--- gforth/Attic/gforth.ds 1994/11/14 19:01:16 1.2 +++ gforth/Attic/gforth.ds 1996/09/23 08:52:47 1.36 @@ -1,27 +1,27 @@ \input texinfo @c -*-texinfo-*- @comment The source is gforth.ds, from which gforth.texi is generated @comment %**start of header (This is for running Texinfo on a region.) -@setfilename gforth-info -@settitle GNU Forth Manual -@setchapternewpage odd +@setfilename gforth.info +@settitle Gforth Manual +@comment @setchapternewpage odd @comment %**end of header (This is for running Texinfo on a region.) @ifinfo -This file documents GNU Forth 0.0 +This file documents Gforth 0.2 -Copyright @copyright{} 1994 GNU Forth Development Group +Copyright @copyright{} 1995,1996 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. - @ignore +@ignore Permission is granted to process this file through TeX and print the results, provided the printed document carries a copying permission notice identical to this one except for the removal of this paragraph (this paragraph not being relevant to the printed manual). - @end ignore +@end ignore Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "Distribution" and "General Public License" are @@ -36,18 +36,22 @@ Copyright @copyright{} 1994 GNU Forth De of in the original English. @end ifinfo +@finalout @titlepage @sp 10 -@center @titlefont{GNU Forth Manual} +@center @titlefont{Gforth Manual} @sp 2 -@center for version 0.0 +@center for version 0.2 @sp 2 @center Anton Ertl +@center Bernd Paysan +@sp 3 +@center This manual is under construction @comment The following two commands start the copyright page. @page @vskip 0pt plus 1filll -Copyright @copyright{} 1994 GNU Forth Development Group +Copyright @copyright{} 1995,1996 Free Software Foundation, Inc. @comment !! Published by ... or You can get a copy of this manual ... @@ -72,47 +76,439 @@ Copyright @copyright{} 1994 GNU Forth De @node Top, License, (dir), (dir) @ifinfo -GNU Forth is a free implementation of ANS Forth available on many -personal machines. This manual corresponds to version 0.0. +Gforth is a free implementation of ANS Forth available on many +personal machines. This manual corresponds to version 0.2. @end ifinfo @menu -* License:: -* Goals:: About the GNU Forth Project -* Other Books:: Things you might want to read -* Invocation:: Starting GNU Forth -* Words:: Forth words available in GNU Forth -* ANS conformance:: Implementation-defined options etc. -* Model:: The abstract machine of GNU Forth -* Emacs and GForth:: The GForth Mode -* Internals:: Implementation details -* Bugs:: How to report them -* Pedigree:: Ancestors of GNU Forth -* Word Index:: An item for each Forth word -* Node Index:: An item for each node +* License:: +* Goals:: About the Gforth Project +* Other Books:: Things you might want to read +* Invocation:: Starting Gforth +* Words:: Forth words available in Gforth +* ANS conformance:: Implementation-defined options etc. +* Model:: The abstract machine of Gforth +* Integrating Gforth:: Forth as scripting language for applications. +* Emacs and Gforth:: The Gforth Mode +* Internals:: Implementation details +* Bugs:: How to report them +* Origin:: Authors and ancestors of Gforth +* Word Index:: An item for each Forth word +* Node Index:: An item for each node @end menu @node License, Goals, Top, Top -@unnumbered License -!! Insert GPL here +@unnumbered GNU GENERAL PUBLIC LICENSE +@center Version 2, June 1991 + +@display +Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc. +675 Mass Ave, Cambridge, MA 02139, USA + +Everyone is permitted to copy and distribute verbatim copies +of this license document, but changing it is not allowed. +@end display + +@unnumberedsec Preamble + + The licenses for most software are designed to take away your +freedom to share and change it. By contrast, the GNU General Public +License is intended to guarantee your freedom to share and change free +software---to make sure the software is free for all its users. This +General Public License applies to most of the Free Software +Foundation's software and to any other program whose authors commit to +using it. (Some other Free Software Foundation software is covered by +the GNU Library General Public License instead.) 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The act of +running the Program is not restricted, and the output from the Program +is covered only if its contents constitute a work based on the +Program (independent of having been made by running the Program). +Whether that is true depends on what the Program does. + +@item +You may copy and distribute verbatim copies of the Program's +source code as you receive it, in any medium, provided that you +conspicuously and appropriately publish on each copy an appropriate +copyright notice and disclaimer of warranty; keep intact all the +notices that refer to this License and to the absence of any warranty; +and give any other recipients of the Program a copy of this License +along with the Program. + +You may charge a fee for the physical act of transferring a copy, and +you may at your option offer warranty protection in exchange for a fee. + +@item +You may modify your copy or copies of the Program or any portion +of it, thus forming a work based on the Program, and copy and +distribute such modifications or work under the terms of Section 1 +above, provided that you also meet all of these conditions: + +@enumerate a +@item +You must cause the modified files to carry prominent notices +stating that you changed the files and the date of any change. + +@item +You must cause any work that you distribute or publish, that in +whole or in part contains or is derived from the Program or any +part thereof, to be licensed as a whole at no charge to all third +parties under the terms of this License. + +@item +If the modified program normally reads commands interactively +when run, you must cause it, when started running for such +interactive use in the most ordinary way, to print or display an +announcement including an appropriate copyright notice and a +notice that there is no warranty (or else, saying that you provide +a warranty) and that users may redistribute the program under +these conditions, and telling the user how to view a copy of this +License. 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In such case, this License incorporates +the limitation as if written in the body of this License. + +@item +The Free Software Foundation may publish revised and/or new versions +of the General Public License from time to time. Such new versions will +be similar in spirit to the present version, but may differ in detail to +address new problems or concerns. + +Each version is given a distinguishing version number. If the Program +specifies a version number of this License which applies to it and ``any +later version'', you have the option of following the terms and conditions +either of that version or of any later version published by the Free +Software Foundation. If the Program does not specify a version number of +this License, you may choose any version ever published by the Free Software +Foundation. + +@item +If you wish to incorporate parts of the Program into other free +programs whose distribution conditions are different, write to the author +to ask for permission. For software which is copyrighted by the Free +Software Foundation, write to the Free Software Foundation; we sometimes +make exceptions for this. Our decision will be guided by the two goals +of preserving the free status of all derivatives of our free software and +of promoting the sharing and reuse of software generally. + +@iftex +@heading NO WARRANTY +@end iftex +@ifinfo +@center NO WARRANTY +@end ifinfo + +@item +BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY +FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN +OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES +PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED +OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF +MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS +TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE +PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, +REPAIR OR CORRECTION. + +@item +IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING +WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR +REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, +INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING +OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED +TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY +YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER +PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE +POSSIBILITY OF SUCH DAMAGES. +@end enumerate + +@iftex +@heading END OF TERMS AND CONDITIONS +@end iftex +@ifinfo +@center END OF TERMS AND CONDITIONS +@end ifinfo + +@page +@unnumberedsec How to Apply These Terms to Your New Programs + + If you develop a new program, and you want it to be of the greatest +possible use to the public, the best way to achieve this is to make it +free software which everyone can redistribute and change under these terms. + + To do so, attach the following notices to the program. It is safest +to attach them to the start of each source file to most effectively +convey the exclusion of warranty; and each file should have at least +the ``copyright'' line and a pointer to where the full notice is found. + +@smallexample +@var{one line to give the program's name and a brief idea of what it does.} +Copyright (C) 19@var{yy} @var{name of author} + +This program is free software; you can redistribute it and/or modify +it under the terms of the GNU General Public License as published by +the Free Software Foundation; either version 2 of the License, or +(at your option) any later version. + +This program is distributed in the hope that it will be useful, +but WITHOUT ANY WARRANTY; without even the implied warranty of +MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +GNU General Public License for more details. + +You should have received a copy of the GNU General Public License +along with this program; if not, write to the Free Software +Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. +@end smallexample + +Also add information on how to contact you by electronic and paper mail. + +If the program is interactive, make it output a short notice like this +when it starts in an interactive mode: + +@smallexample +Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author} +Gnomovision comes with ABSOLUTELY NO WARRANTY; for details +type `show w'. +This is free software, and you are welcome to redistribute it +under certain conditions; type `show c' for details. +@end smallexample + +The hypothetical commands @samp{show w} and @samp{show c} should show +the appropriate parts of the General Public License. Of course, the +commands you use may be called something other than @samp{show w} and +@samp{show c}; they could even be mouse-clicks or menu items---whatever +suits your program. + +You should also get your employer (if you work as a programmer) or your +school, if any, to sign a ``copyright disclaimer'' for the program, if +necessary. Here is a sample; alter the names: + +@smallexample +Yoyodyne, Inc., hereby disclaims all copyright interest in the program +`Gnomovision' (which makes passes at compilers) written by James Hacker. + +@var{signature of Ty Coon}, 1 April 1989 +Ty Coon, President of Vice +@end smallexample + +This General Public License does not permit incorporating your program into +proprietary programs. If your program is a subroutine library, you may +consider it more useful to permit linking proprietary applications with the +library. If this is what you want to do, use the GNU Library General +Public License instead of this License. + +@iftex +@node Preface +@comment node-name, next, previous, up @unnumbered Preface -This manual documents GNU Forth. The reader is expected to know +@cindex Preface +This manual documents Gforth. The reader is expected to know Forth. This manual is primarily a reference manual. @xref{Other Books} for introductory material. @end iftex @node Goals, Other Books, License, Top @comment node-name, next, previous, up -@chapter Goals of GNU Forth +@chapter Goals of Gforth @cindex Goals -The goal of the GNU Forth Project is to develop a standard model for +The goal of the Gforth Project is to develop a standard model for ANSI Forth. This can be split into several subgoals: @itemize @bullet @item -GNU Forth should conform to the ANSI Forth standard. +Gforth should conform to the ANSI Forth standard. @item It should be a model, i.e. it should define all the implementation-dependent things. @@ -121,7 +517,7 @@ It should become standard, i.e. widely a is the most difficult one. @end itemize -To achieve these goals GNU Forth should be +To achieve these goals Gforth should be @itemize @bullet @item Similar to previous models (fig-Forth, F83) @@ -137,32 +533,37 @@ Free. Available on many machines/easy to port. @end itemize -Have we achieved these goals? GNU Forth conforms to the ANS Forth -standard; it may be considered a model, but we have not yet documented +Have we achieved these goals? Gforth conforms to the ANS Forth +standard. It may be considered a model, but we have not yet documented which parts of the model are stable and which parts we are likely to -change; it certainly has not yet become a de facto standard. It has some -similarities and some differences to previous models; It has some -powerful features, but not yet everything that we envisioned; on RISCs -it is as fast as interpreters programmed in assembly, on -register-starved machines it is not so fast, but still faster than any -other C-based interpretive implementation; it is free and available on -many machines. +change. It certainly has not yet become a de facto standard. It has some +similarities and some differences to previous models. It has some +powerful features, but not yet everything that we envisioned. We +certainly have achieved our execution speed goals (@pxref{Performance}). +It is free and available on many machines. @node Other Books, Invocation, Goals, Top @chapter Other books on ANS Forth As the standard is relatively new, there are not many books out yet. It -is not recommended to learn Forth by using GNU Forth and a book that is +is not recommended to learn Forth by using Gforth and a book that is not written for ANS Forth, as you will not know your mistakes from the deviations of the book. There is, of course, the standard, the definite reference if you want to -write ANS Forth programs. It will be available in printed form from -Global Engineering Documents !! somtime in spring or summer 1994. If you -are lucky, you can still get dpANS6 (the draft that was approved as -standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14. +write ANS Forth programs. It is available in printed form from the +National Standards Institute Sales Department (Tel.: USA (212) 642-4900; +Fax.: USA (212) 302-1286) as document @cite{X3.215-1994} for about $200. You +can also get it from Global Engineering Documents (Tel.: USA (800) +854-7179; Fax.: (303) 843-9880) for about $300. + +@cite{dpANS6}, the last draft of the standard, which was then submitted to ANSI +for publication is available electronically and for free in some MS Word +format, and it has been converted to HTML. Some pointers to these +versions can be found through +@*@file{http://www.complang.tuwien.ac.at/projects/forth.html}. -@cite{Forth: The new model} by Jack Woehr (!! Publisher) is an +@cite{Forth: The new model} by Jack Woehr (Prentice-Hall, 1993) is an introductory book based on a draft version of the standard. It does not cover the whole standard. It also contains interesting background information (Jack Woehr was in the ANS Forth Technical Committe). It is @@ -173,7 +574,7 @@ other languages should find it ok. @chapter Invocation You will usually just say @code{gforth}. In many other cases the default -GNU Forth image will be invoked like this: +Gforth image will be invoked like this: @example gforth [files] [-e forth-code] @@ -193,10 +594,12 @@ line. They are: @table @code @item --image-file @var{file} +@item -i @var{file} Loads the Forth image @var{file} instead of the default @file{gforth.fi}. @item --path @var{path} +@item -p @var{path} Uses @var{path} for searching the image file and Forth source code files instead of the default in the environment variable @code{GFORTHPATH} or the path specified at installation time (typically @@ -245,6 +648,12 @@ Forth words, you have to quote them or u after processing the command line (instead of entering interactive mode) append @code{-e bye} to the command line. +If you have several versions of Gforth installed, @code{gforth} will +invoke the version that was installed last. @code{gforth-@var{version}} +invokes a specific version. You may want to use the option +@code{--path}, if your environment contains the variable +@code{GFORTHPATH}. + Not yet implemented: On startup the system first executes the system initialization file (unless the option @code{--no-init-file} is given; note that the system @@ -253,22 +662,24 @@ the user initialization file @file{.gfor option @code{--no-rc} is given; this file is first searched in @file{.