Annotation of gforth/gforth.ds, revision 1.5

1.1       anton       1: \input texinfo   @c -*-texinfo-*-
                      2: @comment The source is gforth.ds, from which gforth.texi is generated
                      3: @comment %**start of header (This is for running Texinfo on a region.)
1.4       anton       4: @setfilename gforth.info
1.1       anton       5: @settitle GNU Forth Manual
1.4       anton       6: @comment @setchapternewpage odd
1.1       anton       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:      
1.4       anton      18: @ignore
1.1       anton      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:      
1.4       anton      24: @end ignore
1.1       anton      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
1.4       anton      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
1.1       anton      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: 
1.4       anton     256: @node Words, ANS conformance, Invocation, Top
1.1       anton     257: @chapter Forth Words
                    258: 
                    259: @menu
1.4       anton     260: * Notation::                    
                    261: * Arithmetic::                  
                    262: * Stack Manipulation::          
                    263: * Memory access::               
                    264: * Control Structures::          
                    265: * Locals::                      
                    266: * Defining Words::              
                    267: * Wordlists::                   
                    268: * Files::                       
                    269: * Blocks::                      
                    270: * Other I/O::                   
                    271: * Programming Tools::           
                    272: * Threading Words::             
1.1       anton     273: @end menu
                    274: 
                    275: @node Notation, Arithmetic, Words, Words
                    276: @section Notation
                    277: 
                    278: The Forth words are described in this section in the glossary notation
                    279: that has become a de-facto standard for Forth texts, i.e.
                    280: 
1.4       anton     281: @format
1.1       anton     282: @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
1.4       anton     283: @end format
1.1       anton     284: @var{Description}
                    285: 
                    286: @table @var
                    287: @item word
                    288: The name of the word. BTW, GNU Forth is case insensitive, so you can
                    289: type the words in in lower case.
                    290: 
                    291: @item Stack effect
                    292: The stack effect is written in the notation @code{@var{before} --
                    293: @var{after}}, where @var{before} and @var{after} describe the top of
                    294: stack entries before and after the execution of the word. The rest of
                    295: the stack is not touched by the word. The top of stack is rightmost,
                    296: i.e., a stack sequence is written as it is typed in. Note that GNU Forth
                    297: uses a separate floating point stack, but a unified stack
                    298: notation. Also, return stack effects are not shown in @var{stack
                    299: effect}, but in @var{Description}. The name of a stack item describes
                    300: the type and/or the function of the item. See below for a discussion of
                    301: the types.
                    302: 
                    303: @item pronunciation
                    304: How the word is pronounced
                    305: 
                    306: @item wordset
                    307: The ANS Forth standard is divided into several wordsets. A standard
                    308: system need not support all of them. So, the fewer wordsets your program
                    309: uses the more portable it will be in theory. However, we suspect that
                    310: most ANS Forth systems on personal machines will feature all
                    311: wordsets. Words that are not defined in the ANS standard have
                    312: @code{gforth} as wordset.
                    313: 
                    314: @item Description
                    315: A description of the behaviour of the word.
                    316: @end table
                    317: 
1.4       anton     318: The type of a stack item is specified by the character(s) the name
                    319: starts with:
1.1       anton     320: 
                    321: @table @code
                    322: @item f
                    323: Bool, i.e. @code{false} or @code{true}.
                    324: @item c
                    325: Char
                    326: @item w
                    327: Cell, can contain an integer or an address
                    328: @item n
                    329: signed integer
                    330: @item u
                    331: unsigned integer
                    332: @item d
                    333: double sized signed integer
                    334: @item ud
                    335: double sized unsigned integer
                    336: @item r
                    337: Float
                    338: @item a_
                    339: Cell-aligned address
                    340: @item c_
                    341: Char-aligned address (note that a Char is two bytes in Windows NT)
                    342: @item f_
                    343: Float-aligned address
                    344: @item df_
                    345: Address aligned for IEEE double precision float
                    346: @item sf_
                    347: Address aligned for IEEE single precision float
                    348: @item xt
                    349: Execution token, same size as Cell
                    350: @item wid
                    351: Wordlist ID, same size as Cell
                    352: @item f83name
                    353: Pointer to a name structure
                    354: @end table
                    355: 
1.4       anton     356: @node Arithmetic, Stack Manipulation, Notation, Words
1.1       anton     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
1.4       anton     366: former, @pxref{Mixed precision}).
                    367: 
                    368: @menu
                    369: * Single precision::            
                    370: * Bitwise operations::          
                    371: * Mixed precision::             operations with single and double-cell integers
                    372: * Double precision::            Double-cell integer arithmetic
                    373: * Floating Point::              
                    374: @end menu
1.1       anton     375: 
1.4       anton     376: @node Single precision, Bitwise operations, Arithmetic, Arithmetic
1.1       anton     377: @subsection Single precision
                    378: doc-+
                    379: doc--
                    380: doc-*
                    381: doc-/
                    382: doc-mod
                    383: doc-/mod
                    384: doc-negate
                    385: doc-abs
                    386: doc-min
                    387: doc-max
                    388: 
1.4       anton     389: @node Bitwise operations, Mixed precision, Single precision, Arithmetic
1.1       anton     390: @subsection Bitwise operations
                    391: doc-and
                    392: doc-or
                    393: doc-xor
                    394: doc-invert
                    395: doc-2*
                    396: doc-2/
                    397: 
1.4       anton     398: @node Mixed precision, Double precision, Bitwise operations, Arithmetic
1.1       anton     399: @subsection Mixed precision
                    400: doc-m+
                    401: doc-*/
                    402: doc-*/mod
                    403: doc-m*
                    404: doc-um*
                    405: doc-m*/
                    406: doc-um/mod
                    407: doc-fm/mod
                    408: doc-sm/rem
                    409: 
1.4       anton     410: @node Double precision, Floating Point, Mixed precision, Arithmetic
1.1       anton     411: @subsection Double precision
                    412: doc-d+
                    413: doc-d-
                    414: doc-dnegate
                    415: doc-dabs
                    416: doc-dmin
                    417: doc-dmax
                    418: 
1.4       anton     419: @node Floating Point,  , Double precision, Arithmetic
                    420: @subsection Floating Point
                    421: 
                    422: Angles in floating point operations are given in radians (a full circle
                    423: has 2 pi radians). Note, that gforth has a separate floating point
                    424: stack, but we use the unified notation.
                    425: 
                    426: Floating point numbers have a number of unpleasant surprises for the
                    427: unwary (e.g., floating point addition is not associative) and even a few
                    428: for the wary. You should not use them unless you know what you are doing
                    429: or you don't care that the results you get are totally bogus. If you
                    430: want to learn about the problems of floating point numbers (and how to
                    431: avoid them), you might start with @cite{Goldberg, What every computer
                    432: scientist should know about floating-point numbers, Computing Surveys
                    433: ?}.
                    434: 
                    435: doc-f+
                    436: doc-f-
                    437: doc-f*
                    438: doc-f/
                    439: doc-fnegate
                    440: doc-fabs
                    441: doc-fmax
                    442: doc-fmin
                    443: doc-floor
                    444: doc-fround
                    445: doc-f**
                    446: doc-fsqrt
                    447: doc-fexp
                    448: doc-fexpm1
                    449: doc-fln
                    450: doc-flnp1
                    451: doc-flog
                    452: doc-fsin
                    453: doc-fcos
                    454: doc-fsincos
                    455: doc-ftan
                    456: doc-fasin
                    457: doc-facos
                    458: doc-fatan
                    459: doc-fatan2
                    460: doc-fsinh
                    461: doc-fcosh
                    462: doc-ftanh
                    463: doc-fasinh
                    464: doc-facosh
                    465: doc-fatanh
                    466: 
                    467: @node Stack Manipulation, Memory access, Arithmetic, Words
1.1       anton     468: @section Stack Manipulation
                    469: 
                    470: gforth has a data stack (aka parameter stack) for characters, cells,
                    471: addresses, and double cells, a floating point stack for floating point
                    472: numbers, a return stack for storing the return addresses of colon
                    473: definitions and other data, and a locals stack for storing local
                    474: variables. Note that while every sane Forth has a separate floating
                    475: point stack, this is not strictly required; an ANS Forth system could
                    476: theoretically keep floating point numbers on the data stack. As an
                    477: additional difficulty, you don't know how many cells a floating point
                    478: number takes. It is reportedly possible to write words in a way that
                    479: they work also for a unified stack model, but we do not recommend trying
1.4       anton     480: it. Instead, just say that your program has an environmental dependency
                    481: on a separate FP stack.
                    482: 
                    483: Also, a Forth system is allowed to keep the local variables on the
1.1       anton     484: return stack. This is reasonable, as local variables usually eliminate
                    485: the need to use the return stack explicitly. So, if you want to produce
                    486: a standard complying program and if you are using local variables in a
                    487: word, forget about return stack manipulations in that word (see the
                    488: standard document for the exact rules).
