined after the @code{BEGIN} (but should be
 1084: visible immediately after the @code{BEGIN}) can only be handled by
 1085: rearranging the loop. E.g., the ``most insidious'' example above can be
 1086: arranged into:
 1087: @example
 1088: BEGIN
 1089:   { x }
 1090:   ... 0=
 1091: WHILE
 1092:   x
 1093: REPEAT
 1094: @end example
 1096: @subsubsection How long do locals live?
 1098: The right answer for the lifetime question would be: A local lives at
 1099: least as long as it can be accessed. For a value-flavoured local this
 1100: means: until the end of its visibility. However, a variable-flavoured
 1101: local could be accessed through its address far beyond its visibility
 1102: scope. Ultimately, this would mean that such locals would have to be
 1103: garbage collected. Since this entails un-Forth-like implementation
 1104: complexities, I adopted the same cowardly solution as some other
 1105: languages (e.g., C): The local lives only as long as it is visible;
 1106: afterwards its address is invalid (and programs that access it
 1107: afterwards are erroneous).
 1109: @subsubsection Programming Style
 1111: The freedom to define locals anywhere has the potential to change
 1112: programming styles dramatically. In particular, the need to use the
 1113: return stack for intermediate storage vanishes. Moreover, all stack
 1114: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
 1115: determined arguments) can be eliminated: If the stack items are in the
 1116: wrong order, just write a locals definition for all of them; then
 1117: write the items in the order you want.
 1119: This seems a little far-fetched and eliminating stack manipulations is
 1120: unlikely to become a conscious programming objective. Still, the
 1121: number of stack manipulations will be reduced dramatically if local
 1122: variables are used liberally (e.g., compare @code{max} in \sect{misc}
 1123: with a traditional implementation of @code{max}).
 1125: This shows one potential benefit of locals: making Forth programs more
 1126: readable. Of course, this benefit will only be realized if the
 1127: programmers continue to honour the principle of factoring instead of
 1128: using the added latitude to make the words longer.
 1130: Using @code{TO} can and should be avoided.  Without @code{TO},
 1131: every value-flavoured local has only a single assignment and many
 1132: advantages of functional languages apply to Forth. I.e., programs are
 1133: easier to analyse, to optimize and to read: It is clear from the
 1134: definition what the local stands for, it does not turn into something
 1135: different later.
 1137: E.g., a definition using @code{TO} might look like this:
 1138: @example
 1139: : strcmp @{ addr1 u1 addr2 u2 -- n @}
 1140:  u1 u2 min 0
 1141:  ?do
 1142:    addr1 c@ addr2 c@ - ?dup
 1143:    if
 1144:      unloop exit
 1145:    then
 1146:    addr1 char+ TO addr1
 1147:    addr2 char+ TO addr2
 1148:  loop
 1149:  u1 u2 - ;
 1150: @end example
 1151: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
 1152: every loop iteration. @code{strcmp} is a typical example of the
 1153: readability problems of using @code{TO}. When you start reading
 1154: @code{strcmp}, you think that @code{addr1} refers to the start of the
 1155: string. Only near the end of the loop you realize that it is something
 1156: else.
 1158: This can be avoided by defining two locals at the start of the loop that
 1159: are initialized with the right value for the current iteration.
 1160: @example
 1161: : strcmp @{ addr1 u1 addr2 u2 -- n @}
 1162:  addr1 addr2
 1163:  u1 u2 min 0 
 1164:  ?do @{ s1 s2 @}
 1165:    s1 c@ s2 c@ - ?dup 
 1166:    if
 1167:      unloop exit
 1168:    then
 1169:    s1 char+ s2 char+
 1170:  loop
 1171:  2drop
 1172:  u1 u2 - ;
 1173: @end example
 1174: Here it is clear from the start that @code{s1} has a different value
 1175: in every loop iteration.