}, then in @file{~}, then in the normal path (see above). -@node Words, , Invocation, Top +@node Words, ANS conformance, Invocation, Top @chapter Forth Words @menu -* Notation:: -* Arithmetic:: -* Stack Manipulation:: -* Memory access:: -* Control Structures:: -* Local Variables:: -* Defining Words:: -* Vocabularies:: -* Files:: -* Blocks:: -* Other I/O:: -* Programming Tools:: +* Notation:: +* Arithmetic:: +* Stack Manipulation:: +* Memory access:: +* Control Structures:: +* Locals:: +* Defining Words:: +* Wordlists:: +* Files:: +* Blocks:: +* Other I/O:: +* Programming Tools:: +* Assembler and Code words:: +* Threading Words:: @end menu @node Notation, Arithmetic, Words, Words @@ -277,28 +688,35 @@ then in @file{~}, then in the normal pat The Forth words are described in this section in the glossary notation that has become a de-facto standard for Forth texts, i.e. -@quotation +@format @var{word} @var{Stack effect} @var{wordset} @var{pronunciation} +@end format @var{Description} -@end quotation @table @var @item word -The name of the word. BTW, GNU Forth is case insensitive, so you can -type the words in in lower case. +The name of the word. BTW, Gforth is case insensitive, so you can +type the words in in lower case (However, @pxref{core-idef}). @item Stack effect The stack effect is written in the notation @code{@var{before} -- @var{after}}, where @var{before} and @var{after} describe the top of stack entries before and after the execution of the word. The rest of the stack is not touched by the word. The top of stack is rightmost, -i.e., a stack sequence is written as it is typed in. Note that GNU Forth +i.e., a stack sequence is written as it is typed in. Note that Gforth uses a separate floating point stack, but a unified stack notation. Also, return stack effects are not shown in @var{stack effect}, but in @var{Description}. The name of a stack item describes the type and/or the function of the item. See below for a discussion of the types. +All words have two stack effects: A compile-time stack effect and a +run-time stack effect. The compile-time stack-effect of most words is +@var{ -- }. If the compile-time stack-effect of a word deviates from +this standard behaviour, or the word does other unusual things at +compile time, both stack effects are shown; otherwise only the run-time +stack effect is shown. + @item pronunciation How the word is pronounced @@ -308,17 +726,20 @@ system need not support all of them. So, uses the more portable it will be in theory. However, we suspect that most ANS Forth systems on personal machines will feature all wordsets. Words that are not defined in the ANS standard have -@code{gforth} as wordset. +@code{gforth} or @code{gforth-internal} as wordset. @code{gforth} +describes words that will work in future releases of Gforth; +@code{gforth-internal} words are more volatile. Environmental query +strings are also displayed like words; you can recognize them by the +@code{environment} in the wordset field. @item Description A description of the behaviour of the word. @end table -The name of a stack item corresponds in the following way with its type: +The type of a stack item is specified by the character(s) the name +starts with: @table @code -@item name starts with -Type @item f Bool, i.e. @code{false} or @code{true}. @item c @@ -334,11 +755,11 @@ double sized signed integer @item ud double sized unsigned integer @item r -Float +Float (on the FP stack) @item a_ Cell-aligned address @item c_ -Char-aligned address (note that a Char is two bytes in Windows NT) +Char-aligned address (note that a Char may have two bytes in Windows NT) @item f_ Float-aligned address @item df_ @@ -351,9 +772,14 @@ Execution token, same size as Cell Wordlist ID, same size as Cell @item f83name Pointer to a name structure +@item " +string in the input stream (not the stack). The terminating character is +a blank by default. If it is not a blank, it is shown in @code{<>} +quotes. + @end table -@node Arithmetic, , Notation, Words +@node Arithmetic, Stack Manipulation, Notation, Words @section Arithmetic Forth arithmetic is not checked, i.e., you will not hear about integer overflow on addition or multiplication, you may hear about division by @@ -363,8 +789,17 @@ corresponds to @code{2 1 -}. Forth offer operators. If you perform division with potentially negative operands, you do not want to use @code{/} or @code{/mod} with its undefined behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the -former). +former, @pxref{Mixed precision}). + +@menu +* Single precision:: +* Bitwise operations:: +* Mixed precision:: operations with single and double-cell integers +* Double precision:: Double-cell integer arithmetic +* Floating Point:: +@end menu +@node Single precision, Bitwise operations, Arithmetic, Arithmetic @subsection Single precision doc-+ doc-- @@ -377,6 +812,7 @@ doc-abs doc-min doc-max +@node Bitwise operations, Mixed precision, Single precision, Arithmetic @subsection Bitwise operations doc-and doc-or @@ -385,6 +821,7 @@ doc-invert doc-2* doc-2/ +@node Mixed precision, Double precision, Bitwise operations, Arithmetic @subsection Mixed precision doc-m+ doc-*/ @@ -396,7 +833,13 @@ doc-um/mod doc-fm/mod doc-sm/rem +@node Double precision, Floating Point, Mixed precision, Arithmetic @subsection Double precision + +The outer (aka text) interpreter converts numbers containing a dot into +a double precision number. Note that only numbers with the dot as last +character are standard-conforming. + doc-d+ doc-d- doc-dnegate @@ -404,10 +847,72 @@ doc-dabs doc-dmin doc-dmax -@node Stack Manipulation,,, +@node Floating Point, , Double precision, Arithmetic +@subsection Floating Point + +The format of floating point numbers recognized by the outer (aka text) +interpreter is: a signed decimal number, possibly containing a decimal +point (@code{.}), followed by @code{E} or @code{e}, optionally followed +by a signed integer (the exponent). E.g., @code{1e} ist the same as +@code{+1.0e+0}. Note that a number without @code{e} +is not interpreted as floating-point number, but as double (if the +number contains a @code{.}) or single precision integer. Also, +conversions between string and floating point numbers always use base +10, irrespective of the value of @code{BASE}. If @code{BASE} contains a +value greater then 14, the @code{E} may be interpreted as digit and the +number will be interpreted as integer, unless it has a signed exponent +(both @code{+} and @code{-} are allowed as signs). + +Angles in floating point operations are given in radians (a full circle +has 2 pi radians). Note, that Gforth has a separate floating point +stack, but we use the unified notation. + +Floating point numbers have a number of unpleasant surprises for the +unwary (e.g., floating point addition is not associative) and even a few +for the wary. You should not use them unless you know what you are doing +or you don't care that the results you get are totally bogus. If you +want to learn about the problems of floating point numbers (and how to +avoid them), you might start with @cite{David Goldberg, What Every +Computer Scientist Should Know About Floating-Point Arithmetic, ACM +Computing Surveys 23(1):5@minus{}48, March 1991}. + +doc-f+ +doc-f- +doc-f* +doc-f/ +doc-fnegate +doc-fabs +doc-fmax +doc-fmin +doc-floor +doc-fround +doc-f** +doc-fsqrt +doc-fexp +doc-fexpm1 +doc-fln +doc-flnp1 +doc-flog +doc-falog +doc-fsin +doc-fcos +doc-fsincos +doc-ftan +doc-fasin +doc-facos +doc-fatan +doc-fatan2 +doc-fsinh +doc-fcosh +doc-ftanh +doc-fasinh +doc-facosh +doc-fatanh + +@node Stack Manipulation, Memory access, Arithmetic, Words @section Stack Manipulation -gforth has a data stack (aka parameter stack) for characters, cells, +Gforth has a data stack (aka parameter stack) for characters, cells, addresses, and double cells, a floating point stack for floating point numbers, a return stack for storing the return addresses of colon definitions and other data, and a locals stack for storing local @@ -417,13 +922,25 @@ theoretically keep floating point number additional difficulty, you don't know how many cells a floating point number takes. It is reportedly possible to write words in a way that they work also for a unified stack model, but we do not recommend trying -it. Also, a Forth system is allowed to keep the local variables on the +it. Instead, just say that your program has an environmental dependency +on a separate FP stack. + +Also, a Forth system is allowed to keep the local variables on the return stack. This is reasonable, as local variables usually eliminate the need to use the return stack explicitly. So, if you want to produce a standard complying program and if you are using local variables in a word, forget about return stack manipulations in that word (see the standard document for the exact rules). +@menu +* Data stack:: +* Floating point stack:: +* Return stack:: +* Locals stack:: +* Stack pointer manipulation:: +@end menu + +@node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation @subsection Data stack doc-drop doc-nip @@ -444,6 +961,7 @@ doc-2tuck doc-2swap doc-2rot +@node Floating point stack, Return stack, Data stack, Stack Manipulation @subsection Floating point stack doc-fdrop doc-fnip @@ -453,6 +971,7 @@ doc-ftuck doc-fswap doc-frot +@node Return stack, Locals stack, Floating point stack, Stack Manipulation @subsection Return stack doc->r doc-r> @@ -463,8 +982,10 @@ doc-2r> doc-2r@ doc-2rdrop +@node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation @subsection Locals stack +@node Stack pointer manipulation, , Locals stack, Stack Manipulation @subsection Stack pointer manipulation doc-sp@ doc-sp! @@ -475,9 +996,16 @@ doc-rp! doc-lp@ doc-lp! -@node Memory access +@node Memory access, Control Structures, Stack Manipulation, Words @section Memory access +@menu +* Stack-Memory transfers:: +* Address arithmetic:: +* Memory block access:: +@end menu + +@node Stack-Memory transfers, Address arithmetic, Memory access, Memory access @subsection Stack-Memory transfers doc-@ @@ -494,6 +1022,7 @@ doc-sf! doc-df@ doc-df! +@node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access @subsection Address arithmetic ANS Forth does not specify the sizes of the data types. Instead, it @@ -509,7 +1038,7 @@ must only occur at specific addresses; e accessed at addresses divisible by 4. Even if a machine allows unaligned accesses, it can usually perform aligned accesses faster. -For the performance-concious: alignment operations are usually only +For the performance-conscious: alignment operations are usually only necessary during the definition of a data structure, not during the (more frequent) accesses to it. @@ -519,9 +1048,12 @@ char-aligned have no use in the standard created. The standard guarantees that addresses returned by @code{CREATE}d words -are cell-aligned; in addition, gforth guarantees that these addresses +are cell-aligned; in addition, Gforth guarantees that these addresses are aligned for all purposes. +Note that the standard defines a word @code{char}, which has nothing to +do with address arithmetic. + doc-chars doc-char+ doc-cells @@ -540,8 +1072,13 @@ doc-dfloats doc-dfloat+ doc-dfalign doc-dfaligned +doc-maxalign +doc-maxaligned +doc-cfalign +doc-cfaligned doc-address-unit-bits +@node Memory block access, , Address arithmetic, Memory access @subsection Memory block access doc-move @@ -555,7 +1092,7 @@ doc-cmove> doc-fill doc-blank -@node Control Structures +@node Control Structures, Locals, Memory access, Words @section Control Structures Control structures in Forth cannot be used in interpret state, only in @@ -563,6 +1100,16 @@ compile state, i.e., in a colon definiti limitation, but have not seen a satisfying way around it yet, although many schemes have been proposed. +@menu +* Selection:: +* Simple Loops:: +* Counted Loops:: +* Arbitrary control structures:: +* Calls and returns:: +* Exception Handling:: +@end menu + +@node Selection, Simple Loops, Control Structures, Control Structures @subsection Selection @example @@ -581,7 +1128,7 @@ ELSE ENDIF @end example -You can use @code{THEN} instead of {ENDIF}. Indeed, @code{THEN} is +You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is standard, and @code{ENDIF} is not, although it is quite popular. We recommend using @code{ENDIF}, because it is less confusing for people who also know other languages (and is not prone to reinforcing negative @@ -600,15 +1147,18 @@ system that only supplies @code{THEN} is Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal and many other programming languages has the meaning 3d.] -We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you -can avoid using @code{?dup}. +Gforth also provides the words @code{?dup-if} and @code{?dup-0=-if}, so +you can avoid using @code{?dup}. Using these alternatives is also more +efficient than using @code{?dup}. Definitions in plain standard Forth +for @code{ENDIF}, @code{?DUP-IF} and @code{?DUP-0=-IF} are provided in +@file{compat/control.fs}. @example @var{n} CASE @var{n1} OF @var{code1} ENDOF @var{n2} OF @var{code2} ENDOF - @dots + @dots{} ENDCASE @end example @@ -617,6 +1167,7 @@ Executes the first @var{codei}, where th the last @code{ENDOF}. It may use @var{n}, which is on top of the stack, but must not consume it. +@node Simple Loops, Counted Loops, Selection, Control Structures @subsection Simple Loops @example @@ -648,6 +1199,7 @@ AGAIN This is an endless loop. +@node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures @subsection Counted Loops The basic counted loop is: @@ -685,13 +1237,21 @@ There are several variations on the coun @code{LEAVE} leaves the innermost counted loop immediately. +If @var{start} is greater than @var{limit}, a @code{?DO} loop is entered +(and @code{LOOP} iterates until they become equal by wrap-around +arithmetic). This behaviour is usually not what you want. Therefore, +Gforth offers @code{+DO} and @code{U+DO} (as replacements for +@code{?DO}), which do not enter the loop if @var{start} is greater than +@var{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for +unsigned loop parameters. + @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the index by @var{n} instead of by 1. The loop is terminated when the border between @var{limit-1} and @var{limit} is crossed. E.g.: -@code{4 0 ?DO i . 