\input texinfo @c -*-texinfo-*- @comment The source is gforth.ds, from which gforth.texi is generated @comment TODO: nac29jan99 - a list of things to add in the next edit: @comment 1. x-ref all ambiguous or implementation-defined features? @comment 2. Describe the use of Auser Avariable AConstant A, etc. @comment 3. words in miscellaneous section need a home. @comment 4. search for TODO for other minor and major works required. @comment 5. [rats] change all @var to @i in Forth source so that info @comment file looks decent. @comment .. would be useful to have a word that identified all deferred words @comment should semantics stuff in intro be moved to another section @comment %**start of header (This is for running Texinfo on a region.) @setfilename gforth.info @settitle Gforth Manual @dircategory GNU programming tools @direntry * Gforth: (gforth). A fast interpreter for the Forth language. @end direntry @comment @setchapternewpage odd @comment TODO this gets left in by HTML converter @macro progstyle {} Programming style note: @end macro @comment %**end of header (This is for running Texinfo on a region.) @comment ---------------------------------------------------------- @comment macros for beautifying glossary entries @comment if these are used, need to strip them out for HTML converter @comment else they get repeated verbatim in HTML output. @comment .. not working yet. @macro GLOSS-START {} @iftex @ninerm @end iftex @end macro @macro GLOSS-END {} @iftex @rm @end iftex @end macro @comment ---------------------------------------------------------- @include version.texi @ifinfo This file documents Gforth @value{VERSION} Copyright @copyright{} 1995-1999 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 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 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 included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "Distribution" and "General Public License" may be included in a translation approved by the author instead of in the original English. @end ifinfo @finalout @titlepage @sp 10 @center @titlefont{Gforth Manual} @sp 2 @center for version @value{VERSION} @sp 2 @center Anton Ertl @center Bernd Paysan @center Jens Wilke @center Neal Crook @sp 3 @center This manual is permanently under construction and was last updated on 04-May-1999 @comment The following two commands start the copyright page. @page @vskip 0pt plus 1filll Copyright @copyright{} 1995--1999 Free Software Foundation, Inc. @comment !! Published by ... or You can get a copy of this manual ... 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. 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 included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "Distribution" and "General Public License" may be included in a translation approved by the author instead of in the original English. @end titlepage @node Top, License, (dir), (dir) @ifinfo Gforth is a free implementation of ANS Forth available on many personal machines. This manual corresponds to version @value{VERSION}. @end ifinfo @menu * License:: The GPL * Goals:: About the Gforth Project * Gforth Environment:: Starting (and exiting) Gforth * Introduction:: An introduction to ANS Forth * Words:: Forth words available in Gforth * Error messages:: How to interpret them * Tools:: Programming tools * 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 * Image Files:: @code{.fi} files contain compiled code * Engine:: The inner interpreter and the primitives * Binding to System Library:: * Cross Compiler:: The Cross Compiler * Bugs:: How to report them * Origin:: Authors and ancestors of Gforth * Forth-related information:: Books and places to look on the WWW * Word Index:: An item for each Forth word * Concept Index:: A menu covering many topics @detailmenu --- The Detailed Node Listing --- Goals of Gforth * Gforth Extensions Sinful?:: Gforth Environment * Invoking Gforth:: * Leaving Gforth:: * Command-line editing:: * Upper and lower case:: * Environment variables:: * Gforth Files:: An Introduction to ANS Forth * Introducing the Text Interpreter:: * Stacks and Postfix notation:: * Your first definition:: * How does that work?:: * Forth is written in Forth:: * Review - elements of a Forth system:: * Where to go next:: * Exercises:: Forth Words * Notation:: * Comments:: * Boolean Flags:: * Arithmetic:: * Stack Manipulation:: * Memory:: * Control Structures:: * Defining Words:: * The Text Interpreter:: * Tokens for Words:: * Word Lists:: * Environmental Queries:: * Files:: * Blocks:: * Other I/O:: * Programming Tools:: * Assembler and Code Words:: * Threading Words:: * Locals:: * Structures:: * Object-oriented Forth:: * Passing Commands to the OS:: * Miscellaneous Words:: Arithmetic * Single precision:: * Bitwise operations:: * Double precision:: Double-cell integer arithmetic * Numeric comparison:: * Mixed precision:: operations with single and double-cell integers * Floating Point:: Stack Manipulation * Data stack:: * Floating point stack:: * Return stack:: * Locals stack:: * Stack pointer manipulation:: Memory * Reserving Data Space:: * Memory Access:: * Address Arithmetic:: * Memory Blocks:: * Dynamic Allocation:: Control Structures * Selection:: * Simple Loops:: * Counted Loops:: * Arbitrary control structures:: * Calls and returns:: * Exception Handling:: Defining Words * Simple Defining Words:: * Colon Definitions:: * User-defined Defining Words:: * Supplying names:: * Interpretation and Compilation Semantics:: The Text Interpreter * Input Sources:: * Number Conversion:: * Interpret/Compile states:: * Literals:: * Interpreter Directives:: Word Lists * Why use word lists?:: * Word list examples:: Files * Forth source files:: * General files:: * Search Paths:: * Forth Search Paths:: * General Search Paths:: Other I/O * Simple numeric output:: * Formatted numeric output:: * String Formats:: * Displaying characters and strings:: * Input:: Programming Tools * Debugging:: Simple and quick. * Assertions:: Making your programs self-checking. * Singlestep Debugger:: Executing your program word by word. Locals * Gforth locals:: * ANS Forth locals:: Gforth locals * Where are locals visible by name?:: * How long do locals live?:: * Programming Style:: * Implementation:: Structures * Why explicit structure support?:: * Structure Usage:: * Structure Naming Convention:: * Structure Implementation:: * Structure Glossary:: Object-oriented Forth * Why object-oriented programming?:: * Object-Oriented Terminology:: * Objects:: * OOF:: * Mini-OOF:: * Comparison with other object models:: The @file{objects.fs} model * Properties of the Objects model:: * Basic Objects Usage:: * The Objects base class:: * Creating objects:: * Object-Oriented Programming Style:: * Class Binding:: * Method conveniences:: * Classes and Scoping:: * Object Interfaces:: * Objects Implementation:: * Objects Glossary:: The @file{oof.fs} model * Properties of the OOF model:: * Basic OOF Usage:: * The OOF base class:: * Class Declaration:: * Class Implementation:: The @file{mini-oof.fs} model * Basic Mini-OOF Usage:: * Mini-OOF Example:: * Mini-OOF Implementation:: Tools * ANS Report:: Report the words used, sorted by wordset. ANS conformance * 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:: The Core Words * core-idef:: Implementation Defined Options * core-ambcond:: Ambiguous Conditions * core-other:: Other System Documentation The optional Block word set * block-idef:: Implementation Defined Options * block-ambcond:: Ambiguous Conditions * block-other:: Other System Documentation The optional Double Number word set * double-ambcond:: Ambiguous Conditions The optional Exception word set * exception-idef:: Implementation Defined Options The optional Facility word set * facility-idef:: Implementation Defined Options * facility-ambcond:: Ambiguous Conditions The optional File-Access word set * file-idef:: Implementation Defined Options * file-ambcond:: Ambiguous Conditions The optional Floating-Point word set * floating-idef:: Implementation Defined Options * floating-ambcond:: Ambiguous Conditions The optional Locals word set * locals-idef:: Implementation Defined Options * locals-ambcond:: Ambiguous Conditions The optional Memory-Allocation word set * memory-idef:: Implementation Defined Options The optional Programming-Tools word set * programming-idef:: Implementation Defined Options * programming-ambcond:: Ambiguous Conditions The optional Search-Order word set * search-idef:: Implementation Defined Options * search-ambcond:: Ambiguous Conditions Image Files * Image Licensing Issues:: Distribution terms for images. * Image File Background:: Why have image files? * Non-Relocatable Image Files:: don't always work. * Data-Relocatable Image Files:: are better. * Fully Relocatable Image Files:: better yet. * Stack and Dictionary Sizes:: Setting the default sizes for an image. * Running Image Files:: @code{gforth -i @var{file}} or @var{file}. * Modifying the Startup Sequence:: and turnkey applications. Fully Relocatable Image Files * gforthmi:: The normal way * cross.fs:: The hard way Engine * Portability:: * Threading:: * Primitives:: * Performance:: Threading * Scheduling:: * Direct or Indirect Threaded?:: * DOES>:: Primitives * Automatic Generation:: * TOS Optimization:: * Produced code:: Cross Compiler * Using the Cross Compiler:: * How the Cross Compiler Works:: Other Forth-related information * Internet resources:: * Books:: * The Forth Interest Group:: * Conferences:: @end detailmenu @end menu @node License, Goals, Top, Top @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.) You can apply it to your programs, too. When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things. To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it. For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights. We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software. Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations. Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all. The precise terms and conditions for copying, distribution and modification follow. @iftex @unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION @end iftex @ifinfo @center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION @end ifinfo @enumerate 0 @item This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The ``Program'', below, refers to any such program or work, and a ``work based on the Program'' means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term ``modification''.) Each licensee is addressed as ``you''. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. 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. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.) @end enumerate These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it. Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program. In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License. @item You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following: @enumerate a @item Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or, @item Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or, @item Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.) @end enumerate The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable. If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code. @item You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance. @item You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it. @item Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License. @item If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program. 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This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License. @item If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. 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 @unnumbered Preface @cindex Preface This manual documents Gforth. Some introductory material is provided for readers who are unfamiliar with Forth or who are migrating to Gforth from other Forth compilers. However, this manual is primarily a reference manual. @end iftex @comment TODO much more blurb here. @c ****************************************************************** @node Goals, Gforth Environment, License, Top @comment node-name, next, previous, up @chapter Goals of Gforth @cindex goals of the Gforth project The goal of the Gforth Project is to develop a standard model for ANS Forth. This can be split into several subgoals: @itemize @bullet @item Gforth should conform to the ANS Forth Standard. @item It should be a model, i.e. it should define all the implementation-dependent things. @item It should become standard, i.e. widely accepted and used. This goal is the most difficult one. @end itemize To achieve these goals Gforth should be @itemize @bullet @item Similar to previous models (fig-Forth, F83) @item Powerful. It should provide for all the things that are considered necessary today and even some that are not yet considered necessary. @item Efficient. It should not get the reputation of being exceptionally slow. @item Free. @item Available on many machines/easy to port. @end itemize 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, but it appears to be quite popular. It has some similarities to and some differences from 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. @menu * Gforth Extensions Sinful?:: @end menu @node Gforth Extensions Sinful?, , Goals, Goals @comment node-name, next, previous, up @section Is it a Sin to use Gforth Extensions? @cindex Gforth extensions If you've been paying attention, you will have realised that there is an ANS (American National Standard) for Forth. As you read through the rest of this manual, you will see documentation for @i{Standard} words, and documentation for some appealing Gforth @i{extensions}. You might ask yourself the question: @i{``Given that there is a standard, would I be committing a sin to use (non-Standard) Gforth extensions?''} The answer to that question is somewhat pragmatic and somewhat philosophical. Consider these points: @itemize @bullet @item A number of the Gforth extensions can be implemented in ANS Forth using files provided in the @file{compat/} directory. These are mentioned in the text in passing. @item Forth has a rich historical precedent for programmers taking advantage of implementation-dependent features of their tools (for example, relying on a knowledge of the dictionary structure). Sometimes these techniques are necessary to extract every last bit of performance from the hardware, sometimes they are just a programming shorthand. @item The best way to break the rules is to know what the rules are. To learn the rules, there is no substitute for studying the text of the Standard itself. In particular, Appendix A of the Standard (@var{Rationale}) provides a valuable insight into the thought processes of the technical committee. @item The best reason to break a rule is because you have to; because it's more productive to do that, because it makes your code run fast enough or because you can see no Standard way to achieve what you want to achieve. @end itemize The tool @file{ans-report.fs} (@pxref{ANS Report}) makes it easy to analyse your program and determine what non-Standard definitions it relies upon. @c ****************************************************************** @node Gforth Environment, Introduction, Goals, Top @chapter Gforth Environment @cindex Gforth environment Note: ultimately, the gforth man page will be auto-generated from the material in this chapter. @menu * Invoking Gforth:: Getting in * Leaving Gforth:: Getting out * Command-line editing:: * Upper and lower case:: * Environment variables:: ..that affect how Gforth starts up * Gforth Files:: What gets installed and where @end menu @comment ---------------------------------------------- @node Invoking Gforth, Leaving Gforth, ,Gforth Environment @section Invoking Gforth @cindex invoking Gforth @cindex running Gforth @cindex command-line options @cindex options on the command line @cindex flags on the command line Gforth is made up of two parts; an executable ``engine'' (named gforth) and an image file. To start it, you will usually just say @code{gforth} -- this automatically loads the default image file. In many other cases the default Gforth image will be invoked like this: @example gforth [files] [-e forth-code] @end example @noindent This interprets the contents of the files and the Forth code in the order they are given. In general, the command line looks like this: @example gforth [initialization options] [image-specific options] @end example The initialization options must come before the rest of the command line. They are: @table @code @cindex -i, command-line option @cindex --image-file, command-line option @item --image-file @i{file} @itemx -i @i{file} Loads the Forth image @i{file} instead of the default @file{gforth.fi} (@pxref{Image Files}). @cindex --path, command-line option @cindex -p, command-line option @item --path @i{path} @itemx -p @i{path} Uses @i{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 (e.g., @file{/usr/local/share/gforth/0.2.0:.}). A path is given as a list of directories, separated by @samp{:} (on Unix) or @samp{;} (on other OSs). @cindex --dictionary-size, command-line option @cindex -m, command-line option @cindex @i{size} parameters for command-line options @cindex size of the dictionary and the stacks @item --dictionary-size @i{size} @itemx -m @i{size} Allocate @i{size} space for the Forth dictionary space instead of using the default specified in the image (typically 256K). The @i{size} specification for this and subsequent options consists of an integer and a unit (e.g., @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element size, in this case Cells), @code{k} (kilobytes), @code{M} (Megabytes), @code{G} (Gigabytes), and @code{T} (Terabytes). If no unit is specified, @code{e} is used. @cindex --data-stack-size, command-line option @cindex -d, command-line option @item --data-stack-size @i{size} @itemx -d @i{size} Allocate @i{size} space for the data stack instead of using the default specified in the image (typically 16K). @cindex --return-stack-size, command-line option @cindex -r, command-line option @item --return-stack-size @i{size} @itemx -r @i{size} Allocate @i{size} space for the return stack instead of using the default specified in the image (typically 15K). @cindex --fp-stack-size, command-line option @cindex -f, command-line option @item --fp-stack-size @i{size} @itemx -f @i{size} Allocate @i{size} space for the floating point stack instead of using the default specified in the image (typically 15.5K). In this case the unit specifier @code{e} refers to floating point numbers. @cindex --locals-stack-size, command-line option @cindex -l, command-line option @item --locals-stack-size @i{size} @itemx -l @i{size} Allocate @i{size} space for the locals stack instead of using the default specified in the image (typically 14.5K). @cindex -h, command-line option @cindex --help, command-line option @item --help @itemx -h Print a message about the command-line options @cindex -v, command-line option @cindex --version, command-line option @item --version @itemx -v Print version and exit @cindex --debug, command-line option @item --debug Print some information useful for debugging on startup. @cindex --offset-image, command-line option @item --offset-image Start the dictionary at a slightly different position than would be used otherwise (useful for creating data-relocatable images, @pxref{Data-Relocatable Image Files}). @cindex --no-offset-im, command-line option @item --no-offset-im Start the dictionary at the normal position. @cindex --clear-dictionary, command-line option @item --clear-dictionary Initialize all bytes in the dictionary to 0 before loading the image (@pxref{Data-Relocatable Image Files}). @cindex --die-on-signal, command-line-option @item --die-on-signal Normally Gforth handles most signals (e.g., the user interrupt SIGINT, or the segmentation violation SIGSEGV) by translating it into a Forth @code{THROW}. With this option, Gforth exits if it receives such a signal. This option is useful when the engine and/or the image might be severely broken (such that it causes another signal before recovering from the first); this option avoids endless loops in such cases. @end table @cindex loading files at startup @cindex executing code on startup @cindex batch processing with Gforth As explained above, the image-specific command-line arguments for the default image @file{gforth.fi} consist of a sequence of filenames and @code{-e @var{forth-code}} options that are interpreted in the sequence in which they are given. The @code{-e @var{forth-code}} or @code{--evaluate @var{forth-code}} option evaluates the Forth code. This option takes only one argument; if you want to evaluate more Forth words, you have to quote them or use @code{-e} several times. To exit after processing the command line (instead of entering interactive mode) append @code{-e bye} to the command line. @cindex versions, invoking other versions of Gforth If you have several versions of Gforth installed, @code{gforth} will invoke the version that was installed last. @code{gforth-@i{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 resulting from using this option may not be ANS Forth conformant). Then the user initialization file @file{.gforth.fs} is executed, unless the option @code{--no-rc} is given; this file is first searched in @file{.}, then in @file{~}, then in the normal path (see above). @comment ---------------------------------------------- @node Leaving Gforth, Command-line editing, Invoking Gforth, Gforth Environment @section Leaving Gforth @cindex Gforth - leaving @cindex leaving Gforth You can leave Gforth by typing @code{bye} or Ctrl-D or (if you invoked Gforth with the @code{--die-on-signal} option) Ctrl-C. When you leave Gforth, all of your definitions and data are discarded. @xref{Image Files} for ways of saving the state of the system before leaving Gforth. doc-bye @comment ---------------------------------------------- @node Command-line editing, Upper and lower case,Leaving Gforth,Gforth Environment @section Command-line editing @cindex command-line editing Gforth maintains a history file that records every line that you type to the text interpreter. This file is preserved between sessions, and is used to provide a command-line recall facility; if you type ctrl-P repeatedly you can recall successively older commands from this (or previous) session(s). The full list of command-line editing facilities is: @itemize @bullet @item ctrl-P (``previous'') (or up-arrow) to recall successively older commands from the history buffer. @item ctrl-N (``next'') (or down-arrow) to recall successively newer commands from the history buffer. @item ctrl-F (or right-arrow) to move the cursor right, non-destructively. @item ctrl-B (or left-arrow) to move the cursor left, non-destructively. @item ctrl-H (backspace) to delete the character to the left of the cursor, closing up the line. @item ctrl-K to delete (``kill'') from the cursor to the end of the line. @item ctrl-A to move the cursor to the start of the line. @item ctrl-E to move the cursor to the end of the line. @item carriage-return or line-feed (ctrl-J, ctrl-M) to submit the current line. @item tab to step through all possible full-word completions of the word currently being typed. @item ctrl-D to terminate Gforth (gracefully, using @code{bye}). @end itemize When editing, displayable characters are inserted to the left of the cursor position; the line is always in ``insert'' (as opposed to ``overstrike'') mode. @cindex history file @cindex @file{.gforth-history} On Unix systems, the history file is @file{~/.gforth-history} by default@footnote{i.e. it is stored in the user's home directory.}. You can find out the name and location of your history file using: @example history-file type \ Unix-class systems history-file type \ Other systems history-dir type @end example If you enter long definitions by hand, you can use a text editor to paste them out of the history file into a Forth source file for reuse at a later time. Gforth never trims the size of the history file, so you should do this periodically, if necessary. @comment this is all defined in history.fs @comment TODO the ctrl-D behaviour can either do a bye or a beep.. how is that option @comment chosen? @comment ---------------------------------------------- @node Upper and lower case, Environment variables,Command-line editing,Gforth Environment @section Upper and lower case @cindex case-sensitivity @cindex upper and lower case Gforth is case-insensitive, so you can enter definitions and invoke Standard words using upper, lower or mixed case (however, @pxref{core-idef, Implementation-defined options, Implementation-defined options}). ANS Forth only @i{requires} implementations to recognise Standard words when they are typed entirely in upper case. Therefore, a Standard program must use upper case for all Standard words@footnote{You can use whatever case you like for words that you define.}. @comment ---------------------------------------------- @node Environment variables, Gforth Files, Upper and lower case,Gforth Environment @section Environment variables @cindex environment variables Gforth uses these environment variables: @itemize @bullet @item @cindex GFORTHHIST - environment variable GFORTHHIST - (Unix systems only) specifies the directory in which to open/create the history file, @file{.gforth-history}. Default: @code{$HOME}. @item @cindex GFORTHPATH - environment variable GFORTHPATH - specifies the path used when searching for the gforth image file and for Forth source-code files. @item @cindex GFORTH - environment variable GFORTH - used by @file{gforthmi} @xref{gforthmi}. @item @cindex GFORTHD - environment variable GFORTHD - used by @file{gforthmi} @xref{gforthmi}. @item @cindex TMP, TEMP - environment variable TMP, TEMP - (non-Unix systems only) used as a potential location for the history file. @end itemize @comment also POSIXELY_CORRECT LINES COLUMNS HOME but no interest in @comment mentioning these. All the Gforth environment variables default to sensible values if they are not set. @comment ---------------------------------------------- @node Gforth Files, ,Environment variables,Gforth Environment @section Gforth files @cindex Gforth files When Gforth is installed on a Unix system it leaves files in these locations: @itemize @bullet @item @file{/usr/local/bin/gforth} @item @file{/usr/local/bin/gforthmi} @item @file{/usr/local/man/man1/gforth.1} - man page. @item @file{/usr/local/info} - the Info version of this manual. @item @file{/usr/local/lib/gforth//..} - Gforth @file{.fi} files. @item @file{/usr/local/share/gforth//TAGS} - Emacs TAGS file. @item @file{/usr/local/share/gforth//..} - Gforth source files. @item @file{../emacs/site-lisp/gforth.el} - Emacs gforth mode. @end itemize @c ****************************************************************** @node Introduction, Words, Gforth Environment, Top @comment node-name, next, previous, up @chapter An Introduction to ANS Forth @cindex Forth - an introduction The primary purpose of this manual is to document Gforth. However, since Forth is not a widely-known language and there is a lack of up-to-date teaching material, it seems worthwhile to provide some introductory material. @xref{Forth-related information} for other sources of Forth-related information. The examples in this section should work on any ANS Forth; the output shown was produced using Gforth. Each example attempts to reproduce the exact output that Gforth produces. If you try out the examples (and you should), what you should type is shown @kbd{like this} and Gforth's response is shown @code{like this}. The single exception is that, where the example shows @kbd{} it means that you should press the ``carriage return'' key. Unfortunately, some output formats for this manual cannot show the difference between @kbd{this} and @code{this} which will make trying out the examples harder (but not impossible). Forth is an unusual language. It provides an interactive development environment which includes both an interpreter and compiler. Forth programming style encourages you to break a problem down into many @cindex factoring small fragments (@dfn{factoring}), and then to develop and test each fragment interactively. Forth advocates assert that breaking the edit-compile-test cycle used by conventional programming languages can lead to great productivity improvements. @menu * Introducing the Text Interpreter:: * Stacks and Postfix notation:: * Your first definition:: * How does that work?:: * Forth is written in Forth:: * Review - elements of a Forth system:: * Where to go next:: * Exercises:: @end menu @comment ---------------------------------------------- @node Introducing the Text Interpreter, Stacks and Postfix notation, Introduction, Introduction @section Introducing the Text Interpreter @cindex text interpreter @cindex outer interpreter When you invoke the Forth image, you will see a startup banner printed and nothing else (if you have Gforth installed on your system, try invoking it now, by typing @kbd{gforth}). Forth is now running its command line interpreter, which is called the @dfn{Text Interpreter} (also known as the @dfn{Outer Interpreter}). (You will learn a lot about the text interpreter as you read through this chapter, but @pxref{The Text Interpreter} for more detail). Although it's not obvious, Forth is actually waiting for your input. Type a number and press the key: @example @kbd{45} ok @end example Rather than give you a prompt to invite you to input something, the text interpreter prints a status message @i{after} it has processed a line of input. The status message in this case (``@code{ ok}'' followed by carriage-return) indicates that the text interpreter was able to process all of your input successfully. Now type something illegal: @example @kbd{qwer341} :1: Undefined word qwer341 ^^^^^^^ $400D2BA8 Bounce $400DBDA8 no.extensions @end example The exact text, other than the ``Undefined word'' may differ slightly on your system, but the effect is the same; when the text interpreter detects an error, it discards any remaining text on a line, resets certain internal state and prints an error message. The text interpreter waits for you to press carriage-return, and then processes your input line. Starting at the beginning of the line, it breaks the line into groups of characters separated by spaces. For each group of characters in turn, it makes two attempts to do something: @itemize @bullet @item It tries to treat it as a command. It does this by searching a @dfn{name dictionary}. If the group of characters matches an entry in the name dictionary, the name dictionary provides the text interpreter with information that allows the text interpreter perform some actions. In Forth jargon, we say that the group @cindex word @cindex definition @cindex execution token @cindex xt of characters names a @dfn{word}, that the dictionary search returns an @dfn{execution token (xt)} corresponding to the @dfn{definition} of the word, and that the text interpreter executes the xt. Often, the terms @dfn{word} and @dfn{definition} are used interchangeably. @item If the text interpreter fails to find a match in the name dictionary, it tries to treat the group of characters as a number in the current number base (when you start up Forth, the current number base is base 10). If the group of characters legitimately represents a number, the text interpreter pushes the number onto a stack (we'll learn more about that in the next section). @end itemize If the text interpreter is unable to do either of these things with any group of characters, it discards the group of characters and the rest of the line, then prints an error message. If the text interpreter reaches the end of the line without error, it prints the status message ``@code{ ok}'' followed by carriage-return. This is the simplest command we can give to the text interpreter: @example @kbd{} ok @end example The text interpreter did everything we asked it to do (nothing) without an error, so it said that everything is ``@code{ ok}''. Try a slightly longer command: @example @kbd{12 dup fred dup} :1: Undefined word 12 dup fred dup ^^^^ $400D2BA8 Bounce $400DBDA8 no.extensions @end example When you press the carriage-return key, the text interpreter starts to work its way along the line: @itemize @bullet @item When it gets to the space after the @code{2}, it takes the group of characters @code{12} and looks them up in the name dictionary@footnote{We can't tell if it found them or not, but assume for now that it did not}. There is no match for this group of characters in the name dictionary, so it tries to treat them as a number. It is able to do this successfully, so it puts the number, 12, ``on the stack'' (whatever that means). @item The text interpreter resumes scanning the line and gets the next group of characters, @code{dup}. It looks it up in the name dictionary and (you'll have to take my word for this) finds it, and executes the word @code{dup} (whatever that means). @item Once again, the text interpreter resumes scanning the line and gets the group of characters @code{fred}. It looks them up in the name dictionary, but can't find them. It tries to treat them as a number, but they don't represent any legal number. @end itemize At this point, the text interpreter gives up and prints an error message. The error message shows exactly how far the text interpreter got in processing the line. In particular, it shows that the text interpreter made no attempt to do anything with the final character group, @code{dup}, even though we have good reason to believe that the text interpreter would have no problem looking that word up and executing it a second time. @comment ---------------------------------------------- @node Stacks and Postfix notation, Your first definition, Introducing the Text Interpreter, Introduction @section Stacks, postfix notation and parameter passing @cindex text interpreter @cindex outer interpreter In procedural programming languages (like C and Pascal), the building-block of programs is the @dfn{function} or @dfn{procedure}. These functions or procedures are called with @dfn{explicit parameters}. For example, in C we might write: @example total = total + new_volume(length,height,depth); @end example @noindent where new_volume is a function-call to another piece of code, and total, length, height and depth are all variables. length, height and depth are parameters to the function-call. In Forth, the equivalent of the function or procedure is the @dfn{definition} and parameters are implicitly passed between definitions using a shared stack that is visible to the programmer. Although Forth does support variables, the existence of the stack means that they are used far less often than in most other programming languages. When the text interpreter encounters a number, it will place (@dfn{push}) it on the stack. There are several stacks (the actual number is implementation-dependent ..) and the particular stack used for any operation is implied unambiguously by the operation being performed. The stack used for all integer operations is called the @dfn{data stack} and, since this is the stack used most commonly, references to ``the data stack'' are often abbreviated to ``the stack''. The stacks have a last-in, first-out (LIFO) organisation. If you type: @example @kbd{1 2 3} ok @end example Then this instructs the text interpreter to placed three numbers on the (data) stack. An analogy for the behaviour of the stack is to take a pack of playing cards and deal out the ace (1), 2 and 3 into a pile on the table. The 3 was the last card onto the pile (``last-in'') and if you take a card off the pile then, unless you're prepared to fiddle a bit, the card that you take off will be the 3 (``first-out''). The number that will be first-out of the stack is called the @dfn{top of stack}, which @cindex TOS definition is often abbreviated to @dfn{TOS}. To understand how parameters are passed in Forth, consider the behaviour of the definition @code{+} (pronounced ``plus''). You will not be surprised to learn that this definition performs addition. More precisely, it adds two number together and produces a result. Where does it get the two numbers from? It takes the top two numbers off the stack. Where does it place the result? On the stack. You can act-out the behaviour of @code{+} with your playing cards like this: @itemize @bullet @item Pick up two cards from the stack on