                    489: 
1.4       anton     490: @menu
                    491: * Data stack::                  
                    492: * Floating point stack::        
                    493: * Return stack::                
                    494: * Locals stack::                
                    495: * Stack pointer manipulation::  
                    496: @end menu
                    497: 
                    498: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
1.1       anton     499: @subsection Data stack
                    500: doc-drop
                    501: doc-nip
                    502: doc-dup
                    503: doc-over
                    504: doc-tuck
                    505: doc-swap
                    506: doc-rot
                    507: doc--rot
                    508: doc-?dup
                    509: doc-pick
                    510: doc-roll
                    511: doc-2drop
                    512: doc-2nip
                    513: doc-2dup
                    514: doc-2over
                    515: doc-2tuck
                    516: doc-2swap
                    517: doc-2rot
                    518: 
1.4       anton     519: @node Floating point stack, Return stack, Data stack, Stack Manipulation
1.1       anton     520: @subsection Floating point stack
                    521: doc-fdrop
                    522: doc-fnip
                    523: doc-fdup
                    524: doc-fover
                    525: doc-ftuck
                    526: doc-fswap
                    527: doc-frot
                    528: 
1.4       anton     529: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
1.1       anton     530: @subsection Return stack
                    531: doc->r
                    532: doc-r>
                    533: doc-r@
                    534: doc-rdrop
                    535: doc-2>r
                    536: doc-2r>
                    537: doc-2r@
                    538: doc-2rdrop
                    539: 
1.4       anton     540: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
1.1       anton     541: @subsection Locals stack
                    542: 
1.4       anton     543: @node Stack pointer manipulation,  , Locals stack, Stack Manipulation
1.1       anton     544: @subsection Stack pointer manipulation
                    545: doc-sp@
                    546: doc-sp!
                    547: doc-fp@
                    548: doc-fp!
                    549: doc-rp@
                    550: doc-rp!
                    551: doc-lp@
                    552: doc-lp!
                    553: 
1.4       anton     554: @node Memory access, Control Structures, Stack Manipulation, Words
1.1       anton     555: @section Memory access
                    556: 
1.4       anton     557: @menu
                    558: * Stack-Memory transfers::      
                    559: * Address arithmetic::          
                    560: * Memory block access::         
                    561: @end menu
                    562: 
                    563: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
1.1       anton     564: @subsection Stack-Memory transfers
                    565: 
                    566: doc-@
                    567: doc-!
                    568: doc-+!
                    569: doc-c@
                    570: doc-c!
                    571: doc-2@
                    572: doc-2!
                    573: doc-f@
                    574: doc-f!
                    575: doc-sf@
                    576: doc-sf!
                    577: doc-df@
                    578: doc-df!
                    579: 
1.4       anton     580: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
1.1       anton     581: @subsection Address arithmetic
                    582: 
                    583: ANS Forth does not specify the sizes of the data types. Instead, it
                    584: offers a number of words for computing sizes and doing address
                    585: arithmetic. Basically, address arithmetic is performed in terms of
                    586: address units (aus); on most systems the address unit is one byte. Note
                    587: that a character may have more than one au, so @code{chars} is no noop
                    588: (on systems where it is a noop, it compiles to nothing).
                    589: 
                    590: ANS Forth also defines words for aligning addresses for specific
                    591: addresses. Many computers require that accesses to specific data types
                    592: must only occur at specific addresses; e.g., that cells may only be
                    593: accessed at addresses divisible by 4. Even if a machine allows unaligned
                    594: accesses, it can usually perform aligned accesses faster. 
                    595: 
                    596: For the performance-concious: alignment operations are usually only
                    597: necessary during the definition of a data structure, not during the
                    598: (more frequent) accesses to it.
                    599: 
                    600: ANS Forth defines no words for character-aligning addresses. This is not
                    601: an oversight, but reflects the fact that addresses that are not
                    602: char-aligned have no use in the standard and therefore will not be
                    603: created.
                    604: 
                    605: The standard guarantees that addresses returned by @code{CREATE}d words
                    606: are cell-aligned; in addition, gforth guarantees that these addresses
                    607: are aligned for all purposes.
                    608: 
                    609: doc-chars
                    610: doc-char+
                    611: doc-cells
                    612: doc-cell+
                    613: doc-align
                    614: doc-aligned
                    615: doc-floats
                    616: doc-float+
                    617: doc-falign
                    618: doc-faligned
                    619: doc-sfloats
                    620: doc-sfloat+
                    621: doc-sfalign
                    622: doc-sfaligned
                    623: doc-dfloats
                    624: doc-dfloat+
                    625: doc-dfalign
                    626: doc-dfaligned
                    627: doc-address-unit-bits
                    628: 
1.4       anton     629: @node Memory block access,  , Address arithmetic, Memory access
1.1       anton     630: @subsection Memory block access
                    631: 
                    632: doc-move
                    633: doc-erase
                    634: 
                    635: While the previous words work on address units, the rest works on
                    636: characters.
                    637: 
                    638: doc-cmove
                    639: doc-cmove>
                    640: doc-fill
                    641: doc-blank
                    642: 
1.4       anton     643: @node Control Structures, Locals, Memory access, Words
1.1       anton     644: @section Control Structures
                    645: 
                    646: Control structures in Forth cannot be used in interpret state, only in
                    647: compile state, i.e., in a colon definition. We do not like this
                    648: limitation, but have not seen a satisfying way around it yet, although
                    649: many schemes have been proposed.
                    650: 
1.4       anton     651: @menu
                    652: * Selection::                   
                    653: * Simple Loops::                
                    654: * Counted Loops::               
                    655: * Arbitrary control structures::  
                    656: * Calls and returns::           
                    657: * Exception Handling::          
                    658: @end menu
                    659: 
                    660: @node Selection, Simple Loops, Control Structures, Control Structures
1.1       anton     661: @subsection Selection
                    662: 
                    663: @example
                    664: @var{flag}
                    665: IF
                    666:   @var{code}
                    667: ENDIF
                    668: @end example
                    669: or
                    670: @example
                    671: @var{flag}
                    672: IF
                    673:   @var{code1}
                    674: ELSE
                    675:   @var{code2}
                    676: ENDIF
                    677: @end example
                    678: 
1.4       anton     679: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
1.1       anton     680: standard, and @code{ENDIF} is not, although it is quite popular. We
                    681: recommend using @code{ENDIF}, because it is less confusing for people
                    682: who also know other languages (and is not prone to reinforcing negative
                    683: prejudices against Forth in these people). Adding @code{ENDIF} to a
                    684: system that only supplies @code{THEN} is simple:
                    685: @example
                    686: : endif   POSTPONE then ; immediate
                    687: @end example
                    688: 
                    689: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
                    690: (adv.)}  has the following meanings:
                    691: @quotation
                    692: ... 2b: following next after in order ... 3d: as a necessary consequence
                    693: (if you were there, then you saw them).
                    694: @end quotation
                    695: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
                    696: and many other programming languages has the meaning 3d.]
                    697: 
                    698: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
                    699: can avoid using @code{?dup}.
                    700: 
                    701: @example
                    702: @var{n}
                    703: CASE
                    704:   @var{n1} OF @var{code1} ENDOF
                    705:   @var{n2} OF @var{code2} ENDOF
1.4       anton     706:   @dots{}
1.1       anton     707: ENDCASE
                    708: @end example
                    709: 
                    710: Executes the first @var{codei}, where the @var{ni} is equal to
                    711: @var{n}. A default case can be added by simply writing the code after
                    712: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
                    713: but must not consume it.
                    714: 
1.4       anton     715: @node Simple Loops, Counted Loops, Selection, Control Structures
1.1       anton     716: @subsection Simple Loops
                    717: 
                    718: @example
                    719: BEGIN
                    720:   @var{code1}
                    721:   @var{flag}
                    722: WHILE
                    723:   @var{code2}
                    724: REPEAT
                    725: @end example
                    726: 
                    727: @var{code1} is executed and @var{flag} is computed. If it is true,
                    728: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
                    729: 
                    730: @example
                    731: BEGIN
                    732:   @var{code}
                    733:   @var{flag}
                    734: UNTIL
                    735: @end example
                    736: 
                    737: @var{code} is executed. The loop is restarted if @code{flag} is false.
                    738: 
                    739: @example
                    740: BEGIN
                    741:   @var{code}
                    742: AGAIN
                    743: @end example
                    744: 
                    745: This is an endless loop.
                    746: 
1.4       anton     747: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
1.1       anton     748: @subsection Counted Loops
                    749: 
                    750: The basic counted loop is:
                    751: @example
                    752: @var{limit} @var{start}
                    753: ?DO
                    754:   @var{body}
                    755: LOOP
                    756: @end example
                    757: 
                    758: This performs one iteration for every integer, starting from @var{start}
                    759: and up to, but excluding @var{limit}. The counter, aka index, can be
                    760: accessed with @code{i}. E.g., the loop
                    761: @example
                    762: 10 0 ?DO
                    763:   i .
                    764: LOOP
                    765: @end example
                    766: prints
                    767: @example
                    768: 0 1 2 3 4 5 6 7 8 9
                    769: @end example
                    770: The index of the innermost loop can be accessed with @code{i}, the index
                    771: of the next loop with @code{j}, and the index of the third loop with
                    772: @code{k}.
                    773: 
                    774: The loop control data are kept on the return stack, so there are some
                    775: restrictions on mixing return stack accesses and counted loop
                    776: words. E.g., if you put values on the return stack outside the loop, you
                    777: cannot read them inside the loop. If you put values on the return stack
                    778: within a loop, you have to remove them before the end of the loop and
                    779: before accessing the index of the loop.