2 +LOOP} prints @code{0 2} +@code{4 0 +DO i . 2 +LOOP} prints @code{0 2} -@code{4 1 ?DO i . 2 +LOOP} prints @code{1 3} +@code{4 1 +DO i . 2 +LOOP} prints @code{1 3} The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative: @@ -699,23 +1259,28 @@ The behaviour of @code{@var{n} +LOOP} is @code{ 0 0 ?DO i . -1 +LOOP} prints nothing -Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative -@var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative -case behaves symmetrical to the positive case: - -@code{-2 0 ?DO i . -1 +LOOP} prints @code{0 -1} - -@code{-1 0 ?DO i . -1 +LOOP} prints @code{0} - -@code{ 0 0 ?DO i . -1 +LOOP} prints nothing - -The loop is terminated when the border between @var{limit@minus{}sgn(n)} and -@var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS -Forth standard. - -@code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even -when the start and the limit value are equal. We do not recommend using -@code{DO}. It will just give you maintenance troubles. +Therefore we recommend avoiding @code{@var{n} +LOOP} with negative +@var{n}. One alternative is @code{@var{u} -LOOP}, which reduces the +index by @var{u} each iteration. The loop is terminated when the border +between @var{limit+1} and @var{limit} is crossed. Gforth also provides +@code{-DO} and @code{U-DO} for down-counting loops. E.g.: + +@code{-2 0 -DO i . 1 -LOOP} prints @code{0 -1} + +@code{-1 0 -DO i . 1 -LOOP} prints @code{0} + +@code{ 0 0 -DO i . 1 -LOOP} prints nothing + +Unfortunately, @code{+DO}, @code{U+DO}, @code{-DO}, @code{U-DO} and +@code{-LOOP} are not in the ANS Forth standard. However, an +implementation for these words that uses only standard words is provided +in @file{compat/loops.fs}. + +@code{?DO} can also be replaced by @code{DO}. @code{DO} always enters +the loop, independent of the loop parameters. Do not use @code{DO}, even +if you know that the loop is entered in any case. Such knowledge tends +to become invalid during maintenance of a program, and then the +@code{DO} will make trouble. @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the @@ -729,33 +1294,35 @@ FOR NEXT @end example This is the preferred loop of native code compiler writers who are too -lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop +lazy to optimize @code{?DO} loops properly. In Gforth, this loop iterates @var{n+1} times; @code{i} produces values starting with @var{n} and ending with 0. Other Forth systems may behave differently, even if -they support @code{FOR} loops. +they support @code{FOR} loops. To avoid problems, don't use @code{FOR} +loops. +@node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures @subsection Arbitrary control structures ANS Forth permits and supports using control structures in a non-nested way. Information about incomplete control structures is stored on the control-flow stack. This stack may be implemented on the Forth data -stack, and this is what we have done in gforth. +stack, and this is what we have done in Gforth. An @i{orig} entry represents an unresolved forward branch, a @i{dest} entry represents a backward branch target. A few words are the basis for building any control structure possible (except control structures that need storage, like calls, coroutines, and backtracking). -if -ahead -then -begin -until -again -cs-pick -cs-roll +doc-if +doc-ahead +doc-then +doc-begin +doc-until +doc-again +doc-cs-pick +doc-cs-roll -On many systems control-flow stack items take one word, in gforth they +On many systems control-flow stack items take one word, in Gforth they currently take three (this may change in the future). Therefore it is a really good idea to manipulate the control flow stack with @code{cs-pick} and @code{cs-roll}, not with data stack manipulation @@ -763,42 +1330,116 @@ words. Some standard control structure words are built from these words: -else -while -repeat +doc-else +doc-while +doc-repeat + +Gforth adds some more control-structure words: + +doc-endif +doc-?dup-if +doc-?dup-0=-if Counted loop words constitute a separate group of words: -?do -do -for -loop -s+loop -+loop -next -leave -?leave -unloop -undo +doc-?do +doc-+do +doc-u+do +doc--do +doc-u-do +doc-do +doc-for +doc-loop +doc-+loop +doc--loop +doc-next +doc-leave +doc-?leave +doc-unloop +doc-done The standard does not allow using @code{cs-pick} and @code{cs-roll} on @i{do-sys}. Our system allows it, but it's your job to ensure that for every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path -through the program (@code{LOOP} etc. compile an @code{UNLOOP}). Also, -you have to ensure that all @code{LEAVE}s are resolved (by using one of -the loop-ending words or @code{UNDO}). +through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the +fall-through path). Also, you have to ensure that all @code{LEAVE}s are +resolved (by using one of the loop-ending words or @code{DONE}). Another group of control structure words are -case -endcase -of -endof +doc-case +doc-endcase +doc-of +doc-endof @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and @code{cs-roll}. -@node Locals +@subsubsection Programming Style + +In order to ensure readability we recommend that you do not create +arbitrary control structures directly, but define new control structure +words for the control structure you want and use these words in your +program. + +E.g., instead of writing + +@example +begin + ... +if [ 1 cs-roll ] + ... +again then +@end example + +we recommend defining control structure words, e.g., + +@example +: while ( dest -- orig dest ) + POSTPONE if + 1 cs-roll ; immediate + +: repeat ( orig dest -- ) + POSTPONE again + POSTPONE then ; immediate +@end example + +and then using these to create the control structure: + +@example +begin + ... +while + ... +repeat +@end example + +That's much easier to read, isn't it? Of course, @code{REPEAT} and +@code{WHILE} are predefined, so in this example it would not be +necessary to define them. + +@node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures +@subsection Calls and returns + +A definition can be called simply be writing the name of the +definition. When the end of the definition is reached, it returns. An +earlier return can be forced using + +doc-exit + +Don't forget to clean up the return stack and @code{UNLOOP} any +outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The +primitive compiled by @code{EXIT} is + +doc-;s + +@node Exception Handling, , Calls and returns, Control Structures +@subsection Exception Handling + +doc-catch +doc-throw + +@node Locals, Defining Words, Control Structures, Words @section Locals Local variables can make Forth programming more enjoyable and Forth @@ -807,10 +1448,18 @@ laden with restrictions. Therefore, we p locals wordset, but also our own, more powerful locals wordset (we implemented the ANS Forth locals wordset through our locals wordset). +The ideas in this section have also been published in the paper +@cite{Automatic Scoping of Local Variables} by M. Anton Ertl, presented +at EuroForth '94; it is available at +@*@file{http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz}. + @menu +* Gforth locals:: +* ANS Forth locals:: @end menu -@subsection gforth locals +@node Gforth locals, ANS Forth locals, Locals, Locals +@subsection Gforth locals Locals can be defined with @@ -853,7 +1502,7 @@ The name of the local may be preceded by Ar Bi f* Ai Br f* f+ ; @end example -GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles +Gforth currently supports cells (@code{W:}, @code{W^}), doubles (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined with @code{W:}, @code{D:} etc.) produces its value and can be changed @@ -873,8 +1522,17 @@ locals are initialized with values from Currently there is no way to define locals with user-defined data structures, but we are working on it. -GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions: +Gforth allows defining locals everywhere in a colon definition. This +poses the following questions: + +@menu +* Where are locals visible by name?:: +* How long do locals live?:: +* Programming Style:: +* Implementation:: +@end menu +@node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals @subsubsection Where are locals visible by name? Basically, the answer is that locals are visible where you would expect @@ -923,17 +1581,17 @@ says. If @code{UNREACHABLE} is used wher lie to the compiler), buggy code will be produced. Another problem with this rule is that at @code{BEGIN}, the compiler -does not know which locals will be visible on the incoming back-edge -. All problems discussed in the following are due to this ignorance of -the compiler (we discuss the problems using @code{BEGIN} loops as -examples; the discussion also applies to @code{?DO} and other +does not know which locals will be visible on the incoming +back-edge. All problems discussed in the following are due to this +ignorance of the compiler (we discuss the problems using @code{BEGIN} +loops as examples; the discussion also applies to @code{?DO} and other loops). Perhaps the most insidious example is: @example AHEAD BEGIN x [ 1 CS-ROLL ] THEN - { x } + @{ x @} ... UNTIL @end example @@ -965,7 +1623,7 @@ compiler. When the branch to the @code{B warns the user if it was too optimisitic: @example IF - { x } + @{ x @} BEGIN \ x ? [ 1 cs-roll ] THEN @@ -981,7 +1639,7 @@ is not used in the wrong area by using e @example IF SCOPE - { x } + @{ x @} ENDSCOPE BEGIN [ 1 cs-roll ] THEN @@ -996,15 +1654,16 @@ If the @code{BEGIN} is not reachable fro @code{AHEAD} or @code{EXIT}), the compiler cannot even make an optimistic guess, as the locals visible after the @code{BEGIN} may be defined later. Therefore, the compiler assumes that no locals are -visible after the @code{BEGIN}. However, the useer can use +visible after the @code{BEGIN}. However, the user can use @code{ASSUME-LIVE} to make the compiler assume that the same locals are -visible at the BEGIN as at the point where the item was created. +visible at the BEGIN as at the point where the top control-flow stack +item was created. doc-assume-live E.g., @example -{ x } +@{ x @} AHEAD ASSUME-LIVE BEGIN @@ -1025,13 +1684,14 @@ rearranging the loop. E.g., the ``most i arranged into: @example BEGIN - { x } + @{ x @} ... 0= WHILE x REPEAT @end example +@node How long do locals live?, Programming Style, Where are locals visible by name?, Gforth locals @subsubsection How long do locals live? The right answer for the lifetime question would be: A local lives at @@ -1045,6 +1705,7 @@ languages (e.g., C): The local lives onl afterwards its address is invalid (and programs that access it afterwards are erroneous). +@node Programming Style, Implementation, How long do locals live?, Gforth locals @subsubsection Programming Style The freedom to define locals anywhere has the potential to change @@ -1056,10 +1717,10 @@ wrong order, just write a locals definit write the items in the order you want. This seems a little far-fetched and eliminating stack manipulations is -unlikely to become a conscious programming objective. Still, the -number of stack manipulations will be reduced dramatically if local -variables are used liberally (e.g., compare @code{max} in \sect{misc} -with a traditional implementation of @code{max}). +unlikely to become a conscious programming objective. Still, the number +of stack manipulations will be reduced dramatically if local variables +are used liberally (e.g., compare @code{max} in @ref{Gforth locals} with +a traditional implementation of @code{max}). This shows one potential benefit of locals: making Forth programs more readable. Of course, this benefit will only be realized if the @@ -1078,8 +1739,8 @@ E.g., a definition using @code{TO} might : strcmp @{ addr1 u1 addr2 u2 -- n @} u1 u2 min 0 ?do - addr1 c@ addr2 c@ - ?dup - if + addr1 c@@ addr2 c@@ - + ?dup-if unloop exit then addr1 char+ TO addr1 @@ -1101,8 +1762,8 @@ are initialized with the right value for addr1 addr2 u1 u2 min 0 ?do @{ s1 s2 @} - s1 c@ s2 c@ - ?dup - if + s1 c@@ s2 c@@ - + ?dup-if unloop exit then s1 char+ s2 char+ @@ -1113,9 +1774,10 @@ are initialized with the right value for Here it is clear from the start that @code{s1} has a different value in every loop iteration. +@node Implementation, , Programming Style, Gforth locals @subsubsection Implementation -GNU Forth uses an extra locals stack. The most compelling reason for +Gforth uses an extra locals stack. The most compelling reason for this is that the return stack is not float-aligned; using an extra stack also eliminates the problems and restrictions of using the return stack as locals stack. Like the other stacks, the locals stack grows toward @@ -1136,8 +1798,8 @@ efficiency reasons, e.g., @code{@@local0 compile the right specialized version, or the general version, as appropriate: -doc-compile-@@local -doc-compile-f@@local +doc-compile-@local +doc-compile-f@local doc-compile-lp+! Combinations of conditional branches and @code{lp+!#} like @@ -1150,10 +1812,10 @@ area and @code{@}} switches it back and initializing code. @code{W:} etc.@ are normal defining words. This special area is cleared at the start of every colon definition. -A special feature of GNU Forths dictionary is used to implement the +A special feature of Gforth's dictionary is used to implement the definition of locals without type specifiers: every wordlist (aka vocabulary) has its own methods for searching -etc. (@xref{dictionary}). For the present purpose we defined a wordlist +etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist with a special search method: When it is searched for a word, it actually creates that word using @code{W:}. @code{@{} changes the search order to first search the wordlist containing @code{@}}, @code{W:} etc., @@ -1190,7 +1852,7 @@ The locals stack pointer is only adjuste @code{lp+!#} orig-locals-size @minus{} new-locals-size @end format The second @code{lp+!#} adjusts the locals stack pointer from the -level at the {\em orig} point to the level after the @code{THEN}. The +level at the @var{orig} point to the level after the @code{THEN}. The first @code{lp+!