the table @item Stare at them intently and ask yourself ``what @i{is} the sum of these two numbers'' @item Decide that the answer is 5 @item Shuffle the two cards back into the pack and find a 5 @item Put a 5 on the remaining ace that's on the table. @end itemize If you don't have a pack of cards handy but you do have Forth running, you can use the definition @code{.s} to show the current state of the stack, without affecting the stack. Type: @example @kbd{clearstack 1 2 3} ok @kbd{.s} <3> 1 2 3 ok @end example The text interpreter looks up the word @code{clearstack} and executes it; it tidies up the stack and removes any entries that may have been left on it by earlier examples. The text interpreter pushes each of the three numbers in turn onto the stack. Finally, the text interpreter looks up the word @code{.s} and executes it. The effect of executing @code{.s} is to print the ``<3>'' (the total number of items on the stack) followed by a list of all the items on the stack; the item on the far right-hand side is the TOS. You can now type: @example @kbd{+ .s} <2> 1 5 ok @end example @noindent which is correct; there are now 2 items on the stack and the result of the addition is 5. If you're playing with cards, try doing a second addition: pick up the two cards, work out that their sum is 6, shuffle them into the pack, look for a 6 and place that on the table. You now have just one item on the stack. What happens if you try to do a third addition? Pick up the first card, pick up the second card -- ah! There is no second card. This is called a @dfn{stack underflow} and consitutes an error. If you try to do the same thing with Forth it will report an error (probably a Stack Underflow or an Invalid Memory Address error). The opposite situation to a stack underflow is a @dfn{stack overflow}, which simply accepts that there is a finite amount of storage space reserved for the stack. To stretch the playing card analogy, if you had enough packs of cards and you piled the cards up on the table, you would eventually be unable to add another card; you'd hit the ceiling. Gforth allows you to set the maximum size of the stacks. In general, the only time that you will get a stack overflow is because a definition has a bug in it and is generating data on the stack uncontrollably. There's one final use for the playing card analogy. If you model your stack using a pack of playing cards, the maximum number of items on your stack will be 52 (I assume you didn't use the Joker). The maximum @i{value} of any item on the stack is 13 (the King). In fact, the only possible numbers are positive integer numbers 1 through 13; you can't have (for example) 0 or 27 or 3.52 or -2. If you change the way you think about some of the cards, you can accommodate different numbers. For example, you could think of the Jack as representing 0, the Queen as representing -1 and the King as representing -2. Your *range* remains unchanged (you can still only represent a total of 13 numbers) but the numbers that you can represent are -2 through 10. In that analogy, the limit was the amount of information that a single stack entry could hold, and Forth has a similar limit. In Forth, the size of a stack entry is called a @dfn{cell}. The actual size of a cell is implementation dependent and affects the maximum value that a stack entry can hold. A Standard Forth provides a cell size of at least 16-bits, and most desktop systems use a cell size of 32-bits. Forth does not do any type checking for you, so you are free to manipulate and combine stack items in any way you wish. A convenient way of treating stack items is as 2's complement signed integers, and that is what Standard words like @code{+} do. Therefore you can type: @example @kbd{-5 12 + .s} <1> 7 ok @end example If you use numbers and definitions like @code{+} in order to turn Forth into a great big pocket calculator, you will realise that it's rather different from a normal calculator. Rather than typing 2 + 3 = you had to type 2 3 + (ignore the fact that you had to use @code{.s} to see the result). The terminology used to describe this difference is to say that your calculator uses @dfn{Infix Notation} (parameters and operators are mixed) whilst Forth uses @dfn{Postfix Notation} (parameters and operators are separate), also called @dfn{Reverse Polish Notation}. Whilst postfix notation might look confusing to begin with, it has several important advantages: @itemize @bullet @item it is unambiguous @item it is more concise @item it fits naturally with a stack-based system @end itemize To examine these claims in more detail, consider these sums: @example 6 + 5 * 4 = 4 * 5 + 6 = @end example If you're just learning maths or your maths is very rusty, you will probably come up with the answer 44 for the first and 26 for the second. If you are a bit of a whizz at maths you will remember the @i{convention} that multiplication takes precendence over addition, and you'd come up with the answer 26 both times. To explain the answer 26 to someone who got the answer 44, you'd probably rewrite the first sum like this: @example 6 + (5 * 4) = @end example If what you really wanted was to perform the addition before the multiplication, you would have to use parentheses to force it. If you did the first two sums on a pocket calculator you would probably get the right answers, unless you were very cautious and entered them using these keystroke sequences: 6 + 5 = * 4 = 4 * 5 = + 6 = Postfix notation is unambiguous because the order that the operators are applied is always explicit; that also means that parentheses are never required. The operators are @i{active} (the act of quoting the operator makes the operation occur) which removes the need for ``=''. The sum 6 + 5 * 4 can be written (in postfix notation) in two equivalent ways: @example 6 5 4 * + or: 5 4 * 6 + @end example An important thing that you should notice about this notation is that the @i{order} of the numbers does not change; if you want to subtract 2 from 10 you type @code{10 2 -}. The reason that Forth uses postfix notation is very simple to explain: it makes the implementation extremely simple, and it follows naturally from using the stack as a mechanism for passing parameters. Another way of thinking about this is to realise that all Forth definitions are @i{active}; they execute as they are encountered by the text interpreter. The result of this is that the syntax of Forth is trivially simple. @comment ---------------------------------------------- @node Your first definition, How does that work?, Stacks and Postfix notation, Introduction @section Your first Forth definition @cindex first definition Until now, the examples we've seen have been trivial; we've just been using Forth as a bigger-than-pocket calculator. Also, each calculation we've shown has been a ``one-off'' -- to repeat it we'd need to type it in again@footnote{That's not quite true. If you press the up-arrow key on your keyboard you should be able to scroll back to any earlier command, edit it and re-enter it.} In this section we'll see how to add new words to Forth's vocabulary. The easiest way to create a new word is to use a @dfn{colon definition}. We'll define a few and try them out before worrying too much about how they work. Try typing in these examples; be careful to copy the spaces accurately: @example : add-two 2 + . ; : greet ." Hello and welcome" ; : demo 5 add-two ; @end example @noindent Now try them out: @example @kbd{greet} Hello and welcome ok @kbd{greet greet} Hello and welcomeHello and welcome ok @kbd{4 add-two} 6 ok @kbd{demo} 7 ok @kbd{9 greet demo add-two} Hello and welcome7 11 ok @end example The first new thing that we've introduced here is the pair of words @code{:} and @code{;}. These are used to start and terminate a new definition, respectively. The first word after the @code{:} is the name for the new definition. As you can see from the examples, a definition is built up of words that have already been defined; Forth makes no distinction between definitions that existed when you started the system up, and those that you define yourself. The examples also introduce the words @code{.} (dot), @code{."} (dot-quote) and @code{dup} (dewp). Dot takes the value from the top of the stack and displays it. It's like @code{.s} except that it only displays the top item of the stack and it is destructive; after it has executed, the number is no longer on the stack. There is always one space printed after the number, and no spaces before it. Dot-quote defines a string (a sequence of characters) that will be printed when the word is executed. The string can contain any printable characters except @code{"}. A @code{"} has a special function; it is not a Forth word but it acts as a delimiter (the way that delimiters work is described in the next section). Finally, @code{dup} duplicates the value at the top of the stack. Try typing @code{5 dup .s} to see what it does. We already know that the text interpreter searches through the dictionary to locate names. If you've followed the examples earlier, you will already have a definition called @code{add-two}. Lets try modifying it by typing in a new definition: @example @kbd{: add-two dup . ." + 2 =" 2 + . ;} redefined add-two ok @end example Forth recognised that we were defining a word that already exists, and printed a message to warn us of that fact. Let's try out the new definition: @example @kbd{9 add-two} 9 + 2 =11 ok @end example @noindent All that we've actually done here, though, is to create a new definition, with a particular name. The fact that there was already a definition with the same name did not make any difference to the way that the new definition was created (except that Forth printed a warning message). The old definition of add-two still exists (try @code{demo} again to see that this is true). Any new definition will use the new definition of @code{add-two}, but old definitions continue to use the version that already existed at the time that they were @code{compiled}. Before you go on to the next section, try defining and redefining some words of your own. @comment ---------------------------------------------- @node How does that work?, Forth is written in Forth, Your first definition, Introduction @section How does that work? @cindex parsing words Now we're going to take another look at the definition of @code{add-two} from the previous section. From our knowledge of the way that the text interpreter works, we would have expected this result when we tried to define @code{add-two}: @example @kbd{: add-two 2 + . " ;} ^^^^^^^ Error: Undefined word @end example The reason that this didn't happen is bound up in the way that @code{:} works. The word @code{:} does two special things. The first special thing that it does prevents the text interpreter from ever seeing the characters @code{add-two}. The text interpreter uses a variable called @cindex modifying >IN @code{>IN} (pronounced ''to-in'') to keep track of where it is in the input line. When it encounters the word @code{:} it behaves in exactly the same way as it does for any other word; it looks it up in the name dictionary, finds its xt and executes it. When @code{:} executes, it looks at the input buffer, finds the word @code{add-two} and advances the value of @code{>IN} to point past it. It then does some other stuff associated with creating the new definition (including creating an entry for @code{add-two} in the name dictionary). When the execution of @code{:} completes, control returns to the text interpreter, which is oblivious to the fact that it has been tricked into ignoring part of the input line. @cindex parsing words Words like @code{:} -- words that advance the value of @code{>IN} and so prevent the text interpreter from acting on the whole of the input line -- are called @dfn{parsing words}. @cindex @code{state} - effect on the text interpreter @cindex text interpreter - effect of state The second special thing that @code{:} does is change the value of a variable called @code{state}, which affects the way that the text interpreter behaves. When Gforth starts up, @code{state} has the value 0, and the text interpreter is said to be @dfn{interpreting}. During a colon definition (started with @code{:}), @code{state} is set to -1 and the text interpreter is said to be @dfn{compiling}. The word @code{;} ends the definition -- one of the things that it does is to change the value of @code{state} back to 0. We have already seen how the text interpreter behaves when it is interpreting; it looks for each character sequence in the dictionary, finds its xt and executes it, or it converts it to a number and pushes it onto the stack, or it fails to do either and generates an error. When the text interpreter is compiling, its behaviour is slightly different; it still looks for each character sequence in the dictionary and finds its xt, or converts it to a number, or fails to do either and generates an error. However, instead of executing the xt or pushing the number onto the stack it lays down (@dfn{compiles}) some magic to make that xt or number get executed or pushed at a later time; at the time that @code{add-two} is @dfn{executed}. Therefore, when you execute @code{add-two} its @dfn{run-time effect} is exactly the same as if you had typed @code{2 + .} outside of a definition, and pressed carriage-return. In Forth, every word or number can be described in terms of three properties: @itemize @bullet @item Its behaviour at @dfn{compile} time @item Its behaviour at @dfn{interpret} time @item Its behaviour at @dfn{execution} time. @end itemize These behaviours are called the @dfn{semantics} of the word or number. The value of @code{state} determines whether the text interpreter will use the compilation or interpretation semantics of a word or number that it encounters. @itemize @bullet @item @cindex interpretation semantics When the text interpreter encounters a word or number in @dfn{interpret} state, it performs the @dfn{interpretation semantics} of the word or number. @item @cindex compilation semantics When the text interpreter encounters a word or number in @dfn{compile} state, it performs the @dfn{compilation semantics} of the word or number. @end itemize @noindent Numbers are always treated in a fixed way: @itemize @bullet @item When the number is @dfn{compiled}, it is appended to the current definition so that its run-time behaviour is to execute. (In other words, the compilation semantics of a number are to postpone its execution semantics until the run-time of the definition that it is being compiled into.) @item When the number is @dfn{interpreted}, its behaviour is to execute. (In other words, the interpretation semantics of a number are to perform its execution semantics.) @item @cindex execution semantics When the number is @dfn{executed}, its behaviour is to push its value onto the stack. (In other words, the execution semantics of a number are to push its value onto the stack.) @end itemize The behaviour of a word is not so regular, but most have @i{default semantics} which means that they behave like this: @itemize @bullet @item The @dfn{compilation semantics} of the word are to append its @dfn{execution semantics} to the current definition (so that its run-time behaviour is to execute). @item The @dfn{interpretation semantics} of the word are to execute. @item The @dfn{execution semantics} of the word are to do something useful. @end itemize The actual behaviour of any particular word depends upon the way in which it was defined. When the text interpreter finds the word in the name dictionary, it not only retrieves the xt for the word, it also retrieves some flags: the @dfn{compile-only} flag and the @dfn{immediate flag}. The compile-only flag indicates that the word has no interpretation semantics; any attempt to interpret a word that has the compile-only flag set will generate an error (for example, @code{IF} has no interpretation semantics). The immediate flag changes the compilation semantics of the word; if it is set, the text interpreter will @dfn{execute} the word rather than @dfn{compiling} @cindex immediate words it. In other words, these so-called @dfn{immediate} words behave like this: @itemize @bullet @item The @dfn{compilation semantics} of the word are to perform its @dfn{execution semantics} (so that its compile-time behaviour is to execute). @item The @dfn{interpretation semantics} of the word are to execute. @item The @dfn{execution semantics} of the word are to do something useful. @end itemize This example shows the difference between an immediate and a non-immediate word: @example : show-state state @@ . ; : show-state-now show-state ; immediate : word1 show-state ; : word2 show-state-now ; @end example The word @code{immediate} after the definition of @code{show-state-now} makes that word an immediate word. These definitions introduce a new word: @code{@@} (pronounced ``fetch''). This word fetches the value of a variable, and leaves it on the stack. Therefore, the behaviour of @code{show-state} is to print a number that represents the current value of @code{state}. When you execute @code{word1}, it prints the number 0, indicating that the system is interpreting. When the text interpreter compiled the definition of @code{word1}, it encountered @code{show-state} whose compilation semantics are to append its execution semantics to the current definition. When you execute @code{word1}, it performs the execution semantics of @code{show-state}. At the time that @code{word1} (and therefore @code{show-state}) are executed, the system is interpreting. When you pressed after entering the definition of @code{word2}, you should have seen the number -1 printed, followed by ``@code{ ok}''. When the text interpreter compiled the definition of @code{word2}, it encountered @code{show-state-now}, an immediate word, whose compilation semantics are therefore to perform its execution semantics. It is executed straight away (even before the text interpreter has moved on to process another group of characters; the @code{;} in this example). The effect of executing it are to display the value of @code{state} @i{at the time that the definition of} @code{word2} @i{is being defined}. Printing -1 demonstrates that the system is compiling at this time. If you execute @code{word2} it does nothing at all. @cindex @code{."}, how it works Before leaving the subject of immediate words, consider the behaviour of @code{."} in the definition of @code{greet}, in the previous section. This word is both a parsing word and an immediate word. Notice that there is a space between @code{."} and the start of the text @code{Hello and welcome}, but that there is no space between the last letter of @code{welcome} and the @code{"} character. The reason for this is that @code{."} is a Forth word; it must have a space after it so that the text interpreter can identify it. The @code{"} is not a Forth word; it is a @dfn{delimiter}. The examples earlier show that, when the string is displayed, there is neither a space before the @code{H} nor after the @code{e}. Since @code{."} is an immediate word, it executes at the time that @code{greet} is defined. When it executes, its behaviour is to search forward in the input line looking for the delimiter. When it finds the delimiter, it updates @code{>IN} to point past the delimiter. It also compiles some magic code into the definition of @code{greet}; the xt of a run-time routine that prints a text string. It compiles the string @code{Hello and welcome} into memory so that it is available to be printed later. When the text interpreter gains control, the next word it finds in the input stream is @code{;} and so it terminates the definition of @code{greet}. @comment ---------------------------------------------- @node Forth is written in Forth, Review - elements of a Forth system, How does that work?, Introduction @section Forth is written in Forth @cindex structure of Forth programs When you start up a Forth compiler, a large number of definitions already exist. In Forth, you develop a new application using bottom-up programming techniques to create new definitions that are defined in terms of existing definitions. As you create each definition you can test and debug it interactively. If you have tried out the examples in this section, you will probably have typed them in by hand; when you leave Gforth, your definitions will be lost. You can avoid this by using a text editor to enter Forth source code into a file, and then loading code from the file using @code{include} (@xref{Forth source files}). A Forth source file is processed by the text interpreter, just as though you had typed it in by hand@footnote{Actually, there are some subtle differences -- see @ref{The Text Interpreter}.}. Gforth also supports the traditional Forth alternative to using text files for program entry (@xref{Blocks}). In common with many, if not most, Forth compilers, most of Gforth is actually written in Forth. All of the @file{.fs} files in the installation directory@footnote{For example, @file{/usr/local/share/gforth..}} are Forth source files, which you can study to see examples of Forth programming. Gforth maintains a history file that records every line that you type to the text interpreter. This file is preserved between sessions, and is used to provide a command-line recall facility. If you enter long definitions by hand, you can use a text editor to paste them out of the history file into a Forth source file for reuse at a later time (@pxref{Command-line editing} for more information). @comment ---------------------------------------------- @node Review - elements of a Forth system, Where to go next, Forth is written in Forth, Introduction @section Review - elements of a Forth system @cindex elements of a Forth system To summarise this chapter: @itemize @bullet @item Forth programs use @dfn{factoring} to break a problem down into small fragments called @dfn{words} or @dfn{definitions}. @item Forth program development is an interactive process. @item The main command loop that accepts input, and controls both interpretation and compilation, is called the @dfn{text interpreter} (also known as the @dfn{outer interpreter}). @item Forth has a very simple syntax, consisting of words and numbers separated by spaces or carriage-return characters. Any additional syntax is imposed by @dfn{parsing words}. @item Forth uses a stack to pass parameters between words. As a result, it uses postfix notation. @item To use a word that has previously been defined, the text interpreter searches for the word in the @dfn{name dictionary}. @item Words have @dfn{interpretation semantics}, @dfn{compilation semantics} and @dfn{execution semantics}. @item The text interpreter uses the value of @code{state} to select between the use of the @dfn{interpretation semantics} and the @dfn{compilation semantics} of a word that it encounters. @item The relationship between the @dfn{interpretation semantics}, @dfn{compilation semantics} and @dfn{execution semantics} for a word depend upon the way in which the word was defined (for example, whether it is an @dfn{immediate} word). @item Forth definitions can be implemented in Forth (called @dfn{high-level definitions}) or in some other way (usually a lower-level language and as a result often called @dfn{low-level definitions}, @dfn{code definitions} or @dfn{primitives}). @item Many Forth systems are implemented mainly in Forth. @end itemize @comment ---------------------------------------------- @node Where to go next,Exercises,Review - elements of a Forth system, Introduction @section Where To Go Next @cindex where to go next Amazing as it may seem, if you have read (and understood) this far, you know almost all the fundamentals about the inner workings of a Forth system. You certainly know enough to be able to read and understand the rest of this manual and the ANS Forth document, to learn more about the facilities that Forth in general and Gforth in particular provide. Even scarier, you know almost enough to implement your own Forth system. However, that's not a good idea just yet.. better to try writing some programs in Gforth. Forth has such a rich vocabulary that it can be hard to know where to start in learning it. This section suggests a few sets of words that are enough to write small but useful programs. Use the word index in this document to learn more about each word, then try it out and try to write small definitions using it. Start by experimenting with these words: @itemize @bullet @item Arithmetic: @code{+ - * / /MOD */ ABS INVERT} @item Comparison: @code{MIN MAX =} @item Logic: @code{AND OR XOR NOT} @item Stack manipulation: @code{DUP DROP SWAP OVER} @item Loops and decisions: @code{IF ELSE ENDIF ?DO I LOOP} @item Input/Output: @code{. ." EMIT CR KEY} @item Defining words: @code{: ; CREATE} @item Memory allocation words: @code{ALLOT ,} @item Tools: @code{SEE WORDS .S MARKER} @end itemize When you have mastered those, go on to: @itemize @bullet @item More defining words: @code{VARIABLE CONSTANT VALUE TO CREATE DOES>} @item Memory access: @code{@@ !} @end itemize When you have mastered these, there's nothing for it but to read through the whole of this manual and find out what you've missed. @comment ---------------------------------------------- @node Exercises, ,Where to go next, Introduction @section Exercises @cindex exercises TODO: provide a set of programming excercises linked into the stuff done already and into other sections of the manual. Provide solutions to all the exercises in a .fs file in the distribution. @c Get some inspiration from Starting Forth and Kelly&Spies. @c excercises: @c 1. take inches and convert to feet and inches. @c 2. take temperature and convert from fahrenheight to celcius; @c may need to care about symmetric vs floored?? @c 3. take input line and do character substitution @c to encipher or decipher @c 4. as above but work on a file for in and out @c 5. take input line and convert to pig-latin @c @c thing of sets of things to exercise then come up with @c problems that need those things. @c ****************************************************************** @node Words, Error messages, Introduction, Top @chapter Forth Words @cindex words @menu * Notation:: * Comments:: * Boolean Flags:: * Arithmetic:: * Stack Manipulation:: * Memory:: * Control Structures:: * Defining Words:: * The Text Interpreter:: * Tokens for Words:: * Word Lists:: * Environmental Queries:: * Files:: * Blocks:: * Other I/O:: * Programming Tools:: * Assembler and Code Words:: * Threading Words:: * Locals:: * Structures:: * Object-oriented Forth:: * Passing Commands to the OS:: * Miscellaneous Words:: @end menu @node Notation, Comments, Words, Words @section Notation @cindex notation of glossary entries @cindex format of glossary entries @cindex glossary notation format @cindex word glossary entry format The Forth words are described in this section in the glossary notation that has become a de-facto standard for Forth texts, i.e., @format @i{word} @i{Stack effect} @i{wordset} @i{pronunciation} @end format @i{Description} @table @var @item word The name of the word. @item Stack effect @cindex stack effect The stack effect is written in the notation @code{@i{before} -- @i{after}}, where @i{before} and @i{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 Gforth uses a separate floating point stack, but a unified stack notation. Also, return stack effects are not shown in @i{stack effect}, but in @i{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 @i{ -- }. 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. @cindex pronounciation of words @item pronunciation How the word is pronounced. @cindex wordset @item wordset The ANS Forth standard is divided into several word sets. A standard system need not support all of them. Therefore, in theory, the fewer word sets your program uses the more portable it will be. However, we suspect that most ANS Forth systems on personal machines will feature all word sets. Words that are not defined in ANS Forth have @code{gforth} or @code{gforth-internal} as word set. @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 word set field. @item Description A description of the behaviour of the word. @end table @cindex types of stack items @cindex stack item types The type of a stack item is specified by the character(s) the name starts with: @table @code @item f @cindex @code{f}, stack item type Boolean flags, i.e. @code{false} or @code{true}. @item c @cindex @code{c}, stack item type Char @item w @cindex @code{w}, stack item type Cell, can contain an integer or an address @item n @cindex @code{n}, stack item type signed integer @item u @cindex @code{u}, stack item type unsigned integer @item d @cindex @code{d}, stack item type double sized signed integer @item ud @cindex @code{ud}, stack item type double sized unsigned integer @item r @cindex @code{r}, stack item type Float (on the FP stack) @item a- @cindex @code{a_}, stack item type Cell-aligned address @item c- @cindex @code{c_}, stack item type Char-aligned address (note that a Char may have two bytes in Windows NT) @item f- @cindex @code{f_}, stack item type Float-aligned address @item df- @cindex @code{df_}, stack item type Address aligned for IEEE double precision float @item sf- @cindex @code{sf_}, stack item type Address aligned for IEEE single precision float @item xt @cindex @code{xt}, stack item type Execution token, same size as Cell @item wid @cindex @code{wid}, stack item type Word list ID, same size as Cell @item f83name @cindex @code{f83name}, stack item type Pointer to a name structure @item " @cindex @code{"}, stack item type string in the input stream (not on 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 Comments, Boolean Flags, Notation, Words @section Comments @cindex comments Forth supports two styles of comment; the traditional @i{in-line} comment, @code{(} and its modern cousin, the @i{comment to end of line}; @code{\}. doc-( doc-\ doc-\G @node Boolean Flags, Arithmetic, Comments, Words @section Boolean Flags @cindex Boolean flags A Boolean flag is cell-sized. A cell with all bits clear represents the flag @code{false} and a flag with all bits set represents the flag @code{true}. Words that check a flag (for example, @code{IF}) will treat a cell that has @i{any} bit set as @code{true}. doc-true doc-false doc-on doc-off @node Arithmetic, Stack Manipulation, Boolean Flags, Words @section Arithmetic @cindex arithmetic words @cindex division with potentially negative operands Forth arithmetic is not checked, i.e., you will not hear about integer overflow on addition or multiplication, you may hear about division by zero if you are lucky. The operator is written after the operands, but the operands are still in the original order. I.e., the infix @code{2-1} corresponds to @code{2 1 -}. Forth offers a variety of division 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, @pxref{Mixed precision}). @comment TODO discuss the different division forms and the std approach @menu * Single precision:: * Bitwise operations:: * Double precision:: Double-cell integer arithmetic * Numeric comparison:: * Mixed precision:: Operations with single and double-cell integers * Floating Point:: @end menu @node Single precision, Bitwise operations, Arithmetic, Arithmetic @subsection Single precision @cindex single precision arithmetic words By default, numbers in Forth are single-precision integers that are 1 cell in size. They can be signed or unsigned, depending upon how you treat them. @xref{Number Conversion} for the rules used by the text interpreter for recognising single-precision integers. doc-+ doc-1+ doc-- doc-1- doc-* doc-/ doc-mod doc-/mod doc-negate doc-abs doc-min doc-max doc-d>s doc-floored @node Bitwise operations, Double precision, Single precision, Arithmetic @subsection Bitwise operations @cindex bitwise operation words doc-and doc-or doc-xor doc-invert doc-lshift doc-rshift doc-2* doc-d2* doc-2/ doc-d2/ @node Double precision, Numeric comparison, Bitwise operations, Arithmetic @subsection Double precision @cindex double precision arithmetic words @xref{Number Conversion} for the rules used by the text interpreter for recognising double-precision integers. A double precision number is represented by a cell pair, with the most significant digit at the TOS. It is trivial to convert an unsigned single to an (unsigned) double; simply push a @code{0} onto the TOS. Since numbers are represented by Gforth using 2's complement arithmetic, converting a signed single to a (signed) double requires sign-extension across the most significant digit. This can be achieved using @code{s>d}. The moral of the story is that you cannot convert a number without knowing whether it represents an unsigned or a signed number. doc-s>d doc-d+ doc-d- doc-dnegate doc-dabs doc-dmin doc-dmax @node Numeric comparison, Mixed precision, Double precision, Arithmetic @subsection Numeric comparison @cindex numeric comparison words doc-< doc-<= doc-<> doc-= doc-> doc->= doc-0< doc-0<= doc-0<> doc-0= doc-0> doc-0>= doc-u< doc-u<= @comment TODO why u<> and u= .. they are the same as <> and = doc-u<> doc-u= doc-u> doc-u>= doc-within doc-d< doc-d<= doc-d<> doc-d= doc-d> doc-d>= doc-d0< doc-d0<= doc-d0<> doc-d0= doc-d0> doc-d0>= doc-du< doc-du<= doc-du<> doc-du= doc-du> doc-du>= @node Mixed precision, Floating Point, Numeric comparison, Arithmetic @subsection Mixed precision @cindex mixed precision arithmetic words doc-m+ doc-*/ doc-*/mod doc-m* doc-um* doc-m*/ doc-um/mod doc-fm/mod doc-sm/rem @node Floating Point, , Mixed precision, Arithmetic @subsection Floating Point @cindex floating point arithmetic words @xref{Number Conversion} for the rules used by the text interpreter for recognising floating-point numbers. @cindex angles in trigonometric operations @cindex trigonometric operations Angles in floating point operations are given in radians (a full circle has 2 pi radians). Gforth has a separate floating point stack, but the documentation uses the unified notation. @cindex floating-point arithmetic, pitfalls 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} (@url{http://www.validgh.com/goldberg/paper.ps}). doc-d>f doc-f>d 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 doc-pi doc-f0< doc-f0<= doc-f0<> doc-f0= doc-f0> doc-f0>= doc-f< doc-f<= doc-f<> doc-f= doc-f> doc-f>= doc-f2* doc-f2/ doc-1/f doc-f~ doc-precision doc-set-precision @node Stack Manipulation, Memory, Arithmetic, Words @section Stack Manipulation @cindex stack manipulation words @cindex floating-point stack in the standard Gforth maintains a number of separate stacks: @cindex data stack @cindex parameter stack @itemize @bullet @item A data stack (also known as the @dfn{parameter stack}) -- for characters, cells, addresses, and double cells. @cindex floating-point stack @item A floating point stack -- for floating point numbers. @cindex return stack @item A return stack -- for storing the return addresses of colon definitions and other data. @cindex locals stack @item A locals stack for storing local variables. @end itemize Whilst every sane Forth has a separate floating-point stack, it is not strictly required; an ANS Forth system could theoretically keep floating-point numbers on the data stack. As an 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. Instead, just say that your program has an environmental dependency on a separate floating-point stack. doc-floating-stack @cindex return stack and locals @cindex locals and return stack A Forth system is allowed to keep 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 compliant program and you are using local variables in a word, forget about return stack manipulations in that word (refer to 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 @cindex data stack manipulation words @cindex stack manipulations words, data stack doc-drop doc-nip doc-dup doc-over doc-tuck doc-swap doc-pick doc-rot doc--rot doc-?dup doc-roll doc-2drop doc-2nip doc-2dup doc-2over doc-2tuck doc-2swap doc-2rot @node Floating point stack, Return stack, Data stack, Stack Manipulation @subsection Floating point stack @cindex floating-point stack manipulation words @cindex stack manipulation words, floating-point stack doc-fdrop doc-fnip doc-fdup doc-fover doc-ftuck doc-fswap doc-fpick doc-frot @node Return stack, Locals stack, Floating point stack, Stack Manipulation @subsection Return stack @cindex return stack manipulation words @cindex stack manipulation words, return stack doc->r doc-r> doc-r@ doc-rdrop doc-2>r doc-2r> doc-2r@ doc-2rdrop @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation @subsection Locals stack @comment TODO @node Stack pointer manipulation, , Locals stack, Stack Manipulation @subsection Stack pointer manipulation @cindex stack pointer manipulation words doc-sp0 doc-s0 doc-sp@ doc-sp! doc-fp0 doc-fp@ doc-fp! doc-rp0 doc-r0 doc-rp@ doc-rp! doc-lp0 doc-l0 doc-lp@ doc-lp! @node Memory, Control Structures, Stack Manipulation, Words @section Memory @cindex memory words @cindex dictionary Forth definitions are organised in memory structures that are collectively called the @dfn{dictionary}. The dictionary can be considered as three logical memory regions: @itemize @bullet @item @cindex code space @cindex code dictionary Code space, also known as the @dfn{code dictionary}. @item @cindex name space @cindex name dictionary Name space, also known as the @dfn{name dictionary}@footnote{Sometimes, the term @dfn{dictionary} is used simply to refer to the name dictionary, because it is the one region that is used for looking up names, just as you would in a conventional dictionary.