 1177: @subsubsection Implementation
 1179: GNU Forth uses an extra locals stack. The most compelling reason for
 1180: this is that the return stack is not float-aligned; using an extra stack
 1181: also eliminates the problems and restrictions of using the return stack
 1182: as locals stack. Like the other stacks, the locals stack grows toward
 1183: lower addresses. A few primitives allow an efficient implementation:
 1185: doc-@local#
 1186: doc-f@local#
 1187: doc-laddr#
 1188: doc-lp+!#
 1189: doc-lp!
 1190: doc->l
 1191: doc-f>l
 1193: In addition to these primitives, some specializations of these
 1194: primitives for commonly occurring inline arguments are provided for
 1195: efficiency reasons, e.g., @code{@@local0} as specialization of
 1196: @code{@@local#} for the inline argument 0. The following compiling words
 1197: compile the right specialized version, or the general version, as
 1198: appropriate:
 1200: doc-compile-@@local
 1201: doc-compile-f@@local
 1202: doc-compile-lp+!
 1204: Combinations of conditional branches and @code{lp+!#} like
 1205: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
 1206: is taken) are provided for efficiency and correctness in loops.
 1208: A special area in the dictionary space is reserved for keeping the
 1209: local variable names. @code{@{} switches the dictionary pointer to this
 1210: area and @code{@}} switches it back and generates the locals
 1211: initializing code. @code{W:} etc.@ are normal defining words. This
 1212: special area is cleared at the start of every colon definition.
 1214: A special feature of GNU Forths dictionary is used to implement the
 1215: definition of locals without type specifiers: every wordlist (aka
 1216: vocabulary) has its own methods for searching
 1217: etc. (@xref{dictionary}). For the present purpose we defined a wordlist
 1218: with a special search method: When it is searched for a word, it
 1219: actually creates that word using @code{W:}. @code{@{} changes the search
 1220: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
 1221: and then the wordlist for defining locals without type specifiers.
 1223: The lifetime rules support a stack discipline within a colon
 1224: definition: The lifetime of a local is either nested with other locals
 1225: lifetimes or it does not overlap them.
 1227: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
 1228: pointer manipulation is generated. Between control structure words
 1229: locals definitions can push locals onto the locals stack. @code{AGAIN}
 1230: is the simplest of the other three control flow words. It has to
 1231: restore the locals stack depth of the corresponding @code{BEGIN}
 1232: before branching. The code looks like this:
 1233: @format
 1234: @code{lp+!#} current-locals-size @minus{} dest-locals-size
 1235: @code{branch} <begin>
 1236: @end format
 1238: @code{UNTIL} is a little more complicated: If it branches back, it
 1239: must adjust the stack just like @code{AGAIN}. But if it falls through,
 1240: the locals stack must not be changed. The compiler generates the
 1241: following code:
 1242: @format
 1243: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
 1244: @end format
 1245: The locals stack pointer is only adjusted if the branch is taken.
 1247: @code{THEN} can produce somewhat inefficient code:
 1248: @format
 1249: @code{lp+!#} current-locals-size @minus{} orig-locals-size
 1250: <orig target>:
 1251: @code{lp+!#} orig-locals-size @minus{} new-locals-size
 1252: @end format
 1253: The second @code{lp+!#} adjusts the locals stack pointer from the
 1254: level at the {\em orig} point to the level after the @code{THEN}. The
 1255: first @code{lp+!#} adjusts the locals stack pointer from the current
 1256: level to the level at the orig point, so the complete effect is an
 1257: adjustment from the current level to the right level after the
 1258: @code{THEN}.
 1260: In a conventional Forth implementation a dest control-flow stack entry
 1261: is just the target address and an orig entry is just the address to be
 1262: patched. Our locals implementation adds a wordlist to every orig or dest
 1263: item. It is the list of locals visible (or assumed visible) at the point
 1264: described by the entry. Our implementation also adds a tag to identify
 1265: the kind of entry, in particular to differentiate between live and dead
 1266: (reachable and unreachable) orig entries.
 1268: A few unusual operations have to be performed on locals wordlists:
 1270: doc-common-list
 1271: doc-sub-list?