                    780: 
                    781: There are several variations on the counted loop:
                    782: 
                    783: @code{LEAVE} leaves the innermost counted loop immediately.
                    784: 
                    785: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
                    786: index by @var{n} instead of by 1. The loop is terminated when the border
                    787: between @var{limit-1} and @var{limit} is crossed. E.g.:
                    788: 
1.2       anton     789: @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
1.1       anton     790: 
1.2       anton     791: @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
1.1       anton     792: 
                    793: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
                    794: 
1.2       anton     795: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
1.1       anton     796: 
1.2       anton     797: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
1.1       anton     798: 
                    799: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
                    800: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
                    801: case behaves symmetrical to the positive case:
                    802: 
1.2       anton     803: @code{-2 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
1.1       anton     804: 
1.2       anton     805: @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0}
1.1       anton     806: 
1.2       anton     807: @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
1.1       anton     808: 
1.2       anton     809: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
1.1       anton     810: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
                    811: Forth standard.
                    812: 
                    813: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
                    814: when the start and the limit value are equal. We do not recommend using
                    815: @code{DO}. It will just give you maintenance troubles.
                    816: 
                    817: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
                    818: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
                    819: return stack so @code{EXIT} can get to its return address.
                    820: 
                    821: Another counted loop is
                    822: @example
                    823: @var{n}
                    824: FOR
                    825:   @var{body}
                    826: NEXT
                    827: @end example
                    828: This is the preferred loop of native code compiler writers who are too
                    829: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
                    830: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
                    831: and ending with 0. Other Forth systems may behave differently, even if
                    832: they support @code{FOR} loops.
                    833: 
1.4       anton     834: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
1.2       anton     835: @subsection Arbitrary control structures
                    836: 
                    837: ANS Forth permits and supports using control structures in a non-nested
                    838: way. Information about incomplete control structures is stored on the
                    839: control-flow stack. This stack may be implemented on the Forth data
                    840: stack, and this is what we have done in gforth.
                    841: 
                    842: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
                    843: entry represents a backward branch target. A few words are the basis for
                    844: building any control structure possible (except control structures that
                    845: need storage, like calls, coroutines, and backtracking).
                    846: 
1.3       anton     847: doc-if
                    848: doc-ahead
                    849: doc-then
                    850: doc-begin
                    851: doc-until
                    852: doc-again
                    853: doc-cs-pick
                    854: doc-cs-roll
1.2       anton     855: 
                    856: On many systems control-flow stack items take one word, in gforth they
                    857: currently take three (this may change in the future). Therefore it is a
                    858: really good idea to manipulate the control flow stack with
                    859: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
                    860: words.
                    861: 
                    862: Some standard control structure words are built from these words:
                    863: 
1.3       anton     864: doc-else
                    865: doc-while
                    866: doc-repeat
1.2       anton     867: 
                    868: Counted loop words constitute a separate group of words:
                    869: 
1.3       anton     870: doc-?do
                    871: doc-do
                    872: doc-for
                    873: doc-loop
                    874: doc-s+loop
                    875: doc-+loop
                    876: doc-next
                    877: doc-leave
                    878: doc-?leave
                    879: doc-unloop
                    880: doc-undo
1.2       anton     881: 
                    882: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
                    883: @i{do-sys}. Our system allows it, but it's your job to ensure that for
                    884: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
1.3       anton     885: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
                    886: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
                    887: resolved (by using one of the loop-ending words or @code{UNDO}).
1.2       anton     888: 
                    889: Another group of control structure words are
                    890: 
1.3       anton     891: doc-case
                    892: doc-endcase
                    893: doc-of
                    894: doc-endof
1.2       anton     895: 
                    896: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
                    897: @code{cs-roll}.
                    898: 
1.3       anton     899: @subsubsection Programming Style
                    900: 
                    901: In order to ensure readability we recommend that you do not create
                    902: arbitrary control structures directly, but define new control structure
                    903: words for the control structure you want and use these words in your
                    904: program.
                    905: 
                    906: E.g., instead of writing
                    907: 
                    908: @example
                    909: begin
                    910:   ...
                    911: if [ 1 cs-roll ]
                    912:   ...
                    913: again then
                    914: @end example
                    915: 
                    916: we recommend defining control structure words, e.g.,
                    917: 
                    918: @example
                    919: : while ( dest -- orig dest )
                    920:  POSTPONE if
                    921:  1 cs-roll ; immediate
                    922: 
                    923: : repeat ( orig dest -- )
                    924:  POSTPONE again
                    925:  POSTPONE then ; immediate
                    926: @end example
                    927: 
                    928: and then using these to create the control structure:
                    929: 
                    930: @example
                    931: begin
                    932:   ...
                    933: while
                    934:   ...
                    935: repeat
                    936: @end example
                    937: 
                    938: That's much easier to read, isn't it? Of course, @code{BEGIN} and
                    939: @code{WHILE} are predefined, so in this example it would not be
                    940: necessary to define them.
                    941: 
1.4       anton     942: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
1.3       anton     943: @subsection Calls and returns
                    944: 
                    945: A definition can be called simply be writing the name of the
                    946: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
                    947: 
                    948: doc-exit
                    949: 
                    950: Don't forget to clean up the return stack and @code{UNLOOP} any
                    951: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
                    952: primitive compiled by @code{EXIT} is
                    953: 
                    954: doc-;s
                    955: 
1.4       anton     956: @node Exception Handling,  , Calls and returns, Control Structures
1.3       anton     957: @subsection Exception Handling
                    958: 
                    959: doc-catch
                    960: doc-throw
                    961: 
1.4       anton     962: @node Locals, Defining Words, Control Structures, Words
1.1       anton     963: @section Locals
                    964: 
1.2       anton     965: Local variables can make Forth programming more enjoyable and Forth
                    966: programs easier to read. Unfortunately, the locals of ANS Forth are
                    967: laden with restrictions. Therefore, we provide not only the ANS Forth
                    968: locals wordset, but also our own, more powerful locals wordset (we
                    969: implemented the ANS Forth locals wordset through our locals wordset).
                    970: 
                    971: @menu
1.4       anton     972: * gforth locals::               
                    973: * ANS Forth locals::            
1.2       anton     974: @end menu
                    975: 
1.4       anton     976: @node gforth locals, ANS Forth locals, Locals, Locals
1.2       anton     977: @subsection gforth locals
                    978: 
                    979: Locals can be defined with
                    980: 
                    981: @example
                    982: @{ local1 local2 ... -- comment @}
                    983: @end example
                    984: or
                    985: @example
                    986: @{ local1 local2 ... @}
                    987: @end example
                    988: 
                    989: E.g.,
                    990: @example
                    991: : max @{ n1 n2 -- n3 @}
                    992:  n1 n2 > if
                    993:    n1
                    994:  else
                    995:    n2
                    996:  endif ;
                    997: @end example
                    998: 
                    999: The similarity of locals definitions with stack comments is intended. A
                   1000: locals definition often replaces the stack comment of a word. The order
                   1001: of the locals corresponds to the order in a stack comment and everything
                   1002: after the @code{--} is really a comment.
                   1003: 
                   1004: This similarity has one disadvantage: It is too easy to confuse locals
                   1005: declarations with stack comments, causing bugs and making them hard to
                   1006: find. However, this problem can be avoided by appropriate coding
                   1007: conventions: Do not use both notations in the same program. If you do,
                   1008: they should be distinguished using additional means, e.g. by position.
                   1009: 
                   1010: The name of the local may be preceded by a type specifier, e.g.,
                   1011: @code{F:} for a floating point value:
                   1012: 
                   1013: @example
                   1014: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
                   1015: \ complex multiplication
                   1016:  Ar Br f* Ai Bi f* f-
                   1017:  Ar Bi f* Ai Br f* f+ ;
                   1018: @end example
                   1019: 
                   1020: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
                   1021: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
                   1022: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
                   1023: with @code{W:}, @code{D:} etc.) produces its value and can be changed
                   1024: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
                   1025: produces its address (which becomes invalid when the variable's scope is
                   1026: left). E.g., the standard word @code{emit} can be defined in therms of
                   1027: @code{type} like this:
                   1028: 
                   1029: @example
                   1030: : emit @{ C^ char* -- @}
                   1031:     char* 1 type ;
                   1032: @end example
                   1033: 
                   1034: A local without type specifier is a @code{W:} local. Both flavours of
                   1035: locals are initialized with values from the data or FP stack.
                   1036: 
                   1037: Currently there is no way to define locals with user-defined data
                   1038: structures, but we are working on it.
                   1039: 
                   1040: GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:
                   1041: 
1.4       anton    1042: @menu
                   1043: * Where are locals visible by name?::  
                   1044: * How long do locals live? ::   
                   1045: * Programming Style::           
                   1046: * Implementation::              
                   1047: @end menu
                   1048: 
                   1049: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1.2       anton    1050: @subsubsection Where are locals visible by name?
                   1051: 
                   1052: Basically, the answer is that locals are visible where you would expect
                   1053: it in block-structured languages, and sometimes a little longer. If you
                   1054: want to restrict the scope of a local, enclose its definition in
                   1055: @code{SCOPE}...@code{ENDSCOPE}.