}. @item @cindex data space Data space @end itemize When you create a colon definition, the text interpreter compiles the code for the definition into the code dictionary and compiles the name of the definition into the name dictionary, together with other information about the definition (such as its execution token). When you create a variable, the execution of @code{variable} will compile some code, assign once cell in data space, and compile the name of the variable into the name dictionary. @cindex memory regions - relationship between them ANS Forth does not specify the relationship between the three memory regions, and specifies that a Standard program must not access code or data space directly -- it may only access data space directly. In addition, the Standard defines what relationships you may and may not rely on when allocating regions in data space. These constraints are simply a reflection of the many diverse techniques that are used to implement Forth systems; understanding and following the requirements of the Standard allows you to write portable programs -- programs that run in the same way on any of these diverse systems. Another way of looking at this is to say that ANS Forth was designed to permit compliant Forth systems to be implemented in many diverse ways. @cindex memory regions - how they are assigned Here are some examples of ways in which name, code and data spaces might be assigned in different Forth implementations: @itemize @bullet @item For a Forth system that runs from RAM under a general-purpose operating system, it can be convenient to interleave name, code and data spaces in a single contiguous memory region. This organisation can be memory-efficient (for example, because the relationship between the name dictionary entry and the associated code dictionary entry can be implicit, rather than requiring an explicit memory pointer to reference from the name dictionary and the code dictionary). This is the organisation used by Gforth, as this example@footnote{The addresses in the example have been truncated to fit it onto the page, and the addresses and data shown will not match the output from your system} shows: @example hex variable fred 123456 fred ! variable jim abcd jim ! : foo + / - ; ' fred 10 - 50 dump ..80: 5C 46 0E 40 84 66 72 65 - 64 20 20 20 20 20 20 20 \F.@.fred ..90: D0 9B 04 08 00 00 00 00 - 56 34 12 00 80 46 0E 40 ........V4...F.@ ..A0: 83 6A 69 6D 20 20 20 20 - D0 9B 04 08 00 00 00 00 .jim ........ ..B0: CD AB 00 00 9C 46 0E 40 - 83 66 6F 6F 20 20 20 20 .....F.@.foo ..C0: 80 9B 04 08 00 00 00 00 - E4 2E 05 08 0C 2F 05 08 ............./.. @end example @item For a high-performance system running on a modern RISC processor with a modified Harvard architecture (one that has a unified main memory but separate instruction and data caches), it is desirable to separate processor instructions from processor data. This encourages a high cache density and therefore a high cache hit rate. The Forth code dictionary is not necessarily made up entirely of processor instructions; its nature is dependent upon the Forth implementation. @item A Forth compiler that runs on a segmented 8086 processor could be designed to interleave the name, code and data spaces within a single 64Kbyte segment. A more common implementation choice is to use a separate 64Kbyte segment for each region, which provides more memory overall but provides an address map in which only the data space is accessible. @item Microprocessors exist that run Forth (or many of the primitives required to implement the Forth virtual machine efficiently) directly. On these processors, the relationship between name, code and data spaces may be imposed as a side-effect of the microarchitecture of the processor. @item A Forth compiler that executes from ROM on an embedded system needs its data space separated from the name and code spaces so that the data space can be mapped to a RAM area. @item A Forth compiler that runs on an embedded system may have a requirement for a small memory footprint. On such a system it can be useful to separate the name space from the data and code spaces; once the application has been compiled, the name dictionary is no longer required@footnote{more strictly speaking, most applications can be designed so that this is the case}. The name dictionary can be deleted entirely, or could be stored in memory on a remote @i{host} system for debug and development purposes. In the latter case, the compiler running on the @i{target} system could implement a protocol across a communication link that would allow it to interrogate the name dictionary. @end itemize @menu * Reserving Data Space:: * Memory Access:: * Address Arithmetic:: * Memory Blocks:: * Dynamic Allocation:: @end menu @node Reserving Data Space, Memory Access, Memory, Memory @subsection Reserving Data Space @cindex reserving data space @cindex data space - reserving some @cindex data space pointer - contiguous regions Data space may be reserved as individual chars or cells or in contiguous regions. These are the rules for reserving contiguous regions in a Standard (i.e., portable) way: @itemize @bullet @item The value of the data-space pointer, @code{here}, always defines the beginning of a contiguous region of data space. @item @code{CREATE} establishes the beginning of a contiguous region of data space (the @code{CREATE}d definition returns the initial address of the region). @item @code{variable} does @i{not} establish the beginning of a contiguous region in data space; @code{variable} followed by @code{allot} is not guaranteed to allocate data space region that is contiguous with the storage allocated by @code{variable}. Instead, use @code{create} -- @xref{Simple Defining Words} for examples. @item Successive calls to @code{allot}, @code{,} (comma), @code{2,} (2-comma), @code{c,} (c-comma) and @code{align} reserve a single contiguous region in data space. The contiguity of the region is interrupted by compiling (or removing) definitions from the dictionary. @item The most recently reserved contiguous region may be released by calling @code{allot} with a negative argument, provided that the region has not been interrupted by compiling (or removing) definitions from the dictionary. @end itemize @cindex data space pointer - alignment These factors affect the alignment of @code{here}, the data space pointer: @itemize @bullet @item If the data-space pointer is aligned@footnote{In ANS Forth-speak, @i{aligned} implictly means @code{CELL}-aligned.} before an @code{allot}, and a whole number of characters are reserved or released, it will remain aligned after the @code{allot}. @item If the data-space pointer is character-aligned before an @code{allot}, and a whole number of cells are reserved or released, it will remain character-aligned after the @code{allot}. @item The initial contents of data space reserved using @code{allot} is undefined. @item Definitions created by @code{create}, @code{variable}, @code{2variable} return aligned addresses. @item After a definition is compiled or @code{align} is executed, the data space pointer is guaranteed to be aligned. @end itemize doc-here doc-unused doc-allot doc-c, doc-f, doc-, doc-2, @cindex user space doc-udp doc-uallot @node Memory Access, Address Arithmetic, Reserving Data Space, Memory @subsection Memory Access @cindex memory access words doc-@ doc-! doc-+! doc-c@ doc-c! doc-2@ doc-2! doc-f@ doc-f! doc-sf@ doc-sf! doc-df@ doc-df! @node Address Arithmetic, Memory Blocks, Memory Access, Memory @subsection Address Arithmetic @cindex address arithmetic words ANS Forth does not specify the sizes of the data types. Instead, it offers a number of words for computing sizes and doing address arithmetic. Address arithmetic is performed in terms of address units (aus); on most systems the address unit is one byte. Note that a character may have more than one au, so @code{chars} is no noop (on systems where it is a noop, it compiles to nothing). @cindex alignment of addresses for types ANS Forth also defines words for aligning addresses for specific types. Many computers require that accesses to specific data types must only occur at specific addresses; e.g., that cells may only be accessed at addresses divisible by 4. Even if a machine allows unaligned accesses, it can usually perform aligned accesses faster. 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. ANS Forth defines no words for character-aligning addresses. This is not an oversight, but reflects the fact that addresses that are not char-aligned have no use in the standard and therefore will not be created. @cindex @code{CREATE} and alignment ANS Forth guarantees that addresses returned by @code{CREATE}d words are cell-aligned; in addition, Gforth guarantees that these addresses are aligned for all purposes. Note that the ANS Forth word @code{char} has nothing to do with address arithmetic. doc-chars doc-char+ doc-cells doc-cell+ doc-cell doc-align doc-aligned doc-floats doc-float+ doc-float doc-falign doc-faligned doc-sfloats doc-sfloat+ doc-sfalign doc-sfaligned doc-dfloats doc-dfloat+ doc-dfalign doc-dfaligned doc-maxalign doc-maxaligned doc-cfalign doc-cfaligned doc-address-unit-bits @node Memory Blocks, Dynamic Allocation, Address Arithmetic, Memory @subsection Memory Blocks @cindex memory block words @cindex character strings - moving and copying Memory blocks often represent character strings; @xref{String Formats} for ways of storing character strings in memory. @xref{Displaying characters and strings} for other string-processing words. Some of these words work on address units (increments of @code{CELL}), and expect a @code{CELL}-aligned address. Others work on character units (increments of @code{CHAR}), and expect a @code{CHAR}-aligned address. Choose the correct operation depending upon your data type. If you are moving a block of memory (for example, a region reserved by @code{allot}) it is safe to use @code{move}, and it should be faster than using @code{cmove}. If you are moving (for example) a string compiled using @code{S"}, it is not portable to use @code{move}; the alignment of the string in memory could change, and the relationship between @code{CELL} and @code{CHAR} could change. When copying characters between overlapping memory regions, choose carefully between @code{cmove} and @code{cmove>}. You can only use any of these words @i{portably} to access data space. @comment TODO - think the naming of the arguments is wrong for move @comment well, really it seems to be the Standard that's wrong; it @comment describes MOVE as a word that requires a CELL-aligned source @comment and destination address but a xtranfer count that need not @comment be a multiple of CELL. doc-move doc-erase doc-cmove doc-cmove> doc-fill doc-blank doc-compare doc-search doc--trailing doc-/string @comment TODO examples @node Dynamic Allocation, ,Memory Blocks, Memory @subsection Dynamic Allocation of Memory @cindex dynamic allocation of memory @cindex memory-allocation word set The ANS Forth memory-allocation word set allows memory regions to be dynamically assigned, resized and released without affecting the data space pointer. In Gforth, these words are implemented using the standard C library calls malloc(), free() and resize(). doc-allocate doc-free doc-resize @node Control Structures, Defining Words, Memory, Words @section Control Structures @cindex control structures Control structures in Forth cannot be used in interpret state, only in compile state@footnote{To be precise, they have no interpretation semantics (@pxref{Interpretation and Compilation Semantics}).}, i.e., in a colon definition. We do not like this limitation, but have not seen a satisfying way around it yet, although many schemes have been proposed. @menu * Selection:: IF.. ELSE.. ENDIF * Simple Loops:: BEGIN.. * Counted Loops:: DO * Arbitrary control structures:: * Calls and returns:: * Exception Handling:: @end menu @node Selection, Simple Loops, Control Structures, Control Structures @subsection Selection @cindex selection control structures @cindex control structures for selection @cindex @code{IF} control structure @example @i{flag} IF @i{code} ENDIF @end example @noindent or @example @i{flag} IF @i{code1} ELSE @i{code2} ENDIF @end example 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 prejudices against Forth in these people). Adding @code{ENDIF} to a system that only supplies @code{THEN} is simple: @example : ENDIF POSTPONE THEN ; immediate @end example [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then (adv.)} has the following meanings: @quotation ... 2b: following next after in order ... 3d: as a necessary consequence (if you were there, then you saw them). @end quotation Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal and many other programming languages has the meaning 3d.] 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 ANS Forth for @code{ENDIF}, @code{?DUP-IF} and @code{?DUP-0=-IF} are provided in @file{compat/control.fs}. @cindex @code{CASE} control structure @example @i{n} CASE @i{n1} OF @i{code1} ENDOF @i{n2} OF @i{code2} ENDOF @dots{} ENDCASE @end example Executes the first @i{codei}, where the @i{ni} is equal to @i{n}. A default case can be added by simply writing the code after the last @code{ENDOF}. It may use @i{n}, which is on top of the stack, but must not consume it. @node Simple Loops, Counted Loops, Selection, Control Structures @subsection Simple Loops @cindex simple loops @cindex loops without count @cindex @code{WHILE} loop @example BEGIN @i{code1} @i{flag} WHILE @i{code2} REPEAT @end example @i{code1} is executed and @i{flag} is computed. If it is true, @i{code2} is executed and the loop is restarted; If @i{flag} is false, execution continues after the @code{REPEAT}. @cindex @code{UNTIL} loop @example BEGIN @i{code} @i{flag} UNTIL @end example @i{code} is executed. The loop is restarted if @code{flag} is false. @cindex endless loop @cindex loops, endless @example BEGIN @i{code} AGAIN @end example This is an endless loop. @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures @subsection Counted Loops @cindex counted loops @cindex loops, counted @cindex @code{DO} loops The basic counted loop is: @example @i{limit} @i{start} ?DO @i{body} LOOP @end example This performs one iteration for every integer, starting from @i{start} and up to, but excluding @i{limit}. The counter, or @i{index}, can be accessed with @code{i}. For example, the loop: @example 10 0 ?DO i . LOOP @end example @noindent prints @code{0 1 2 3 4 5 6 7 8 9} The index of the innermost loop can be accessed with @code{i}, the index of the next loop with @code{j}, and the index of the third loop with @code{k}. doc-i doc-j doc-k The loop control data are kept on the return stack, so there are some restrictions on mixing return stack accesses and counted loop words. In particuler, if you put values on the return stack outside the loop, you cannot read them inside the loop@footnote{well, not in a way that is portable.}. If you put values on the return stack within a loop, you have to remove them before the end of the loop and before accessing the index of the loop. There are several variations on the counted loop: @itemize @bullet @item @code{LEAVE} leaves the innermost counted loop immediately; execution continues after the associated @code{LOOP} or @code{NEXT}. For example: @example 10 0 ?DO i DUP . 3 = IF LEAVE THEN LOOP @end example prints @code{0 1 2 3} @item @code{UNLOOP} prepares for an abnormal loop exit, e.g., via @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the return stack so @code{EXIT} can get to its return address. For example: @example : demo 10 0 ?DO i DUP . 3 = IF UNLOOP EXIT THEN LOOP ." Done" ; @end example prints @code{0 1 2 3} @item If @i{start} is greater than @i{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 @i{start} is greater than @i{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for unsigned loop parameters. @item @code{?DO} can 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. @item @code{LOOP} can be replaced with @code{@i{n} +LOOP}; this updates the index by @i{n} instead of by 1. The loop is terminated when the border between @i{limit-1} and @i{limit} is crossed. E.g.: @example 4 0 +DO i . 2 +LOOP @end example @noindent prints @code{0 2} @example 4 1 +DO i . 2 +LOOP @end example @noindent prints @code{1 3} @cindex negative increment for counted loops @cindex counted loops with negative increment The behaviour of @code{@i{n} +LOOP} is peculiar when @i{n} is negative: @example -1 0 ?DO i . -1 +LOOP @end example @noindent prints @code{0 -1} @example 0 0 ?DO i . -1 +LOOP @end example prints nothing. Therefore we recommend avoiding @code{@i{n} +LOOP} with negative @i{n}. One alternative is @code{@i{u} -LOOP}, which reduces the index by @i{u} each iteration. The loop is terminated when the border between @i{limit+1} and @i{limit} is crossed. Gforth also provides @code{-DO} and @code{U-DO} for down-counting loops. E.g.: @example -2 0 -DO i . 1 -LOOP @end example @noindent prints @code{0 -1} @example -1 0 -DO i . 1 -LOOP @end example @noindent prints @code{0} @example 0 0 -DO i . 1 -LOOP @end example @noindent prints nothing. @end itemize Unfortunately, @code{+DO}, @code{U+DO}, @code{-DO}, @code{U-DO} and @code{-LOOP} are not defined in ANS Forth. However, an implementation for these words that uses only standard words is provided in @file{compat/loops.fs}. @cindex @code{FOR} loops Another counted loop is: @example @i{n} FOR @i{body} NEXT @end example This is the preferred loop of native code compiler writers who are too lazy to optimize @code{?DO} loops properly. This loop structure is not defined in ANS Forth. In Gforth, this loop iterates @i{n+1} times; @code{i} produces values starting with @i{n} and ending with 0. Other Forth systems may behave differently, even if 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 @cindex control structures, user-defined @cindex control-flow stack 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. @cindex @code{orig}, control-flow stack item @cindex @code{dest}, control-flow stack item 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). doc-if doc-ahead doc-then doc-begin doc-until doc-again doc-cs-pick doc-cs-roll The Standard words @code{CS-PICK} and @code{CS-ROLL} allow you to manipulate the control-flow stack in a portable way. Without them, you would need to know how many stack items are occupied by a control-flow entry (many systems use one cell. In Gforth they currently take three, but this may change in the future). Some standard control structure words are built from these words: 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: 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}. Gforth 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 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: 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}. @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. For example, instead of writing: @example BEGIN ... IF [ 1 CS-ROLL ] ... AGAIN THEN @end example @noindent 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 @noindent 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 @cindex calling a definition @cindex returning from a definition @cindex recursive definitions A definition can be called simply be writing the name of the definition to be called. Normally a definition is invisible during its own definition. If you want to write a directly recursive definition, you can use @code{recursive} to make the current definition visible, or @code{recurse} to call the current definition directly. doc-recursive doc-recurse @comment TODO add example of the two recursion methods @quotation @progstyle I prefer using @code{recursive} to @code{recurse}, because calling the definition by name is more descriptive (if the name is well-chosen) than the somewhat cryptic @code{recurse}. E.g., in a quicksort implementation, it is much better to read (and think) ``now sort the partitions'' than to read ``now do a recursive call''. @end quotation For mutual recursion, use @code{Defer}red words, like this: @example Defer foo : bar ( ... -- ... ) ... foo ... ; :noname ( ... -- ... ) ... bar ... ; IS foo @end example The current definition returns control to the calling definition when the end of the definition is reached or @code{EXIT} is encountered. Deferred words are discussed in more detail in @ref{Simple Defining Words}. doc-exit doc-;s @node Exception Handling, , Calls and returns, Control Structures @subsection Exception Handling @cindex exceptions If your program detects a fatal error condition, the simplest action that it can take is to @code{quit}. This resets the return stack and restarts the text interpreter, but does not print any error message. The next stage in severity is to execute @code{abort}, which has the same effect as @code{quit}, with the addition that it resets the data stack. A slightly more sophisticated approach is use use @code{abort"}, which compiles a string to be used as an error message and does a conditional @code{abort} at run-time. For example: @example @kbd{: checker abort" That flag was true" ." A false flag" ;} ok @kbd{0 checker} A false flag ok @kbd{1 checker} :1: That flag was true 1 checker ^^^^^^^ $400D1648 throw $400E4660 @end example These simple techniques allow a program to react to a fatal error condition, but they are not exactly user-friendly. The ANS Forth Exception word set provides the pair of words @code{throw} and @code{catch}, which can be used to provide sophisticated error-handling. @code{catch} has a similar behaviour to @code{execute}, in that it takes an @i{xt} as a parameter and starts execution of the xt. However, before passing control to the xt, @code{catch} pushes an @dfn{exception frame} onto the @dfn{exception stack}. This exception frame is used to restore the system to a known state if a detected error occurs during the execution of the xt. A typical way to use @code{catch} would be: @example ... ['] foo catch IF ... @end example Whilst @code{foo} executes, it can call other words to any level of nesting, as usual. If @code{foo} (and all the words that it calls) execute successfully, control will ultimately passes to the word following the @code{catch}, and there will be a @code{true} flag (0) at TOS. However, if any word detects an error, it can terminate the execution of @code{foo} by pushing an error code onto the stack and then performing a @code{throw}. The execution of @code{throw} will pass control to the word following the @code{catch}, but this time the TOS will hold the error code. Therefore, the @code{IF} in the example can be used to determine whether @code{foo} executed successfully. This simple example shows how you can use @code{throw} and @code{catch} to ``take over'' exception handling from the system: @example : my-div ['] / catch if ." DIVIDE ERROR" else ." OK.. " . then ; @end example The next example is more sophisticated and shows a multi-level @code{throw} and @code{catch}. To understand this example, start at the definition of @code{top-level} and work backwards: @example : lowest-level ( -- c ) key dup 27 = if 1 throw \ ESCAPE key pressed else ." lowest-level successfull" CR then ; : lower-level ( -- c ) lowest-level \ at this level consider a CTRL-U to be a fatal error dup 21 = if \ CTRL-U 2 throw else ." lower-level successfull" CR then ; : low-level ( -- c ) ['] lower-level catch ?dup if \ error occurred - do we recognise it? dup 1 = if \ ESCAPE key pressed.. pretend it was an E [char] E else throw \ propogate the error upwards then then ." low-level successfull" CR ; : top-level ( -- ) CR ['] low-level catch \ CATCH is used like EXECUTE ?dup if \ error occurred.. ." Error " . ." occurred - contact your supplier" else ." The '" emit ." ' key was pressed" CR then ; @end example The ANS Forth document assigns @code{throw} codes thus: @itemize @bullet @item codes in the range -1 -- -255 are reserved to be assigned by the Standard. Assignments for codes in the range -1 -- -58 are currently documented in the Standard. In particular, @code{-1 throw} is equivalent to @code{abort} and @code{-2 throw} is equivalent to @code{abort"}. @item codes in the range -256 -- -4095 are reserved to be assigned by the system. @item all other codes may be assigned by programs. @end itemize Gforth provides the word @code{exception} as a mechanism for assigning system throw codes to applications. This allows multiple applications to co-exist in memory without any clash of @code{throw} codes. A definition of @code{exception} in ANS Forth is provided in @file{compat/exception.fs}. doc-quit doc-abort doc-abort" doc-catch doc-throw doc---exception-exception @c ------------------------------------------------------------- @node Defining Words, The Text Interpreter, Control Structures, Words @section Defining Words @cindex defining words @menu * Simple Defining Words:: Variables, values and constants * 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 @cindex simple defining words @cindex defining words, simple Defining words are used to create new entries in the dictionary. The simplest defining word is @code{CREATE}. @code{CREATE} is used like this: @example CREATE new-word1 @end example @code{CREATE} is a parsing word that generates a dictionary entry for @code{new-word1}. When @code{new-word1} is executed, all that it does is leave an address on the stack. The address represents the value of the data space pointer (@code{HERE}) at the time that @code{new-word1} was defined. Therefore, @code{CREATE} is a way of associating a name with the address of a region of memory. By extending this example to reserve some memory in data space, we end up with a @i{variable}. Here are two different ways to do it: @example CREATE new-word2 1 cells allot \ reserve 1 cell - initial value undefined CREATE new-word3 4 , \ reserve 1 cell and initialise it (to 4) @end example The variable can be examined and modified using @code{@@} (``fetch'') and @code{!} (``store'') like this: @example new-word2 @@ . \ get address, fetch from it and display 1234 new-word2 ! \ new value, get address, store to it @end example As a final refinement, the whole code sequence can be wrapped up in a defining word (pre-empting the subject of the next section), making it easier to create new variables: @example : myvariable ( "name" -- a-addr ) CREATE 1 cells allot ; myvariable foo myvariable joe 45 3 * foo ! \ set foo to 135 1234 joe ! \ set joe to 1234 3 joe +! \ increment joe by 3.. to 1237 @end example Not surprisingly, there is no need to define @code{myvariable}, since Forth already has a definition @code{Variable}. It behaves in exactly the same way as @code{myvariable} but it is implemented in an optimised way. Forth also provides @code{2Variable} and @code{fvariable} for double and floating-point variables, respectively. @cindex arrays A similar mechanism can be used to create arrays. For example, an 80-character text input buffer: @example CREATE text-buf 80 chars allot text-buf 0 chars c@@ \ the 1st character (offset 0) text-buf 3 chars c@@ \ the 4th character (offset 3) @end example You can build arbitrarily complex data structures by allocating appropriate areas of memory. @xref{Structures} for further discussions of this, and to learn about some Gforth tools that make it easier. @cindex user variables @cindex user space The defining word @code{User} behaves in the same way as @code{Variable}. The difference is that it reserves space in @i{user (data) space} rather than normal data space. In a Forth system that has a multi-tasker, each task has its own set of user variables. @comment TODO is that stuff about user variables strictly correct? Is it @comment just terminal tasks that have user variables? @comment should document tasker.fs (with some examples) elsewhere @comment in this manual, then expand on user space and user variables. After @code{CREATE} and @code{Variable}s, the next defining word to consider is @code{Constant}. @code{Constant} allows you to declare a fixed value and refer to it by name. For example: @example 12 Constant INCHES-PER-FOOT 3E+08 fconstant SPEED-O-LIGHT @end example A @code{Variable} can be both read and written, so its run-time behaviour is to supply an address through which its current value can be manipulated. In contrast, the value of a @code{Constant} cannot be changed once it has been declared@footnote{Well, often it can be -- but not in a Standard, portable way. It's safer to use a @code{Value} (read on).} so it's not necessary to supply the address -- it is more efficient to return the value of the constant directly. That's exactly what happens; the run-time effect of a constant is to put its value on the top of the stack (@ref{User-defined Defining Words} describes one way of implementing @code{Constant}). Gforth also provides @code{2Constant} and @code{fconstant} for defining double and floating-point constants, respectively. Constants in Forth behave differently from their equivalents in other programming languages. In other languages, a constant (such as an EQU in assembler or a #define in C) only exists at compile-time; in the executable program the constant has been translated into an absolute number and, unless you are using a symbolic debugger, it's impossible to know what abstract thing that number represents. In Forth a constant has an entry in the name dictionary and remains there after the code that uses it has been defined. In fact, it must remain in the dictionary since it has run-time duties to perform. For example: @example 12 Constant INCHES-PER-FOOT : FEET-TO-INCHES ( n1 -- n2 ) INCHES-PER-FOOT * ; @end example @cindex in-lining of constants When @code{FEET-TO-INCHES} is executed, it will in turn execute the xt associated with the constant @code{INCHES-PER-FOOT}. If you use @code{see} to decompile the definition of @code{FEET-TO-INCHES}, you can see that it makes a call to @code{INCHES-PER-FOOT}. Some Forth compilers attempt to optimise constants by in-lining them where they are used. You can force Gforth to in-line a constant like this: @example : FEET-TO-INCHES ( n1 -- n2 ) [ INCHES-PER-FOOT ] LITERAL * ; @end example If you use @code{see} to decompile @i{this} version of @code{FEET-TO-INCHES}, you can see that @code{INCHES-PER-FOOT} is no longer present. @xref{Interpret/Compile states} and @xref{Literals} explain to this works. In-lining constants in this way might improve execution time fractionally, and can ensure that a constant is now only referenced at compile-time. However, the definition of the constant still remains in the dictionary. Some Forth compilers provide a mechanism for controlling a second dictionary for holding transient words such that this second dictionary can be deleted later in order to recover memory space. However, there is no standard way of doing this. One aspect of constants and variables that can sometimes be confusing is that they have different stack effects; one returns its value whilst the other returns the address of its value. The defining word @code{Value} provides an alternative to @code{Variable}, and has the same stack effect as a constant. A @code{Value} needs an additional word, @code{TO} to allow its value to be changed. Here are some examples: @example 12 Value APPLES \ a Value is initialised when it is declared.. like a \ constant but unlike a variable 34 TO APPLES \ Change the value of APPLES. TO is a parsing word APPLES \ puts 34 on the top of the stack. @end example The defining word @code{Defer} allows you to define a word by name without defining its behaviour; the definition of its behaviour is deferred. Here are two situation where this can be useful: @itemize @bullet @item Where you want to allow the behaviour of a word to be altered later, and for all precompiled references to the word to change when its behaviour is changed. @item For mutual recursion; @xref{Calls and returns}. @end itemize In the following example, @code{foo} always invokes the version of @code{greet} that prints ``@code{Good morning}'' whilst @code{bar} always invokes the version that prints ``@code{Hello}''. There is no way of getting @code{foo} to use the later version without re-ordering the source code and recompilng it. @example : greet ." Good morning" ; : foo ... greet ... ; : greet ." Hello" ; : bar ... greet ... ; @end example This problem can be solved by defining @code{greet} as a @code{Defer}red word. The behaviour of a @code{Defer}red word can be defined and redefined at any time by using @code{IS} to associate the xt of a previously-defined word with it. The previous example becomes: @example Defer greet : foo ... greet ... ; : bar ... greet ... ; : greet1 ." Good morning" ; : greet2 ." Hello" ; ' greet2 IS greet \ make greet behave like greet2 @end example A deferred word can only inherit default semantics from the xt (because that is all that an xt can represent -- @pxref{Tokens for Words} for more discussion of this). However, the semantics of the deferred word itself can be modified at the time that it is defined. For example: @example : bar .... ; compile-only Defer fred immediate Defer jim ' bar IS jim \ jim has default semantics ' bar IS fred \ fred is immediate @end example The defining word @code{Alias} allows you to define a word by name that has the same behaviour as some other word. Here are two situation where this can be useful: @itemize @bullet @item When you want access to a word's definition from a different word list (for an example of this, see the definition of the @code{Root} word list in the Gforth source). @item When you want to create a synonym; a definition that can be known by either of two names (for example, @code{THEN} and @code{ENDIF} are aliases). @end itemize The word whose behaviour the alias is to inherit is represented by an xt. Therefore, the alias can only inherits default semantics from its ancestor. The semantics of the alias itself can be modified at the time that it is defined. For example: @example : foo ... ; immediate ' foo Alias bar \ bar is not an immediate word ' foo Alias fooby immediate \ fooby is an immediate word @end example Words that are aliases have the same xt. Their semantics can differ because the rules about a word's semantics are stored in the name dictionary, and the aliases each have their own dictionary entry. It follows that words that are aliases have different name tokens and may have the same or different compilation tokens. Once again, see @ref{Tokens for Words} for more discussions of this. doc-create doc-variable doc-2variable doc-fvariable doc-user doc-constant doc-2constant doc-fconstant doc-value doc-to doc-defer doc-is doc-alias @comment TODO document these: what's defers [is] doc-what's doc-defers Definitions in ANS Forth for @code{defer}, @code{} and @code{[is]} are provided in @file{compat/defer.fs}. @node Colon Definitions, User-defined Defining Words, Simple Defining Words, Defining Words @subsection Colon Definitions @cindex colon definitions @example : name ( ... -- ... ) word1 word2 word3 ; @end example @noindent 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{Your first definition} for simple examples of colon definitions, then @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 @cindex user-defined defining words @cindex defining words, user-defined You can create a new defining word by wrapping defining-time code around an existing defining word and putting the sequence in a colon definition. For example, suppose that you have a word @code{stats} that gathers statistics about colon definitions given the @i{xt} of the definition, and you want every colon definition in your application to make a call to @code{stats}. You can define and use a new version of @code{:} like this: @example : stats ( xt -- ) DUP ." (Gathering statistics for " . ." )" ... ; \ other code : my: : lastxt postpone literal ['] stats compile, ; my: foo + - ; @end example When @code{foo} is defined using @code{my:} these steps occur: @itemize @bullet @item @code{my:} is executed. @item The @code{:} within the definition (the one between @code{my:} and @code{lastxt}) is executed, and does just what it always does; it parses the input stream for a name, builds a dictionary header for the name @code{foo} and switches @code{state} from interpret to compile. @item The word @code{lastxt} is executed. It puts the @i{xt} for the word that is being defined -- @code{foo} -- onto the stack. @item The code that was produced by @code{postpone literal} is executed; this causes the value on the stack to be compiled as a literal in the code area of @code{foo}. @item The code @code{['] stats} compiles a literal into the definition of @code{my:}. When @code{compile,} is executed, that literal -- the execution token for @code{stats} -- is layed down in the code area of @code{foo} , following the literal@footnote{Strictly speaking, the mechanism that @code{compile,} uses to convert an @i{xt} into something in the code area is implementation-dependent. A threaded implementation might spit out the execution token directly whilst another implementation might spit out a native code sequence.