#} adjusts the locals stack pointer from the current level to the level at the orig point, so the complete effect is an adjustment from the current level to the right level after the @@ -1240,11 +1902,12 @@ this may lead to increased space needs f usually less than reclaiming this space would cost in code size. +@node ANS Forth locals, , Gforth locals, Locals @subsection ANS Forth locals The ANS Forth locals wordset does not define a syntax for locals, but words that make it possible to define various syntaxes. One of the -possible syntaxes is a subset of the syntax we used in the gforth locals +possible syntaxes is a subset of the syntax we used in the Gforth locals wordset, i.e.: @example @@ -1260,27 +1923,27 @@ restrictions are: @itemize @bullet @item -Locals can only be cell-sized values (no type specifers are allowed). +Locals can only be cell-sized values (no type specifiers are allowed). @item Locals can be defined only outside control structures. @item Locals can interfere with explicit usage of the return stack. For the exact (and long) rules, see the standard. If you don't use return stack -accessing words in a definition using locals, you will we all right. The +accessing words in a definition using locals, you will be all right. The purpose of this rule is to make locals implementation on the return stack easier. @item The whole definition must be in one line. @end itemize -Locals defined in this way behave like @code{VALUE}s -(@xref{values}). I.e., they are initialized from the stack. Using their +Locals defined in this way behave like @code{VALUE}s (@xref{Simple +Defining Words}). I.e., they are initialized from the stack. Using their name produces their value. Their value can be changed using @code{TO}. -Since this syntax is supported by gforth directly, you need not do +Since this syntax is supported by Gforth directly, you need not do anything to use it. If you want to port a program using this syntax to -another ANS Forth system, use @file{anslocal.fs} to implement the syntax -on the other system. +another ANS Forth system, use @file{compat/anslocal.fs} to implement the +syntax on the other system. Note that a syntax shown in the standard, section A.13 looks similar, but is quite different in having the order of locals @@ -1292,13 +1955,2325 @@ doc-(local) The ANS Forth locals extension wordset defines a syntax, but it is so awful that we strongly recommend not to use it. We have implemented this -syntax to make porting to gforth easy, but do not document it here. The +syntax to make porting to Gforth easy, but do not document it here. The problem with this syntax is that the locals are defined in an order reversed with respect to the standard stack comment notation, making programs harder to read, and easier to misread and miswrite. The only merit of this syntax is that it is easy to implement using the ANS Forth locals wordset. +@node Defining Words, Wordlists, Locals, Words +@section Defining Words + +@menu +* Simple Defining Words:: +* Colon Definitions:: +* User-defined Defining Words:: +* Supplying names:: +* Interpretation and Compilation Semantics:: +@end menu + +@node Simple Defining Words, Colon Definitions, Defining Words, Defining Words +@subsection Simple Defining Words + +doc-constant +doc-2constant +doc-fconstant +doc-variable +doc-2variable +doc-fvariable +doc-create +doc-user +doc-value +doc-to +doc-defer +doc-is + +@node Colon Definitions, User-defined Defining Words, Simple Defining Words, Defining Words +@subsection Colon Definitions + +@example +: name ( ... -- ... ) + word1 word2 word3 ; +@end example + +creates a word called @code{name}, that, upon execution, executes +@code{word1 word2 word3}. @code{name} is a @dfn{(colon) definition}. + +The explanation above is somewhat superficial. @xref{Interpretation and +Compilation Semantics} for an in-depth discussion of some of the issues +involved. + +doc-: +doc-; + +@node User-defined Defining Words, Supplying names, Colon Definitions, Defining Words +@subsection User-defined Defining Words + +You can create new defining words simply by wrapping defining-time code +around existing defining words and putting the sequence in a colon +definition. + +If you want the words defined with your defining words to behave +differently from words defined with standard defining words, you can +write your defining word like this: + +@example +: def-word ( "name" -- ) + Create @var{code1} +DOES> ( ... -- ... ) + @var{code2} ; + +def-word name +@end example + +Technically, this fragment defines a defining word @code{def-word}, and +a word @code{name}; when you execute @code{name}, the address of the +body of @code{name} is put on the data stack and @var{code2} is executed +(the address of the body of @code{name} is the address @code{HERE} +returns immediately after the @code{CREATE}). + +In other words, if you make the following definitions: + +@example +: def-word1 ( "name" -- ) + Create @var{code1} ; + +: action1 ( ... -- ... ) + @var{code2} ; + +def-word name1 +@end example + +Using @code{name1 action1} is equivalent to using @code{name}. + +E.g., you can implement @code{Constant} in this way: + +@example +: constant ( w "name" -- ) + create , +DOES> ( -- w ) + @@ ; +@end example + +When you create a constant with @code{5 constant five}, first a new word +@code{five} is created, then the value 5 is laid down in the body of +@code{five} with @code{,}. When @code{five} is invoked, the address of +the body is put on the stack, and @code{@@} retrieves the value 5. + +In the example above the stack comment after the @code{DOES>} specifies +the stack effect of the defined words, not the stack effect of the +following code (the following code expects the address of the body on +the top of stack, which is not reflected in the stack comment). This is +the convention that I use and recommend (it clashes a bit with using +locals declarations for stack effect specification, though). + +@subsubsection Applications of @code{CREATE..DOES>} + +You may wonder how to use this feature. Here are some usage patterns: + +When you see a sequence of code occurring several times, and you can +identify a meaning, you will factor it out as a colon definition. When +you see similar colon definitions, you can factor them using +@code{CREATE..DOES>}. E.g., an assembler usually defines several words +that look very similar: +@example +: ori, ( reg-taget reg-source n -- ) + 0 asm-reg-reg-imm ; +: andi, ( reg-taget reg-source n -- ) + 1 asm-reg-reg-imm ; +@end example + +This could be factored with: +@example +: reg-reg-imm ( op-code -- ) + create , +DOES> ( reg-taget reg-source n -- ) + @@ asm-reg-reg-imm ; + +0 reg-reg-imm ori, +1 reg-reg-imm andi, +@end example + +Another view of @code{CREATE..DOES>} is to consider it as a crude way to +supply a part of the parameters for a word (known as @dfn{currying} in +the functional language community). E.g., @code{+} needs two +parameters. Creating versions of @code{+} with one parameter fixed can +be done like this: +@example +: curry+ ( n1 -- ) + create , +DOES> ( n2 -- n1+n2 ) + @@ + ; + + 3 curry+ 3+ +-2 curry+ 2- +@end example + +@subsubsection The gory details of @code{CREATE..DOES>} + +doc-does> + +This means that you need not use @code{CREATE} and @code{DOES>} in the +same definition; E.g., you can put the @code{DOES>}-part in a separate +definition. This allows us to, e.g., select among different DOES>-parts: +@example +: does1 +DOES> ( ... -- ... ) + ... ; + +: does2 +DOES> ( ... -- ... ) + ... ; + +: def-word ( ... -- ... ) + create ... + IF + does1 + ELSE + does2 + ENDIF ; +@end example + +In a standard program you can apply a @code{DOES>}-part only if the last +word was defined with @code{CREATE}. In Gforth, the @code{DOES>}-part +will override the behaviour of the last word defined in any case. In a +standard program, you can use @code{DOES>} only in a colon +definition. In Gforth, you can also use it in interpretation state, in a +kind of one-shot mode: +@example +CREATE name ( ... -- ... ) + @var{initialization} +DOES> + @var{code} ; +@end example +This is equivalwent to the standard +@example +:noname +DOES> + @var{code} ; +CREATE name EXECUTE ( ... -- ... ) + @var{initialization} +@end example + +You can get the address of the body of a word with + +doc->body + +@node Supplying names, Interpretation and Compilation Semantics, User-defined Defining Words, Defining Words +@subsection Supplying names for the defined words + +By default, defining words take the names for the defined words from the +input stream. Sometimes you want to supply the name from a string. You +can do this with + +doc-nextname + +E.g., + +@example +s" foo" nextname create +@end example +is equivalent to +@example +create foo +@end example + +Sometimes you want to define a word without a name. You can do this with + +doc-noname + +To make any use of the newly defined word, you need its execution +token. You can get it with + +doc-lastxt + +E.g., you can initialize a deferred word with an anonymous colon +definition: +@example +Defer deferred +noname : ( ... -- ... ) + ... ; +lastxt IS deferred +@end example + +@code{lastxt} also works when the last word was not defined as +@code{noname}. + +The standard has also recognized the need for anonymous words and +provides + +doc-:noname + +This leaves the execution token for the word on the stack after the +closing @code{;}. You can rewrite the last example with @code{:noname}: +@example +Defer deferred +:noname ( ... -- ... ) + ... ; +IS deferred +@end example + +@node Interpretation and Compilation Semantics, , Supplying names, Defining Words +@subsection Interpretation and Compilation Semantics + +The @dfn{interpretation semantics} of a word are what the text +interpreter does when it encounters the word in interpret state. It also +appears in some other contexts, e.g., the execution token returned by +@code{' @var{word}} identifies the interpretation semantics of +@var{word} (in other words, @code{' @var{word} execute} is equivalent to +interpret-state text interpretation of @code{@var{word}}). + +The @dfn{compilation semantics} of a word are what the text interpreter +does when it encounters the word in compile state. It also appears in +other contexts, e.g, @code{POSTPONE @var{word}} compiles@footnote{In +standard terminology, ``appends to the current definition''.} the +compilation semantics of @var{word}. + +The standard also talks about @dfn{execution semantics}. They are used +only for defining the interpretation and compilation semantics of many +words. By default, the interpretation semantics of a word are to +@code{execute} its execution semantics, and the compilation semantics of +a word are to @code{compile,} its execution semantics.@footnote{In +standard terminology: The default interpretation semantics are its +execution semantics; the default compilation semantics are to append its +execution semantics to the execution semantics of the current +definition.} + +You can change the compilation semantics into @code{execute}ing the +execution semantics with + +doc-immediate + +You can remove the interpretation semantics of a word with + +doc-compile-only +doc-restrict + +Note that ticking (@code{'}) compile-only words gives an error +(``Interpreting a compile-only word''). + +Gforth also allows you to define words with arbitrary combinations of +interpretation and compilation semantics. + +doc-interpret/compile: + +This feature was introduced for implementing @code{TO} and @code{S"}. I +recommend that you do not define such words, as cute as they may be: +they make it hard to get at both parts of the word in some contexts. +E.g., assume you want to get an execution token for the compilation +part. Instead, define two words, one that embodies the interpretation +part, and one that embodies the compilation part. + +There is, however, a potentially useful application of this feature: +Providing differing implementations for the default semantics. While +this introduces redundancy and is therefore usually a bad idea, a +performance improvement may be worth the trouble. E.g., consider the +word @code{foobar}: + +@example +: foobar + foo bar ; +@end example + +Let us assume that @code{foobar} is called so frequently that the +calling overhead would take a significant amount of the run-time. We can +optimize it with @code{interpret/compile:}: + +@example +:noname + foo bar ; +:noname + POSTPONE foo POSTPONE bar ; +interpret/compile: foobar +@end example + +This definition has the same interpretation semantics and essentially +the same compilation semantics as the simple definition of +@code{foobar}, but the implementation of the compilation semantics is +more efficient with respect to run-time. + +Some people try to use state-smart words to emulate the feature provided +by @code{interpret/compile:} (words are state-smart if they check +@code{STATE} during execution). E.g., they would try to code +@code{foobar} like this: + +@example +: foobar + STATE @@ + IF ( compilation state ) + POSTPONE foo POSTPONE bar + ELSE + foo bar + ENDIF ; immediate +@end example + +While this works if @code{foobar} is processed only by the text +interpreter, it does not work in other contexts (like @code{'} or +@code{POSTPONE}). E.g., @code{' foobar} will produce an execution token +for a state-smart word, not for the interpretation semantics of the +original @code{foobar}; when you execute this execution token (directly +with @code{EXECUTE} or indirectly through @code{COMPILE,}) in compile +state, the result will not be what you expected (i.e., it will not +perform @code{foo bar}). State-smart words are a bad idea. Simply don't +write them! + +It is also possible to write defining words that define words with +arbitrary combinations of interpretation and compilation semantics (or, +preferably, arbitrary combinations of implementations of the default +semantics). In general, this looks like: + +@example +: def-word + create-interpret/compile + @var{code1} +interpretation> + @var{code2} + + @var{code3} + ( -- n ) + @@ + ( compilation. -- ; run-time. -- n ) + @@ postpone literal + +doc- +doc-body} also gives you the body of a word created with +@code{create-interpret/compile}. + +@node Wordlists, Files, Defining Words, Words +@section Wordlists + +@node Files, Blocks, Wordlists, Words +@section Files + +@node Blocks, Other I/O, Files, Words +@section Blocks + +@node Other I/O, Programming Tools, Blocks, Words +@section Other I/O + +@node Programming Tools, Assembler and Code words, Other I/O, Words +@section Programming Tools + +@menu +* Debugging:: Simple and quick. +* Assertions:: Making your programs self-checking. +@end menu + +@node Debugging, Assertions, Programming Tools, Programming Tools +@subsection Debugging + +The simple debugging aids provided in @file{debugging.fs} +are meant to support a different style of debugging than the +tracing/stepping debuggers used in languages with long turn-around +times. + +A much better (faster) way in fast-compilig languages is to add +printing code at well-selected places, let the program run, look at +the output, see where things went wrong, add more printing code, etc., +until the bug is found. + +The word @code{~~} is easy to insert. It just prints debugging +information (by default the source location and the stack contents). It +is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to +query-replace them with nothing). The deferred words +@code{printdebugdata} and @code{printdebugline} control the output of +@code{~~}. The default source location output format works well with +Emacs' compilation mode, so you can step through the program at the +source level using @kbd{C-x `} (the advantage over a stepping debugger +is that you can step in any direction and you know where the crash has +happened or where the strange data has occurred). + +Note that the default actions clobber the contents of the pictured +numeric output string, so you should not use @code{~~}, e.g., between +@code{<#} and @code{#>}. + +doc-~~ +doc-printdebugdata +doc-printdebugline + +@node Assertions, , Debugging, Programming Tools +@subsection Assertions + +It is a good idea to make your programs self-checking, in particular, if +you use an assumption (e.g., that a certain field of a data structure is +never zero) that may become wrong during maintenance. Gforth supports +assertions for this purpose. They are used like this: + +@example +assert( @var{flag} ) +@end example + +The code between @code{assert(} and @code{)} should compute a flag, that +should be true if everything is alright and false otherwise. It should +not change anything else on the stack. The overall stack effect of the +assertion is @code{( -- )}. E.g. + +@example +assert( 1 1 + 2 = ) \ what we learn in school +assert( dup 0<> ) \ assert that the top of stack is not zero +assert( false ) \ this code should not be reached +@end example + +The need for assertions is different at different times. During +debugging, we want more checking, in production we sometimes care more +for speed. Therefore, assertions can be turned off, i.e., the assertion +becomes a comment. Depending on the importance of an assertion and the +time it takes to check it, you may want to turn off some assertions and +keep others turned on. Gforth provides several levels of assertions for +this purpose: + +doc-assert0( +doc-assert1( +doc-assert2( +doc-assert3( +doc-assert( +doc-) + +@code{Assert(} is the same as @code{assert1(}. The variable +@code{assert-level} specifies the highest assertions that are turned +on. I.e., at the default @code{assert-level} of one, @code{assert0(} and +@code{assert1(} assertions perform checking, while @code{assert2(} and +@code{assert3(} assertions are treated as comments. + +Note that the @code{assert-level} is evaluated at compile-time, not at +run-time. I.e., you cannot turn assertions on or off at run-time, you +have to set the @code{assert-level} appropriately before compiling a +piece of code. You can compile several pieces of code at several +@code{assert-level}s (e.g., a trusted library at level 1 and newly +written code at level 3). + +doc-assert-level + +If an assertion fails, a message compatible with Emacs' compilation mode +is produced and the execution is aborted (currently with @code{ABORT"}. +If there is interest, we will introduce a special throw code. But if you +intend to @code{catch} a specific condition, using @code{throw} is +probably more appropriate than an assertion). + +@node Assembler and Code words, Threading Words, Programming Tools, Words +@section Assembler and Code words + +Gforth provides some words for defining primitives (words written in +machine code), and for defining the the machine-code equivalent of +@code{DOES>}-based defining words. However, the machine-independent +nature of Gforth poses a few problems: First of all. Gforth runs on +several architectures, so it can provide no standard assembler. What's +worse is that the register allocation not only depends on the processor, +but also on the @code{gcc} version and options used. + +The words that Gforth offers encapsulate some system dependences (e.g., the +header structure), so a system-independent assembler may be used in +Gforth. If you do not have an assembler, you can compile machine code +directly with @code{,} and @code{c,}. + +doc-assembler +doc-code +doc-end-code +doc-;code +doc-flush-icache + +If @code{flush-icache} does not work correctly, @code{code} words +etc. will not work (reliably), either. + +These words are rarely used. Therefore they reside in @code{code.fs}, +which is usually not loaded (except @code{flush-icache}, which is always +present). You can load them with @code{require code.fs}. + +In the assembly code you will want to refer to the inner interpreter's +registers (e.g., the data stack pointer) and you may want to use other +registers for temporary storage. Unfortunately, the register allocation +is installation-dependent. + +The easiest solution is to use explicit register declarations +(@pxref{Explicit Reg Vars, , Variables in Specified Registers, gcc.info, +GNU C Manual}) for all of the inner interpreter's registers: You have to +compile Gforth with @code{-DFORCE_REG} (configure option +@code{--enable-force-reg}) and the appropriate declarations must be +present in the @code{machine.h} file (see @code{mips.h} for an example; +you can find a full list of all declarable register symbols with +@code{grep register engine.c}). If you give explicit registers to all +variables that are declared at the beginning of @code{engine()}, you +should be able to use the other caller-saved registers for temporary +storage. Alternatively, you can use the @code{gcc} option +@code{-ffixed-REG} (@pxref{Code Gen Options, , Options for Code +Generation Conventions, gcc.info, GNU C Manual}) to reserve a register +(however, this restriction on register allocation may slow Gforth +significantly). + +If this solution is not viable (e.g., because @code{gcc} does not allow +you to explicitly declare all the registers you need), you have to find +out by looking at the code where the inner interpreter's registers +reside and which registers can be used for temporary storage. You can +get an assembly listing of the engine's code with @code{make engine.s}. + +In any case, it is good practice to abstract your assembly code from the +actual register allocation. E.g., if the data stack pointer resides in +register @code{$17}, create an alias for this register called @code{sp}, +and use that in your assembly code. + +Another option for implementing normal and defining words efficiently +is: adding the wanted functionality to the source of Gforth. For normal +words you just have to edit @file{primitives} (@pxref{Automatic +Generation}), defining words (equivalent to @code{;CODE} words, for fast +defined words) may require changes in @file{engine.c}, @file{kernal.fs}, +@file{prims2x.fs}, and possibly @file{cross.fs}. + + +@node Threading Words, , Assembler and Code words, Words +@section Threading Words + +These words provide access to code addresses and other threading stuff +in Gforth (and, possibly, other interpretive Forths). It more or less +abstracts away the differences between direct and indirect threading +(and, for direct threading, the machine dependences). However, at +present this wordset is still inclomplete. It is also pretty low-level; +some day it will hopefully be made unnecessary by an internals words set +that abstracts implementation details away completely. + +doc->code-address +doc->does-code +doc-code-address! +doc-does-code! +doc-does-handler! +doc-/does-handler + +The code addresses produced by various defining words are produced by +the following words: + +doc-docol: +doc-docon: +doc-dovar: +doc-douser: +doc-dodefer: +doc-dofield: + +You can recognize words defined by a @code{CREATE}...@code{DOES>} word +with @code{>DOES-CODE}. If the word was defined in that way, the value +returned is different from 0 and identifies the @code{DOES>} used by the +defining word. + +@node ANS conformance, Model, Words, Top +@chapter ANS conformance + +To the best of our knowledge, Gforth is an + +ANS Forth System +@itemize @bullet +@item providing the Core Extensions word set +@item providing the Block word set +@item providing the Block Extensions word set +@item providing the Double-Number word set +@item providing the Double-Number Extensions word set +@item providing the Exception word set +@item providing the Exception Extensions word set +@item providing the Facility word set +@item providing @code{MS} and @code{TIME&DATE} from the Facility Extensions word set +@item providing the File Access word set +@item providing the File Access Extensions word set +@item providing the Floating-Point word set +@item providing the Floating-Point Extensions word set +@item providing the Locals word set +@item providing the Locals Extensions word set +@item providing the Memory-Allocation word set +@item providing the Memory-Allocation Extensions word set (that one's easy) +@item providing the Programming-Tools word set +@item providing @code{;CODE}, @code{AHEAD}, @code{ASSEMBLER}, @code{BYE}, @code{CODE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set +@item providing the Search-Order word set +@item providing the Search-Order Extensions word set +@item providing the String word set +@item providing the String Extensions word set (another easy one) +@end itemize + +In addition, ANS Forth systems are required to document certain +implementation choices. This chapter tries to meet these +requirements. In many cases it gives a way to ask the system for the +information instead of providing the information directly, in +particular, if the information depends on the processor, the operating +system or the installation options chosen, or if they are likely to +change during the maintenance of Gforth. + +@comment The framework for the rest has been taken from pfe. + +@menu +* The Core Words:: +* The optional Block word set:: +* The optional Double Number word set:: +* The optional Exception word set:: +* The optional Facility word set:: +* The optional File-Access word set:: +* The optional Floating-Point word set:: +* The optional Locals word set:: +* The optional Memory-Allocation word set:: +* The optional Programming-Tools word set:: +* The optional Search-Order word set:: +@end menu + + +@c ===================================================================== +@node The Core Words, The optional Block word set, ANS conformance, ANS conformance +@comment node-name, next, previous, up +@section The Core Words +@c ===================================================================== + +@menu +* core-idef:: Implementation Defined Options +* core-ambcond:: Ambiguous Conditions +* core-other:: Other System Documentation +@end menu + +@c --------------------------------------------------------------------- +@node core-idef, core-ambcond, The Core Words, The Core Words +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item (Cell) aligned addresses: +processor-dependent. Gforth's alignment words perform natural alignment +(e.g., an address aligned for a datum of size 8 is divisible by +8). Unaligned accesses usually result in a @code{-23 THROW}. + +@item @code{EMIT} and non-graphic characters: +The character is output using the C library function (actually, macro) +@code{putc}. + +@item character editing of @code{ACCEPT} and @code{EXPECT}: +This is modeled on the GNU readline library (@pxref{Readline +Interaction, , Command Line Editing, readline, The GNU Readline +Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by +producing a full word completion every time you type it (instead of +producing the common prefix of all completions). + +@item character set: +The character set of your computer and display device. Gforth is +8-bit-clean (but some other component in your system may make trouble). + +@item Character-aligned address requirements: +installation-dependent. Currently a character is represented by a C +@code{unsigned char}; in the future we might switch to @code{wchar_t} +(Comments on that requested). + +@item character-set extensions and matching of names: +Any character except the ASCII NUL charcter can be used in a +name. Matching is case-insensitive (except in @code{TABLE}s. The +matching is performed using the C function @code{strncasecmp}, whose +function is probably influenced by the locale. E.g., the @code{C} locale +does not know about accents and umlauts, so they are matched +case-sensitively in that locale. For portability reasons it is best to +write programs such that they work in the @code{C} locale. Then one can +use libraries written by a Polish programmer (who might use words +containing ISO Latin-2 encoded characters) and by a French programmer +(ISO Latin-1) in the same program (of course, @code{WORDS} will produce +funny results for some of the words (which ones, depends on the font you +are using)). Also, the locale you prefer may not be available in other +operating systems. Hopefully, Unicode will solve these problems one day. + +@item conditions under which control characters match a space delimiter: +If @code{WORD} is called with the space character as a delimiter, all +white-space characters (as identified by the C macro @code{isspace()}) +are delimiters. @code{PARSE}, on the other hand, treats space like other +delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves +like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer +interpreter (aka text interpreter) by default, treats all white-space +characters as delimiters. + +@item format of the control flow stack: +The data stack is used as control flow stack. The size of a control flow +stack item in cells is given by the constant @code{cs-item-size}. At the +time of this writing, an item consists of a (pointer to a) locals list +(third), an address in the code (second), and a tag for identifying the +item (TOS). The following tags are used: @code{defstart}, +@code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest}, +@code{scopestart}. + +@item conversion of digits > 35 +The characters @code{[\]^_'} are the digits with the decimal value +36@minus{}41. There is no way to input many of the larger digits. + +@item display after input terminates in @code{ACCEPT} and @code{EXPECT}: +The cursor is moved to the end of the entered string. If the input is +terminated using the @kbd{Return} key, a space is typed. + +@item exception abort sequence of @code{ABORT"}: +The error string is stored into the variable @code{"error} and a +@code{-2 throw} is performed. + +@item input line terminator: +For interactive input, @kbd{C-m} (CR) and @kbd{C-j} (LF) terminate +lines. One of these characters is typically produced when you type the +@kbd{Enter} or @kbd{Return} key. + +@item maximum size of a counted string: +@code{s" /counted-string" environment? drop .}. Currently 255 characters +on all ports, but this may change. + +@item maximum size of a parsed string: +Given by the constant @code{/line}. Currently 255 characters. + +@item maximum size of a definition name, in characters: +31 + +@item maximum string length for @code{ENVIRONMENT?}, in characters: +31 + +@item method of selecting the user input device: +The user input device is the standard input. There is currently no way to +change it from within Gforth. However, the input can typically be +redirected in the command line that starts Gforth. + +@item method of selecting the user output device: +@code{EMIT} and @code{TYPE} output to the file-id stored in the value +@code{outfile-id} (@code{stdout} by default). Gforth uses buffered +output, so output on a terminal does not become visible before the next +newline or buffer overflow. Output on non-terminals is invisible until +the buffer overflows. + +@item methods of dictionary compilation: +What are we expected to document here? + +@item number of bits in one address unit: +@code{s" address-units-bits" environment? drop .}. 8 in all current +ports. + +@item number representation and arithmetic: +Processor-dependent. Binary two's complement on all current ports. + +@item ranges for integer types: +Installation-dependent. Make environmental queries for @code{MAX-N}, +@code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for +unsigned (and positive) types is 0. The lower bound for signed types on +two's complement and one's complement machines machines can be computed +by adding 1 to the upper bound. + +@item read-only data space regions: +The whole Forth data space is writable. + +@item size of buffer at @code{WORD}: +@code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is +shared with the pictured numeric output string. If overwriting +@code{PAD} is acceptable, it is as large as the remaining dictionary +space, although only as much can be sensibly used as fits in a counted +string. + +@item size of one cell in address units: +@code{1 cells .