 1272: doc-list-size
 1274: Several features of our locals wordlist implementation make these
 1275: operations easy to implement: The locals wordlists are organised as
 1276: linked lists; the tails of these lists are shared, if the lists
 1277: contain some of the same locals; and the address of a name is greater
 1278: than the address of the names behind it in the list.
 1280: Another important implementation detail is the variable
 1281: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
 1282: determine if they can be reached directly or only through the branch
 1283: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
 1284: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
 1285: definition, by @code{BEGIN} and usually by @code{THEN}.
 1287: Counted loops are similar to other loops in most respects, but
 1288: @code{LEAVE} requires special attention: It performs basically the same
 1289: service as @code{AHEAD}, but it does not create a control-flow stack
 1290: entry. Therefore the information has to be stored elsewhere;
 1291: traditionally, the information was stored in the target fields of the
 1292: branches created by the @code{LEAVE}s, by organizing these fields into a
 1293: linked list. Unfortunately, this clever trick does not provide enough
 1294: space for storing our extended control flow information. Therefore, we
 1295: introduce another stack, the leave stack. It contains the control-flow
 1296: stack entries for all unresolved @code{LEAVE}s.
 1298: Local names are kept until the end of the colon definition, even if
 1299: they are no longer visible in any control-flow path. In a few cases
 1300: this may lead to increased space needs for the locals name area, but
 1301: usually less than reclaiming this space would cost in code size.
 1304: @subsection ANS Forth locals
 1306: The ANS Forth locals wordset does not define a syntax for locals, but
 1307: words that make it possible to define various syntaxes. One of the
 1308: possible syntaxes is a subset of the syntax we used in the gforth locals
 1309: wordset, i.e.:
 1311: @example
 1312: @{ local1 local2 ... -- comment @}
 1313: @end example
 1314: or
 1315: @example
 1316: @{ local1 local2 ... @}
 1317: @end example
 1319: The order of the locals corresponds to the order in a stack comment. The
 1320: restrictions are:
 1322: @itemize @bullet
 1323: @item
 1324: Locals can only be cell-sized values (no type specifers are allowed).
 1325: @item
 1326: Locals can be defined only outside control structures.
 1327: @item
 1328: Locals can interfere with explicit usage of the return stack. For the
 1329: exact (and long) rules, see the standard. If you don't use return stack
 1330: accessing words in a definition using locals, you will we all right. The
 1331: purpose of this rule is to make locals implementation on the return
 1332: stack easier.
 1333: @item
 1334: The whole definition must be in one line.
 1335: @end itemize
 1337: Locals defined in this way behave like @code{VALUE}s
 1338: (@xref{values}). I.e., they are initialized from the stack. Using their
 1339: name produces their value. Their value can be changed using @code{TO}.
 1341: Since this syntax is supported by gforth directly, you need not do
 1342: anything to use it. If you want to port a program using this syntax to
 1343: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
 1344: on the other system.
 1346: Note that a syntax shown in the standard, section A.13 looks
 1347: similar, but is quite different in having the order of locals
 1348: reversed. Beware!
 1350: The ANS Forth locals wordset itself consists of the following word
 1352: doc-(local)
 1354: The ANS Forth locals extension wordset defines a syntax, but it is so
 1355: awful that we strongly recommend not to use it. We have implemented this
 1356: syntax to make porting to gforth easy, but do not document it here. The
 1357: problem with this syntax is that the locals are defined in an order
 1358: reversed with respect to the standard stack comment notation, making
 1359: programs harder to read, and easier to misread and miswrite. The only
 1360: merit of this syntax is that it is easy to implement using the ANS Forth
 1361: locals wordset.
 1363: @node Internals
 1364: @chapter Internals
 1366: Reading this section is not necessary for programming with gforth. It
 1367: should be helpful for finding your way in the gforth sources.