}. @item At this point, the execution of @code{my:} is complete, and control returns to the text interpreter. The text interpreter is in compile state, so subsequent text @code{+ -} is compiled into the definition of @code{foo} and the @code{;} terminates the definition as always. @end itemize You can use @code{see} to decompile a word that was defined using @code{my:} and see how it is different from a normal @code{:} definition. For example: @example : bar + - ; \ like foo but using : rather than my: see bar : bar + - ; see foo : foo 107645672 stats + - ; \ use ' stats . to show that 107645672 is the xt for stats @end example Rather than edit your application's source code to change every @code{:} to a @code{my:}, use a deferred word: @example : real: : ; \ retain access to the original defer : \ redefine as a deferred word ' my: IS : \ use special version of : \ \ load application here \ ' real: IS : \ go back to the original @end example You can use techniques like this to make new defining words in terms of @i{any} existing defining word. @cindex defining defining words @cindex @code{CREATE} ... @code{DOES>} 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 @i{code1} DOES> ( ... -- ... ) @i{code2} ; def-word name @end example @cindex child words This fragment defines a @dfn{defining word} @code{def-word} and then executes it. When @code{def-word} executes, it @code{CREATE}s a new word, @code{name}, and executes the code @i{code1}. The code @i{code2} is not executed at this time. The word @code{name} is sometimes called a @dfn{child} of @code{def-word}. When you execute @code{name}, the address of the body of @code{name} is put on the data stack and @i{code2} is executed (the address of the body of @code{name} is the address @code{HERE} returns immediately after the @code{CREATE}). @cindex atavism in child words You can use @code{def-word} to define a set of child word that behave differently, though atavistically; they all have a common run-time behaviour determined by @i{code2}. Typically, the @i{code1} sequence builds a data area in the body of the child word. The structure of the data is common to all children of @code{def-word}, but the data values are specific -- and private -- to each child word. When a child word is executed, the address of its private data area is passed as a parameter on TOS to be used and manipulated@footnote{It is legitimate both to read and write to this data area.} by @i{code2}. The two fragments of code that make up the defining words act (are executed) at two completely separate times: @itemize @bullet @item At @i{define time}, the defining word executes @i{code1} to generate a child word @item At @i{child execution time}, when a child word is invoked, @i{code2} is executed, using parameters (data) that are private and specific to the child word. @end itemize @c NAC I think this is a really bad example, because it diminishes @c rather than emphasising the fact that some important stuff happens @c at define time, and other important stuff happens at child-invocation @c time, and that those two times are potentially very different. @c @c In other words, if you make the following definitions: @c @example @c : def-word1 ( "name" -- ) @c CREATE @i{code1} ; @c @c : action1 ( ... -- ... ) @c @i{code2} ; @c @c def-word1 name1 @c @end example @c @c Using @code{name1 action1} is equivalent to using @code{name}. The classic example is that you can define @code{CONSTANT} in this way: @example : CONSTANT ( w "name" -- ) CREATE , DOES> ( -- w ) @@ ; @end example @comment There is a beautiful description of how this works and what @comment it does in the Forthwrite 100th edition.. as well as an elegant @comment commentary on the Counting Fruits problem. When you create a constant with @code{5 CONSTANT five}, a set of define-time actions take place; 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. The word @code{five} has no code of its own; it simply contains a data field and a pointer to the code that follows @code{DOES>} in its defining word. That makes words created in this way very compact. The final example in this section is intended to remind you that space reserved in @code{CREATE}d words is @i{data} space and therefore can be both read and written by a Standard program@footnote{Exercise: use this example as a starting point for your own implementation of @code{Value} and @code{TO} -- if you get stuck, investigate the behaviour of @code{'} and @code{[']}.}: @example : foo ( "name" -- ) CREATE -1 , DOES> ( -- ) @@ .; foo first-word foo second-word 123 ' first-word >BODY ! @end example If @code{first-word} had been a @code{CREATE}d word, we could simply have executed it to get the address of its data field. However, since it was defined to have @code{DOES>} actions, its execution semantics are to perform those @code{DOES>} actions. To get the address of its data field it's necessary to use @code{'} to get its xt, then @code{>BODY} to translate the xt into the address of the data field. When you execute @code{first-word}, it will display @code{123}. When you execute @code{second-word} it will display @code{-1}. @cindex stack effect of @code{DOES>}-parts @cindex @code{DOES>}-parts, stack effect In the examples 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>} @cindex @code{CREATE} ... @code{DOES>}, applications You may wonder how to use this feature. Here are some usage patterns: @cindex factoring similar colon definitions 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-target reg-source n -- ) 0 asm-reg-reg-imm ; : andi, ( reg-target reg-source n -- ) 1 asm-reg-reg-imm ; @end example @noindent This could be factored with: @example : reg-reg-imm ( op-code -- ) CREATE , DOES> ( reg-target reg-source n -- ) @@ asm-reg-reg-imm ; 0 reg-reg-imm ori, 1 reg-reg-imm andi, @end example @cindex currying 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>} @cindex @code{CREATE} ... @code{DOES>}, details doc-does> @cindex @code{DOES>} in a separate definition This means that you need not use @code{CREATE} and @code{DOES>} in the same definition; you can put the @code{DOES>}-part in a separate definition. This allows us to, e.g., select among different @code{DOES>}-parts: @example : does1 DOES> ( ... -- ... ) ... ; : does2 DOES> ( ... -- ... ) ... ; : def-word ( ... -- ... ) create ... IF does1 ELSE does2 ENDIF ; @end example In this example, the selection of whether to use @code{does1} or @code{does2} is made at compile-time; at the time that the child word is @code{CREATE}d. @cindex @code{DOES>} in interpretation state 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; for example: @example CREATE name ( ... -- ... ) @i{initialization} DOES> @i{code} ; @end example @noindent is equivalent to the standard: @example :noname DOES> @i{code} ; CREATE name EXECUTE ( ... -- ... ) @i{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 the name of a defined word @cindex names for defined words @cindex defining words, name parameter @cindex defining words, name given in a string By default, a defining word takes the name for the defined word from the input stream. Sometimes you want to supply the name from a string. You can do this with: doc-nextname For example: @example s" foo" nextname create @end example @noindent is equivalent to: @example create foo @end example @cindex defining words without name Sometimes you want to define an @dfn{anonymous word}; a word without a name. You can do this with: doc-:noname This leaves the execution token for the word on the stack after the closing @code{;}. Here's an example in which a deferred word is initialised with an @code{xt} from an anonymous colon definition: @example Defer deferred :noname ( ... -- ... ) ... ; IS deferred @end example @noindent Gforth provides an alternative way of doing this, using two separate words: doc-noname @cindex execution token of last defined word doc-lastxt @noindent The previous example can be rewritten using @code{noname} and @code{lastxt}: @example Defer deferred noname : ( ... -- ... ) ... ; lastxt IS deferred @end example @noindent @code{lastxt} also works when the last word was not defined as @code{noname}. It also has the useful property that is is valid as soon as the header for a definition has been build. Thus: @example lastxt . : foo [ lastxt . ] ; ' foo . @end example @noindent prints 3 numbers; the last two are the same. @node Interpretation and Compilation Semantics, , Supplying names, Defining Words @subsection Interpretation and Compilation Semantics @cindex semantics, interpretation and compilation @cindex interpretation 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{' @i{word}} identifies the interpretation semantics of @i{word} (in other words, @code{' @i{word} execute} is equivalent to interpret-state text interpretation of @code{@i{word}}). @cindex compilation semantics 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 @i{word}} compiles@footnote{In standard terminology, ``appends to the current definition''.} the compilation semantics of @i{word}. @cindex execution semantics 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.} @comment TODO expand, make it co-operate with new sections on text interpreter. @cindex immediate words @cindex compile-only words You can change the semantics of the most-recently defined word: doc-immediate doc-compile-only doc-restrict Note that ticking (@code{'}) a compile-only word 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. Once you have done that, you can define a combined word with @code{interpret/compile:} for the convenience of your users. You might try to use this feature to provide an optimizing implementation of the default compilation semantics of a word. For example, by defining: @example :noname foo bar ; :noname POSTPONE foo POSTPONE bar ; interpret/compile: opti-foobar @end example @noindent as an optimizing version of: @example : foobar foo bar ; @end example Unfortunately, this does not work correctly with @code{[compile]}, because @code{[compile]} assumes that the compilation semantics of all @code{interpret/compile:} words are non-default. I.e., @code{[compile] opti-foobar} would compile compilation semantics, whereas @code{[compile] foobar} would compile interpretation semantics. @cindex state-smart words (are a bad idea) Some people try to use @dfn{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 Although this works if @code{foobar} is only processed 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@footnote{For a more detailed discussion of this topic, see @cite{@code{State}-smartness -- Why it is Evil and How to Exorcise it} by Anton Ertl; presented at EuroForth '98 and available from @url{http://www.complang.tuwien.ac.at/papers/}}! @cindex defining words with arbitrary semantics combinations It is also possible to write defining words that define words with arbitrary combinations of interpretation and compilation semantics. In general, they look like this: @example : def-word create-interpret/compile @i{code1} interpretation> @i{code2} @i{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}. doc-postpone @comment TODO -- expand glossary text for POSTPONE @c ---------------------------------------------------------- @node The Text Interpreter, Tokens for Words, Defining Words, Words @section The Text Interpreter @cindex interpreter - outer @cindex text interpreter @cindex outer interpreter The text interpreter@footnote{This is an expanded version of the material in @ref{Introducing the Text Interpreter}.} is an endless loop that processes input from the current input device. A popular implementation technique for Forth is to implement a @dfn{forth virtual machine} using a loop called the @dfn{inner interpreter}. Because of this naming, the text interpreter is also known as the @dfn{outer interpreter}. @cindex interpret state @cindex compile state The text interpreter operates in one of two states: @dfn{interpret state} and @dfn{compile state}. The current state is defined by the aptly-named variable, @code{state}. This section starts by describing how the text interpreter behaves when it is in interpret state, processing input from the user input device -- the keyboard. This is the mode that a Forth system is in after it starts up. @cindex input buffer @cindex terminal input buffer The text interpreter works from an area of memory called the @dfn{input buffer}@footnote{When the text interpreter is processing input from the keyboard, this area of memory is called the @dfn{terminal input buffer} (TIB) and is addressed by the (obsolescent) words @code{TIB} and @code{#TIB}.}, which stores your keyboard input when you press the key. Starting at the beginning of the input buffer, it skips leading spaces (called @dfn{delimiters}) then parses a string (a sequence of non-space characters) until it reaches either a space character or the end of the buffer. Having parsed a string, it makes two attempts to process it: @cindex dictionary @itemize @bullet @item It looks for the string in a @dfn{dictionary} of definitions. If the string is found, the string names a @dfn{definition} (also known as a @dfn{word}) and the dictionary search returns information that allows the text interpreter to perform the word's @dfn{interpretation semantics}. In most cases, this simply means that the word will be executed. @item If the string is not found in the dictionary, the text interpreter attempts to treat it as a number, using the rules described in @ref{Number Conversion}. If the string represents a legal number in the current radix, the number is pushed onto a parameter stack (the data stack for integers, the floating-point stack for floating-point numbers). @end itemize If both attempts fail, or if the word is found in the dictionary but has no interpretation semantics@footnote{This happens if the word was defined as @code{COMPILE-ONLY}.} the text interpreter discards the remainder of the input buffer, issues an error message and waits for more input. If one of the attempts succeeds, the text interpreter repeats the parsing process until the whole of the input buffer has been processed, at which point it prints the status message ``@code{ ok}'' and waits for more input. @cindex parse area The text interpreter keeps track of its position in the input buffer by updating a variable called @code{>IN} (pronounced ``to-in''). The value of @code{>IN} starts out as 0, indicating an offset of 0 from the start of the input buffer. The region from offset @code{>IN @@} to the end of the input buffer is called the @dfn{parse area}@footnote{In other words, the text interpreter processes the contents of the input buffer by parsing strings from the parse area until the parse area is empty.}. This example shows how @code{>IN} changes as the text interpreter parses the input buffer: @example : remaining >IN @@ SOURCE 2 PICK - -ROT + SWAP CR ." ->" TYPE ." <-" ; IMMEDIATE 1 2 3 remaining + remaining . : foo 1 2 3 remaining SWAP remaining ; @end example @noindent The result is: @example ->+ remaining .<- ->.<-5 ok ->SWAP remaining ;-< ->;<- ok @end example @cindex parsing words The value of @code{>IN} can also be modified by a word in the input buffer that is executed by the text interpreter. This means that a word can ``trick'' the text interpreter into either skipping a section of the input buffer@footnote{This is how parsing words work.} or into parsing a section twice. For example: @example : lat ." <>" ; : flat ." <>" >IN DUP @@ 3 - SWAP ! ; @end example @noindent When @code{flat} is executed, this output is produced@footnote{Exercise for the reader: what would happen if the @code{3} were replaced with @code{4}?}: @example <><> @end example @noindent Two important notes about the behaviour of the text interpreter: @itemize @bullet @item It processes each input string to completion before parsing additional characters from the input buffer. @item It treats the input buffer as a read-only region (and so must your code). @end itemize @noindent When the text interpreter is in compile state, its behaviour changes in these ways: @itemize @bullet @item If a parsed string is found in the dictionary, the text interpreter will perform the word's @dfn{compilation semantics}. In most cases, this simply means that the execution semantics of the word will be appended to the current definition. @item When a number is encountered, it is compiled into the current definition (as a literal) rather than being pushed onto a parameter stack. @item If an error occurs, @code{state} is modified to put the text interpreter back into interpret state. @item Each time a line is entered from the keyboard, Gforth prints ``@code{ compiled}'' rather than `` @code{ok}''. @end itemize @cindex text interpreter - input sources When the text interpreter is using an input device other than the keyboard, its behaviour changes in these ways: @itemize @bullet @item When the parse area is empty, the text interpreter attempts to refill the input buffer from the input source. When the input source is exhausted, the input source is set back to the user input device. @item It doesn't print out ``@code{ ok}'' or ``@code{ compiled}'' messages each time the parse area is emptied. @item If an error occurs, the input source is set back to the user input device. @end itemize @ref{Input Sources} describes this in more detail. doc->in doc-source doc-tib doc-#tib @menu * Input Sources:: * Number Conversion:: * Interpret/Compile states:: * Literals:: * Interpreter Directives:: @end menu @node Input Sources, Number Conversion, The Text Interpreter, The Text Interpreter @subsection Input Sources @cindex input sources @cindex text interpreter - input sources By default, the text interpreter accepts input from the user input device (the keyboard) when Forth starts up. The text interpreter can process input from any of these sources: @itemize @bullet @item The user input device -- the keyboard. @item A file, using the words described in @ref{Forth source files}. @item A block, using the words described in @ref{Blocks}. @item A text string, using @code{evaluate}. @end itemize A program can identify the current input device from the values of @code{source-id} and @code{blk}. doc-source-id doc-blk doc-save-input doc-restore-input doc-evaluate @node Number Conversion, Interpret/Compile states, Input Sources, The Text Interpreter @subsection Number Conversion @cindex number conversion @cindex double-cell numbers, input format @cindex input format for double-cell numbers @cindex single-cell numbers, input format @cindex input format for single-cell numbers @cindex floating-point numbers, input format @cindex input format for floating-point numbers This section describes the rules that the text interpreter uses when it tries to convert a string into a number. Let represent any character that is a legal digit in the current number base@footnote{For example, 0-9 when the number base is decimal or 0-9, A-F when the number base is hexadecimal.}. Let represent any character in the range 0-9. Let @{@i{a b}@} represent the @i{optional} presence of any of the characters in the braces (@i{a} or @i{b} or neither). Let * represent any number of instances of the previous character (including none). Let any other character represent itself. @noindent Now, the conversion rules are: @itemize @bullet @item A string of the form * is treated as a single-precision (cell-sized) positive integer. Examples are 0 123 6784532 32343212343456 42 @item A string of the form -* is treated as a single-precision (cell-sized) negative integer, and is represented using 2's-complement arithmetic. Examples are -45 -5681 -0 @item A string of the form *.* is treated as a double-precision (double-cell-sized) positive integer. Examples are 3465. 3.465 34.65 (all three of these represent the same number). @item A string of the form -*.* is treated as a double-precision (double-cell-sized) negative integer, and is represented using 2's-complement arithmetic. Examples are -3465. -3.465 -34.65 (all three of these represent the same number). @item A string of the form @{+ -@}@{.@}*@{e E@}@{+ -@}* is treated as a floating-point number. Examples are 1e0 1.e 1.e0 +1e+0 (which all represent the same number) +12.E-4 @end itemize By default, the number base used for integer number conversion is given by the contents of the variable @code{BASE}. Base 10 (decimal) is always used for floating-point number conversion. doc-dpl doc-base doc-hex doc-decimal @cindex '-prefix for character strings @cindex &-prefix for decimal numbers @cindex %-prefix for binary numbers @cindex $-prefix for hexadecimal numbers Gforth allows you to override the value of @code{BASE} by using a prefix@footnote{Some Forth implementations provide a similar scheme by implementing @code{$} etc. as parsing words that process the subsequent number in the input stream and push it onto the stack. For example, see @cite{Number Conversion and Literals}, by Wil Baden; Forth Dimensions 20(3) pages 26--27. In such implementations, unlike in Gforth, a space is required between the prefix and the number.} before the first digit of an (integer) number. Four prefixes are supported: @itemize @bullet @item @code{&} -- decimal number @item @code{%} -- binary number @item @code{$} -- hexadecimal number @item @code{'} -- base 256 number @end itemize Here are some examples, with the equivalent decimal number shown after in braces: -$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision number), 'AB (16706; ascii A is 65, ascii B is 66, number is 65*256 + 66), 'ab (24930; ascii a is 97, ascii B is 98, number is 97*256 + 98), &905 (905), $abc (2478), $ABC (2478). @cindex number conversion - traps for the unwary @noindent Number conversion has a number of traps for the unwary: @itemize @bullet @item You cannot determine the current number base using the code sequence @code{BASE @@ .} -- the number base is always 10 in the current number base. Instead, use something like @code{BASE @@ DECIMAL DUP . BASE !} @item If the number base is set to a value greater than 14 (for example, hexadecimal), the number 123E4 is ambiguous; the conversion rules allow it to be intepreted as either a single-precision integer or a floating-point number (Gforth treats it as an integer). The ambiguity can be resolved by explicitly stating the sign of the mantissa and/or exponent: 123E+4 or +123E4 -- if the number base is decimal, no ambiguity arises; either representation will be treated as a floating-point number. @item There is a word @code{bin} but it does @i{not} set the number base! It is used to specify file types. @item ANS Forth requires the @code{.} of a double-precision number to be the final character in the string. Allowing the @code{.} to be anywhere after the first digit is a Gforth extension. @item The number conversion process does not check for overflow. @item In Gforth, number conversion to floating-point numbers always use base 10, irrespective of the value of @code{BASE}. In ANS Forth, conversion to floating-point numbers whilst the value of @code{BASE} is not 10 is an ambiguous condition. @end itemize @ref{Input} describes words that you can use to read numbers into your programs. @node Interpret/Compile states, Literals, Number Conversion, The Text Interpreter @subsection Interpret/Compile states @cindex Interpret/Compile states A standard program is not permitted to change @code{state} explicitly. However, it can change @code{state} implicitly, using the words @code{[} and @code{]}. When @code{[} is executed it switches @code{state} to interpret state, and therefore the text interpreter starts interpreting. When @code{]} is executed it switches @code{state} to compile state and therefore the text interpreter starts compiling. The most common usage for these words is to compile literals, as shown in @ref{Literals}. However, they give you the freedom to switch modes at will. Here is an example of building a jump-table of execution tokens: @example : AA ." this is A" ; : BB ." this is B" ; : CC ." this is C" ; create table ' aa COMPILE, ' bb COMPILE, ' cc COMPILE, : go ( n -- ) \ n is offset into table.. 0 for 1st entry cells table + @ execute ; @end example @noindent Now @code{0 go} will display ``@code{this is A}''. The table can be built far more neatly@footnote{The source code is neater.. what is compiled in memory in each case is identical.} like this: @example create table ] aa bb cc [ @end example The problem with this code is that it is not portable; it will only work on systems where code space and data space co-incide. The reason is that both tables @i{compile} execution tokens -- into code space. The Standard only allows data space to be assigned for a @code{CREATE}d word. In addition, the Standard only allows @code{@@} to access data space, whilst this example is using it to access code space. The only portable, Standard way to build this table is to build it in data space, like this: @example create table ' aa , ' bb , ' cc , @end example @noindent A similar technique can be used to build a table of constants: @example create primes 1 , 3 , 5 , 7 , 11 , @end example doc-state doc-[ doc-] @node Literals, Interpreter Directives, Interpret/Compile states, The Text Interpreter @subsection Literals @cindex Literals Often, you want to use a number within a colon definition. When you do this, the text interpreter automatically compiles the number as a @i{literal}. A literal is a number whose run-time effect is to be pushed onto the stack. If you had to do some maths to generate the number, you might write it like this: @example : HOUR-TO-SEC ( n1 -- n2 ) 60 * \ to minutes 60 * ; \ to seconds @end example It is very clear what this definition is doing, but it's inefficient since it is performing 2 multiples at run-time. An alternative would be to write: @example : HOUR-TO-SEC ( n1 -- n2 ) 3600 * ; \ to seconds @end example Which does the same thing, and has the advantage of using a single multiply. Ideally, we'd like the efficiency of the second with the readability of the first. @code{Literal} allows us to achieve that. It takes a number from the stack and lays it down in the current definition just as though the number had been typed directly into the definition. Our first attempt might look like this: @example 60 \ mins per hour 60 * \ seconds per minute : HOUR-TO-SEC ( n1 -- n2 ) Literal * ; \ to seconds @end example But this produces the error message @code{unstructured}. What happened? The stack notation for @code{:} is (@i{ -- colon-sys}) and the size of @i{colon-sys} is implementation-defined. In other words, once we start a colon definition we can't portably access anything that was on the stack before the definition began@footnote{@cite{Two Problems in ANS Forth}, by Thomas Worthington; Forth Dimensions 20(2) pages 32--34 describes some situations where you might want to access stack items above colon-sys, and provides a solution to the problem.}. The correct way of solving this problem in this instance is to use @code{[ ]} like this: @example : HOUR-TO-SEC ( n1 -- n2 ) [ 60 \ minutes per hour 60 * ] \ seconds per minute LITERAL * ; \ to seconds @end example doc-literal doc-]L doc-2literal doc-fliteral @node Interpreter Directives, , Literals, The Text Interpreter @subsection Interpreter Directives @cindex interpreter directives These words are usually used in interpret state; typically to control which parts of a source file are processed by the text interpreter. There are only a few ANS Forth Standard words, but Gforth supplements these with a rich set of immediate control structure words to compensate for the fact that the non-immediate versions can only be used in compile state (@pxref{Control Structures}). Typical usages: @example FALSE Constant ASSEMBLER . . ASSEMBLER [IF] : ASSEMBLER-FEATURE ... ; [ENDIF] . . : SEE ... \ general-purpose SEE code [ ASSEMBLER [IF] ] ... \ assembler-specific SEE code [ [ENDIF] ] ; @end example doc-[IF] doc-[ELSE] doc-[THEN] doc-[ENDIF] doc-[IFDEF] doc-[IFUNDEF] doc-[?DO] doc-[DO] doc-[FOR] doc-[LOOP] doc-[+LOOP] doc-[NEXT] doc-[BEGIN] doc-[UNTIL] doc-[AGAIN] doc-[WHILE] doc-[REPEAT] @c ------------------------------------------------------------- @node Tokens for Words, Word Lists, The Text Interpreter, Words @section Tokens for Words @cindex tokens for words This section describes the creation and use of tokens that represent words. Named words have information stored in their name dictionary entries to indicate any non-default semantics (@pxref{Interpretation and Compilation Semantics}). The semantics can be modified, using @code{immediate} and/or @code{compile-only}, at the time that the words are defined. Unnamed words have (by definition) no name dictionary entry, and therefore must have default semantics. Named words have interpretation and compilation semantics. Unnamed words just have execution semantics. @cindex xt @cindex execution token The execution semantics of an unnamed word are represented by an @dfn{execution token} (@i{xt}). As explained in @ref{Supplying names}, the execution token of the last word defined can be produced with @code{lastxt}. The interpretation semantics of a named word are also represented by an execution token. You can produce the execution token using @code{'} or @code{[']}. A simple example shows the difference between the two: @example : greet ( -- ) ." Hello" ; : foo ( -- xt ) ['] greet ; \ ['] parses greet at compile-time : bar ( -- ) ' EXECUTE ; \ ' parses at run-time \ the next four lines all do the same thing foo EXECUTE greet ' greet EXECUTE boo greet @end example An execution token occupies one cell. @cindex code field address @cindex CFA In Gforth, the abstract data type @i{execution token} is implemented as a code field address (CFA). @comment TODO note that the standard does not say what it represents.. @comment and you cannot necessarily compile it in all Forths (eg native @comment compilers?). For literals, use @code{'} in interpreted code and @code{[']} in compiled code. Gforth's @code{'} and @code{[']} behave somewhat unusually by complaining about compile-only words. To get the execution token for a compile-only word @i{name}, use @code{COMP' @i{name} DROP} or @code{[COMP'] @i{name} DROP}. @cindex compilation token The compilation semantics of a named word are represented by a @dfn{compilation token} consisting of two cells: @i{w xt}. The top cell @i{xt} is an execution token. The compilation semantics represented by the compilation token can be performed with @code{execute}, which consumes the whole compilation token, with an additional stack effect determined by the represented compilation semantics. At present, the @i{w} part of a compilation token is an execution token, and the @i{xt} part represents either @code{execute} or @code{compile,}@footnote{Depending upon the compilation semantics of the word. If the word has default compilation semantics, the @i{xt} will represent @code{compile,}. If the word is @code{immediate}, the @i{xt} will represent @code{execute}.}. However, don't rely on that knowledge, unless necessary; future versions of Gforth may introduce unusual compilation tokens (e.g., a compilation token that represents the compilation semantics of a literal). You can compile the compilation semantics with @code{postpone,}. I.e., @code{COMP' @i{word} postpone,} is equivalent to @code{postpone @i{word}}. @cindex name token @cindex name field address @cindex NFA Named words are also represented by the @dfn{name token}, (@i{nt}). In Gforth, the abstract data type @emph{name token} is implemented as a name field address (NFA). doc-execute doc-compile, doc-['] doc-' doc-[comp'] doc-comp' doc-postpone, doc-find-name doc-name>int doc-name?int doc-name>comp doc-name>string @c ------------------------------------------------------------- @node Word Lists, Environmental Queries, Tokens for Words, Words @section Word Lists @cindex word lists @cindex name dictionary @cindex wid All definitions other than those created by @code{:noname} have an entry in the name dictionary. The name dictionary is fragmented into a number of parts, called @dfn{word lists}. A word list is identified by a cell-sized word list identifier (@i{wid}) in much the same way as a file is identified by a file handle. The numerical value of the wid has no (portable) meaning, and might change from session to session. @cindex compilation word list At any one time, a single word list is defined as the word list to which all new definitions will be added -- this is called the @dfn{compilation word list}. When Gforth is started, the compilation word list is the word list called @code{FORTH-WORDLIST}. @cindex search order stack Forth maintains a stack of word lists, representing the @dfn{search order}. When the name dictionary is searched (for example, when attempting to find a word's execution token during compilation), only those word lists that are currently in the search order are searched. The most recently-defined word in the word list at the top of the word list stack is searched first, and the search proceeds until either the word is located or the oldest definition in the word list at the bottom of the stack is reached. Definitions of the word may exist in more than one word lists; the search order determines which version will be found. The ANS Forth ``Search order'' word set is intended to provide a set of low-level tools that allow various different schemes to be implemented. Gforth provides @code{vocabulary}, a traditional Forth word. @file{compat/vocabulary.fs} provides an implementation in ANS Standard Forth. @comment TODO: locals section refers to here, saying that every word list (aka @comment vocabulary) has its own methods for searching etc. Need to document that. @comment the thisone- prefix is used to pick out the true definition of a @comment word from the source files, rather than some alias. doc-forth-wordlist doc-definitions doc-get-current doc-set-current doc-get-order doc---thisone-set-order doc-wordlist doc-also doc---thisone-forth doc-only doc---thisone-order doc-previous doc-find doc-search-wordlist doc-words doc-vlist doc-mappedwordlist doc-root doc-vocabulary doc-seal doc-vocs doc-current doc-context @menu * Why use word lists?:: * Word list examples:: @end menu @node Why use word lists?, Word list examples, Word Lists, Word Lists @subsection Why use word lists? @cindex word lists - why use them? Here are some reasons for using multiple word lists: @itemize @bullet @item To improve compilation speed by reducing the number of name dictionary entries that must be searched. This is achieved by creating a new word list that contains all of the definitions that are used in the definition of a Forth system but which would not usually be used by programs running on that system. That word list would be on the search list when the Forth system was compiled but would be removed from the search list for normal operation. This can be a useful technique for low-performance systems (for example, 8-bit processors in embedded systems) but is unlikely to be necessary in high-performance desktop systems. @item To prevent a set of words from being used outside the context in which they are valid. Two classic examples of this are an integrated editor (all of the edit commands are defined in a separate word list; the search order is set to the editor word list when the editor is invoked; the old search order is restored when the editor is terminated) and an integrated assembler (the op-codes for the machine are defined in a separate word list which is used when a @code{CODE} word is defined). @item To prevent a name-space clash between multiple definitions with the same name. For example, when building a cross-compiler you might have a word @code{IF} that generates conditional code for your target system. By placing this definition in a different word list you can control whether the host system's @code{IF} or the target system's @code{IF} get used in any particular context by controlling the order of the word lists on the search order stack. @end itemize @node Word list examples, ,Why use word lists?, Word Lists @subsection Word list examples @cindex word lists - examples Here is an example of creating and using a new wordlist using ANS Forth Standard words: @example wordlist constant my-new-words-wordlist : my-new-words get-order nip my-new-words-wordlist swap set-order ; \ add it to the search order also my-new-words \ alternatively, add it to the search order and make it \ the compilation word list also my-new-words definitions \ type "order" to see the problem @end example The problem with this example is that @code{order} has no way to associate the name @code{my-new-words} with the wid of the word list (in Gforth, @code{order} and @code{vocs} will display @code{???