                   1056: 
                   1057: doc-scope
                   1058: doc-endscope
                   1059: 
                   1060: These words behave like control structure words, so you can use them
                   1061: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
                   1062: arbitrary ways.
                   1063: 
                   1064: If you want a more exact answer to the visibility question, here's the
                   1065: basic principle: A local is visible in all places that can only be
                   1066: reached through the definition of the local@footnote{In compiler
                   1067: construction terminology, all places dominated by the definition of the
                   1068: local.}. In other words, it is not visible in places that can be reached
                   1069: without going through the definition of the local. E.g., locals defined
                   1070: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
                   1071: defined in @code{BEGIN}...@code{UNTIL} are visible after the
                   1072: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
                   1073: 
                   1074: The reasoning behind this solution is: We want to have the locals
                   1075: visible as long as it is meaningful. The user can always make the
                   1076: visibility shorter by using explicit scoping. In a place that can
                   1077: only be reached through the definition of a local, the meaning of a
                   1078: local name is clear. In other places it is not: How is the local
                   1079: initialized at the control flow path that does not contain the
                   1080: definition? Which local is meant, if the same name is defined twice in
                   1081: two independent control flow paths?
                   1082: 
                   1083: This should be enough detail for nearly all users, so you can skip the
                   1084: rest of this section. If you relly must know all the gory details and
                   1085: options, read on.
                   1086: 
                   1087: In order to implement this rule, the compiler has to know which places
                   1088: are unreachable. It knows this automatically after @code{AHEAD},
                   1089: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
                   1090: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
                   1091: compiler that the control flow never reaches that place. If
                   1092: @code{UNREACHABLE} is not used where it could, the only consequence is
                   1093: that the visibility of some locals is more limited than the rule above
                   1094: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
                   1095: lie to the compiler), buggy code will be produced.
                   1096: 
                   1097: Another problem with this rule is that at @code{BEGIN}, the compiler
1.3       anton    1098: does not know which locals will be visible on the incoming
                   1099: back-edge. All problems discussed in the following are due to this
                   1100: ignorance of the compiler (we discuss the problems using @code{BEGIN}
                   1101: loops as examples; the discussion also applies to @code{?DO} and other
1.2       anton    1102: loops). Perhaps the most insidious example is:
                   1103: @example
                   1104: AHEAD
                   1105: BEGIN
                   1106:   x
                   1107: [ 1 CS-ROLL ] THEN
1.4       anton    1108:   @{ x @}
1.2       anton    1109:   ...
                   1110: UNTIL
                   1111: @end example
                   1112: 
                   1113: This should be legal according to the visibility rule. The use of
                   1114: @code{x} can only be reached through the definition; but that appears
                   1115: textually below the use.
                   1116: 
                   1117: From this example it is clear that the visibility rules cannot be fully
                   1118: implemented without major headaches. Our implementation treats common
                   1119: cases as advertised and the exceptions are treated in a safe way: The
                   1120: compiler makes a reasonable guess about the locals visible after a
                   1121: @code{BEGIN}; if it is too pessimistic, the
                   1122: user will get a spurious error about the local not being defined; if the
                   1123: compiler is too optimistic, it will notice this later and issue a
                   1124: warning. In the case above the compiler would complain about @code{x}
                   1125: being undefined at its use. You can see from the obscure examples in
                   1126: this section that it takes quite unusual control structures to get the
                   1127: compiler into trouble, and even then it will often do fine.
                   1128: 
                   1129: If the @code{BEGIN} is reachable from above, the most optimistic guess
                   1130: is that all locals visible before the @code{BEGIN} will also be
                   1131: visible after the @code{BEGIN}. This guess is valid for all loops that
                   1132: are entered only through the @code{BEGIN}, in particular, for normal
                   1133: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
                   1134: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
                   1135: compiler. When the branch to the @code{BEGIN} is finally generated by
                   1136: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
                   1137: warns the user if it was too optimisitic:
                   1138: @example
                   1139: IF
1.4       anton    1140:   @{ x @}
1.2       anton    1141: BEGIN
                   1142:   \ x ? 
                   1143: [ 1 cs-roll ] THEN
                   1144:   ...
                   1145: UNTIL
                   1146: @end example
                   1147: 
                   1148: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
                   1149: optimistically assumes that it lives until the @code{THEN}. It notices
                   1150: this difference when it compiles the @code{UNTIL} and issues a
                   1151: warning. The user can avoid the warning, and make sure that @code{x}
                   1152: is not used in the wrong area by using explicit scoping:
                   1153: @example
                   1154: IF
                   1155:   SCOPE
1.4       anton    1156:   @{ x @}
1.2       anton    1157:   ENDSCOPE
                   1158: BEGIN
                   1159: [ 1 cs-roll ] THEN
                   1160:   ...
                   1161: UNTIL
                   1162: @end example
                   1163: 
                   1164: Since the guess is optimistic, there will be no spurious error messages
                   1165: about undefined locals.
                   1166: 
                   1167: If the @code{BEGIN} is not reachable from above (e.g., after
                   1168: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
                   1169: optimistic guess, as the locals visible after the @code{BEGIN} may be
                   1170: defined later. Therefore, the compiler assumes that no locals are
                   1171: visible after the @code{BEGIN}. However, the useer can use
                   1172: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
                   1173: visible at the BEGIN as at the point where the item was created.
                   1174: 
                   1175: doc-assume-live
                   1176: 
                   1177: E.g.,
                   1178: @example
1.4       anton    1179: @{ x @}
1.2       anton    1180: AHEAD
                   1181: ASSUME-LIVE
                   1182: BEGIN
                   1183:   x
                   1184: [ 1 CS-ROLL ] THEN
                   1185:   ...
                   1186: UNTIL
                   1187: @end example
                   1188: 
                   1189: Other cases where the locals are defined before the @code{BEGIN} can be
                   1190: handled by inserting an appropriate @code{CS-ROLL} before the
                   1191: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
                   1192: behind the @code{ASSUME-LIVE}).
                   1193: 
                   1194: Cases where locals are defined after the @code{BEGIN} (but should be
                   1195: visible immediately after the @code{BEGIN}) can only be handled by
                   1196: rearranging the loop. E.g., the ``most insidious'' example above can be
                   1197: arranged into:
                   1198: @example
                   1199: BEGIN
1.4       anton    1200:   @{ x @}
1.2       anton    1201:   ... 0=
                   1202: WHILE
                   1203:   x
                   1204: REPEAT
                   1205: @end example
                   1206: 
1.4       anton    1207: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1.2       anton    1208: @subsubsection How long do locals live?
                   1209: 
                   1210: The right answer for the lifetime question would be: A local lives at
                   1211: least as long as it can be accessed. For a value-flavoured local this
                   1212: means: until the end of its visibility. However, a variable-flavoured
                   1213: local could be accessed through its address far beyond its visibility
                   1214: scope. Ultimately, this would mean that such locals would have to be
                   1215: garbage collected. Since this entails un-Forth-like implementation
                   1216: complexities, I adopted the same cowardly solution as some other
                   1217: languages (e.g., C): The local lives only as long as it is visible;
                   1218: afterwards its address is invalid (and programs that access it
                   1219: afterwards are erroneous).
                   1220: 
1.4       anton    1221: @node Programming Style, Implementation, How long do locals live?, gforth locals
1.2       anton    1222: @subsubsection Programming Style
                   1223: 
                   1224: The freedom to define locals anywhere has the potential to change
                   1225: programming styles dramatically. In particular, the need to use the
                   1226: return stack for intermediate storage vanishes. Moreover, all stack
                   1227: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
                   1228: determined arguments) can be eliminated: If the stack items are in the
                   1229: wrong order, just write a locals definition for all of them; then
                   1230: write the items in the order you want.
                   1231: 
                   1232: This seems a little far-fetched and eliminating stack manipulations is
1.4       anton    1233: unlikely to become a conscious programming objective. Still, the number
                   1234: of stack manipulations will be reduced dramatically if local variables
                   1235: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
                   1236: a traditional implementation of @code{max}).
1.2       anton    1237: 
                   1238: This shows one potential benefit of locals: making Forth programs more
                   1239: readable. Of course, this benefit will only be realized if the
                   1240: programmers continue to honour the principle of factoring instead of
                   1241: using the added latitude to make the words longer.
                   1242: 
                   1243: Using @code{TO} can and should be avoided.  Without @code{TO},
                   1244: every value-flavoured local has only a single assignment and many
                   1245: advantages of functional languages apply to Forth. I.e., programs are
                   1246: easier to analyse, to optimize and to read: It is clear from the
                   1247: definition what the local stands for, it does not turn into something
                   1248: different later.
                   1249: 
                   1250: E.g., a definition using @code{TO} might look like this:
                   1251: @example
                   1252: : strcmp @{ addr1 u1 addr2 u2 -- n @}
                   1253:  u1 u2 min 0
                   1254:  ?do
                   1255:    addr1 c@ addr2 c@ - ?dup
                   1256:    if
                   1257:      unloop exit
                   1258:    then
                   1259:    addr1 char+ TO addr1
                   1260:    addr2 char+ TO addr2
                   1261:  loop
                   1262:  u1 u2 - ;
                   1263: @end example
                   1264: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
                   1265: every loop iteration. @code{strcmp} is a typical example of the
                   1266: readability problems of using @code{TO}. When you start reading
                   1267: @code{strcmp}, you think that @code{addr1} refers to the start of the
                   1268: string. Only near the end of the loop you realize that it is something
                   1269: else.