 1369: @section Portability
 1371: One of the main goals of the effort is availability across a wide range
 1372: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
 1373: this goal by manually coding the engine in assembly language for several
 1374: then-popular processors. This approach is very labor-intensive and the
 1375: results are short-lived due to progress in computer architecture.
 1377: Others have avoided this problem by coding in C, e.g., Mitch Bradley
 1378: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
 1379: particularly popular for UNIX-based Forths due to the large variety of
 1380: architectures of UNIX machines. Unfortunately an implementation in C
 1381: does not mix well with the goals of efficiency and with using
 1382: traditional techniques: Indirect or direct threading cannot be expressed
 1383: in C, and switch threading, the fastest technique available in C, is
 1384: significantly slower. Another problem with C is that it's very
 1385: cumbersome to express double integer arithmetic.
 1387: Fortunately, there is a portable language that does not have these
 1388: limitations: GNU C, the version of C processed by the GNU C compiler
 1389: (@pxref{C Extensions, , Extensions to the C Language Family,,
 1390: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
 1391: Labels as Values,, GNU C Manual}) makes direct and indirect
 1392: threading possible, its @code{long long} type (@pxref{Long Long, ,
 1393: Double-Word Integers,, GNU C Manual}) corresponds to Forths
 1394: double numbers. GNU C is available for free on all important (and many
 1395: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
 1396: the Atari ST, so a Forth written in GNU C can run on all these
 1397: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
 1398: available on the Mac (@pxref{Boycott, , Protect Your Freedom--Fight
 1399: ``Look And Feel'',, GNU C Manual}).}.
 1401: Writing in a portable language has the reputation of producing code that
 1402: is slower than assembly. For our Forth engine we repeatedly looked at
 1403: the code produced by the compiler and eliminated most compiler-induced
 1404: inefficiencies by appropriate changes in the source-code.
 1406: However, register allocation cannot be portably influenced by the
 1407: programmer, leading to some inefficiencies on register-starved
 1408: machines. We use explicit register declarations (@pxref{Explicit Reg
 1409: Vars, , Variables in Specified Registers,, GNU C Manual}) to
 1410: improve the speed on some machines. They are turned on by using the
 1411: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
 1412: only depends on the machine, but also on the compiler version: On some
 1413: machines some compiler versions produce incorrect code when certain
 1414: explicit register declarations are used. So by default
 1415: @code{-DFORCE_REG} is not used.
 1417: @section Threading
 1419: GNU C's labels as values extension (available since @code{gcc-2.0},
 1420: @pxref{Labels as Values, , Labels as Values,, GNU C Manual})
 1421: makes it possible to take the address of @var{label} by writing
 1422: @code{&&@var{label}}.  This address can then be used in a statement like
 1423: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
 1424: @code{goto x}.
 1426: With this feature an indirect threaded NEXT looks like:
 1427: @example
 1428: cfa = *ip++;
 1429: ca = *cfa;
 1430: goto *ca;
 1431: @end example
 1432: For those unfamiliar with the names: @code{ip} is the Forth instruction
 1433: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
 1434: execution token and points to the code field of the next word to be
 1435: executed; The @code{ca} (code address) fetched from there points to some
 1436: executable code, e.g., a primitive or the colon definition handler
 1437: @code{docol}.