} for a wid that has no associated name). There is no Standard way of associating a name with a wid. In Gforth, this example can be re-coded using @code{vocabulary}, which associates a name with a wid: @example vocabulary my-new-words \ add it to the search order my-new-words \ alternatively, add it to the search order and make it \ the compilation word list my-new-words definitions \ type "order" to see that the problem is solved @end example @c ------------------------------------------------------------- @node Environmental Queries, Files, Word Lists, Words @section Environmental Queries @cindex environmental queries ANS Forth introduced the idea of ``environmental queries'' as a way for a program running on a system to determine certain characteristics of the system. The Standard specifies a number of strings that might be recognised by a system. The Standard requires that the name space used for environmental queries be distinct from the name space used for definitions. Typically, environmental queries are supported by creating a set of definitions in a word list that is @i{only} used during environmental queries; that is what Gforth does. There is no Standard way of adding definitions to the set of recognised environmental queries, but any implementation that supports the loading of optional word sets must have some mechanism for doing this (after loading the word set, the associated environmental query string must return @code{true}). In Gforth, the word list used to honour environmental queries can be manipulated just like any other word list. doc-environment? doc-environment-wordlist doc-gforth doc-os-class Note that, whilst the documentation for (e.g.) @code{gforth} shows it returning two items on the stack, querying it using @code{environment?} will return an additional item; the @code{true} flag that shows that the string was recognised. @comment TODO Document the standard strings or note where they are documented herein Here are some examples of using environmental queries: @example s" address-unit-bits" environment? 0= [IF] cr .( environmental attribute address-units-bits unknown... ) cr [THEN] s" block" environment? [IF] DROP include block.fs [THEN] s" gforth" environment? [IF] 2DROP include compat/vocabulary.fs [THEN] s" gforth" environment? [IF] .( Gforth version ) TYPE [ELSE] .( Not Gforth..) [THEN] @end example Here is an example of adding a definition to the environment word list: @example get-current environment-wordlist set-current true constant block true constant block-ext set-current @end example You can see what definitions are in the environment word list like this: @example get-order 1+ environment-wordlist swap set-order words previous @end example @c ------------------------------------------------------------- @node Files, Blocks, Environmental Queries, Words @section Files @cindex files @cindex I/O - file-handling Gforth provides facilities for accessing files that are stored in the host operating system's file-system. Files that are processed by Gforth can be divided into two categories: @itemize @bullet @item Files that are processed by the Text Interpreter (@dfn{Forth source files}). @item Files that are processed by some other program (@dfn{general files}). @end itemize @menu * Forth source files:: * General files:: * Search Paths:: * Forth Search Paths:: * General Search Paths:: @end menu @c ------------------------------------------------------------- @node Forth source files, General files, Files, Files @subsection Forth source files @cindex including files @cindex Forth source files The simplest way to interpret the contents of a file is to use one of these two formats: @example include mysource.fs s" mysource.fs" included @end example Sometimes you want to include a file only if it is not included already (by, say, another source file). In that case, you can use one of these fomats: @example require mysource.fs needs mysource.fs s" mysource.fs" required @end example @cindex stack effect of included files @cindex including files, stack effect I recommend that you write your source files such that interpreting them does not change the stack. This allows using these files with @code{required} and friends without complications. For example: @example 1 require foo.fs drop @end example doc-include-file doc-included doc-included? doc-include doc-required doc-require doc-needs doc-init-included-files A definition in ANS Forth for @code{required} is provided in @file{compat/required.fs}. @c ------------------------------------------------------------- @node General files, Search Paths, Forth source files, Files @subsection General files @cindex general files @cindex file-handling Files are opened/created by name and type. The following types are recognised: doc-r/o doc-r/w doc-w/o doc-bin When a file is opened/created, it returns a file identifier, @i{wfileid} that is used for all other file commands. All file commands also return a status value, @i{wior}, that is 0 for a successful operation and an implementation-defined non-zero value in the case of an error. doc-open-file doc-create-file doc-close-file doc-delete-file doc-rename-file doc-read-file doc-read-line doc-write-file doc-write-line doc-emit-file doc-flush-file doc-file-status doc-file-position doc-reposition-file doc-file-size doc-resize-file @c --------------------------------------------------------- @node Search Paths, Forth Search Paths, General files, Files @subsection Search Paths @cindex path for @code{included} @cindex file search path @cindex @code{include} search path @cindex search path for files If you specify an absolute filename (i.e., a filename starting with @file{/} or @file{~}, or with @file{:} in the second position (as in @samp{C:...})) for @code{included} and friends, that file is included just as you would expect. For relative filenames, Gforth uses a search path similar to Forth's search order (@pxref{Word Lists}). It tries to find the given filename in the directories present in the path, and includes the first one it finds. There are separate search paths for Forth source files and general files. If the search path contains the directory @file{.} (as it should), this refers to the directory that the present file was @code{included} from. This allows files to include other files relative to their own position (irrespective of the current working directory or the absolute position). This feature is essential for libraries consisting of several files, where a file may include other files from the library. It corresponds to @code{#include "..."} in C. If the current input source is not a file, @file{.} refers to the directory of the innermost file being included, or, if there is no file being included, to the current working directory. Use @file{~+} to refer to the current working directory (as in the @code{bash}). If the filename starts with @file{./}, the search path is not searched (just as with absolute filenames), and the @file{.} has the same meaning as described above. @c --------------------------------------------------------- @node Forth Search Paths, General Search Paths, Search Paths, Files @subsubsection Forth Search Paths @cindex search path control - Forth The search path is initialized when you start Gforth (@pxref{Invoking Gforth}). You can display it and change it using these words: doc-.fpath doc-fpath+ doc-fpath= doc-open-fpath-file Here is an example of using @code{fpath} and @code{require}: @example fpath= /usr/lib/forth/|./ require timer.fs @end example @c --------------------------------------------------------- @node General Search Paths, , Forth Search Paths, Files @subsubsection General Search Paths @cindex search path control - for user applications Your application may need to search files in several directories, like @code{included} does. To facilitate this, Gforth allows you to define and use your own search paths, by providing generic equivalents of the Forth search path words: doc-.path doc-path+ doc-path= doc-open-path-file Here's an example of creating a search path: @example \ Make a buffer for the path: create mypath 100 chars , \ maximum length (is checked) 0 , \ real len 100 chars allot \ space for path @end example @c ------------------------------------------------------------- @node Blocks, Other I/O, Files, Words @section Blocks @cindex I/O - blocks @cindex blocks When you run Gforth on a modern desk-top computer, it runs under the control of an operating system which provides certain services. One of these services is @var{file services}, which allows Forth source code and data to be stored in files and read into Gforth (@pxref{Files}). Traditionally, Forth has been an important programming language on systems where it has interfaced directly to the underlying hardware with no intervening operating system. Forth provides a mechanism, called @dfn{blocks}, for accessing mass storage on such systems. A block is a 1024-byte data area, which can be used to hold data or Forth source code. No structure is imposed on the contents of the block. A block is identified by its number; blocks are numbered contiguously from 1 to an implementation-defined maximum. A typical system that used blocks but no operating system might use a single floppy-disk drive for mass storage, with the disks formatted to provide 256-byte sectors. Blocks would be implemented by assigning the first four sectors of the disk to block 1, the second four sectors to block 2 and so on, up to the limit of the capacity of the disk. The disk would not contain any file system information, just the set of blocks. @cindex blocks file On systems that do provide file services, blocks are typically implemented by storing a sequence of blocks within a single @dfn{blocks file}. The size of the blocks file will be an exact multiple of 1024 bytes, corresponding to the number of blocks it contains. This is the mechanism that Gforth uses. @cindex @file{blocks.fb} Only 1 blocks file can be open at a time. If you use block words without having specified a blocks file, Gforth defaults to the blocks file @file{blocks.fb}. Gforth uses the Forth search path when attempting to locate a blocks file (@pxref{Forth Search Paths}). @cindex block buffers When you read and write blocks under program control, Gforth uses a number of @dfn{block buffers} as intermediate storage. These buffers are not used when you use @code{load} to interpret the contents of a block. The behaviour of the block buffers is directly analagous to that of a cache. Each block buffer has three states: @itemize @bullet @item Unassigned @item Assigned-clean @item Assigned-dirty @end itemize Initially, all block buffers are @i{unassigned}. In order to access a block, the block (specified by its block number) must be assigned to a block buffer. The assignment of a block to a block buffer is performed by @code{block} or @code{buffer}. Use @code{block} when you wish to modify the existing contents of a block. Use @code{buffer} when you don't care about the existing contents of the block@footnote{The ANS Forth definition of @code{block} is intended not to cause disk I/O; if the data associated with the particular block is already stored in a block buffer due to an earlier @code{block} command, @code{buffer} will return that block buffer and the existing contents of the block will be available. Otherwise, @code{buffer} will simply assign a new, empty block buffer for the block.}. Once a block has been assigned to a block buffer, the block buffer state becomes @i{assigned-clean}. Data can now be manipulated within the block buffer. When the contents of a block buffer is changed it is necessary, @i{before calling} @code{block} @i{or} @code{buffer} @i{again}, to either abandon the changes (by doing nothing) or commit the changes, using @code{update}. Using @code{update} does not change the blocks file; it simply changes a block buffer's state to @i{assigned-dirty}. The word @code{flush} causes all @i{assigned-dirty} blocks to be written back to the blocks file on disk. Leaving Gforth using @code{bye} also causes a @code{flush} to be performed. In Gforth, @code{block} and @code{buffer} use a @i{direct-mapped} algorithm to assign a block buffer to a block. That means that any particular block can only be assigned to one specific block buffer, called (for the particular operation) the @i{victim buffer}. If the victim buffer is @i{unassigned} or @i{assigned-clean} it can be allocated to the new block immediately. If it is @i{assigned-dirty} its current contents must be written out to disk before it can be allocated to the new block. Although no structure is imposed on the contents of a block, it is traditional to display the contents as 16 lines each of 64 characters. A block provides a single, continuous stream of input (for example, it acts as a single parse area) -- there are no end-of-line characters within a block, and no end-of-file character at the end of a block. There are two consequences of this: @itemize @bullet @item The last character of one line wraps straight into the first character of the following line @item The word @code{\} -- comment to end of line -- requires special treatment; in the context of a block it causes all characters until the end of the current 64-character ``line'' to be ignored. @end itemize In Gforth, when you use @code{block} with a non-existent block number, the current block file will be extended to the appropriate size and the block buffer will be initialised with spaces. Gforth doesn't encourage the use of blocks; the mechanism is only provided for backward compatibility -- ANS Forth requires blocks to be available when files are. Common techniques that are used when working with blocks include: @itemize @bullet @item A screen editor that allows you to edit blocks without leaving the Forth environment. @item Shadow screens; where every code block has an associated block containing comments (for example: code in odd block numbers, comments in even block numbers). Typically, the block editor provides a convenient mechanism to toggle between code and comments. @item Load blocks; a single block (typically block 1) contains a number of @code{thru} commands which @code{load} the whole of the application. @item Chaining blocks; a block terminates with a @code{-->} so that a whole application can be @code{load}ed by @code{load}ing a single block. @end itemize See Frank Sergeant's Pygmy Forth to see just how well blocks can be integrated into a Forth programming environment. @comment TODO what about errors on open-blocks? doc-open-blocks doc-use doc-get-block-fid doc-block-position doc-scr doc-list doc---block-block doc-buffer doc-update doc-updated? doc-save-buffers doc-empty-buffers doc-empty-buffer doc-flush doc-load doc-thru doc-+load doc-+thru doc---block---> doc-block-included @c ------------------------------------------------------------- @node Other I/O, Programming Tools, Blocks, Words @section Other I/O @cindex I/O - keyboard and display @menu * Simple numeric output:: Predefined formats * Formatted numeric output:: Formatted (pictured) output * String Formats:: How Forth stores strings in memory * Displaying characters and strings:: Other stuff * Input:: Input @end menu @node Simple numeric output, Formatted numeric output, Other I/O, Other I/O @subsection Simple numeric output @cindex numeric output - simple/free-format The simplest output functions are those that display numbers from the data or floating-point stacks. Floating-point output is always displayed using base 10. Numbers displayed from the data stack use the value stored in @code{base}. doc-. doc-dec. doc-hex. doc-u. doc-.r doc-u.r doc-d. doc-ud. doc-d.r doc-ud.r doc-f. doc-fe. doc-fs. Examples of printing the number 1234.5678E23 in the different floating-point output formats are shown below: @example f. 123456779999999000000000000. fe. 123.456779999999E24 fs. 1.23456779999999E26 @end example @node Formatted numeric output, String Formats, Simple numeric output, Other I/O @subsection Formatted numeric output @cindex formatted numeric output @cindex pictured numeric output @cindex numeric output - formatted Forth traditionally uses a technique called @dfn{pictured numeric output} for formatted printing of integers. In this technique, digits are extracted from the number (using the current output radix defined by @code{base}), converted to ASCII codes and appended to a string that is built in a scratch-pad area of memory (@pxref{core-idef, Implementation-defined options, Implementation-defined options}). Arbitrary characters can be appended to the string during the extraction process. The completed string is specified by an address and length and can be manipulated (@code{TYPE}ed, copied, modified) under program control. All of the words described in the previous section for simple numeric output are implemented in Gforth using pictured numeric output. Three important things to remember about Pictured Numeric Output: @itemize @bullet @item It always operates on double-precision numbers; to display a single-precision number, convert it first (@pxref{Double precision} for ways of doing this). @item It always treats the double-precision number as though it were unsigned. The examples below show ways of printing signed numbers. @item The string is built up from right to left; least significant digit first. @end itemize doc-<# doc-# doc-#s doc-hold doc-sign doc-#> doc-represent Here are some examples of using pictured numeric output: @example : my-u. ( u -- ) \ Simplest use of pns.. behaves like Standard u. 0 \ convert to unsigned double <# \ start conversion #s \ convert all digits #> \ complete conversion TYPE SPACE ; \ display, with trailing space : cents-only ( u -- ) 0 \ convert to unsigned double <# \ start conversion # # \ convert two least-significant digits #> \ complete conversion, discard other digits TYPE SPACE ; \ display, with trailing space : dollars-and-cents ( u -- ) 0 \ convert to unsigned double <# \ start conversion # # \ convert two least-significant digits [char] . hold \ insert decimal point #s \ convert remaining digits [char] $ hold \ append currency symbol #> \ complete conversion TYPE SPACE ; \ display, with trailing space : my-. ( n -- ) \ handling negatives.. behaves like Standard . s>d \ convert to signed double swap over dabs \ leave sign byte followed by unsigned double <# \ start conversion #s \ convert all digits rot sign \ get at sign byte, append "-" if needed #> \ complete conversion TYPE SPACE ; \ display, with trailing space : account. ( n -- ) \ accountants don't like minus signs, they use braces \ for negative numbers s>d \ convert to signed double swap over dabs \ leave sign byte followed by unsigned double <# \ start conversion 2 pick \ get copy of sign byte 0< IF [char] ) hold THEN \ right-most character of output #s \ convert all digits rot \ get at sign byte 0< IF [char] ( hold THEN #> \ complete conversion TYPE SPACE ; \ display, with trailing space @end example Here are some examples of using these words: @example 1 my-u. 1 hex -1 my-u. decimal FFFFFFFF 1 cents-only 01 1234 cents-only 34 2 dollars-and-cents $0.02 1234 dollars-and-cents $12.34 123 my-. 123 -123 my. -123 123 account. 123 -456 account. (456) @end example @node String Formats, Displaying characters and strings, Formatted numeric output, Other I/O @subsection String Formats @cindex strings - see character strings @cindex character strings - formats @cindex I/O - see character strings Forth commonly uses two different methods for representing character strings: @itemize @bullet @item @cindex address of counted string As a @dfn{counted string}, represented by a @i{c-addr}. The char addressed by @i{c-addr} contains a character-count, @i{n}, of the string and the string occupies the subsequent @i{n} char addresses in memory. @item As cell pair on the stack; @i{c-addr u}, where @i{u} is the length of the string in characters, and @i{c-addr} is the address of the first byte of the string. @end itemize ANS Forth encourages the use of the second format when representing strings on the stack, whilst conceeding that the counted string format remains useful as a way of storing strings in memory. doc-count @xref{Memory Blocks} for words that move, copy and search for strings. @xref{Displaying characters and strings,} for words that display characters and strings. @node Displaying characters and strings, Input, String Formats, Other I/O @subsection Displaying characters and strings @cindex characters - compiling and displaying @cindex character strings - compiling and displaying This section starts with a glossary of Forth words and ends with a set of examples. doc-bl doc-space doc-spaces doc-emit doc-toupper doc-." doc-.( doc-type doc-cr @cindex cursor control doc-at-xy doc-page doc-s" doc-c" doc-char doc-[char] doc-sliteral As an example, consider the following text, stored in a file @file{test.fs}: @example .( text-1) : my-word ." text-2" cr .( text-3) ; ." text-4" : my-char [char] ALPHABET emit char emit ; @end example When you load this code into Gforth, the following output is generated: @example @kbd{include test.fs } text-1text-3text-4 ok @end example @itemize @bullet @item Messages @code{text-1} and @code{text-3} are displayed because @code{.(} is an immediate word; it behaves in the same way whether it is used inside or outside a colon definition. @item Message @code{text-4} is displayed because of Gforth's added interpretation semantics for @code{."}. @item Message @code{text-2} is @i{not} displayed, because the text interpreter performs the compilation semantics for @code{."} within the definition of @code{my-word}. @end itemize Here are some examples of executing @code{my-word} and @code{my-char}: @example @kbd{my-word } text-2 ok @kbd{my-char fred } Af ok @kbd{my-char jim } Aj ok @end example @itemize @bullet @item Message @code{text-2} is displayed because of the run-time behaviour of @code{."}. @item @code{[char]} compiles the ``A'' from ``ALPHABET'' and puts its display code on the stack at run-time. @code{emit} always displays the character when @code{my-char} is executed. @item @code{char} parses a string at run-time and the second @code{emit} displays the first character of the string. @item If you type @code{see my-char} you can see that @code{[char]} discarded the text ``LPHABET'' and only compiled the display code for ``A'' into the definition of @code{my-char}. @end itemize @node Input, , Displaying characters and strings, Other I/O @subsection Input @cindex input @cindex I/O - see input @cindex parsing a string @xref{String Formats} for ways of storing character strings in memory. @comment TODO examples for >number >float accept key key? pad parse word refill @comment then index them doc-key doc-key? doc->number doc->float doc-accept doc-pad doc-parse doc-word doc-sword doc-refill @comment obsolescent words.. doc-convert doc-query doc-expect doc-span @c ------------------------------------------------------------- @node Programming Tools, Assembler and Code Words, Other I/O, Words @section Programming Tools @cindex programming tools @menu * Debugging:: Simple and quick. * Assertions:: Making your programs self-checking. * Singlestep Debugger:: Executing your program word by word. @end menu @node Debugging, Assertions, Programming Tools, Programming Tools @subsection Debugging @cindex debugging Languages with a slow edit/compile/link/test development loop tend to require sophisticated tracing/stepping debuggers to facilate productive debugging. A much better (faster) way in fast-compiling 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 simple debugging aids provided in @file{debugs.fs} are meant to support this style of debugging. In addition, there are words for non-destructively inspecting the stack and memory: doc-.s doc-f.s There is a word @code{.r} but it does @i{not} display the return stack! It is used for formatted numeric output. doc-depth doc-fdepth doc-clearstack doc-? doc-dump The word @code{~~} prints debugging information (by default the source location and the stack contents). It is easy to insert. If you use Emacs 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). The default actions of @code{~~} 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 doc-see doc-marker Here's an example of using @code{marker} at the start of a source file that you are debugging; it ensures that you only ever have one copy of the file's definitions compiled at any time: @example [IFDEF] my-code my-code [ENDIF] marker my-code init-included-files \ .. definitions start here \ . \ . \ end @end example @node Assertions, Singlestep Debugger, Debugging, Programming Tools @subsection Assertions @cindex assertions It is a good idea to make your programs self-checking, especially if you make an assumption that may become invalid during maintenance (for example, that a certain field of a data structure is never zero). Gforth supports @dfn{assertions} for this purpose. They are used like this: @example assert( @i{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-) 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. The value of @code{assert-level} is evaluated at compile-time, not at run-time. Therefore 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 different pieces of code at different @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). Definitions in ANS Forth for these assertion words are provided in @file{compat/assert.fs}. @node Singlestep Debugger, , Assertions, Programming Tools @subsection Singlestep Debugger @cindex singlestep Debugger @cindex debugging Singlestep @cindex @code{dbg} @cindex @code{BREAK:} @cindex @code{BREAK"} When you create a new word there's often the need to check whether it behaves correctly or not. You can do this by typing @code{dbg badword}. A debug session might look like this: @example : badword 0 DO i . LOOP ; ok 2 dbg badword : badword Scanning code... Nesting debugger ready! 400D4738 8049BC4 0 -> [ 2 ] 00002 00000 400D4740 8049F68 DO -> [ 0 ] 400D4744 804A0C8 i -> [ 1 ] 00000 400D4748 400C5E60 . -> 0 [ 0 ] 400D474C 8049D0C LOOP -> [ 0 ] 400D4744 804A0C8 i -> [ 1 ] 00001 400D4748 400C5E60 . -> 1 [ 0 ] 400D474C 8049D0C LOOP -> [ 0 ] 400D4758 804B384 ; -> ok @end example Each line displayed is one step. You always have to hit return to execute the next word that is displayed. If you don't want to execute the next word in a whole, you have to type @kbd{n} for @code{nest}. Here is an overview what keys are available: @table @i @item Next; Execute the next word. @item n Nest; Single step through next word. @item u Unnest; Stop debugging and execute rest of word. If we got to this word with nest, continue debugging with the calling word. @item d Done; Stop debugging and execute rest. @item s Stop; Abort immediately. @end table Debugging large application with this mechanism is very difficult, because you have to nest very deeply into the program before the interesting part begins. This takes a lot of time. To do it more directly put a @code{BREAK:} command into your source code. When program execution reaches @code{BREAK:} the single step debugger is invoked and you have all the features described above. If you have more than one part to debug it is useful to know where the program has stopped at the moment. You can do this by the @code{BREAK" string"} command. This behaves like @code{BREAK:} except that string is typed out when the ``breakpoint'' is reached. doc-dbg doc-BREAK: doc-BREAK" @c ------------------------------------------------------------- @node Assembler and Code Words, Threading Words, Programming Tools, Words @section Assembler and Code Words @cindex assembler @cindex code words Gforth provides some words for defining primitives (words written in machine code), and for defining 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,}@footnote{This isn't portable, because these words emit stuff in @i{data} space; it works because Gforth has unified code/data spaces. Assembler isn't likely to be portable anyway.}. 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. The typical usage of these @code{code} words can be shown most easily by analogy to the equivalent high-level defining words: @example : foo code foo ; end-code : bar : bar CREATE CREATE DOES> ;code ; end-code @end example @code{flush-icache} is always present. The other words are rarely used and reside in @code{code.fs}, which is usually not loaded. You can load it with @code{require code.fs}. @cindex registers of the inner interpreter 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. @cindex code words, portable Another option for implementing normal and defining words efficiently is to add the desired 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{kernel.fs}, @file{prims2x.fs}, and possibly @file{cross.fs}. @c ------------------------------------------------------------- @node Threading Words, Locals, Assembler and Code Words, Words @section Threading Words @cindex threading words @cindex code address 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 incomplete. It is also pretty low-level; some day it will hopefully be made unnecessary by an internals wordset that abstracts implementation details away completely. doc-threading-method 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 non-zero and identifies the @code{DOES>} used by the defining word. @comment TODO should that be ``identifies the xt of the DOES> ??'' @c ------------------------------------------------------------- @node Locals, Structures, Threading Words, Words @section Locals @cindex locals Local variables can make Forth programming more enjoyable and Forth programs easier to read. Unfortunately, the locals of ANS Forth are laden with restrictions. Therefore, we provide not only the ANS Forth 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 @*@url{http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz}. @menu * Gforth locals:: * ANS Forth locals:: @end menu @node Gforth locals, ANS Forth locals, Locals, Locals @subsection Gforth locals @cindex Gforth locals @cindex locals, Gforth style Locals can be defined with @example @{ local1 local2 ... -- comment @} @end example or @example @{ local1 local2 ... @} @end example E.g., @example : max @{ n1 n2 -- n3 @} n1 n2 > if n1 else n2 endif ; @end example The similarity of locals definitions with stack comments is intended. A locals definition often replaces the stack comment of a word. The order of the locals corresponds to the order in a stack comment and everything after the @code{--} is really a comment. This similarity has one disadvantage: It is too easy to confuse locals declarations with stack comments, causing bugs and making them hard to find. However, this problem can be avoided by appropriate coding conventions: Do not use both notations in the same program. If you do, they should be distinguished using additional means, e.g. by position. @cindex types of locals @cindex locals types The name of the local may be preceded by a type specifier, e.g., @code{F:} for a floating point value: @example : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @} \ complex multiplication Ar Br f* Ai Bi f* f- Ar Bi f* Ai Br f* f+ ; @end example @cindex flavours of locals @cindex locals flavours @cindex value-flavoured locals @cindex variable-flavoured locals 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 with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.) produces its address (which becomes invalid when the variable's scope is left). E.g., the standard word @code{emit} can be defined in terms of @code{type} like this: @example : emit @{ C^ char* -- @} char* 1 type ; @end example @cindex default type of locals @cindex locals, default type A local without type specifier is a @code{W:} local. Both flavours of locals are initialized with values from the data or FP stack. Currently there is no way to define locals with user-defined data structures, but we are working on it. 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? @cindex locals visibility @cindex visibility of locals @cindex scope of locals Basically, the answer is that locals are visible where you would expect it in block-structured languages, and sometimes a little longer. If you want to restrict the scope of a local, enclose its definition in @code{SCOPE}...@code{ENDSCOPE}. doc-scope doc-endscope These words behave like control structure words, so you can use them with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in arbitrary ways. If you want a more exact answer to the visibility question, here's the basic principle: A local is visible in all places that can only be reached through the definition of the local@footnote{In compiler construction terminology, all places dominated by the definition of the local.}. In other words, it is not visible in places that can be reached without going through the definition of the local. E.g., locals defined in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals defined in @code{BEGIN}...@code{UNTIL} are visible after the @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}). The reasoning behind this solution is: We want to have the locals visible as long as it is meaningful. The user can always make the visibility shorter by using explicit scoping. In a place that can only be reached through the definition of a local, the meaning of a local name is clear. In other places it is not: How is the local initialized at the control flow path that does not contain the definition? Which local is meant, if the same name is defined twice in two independent control flow paths? This should be enough detail for nearly all users, so you can skip the rest of this section. If you really must know all the gory details and options, read on. In order to implement this rule, the compiler has to know which places are unreachable. It knows this automatically after @code{AHEAD}, @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the compiler that the control flow never reaches that place. If @code{UNREACHABLE} is not used where it could, the only consequence is that the visibility of some locals is more limited than the rule above says. If @code{UNREACHABLE} is used where it should not (i.e., if you lie to the compiler), buggy code will be produced. doc-unreachable 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 loops). Perhaps the most insidious example is: @example AHEAD BEGIN x [ 1 CS-ROLL ] THEN @{ x @} ... UNTIL @end example This should be legal according to the visibility rule. The use of @code{x} can only be reached through the definition; but that appears textually below the use. From this example it is clear that the visibility rules cannot be fully implemented without major headaches. Our implementation treats common cases as advertised and the exceptions are treated in a safe way: The compiler makes a reasonable guess about the locals visible after a @code{BEGIN}; if it is too pessimistic, the user will get a spurious error about the local not being defined; if the compiler is too optimistic, it will notice this later and issue a warning. In the case above the compiler would complain about @code{x} being undefined at its use. You can see from the obscure examples in this section that it takes quite unusual control structures to get the compiler into trouble, and even then it will often do fine. If the @code{BEGIN} is reachable from above, the most optimistic guess is that all locals visible before the @code{BEGIN} will also be visible after the @code{BEGIN}. This guess is valid for all loops that are entered only through the @code{BEGIN}, in particular, for normal @code{BEGIN}...