}. + +@item size of one character in address units: +@code{1 chars .}. 1 on all current ports. + +@item size of the keyboard terminal buffer: +Varies. You can determine the size at a specific time using @code{lp@@ +tib - .}. It is shared with the locals stack and TIBs of files that +include the current file. You can change the amount of space for TIBs +and locals stack at Gforth startup with the command line option +@code{-l}. + +@item size of the pictured numeric output buffer: +@code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is +shared with @code{WORD}. + +@item size of the scratch area returned by @code{PAD}: +The remainder of dictionary space. You can even use the unused part of +the data stack space. The current size can be computed with @code{sp@@ +pad - .}. + +@item system case-sensitivity characteristics: +Dictionary searches are case insensitive (except in +@code{TABLE}s). However, as explained above under @i{character-set +extensions}, the matching for non-ASCII characters is determined by the +locale you are using. In the default @code{C} locale all non-ASCII +characters are matched case-sensitively. + +@item system prompt: +@code{ ok} in interpret state, @code{ compiled} in compile state. + +@item division rounding: +installation dependent. @code{s" floored" environment? drop .}. We leave +the choice to @code{gcc} (what to use for @code{/}) and to you (whether to use +@code{fm/mod}, @code{sm/rem} or simply @code{/}). + +@item values of @code{STATE} when true: +-1. + +@item values returned after arithmetic overflow: +On two's complement machines, arithmetic is performed modulo +2**bits-per-cell for single arithmetic and 4**bits-per-cell for double +arithmetic (with appropriate mapping for signed types). Division by zero +typically results in a @code{-55 throw} (Floating-point unidentified +fault), although a @code{-10 throw} (divide by zero) would be more +appropriate. + +@item whether the current definition can be found after @t{DOES>}: +No. + +@end table + +@c --------------------------------------------------------------------- +@node core-ambcond, core-other, core-idef, The Core Words +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item a name is neither a word nor a number: +@code{-13 throw} (Undefined word). Actually, @code{-13 bounce}, which +preserves the data and FP stack, so you don't lose more work than +necessary. + +@item a definition name exceeds the maximum length allowed: +@code{-19 throw} (Word name too long) + +@item addressing a region not inside the various data spaces of the forth system: +The stacks, code space and name space are accessible. Machine code space is +typically readable. Accessing other addresses gives results dependent on +the operating system. On decent systems: @code{-9 throw} (Invalid memory +address). + +@item argument type incompatible with parameter: +This is usually not caught. Some words perform checks, e.g., the control +flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type +mismatch). + +@item attempting to obtain the execution token of a word with undefined execution semantics: +@code{-14 throw} (Interpreting a compile-only word). In some cases, you +get an execution token for @code{compile-only-error} (which performs a +@code{-14 throw} when executed). + +@item dividing by zero: +typically results in a @code{-55 throw} (floating point unidentified +fault), although a @code{-10 throw} (divide by zero) would be more +appropriate. + +@item insufficient data stack or return stack space: +Not checked. This typically results in mysterious illegal memory +accesses, producing @code{-9 throw} (Invalid memory address) or +@code{-23 throw} (Address alignment exception). + +@item insufficient space for loop control parameters: +like other return stack overflows. + +@item insufficient space in the dictionary: +Not checked. Similar results as stack overflows. However, typically the +error appears at a different place when one inserts or removes code. + +@item interpreting a word with undefined interpretation semantics: +For some words, we defined interpretation semantics. For the others: +@code{-14 throw} (Interpreting a compile-only word). + +@item modifying the contents of the input buffer or a string literal: +These are located in writable memory and can be modified. + +@item overflow of the pictured numeric output string: +Not checked. + +@item parsed string overflow: +@code{PARSE} cannot overflow. @code{WORD} does not check for overflow. + +@item producing a result out of range: +On two's complement machines, arithmetic is performed modulo +2**bits-per-cell for single arithmetic and 4**bits-per-cell for double +arithmetic (with appropriate mapping for signed types). Division by zero +typically results in a @code{-55 throw} (floatingpoint unidentified +fault), although a @code{-10 throw} (divide by zero) would be more +appropriate. @code{convert} and @code{>number} currently overflow +silently. + +@item reading from an empty data or return stack: +The data stack is checked by the outer (aka text) interpreter after +every word executed. If it has underflowed, a @code{-4 throw} (Stack +underflow) is performed. Apart from that, the stacks are not checked and +underflows can result in similar behaviour as overflows (of adjacent +stacks). + +@item unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name: +@code{Create} and its descendants perform a @code{-16 throw} (Attempt to +use zero-length string as a name). Words like @code{'} probably will not +find what they search. Note that it is possible to create zero-length +names with @code{nextname} (should it not?). + +@item @code{>IN} greater than input buffer: +The next invocation of a parsing word returns a string wih length 0. + +@item @code{RECURSE} appears after @code{DOES>}: +Compiles a recursive call to the defining word, not to the defined word. + +@item argument input source different than current input source for @code{RESTORE-INPUT}: +@code{-12 THROW}. Note that, once an input file is closed (e.g., because +the end of the file was reached), its source-id may be +reused. Therefore, restoring an input source specification referencing a +closed file may lead to unpredictable results instead of a @code{-12 +THROW}. + +In the future, Gforth may be able to restore input source specifications +from other than the current input soruce. + +@item data space containing definitions gets de-allocated: +Deallocation with @code{allot} is not checked. This typically resuls in +memory access faults or execution of illegal instructions. + +@item data space read/write with incorrect alignment: +Processor-dependent. Typically results in a @code{-23 throw} (Address +alignment exception). Under Linux on a 486 or later processor with +alignment turned on, incorrect alignment results in a @code{-9 throw} +(Invalid memory address). There are reportedly some processors with +alignment restrictions that do not report them. + +@item data space pointer not properly aligned, @code{,}, @code{C,}: +Like other alignment errors. + +@item less than u+2 stack items (@code{PICK} and @code{ROLL}): +Not checked. May cause an illegal memory access. + +@item loop control parameters not available: +Not checked. The counted loop words simply assume that the top of return +stack items are loop control parameters and behave accordingly. + +@item most recent definition does not have a name (@code{IMMEDIATE}): +@code{abort" last word was headerless"}. + +@item name not defined by @code{VALUE} used by @code{TO}: +@code{-32 throw} (Invalid name argument) (unless name was defined by +@code{CONSTANT}; then it just changes the constant). + +@item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}): +@code{-13 throw} (Undefined word) + +@item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}): +Gforth behaves as if they were of the same type. I.e., you can predict +the behaviour by interpreting all parameters as, e.g., signed. + +@item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}: +Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} performs the +compilation semantics of @code{TO}. + +@item String longer than a counted string returned by @code{WORD}: +Not checked. The string will be ok, but the count will, of course, +contain only the least significant bits of the length. + +@item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}): +Processor-dependent. Typical behaviours are returning 0 and using only +the low bits of the shift count. + +@item word not defined via @code{CREATE}: +@code{>BODY} produces the PFA of the word no matter how it was defined. + +@code{DOES>} changes the execution semantics of the last defined word no +matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to +@code{CREATE , DOES>}. + +@item words improperly used outside @code{<#} and @code{#>}: +Not checked. As usual, you can expect memory faults. + +@end table + + +@c --------------------------------------------------------------------- +@node core-other, , core-ambcond, The Core Words +@subsection Other system documentation +@c --------------------------------------------------------------------- + +@table @i + +@item nonstandard words using @code{PAD}: +None. + +@item operator's terminal facilities available: +After processing the command line, Gforth goes into interactive mode, +and you can give commands to Gforth interactively. The actual facilities +available depend on how you invoke Gforth. + +@item program data space available: +@code{sp@@ here - .} gives the space remaining for dictionary and data +stack together. + +@item return stack space available: +By default 16 KBytes. The default can be overridden with the @code{-r} +switch (@pxref{Invocation}) when Gforth starts up. + +@item stack space available: +@code{sp@@ here - .} gives the space remaining for dictionary and data +stack together. + +@item system dictionary space required, in address units: +Type @code{here forthstart - .} after startup. At the time of this +writing, this gives 70108 (bytes) on a 32-bit system. +@end table + + +@c ===================================================================== +@node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance +@section The optional Block word set +@c ===================================================================== + +@menu +* block-idef:: Implementation Defined Options +* block-ambcond:: Ambiguous Conditions +* block-other:: Other System Documentation +@end menu + + +@c --------------------------------------------------------------------- +@node block-idef, block-ambcond, The optional Block word set, The optional Block word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item the format for display by @code{LIST}: +First the screen number is displayed, then 16 lines of 64 characters, +each line preceded by the line number. + +@item the length of a line affected by @code{\}: +64 characters. +@end table + + +@c --------------------------------------------------------------------- +@node block-ambcond, block-other, block-idef, The optional Block word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item correct block read was not possible: +Typically results in a @code{throw} of some OS-derived value (between +-512 and -2048). If the blocks file was just not long enough, blanks are +supplied for the missing portion. + +@item I/O exception in block transfer: +Typically results in a @code{throw} of some OS-derived value (between +-512 and -2048). + +@item invalid block number: +@code{-35 throw} (Invalid block number) + +@item a program directly alters the contents of @code{BLK}: +The input stream is switched to that other block, at the same +position. If the storing to @code{BLK} happens when interpreting +non-block input, the system will get quite confused when the block ends. + +@item no current block buffer for @code{UPDATE}: +@code{UPDATE} has no effect. + +@end table + + +@c --------------------------------------------------------------------- +@node block-other, , block-ambcond, The optional Block word set +@subsection Other system documentation +@c --------------------------------------------------------------------- + +@table @i + +@item any restrictions a multiprogramming system places on the use of buffer addresses: +No restrictions (yet). + +@item the number of blocks available for source and data: +depends on your disk space. + +@end table + + +@c ===================================================================== +@node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance +@section The optional Double Number word set +@c ===================================================================== + +@menu +* double-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node double-ambcond, , The optional Double Number word set, The optional Double Number word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item @var{d} outside of range of @var{n} in @code{D>S}: +The least significant cell of @var{d} is produced. + +@end table + + +@c ===================================================================== +@node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance +@section The optional Exception word set +@c ===================================================================== + +@menu +* exception-idef:: Implementation Defined Options +@end menu + + +@c --------------------------------------------------------------------- +@node exception-idef, , The optional Exception word set, The optional Exception word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i +@item @code{THROW}-codes used in the system: +The codes -256@minus{}-511 are used for reporting signals (see +@file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors +(for file and memory allocation operations). The mapping from OS error +numbers to throw code is -512@minus{}@var{errno}. One side effect of +this mapping is that undefined OS errors produce a message with a +strange number; e.g., @code{-1000 THROW} results in @code{Unknown error +488} on my system. +@end table + +@c ===================================================================== +@node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance +@section The optional Facility word set +@c ===================================================================== + +@menu +* facility-idef:: Implementation Defined Options +* facility-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item encoding of keyboard events (@code{EKEY}): +Not yet implemeted. + +@item duration of a system clock tick +System dependent. With respect to @code{MS}, the time is specified in +microseconds. How well the OS and the hardware implement this, is +another question. + +@item repeatability to be expected from the execution of @code{MS}: +System dependent. On Unix, a lot depends on load. If the system is +lightly loaded, and the delay is short enough that Gforth does not get +swapped out, the performance should be acceptable. Under MS-DOS and +other single-tasking systems, it should be good. + +@end table + + +@c --------------------------------------------------------------------- +@node facility-ambcond, , facility-idef, The optional Facility word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item @code{AT-XY} can't be performed on user output device: +Largely