 1439: Direct threading is even simpler:
 1440: @example
 1441: ca = *ip++;
 1442: goto *ca;
 1443: @end example
 1445: Of course we have packaged the whole thing neatly in macros called
 1446: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
 1448: @subsection Scheduling
 1450: There is a little complication: Pipelined and superscalar processors,
 1451: i.e., RISC and some modern CISC machines can process independent
 1452: instructions while waiting for the results of an instruction. The
 1453: compiler usually reorders (schedules) the instructions in a way that
 1454: achieves good usage of these delay slots. However, on our first tries
 1455: the compiler did not do well on scheduling primitives. E.g., for
 1456: @code{+} implemented as
 1457: @example
 1458: n=sp[0]+sp[1];
 1459: sp++;
 1460: sp[0]=n;
 1461: NEXT;
 1462: @end example
 1463: the NEXT comes strictly after the other code, i.e., there is nearly no
 1464: scheduling. After a little thought the problem becomes clear: The
 1465: compiler cannot know that sp and ip point to different addresses (and
 1466: the version of @code{gcc} we used would not know it even if it could),
 1467: so it could not move the load of the cfa above the store to the
 1468: TOS. Indeed the pointers could be the same, if code on or very near the
 1469: top of stack were executed. In the interest of speed we chose to forbid
 1470: this probably unused ``feature'' and helped the compiler in scheduling:
 1471: NEXT is divided into the loading part (@code{NEXT_P1}) and the goto part
 1472: (@code{NEXT_P2}). @code{+} now looks like:
 1473: @example
 1474: n=sp[0]+sp[1];
 1475: sp++;
 1476: NEXT_P1;
 1477: sp[0]=n;
 1478: NEXT_P2;
 1479: @end example
 1480: This can be scheduled optimally by the compiler (see \sect{TOS}).
 1482: This division can be turned off with the switch @code{-DCISC_NEXT}. This
 1483: switch is on by default on machines that do not profit from scheduling
 1484: (e.g., the 80386), in order to preserve registers.
 1486: @subsection Direct or Indirect Threaded?
 1488: Both! After packaging the nasty details in macro definitions we
 1489: realized that we could switch between direct and indirect threading by
 1490: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
 1491: defining a few machine-specific macros for the direct-threading case.
 1492: On the Forth level we also offer access words that hide the
 1493: differences between the threading methods (@pxref{Threading Words}).
 1495: Indirect threading is implemented completely
 1496: machine-independently. Direct threading needs routines for creating
 1497: jumps to the executable code (e.g. to docol or dodoes). These routines
 1498: are inherently machine-dependent, but they do not amount to many source
 1499: lines. I.e., even porting direct threading to a new machine is a small
 1500: effort.
 1502: @subsection DOES>
 1503: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
 1504: the chunk of code executed by every word defined by a
 1505: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
 1506: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
 1507: DOES-code)? There are two solutions:
 1509: In fig-Forth the code field points directly to the dodoes and the
 1510: DOES-code address is stored in the cell after the code address
 1511: (i.e. at cfa cell+). It may seem that this solution is illegal in the
 1512: Forth-79 and all later standards, because in fig-Forth this address
 1513: lies in the body (which is illegal in these standards). However, by
 1514: making the code field larger for all words this solution becomes legal
 1515: again. We use this approach for the indirect threaded version. Leaving
 1516: a cell unused in most words is a bit wasteful, but on the machines we
 1517: are targetting this is hardly a problem. The other reason for having a
 1518: code field size of two cells is to avoid having different image files
 1519: for direct and indirect threaded systems (@pxref{image-format}).
 1521: The other approach is that the code field points or jumps to the cell
 1522: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
 1523: this address. @code{dodoes} can then get the DOES-code address by
 1524: computing the code address, i.e., the address of the jump to dodoes,
 1525: and add the length of that jump field. A variant of this is to have a
 1526: call to @code{dodoes} after the @code{DOES>}; then the return address
 1527: (which can be found in the return register on RISCs) is the DOES-code
 1528: address. Since the two cells available in the code field are usually
 1529: used up by the jump to the code address in direct threading, we use
 1530: this approach for direct threading. We did not want to add another
 1531: cell to the code field.