@code{WHILE}...@code{REPEAT} and @code{BEGIN}...@code{UNTIL} loops and it is implemented in our compiler. When the branch to the @code{BEGIN} is finally generated by @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and warns the user if it was too optimistic: @example IF @{ x @} BEGIN \ x ? [ 1 cs-roll ] THEN ... UNTIL @end example Here, @code{x} lives only until the @code{BEGIN}, but the compiler optimistically assumes that it lives until the @code{THEN}. It notices this difference when it compiles the @code{UNTIL} and issues a warning. The user can avoid the warning, and make sure that @code{x} is not used in the wrong area by using explicit scoping: @example IF SCOPE @{ x @} ENDSCOPE BEGIN [ 1 cs-roll ] THEN ... UNTIL @end example Since the guess is optimistic, there will be no spurious error messages about undefined locals. If the @code{BEGIN} is not reachable from above (e.g., after @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 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 top control-flow stack item was created. doc-assume-live E.g., @example @{ x @} AHEAD ASSUME-LIVE BEGIN x [ 1 CS-ROLL ] THEN ... UNTIL @end example Other cases where the locals are defined before the @code{BEGIN} can be handled by inserting an appropriate @code{CS-ROLL} before the @code{ASSUME-LIVE} (and changing the control-flow stack manipulation behind the @code{ASSUME-LIVE}). Cases where locals are defined after the @code{BEGIN} (but should be visible immediately after the @code{BEGIN}) can only be handled by rearranging the loop. E.g., the ``most insidious'' example above can be arranged into: @example BEGIN @{ 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? @cindex locals lifetime @cindex lifetime of locals The right answer for the lifetime question would be: A local lives at least as long as it can be accessed. For a value-flavoured local this means: until the end of its visibility. However, a variable-flavoured local could be accessed through its address far beyond its visibility scope. Ultimately, this would mean that such locals would have to be garbage collected. Since this entails un-Forth-like implementation complexities, I adopted the same cowardly solution as some other languages (e.g., C): The local lives only as long as it is visible; 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 @cindex locals programming style @cindex programming style, locals The freedom to define locals anywhere has the potential to change programming styles dramatically. In particular, the need to use the return stack for intermediate storage vanishes. Moreover, all stack manipulations (except @code{PICK}s and @code{ROLL}s with run-time determined arguments) can be eliminated: If the stack items are in the wrong order, just write a locals definition for all of them; then 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 @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 programmers continue to honour the principle of factoring instead of using the added latitude to make the words longer. @cindex single-assignment style for locals Using @code{TO} can and should be avoided. Without @code{TO}, every value-flavoured local has only a single assignment and many advantages of functional languages apply to Forth. I.e., programs are easier to analyse, to optimize and to read: It is clear from the definition what the local stands for, it does not turn into something different later. E.g., a definition using @code{TO} might look like this: @example : strcmp @{ addr1 u1 addr2 u2 -- n @} u1 u2 min 0 ?do addr1 c@@ addr2 c@@ - ?dup-if unloop exit then addr1 char+ TO addr1 addr2 char+ TO addr2 loop u1 u2 - ; @end example Here, @code{TO} is used to update @code{addr1} and @code{addr2} at every loop iteration. @code{strcmp} is a typical example of the readability problems of using @code{TO}. When you start reading @code{strcmp}, you think that @code{addr1} refers to the start of the string. Only near the end of the loop you realize that it is something else. This can be avoided by defining two locals at the start of the loop that are initialized with the right value for the current iteration. @example : strcmp @{ addr1 u1 addr2 u2 -- n @} addr1 addr2 u1 u2 min 0 ?do @{ s1 s2 @} s1 c@@ s2 c@@ - ?dup-if unloop exit then s1 char+ s2 char+ loop 2drop u1 u2 - ; @end example 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 @cindex locals implementation @cindex implementation of locals @cindex locals stack 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 lower addresses. A few primitives allow an efficient implementation: doc-@local# doc-f@local# doc-laddr# doc-lp+!# doc-lp! doc->l doc-f>l In addition to these primitives, some specializations of these primitives for commonly occurring inline arguments are provided for efficiency reasons, e.g., @code{@@local0} as specialization of @code{@@local#} for the inline argument 0. The following compiling words compile the right specialized version, or the general version, as appropriate: doc-compile-@local doc-compile-f@local doc-compile-lp+! Combinations of conditional branches and @code{lp+!#} like @code{?branch-lp+!#} (the locals pointer is only changed if the branch is taken) are provided for efficiency and correctness in loops. A special area in the dictionary space is reserved for keeping the local variable names. @code{@{} switches the dictionary pointer to this area and @code{@}} switches it back and generates the locals initializing code. @code{W:} etc.@ are normal defining words. This special area is cleared at the start of every colon definition. @cindex word list for defining locals A special feature of Gforth's dictionary is used to implement the definition of locals without type specifiers: every word list (aka vocabulary) has its own methods for searching etc. (@pxref{Word Lists}). For the present purpose we defined a word list 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 word list containing @code{@}}, @code{W:} etc., and then the word list for defining locals without type specifiers. The lifetime rules support a stack discipline within a colon definition: The lifetime of a local is either nested with other locals lifetimes or it does not overlap them. At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack pointer manipulation is generated. Between control structure words locals definitions can push locals onto the locals stack. @code{AGAIN} is the simplest of the other three control flow words. It has to restore the locals stack depth of the corresponding @code{BEGIN} before branching. The code looks like this: @format @code{lp+!#} current-locals-size @minus{} dest-locals-size @code{branch} @end format @code{UNTIL} is a little more complicated: If it branches back, it must adjust the stack just like @code{AGAIN}. But if it falls through, the locals stack must not be changed. The compiler generates the following code: @format @code{?branch-lp+!#} current-locals-size @minus{} dest-locals-size @end format The locals stack pointer is only adjusted if the branch is taken. @code{THEN} can produce somewhat inefficient code: @format @code{lp+!#} current-locals-size @minus{} orig-locals-size : @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 @i{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 @code{THEN}. @cindex locals information on the control-flow stack @cindex control-flow stack items, locals information In a conventional Forth implementation a dest control-flow stack entry is just the target address and an orig entry is just the address to be patched. Our locals implementation adds a word list to every orig or dest item. It is the list of locals visible (or assumed visible) at the point described by the entry. Our implementation also adds a tag to identify the kind of entry, in particular to differentiate between live and dead (reachable and unreachable) orig entries. A few unusual operations have to be performed on locals word lists: doc-common-list doc-sub-list? doc-list-size Several features of our locals word list implementation make these operations easy to implement: The locals word lists are organised as linked lists; the tails of these lists are shared, if the lists contain some of the same locals; and the address of a name is greater than the address of the names behind it in the list. Another important implementation detail is the variable @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to determine if they can be reached directly or only through the branch that they resolve. @code{dead-code} is set by @code{UNREACHABLE}, @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon definition, by @code{BEGIN} and usually by @code{THEN}. Counted loops are similar to other loops in most respects, but @code{LEAVE} requires special attention: It performs basically the same service as @code{AHEAD}, but it does not create a control-flow stack entry. Therefore the information has to be stored elsewhere; traditionally, the information was stored in the target fields of the branches created by the @code{LEAVE}s, by organizing these fields into a linked list. Unfortunately, this clever trick does not provide enough space for storing our extended control flow information. Therefore, we introduce another stack, the leave stack. It contains the control-flow stack entries for all unresolved @code{LEAVE}s. Local names are kept until the end of the colon definition, even if they are no longer visible in any control-flow path. In a few cases this may lead to increased space needs for the locals name area, but usually less than reclaiming this space would cost in code size. @node ANS Forth locals, , Gforth locals, Locals @subsection ANS Forth locals @cindex locals, ANS Forth style 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 wordset, i.e.: @example @{ local1 local2 ... -- comment @} @end example @noindent or @example @{ local1 local2 ... @} @end example The order of the locals corresponds to the order in a stack comment. The restrictions are: @itemize @bullet @item 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 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{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 anything to use it. If you want to port a program using this syntax to 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 reversed. Beware! The ANS Forth locals wordset itself consists of a word: doc-(local) The ANS Forth locals extension wordset defines a syntax using @code{locals|}, 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 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. @c ---------------------------------------------------------- @node Structures, Object-oriented Forth, Locals, Words @section Structures @cindex structures @cindex records This section presents the structure package that comes with Gforth. A version of the package implemented in ANS Forth is available in @file{compat/struct.fs}. This package was inspired by a posting on comp.lang.forth in 1989 (unfortunately I don't remember, by whom; possibly John Hayes). A version of this section has been published in ???. Marcel Hendrix provided helpful comments. @menu * Why explicit structure support?:: * Structure Usage:: * Structure Naming Convention:: * Structure Implementation:: * Structure Glossary:: @end menu @node Why explicit structure support?, Structure Usage, Structures, Structures @subsection Why explicit structure support? @cindex address arithmetic for structures @cindex structures using address arithmetic If we want to use a structure containing several fields, we could simply reserve memory for it, and access the fields using address arithmetic (@pxref{Address Arithmetic}). As an example, consider a structure with the following fields @table @code @item a is a float @item b is a cell @item c is a float @end table Given the (float-aligned) base address of the structure we get the address of the field @table @code @item a without doing anything further. @item b with @code{float+} @item c with @code{float+ cell+ faligned} @end table It is easy to see that this can become quite tiring. Moreover, it is not very readable, because seeing a @code{cell+} tells us neither which kind of structure is accessed nor what field is accessed; we have to somehow infer the kind of structure, and then look up in the documentation, which field of that structure corresponds to that offset. Finally, this kind of address arithmetic also causes maintenance troubles: If you add or delete a field somewhere in the middle of the structure, you have to find and change all computations for the fields afterwards. So, instead of using @code{cell+} and friends directly, how about storing the offsets in constants: @example 0 constant a-offset 0 float+ constant b-offset 0 float+ cell+ faligned c-offset @end example Now we can get the address of field @code{x} with @code{x-offset +}. This is much better in all respects. Of course, you still have to change all later offset definitions if you add a field. You can fix this by declaring the offsets in the following way: @example 0 constant a-offset a-offset float+ constant b-offset b-offset cell+ faligned constant c-offset @end example Since we always use the offsets with @code{+}, we could use a defining word @code{cfield} that includes the @code{+} in the action of the defined word: @example : cfield ( n "name" -- ) create , does> ( name execution: addr1 -- addr2 ) @@ + ; 0 cfield a 0 a float+ cfield b 0 b cell+ faligned cfield c @end example Instead of @code{x-offset +}, we now simply write @code{x}. The structure field words now can be used quite nicely. However, their definition is still a bit cumbersome: We have to repeat the name, the information about size and alignment is distributed before and after the field definitions etc. The structure package presented here addresses these problems. @node Structure Usage, Structure Naming Convention, Why explicit structure support?, Structures @subsection Structure Usage @cindex structure usage @cindex @code{field} usage @cindex @code{struct} usage @cindex @code{end-struct} usage You can define a structure for a (data-less) linked list with: @example struct cell% field list-next end-struct list% @end example With the address of the list node on the stack, you can compute the address of the field that contains the address of the next node with @code{list-next}. E.g., you can determine the length of a list with: @example : list-length ( list -- n ) \ "list" is a pointer to the first element of a linked list \ "n" is the length of the list 0 BEGIN ( list1 n1 ) over WHILE ( list1 n1 ) 1+ swap list-next @@ swap REPEAT nip ; @end example You can reserve memory for a list node in the dictionary with @code{list% %allot}, which leaves the address of the list node on the stack. For the equivalent allocation on the heap you can use @code{list% %alloc} (or, for an @code{allocate}-like stack effect (i.e., with ior), use @code{list% %allocate}). You can get the the size of a list node with @code{list% %size} and its alignment with @code{list% %alignment}. Note that in ANS Forth the body of a @code{create}d word is @code{aligned} but not necessarily @code{faligned}; therefore, if you do a: @example create @emph{name} foo% %allot @end example @noindent then the memory alloted for @code{foo%} is guaranteed to start at the body of @code{@emph{name}} only if @code{foo%} contains only character, cell and double fields. @cindex strcutures containing structures You can include a structure @code{foo%} as a field of another structure, like this: @example struct ... foo% field ... ... end-struct ... @end example @cindex structure extension @cindex extended records Instead of starting with an empty structure, you can extend an existing structure. E.g., a plain linked list without data, as defined above, is hardly useful; You can extend it to a linked list of integers, like this:@footnote{This feature is also known as @emph{extended records}. It is the main innovation in the Oberon language; in other words, adding this feature to Modula-2 led Wirth to create a new language, write a new compiler etc. Adding this feature to Forth just required a few lines of code.} @example list% cell% field intlist-int end-struct intlist% @end example @code{intlist%} is a structure with two fields: @code{list-next} and @code{intlist-int}. @cindex structures containing arrays You can specify an array type containing @emph{n} elements of type @code{foo%} like this: @example foo% @emph{n} * @end example You can use this array type in any place where you can use a normal type, e.g., when defining a @code{field}, or with @code{%allot}. @cindex first field optimization The first field is at the base address of a structure and the word for this field (e.g., @code{list-next}) actually does not change the address on the stack. You may be tempted to leave it away in the interest of run-time and space efficiency. This is not necessary, because the structure package optimizes this case and compiling such words does not generate any code. So, in the interest of readability and maintainability you should include the word for the field when accessing the field. @node Structure Naming Convention, Structure Implementation, Structure Usage, Structures @subsection Structure Naming Convention @cindex structure naming convention The field names that come to (my) mind are often quite generic, and, if used, would cause frequent name clashes. E.g., many structures probably contain a @code{counter} field. The structure names that come to (my) mind are often also the logical choice for the names of words that create such a structure. Therefore, I have adopted the following naming conventions: @itemize @bullet @cindex field naming convention @item The names of fields are of the form @code{@emph{struct}-@emph{field}}, where @code{@emph{struct}} is the basic name of the structure, and @code{@emph{field}} is the basic name of the field. You can think of field words as converting the (address of the) structure into the (address of the) field. @cindex structure naming convention @item The names of structures are of the form @code{@emph{struct}%}, where @code{@emph{struct}} is the basic name of the structure. @end itemize This naming convention does not work that well for fields of extended structures; e.g., the integer list structure has a field @code{intlist-int}, but has @code{list-next}, not @code{intlist-next}. @node Structure Implementation, Structure Glossary, Structure Naming Convention, Structures @subsection Structure Implementation @cindex structure implementation @cindex implementation of structures The central idea in the implementation is to pass the data about the structure being built on the stack, not in some global variable. Everything else falls into place naturally once this design decision is made. The type description on the stack is of the form @emph{align size}. Keeping the size on the top-of-stack makes dealing with arrays very simple. @code{field} is a defining word that uses @code{Create} and @code{DOES>}. The body of the field contains the offset of the field, and the normal @code{DOES>} action is simply: @example @ + @end example @noindent i.e., add the offset to the address, giving the stack effect @i{addr1 -- addr2} for a field. @cindex first field optimization, implementation This simple structure is slightly complicated by the optimization for fields with offset 0, which requires a different @code{DOES>}-part (because we cannot rely on there being something on the stack if such a field is invoked during compilation). Therefore, we put the different @code{DOES>}-parts in separate words, and decide which one to invoke based on the offset. For a zero offset, the field is basically a noop; it is immediate, and therefore no code is generated when it is compiled. @node Structure Glossary, , Structure Implementation, Structures @subsection Structure Glossary @cindex structure glossary doc-%align doc-%alignment doc-%alloc doc-%allocate doc-%allot doc-cell% doc-char% doc-dfloat% doc-double% doc-end-struct doc-field doc-float% doc-naligned doc-sfloat% doc-%size doc-struct @c ------------------------------------------------------------- @node Object-oriented Forth, Passing Commands to the OS, Structures, Words @section Object-oriented Forth Gforth comes with three packages for object-oriented programming: @file{objects.fs}, @file{oof.fs}, and @file{mini-oof.fs}; none of them is preloaded, so you have to @code{include} them before use. The most important differences between these packages (and others) are discussed in @ref{Comparison with other object models}. All packages are written in ANS Forth and can be used with any other ANS Forth. @menu * Why object-oriented programming?:: * Object-Oriented Terminology:: * Objects:: * OOF:: * Mini-OOF:: * Comparison with other object models:: @end menu @node Why object-oriented programming?, Object-Oriented Terminology, , Object-oriented Forth @subsubsection Why object-oriented programming? @cindex object-oriented programming motivation @cindex motivation for object-oriented programming Often we have to deal with several data structures (@emph{objects}), that have to be treated similarly in some respects, but differently in others. Graphical objects are the textbook example: circles, triangles, dinosaurs, icons, and others, and we may want to add more during program development. We want to apply some operations to any graphical object, e.g., @code{draw} for displaying it on the screen. However, @code{draw} has to do something different for every kind of object. @comment TODO add some other operations eg perimeter, area @comment and tie in to concrete examples later.. We could implement @code{draw} as a big @code{CASE} control structure that executes the appropriate code depending on the kind of object to be drawn. This would be not be very elegant, and, moreover, we would have to change @code{draw} every time we add a new kind of graphical object (say, a spaceship). What we would rather do is: When defining spaceships, we would tell the system: ``Here's how you @code{draw} a spaceship; you figure out the rest''. This is the problem that all systems solve that (rightfully) call themselves object-oriented; the object-oriented packages presented here solve this problem (and not much else). @comment TODO ?list properties of oo systems.. oo vs o-based? @node Object-Oriented Terminology, Objects, Why object-oriented programming?, Object-oriented Forth @subsubsection Object-Oriented Terminology @cindex object-oriented terminology @cindex terminology for object-oriented programming This section is mainly for reference, so you don't have to understand all of it right away. The terminology is mainly Smalltalk-inspired. In short: @table @emph @cindex class @item class a data structure definition with some extras. @cindex object @item object an instance of the data structure described by the class definition. @cindex instance variables @item instance variables fields of the data structure. @cindex selector @cindex method selector @cindex virtual function @item selector (or @emph{method selector}) a word (e.g., @code{draw}) that performs an operation on a variety of data structures (classes). A selector describes @emph{what} operation to perform. In C++ terminology: a (pure) virtual function. @cindex method @item method the concrete definition that performs the operation described by the selector for a specific class. A method specifies @emph{how} the operation is performed for a specific class. @cindex selector invocation @cindex message send @cindex invoking a selector @item selector invocation a call of a selector. One argument of the call (the TOS (top-of-stack)) is used for determining which method is used. In Smalltalk terminology: a message (consisting of the selector and the other arguments) is sent to the object. @cindex receiving object @item receiving object the object used for determining the method executed by a selector invocation. In the @file{objects.fs} model, it is the object that is on the TOS when the selector is invoked. (@emph{Receiving} comes from the Smalltalk @emph{message} terminology.) @cindex child class @cindex parent class @cindex inheritance @item child class a class that has (@emph{inherits}) all properties (instance variables, selectors, methods) from a @emph{parent class}. In Smalltalk terminology: The subclass inherits from the superclass. In C++ terminology: The derived class inherits from the base class. @end table @c If you wonder about the message sending terminology, it comes from @c a time when each object had it's own task and objects communicated via @c message passing; eventually the Smalltalk developers realized that @c they can do most things through simple (indirect) calls. They kept the @c terminology. @node Objects, OOF, Object-Oriented Terminology, Object-oriented Forth @subsection The @file{objects.fs} model @cindex objects @cindex object-oriented programming @cindex @file{objects.fs} @cindex @file{oof.fs} This section describes the @file{objects.fs} package. This material also has been published in @cite{Yet Another Forth Objects Package} by Anton Ertl and appeared in Forth Dimensions 19(2), pages 37--43 (@url{http://www.complang.tuwien.ac.at/forth/objects/objects.html}). @c McKewan's and Zsoter's packages This section assumes that you have read @ref{Structures}. The techniques on which this model is based have been used to implement the parser generator, Gray, and have also been used in Gforth for implementing the various flavours of word lists (hashed or not, case-sensitive or not, special-purpose word lists for locals etc.). @menu * Properties of the Objects model:: * Basic Objects Usage:: * The Objects base class:: * Creating objects:: * Object-Oriented Programming Style:: * Class Binding:: * Method conveniences:: * Classes and Scoping:: * Object Interfaces:: * Objects Implementation:: * Objects Glossary:: @end menu Marcel Hendrix provided helpful comments on this section. Andras Zsoter and Bernd Paysan helped me with the related works section. @node Properties of the Objects model, Basic Objects Usage, Objects, Objects @subsubsection Properties of the @file{objects.fs} model @cindex @file{objects.fs} properties @itemize @bullet @item It is straightforward to pass objects on the stack. Passing selectors on the stack is a little less convenient, but possible. @item Objects are just data structures in memory, and are referenced by their address. You can create words for objects with normal defining words like @code{constant}. Likewise, there is no difference between instance variables that contain objects and those that contain other data. @item Late binding is efficient and easy to use. @item It avoids parsing, and thus avoids problems with state-smartness and reduced extensibility; for convenience there are a few parsing words, but they have non-parsing counterparts. There are also a few defining words that parse. This is hard to avoid, because all standard defining words parse (except @code{:noname}); however, such words are not as bad as many other parsing words, because they are not state-smart. @item It does not try to incorporate everything. It does a few things and does them well (IMO). In particular, this model was not designed to support information hiding (although it has features that may help); you can use a separate package for achieving this. @item It is layered; you don't have to learn and use all features to use this model. Only a few features are necessary (@xref{Basic Objects Usage}, @xref{The Objects base class}, @xref{Creating objects}.), the others are optional and independent of each other. @item An implementation in ANS Forth is available. @end itemize @node Basic Objects Usage, The Objects base class, Properties of the Objects model, Objects @subsubsection Basic @file{objects.fs} Usage @cindex basic objects usage @cindex objects, basic usage You can define a class for graphical objects like this: @cindex @code{class} usage @cindex @code{end-class} usage @cindex @code{selector} usage @example object class \ "object" is the parent class selector draw ( x y graphical -- ) end-class graphical @end example This code defines a class @code{graphical} with an operation @code{draw}. We can perform the operation @code{draw} on any @code{graphical} object, e.g.: @example 100 100 t-rex draw @end example @noindent where @code{t-rex} is a word (say, a constant) that produces a graphical object. @comment TODO add a 2nd operation eg perimeter.. and use for @comment a concrete example @cindex abstract class How do we create a graphical object? With the present definitions, we cannot create a useful graphical object. The class @code{graphical} describes graphical objects in general, but not any concrete graphical object type (C++ users would call it an @emph{abstract class}); e.g., there is no method for the selector @code{draw} in the class @code{graphical}. For concrete graphical objects, we define child classes of the class @code{graphical}, e.g.: @cindex @code{overrides} usage @cindex @code{field} usage in class definition @example graphical class \ "graphical" is the parent class cell% field circle-radius :noname ( x y circle -- ) circle-radius @@ draw-circle ; overrides draw :noname ( n-radius circle -- ) circle-radius ! ; overrides construct end-class circle @end example Here we define a class @code{circle} as a child of @code{graphical}, with field @code{circle-radius} (which behaves just like a field (@pxref{Structures}); it defines (using @code{overrides}) new methods for the selectors @code{draw} and @code{construct} (@code{construct} is defined in @code{object}, the parent class of @code{graphical}). Now we can create a circle on the heap (i.e., @code{allocate}d memory) with: @cindex @code{heap-new} usage @example 50 circle heap-new constant my-circle @end example @noindent @code{heap-new} invokes @code{construct}, thus initializing the field @code{circle-radius} with 50. We can draw this new circle at (100,100) with: @example 100 100 my-circle draw @end example @cindex selector invocation, restrictions @cindex class definition, restrictions Note: You can only invoke a selector if the object on the TOS (the receiving object) belongs to the class where the selector was defined or one of its descendents; e.g., you can invoke @code{draw} only for objects belonging to @code{graphical} or its descendents (e.g., @code{circle}). Immediately before @code{end-class}, the search order has to be the same as immediately after @code{class}. @node The Objects base class, Creating objects, Basic Objects Usage, Objects @subsubsection The @file{object.fs} base class @cindex @code{object} class When you define a class, you have to specify a parent class. So how do you start defining classes? There is one class available from the start: @code{object}. It is ancestor for all classes and so is the only class that has no parent. It has two selectors: @code{construct} and @code{print}. @node Creating objects, Object-Oriented Programming Style, The Objects base class, Objects @subsubsection Creating objects @cindex creating objects @cindex object creation @cindex object allocation options @cindex @code{heap-new} discussion @cindex @code{dict-new} discussion @cindex @code{construct} discussion You can create and initialize an object of a class on the heap with @code{heap-new} ( ... class -- object ) and in the dictionary (allocation with @code{allot}) with @code{dict-new} ( ... class -- object ). Both words invoke @code{construct}, which consumes the stack items indicated by "..." above. @cindex @code{init-object} discussion @cindex @code{class-inst-size} discussion If you want to allocate memory for an object yourself, you can get its alignment and size with @code{class-inst-size 2@@} ( class -- align size ). Once you have memory for an object, you can initialize it with @code{init-object} ( ... class object -- ); @code{construct} does only a part of the necessary work. @node Object-Oriented Programming Style, Class Binding, Creating objects, Objects @subsubsection Object-Oriented Programming Style @cindex object-oriented programming style This section is not exhaustive. @cindex stack effects of selectors @cindex selectors and stack effects In general, it is a good idea to ensure that all methods for the same selector have the same stack effect: when you invoke a selector, you often have no idea which method will be invoked, so, unless all methods have the same stack effect, you will not know the stack effect of the selector invocation. One exception to this rule is methods for the selector @code{construct}. We know which method is invoked, because we specify the class to be constructed at the same place. Actually, I defined @code{construct} as a selector only to give the users a convenient way to specify initialization. The way it is used, a mechanism different from selector invocation would be more natural (but probably would take more code and more space to explain). @node Class Binding, Method conveniences, Object-Oriented Programming Style, Objects @subsubsection Class Binding @cindex class binding @cindex early binding @cindex late binding Normal selector invocations determine the method at run-time depending on the class of the receiving object. This run-time selection is called @i{late binding}. Sometimes it's preferable to invoke a different method. For example, you might want to use the simple method for @code{print}ing @code{object}s instead of the possibly long-winded @code{print} method of the receiver class. You can achieve this by replacing the invocation of @code{print} with: @cindex @code{[bind]} usage @example [bind] object print @end example @noindent in compiled code or: @cindex @code{bind} usage @example bind object print @end example @cindex class binding, alternative to @noindent in interpreted code. Alternatively, you can define the method with a name (e.g., @code{print-object}), and then invoke it through the name. Class binding is just a (often more convenient) way to achieve the same effect; it avoids name clutter and allows you to invoke methods directly without naming them first. @cindex superclass binding @cindex parent class binding A frequent use of class binding is this: When we define a method for a selector, we often want the method to do what the selector does in the parent class, and a little more. There is a special word for this purpose: @code{[parent]}; @code{[parent] @emph{selector}} is equivalent to @code{[bind] @emph{parent selector}}, where @code{@emph{parent}} is the parent class of the current class. E.g., a method definition might look like: @cindex @code{[parent]} usage @example :noname dup [parent] foo \ do parent's foo on the receiving object ... \ do some more ; overrides foo @end example @cindex class binding as optimization In @cite{Object-oriented programming in ANS Forth} (Forth Dimensions, March 1997), Andrew McKewan presents class binding as an optimization technique. I recommend not using it for this purpose unless you are in an emergency. Late binding is pretty fast with this model anyway, so the benefit of using class binding is small; the cost of using class binding where it is not appropriate is reduced maintainability. While we are at programming style questions: You should bind selectors only to ancestor classes of the receiving object. E.g., say, you know that the receiving object is of class @code{foo} or its descendents; then you should bind only to @code{foo} and its ancestors. @node Method conveniences, Classes and Scoping, Class Binding, Objects @subsubsection Method conveniences @cindex method conveniences In a method you usually access the receiving object pretty often. If you define the method as a plain colon definition (e.g., with @code{:noname}), you may have to do a lot of stack gymnastics. To avoid this, you can define the method with @code{m: ... ;m}. E.g., you could define the method for @code{draw}ing a @code{circle} with @cindex @code{this} usage @cindex @code{m:} usage @cindex @code{;m} usage @example m: ( x y circle -- ) ( x y ) this circle-radius @@ draw-circle ;m @end example @cindex @code{exit} in @code{m: ... ;m} @cindex @code{exitm} discussion @cindex @code{catch} in @code{m: ... ;m} When this method is executed, the receiver object is removed from the stack; you can access it with @code{this} (admittedly, in this example the use of @code{m: ... ;m} offers no advantage). Note that I specify the stack effect for the whole method (i.e. including the receiver object), not just for the code between @code{m:} and @code{;m}. You cannot use @code{exit} in @code{m:...;m}; instead, use @code{exitm}.