                   1270: 
                   1271: This can be avoided by defining two locals at the start of the loop that
                   1272: are initialized with the right value for the current iteration.
                   1273: @example
                   1274: : strcmp @{ addr1 u1 addr2 u2 -- n @}
                   1275:  addr1 addr2
                   1276:  u1 u2 min 0 
                   1277:  ?do @{ s1 s2 @}
                   1278:    s1 c@ s2 c@ - ?dup 
                   1279:    if
                   1280:      unloop exit
                   1281:    then
                   1282:    s1 char+ s2 char+
                   1283:  loop
                   1284:  2drop
                   1285:  u1 u2 - ;
                   1286: @end example
                   1287: Here it is clear from the start that @code{s1} has a different value
                   1288: in every loop iteration.
                   1289: 
1.4       anton    1290: @node Implementation,  , Programming Style, gforth locals
1.2       anton    1291: @subsubsection Implementation
                   1292: 
                   1293: GNU Forth uses an extra locals stack. The most compelling reason for
                   1294: this is that the return stack is not float-aligned; using an extra stack
                   1295: also eliminates the problems and restrictions of using the return stack
                   1296: as locals stack. Like the other stacks, the locals stack grows toward
                   1297: lower addresses. A few primitives allow an efficient implementation:
                   1298: 
                   1299: doc-@local#
                   1300: doc-f@local#
                   1301: doc-laddr#
                   1302: doc-lp+!#
                   1303: doc-lp!
                   1304: doc->l
                   1305: doc-f>l
                   1306: 
                   1307: In addition to these primitives, some specializations of these
                   1308: primitives for commonly occurring inline arguments are provided for
                   1309: efficiency reasons, e.g., @code{@@local0} as specialization of
                   1310: @code{@@local#} for the inline argument 0. The following compiling words
                   1311: compile the right specialized version, or the general version, as
                   1312: appropriate:
                   1313: 
                   1314: doc-compile-@@local
                   1315: doc-compile-f@@local
                   1316: doc-compile-lp+!
                   1317: 
                   1318: Combinations of conditional branches and @code{lp+!#} like
                   1319: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
                   1320: is taken) are provided for efficiency and correctness in loops.
                   1321: 
                   1322: A special area in the dictionary space is reserved for keeping the
                   1323: local variable names. @code{@{} switches the dictionary pointer to this
                   1324: area and @code{@}} switches it back and generates the locals
                   1325: initializing code. @code{W:} etc.@ are normal defining words. This
                   1326: special area is cleared at the start of every colon definition.
                   1327: 
                   1328: A special feature of GNU Forths dictionary is used to implement the
                   1329: definition of locals without type specifiers: every wordlist (aka
                   1330: vocabulary) has its own methods for searching
1.4       anton    1331: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1.2       anton    1332: with a special search method: When it is searched for a word, it
                   1333: actually creates that word using @code{W:}. @code{@{} changes the search
                   1334: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
                   1335: and then the wordlist for defining locals without type specifiers.
                   1336: 
                   1337: The lifetime rules support a stack discipline within a colon
                   1338: definition: The lifetime of a local is either nested with other locals
                   1339: lifetimes or it does not overlap them.
                   1340: 
                   1341: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
                   1342: pointer manipulation is generated. Between control structure words
                   1343: locals definitions can push locals onto the locals stack. @code{AGAIN}
                   1344: is the simplest of the other three control flow words. It has to
                   1345: restore the locals stack depth of the corresponding @code{BEGIN}
                   1346: before branching. The code looks like this:
                   1347: @format
                   1348: @code{lp+!#} current-locals-size @minus{} dest-locals-size
                   1349: @code{branch} <begin>
                   1350: @end format
                   1351: 
                   1352: @code{UNTIL} is a little more complicated: If it branches back, it
                   1353: must adjust the stack just like @code{AGAIN}. But if it falls through,
                   1354: the locals stack must not be changed. The compiler generates the
                   1355: following code:
                   1356: @format
                   1357: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
                   1358: @end format
                   1359: The locals stack pointer is only adjusted if the branch is taken.
                   1360: 
                   1361: @code{THEN} can produce somewhat inefficient code:
                   1362: @format
                   1363: @code{lp+!#} current-locals-size @minus{} orig-locals-size
                   1364: <orig target>:
                   1365: @code{lp+!#} orig-locals-size @minus{} new-locals-size
                   1366: @end format
                   1367: The second @code{lp+!#} adjusts the locals stack pointer from the
1.4       anton    1368: level at the @var{orig} point to the level after the @code{THEN}. The
1.2       anton    1369: first @code{lp+!#} adjusts the locals stack pointer from the current
                   1370: level to the level at the orig point, so the complete effect is an
                   1371: adjustment from the current level to the right level after the
                   1372: @code{THEN}.
                   1373: 
                   1374: In a conventional Forth implementation a dest control-flow stack entry
                   1375: is just the target address and an orig entry is just the address to be
                   1376: patched. Our locals implementation adds a wordlist to every orig or dest
                   1377: item. It is the list of locals visible (or assumed visible) at the point
                   1378: described by the entry. Our implementation also adds a tag to identify
                   1379: the kind of entry, in particular to differentiate between live and dead
                   1380: (reachable and unreachable) orig entries.
                   1381: 
                   1382: A few unusual operations have to be performed on locals wordlists:
                   1383: 
                   1384: doc-common-list
                   1385: doc-sub-list?
                   1386: doc-list-size
                   1387: 
                   1388: Several features of our locals wordlist implementation make these
                   1389: operations easy to implement: The locals wordlists are organised as
                   1390: linked lists; the tails of these lists are shared, if the lists
                   1391: contain some of the same locals; and the address of a name is greater
                   1392: than the address of the names behind it in the list.
                   1393: 
                   1394: Another important implementation detail is the variable
                   1395: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
                   1396: determine if they can be reached directly or only through the branch
                   1397: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
                   1398: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
                   1399: definition, by @code{BEGIN} and usually by @code{THEN}.
                   1400: 
                   1401: Counted loops are similar to other loops in most respects, but
                   1402: @code{LEAVE} requires special attention: It performs basically the same
                   1403: service as @code{AHEAD}, but it does not create a control-flow stack
                   1404: entry. Therefore the information has to be stored elsewhere;
                   1405: traditionally, the information was stored in the target fields of the
                   1406: branches created by the @code{LEAVE}s, by organizing these fields into a
                   1407: linked list. Unfortunately, this clever trick does not provide enough
                   1408: space for storing our extended control flow information. Therefore, we
                   1409: introduce another stack, the leave stack. It contains the control-flow
                   1410: stack entries for all unresolved @code{LEAVE}s.
                   1411: 
                   1412: Local names are kept until the end of the colon definition, even if
                   1413: they are no longer visible in any control-flow path. In a few cases
                   1414: this may lead to increased space needs for the locals name area, but
                   1415: usually less than reclaiming this space would cost in code size.
                   1416: 
                   1417: 
1.4       anton    1418: @node ANS Forth locals,  , gforth locals, Locals
1.2       anton    1419: @subsection ANS Forth locals
                   1420: 
                   1421: The ANS Forth locals wordset does not define a syntax for locals, but
                   1422: words that make it possible to define various syntaxes. One of the
                   1423: possible syntaxes is a subset of the syntax we used in the gforth locals
                   1424: wordset, i.e.:
                   1425: 
                   1426: @example
                   1427: @{ local1 local2 ... -- comment @}
                   1428: @end example
                   1429: or
                   1430: @example
                   1431: @{ local1 local2 ... @}
                   1432: @end example
                   1433: 
                   1434: The order of the locals corresponds to the order in a stack comment. The
                   1435: restrictions are:
1.1       anton    1436: 
1.2       anton    1437: @itemize @bullet
                   1438: @item
                   1439: Locals can only be cell-sized values (no type specifers are allowed).
                   1440: @item
                   1441: Locals can be defined only outside control structures.
                   1442: @item
                   1443: Locals can interfere with explicit usage of the return stack. For the
                   1444: exact (and long) rules, see the standard. If you don't use return stack
                   1445: accessing words in a definition using locals, you will we all right. The
                   1446: purpose of this rule is to make locals implementation on the return
                   1447: stack easier.
                   1448: @item
                   1449: The whole definition must be in one line.
                   1450: @end itemize
                   1451: 
                   1452: Locals defined in this way behave like @code{VALUE}s
1.4       anton    1453: (@xref{Values}). I.e., they are initialized from the stack. Using their
1.2       anton    1454: name produces their value. Their value can be changed using @code{TO}.
                   1455: 
                   1456: Since this syntax is supported by gforth directly, you need not do
                   1457: anything to use it. If you want to port a program using this syntax to
                   1458: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
                   1459: on the other system.
                   1460: 
                   1461: Note that a syntax shown in the standard, section A.13 looks
                   1462: similar, but is quite different in having the order of locals
                   1463: reversed. Beware!