 1533: @section Primitives
 1535: @subsection Automatic Generation
 1537: Since the primitives are implemented in a portable language, there is no
 1538: longer any need to minimize the number of primitives. On the contrary,
 1539: having many primitives is an advantage: speed. In order to reduce the
 1540: number of errors in primitives and to make programming them easier, we
 1541: provide a tool, the primitive generator (@file{prims2x.fs}), that
 1542: automatically generates most (and sometimes all) of the C code for a
 1543: primitive from the stack effect notation.  The source for a primitive
 1544: has the following form:
 1546: @format
 1547: @var{Forth-name}	@var{stack-effect}	@var{category}	[@var{pronounc.}]
 1548: [@code{""}@var{glossary entry}@code{""}]
 1549: @var{C code}
 1550: [@code{:}
 1551: @var{Forth code}]
 1552: @end format
 1554: The items in brackets are optional. The category and glossary fields
 1555: are there for generating the documentation, the Forth code is there
 1556: for manual implementations on machines without GNU C. E.g., the source
 1557: for the primitive @code{+} is:
 1558: @example
 1559: +    n1 n2 -- n    core    plus
 1560: n = n1+n2;
 1561: @end example
 1563: This looks like a specification, but in fact @code{n = n1+n2} is C
 1564: code. Our primitive generation tool extracts a lot of information from
 1565: the stack effect notations@footnote{We use a one-stack notation, even
 1566: though we have separate data and floating-point stacks; The separate
 1567: notation can be generated easily from the unified notation.}: The number
 1568: of items popped from and pushed on the stack, their type, and by what
 1569: name they are referred to in the C code. It then generates a C code
 1570: prelude and postlude for each primitive. The final C code for @code{+}
 1571: looks like this:
 1573: @example
 1574: I_plus:	/* + ( n1 n2 -- n ) */  /* label, stack effect */
 1575: /*  */                          /* documentation */
 1576: {
 1577: DEF_CA                          /* definition of variable ca (indirect threading) */
 1578: Cell n1;                        /* definitions of variables */
 1579: Cell n2;
 1580: Cell n;
 1581: n1 = (Cell) sp[1];              /* input */
 1582: n2 = (Cell) TOS;
 1583: sp += 1;                        /* stack adjustment */
 1584: NAME("+")                       /* debugging output (with -DDEBUG) */
 1585: {
 1586: n = n1+n2;                      /* C code taken from the source */
 1587: }
 1588: NEXT_P1;                        /* NEXT part 1 */
 1589: TOS = (Cell)n;                  /* output */
 1590: NEXT_P2;                        /* NEXT part 2 */
 1591: }
 1592: @end example
 1594: This looks long and inefficient, but the GNU C compiler optimizes quite
 1595: well and produces optimal code for @code{+} on, e.g., the R3000 and the
 1596: HP RISC machines: Defining the @code{n}s does not produce any code, and
 1597: using them as intermediate storage also adds no cost.
 1599: There are also other optimizations, that are not illustrated by this
 1600: example: Assignments between simple variables are usually for free (copy
 1601: propagation). If one of the stack items is not used by the primitive
 1602: (e.g.  in @code{drop}), the compiler eliminates the load from the stack
 1603: (dead code elimination). On the other hand, there are some things that
 1604: the compiler does not do, therefore they are performed by
 1605: @file{prims2x.fs}: The compiler does not optimize code away that stores
 1606: a stack item to the place where it just came from (e.g., @code{over}).
 1608: While programming a primitive is usually easy, there are a few cases
 1609: where the programmer has to take the actions of the generator into
 1610: account, most notably @code{?dup}, but also words that do not (always)
 1611: fall through to NEXT.
 1613: @subsection TOS Optimization
 1615: An important optimization for stack machine emulators, e.g., Forth
 1616: engines, is keeping  one or more of the top stack items in
 1617: registers.  If a word has the stack effect {@var{in1}...@var{inx} @code{--}
 1618: @var{out1}...@var{outy}}, keeping the top @var{n} items in registers
 1619: @itemize
 1620: @item
 1621: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
 1622: due to fewer loads from and stores to the stack.
 1623: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
 1624: @var{y<n}, due to additional moves between registers.
 1625: @end itemize
 1627: In particular, keeping one item in a register is never a disadvantage,
 1628: if there are enough registers. Keeping two items in registers is a
 1629: disadvantage for frequent words like @code{?branch}, constants,
 1630: variables, literals and @code{i}. Therefore our generator only produces
 1631: code that keeps zero or one items in registers. The generated C code
 1632: covers both cases; the selection between these alternatives is made at
 1633: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
 1634: code for @code{+} is just a simple variable name in the one-item case,
 1635: otherwise it is a macro that expands into @code{sp[0]}. Note that the
 1636: GNU C compiler tries to keep simple variables like @code{TOS} in
 1637: registers, and it usually succeeds, if there are enough registers.