@footnote{Moreover, for any word that calls @code{catch} and was defined before loading @code{objects.fs}, you have to redefine it like I redefined @code{catch}: @code{: catch this >r catch r> to-this ;}} @cindex @code{inst-var} usage You will frequently use sequences of the form @code{this @emph{field}} (in the example above: @code{this circle-radius}). If you use the field only in this way, you can define it with @code{inst-var} and eliminate the @code{this} before the field name. E.g., the @code{circle} class above could also be defined with: @example graphical class cell% inst-var radius m: ( x y circle -- ) radius @@ draw-circle ;m overrides draw m: ( n-radius circle -- ) radius ! ;m overrides construct end-class circle @end example @code{radius} can only be used in @code{circle} and its descendent classes and inside @code{m:...;m}. @cindex @code{inst-value} usage You can also define fields with @code{inst-value}, which is to @code{inst-var} what @code{value} is to @code{variable}. You can change the value of such a field with @code{[to-inst]}. E.g., we could also define the class @code{circle} like this: @example graphical class inst-value radius m: ( x y circle -- ) radius draw-circle ;m overrides draw m: ( n-radius circle -- ) [to-inst] radius ;m overrides construct end-class circle @end example @node Classes and Scoping, Object Interfaces, Method conveniences, Objects @subsubsection Classes and Scoping @cindex classes and scoping @cindex scoping and classes Inheritance is frequent, unlike structure extension. This exacerbates the problem with the field name convention (@pxref{Structure Naming Convention}): One always has to remember in which class the field was originally defined; changing a part of the class structure would require changes for renaming in otherwise unaffected code. @cindex @code{inst-var} visibility @cindex @code{inst-value} visibility To solve this problem, I added a scoping mechanism (which was not in my original charter): A field defined with @code{inst-var} (or @code{inst-value}) is visible only in the class where it is defined and in the descendent classes of this class. Using such fields only makes sense in @code{m:}-defined methods in these classes anyway. This scoping mechanism allows us to use the unadorned field name, because name clashes with unrelated words become much less likely. @cindex @code{protected} discussion @cindex @code{private} discussion Once we have this mechanism, we can also use it for controlling the visibility of other words: All words defined after @code{protected} are visible only in the current class and its descendents. @code{public} restores the compilation (i.e. @code{current}) word list that was in effect before. If you have several @code{protected}s without an intervening @code{public} or @code{set-current}, @code{public} will restore the compilation word list in effect before the first of these @code{protected}s. @node Object Interfaces, Objects Implementation, Classes and Scoping, Objects @subsubsection Object Interfaces @cindex object interfaces @cindex interfaces for objects In this model you can only call selectors defined in the class of the receiving objects or in one of its ancestors. If you call a selector with a receiving object that is not in one of these classes, the result is undefined; if you are lucky, the program crashes immediately. @cindex selectors common to hardly-related classes Now consider the case when you want to have a selector (or several) available in two classes: You would have to add the selector to a common ancestor class, in the worst case to @code{object}. You may not want to do this, e.g., because someone else is responsible for this ancestor class. The solution for this problem is interfaces. An interface is a collection of selectors. If a class implements an interface, the selectors become available to the class and its descendents. A class can implement an unlimited number of interfaces. For the problem discussed above, we would define an interface for the selector(s), and both classes would implement the interface. As an example, consider an interface @code{storage} for writing objects to disk and getting them back, and a class @code{foo} that implements it. The code would look like this: @cindex @code{interface} usage @cindex @code{end-interface} usage @cindex @code{implementation} usage @example interface selector write ( file object -- ) selector read1 ( file object -- ) end-interface storage bar class storage implementation ... overrides write ... overrides read ... end-class foo @end example @noindent (I would add a word @code{read} @i{( file -- object )} that uses @code{read1} internally, but that's beyond the point illustrated here.) Note that you cannot use @code{protected} in an interface; and of course you cannot define fields. In the Neon model, all selectors are available for all classes; therefore it does not need interfaces. The price you pay in this model is slower late binding, and therefore, added complexity to avoid late binding. @node Objects Implementation, Objects Glossary, Object Interfaces, Objects @subsubsection @file{objects.fs} Implementation @cindex @file{objects.fs} implementation @cindex @code{object-map} discussion An object is a piece of memory, like one of the data structures described with @code{struct...end-struct}. It has a field @code{object-map} that points to the method map for the object's class. @cindex method map @cindex virtual function table The @emph{method map}@footnote{This is Self terminology; in C++ terminology: virtual function table.} is an array that contains the execution tokens (@i{xt}s) of the methods for the object's class. Each selector contains an offset into a method map. @cindex @code{selector} implementation, class @code{selector} is a defining word that uses @code{CREATE} and @code{DOES>}. The body of the selector contains the offset; the @code{does>} action for a class selector is, basically: @example ( object addr ) @@ over object-map @@ + @@ execute @end example Since @code{object-map} is the first field of the object, it does not generate any code. As you can see, calling a selector has a small, constant cost. @cindex @code{current-interface} discussion @cindex class implementation and representation A class is basically a @code{struct} combined with a method map. During the class definition the alignment and size of the class are passed on the stack, just as with @code{struct}s, so @code{field} can also be used for defining class fields. However, passing more items on the stack would be inconvenient, so @code{class} builds a data structure in memory, which is accessed through the variable @code{current-interface}. After its definition is complete, the class is represented on the stack by a pointer (e.g., as parameter for a child class definition). A new class starts off with the alignment and size of its parent, and a copy of the parent's method map. Defining new fields extends the size and alignment; likewise, defining new selectors extends the method map. @code{overrides} just stores a new @i{xt} in the method map at the offset given by the selector. @cindex class binding, implementation Class binding just gets the @i{xt} at the offset given by the selector from the class's method map and @code{compile,}s (in the case of @code{[bind]}) it. @cindex @code{this} implementation @cindex @code{catch} and @code{this} @cindex @code{this} and @code{catch} I implemented @code{this} as a @code{value}. At the start of an @code{m:...;m} method the old @code{this} is stored to the return stack and restored at the end; and the object on the TOS is stored @code{TO this}. This technique has one disadvantage: If the user does not leave the method via @code{;m}, but via @code{throw} or @code{exit}, @code{this} is not restored (and @code{exit} may crash). To deal with the @code{throw} problem, I have redefined @code{catch} to save and restore @code{this}; the same should be done with any word that can catch an exception. As for @code{exit}, I simply forbid it (as a replacement, there is @code{exitm}). @cindex @code{inst-var} implementation @code{inst-var} is just the same as @code{field}, with a different @code{DOES>} action: @example @@ this + @end example Similar for @code{inst-value}. @cindex class scoping implementation Each class also has a word list that contains the words defined with @code{inst-var} and @code{inst-value}, and its protected words. It also has a pointer to its parent. @code{class} pushes the word lists of the class and all its ancestors onto the search order stack, and @code{end-class} drops them. @cindex interface implementation An interface is like a class without fields, parent and protected words; i.e., it just has a method map. If a class implements an interface, its method map contains a pointer to the method map of the interface. The positive offsets in the map are reserved for class methods, therefore interface map pointers have negative offsets. Interfaces have offsets that are unique throughout the system, unlike class selectors, whose offsets are only unique for the classes where the selector is available (invokable). This structure means that interface selectors have to perform one indirection more than class selectors to find their method. Their body contains the interface map pointer offset in the class method map, and the method offset in the interface method map. The @code{does>} action for an interface selector is, basically: @example ( object selector-body ) 2dup selector-interface @@ ( object selector-body object interface-offset ) swap object-map @@ + @@ ( object selector-body map ) swap selector-offset @@ + @@ execute @end example where @code{object-map} and @code{selector-offset} are first fields and generate no code. As a concrete example, consider the following code: @example interface selector if1sel1 selector if1sel2 end-interface if1 object class if1 implementation selector cl1sel1 cell% inst-var cl1iv1 ' m1 overrides construct ' m2 overrides if1sel1 ' m3 overrides if1sel2 ' m4 overrides cl1sel2 end-class cl1 create obj1 object dict-new drop create obj2 cl1 dict-new drop @end example The data structure created by this code (including the data structure for @code{object}) is shown in the figure, assuming a cell size of 4. @comment TODO add this diagram.. @node Objects Glossary, , Objects Implementation, Objects @subsubsection @file{objects.fs} Glossary @cindex @file{objects.fs} Glossary doc---objects-bind doc---objects- doc---objects-bind' doc---objects-[bind] doc---objects-class doc---objects-class->map doc---objects-class-inst-size doc---objects-class-override! doc---objects-construct doc---objects-current' doc---objects-[current] doc---objects-current-interface doc---objects-dict-new doc---objects-drop-order doc---objects-end-class doc---objects-end-class-noname doc---objects-end-interface doc---objects-end-interface-noname doc---objects-exitm doc---objects-heap-new doc---objects-implementation doc---objects-init-object doc---objects-inst-value doc---objects-inst-var doc---objects-interface doc---objects-;m doc---objects-m: doc---objects-method doc---objects-object doc---objects-overrides doc---objects-[parent] doc---objects-print doc---objects-protected doc---objects-public doc---objects-push-order doc---objects-selector doc---objects-this doc---objects- doc---objects-[to-inst] doc---objects-to-this doc---objects-xt-new @c ------------------------------------------------------------- @node OOF, Mini-OOF, Objects, Object-oriented Forth @subsection The @file{oof.fs} model @cindex oof @cindex object-oriented programming @cindex @file{objects.fs} @cindex @file{oof.fs} This section describes the @file{oof.fs} package. The package described in this section has been used in bigFORTH since 1991, and used for two large applications: a chromatographic system used to create new medicaments, and a graphic user interface library (MINOS). You can find a description (in German) of @file{oof.fs} in @cite{Object oriented bigFORTH} by Bernd Paysan, published in @cite{Vierte Dimension} 10(2), 1994. @menu * Properties of the OOF model:: * Basic OOF Usage:: * The OOF base class:: * Class Declaration:: * Class Implementation:: @end menu @node Properties of the OOF model, Basic OOF Usage, OOF, OOF @subsubsection Properties of the @file{oof.fs} model @cindex @file{oof.fs} properties @itemize @bullet @item This model combines object oriented programming with information hiding. It helps you writing large application, where scoping is necessary, because it provides class-oriented scoping. @item Named objects, object pointers, and object arrays can be created, selector invocation uses the ``object selector'' syntax. Selector invocation to objects and/or selectors on the stack is a bit less convenient, but possible. @item Selector invocation and instance variable usage of the active object is straightforward, since both make use of the active object. @item Late binding is efficient and easy to use. @item State-smart objects parse selectors. However, extensibility is provided using a (parsing) selector @code{postpone} and a selector @code{'}. @item An implementation in ANS Forth is available. @end itemize @node Basic OOF Usage, The OOF base class, Properties of the OOF model, OOF @subsubsection Basic @file{oof.fs} Usage @cindex @file{oof.fs} usage This section uses the same example as for @code{objects} (@pxref{Basic Objects Usage}). You can define a class for graphical objects like this: @cindex @code{class} usage @cindex @code{class;} usage @cindex @code{method} usage @example object class graphical \ "object" is the parent class method draw ( x y graphical -- ) class; @end example This code defines a class @code{graphical} with an operation @code{draw}. We can perform the operation @code{draw} on any @code{graphical} object, e.g.: @example 100 100 t-rex draw @end example @noindent where @code{t-rex} is an object or object pointer, created with e.g. @code{graphical : t-rex}. @cindex abstract class How do we create a graphical object? With the present definitions, we cannot create a useful graphical object. The class @code{graphical} describes graphical objects in general, but not any concrete graphical object type (C++ users would call it an @emph{abstract class}); e.g., there is no method for the selector @code{draw} in the class @code{graphical}. For concrete graphical objects, we define child classes of the class @code{graphical}, e.g.: @example graphical class circle \ "graphical" is the parent class cell var circle-radius how: : draw ( x y -- ) circle-radius @@ draw-circle ; : init ( n-radius -- ( circle-radius ! ; class; @end example Here we define a class @code{circle} as a child of @code{graphical}, with a field @code{circle-radius}; it defines new methods for the selectors @code{draw} and @code{init} (@code{init} is defined in @code{object}, the parent class of @code{graphical}). Now we can create a circle in the dictionary with: @example 50 circle : my-circle @end example @noindent @code{:} invokes @code{init}, thus initializing the field @code{circle-radius} with 50. We can draw this new circle at (100,100) with: @example 100 100 my-circle draw @end example @cindex selector invocation, restrictions @cindex class definition, restrictions Note: You can only invoke a selector if the receiving object belongs to the class where the selector was defined or one of its descendents; e.g., you can invoke @code{draw} only for objects belonging to @code{graphical} or its descendents (e.g., @code{circle}). The scoping mechanism will check if you try to invoke a selector that is not defined in this class hierarchy, so you'll get an error at compilation time. @node The OOF base class, Class Declaration, Basic OOF Usage, OOF @subsubsection The @file{oof.fs} base class @cindex @file{oof.fs} base class When you define a class, you have to specify a parent class. So how do you start defining classes? There is one class available from the start: @code{object}. You have to use it as ancestor for all classes. It is the only class that has no parent. Classes are also objects, except that they don't have instance variables; class manipulation such as inheritance or changing definitions of a class is handled through selectors of the class @code{object}. @code{object} provides a number of selectors: @itemize @bullet @item @code{class} for subclassing, @code{definitions} to add definitions later on, and @code{class?} to get type informations (is the class a subclass of the class passed on the stack?). doc---object-class doc---object-definitions doc---object-class? @item @code{init} and @code{dispose} as constructor and destructor of the object. @code{init} is invocated after the object's memory is allocated, while @code{dispose} also handles deallocation. Thus if you redefine @code{dispose}, you have to call the parent's dispose with @code{super dispose}, too. doc---object-init doc---object-dispose @item @code{new}, @code{new[]}, @code{:}, @code{ptr}, @code{asptr}, and @code{[]} to create named and unnamed objects and object arrays or object pointers. doc---object-new doc---object-new[] doc---object-: doc---object-ptr doc---object-asptr doc---object-[] @item @code{::} and @code{super} for explicit scoping. You should use explicit scoping only for super classes or classes with the same set of instance variables. Explicitly-scoped selectors use early binding. doc---object-:: doc---object-super @item @code{self} to get the address of the object doc---object-self @item @code{bind}, @code{bound}, @code{link}, and @code{is} to assign object pointers and instance defers. doc---object-bind doc---object-bound doc---object-link doc---object-is @item @code{'} to obtain selector tokens, @code{send} to invocate selectors form the stack, and @code{postpone} to generate selector invocation code. doc---object-' doc---object-postpone @item @code{with} and @code{endwith} to select the active object from the stack, and enable its scope. Using @code{with} and @code{endwith} also allows you to create code using selector @code{postpone} without being trapped by the state-smart objects. doc---object-with doc---object-endwith @end itemize @node Class Declaration, Class Implementation, The OOF base class, OOF @subsubsection Class Declaration @cindex class declaration @itemize @bullet @item Instance variables doc---oof-var @item Object pointers doc---oof-ptr doc---oof-asptr @item Instance defers doc---oof-defer @item Method selectors doc---oof-early doc---oof-method @item Class-wide variables doc---oof-static @item End declaration doc---oof-how: doc---oof-class; @end itemize @c ------------------------------------------------------------- @node Class Implementation, , Class Declaration, OOF @subsubsection Class Implementation @cindex class implementation @c ------------------------------------------------------------- @node Mini-OOF, Comparison with other object models, OOF, Object-oriented Forth @subsection The @file{mini-oof.fs} model @cindex mini-oof Gforth's third object oriented Forth package is a 12-liner. It uses a mixture of the @file{object.fs} and the @file{oof.fs} syntax, and reduces to the bare minimum of features. This is based on a posting of Bernd Paysan in comp.arch. @menu * Basic Mini-OOF Usage:: * Mini-OOF Example:: * Mini-OOF Implementation:: @end menu @c ------------------------------------------------------------- @node Basic Mini-OOF Usage, Mini-OOF Example, , Mini-OOF @subsubsection Basic @file{mini-oof.fs} Usage @cindex mini-oof usage There is a base class (@code{class}, which allocates one cell for the object pointer) plus seven other words: to define a method, a variable, a class; to end a class, to resolve binding, to allocate an object and to compile a class method. @comment TODO better description of the last one doc-object doc-method doc-var doc-class doc-end-class doc-defines doc-new doc-:: @c ------------------------------------------------------------- @node Mini-OOF Example, Mini-OOF Implementation, Basic Mini-OOF Usage, Mini-OOF @subsubsection Mini-OOF Example @cindex mini-oof example A short example shows how to use this package. This example, in slightly extended form, is supplied as @file{moof-exm.fs} @comment TODO could flesh this out with some comments from the Forthwrite article @example object class method init method draw end-class graphical @end example This code defines a class @code{graphical} with an operation @code{draw}. We can perform the operation @code{draw} on any @code{graphical} object, e.g.: @example 100 100 t-rex draw @end example where @code{t-rex} is an object or object pointer, created with e.g. @code{graphical new Constant t-rex}. For concrete graphical objects, we define child classes of the class @code{graphical}, e.g.: @example graphical class cell var circle-radius end-class circle \ "graphical" is the parent class :noname ( x y -- ) circle-radius @@ draw-circle ; circle defines draw :noname ( r -- ) circle-radius ! ; circle defines init @end example There is no implicit init method, so we have to define one. The creation code of the object now has to call init explicitely. @example circle new Constant my-circle 50 my-circle init @end example It is also possible to add a function to create named objects with automatic call of @code{init}, given that all objects have @code{init} on the same place: @example : new: ( .. o "name" -- ) new dup Constant init ; 80 circle new: large-circle @end example We can draw this new circle at (100,100) with: @example 100 100 my-circle draw @end example @node Mini-OOF Implementation, , Mini-OOF Example, Mini-OOF @subsubsection @file{mini-oof.fs} Implementation Object-oriented systems with late binding typically use a ``vtable''-approach: the first variable in each object is a pointer to a table, which contains the methods as function pointers. The vtable may also contain other information. So first, let's declare methods: @example : method ( m v -- m' v ) Create over , swap cell+ swap DOES> ( ... o -- ... ) @ over @ + @ execute ; @end example During method declaration, the number of methods and instance variables is on the stack (in address units). @code{method} creates one method and increments the method number. To execute a method, it takes the object, fetches the vtable pointer, adds the offset, and executes the @i{xt} stored there. Each method takes the object it is invoked from as top of stack parameter. The method itself should consume that object. Now, we also have to declare instance variables @example : var ( m v size -- m v' ) Create over , + DOES> ( o -- addr ) @ + ; @end example As before, a word is created with the current offset. Instance variables can have different sizes (cells, floats, doubles, chars), so all we do is take the size and add it to the offset. If your machine has alignment restrictions, put the proper @code{aligned} or @code{faligned} before the variable, to adjust the variable offset. That's why it is on the top of stack. We need a starting point (the base object) and some syntactic sugar: @example Create object 1 cells , 2 cells , : class ( class -- class methods vars ) dup 2@ ; @end example For inheritance, the vtable of the parent object has to be copied when a new, derived class is declared. This gives all the methods of the parent class, which can be overridden, though. @example : end-class ( class methods vars -- ) Create here >r , dup , 2 cells ?DO ['] noop , 1 cells +LOOP cell+ dup cell+ r> rot @ 2 cells /string move ; @end example The first line creates the vtable, initialized with @code{noop}s. The second line is the inheritance mechanism, it copies the xts from the parent vtable. We still have no way to define new methods, let's do that now: @example : defines ( xt class -- ) ' >body @ + ! ; @end example To allocate a new object, we need a word, too: @example : new ( class -- o ) here over @ allot swap over ! ; @end example Sometimes derived classes want to access the method of the parent object. There are two ways to achieve this with Mini-OOF: first, you could use named words, and second, you could look up the vtable of the parent object. @example : :: ( class "name" -- ) ' >body @ + @ compile, ; @end example Nothing can be more confusing than a good example, so here is one. First let's declare a text object (called @code{button}), that stores text and position: @example object class cell var text cell var len cell var x cell var y method init method draw end-class button @end example @noindent Now, implement the two methods, @code{draw} and @code{init}: @example :noname ( o -- ) >r r@ x @ r@ y @ at-xy r@ text @ r> len @ type ; button defines draw :noname ( addr u o -- ) >r 0 r@ x ! 0 r@ y ! r@ len ! r> text ! ; button defines init @end example @noindent To demonstrate inheritance, we define a class @code{bold-button}, with no new data and no new methods: @example button class end-class bold-button : bold 27 emit ." [1m" ; : normal 27 emit ." [0m" ; @end example @noindent The class @code{bold-button} has a different draw method to @code{button}, but the new method is defined in terms of the draw method for @code{button}: @example :noname bold [ button :: draw ] normal ; bold-button defines draw @end example @noindent Finally, create two objects and apply methods: @example button new Constant foo s" thin foo" foo init page foo draw bold-button new Constant bar s" fat bar" bar init 1 bar y ! bar draw @end example @node Comparison with other object models, , Mini-OOF, Object-oriented Forth @subsubsection Comparison with other object models @cindex comparison of object models @cindex object models, comparison Many object-oriented Forth extensions have been proposed (@cite{A survey of object-oriented Forths} (SIGPLAN Notices, April 1996) by Bradford J. Rodriguez and W. F. S. Poehlman lists 17). This section discusses the relation of the object models described here to two well-known and two closely-related (by the use of method maps) models. @cindex Neon model The most popular model currently seems to be the Neon model (see @cite{Object-oriented programming in ANS Forth} (Forth Dimensions, March 1997) by Andrew McKewan) but this model has a number of limitations @footnote{A longer version of this critique can be found in @cite{On Standardizing Object-Oriented Forth Extensions} (Forth Dimensions, May 1997) by Anton Ertl.}: @itemize @bullet @item It uses a @code{@emph{selector object}} syntax, which makes it unnatural to pass objects on the stack. @item It requires that the selector parses the input stream (at compile time); this leads to reduced extensibility and to bugs that are+ hard to find. @item It allows using every selector to every object; this eliminates the need for classes, but makes it harder to create efficient implementations. @end itemize @cindex Pountain's object-oriented model Another well-known publication is @cite{Object-Oriented Forth} (Academic Press, London, 1987) by Dick Pountain. However, it is not really about object-oriented programming, because it hardly deals with late binding. Instead, it focuses on features like information hiding and overloading that are characteristic of modular languages like Ada (83). @cindex Zsoter's object-oriented model In @cite{Does late binding have to be slow?} (Forth Dimensions 18(1) 1996, pages 31-35) Andras Zsoter describes a model that makes heavy use of an active object (like @code{this} in @file{objects.fs}): The active object is not only used for accessing all fields, but also specifies the receiving object of every selector invocation; you have to change the active object explicitly with @code{@{ ... @}}, whereas in @file{objects.fs} it changes more or less implicitly at @code{m: ... ;m}. Such a change at the method entry point is unnecessary with the Zsoter's model, because the receiving object is the active object already. On the other hand, the explicit change is absolutely necessary in that model, because otherwise no one could ever change the active object. An ANS Forth implementation of this model is available at @url{http://www.forth.org/fig/oopf.html}. @cindex @file{oof.fs}, differences to other models The @file{oof.fs} model combines information hiding and overloading resolution (by keeping names in various word lists) with object-oriented programming. It sets the active object implicitly on method entry, but also allows explicit changing (with @code{>o...o>} or with @code{with...endwith}). It uses parsing and state-smart objects and classes for resolving overloading and for early binding: the object or class parses the selector and determines the method from this. If the selector is not parsed by an object or class, it performs a call to the selector for the active object (late binding), like Zsoter's model. Fields are always accessed through the active object. The big disadvantage of this model is the parsing and the state-smartness, which reduces extensibility and increases the opportunities for subtle bugs; essentially, you are only safe if you never tick or @code{postpone} an object or class (Bernd disagrees, but I (Anton) am not convinced). @cindex @file{mini-oof.fs}, differences to other models The @file{mini-oof.fs} model is quite similar to a very stripped-down version of the @file{objects.fs} model, but syntactically it is a mixture of the @file{objects.fs} and @file{oof.fs} models. @c ------------------------------------------------------------- @node Passing Commands to the OS, Miscellaneous Words, Object-oriented Forth, Words @section Passing Commands to the Operating System @cindex operating system - passing commands @cindex shell commands Gforth allows you to pass an arbitrary string to the host operating system shell (if such a thing exists) for execution. doc-sh doc-system doc-$? doc-getenv @c ------------------------------------------------------------- @node Miscellaneous Words, , Passing Commands to the OS, Words @section Miscellaneous Words @cindex miscellaneous words @comment TODO find homes for these These section lists the ANS Forth words that are not documented elsewhere in this manual. Ultimately, they all need proper homes. doc-ms doc-time&date doc-[compile] The following ANS Forth words are not currently supported by Gforth (@pxref{ANS conformance}): @code{EDITOR} @code{EKEY} @code{EKEY>CHAR} @code{EKEY?} @code{EMIT?} @code{FORGET} @c ****************************************************************** @node Error messages, Tools, Words, Top @chapter Error messages @cindex error messages @cindex backtrace A typical Gforth error message looks like this: @example in file included from :-1 in file included from ./yyy.fs:1 ./xxx.fs:4: Invalid memory address bar ^^^ $400E664C @@ $400E6664 foo @end example The message identifying the error is @code{Invalid memory address}. The error happened when text-interpreting line 4 of the file @file{./xxx.fs}. This line is given (it contains @code{bar}), and the word on the line where the error happened, is pointed out (with @code{^^^}). The file containing the error was included in line 1 of @file{./yyy.fs}, and @file{yyy.fs} was included from a non-file (in this case, by giving @file{yyy.fs} as command-line parameter to Gforth). At the end of the error message you find a return stack dump that can be interpreted as a backtrace (possibly empty). On top you find the top of the return stack when the @code{throw} happened, and at the bottom you find the return stack entry just above the return stack of the topmost text interpreter. To the right of most return stack entries you see a guess for the word that pushed that return stack entry as its return address. This gives a backtrace. In our case we see that @code{bar} called @code{foo}, and @code{foo} called @code{@@} (and @code{@@} had an @emph{Invalid memory address} exception). Note that the backtrace is not perfect: We don't know which return stack entries are return addresses (so we may get false positives); and in some cases (e.g., for @code{abort"}) we cannot determine from the return address the word that pushed the return address, so for some return addresses you see no names in the return stack dump. @cindex @code{catch} and backtraces The return stack dump represents the return stack at the time when a specific @code{throw} was executed. In programs that make use of @code{catch}, it is not necessarily clear which @code{throw} should be used for the return stack dump (e.g., consider one @code{throw} that indicates an error, which is caught, and during recovery another error happens; which @code{throw} should be used for the stack dump). Gforth presents the return stack dump for the first @code{throw} after the last executed (not returned-to) @code{catch}; this works well in the usual case. @cindex @code{gforth-fast} and backtraces @cindex @code{gforth-fast}, difference from @code{gforth} @cindex backtraces with @code{gforth-fast} @cindex return stack dump with @code{gforth-fast} @code{gforth} is able to do a return stack dump for throws generated from primitives (e.g., invalid memory address, stack empty etc.); @code{gforth-fast} is only able to do a return stack dump from a directly called @code{throw} (including @code{abort} etc.). This is the only difference (apart from a speed difference of about 30%) between @code{gforth} and @code{gforth-fast}. Given an exception caused by a primitive in @code{gforth-fast}, you will typically see no return stack dump at all; however, if the exception is caught by @code{catch} (e.g., for restoring some state), and then @code{throw}n again, the return stack dump will be for the first such @code{throw}. @c ****************************************************************** @node Tools, ANS conformance, Error messages, Top @chapter Tools @menu * ANS Report:: Report the words used, sorted by wordset. @end menu See also @ref{Emacs and Gforth}. @node ANS Report, , Tools, Tools @section @file{ans-report.fs}: Report the words used, sorted by wordset @cindex @file{ans-report.fs} @cindex report the words used in your program @cindex words used in your program If you want to label a Forth program as ANS Forth Program, you must document which wordsets the program uses; for extension wordsets, it is helpful to list the words the program requires from these wordsets (because Forth systems are allowed to provide only some words of them). The @file{ans-report.fs} tool makes it easy for you to determine which words from which wordset and which non-ANS words your application uses. You simply have to include @file{ans-report.fs} before loading the program you want to check. After loading your program, you can get the report with @code{print-ans-report}. A typical use is to run this as batch job like this: @example gforth ans-report.fs myprog.fs -e "print-ans-report bye" @end example The output looks like this (for @file{compat/control.fs}): @example The program uses the following words from CORE : : POSTPONE THEN ; immediate ?dup IF 0= from BLOCK-EXT : \ from FILE : ( @end example @subsection Caveats Note that @file{ans-report.fs} just checks which words are used, not whether they are used in an ANS Forth conforming way! Some words are defined in several wordsets in the standard. @file{ans-report.fs} reports them for only one of the wordsets, and not necessarily the one you expect. It depends on usage which wordset is the right one to specify. E.g., if you only use the compilation semantics of @code{S"}, it is a Core word; if you also use its interpretation semantics, it is a File word. @c ****************************************************************** @node ANS conformance, Model, Tools, Top @chapter ANS conformance @cindex ANS conformance of Gforth 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 @cindex system documentation 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 ===================================================================== @cindex core words, system documentation @cindex system documentation, core words @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 --------------------------------------------------------------------- @cindex core words, implementation-defined options @cindex implementation-defined options, core words @table @i @item (Cell) aligned addresses: @cindex cell-aligned addresses @cindex 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: @cindex @code{EMIT} and non-graphic characters @cindex non-graphic characters and @code{EMIT} The character is output using the C library function (actually, macro) @code{putc}. @item character editing of @code{ACCEPT} and @code{EXPECT}: @cindex character editing of @code{ACCEPT} and @code{EXPECT} @cindex editing in @code{ACCEPT} and @code{EXPECT} @cindex @code{ACCEPT}, editing @cindex @code{EXPECT}, editing 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). @xref{Command-line editing}. @item character set: @cindex 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: @cindex 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: @cindex character-set extensions and matching of names @cindex case-sensitivity for name lookup @cindex name lookup, case-sensitivity @cindex locale and case-sensitivity Any character except the ASCII NUL character 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: @cindex space delimiters @cindex control characters as delimiters 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: @cindex control-flow stack, format 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 @cindex 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}: @cindex @code{EXPECT}, display after end of input @cindex @code{ACCEPT}, display after end of input 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"}: @cindex exception abort sequence of @code{ABORT"} @cindex @code{ABORT"}, exception abort sequence The error string is stored into the variable @code{"error} and a @code{-2 throw} is performed. @item input line terminator: @cindex input line terminator @cindex line terminator on input @cindex newline character on input 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: @cindex maximum size of a counted string @cindex counted string, maximum size @code{s" /counted-string" environment? drop .