terminal dependant. No range checks are done on the arguments. +No errors are reported. You may see some garbage appearing, you may see +simply nothing happen. + +@end table + + +@c ===================================================================== +@node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance +@section The optional File-Access word set +@c ===================================================================== + +@menu +* file-idef:: Implementation Defined Options +* file-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item File access methods used: +@code{R/O}, @code{R/W} and @code{BIN} work as you would +expect. @code{W/O} translates into the C file opening mode @code{w} (or +@code{wb}): The file is cleared, if it exists, and created, if it does +not (both with @code{open-file} and @code{create-file}). Under Unix +@code{create-file} creates a file with 666 permissions modified by your +umask. + +@item file exceptions: +The file words do not raise exceptions (except, perhaps, memory access +faults when you pass illegal addresses or file-ids). + +@item file line terminator: +System-dependent. Gforth uses C's newline character as line +terminator. What the actual character code(s) of this are is +system-dependent. + +@item file name format +System dependent. Gforth just uses the file name format of your OS. + +@item information returned by @code{FILE-STATUS}: +@code{FILE-STATUS} returns the most powerful file access mode allowed +for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file +cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable +along with the retured mode. + +@item input file state after an exception when including source: +All files that are left via the exception are closed. + +@item @var{ior} values and meaning: +The @var{ior}s returned by the file and memory allocation words are +intended as throw codes. They typically are in the range +-512@minus{}-2047 of OS errors. The mapping from OS error numbers to +@var{ior}s is -512@minus{}@var{errno}. + +@item maximum depth of file input nesting: +limited by the amount of return stack, locals/TIB stack, and the number +of open files available. This should not give you troubles. + +@item maximum size of input line: +@code{/line}. Currently 255. + +@item methods of mapping block ranges to files: +Currently, the block words automatically access the file +@file{blocks.fb} in the currend working directory. More sophisticated +methods could be implemented if there is demand (and a volunteer). + +@item number of string buffers provided by @code{S"}: +1 + +@item size of string buffer used by @code{S"}: +@code{/line}. currently 255. + +@end table + +@c --------------------------------------------------------------------- +@node file-ambcond, , file-idef, The optional File-Access word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item attempting to position a file outside it's boundaries: +@code{REPOSITION-FILE} is performed as usual: Afterwards, +@code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}. + +@item attempting to read from file positions not yet written: +End-of-file, i.e., zero characters are read and no error is reported. + +@item @var{file-id} is invalid (@code{INCLUDE-FILE}): +An appropriate exception may be thrown, but a memory fault or other +problem is more probable. + +@item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}): +The @var{ior} produced by the operation, that discovered the problem, is +thrown. + +@item named file cannot be opened (@code{included}): +The @var{ior} produced by @code{open-file} is thrown. + +@item requesting an unmapped block number: +There are no unmapped legal block numbers. On some operating systems, +writing a block with a large number may overflow the file system and +have an error message as consequence. + +@item using @code{source-id} when @code{blk} is non-zero: +@code{source-id} performs its function. Typically it will give the id of +the source which loaded the block. (Better ideas?) + +@end table + + +@c ===================================================================== +@node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance +@section The optional Floating-Point word set +@c ===================================================================== + +@menu +* floating-idef:: Implementation Defined Options +* floating-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item format and range of floating point numbers: +System-dependent; the @code{double} type of C. + +@item results of @code{REPRESENT} when @var{float} is out of range: +System dependent; @code{REPRESENT} is implemented using the C library +function @code{ecvt()} and inherits its behaviour in this respect. + +@item rounding or truncation of floating-point numbers: +System dependent; the rounding behaviour is inherited from the hosting C +compiler. IEEE-FP-based (i.e., most) systems by default round to +nearest, and break ties by rounding to even (i.e., such that the last +bit of the mantissa is 0). + +@item size of floating-point stack: +@code{s" FLOATING-STACK" environment? drop .}. Can be changed at startup +with the command-line option @code{-f}. + +@item width of floating-point stack: +@code{1 floats}. + +@end table + + +@c --------------------------------------------------------------------- +@node floating-ambcond, , floating-idef, The optional Floating-Point word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item @code{df@@} or @code{df!} used with an address that is not double-float aligned: +System-dependent. Typically results in an alignment fault like other +alignment violations. + +@item @code{f@@} or @code{f!} used with an address that is not float aligned: +System-dependent. Typically results in an alignment fault like other +alignment violations. + +@item Floating-point result out of range: +System-dependent. Can result in a @code{-55 THROW} (Floating-point +unidentified fault), or can produce a special value representing, e.g., +Infinity. + +@item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned: +System-dependent. Typically results in an alignment fault like other +alignment violations. + +@item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}): +The floating-point number is converted into decimal nonetheless. + +@item Both arguments are equal to zero (@code{FATAN2}): +System-dependent. @code{FATAN2} is implemented using the C library +function @code{atan2()}. + +@item Using ftan on an argument @var{r1} where cos(@var{r1}) is zero: +System-dependent. Anyway, typically the cos of @var{r1} will not be zero +because of small errors and the tan will be a very large (or very small) +but finite number. + +@item @var{d} cannot be presented precisely as a float in @code{D>F}: +The result is rounded to the nearest float. + +@item dividing by zero: +@code{-55 throw} (Floating-point unidentified fault) + +@item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}): +System dependent. On IEEE-FP based systems the number is converted into +an infinity. + +@item @var{float}<1 (@code{facosh}): +@code{-55 throw} (Floating-point unidentified fault) + +@item @var{float}=<-1 (@code{flnp1}): +@code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems +negative infinity is typically produced for @var{float}=-1. + +@item @var{float}=<0 (@code{fln}, @code{flog}): +@code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems +negative infinity is typically produced for @var{float}=0. + +@item @var{float}<0 (@code{fasinh}, @code{fsqrt}): +@code{-55 throw} (Floating-point unidentified fault). @code{fasinh} +produces values for these inputs on my Linux box (Bug in the C library?) + +@item |@var{float}|>1 (@code{facos}, @code{fasin}, @code{fatanh}): +@code{-55 throw} (Floating-point unidentified fault). + +@item integer part of float cannot be represented by @var{d} in @code{f>d}: +@code{-55 throw} (Floating-point unidentified fault). + +@item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}): +This does not happen. +@end table + + + +@c ===================================================================== +@node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance +@section The optional Locals word set +@c ===================================================================== + +@menu +* locals-idef:: Implementation Defined Options +* locals-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item maximum number of locals in a definition: +@code{s" #locals" environment? drop .}. Currently 15. This is a lower +bound, e.g., on a 32-bit machine there can be 41 locals of up to 8 +characters. The number of locals in a definition is bounded by the size +of locals-buffer, which contains the names of the locals. + +@end table + + +@c --------------------------------------------------------------------- +@node locals-ambcond, , locals-idef, The optional Locals word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item executing a named local in interpretation state: +@code{-14 throw} (Interpreting a compile-only word). + +@item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}): +@code{-32 throw} (Invalid name argument) + +@end table + + +@c ===================================================================== +@node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance +@section The optional Memory-Allocation word set +@c ===================================================================== + +@menu +* memory-idef:: Implementation Defined Options +@end menu + + +@c --------------------------------------------------------------------- +@node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item values and meaning of @var{ior}: +The @var{ior}s returned by the file and memory allocation words are +intended as throw codes. They typically are in the range +-512@minus{}-2047 of OS errors. The mapping from OS error numbers to +@var{ior}s is -512@minus{}@var{errno}. + +@end table + +@c ===================================================================== +@node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance +@section The optional Programming-Tools word set +@c ===================================================================== + +@menu +* programming-idef:: Implementation Defined Options +* programming-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item ending sequence for input following @code{;code} and @code{code}: +Not implemented (yet). + +@item manner of processing input following @code{;code} and @code{code}: +Not implemented (yet). + +@item search order capability for @code{EDITOR} and @code{ASSEMBLER}: +Not implemented (yet). If they were implemented, they would use the +search order wordset. + +@item source and format of display by @code{SEE}: +The source for @code{see} is the intermediate code used by the inner +interpreter. The current @code{see} tries to output Forth source code +as well as possible. + +@end table + +@c --------------------------------------------------------------------- +@node programming-ambcond, , programming-idef, The optional Programming-Tools word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item deleting the compilation wordlist (@code{FORGET}): +Not implemented (yet). + +@item fewer than @var{u}+1 items on the control flow stack (@code{CS-PICK}, @code{CS-ROLL}): +This typically results in an @code{abort"} with a descriptive error +message (may change into a @code{-22 throw} (Control structure mismatch) +in the future). You may also get a memory access error. If you are +unlucky, this ambiguous condition is not caught. + +@item @var{name} can't be found (@code{forget}): +Not implemented (yet). + +@item @var{name} not defined via @code{CREATE}: +@code{;code} is not implemented (yet). If it were, it would behave like +@code{DOES>} in this respect, i.e., change the execution semantics of +the last defined word no matter how it was defined. + +@item @code{POSTPONE} applied to @code{[IF]}: +After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is +equivalent to @code{[IF]}. + +@item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}: +Continue in the same state of conditional compilation in the next outer +input source. Currently there is no warning to the user about this. + +@item removing a needed definition (@code{FORGET}): +Not implemented (yet). + +@end table + + +@c ===================================================================== +@node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance +@section The optional Search-Order word set +@c ===================================================================== + +@menu +* search-idef:: Implementation Defined Options +* search-ambcond:: Ambiguous Conditions +@end menu + + +@c --------------------------------------------------------------------- +@node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set +@subsection Implementation Defined Options +@c --------------------------------------------------------------------- + +@table @i + +@item maximum number of word lists in search order: +@code{s" wordlists" environment? drop .}. Currently 16. + +@item minimum search order: +@code{root root}. + +@end table + +@c --------------------------------------------------------------------- +@node search-ambcond, , search-idef, The optional Search-Order word set +@subsection Ambiguous conditions +@c --------------------------------------------------------------------- + +@table @i + +@item changing the compilation wordlist (during compilation): +The word is entered into the wordlist that was the compilation wordlist +at the start of the definition. Any changes to the name field (e.g., +@code{immediate}) or the code field (e.g., when executing @code{DOES>}) +are applied to the latest defined word (as reported by @code{last} or +@code{lastxt}), if possible, irrespective of the compilation wordlist. + +@item search order empty (@code{previous}): +@code{abort" Vocstack empty"}. + +@item too many word lists in search order (@code{also}): +@code{abort" Vocstack full"}. + +@end table + +@node Model, Integrating Gforth, ANS conformance, Top +@chapter Model + +This chapter has yet to be written. It will contain information, on +which internal structures you can rely. + +@node Integrating Gforth, Emacs and Gforth, Model, Top +@chapter Integrating Gforth into C programs + +This is not yet implemented. + +Several people like to use Forth as scripting language for applications +that are otherwise written in C, C++, or some other language. + +The Forth system ATLAST provides facilities for embedding it into +applications; unfortunately it has several disadvantages: most +importantly, it is not based on ANS Forth, and it is apparently dead +(i.e., not developed further and not supported). The facilities +provided by Gforth in this area are inspired by ATLASTs facilities, so +making the switch should not be hard. + +We also tried to design the interface such that it can easily be +implemented by other Forth systems, so that we may one day arrive at a +standardized interface. Such a standard interface would allow you to +replace the Forth system without having to rewrite C code. + +You embed the Gforth interpreter by linking with the library +@code{libgforth.a} (give the compiler the option @code{-lgforth}). All +global symbols in this library that belong to the interface, have the +prefix @code{forth_}. (Global symbols that are used internally have the +prefix @code{gforth_}). + +You can include the declarations of Forth types and the functions and +variables of the interface with @code{#include }. + +Types. + +Variables. + +Data and FP Stack pointer. Area sizes. + +functions. + +forth_init(imagefile) +forth_evaluate(string) exceptions? +forth_goto(address) (or forth_execute(xt)?) +forth_continue() (a corountining mechanism) + +Adding primitives. + +No checking. + +Signals? + +Accessing the Stacks + +@node Emacs and Gforth, Internals, Integrating Gforth, Top +@chapter Emacs and Gforth + +Gforth comes with @file{gforth.el}, an improved version of +@file{forth.el} by Goran Rydqvist (included in the TILE package). The +improvements are a better (but still not perfect) handling of +indentation. I have also added comment paragraph filling (@kbd{M-q}), +commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and +removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the +stuff I do not use alone, even though some of it only makes sense for +TILE. To get a description of these features, enter Forth mode and type +@kbd{C-h m}. + +In addition, Gforth supports Emacs quite well: The source code locations +given in error messages, debugging output (from @code{~~}) and failed +assertion messages are in the right format for Emacs' compilation mode +(@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs +Manual}) so the source location corresponding to an error or other +message is only a few keystrokes away (@kbd{C-x `} for the next error, +@kbd{C-c C-c} for the error under the cursor). + +Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file +(@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that +contains the definitions of all words defined afterwards. You can then +find the source for a word using @kbd{M-.