 1639: The primitive generator performs the TOS optimization for the
 1640: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
 1641: operations the benefit of this optimization is even larger:
 1642: floating-point operations take quite long on most processors, but can be
 1643: performed in parallel with other operations as long as their results are
 1644: not used. If the FP-TOS is kept in a register, this works. If
 1645: it is kept on the stack, i.e., in memory, the store into memory has to
 1646: wait for the result of the floating-point operation, lengthening the
 1647: execution time of the primitive considerably.
 1649: The TOS optimization makes the automatic generation of primitives a
 1650: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
 1651: @code{TOS} is not sufficient. There are some special cases to
 1652: consider:
 1653: @itemize
 1654: @item In the case of @code{dup ( w -- w w )} the generator must not
 1655: eliminate the store to the original location of the item on the stack,
 1656: if the TOS optimization is turned on.
 1657: @item Primitives with stack effects of the form {@code{--}
 1658: @var{out1}...@var{outy}} must store the TOS to the stack at the start.
 1659: Likewise, primitives with the stack effect {@var{in1}...@var{inx} @code{--}}
 1660: must load the TOS from the stack at the end. But for the null stack
 1661: effect @code{--} no stores or loads should be generated.
 1662: @end itemize
 1664: @subsection Produced code
 1666: To see what assembly code is produced for the primitives on your machine
 1667: with your compiler and your flag settings, type @code{make engine.s} and
 1668: look at the resulting file @file{engine.c}.
 1670: @section System Architecture
 1672: Our Forth system consists not only of primitives, but also of
 1673: definitions written in Forth. Since the Forth compiler itself belongs
 1674: to those definitions, it is not possible to start the system with the
 1675: primitives and the Forth source alone. Therefore we provide the Forth
 1676: code as an image file in nearly executable form. At the start of the
 1677: system a C routine loads the image file into memory, sets up the
 1678: memory (stacks etc.) according to information in the image file, and
 1679: starts executing Forth code.
 1681: The image file format is a compromise between the goals of making it
 1682: easy to generate image files and making them portable. The easiest way
 1683: to generate an image file is to just generate a memory dump. However,
 1684: this kind of image file cannot be used on a different machine, or on
 1685: the next version of the engine on the same machine, it even might not
 1686: work with the same engine compiled by a different version of the C
 1687: compiler. We would like to have as few versions of the image file as
 1688: possible, because we do not want to distribute many versions of the
 1689: same image file, and to make it easy for the users to use their image
 1690: files on many machines. We currently need to create a different image
 1691: file for machines with different cell sizes and different byte order
 1692: (little- or big-endian)@footnote{We consider adding information to the
 1693: image file that enables the loader to change the byte order.}.
 1695: Forth code that is going to end up in a portable image file has to
 1696: comply to some restrictions: addresses have to be stored in memory
 1697: with special words (@code{A!}, @code{A,}, etc.) in order to make the
 1698: code relocatable. Cells, floats, etc., have to be stored at the
 1699: natural alignment boundaries@footnote{E.g., store floats (8 bytes) at
 1700: an address dividable by~8. This happens automatically in our system
 1701: when you use the ANSI alignment words.}, in order to avoid alignment
 1702: faults on machines with stricter alignment. The image file is produced
 1703: by a metacompiler (@file{cross.fs}).
 1705: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
 1706: file is not directly executable, but has to undergo some manipulations
 1707: during loading. Address relocation is performed at image load-time, not
 1708: at run-time. The loader also has to replace tokens standing for
 1709: primitive calls with the appropriate code-field addresses (or code
 1710: addresses in the case of direct threading).
 1712: @contents
 1713: @bye

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