                   1464: 
                   1465: The ANS Forth locals wordset itself consists of the following word
                   1466: 
                   1467: doc-(local)
                   1468: 
                   1469: The ANS Forth locals extension wordset defines a syntax, but it is so
                   1470: awful that we strongly recommend not to use it. We have implemented this
                   1471: syntax to make porting to gforth easy, but do not document it here. The
                   1472: problem with this syntax is that the locals are defined in an order
                   1473: reversed with respect to the standard stack comment notation, making
                   1474: programs harder to read, and easier to misread and miswrite. The only
                   1475: merit of this syntax is that it is easy to implement using the ANS Forth
                   1476: locals wordset.
1.3       anton    1477: 
1.4       anton    1478: @node Defining Words, Wordlists, Locals, Words
                   1479: @section Defining Words
                   1480: 
                   1481: @node Values,  , Defining Words, Defining Words
                   1482: @subsection Values
                   1483: 
                   1484: @node Wordlists, Files, Defining Words, Words
                   1485: @section Wordlists
                   1486: 
                   1487: @node Files, Blocks, Wordlists, Words
                   1488: @section Files
                   1489: 
                   1490: @node Blocks, Other I/O, Files, Words
                   1491: @section Blocks
                   1492: 
                   1493: @node Other I/O, Programming Tools, Blocks, Words
                   1494: @section Other I/O
                   1495: 
                   1496: @node Programming Tools, Threading Words, Other I/O, Words
                   1497: @section Programming Tools
                   1498: 
1.5     ! anton    1499: @menu
        !          1500: * Debugging::                   Simple and quick.
        !          1501: * Assertions::                  Making your programs self-checking.
        !          1502: @end menu
        !          1503: 
        !          1504: @node Debugging, Assertions, Programming Tools, Programming Tools
1.4       anton    1505: @subsection Debugging
                   1506: 
                   1507: The simple debugging aids provided in @file{debugging.fs}
                   1508: are meant to support a different style of debugging than the
                   1509: tracing/stepping debuggers used in languages with long turn-around
                   1510: times.
                   1511: 
                   1512: A much better (faster) way in fast-compilig languages is to add
                   1513: printing code at well-selected places, let the program run, look at
                   1514: the output, see where things went wrong, add more printing code, etc.,
                   1515: until the bug is found.
                   1516: 
                   1517: The word @code{~~} is easy to insert. It just prints debugging
                   1518: information (by default the source location and the stack contents). It
                   1519: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
                   1520: query-replace them with nothing). The deferred words
                   1521: @code{printdebugdata} and @code{printdebugline} control the output of
                   1522: @code{~~}. The default source location output format works well with
                   1523: Emacs' compilation mode, so you can step through the program at the
1.5     ! anton    1524: source level using @kbd{C-x `} (the advantage over a stepping debugger
        !          1525: is that you can step in any direction and you know where the crash has
        !          1526: happened or where the strange data has occurred).
1.4       anton    1527: 
                   1528: Note that the default actions clobber the contents of the pictured
                   1529: numeric output string, so you should not use @code{~~}, e.g., between
                   1530: @code{<#} and @code{#>}.
                   1531: 
                   1532: doc-~~
                   1533: doc-printdebugdata
                   1534: doc-printdebugline
                   1535: 
1.5     ! anton    1536: @node Assertions,  , Debugging, Programming Tools
1.4       anton    1537: @subsection Assertions
                   1538: 
1.5     ! anton    1539: It is a good idea to make your programs self-checking, in particular, if
        !          1540: you use an assumption (e.g., that a certain field of a data structure is
        !          1541: never zero) that may become wrong during maintenance. GForth supports
        !          1542: assertions for this purpose. They are used like this:
        !          1543: 
        !          1544: @example
        !          1545: assert( @var{flag} )
        !          1546: @end example
        !          1547: 
        !          1548: The code between @code{assert(} and @code{)} should compute a flag, that
        !          1549: should be true if everything is alright and false otherwise. It should
        !          1550: not change anything else on the stack. The overall stack effect of the
        !          1551: assertion is @code{( -- )}. E.g.
        !          1552: 
        !          1553: @example
        !          1554: assert( 1 1 + 2 = ) \ what we learn in school
        !          1555: assert( dup 0<> ) \ assert that the top of stack is not zero
        !          1556: assert( false ) \ this code should not be reached
        !          1557: @end example
        !          1558: 
        !          1559: The need for assertions is different at different times. During
        !          1560: debugging, we want more checking, in production we sometimes care more
        !          1561: for speed. Therefore, assertions can be turned off, i.e., the assertion
        !          1562: becomes a comment. Depending on the importance of an assertion and the
        !          1563: time it takes to check it, you may want to turn off some assertions and
        !          1564: keep others turned on. GForth provides several levels of assertions for
        !          1565: this purpose:
        !          1566: 
        !          1567: doc-assert0(
        !          1568: doc-assert1(
        !          1569: doc-assert2(
        !          1570: doc-assert3(
        !          1571: doc-assert(
        !          1572: doc-)
        !          1573: 
        !          1574: @code{Assert(} is the same as @code{assert1(}. The variable
        !          1575: @code{assert-level} specifies the highest assertions that are turned
        !          1576: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
        !          1577: @code{assert1(} assertions perform checking, while @code{assert2(} and
        !          1578: @code{assert3(} assertions are treated as comments.
        !          1579: 
        !          1580: Note that the @code{assert-level} is evaluated at compile-time, not at
        !          1581: run-time. I.e., you cannot turn assertions on or off at run-time, you
        !          1582: have to set the @code{assert-level} appropriately before compiling a
        !          1583: piece of code. You can compile several pieces of code at several
        !          1584: @code{assert-level}s (e.g., a trusted library at level 1 and newly
        !          1585: written code at level 3).
        !          1586: 
        !          1587: doc-assert-level
        !          1588: 
        !          1589: If an assertion fails, a message compatible with Emacs' compilation mode
        !          1590: is produced and the execution is aborted (currently with @code{ABORT"}.
        !          1591: If there is interest, we will introduce a special throw code. But if you
        !          1592: intend to @code{catch} a specific condition, using @code{throw} is
        !          1593: probably more appropriate than an assertion).
        !          1594: 
1.4       anton    1595: @node Threading Words,  , Programming Tools, Words
                   1596: @section Threading Words
                   1597: 
                   1598: These words provide access to code addresses and other threading stuff
                   1599: in gforth (and, possibly, other interpretive Forths). It more or less
                   1600: abstracts away the differences between direct and indirect threading
                   1601: (and, for direct threading, the machine dependences). However, at
                   1602: present this wordset is still inclomplete. It is also pretty low-level;
                   1603: some day it will hopefully be made unnecessary by an internals words set
                   1604: that abstracts implementation details away completely.
                   1605: 
                   1606: doc->code-address
                   1607: doc->does-code
                   1608: doc-code-address!
                   1609: doc-does-code!
                   1610: doc-does-handler!
                   1611: doc-/does-handler
                   1612: 
                   1613: @node ANS conformance, Model, Words, Top
                   1614: @chapter ANS conformance
                   1615: 
                   1616: @node Model, Emacs and GForth, ANS conformance, Top
                   1617: @chapter Model
                   1618: 
                   1619: @node Emacs and GForth, Internals, Model, Top
                   1620: @chapter Emacs and GForth
                   1621: 
                   1622: GForth comes with @file{gforth.el}, an improved version of
                   1623: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
                   1624: improvements are a better (but still not perfect) handling of
                   1625: indentation. I have also added comment paragraph filling (@kbd{M-q}),
                   1626: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-x |}) regions and
                   1627: removing debugging tracers (@kbd{C-x ~}). I left the stuff I do not use
                   1628: alone, even though some of it only makes sense for TILE. To get a
                   1629: description of these features, enter Forth mode and type @kbd{C-h m}.
                   1630: 
                   1631: In addition, GForth supports Emacs quite well: The source code locations
                   1632: given in error messages, debugging output (from @code{~~}) and failed
                   1633: assertion messages are in the right format for Emacs' compilation mode
                   1634: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
                   1635: Manual}) so the source location corresponding to an error or other
                   1636: message is only a few keystrokes away (@kbd{C-x `} for the next error,
                   1637: @kbd{C-c C-c} for the error under the cursor).
                   1638: 
                   1639: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
                   1640: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
                   1641: contains the definitions of all words defined afterwards. You can then
                   1642: find the source for a word using @kbd{M-.}. Note that emacs can use
                   1643: several tags files at the same time (e.g., one for the gforth sources
                   1644: and one for your program).
                   1645: 
                   1646: To get all these benefits, add the following lines to your @file{.emacs}
                   1647: file:
                   1648: 
                   1649: @example
                   1650: (autoload 'forth-mode "gforth.el")
                   1651: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
                   1652: @end example
                   1653: 
                   1654: @node Internals, Bugs, Emacs and GForth, Top
1.3       anton    1655: @chapter Internals
                   1656: 
                   1657: Reading this section is not necessary for programming with gforth. It
                   1658: should be helpful for finding your way in the gforth sources.