}. Note that emacs can use +several tags files at the same time (e.g., one for the Gforth sources +and one for your program, @pxref{Select Tags Table,,Selecting a Tags +Table,emacs, Emacs Manual}). The TAGS file for the preloaded words is +@file{$(datadir)/gforth/$(VERSION)/TAGS} (e.g., +@file{/usr/local/share/gforth/0.2.0/TAGS}). + +To get all these benefits, add the following lines to your @file{.emacs} +file: + +@example +(autoload 'forth-mode "gforth.el") +(setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist)) +@end example + +@node Internals, Bugs, Emacs and Gforth, Top +@chapter Internals + +Reading this section is not necessary for programming with Gforth. It +should be helpful for finding your way in the Gforth sources. + +The ideas in this section have also been published in the papers +@cite{ANS fig/GNU/??? Forth} (in German) by Bernd Paysan, presented at +the Forth-Tagung '93 and @cite{A Portable Forth Engine} by M. Anton +Ertl, presented at EuroForth '93; the latter is available at +@*@file{http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z}. + +@menu +* Portability:: +* Threading:: +* Primitives:: +* System Architecture:: +* Performance:: +@end menu + +@node Portability, Threading, Internals, Internals +@section Portability + +One of the main goals of the effort is availability across a wide range +of personal machines. fig-Forth, and, to a lesser extent, F83, achieved +this goal by manually coding the engine in assembly language for several +then-popular processors. This approach is very labor-intensive and the +results are short-lived due to progress in computer architecture. + +Others have avoided this problem by coding in C, e.g., Mitch Bradley +(cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is +particularly popular for UNIX-based Forths due to the large variety of +architectures of UNIX machines. Unfortunately an implementation in C +does not mix well with the goals of efficiency and with using +traditional techniques: Indirect or direct threading cannot be expressed +in C, and switch threading, the fastest technique available in C, is +significantly slower. Another problem with C is that it's very +cumbersome to express double integer arithmetic. + +Fortunately, there is a portable language that does not have these +limitations: GNU C, the version of C processed by the GNU C compiler +(@pxref{C Extensions, , Extensions to the C Language Family, gcc.info, +GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, , +Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect +threading possible, its @code{long long} type (@pxref{Long Long, , +Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forth's +double numbers@footnote{Unfortunately, long longs are not implemented +properly on all machines (e.g., on alpha-osf1, long longs are only 64 +bits, the same size as longs (and pointers), but they should be twice as +long according to @ref{Long Long, , Double-Word Integers, gcc.info, GNU +C Manual}). So, we had to implement doubles in C after all. Still, on +most machines we can use long longs and achieve better performance than +with the emulation package.}. GNU C is available for free on all +important (and many unimportant) UNIX machines, VMS, 80386s running +MS-DOS, the Amiga, and the Atari ST, so a Forth written in GNU C can run +on all these machines. + +Writing in a portable language has the reputation of producing code that +is slower than assembly. For our Forth engine we repeatedly looked at +the code produced by the compiler and eliminated most compiler-induced +inefficiencies by appropriate changes in the source-code. + +However, register allocation cannot be portably influenced by the +programmer, leading to some inefficiencies on register-starved +machines. We use explicit register declarations (@pxref{Explicit Reg +Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to +improve the speed on some machines. They are turned on by using the +@code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not +only depends on the machine, but also on the compiler version: On some +machines some compiler versions produce incorrect code when certain +explicit register declarations are used. So by default +@code{-DFORCE_REG} is not used. + +@node Threading, Primitives, Portability, Internals +@section Threading + +GNU C's labels as values extension (available since @code{gcc-2.0}, +@pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual}) +makes it possible to take the address of @var{label} by writing +@code{&&@var{label}}. This address can then be used in a statement like +@code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as +@code{goto x}. + +With this feature an indirect threaded NEXT looks like: +@example +cfa = *ip++; +ca = *cfa; +goto *ca; +@end example +For those unfamiliar with the names: @code{ip} is the Forth instruction +pointer; the @code{cfa} (code-field address) corresponds to ANS Forths +execution token and points to the code field of the next word to be +executed; The @code{ca} (code address) fetched from there points to some +executable code, e.g., a primitive or the colon definition handler +@code{docol}. + +Direct threading is even simpler: +@example +ca = *ip++; +goto *ca; +@end example + +Of course we have packaged the whole thing neatly in macros called +@code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa). + +@menu +* Scheduling:: +* Direct or Indirect Threaded?:: +* DOES>:: +@end menu + +@node Scheduling, Direct or Indirect Threaded?, Threading, Threading +@subsection Scheduling + +There is a little complication: Pipelined and superscalar processors, +i.e., RISC and some modern CISC machines can process independent +instructions while waiting for the results of an instruction. The +compiler usually reorders (schedules) the instructions in a way that +achieves good usage of these delay slots. However, on our first tries +the compiler did not do well on scheduling primitives. E.g., for +@code{+} implemented as +@example +n=sp[0]+sp[1]; +sp++; +sp[0]=n; +NEXT; +@end example +the NEXT comes strictly after the other code, i.e., there is nearly no +scheduling. After a little thought the problem becomes clear: The +compiler cannot know that sp and ip point to different addresses (and +the version of @code{gcc} we used would not know it even if it was +possible), so it could not move the load of the cfa above the store to +the TOS. Indeed the pointers could be the same, if code on or very near +the top of stack were executed. In the interest of speed we chose to +forbid this probably unused ``feature'' and helped the compiler in +scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and +the goto part (@code{NEXT_P2}). @code{+} now looks like: +@example +n=sp[0]+sp[1]; +sp++; +NEXT_P1; +sp[0]=n; +NEXT_P2; +@end example +This can be scheduled optimally by the compiler. + +This division can be turned off with the switch @code{-DCISC_NEXT}. This +switch is on by default on machines that do not profit from scheduling +(e.g., the 80386), in order to preserve registers. + +@node Direct or Indirect Threaded?, DOES>, Scheduling, Threading +@subsection Direct or Indirect Threaded? + +Both! After packaging the nasty details in macro definitions we +realized that we could switch between direct and indirect threading by +simply setting a compilation flag (@code{-DDIRECT_THREADED}) and +defining a few machine-specific macros for the direct-threading case. +On the Forth level we also offer access words that hide the +differences between the threading methods (@pxref{Threading Words}). + +Indirect threading is implemented completely +machine-independently. Direct threading needs routines for creating +jumps to the executable code (e.g. to docol or dodoes). These routines +are inherently machine-dependent, but they do not amount to many source +lines. I.e., even porting direct threading to a new machine is a small +effort. + +@node DOES>, , Direct or Indirect Threaded?, Threading +@subsection DOES> +One of the most complex parts of a Forth engine is @code{dodoes}, i.e., +the chunk of code executed by every word defined by a +@code{CREATE}...@code{DOES>} pair. The main problem here is: How to find +the Forth code to be executed, i.e. the code after the @code{DOES>} (the +DOES-code)? There are two solutions: + +In fig-Forth the code field points directly to the dodoes and the +DOES-code address is stored in the cell after the code address +(i.e. at cfa cell+). It may seem that this solution is illegal in the +Forth-79 and all later standards, because in fig-Forth this address +lies in the body (which is illegal in these standards). However, by +making the code field larger for all words this solution becomes legal +again. We use this approach for the indirect threaded version. Leaving +a cell unused in most words is a bit wasteful, but on the machines we +are targetting this is hardly a problem. The other reason for having a +code field size of two cells is to avoid having different image files +for direct and indirect threaded systems (@pxref{System Architecture}). + +The other approach is that the code field points or jumps to the cell +after @code{DOES}. In this variant there is a jump to @code{dodoes} at +this address. @code{dodoes} can then get the DOES-code address by +computing the code address, i.e., the address of the jump to dodoes, +and add the length of that jump field. A variant of this is to have a +call to @code{dodoes} after the @code{DOES>}; then the return address +(which can be found in the return register on RISCs) is the DOES-code +address. Since the two cells available in the code field are usually +used up by the jump to the code address in direct threading, we use +this approach for direct threading. We did not want to add another +cell to the code field. + +@node Primitives, System Architecture, Threading, Internals +@section Primitives + +@menu +* Automatic Generation:: +* TOS Optimization:: +* Produced code:: +@end menu + +@node Automatic Generation, TOS Optimization, Primitives, Primitives +@subsection Automatic Generation + +Since the primitives are implemented in a portable language, there is no +longer any need to minimize the number of primitives. On the contrary, +having many primitives is an advantage: speed. In order to reduce the +number of errors in primitives and to make programming them easier, we +provide a tool, the primitive generator (@file{prims2x.fs}), that +automatically generates most (and sometimes all) of the C code for a +primitive from the stack effect notation. The source for a primitive +has the following form: + +@format +@var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}] +[@code{""}@var{glossary entry}@code{""}] +@var{C code} +[@code{:} +@var{Forth code}] +@end format + +The items in brackets are optional. The category and glossary fields +are there for generating the documentation, the Forth code is there +for manual implementations on machines without GNU C. E.g., the source +for the primitive @code{+} is: +@example ++ n1 n2 -- n core plus +n = n1+n2; +@end example + +This looks like a specification, but in fact @code{n = n1+n2} is C +code. Our primitive generation tool extracts a lot of information from +the stack effect notations@footnote{We use a one-stack notation, even +though we have separate data and floating-point stacks; The separate +notation can be generated easily from the unified notation.}: The number +of items popped from and pushed on the stack, their type, and by what +name they are referred to in the C code. It then generates a C code +prelude and postlude for each primitive. The final C code for @code{+} +looks like this: + +@example +I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */ +/* */ /* documentation */ +@{ +DEF_CA /* definition of variable ca (indirect threading) */ +Cell n1; /* definitions of variables */ +Cell n2; +Cell n; +n1 = (Cell) sp[1]; /* input */ +n2 = (Cell) TOS; +sp += 1; /* stack adjustment */ +NAME("+") /* debugging output (with -DDEBUG) */ +@{ +n = n1+n2; /* C code taken from the source */ +@} +NEXT_P1; /* NEXT part 1 */ +TOS = (Cell)n; /* output */ +NEXT_P2; /* NEXT part 2 */ +@} +@end example + +This looks long and inefficient, but the GNU C compiler optimizes quite +well and produces optimal code for @code{+} on, e.g., the R3000 and the +HP RISC machines: Defining the @code{n}s does not produce any code, and +using them as intermediate storage also adds no cost. + +There are also other optimizations, that are not illustrated by this +example: Assignments between simple variables are usually for free (copy +propagation). If one of the stack items is not used by the primitive +(e.g. in @code{drop}), the compiler eliminates the load from the stack +(dead code elimination). On the other hand, there are some things that +the compiler does not do, therefore they are performed by +@file{prims2x.fs}: The compiler does not optimize code away that stores +a stack item to the place where it just came from (e.g., @code{over}). + +While programming a primitive is usually easy, there are a few cases +where the programmer has to take the actions of the generator into +account, most notably @code{?dup}, but also words that do not (always) +fall through to NEXT. + +@node TOS Optimization, Produced code, Automatic Generation, Primitives +@subsection TOS Optimization + +An important optimization for stack machine emulators, e.g., Forth +engines, is keeping one or more of the top stack items in +registers. If a word has the stack effect @var{in1}...@var{inx} @code{--} +@var{out1}...@var{outy}, keeping the top @var{n} items in registers +@itemize @bullet +@item +is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n}, +due to fewer loads from and stores to the stack. +@item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x