                   1659: 
1.4       anton    1660: @menu
                   1661: * Portability::                 
                   1662: * Threading::                   
                   1663: * Primitives::                  
                   1664: * System Architecture::         
                   1665: @end menu
                   1666: 
                   1667: @node Portability, Threading, Internals, Internals
1.3       anton    1668: @section Portability
                   1669: 
                   1670: One of the main goals of the effort is availability across a wide range
                   1671: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
                   1672: this goal by manually coding the engine in assembly language for several
                   1673: then-popular processors. This approach is very labor-intensive and the
                   1674: results are short-lived due to progress in computer architecture.
                   1675: 
                   1676: Others have avoided this problem by coding in C, e.g., Mitch Bradley
                   1677: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
                   1678: particularly popular for UNIX-based Forths due to the large variety of
                   1679: architectures of UNIX machines. Unfortunately an implementation in C
                   1680: does not mix well with the goals of efficiency and with using
                   1681: traditional techniques: Indirect or direct threading cannot be expressed
                   1682: in C, and switch threading, the fastest technique available in C, is
                   1683: significantly slower. Another problem with C is that it's very
                   1684: cumbersome to express double integer arithmetic.
                   1685: 
                   1686: Fortunately, there is a portable language that does not have these
                   1687: limitations: GNU C, the version of C processed by the GNU C compiler
                   1688: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
                   1689: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
                   1690: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
                   1691: threading possible, its @code{long long} type (@pxref{Long Long, ,
                   1692: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
                   1693: double numbers. GNU C is available for free on all important (and many
                   1694: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
                   1695: the Atari ST, so a Forth written in GNU C can run on all these
                   1696: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
1.5     ! anton    1697: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
1.3       anton    1698: ``Look And Feel'', gcc.info, GNU C Manual}).}.
                   1699: 
                   1700: Writing in a portable language has the reputation of producing code that
                   1701: is slower than assembly. For our Forth engine we repeatedly looked at
                   1702: the code produced by the compiler and eliminated most compiler-induced
                   1703: inefficiencies by appropriate changes in the source-code.
                   1704: 
                   1705: However, register allocation cannot be portably influenced by the
                   1706: programmer, leading to some inefficiencies on register-starved
                   1707: machines. We use explicit register declarations (@pxref{Explicit Reg
                   1708: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
                   1709: improve the speed on some machines. They are turned on by using the
                   1710: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
                   1711: only depends on the machine, but also on the compiler version: On some
                   1712: machines some compiler versions produce incorrect code when certain
                   1713: explicit register declarations are used. So by default
                   1714: @code{-DFORCE_REG} is not used.
                   1715: 
1.4       anton    1716: @node Threading, Primitives, Portability, Internals
1.3       anton    1717: @section Threading
                   1718: 
                   1719: GNU C's labels as values extension (available since @code{gcc-2.0},
                   1720: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
                   1721: makes it possible to take the address of @var{label} by writing
                   1722: @code{&&@var{label}}.  This address can then be used in a statement like
                   1723: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
                   1724: @code{goto x}.
                   1725: 
                   1726: With this feature an indirect threaded NEXT looks like:
                   1727: @example
                   1728: cfa = *ip++;
                   1729: ca = *cfa;
                   1730: goto *ca;
                   1731: @end example
                   1732: For those unfamiliar with the names: @code{ip} is the Forth instruction
                   1733: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
                   1734: execution token and points to the code field of the next word to be
                   1735: executed; The @code{ca} (code address) fetched from there points to some
                   1736: executable code, e.g., a primitive or the colon definition handler
                   1737: @code{docol}.
                   1738: 
                   1739: Direct threading is even simpler:
                   1740: @example
                   1741: ca = *ip++;
                   1742: goto *ca;
                   1743: @end example
                   1744: 
                   1745: Of course we have packaged the whole thing neatly in macros called
                   1746: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
                   1747: 
1.4       anton    1748: @menu
                   1749: * Scheduling::                  
                   1750: * Direct or Indirect Threaded?::  
                   1751: * DOES>::                       
                   1752: @end menu
                   1753: 
                   1754: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1.3       anton    1755: @subsection Scheduling
                   1756: 
                   1757: There is a little complication: Pipelined and superscalar processors,
                   1758: i.e., RISC and some modern CISC machines can process independent
                   1759: instructions while waiting for the results of an instruction. The
                   1760: compiler usually reorders (schedules) the instructions in a way that
                   1761: achieves good usage of these delay slots. However, on our first tries
                   1762: the compiler did not do well on scheduling primitives. E.g., for
                   1763: @code{+} implemented as
                   1764: @example
                   1765: n=sp[0]+sp[1];
                   1766: sp++;
                   1767: sp[0]=n;
                   1768: NEXT;
                   1769: @end example
                   1770: the NEXT comes strictly after the other code, i.e., there is nearly no
                   1771: scheduling. After a little thought the problem becomes clear: The
                   1772: compiler cannot know that sp and ip point to different addresses (and
1.4       anton    1773: the version of @code{gcc} we used would not know it even if it was
                   1774: possible), so it could not move the load of the cfa above the store to
                   1775: the TOS. Indeed the pointers could be the same, if code on or very near
                   1776: the top of stack were executed. In the interest of speed we chose to
                   1777: forbid this probably unused ``feature'' and helped the compiler in
                   1778: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
                   1779: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1.3       anton    1780: @example
                   1781: n=sp[0]+sp[1];
                   1782: sp++;
                   1783: NEXT_P1;
                   1784: sp[0]=n;
                   1785: NEXT_P2;
                   1786: @end example
1.4       anton    1787: This can be scheduled optimally by the compiler.
1.3       anton    1788: 
                   1789: This division can be turned off with the switch @code{-DCISC_NEXT}. This
                   1790: switch is on by default on machines that do not profit from scheduling
                   1791: (e.g., the 80386), in order to preserve registers.
                   1792: 
1.4       anton    1793: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1.3       anton    1794: @subsection Direct or Indirect Threaded?
                   1795: 
                   1796: Both! After packaging the nasty details in macro definitions we
                   1797: realized that we could switch between direct and indirect threading by
                   1798: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
                   1799: defining a few machine-specific macros for the direct-threading case.
                   1800: On the Forth level we also offer access words that hide the
                   1801: differences between the threading methods (@pxref{Threading Words}).
                   1802: 
                   1803: Indirect threading is implemented completely
                   1804: machine-independently. Direct threading needs routines for creating
                   1805: jumps to the executable code (e.g. to docol or dodoes). These routines
                   1806: are inherently machine-dependent, but they do not amount to many source
                   1807: lines. I.e., even porting direct threading to a new machine is a small
                   1808: effort.
                   1809: 
1.4       anton    1810: @node DOES>,  , Direct or Indirect Threaded?, Threading
1.3       anton    1811: @subsection DOES>
                   1812: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
                   1813: the chunk of code executed by every word defined by a
                   1814: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
                   1815: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
                   1816: DOES-code)? There are two solutions:
                   1817: 
                   1818: In fig-Forth the code field points directly to the dodoes and the
                   1819: DOES-code address is stored in the cell after the code address
                   1820: (i.e. at cfa cell+). It may seem that this solution is illegal in the
                   1821: Forth-79 and all later standards, because in fig-Forth this address
                   1822: lies in the body (which is illegal in these standards). However, by
                   1823: making the code field larger for all words this solution becomes legal
                   1824: again. We use this approach for the indirect threaded version. Leaving
                   1825: a cell unused in most words is a bit wasteful, but on the machines we
                   1826: are targetting this is hardly a problem. The other reason for having a
                   1827: code field size of two cells is to avoid having different image files
1.4       anton    1828: for direct and indirect threaded systems (@pxref{System Architecture}).
1.3       anton    1829: 
                   1830: The other approach is that the code field points or jumps to the cell
                   1831: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
                   1832: this address. @code{dodoes} can then get the DOES-code address by
                   1833: computing the code address, i.e., the address of the jump to dodoes,
                   1834: and add the length of that jump field. A variant of this is to have a
                   1835: call to @code{dodoes} after the @code{DOES>}; then the return address
                   1836: (which can be found in the return register on RISCs) is the DOES-code
                   1837: address. Since the two cells available in the code field are usually
                   1838: used up by the jump to the code address in direct threading, we use
                   1839: this approach for direct threading. We did not want to add another
                   1840: cell to the code field.
                   1841: 
1.4       anton    1842: @node Primitives, System Architecture, Threading, Internals
1.3       anton    1843: @section Primitives
                   1844: 
1.4       anton    1845: @menu
                   1846: * Automatic Generation::        
                   1847: * TOS Optimization::            
                   1848: * Produced code::               
                   1849: @end menu
                   1850: 
                   1851: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1.3       anton    1852: @subsection Automatic Generation
                   1853: 
                   1854: Since the primitives are implemented in a portable language, there is no
                   1855: longer any need to minimize the number of primitives. On the contrary,
                   1856: having many primitives is an advantage: speed. In order to reduce the
                   1857: number of errors in primitives and to make programming them easier, we
                   1858: provide a tool, the primitive generator (@file{prims2x.fs}), that
                   1859: automatically generates most (and sometimes all) of the C code for a
                   1860: primitive from the stack effect notation.  The source for a primitive
                   1861: has the following form:
                   1862: 
                   1863: @format
                   1864: @var{Forth-name}       @var{stack-effect}      @var{category}  [@var{pronounc.}]
                   1865: [@code{""}@var{glossary entry}@code{""}]
                   1866: @var{C code}
                   1867: [@code{:}
                   1868: @var{Forth code}]
                   1869: @end format
                   1870: 
                   1871: The items in brackets are optional. The category and glossary fields
                   1872: are there for generating the documentation, the Forth code is there
                   1873: for manual implementations on machines without GNU C. E.g., the source
                   1874: for the primitive @code{+} is:
                   1875: @example
                   1876: +    n1 n2 -- n    core    plus
                   1877: n = n1+n2;
                   1878: @end example
                   1879: 
                   1880: This looks like a specification, but in fact @code{n = n1+n2} is C
                   1881: code. Our primitive generation tool extracts a lot of information from
                   1882: the stack effect notations@footnote{We use a one-stack notation, even
                   1883: though we have separate data and floating-point stacks; The separate
                   1884: notation can be generated easily from the unified notation.}: The number
                   1885: of items popped from and pushed on the stack, their type, and by what
                   1886: name they are referred to in the C code. It then generates a C code
                   1887: prelude and postlude for each primitive. The final C code for @code{+}
                   1888: looks like this:
                   1889: 
                   1890: @example
                   1891: I_plus:        /* + ( n1 n2 -- n ) */  /* label, stack effect */
                   1892: /*  */                          /* documentation */
1.4       anton    1893: @{
1.3       anton    1894: DEF_CA                          /* definition of variable ca (indirect threading) */
                   1895: Cell n1;                        /* definitions of variables */
                   1896: Cell n2;
                   1897: Cell n;
                   1898: n1 = (Cell) sp[1];              /* input */
                   1899: n2 = (Cell) TOS;
                   1900: sp += 1;                        /* stack adjustment */
                   1901: NAME("+")                       /* debugging output (with -DDEBUG) */
1.4       anton    1902: @{
1.3       anton    1903: n = n1+n2;                      /* C code taken from the source */
1.4       anton    1904: @}
1.3       anton    1905: NEXT_P1;                        /* NEXT part 1 */
                   1906: TOS = (Cell)n;                  /* output */
                   1907: NEXT_P2;                        /* NEXT part 2 */
1.4       anton    1908: @}
1.3       anton    1909: @end example
                   1910: 
                   1911: This looks long and inefficient, but the GNU C compiler optimizes quite
                   1912: well and produces optimal code for @code{+} on, e.g., the R3000 and the
                   1913: HP RISC machines: Defining the @code{n}s does not produce any code, and
                   1914: using them as intermediate storage also adds no cost.
                   1915: 
                   1916: There are also other optimizations, that are not illustrated by this
                   1917: example: Assignments between simple variables are usually for free (copy
                   1918: propagation). If one of the stack items is not used by the primitive
                   1919: (e.g.  in @code{drop}), the compiler eliminates the load from the stack
                   1920: (dead code elimination). On the other hand, there are some things that
                   1921: the compiler does not do, therefore they are performed by
                   1922: @file{prims2x.fs}: The compiler does not optimize code away that stores
                   1923: a stack item to the place where it just came from (e.g., @code{over}).
                   1924: 
                   1925: While programming a primitive is usually easy, there are a few cases
                   1926: where the programmer has to take the actions of the generator into
                   1927: account, most notably @code{?dup}, but also words that do not (always)
                   1928: fall through to NEXT.
                   1929: 
1.4       anton    1930: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1.3       anton    1931: @subsection TOS Optimization
                   1932: 
                   1933: An important optimization for stack machine emulators, e.g., Forth
                   1934: engines, is keeping  one or more of the top stack items in
1.4       anton    1935: registers.  If a word has the stack effect @var{in1}...@var{inx} @code{--}
                   1936: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1.3       anton    1937: @itemize
                   1938: @item
                   1939: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
                   1940: due to fewer loads from and stores to the stack.
                   1941: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
                   1942: @var{y<n}, due to additional moves between registers.
                   1943: @end itemize
                   1944: 
                   1945: In particular, keeping one item in a register is never a disadvantage,
                   1946: if there are enough registers. Keeping two items in registers is a
                   1947: disadvantage for frequent words like @code{?branch}, constants,
                   1948: variables, literals and @code{i}. Therefore our generator only produces
                   1949: code that keeps zero or one items in registers. The generated C code
                   1950: covers both cases; the selection between these alternatives is made at
                   1951: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
                   1952: code for @code{+} is just a simple variable name in the one-item case,
                   1953: otherwise it is a macro that expands into @code{sp[0]}. Note that the
                   1954: GNU C compiler tries to keep simple variables like @code{TOS} in
                   1955: registers, and it usually succeeds, if there are enough registers.
                   1956: 
                   1957: The primitive generator performs the TOS optimization for the
                   1958: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
                   1959: operations the benefit of this optimization is even larger:
                   1960: floating-point operations take quite long on most processors, but can be
                   1961: performed in parallel with other operations as long as their results are
                   1962: not used. If the FP-TOS is kept in a register, this works. If
                   1963: it is kept on the stack, i.e., in memory, the store into memory has to
                   1964: wait for the result of the floating-point operation, lengthening the
                   1965: execution time of the primitive considerably.
                   1966: 
                   1967: The TOS optimization makes the automatic generation of primitives a
                   1968: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
                   1969: @code{TOS} is not sufficient. There are some special cases to
                   1970: consider:
                   1971: @itemize
                   1972: @item In the case of @code{dup ( w -- w w )} the generator must not
                   1973: eliminate the store to the original location of the item on the stack,
                   1974: if the TOS optimization is turned on.
1.4       anton    1975: @item Primitives with stack effects of the form @code{--}
                   1976: @var{out1}...@var{outy} must store the TOS to the stack at the start.
                   1977: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1.3       anton    1978: must load the TOS from the stack at the end. But for the null stack
                   1979: effect @code{--} no stores or loads should be generated.
                   1980: @end itemize
                   1981: 
1.4       anton    1982: @node Produced code,  , TOS Optimization, Primitives
1.3       anton    1983: @subsection Produced code
                   1984: 
                   1985: To see what assembly code is produced for the primitives on your machine
                   1986: with your compiler and your flag settings, type @code{make engine.s} and
1.4       anton    1987: look at the resulting file @file{engine.s}.
1.3       anton    1988: 
1.4       anton    1989: @node System Architecture,  , Primitives, Internals
1.3       anton    1990: @section System Architecture
                   1991: 
                   1992: Our Forth system consists not only of primitives, but also of
                   1993: definitions written in Forth. Since the Forth compiler itself belongs
                   1994: to those definitions, it is not possible to start the system with the
                   1995: primitives and the Forth source alone. Therefore we provide the Forth
                   1996: code as an image file in nearly executable form. At the start of the
                   1997: system a C routine loads the image file into memory, sets up the
                   1998: memory (stacks etc.) according to information in the image file, and
                   1999: starts executing Forth code.
                   2000: 
                   2001: The image file format is a compromise between the goals of making it
                   2002: easy to generate image files and making them portable. The easiest way
                   2003: to generate an image file is to just generate a memory dump. However,
                   2004: this kind of image file cannot be used on a different machine, or on
                   2005: the next version of the engine on the same machine, it even might not
                   2006: work with the same engine compiled by a different version of the C
                   2007: compiler. We would like to have as few versions of the image file as
                   2008: possible, because we do not want to distribute many versions of the
                   2009: same image file, and to make it easy for the users to use their image
                   2010: files on many machines. We currently need to create a different image
                   2011: file for machines with different cell sizes and different byte order
                   2012: (little- or big-endian)@footnote{We consider adding information to the
                   2013: image file that enables the loader to change the byte order.}.
                   2014: 
                   2015: Forth code that is going to end up in a portable image file has to
1.4       anton    2016: comply to some restrictions: addresses have to be stored in memory with
                   2017: special words (@code{A!}, @code{A,}, etc.) in order to make the code
                   2018: relocatable. Cells, floats, etc., have to be stored at the natural
                   2019: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
                   2020: dividable by~8. This happens automatically in our system when you use
                   2021: the ANS Forth alignment words.}, in order to avoid alignment faults on
                   2022: machines with stricter alignment. The image file is produced by a
                   2023: metacompiler (@file{cross.fs}).
1.3       anton    2024: 
                   2025: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
                   2026: file is not directly executable, but has to undergo some manipulations
                   2027: during loading. Address relocation is performed at image load-time, not
                   2028: at run-time. The loader also has to replace tokens standing for
                   2029: primitive calls with the appropriate code-field addresses (or code
                   2030: addresses in the case of direct threading).
1.4       anton    2031: 
                   2032: @node Bugs, Pedigree, Internals, Top
                   2033: @chapter Bugs
                   2034: 
                   2035: @node Pedigree, Word Index, Bugs, Top
                   2036: @chapter Pedigree
                   2037: 
                   2038: @node Word Index, Node Index, Pedigree, Top
                   2039: @chapter Word Index
                   2040: 
                   2041: @node Node Index,  , Word Index, Top
                   2042: @chapter Node Index
1.1       anton    2043: 
                   2044: @contents
                   2045: @bye
                   2046: 

FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>