}. Currently 255 characters on all ports, but this may change. @item maximum size of a parsed string: @cindex maximum size of a parsed string @cindex parsed string, maximum size Given by the constant @code{/line}. Currently 255 characters. @item maximum size of a definition name, in characters: @cindex maximum size of a definition name, in characters @cindex name, maximum length 31 @item maximum string length for @code{ENVIRONMENT?}, in characters: @cindex maximum string length for @code{ENVIRONMENT?}, in characters @cindex @code{ENVIRONMENT?} string length, maximum 31 @item method of selecting the user input device: @cindex user input device, method of selecting 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: @cindex user output device, method of selecting @code{EMIT} and @code{TYPE} output to the file-id stored in the value @code{outfile-id} (@code{stdout} by default). Gforth uses unbuffered output when the user output device is a terminal, otherwise the output is buffered. @item methods of dictionary compilation: What are we expected to document here? @item number of bits in one address unit: @cindex number of bits in one address unit @cindex address unit, size in bits @code{s" address-units-bits" environment? drop .}. 8 in all current ports. @item number representation and arithmetic: @cindex number representation and arithmetic Processor-dependent. Binary two's complement on all current ports. @item ranges for integer types: @cindex ranges for integer types @cindex integer types, ranges 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: @cindex read-only data space regions @cindex data-space, read-only regions The whole Forth data space is writable. @item size of buffer at @code{WORD}: @cindex size of buffer at @code{WORD} @cindex @code{WORD} buffer size @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: @cindex cell size @code{1 cells .}. @item size of one character in address units: @cindex char size @code{1 chars .}. 1 on all current ports. @item size of the keyboard terminal buffer: @cindex size of the keyboard terminal buffer @cindex terminal buffer, size 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: @cindex size of the pictured numeric output buffer @cindex pictured numeric output buffer, size @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}: @cindex size of the scratch area returned by @code{PAD} @cindex @code{PAD} size The remainder of dictionary space. @code{unused pad here - - .}. @item system case-sensitivity characteristics: @cindex 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: @cindex system prompt @cindex prompt @code{ ok} in interpret state, @code{ compiled} in compile state. @item division rounding: @cindex 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: @cindex @code{STATE} values -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>}: @cindex @t{DOES>}, visibility of current definition No. @end table @c --------------------------------------------------------------------- @node core-ambcond, core-other, core-idef, The Core Words @subsection Ambiguous conditions @c --------------------------------------------------------------------- @cindex core words, ambiguous conditions @cindex ambiguous conditions, core words @table @i @item a name is neither a word nor a number: @cindex name not found @cindex undefined word @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: @cindex word name too long @code{-19 throw} (Word name too long) @item addressing a region not inside the various data spaces of the forth system: @cindex Invalid memory address 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: @cindex argument type mismatch 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: @cindex Interpreting a compile-only word, for @code{'} etc. @cindex execution token of words 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: @cindex dividing by zero @cindex floating point unidentified fault, integer division On better platforms, this produces a @code{-10 throw} (Division by zero); on other systems, this typically results in a @code{-55 throw} (Floating-point unidentified fault). @item insufficient data stack or return stack space: @cindex insufficient data stack or return stack space @cindex stack overflow @cindex address alignment exception, stack overflow @cindex Invalid memory address, stack overflow Depending on the operating system, the installation, and the invocation of Gforth, this is either checked by the memory management hardware, or it is not checked. If it is checked, you typically get a @code{-3 throw} (Stack overflow), @code{-5 throw} (Return stack overflow), or @code{-9 throw} (Invalid memory address) (depending on the platform and how you achieved the overflow) as soon as the overflow happens. If it is not checked, overflows typically result in mysterious illegal memory accesses, producing @code{-9 throw} (Invalid memory address) or @code{-23 throw} (Address alignment exception); they might also destroy the internal data structure of @code{ALLOCATE} and friends, resulting in various errors in these words. @item insufficient space for loop control parameters: @cindex insufficient space for loop control parameters like other return stack overflows. @item insufficient space in the dictionary: @cindex insufficient space in the dictionary @cindex dictionary overflow If you try to allot (either directly with @code{allot}, or indirectly with @code{,}, @code{create} etc.) more memory than available in the dictionary, you get a @code{-8 throw} (Dictionary overflow). If you try to access memory beyond the end of the dictionary, the results are similar to stack overflows. @item interpreting a word with undefined interpretation semantics: @cindex interpreting a word with undefined interpretation semantics @cindex Interpreting a compile-only word For some words, we have 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: @cindex 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: @cindex overflow of the pictured numeric output string @cindex pictured numeric output string, overflow @code{-17 throw} (Pictured numeric ouput string overflow). @item parsed string overflow: @cindex parsed string overflow @code{PARSE} cannot overflow. @code{WORD} does not check for overflow. @item producing a result out of range: @cindex 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{-10 throw} (divide by zero) or @code{-55 throw} (floating point unidentified fault). @code{convert} and @code{>number} currently overflow silently. @item reading from an empty data or return stack: @cindex stack empty @cindex stack underflow @cindex return stack underflow 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, stacks may be checked or not, depending on operating system, installation, and invocation. If they are caught by a check, they typically result in @code{-4 throw} (Stack underflow), @code{-6 throw} (Return stack underflow) or @code{-9 throw} (Invalid memory address), depending on the platform and which stack underflows and by how much. Note that even if the system uses checking (through the MMU), your program may have to underflow by a significant number of stack items to trigger the reaction (the reason for this is that the MMU, and therefore the checking, works with a page-size granularity). If there is no checking, the symptoms resulting from an underflow are similar to those from an overflow. Unbalanced return stack errors result in a variaty of symptoms, including @code{-9 throw} (Invalid memory address) and Illegal Instruction (typically @code{-260 throw}). @item unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name: @cindex unexpected end of the input buffer @cindex zero-length string as a name @cindex Attempt to use 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: @cindex @code{>IN} greater than input buffer The next invocation of a parsing word returns a string with length 0. @item @code{RECURSE} appears after @code{DOES>}: @cindex @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}: @cindex argument input source different than current input source for @code{RESTORE-INPUT} @cindex argument type mismatch, @code{RESTORE-INPUT} @cindex @code{RESTORE-INPUT}, Argument type mismatch @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 source. @item data space containing definitions gets de-allocated: @cindex data space containing definitions gets de-allocated Deallocation with @code{allot} is not checked. This typically results in memory access faults or execution of illegal instructions. @item data space read/write with incorrect alignment: @cindex data space read/write with incorrect alignment @cindex alignment faults @cindex address alignment exception Processor-dependent. Typically results in a @code{-23 throw} (Address alignment exception). Under Linux-Intel 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 violations. @item data space pointer not properly aligned, @code{,}, @code{C,}: @cindex 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}): Like other stack underflows. @item loop control parameters not available: @cindex 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}): @cindex most recent definition does not have a name (@code{IMMEDIATE}) @cindex last word was headerless @code{abort" last word was headerless"}. @item name not defined by @code{VALUE} used by @code{TO}: @cindex name not defined by @code{VALUE} used by @code{TO} @cindex @code{TO} on non-@code{VALUE}s @cindex Invalid name argument, @code{TO} @code{-32 throw} (Invalid name argument) (unless name is a local or was defined by @code{CONSTANT}; in the latter case it just changes the constant). @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}): @cindex name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}) @cindex undefined word, @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}): @cindex 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}: @cindex @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}: @cindex string longer than a counted string returned by @code{WORD} @cindex @code{WORD}, string overflow 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}): @cindex @code{LSHIFT}, large shift counts @cindex @code{RSHIFT}, large shift counts Processor-dependent. Typical behaviours are returning 0 and using only the low bits of the shift count. @item word not defined via @code{CREATE}: @cindex @code{>BODY} of non-@code{CREATE}d words @code{>BODY} produces the PFA of the word no matter how it was defined. @cindex @code{DOES>} of non-@code{CREATE}d words @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 --------------------------------------------------------------------- @cindex other system documentation, core words @cindex core words, other system documentation @table @i @item nonstandard words using @code{PAD}: @cindex @code{PAD} use by nonstandard words None. @item operator's terminal facilities available: @cindex 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: @cindex program data space available @cindex data space available @code{UNUSED .} gives the remaining dictionary space. The total dictionary space can be specified with the @code{-m} switch (@pxref{Invoking Gforth}) when Gforth starts up. @item return stack space available: @cindex return stack space available You can compute the total return stack space in cells with @code{s" RETURN-STACK-CELLS" environment? drop .}. You can specify it at startup time with the @code{-r} switch (@pxref{Invoking Gforth}). @item stack space available: @cindex stack space available You can compute the total data stack space in cells with @code{s" STACK-CELLS" environment? drop .}. You can specify it at startup time with the @code{-d} switch (@pxref{Invoking Gforth}). @item system dictionary space required, in address units: @cindex system dictionary space required, in address units Type @code{here forthstart - .} after startup. At the time of this writing, this gives 80080 (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 ===================================================================== @cindex system documentation, block words @cindex block words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, block words @cindex block words, implementation-defined options @table @i @item the format for display by @code{LIST}: @cindex @code{LIST} display format 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{\}: @cindex length of a line affected by @code{\} @cindex @code{\}, line length in blocks 64 characters. @end table @c --------------------------------------------------------------------- @node block-ambcond, block-other, block-idef, The optional Block word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @cindex block words, ambiguous conditions @cindex ambiguous conditions, block words @table @i @item correct block read was not possible: @cindex block read 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: @cindex I/O exception in block transfer @cindex block transfer, I/O exception Typically results in a @code{throw} of some OS-derived value (between -512 and -2048). @item invalid block number: @cindex invalid block number @cindex block number invalid @code{-35 throw} (Invalid block number) @item a program directly alters the contents of @code{BLK}: @cindex @code{BLK}, altering @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}: @cindex @code{UPDATE}, no current block buffer @code{UPDATE} has no effect. @end table @c --------------------------------------------------------------------- @node block-other, , block-ambcond, The optional Block word set @subsection Other system documentation @c --------------------------------------------------------------------- @cindex other system documentation, block words @cindex block words, other system documentation @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 ===================================================================== @cindex system documentation, double words @cindex double words, system documentation @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 --------------------------------------------------------------------- @cindex double words, ambiguous conditions @cindex ambiguous conditions, double words @table @i @item @i{d} outside of range of @i{n} in @code{D>S}: @cindex @code{D>S}, @i{d} out of range of @i{n} The least significant cell of @i{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 ===================================================================== @cindex system documentation, exception words @cindex exception words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, exception words @cindex exception words, implementation-defined options @table @i @item @code{THROW}-codes used in the system: @cindex @code{THROW}-codes used in the system The codes -256@minus{}-511 are used for reporting signals. The mapping from OS signal numbers to throw codes is -256@minus{}@i{signal}. The codes -512@minus{}-2047 are used for OS errors (for file and memory allocation operations). The mapping from OS error numbers to throw codes is -512@minus{}@code{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 ===================================================================== @cindex system documentation, facility words @cindex facility words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, facility words @cindex facility words, implementation-defined options @table @i @item encoding of keyboard events (@code{EKEY}): @cindex keyboard events, encoding in @code{EKEY} @cindex @code{EKEY}, encoding of keyboard events Not yet implemented. @item duration of a system clock tick: @cindex duration of a system clock tick @cindex clock tick duration 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}: @cindex repeatability to be expected from the execution of @code{MS} @cindex @code{MS}, repeatability to be expected 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 --------------------------------------------------------------------- @cindex facility words, ambiguous conditions @cindex ambiguous conditions, facility words @table @i @item @code{AT-XY} can't be performed on user output device: @cindex @code{AT-XY} can't be performed on user output device Largely terminal dependent. 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 ===================================================================== @cindex system documentation, file words @cindex file words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, file words @cindex file words, implementation-defined options @table @i @item file access methods used: @cindex 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 (with both @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: @cindex 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: @cindex 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: @cindex file name format System dependent. Gforth just uses the file name format of your OS. @item information returned by @code{FILE-STATUS}: @cindex @code{FILE-STATUS}, returned information @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 returned mode. @item input file state after an exception when including source: @cindex exception when including source All files that are left via the exception are closed. @item @i{ior} values and meaning: @cindex @i{ior} values and meaning The @i{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 @i{ior}s is -512@minus{}@i{errno}. @item maximum depth of file input nesting: @cindex maximum depth of file input nesting @cindex file input nesting, maximum depth 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: @cindex maximum size of input line @cindex input line size, maximum @code{/line}. Currently 255. @item methods of mapping block ranges to files: @cindex mapping block ranges to files @cindex files containing blocks @cindex blocks in files By default, blocks are accessed in the file @file{blocks.fb} in the current working directory. The file can be switched with @code{USE}. @item number of string buffers provided by @code{S"}: @cindex @code{S"}, number of string buffers 1 @item size of string buffer used by @code{S"}: @cindex @code{S"}, size of string buffer @code{/line}. currently 255. @end table @c --------------------------------------------------------------------- @node file-ambcond, , file-idef, The optional File-Access word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @cindex file words, ambiguous conditions @cindex ambiguous conditions, file words @table @i @item attempting to position a file outside its boundaries: @cindex @code{REPOSITION-FILE}, outside the file'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: @cindex reading from file positions not yet written End-of-file, i.e., zero characters are read and no error is reported. @item @i{file-id} is invalid (@code{INCLUDE-FILE}): @cindex @code{INCLUDE-FILE}, @i{file-id} is invalid An appropriate exception may be thrown, but a memory fault or other problem is more probable. @item I/O exception reading or closing @i{file-id} (@code{INCLUDE-FILE}, @code{INCLUDED}): @cindex @code{INCLUDE-FILE}, I/O exception reading or closing @i{file-id} @cindex @code{INCLUDED}, I/O exception reading or closing @i{file-id} The @i{ior} produced by the operation, that discovered the problem, is thrown. @item named file cannot be opened (@code{INCLUDED}): @cindex @code{INCLUDED}, named file cannot be opened The @i{ior} produced by @code{open-file} is thrown. @item requesting an unmapped block number: @cindex unmapped block numbers 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: @cindex @code{SOURCE-ID}, behaviour 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 ===================================================================== @cindex system documentation, floating-point words @cindex floating-point words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, floating-point words @cindex floating-point words, implementation-defined options @table @i @item format and range of floating point numbers: @cindex format and range of floating point numbers @cindex floating point numbers, format and range System-dependent; the @code{double} type of C. @item results of @code{REPRESENT} when @i{float} is out of range: @cindex @code{REPRESENT}, results when @i{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: @cindex rounding of floating-point numbers @cindex truncation of floating-point numbers @cindex floating-point numbers, rounding or truncation 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: @cindex floating-point stack size @code{s" FLOATING-STACK" environment? drop .} gives the total size of the floating-point stack (in floats). You can specify this on startup with the command-line option @code{-f} (@pxref{Invoking Gforth}). @item width of floating-point stack: @cindex floating-point stack width @code{1 floats}. @end table @c --------------------------------------------------------------------- @node floating-ambcond, , floating-idef, The optional Floating-Point word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @cindex floating-point words, ambiguous conditions @cindex ambiguous conditions, floating-point words @table @i @item @code{df@@} or @code{df!} used with an address that is not double-float aligned: @cindex @code{df@@} or @code{df!} used with an address that is not double-float aligned System-dependent. Typically results in a @code{-23 THROW} like other alignment violations. @item @code{f@@} or @code{f!} used with an address that is not float aligned: @cindex @code{f@@} used with an address that is not float aligned @cindex @code{f!} used with an address that is not float aligned System-dependent. Typically results in a @code{-23 THROW} like other alignment violations. @item floating-point result out of range: @cindex 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: @cindex @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 @code{BASE} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}): @cindex @code{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}): @cindex @code{FATAN2}, both arguments are equal to zero System-dependent. @code{FATAN2} is implemented using the C library function @code{atan2()}. @item Using @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero: @cindex @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero System-dependent. Anyway, typically the cos of @i{r1} will not be zero because of small errors and the tan will be a very large (or very small) but finite number. @item @i{d} cannot be presented precisely as a float in @code{D>F}: @cindex @code{D>F}, @i{d} cannot be presented precisely as a float The result is rounded to the nearest float. @item dividing by zero: @cindex dividing by zero, floating-point @cindex floating-point dividing by zero @cindex floating-point unidentified fault, FP divide-by-zero @code{-55 throw} (Floating-point unidentified fault) @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}): @cindex 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 @i{float}<1 (@code{FACOSH}): @cindex @code{FACOSH}, @i{float}<1 @cindex floating-point unidentified fault, @code{FACOSH} @code{-55 throw} (Floating-point unidentified fault) @item @i{float}=<-1 (@code{FLNP1}): @cindex @code{FLNP1}, @i{float}=<-1 @cindex floating-point unidentified fault, @code{FLNP1} @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems negative infinity is typically produced for @i{float}=-1. @item @i{float}=<0 (@code{FLN}, @code{FLOG}): @cindex @code{FLN}, @i{float}=<0 @cindex @code{FLOG}, @i{float}=<0 @cindex floating-point unidentified fault, @code{FLN} or @code{FLOG} @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems negative infinity is typically produced for @i{float}=0. @item @i{float}<0 (@code{FASINH}, @code{FSQRT}): @cindex @code{FASINH}, @i{float}<0 @cindex @code{FSQRT}, @i{float}<0 @cindex floating-point unidentified fault, @code{FASINH} or @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 |@i{float}|>1 (@code{FACOS}, @code{FASIN}, @code{FATANH}): @cindex @code{FACOS}, |@i{float}|>1 @cindex @code{FASIN}, |@i{float}|>1 @cindex @code{FATANH}, |@i{float}|>1 @cindex floating-point unidentified fault, @code{FACOS}, @code{FASIN} or @code{FATANH} @code{-55 throw} (Floating-point unidentified fault). @item integer part of float cannot be represented by @i{d} in @code{F>D}: @cindex @code{F>D}, integer part of float cannot be represented by @i{d} @cindex floating-point unidentified fault, @code{F>D} @code{-55 throw} (Floating-point unidentified fault). @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}): @cindex 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 ===================================================================== @cindex system documentation, locals words @cindex locals words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, locals words @cindex locals words, implementation-defined options @table @i @item maximum number of locals in a definition: @cindex maximum number of locals in a definition @cindex locals, maximum number 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 --------------------------------------------------------------------- @cindex locals words, ambiguous conditions @cindex ambiguous conditions, locals words @table @i @item executing a named local in interpretation state: @cindex local in interpretation state @cindex Interpreting a compile-only word, for a local Locals have no interpretation semantics. If you try to perform the interpretation semantics, you will get a @code{-14 throw} somewhere (Interpreting a compile-only word). If you perform the compilation semantics, the locals access will be compiled (irrespective of state). @item @i{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}): @cindex name not defined by @code{VALUE} or @code{(LOCAL)} used by @code{TO} @cindex @code{TO} on non-@code{VALUE}s and non-locals @cindex Invalid name argument, @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 ===================================================================== @cindex system documentation, memory-allocation words @cindex memory-allocation words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, memory-allocation words @cindex memory-allocation words, implementation-defined options @table @i @item values and meaning of @i{ior}: @cindex @i{ior} values and meaning The @i{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 @i{ior}s is -512@minus{}@i{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 ===================================================================== @cindex system documentation, programming-tools words @cindex programming-tools words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, programming-tools words @cindex programming-tools words, implementation-defined options @table @i @item ending sequence for input following @code{;CODE} and @code{CODE}: @cindex @code{;CODE} ending sequence @cindex @code{CODE} ending sequence @code{END-CODE} @item manner of processing input following @code{;CODE} and @code{CODE}: @cindex @code{;CODE}, processing input @cindex @code{CODE}, processing input The @code{ASSEMBLER} vocabulary is pushed on the search order stack, and the input is processed by the text interpreter, (starting) in interpret state. @item search order capability for @code{EDITOR} and @code{ASSEMBLER}: @cindex @code{ASSEMBLER}, search order capability The ANS Forth search order word set. @item source and format of display by @code{SEE}: @cindex @code{SEE}, source and format of output 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 --------------------------------------------------------------------- @cindex programming-tools words, ambiguous conditions @cindex ambiguous conditions, programming-tools words @table @i @item deleting the compilation word list (@code{FORGET}): @cindex @code{FORGET}, deleting the compilation word list Not implemented (yet). @item fewer than @i{u}+1 items on the control-flow stack (@code{CS-PICK}, @code{CS-ROLL}): @cindex @code{CS-PICK}, fewer than @i{u}+1 items on the control flow-stack @cindex @code{CS-ROLL}, fewer than @i{u}+1 items on the control flow-stack @cindex control-flow stack underflow 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 @i{name} can't be found (@code{FORGET}): @cindex @code{FORGET}, @i{name} can't be found Not implemented (yet). @item @i{name} not defined via @code{CREATE}: @cindex @code{;CODE}, @i{name} not defined via @code{CREATE} @code{;CODE} behaves like @code{DOES>} in this respect, i.e., it changes the execution semantics of the last defined word no matter how it was defined. @item @code{POSTPONE} applied to @code{[IF]}: @cindex @code{POSTPONE} applied to @code{[IF]} @cindex @code{[IF]} and @code{POSTPONE} 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]}: @cindex @code{[IF]}, 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}): @cindex @code{FORGET}, removing a needed definition 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 ===================================================================== @cindex system documentation, search-order words @cindex search-order words, system documentation @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 --------------------------------------------------------------------- @cindex implementation-defined options, search-order words @cindex search-order words, implementation-defined options @table @i @item maximum number of word lists in search order: @cindex maximum number of word lists in search order @cindex search order, maximum depth @code{s" wordlists" environment? drop .}. Currently 16. @item minimum search order: @cindex minimum search order @cindex search order, minimum @code{root root}. @end table @c --------------------------------------------------------------------- @node search-ambcond, , search-idef, The optional Search-Order word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @cindex search-order words, ambiguous conditions @cindex ambiguous conditions, search-order words @table @i @item changing the compilation word list (during compilation): @cindex changing the compilation word list (during compilation) @cindex compilation word list, change before definition ends The word is entered into the word list that was the compilation word list 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 word list. @item search order empty (@code{previous}): @cindex @code{previous}, search order empty @cindex vocstack empty, @code{previous} @code{abort" Vocstack empty"}. @item too many word lists in search order (@code{also}): @cindex @code{also}, too many word lists in search order @cindex vocstack full, @code{also} @code{abort" Vocstack full"}. @end table @c *************************************************************** @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. @c *************************************************************** @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 ATLAST's 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 @c ****************************************************************** @node Emacs and Gforth, Image Files, Integrating Gforth, Top @chapter Emacs and Gforth @cindex Emacs and Gforth @cindex @file{gforth.el} @cindex @file{forth.el} @cindex Rydqvist, Goran @cindex comment editing commands @cindex @code{\}, editing with Emacs @cindex debug tracer editing commands @cindex @code{~~}, removal with Emacs @cindex Forth mode in Emacs Gforth comes with @file{gforth.el}, an improved version of @file{forth.el} by Goran Rydqvist (included in the TILE package). The improvements are: @itemize @bullet @item A better (but still not perfect) handling of indentation. @item Comment paragraph filling (@kbd{M-q}) @item Commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) of regions @item Removal of debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). @end itemize 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}. @cindex source location of error or debugging output in Emacs @cindex error output, finding the source location in Emacs @cindex debugging output, finding the source location in Emacs 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). @cindex @file{TAGS} file @cindex @file{etags.fs} @cindex viewing the source of a word in Emacs Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file will be produced (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) 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}). @cindex @file{.emacs} 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 @c ****************************************************************** @node Image Files, Engine, Emacs and Gforth, Top @chapter Image Files @cindex image file @cindex @file{.fi} files @cindex precompiled Forth code @cindex dictionary in persistent form @cindex persistent form of dictionary An image file is a file containing an image of the Forth dictionary, i.e., compiled Forth code and data residing in the dictionary. By convention, we use the extension @code{.fi} for image files. @menu * Image Licensing Issues:: Distribution terms for images. * Image File Background:: Why have image files? * Non-Relocatable Image Files:: don't always work. * Data-Relocatable Image Files:: are better. * Fully Relocatable Image Files:: better yet. * Stack and Dictionary Sizes:: Setting the default sizes for an image. * Running Image Files:: @code{gforth -i @i{file}} or @i{file}. * Modifying the Startup Sequence:: and turnkey applications. @end menu @node Image Licensing Issues, Image File Background, Image Files, Image Files @section Image Licensing Issues @cindex license for images @cindex image license An image created with @code{gforthmi} (@pxref{gforthmi}) or @code{savesystem} (@pxref{Non-Relocatable Image Files}) includes the original image; i.e., according to copyright law it is a derived work of the original image. Since Gforth is distributed under the GNU GPL, the newly created image falls under the GNU GPL, too. In particular, this means that if you distribute the image, you have to make all of the sources for the image available, including those you wrote. For details see @ref{License, , GNU General Public License (Section 3)}. If you create an image with @code{cross} (@pxref{cross.fs}), the image contains only code compiled from the sources you gave it; if none of these sources is under the GPL, the terms discussed above do not apply to the image. However, if your image needs an engine (a gforth binary) that is under the GPL, you should make sure that you distribute both in a way that is at most a @emph{mere aggregation}, if you don't want the terms of the GPL to apply to the image. @node Image File Background, Non-Relocatable Image Files, Image Licensing Issues, Image Files @section Image File Background @cindex image file background Our Forth system consists not only of primitives, but also of definitions written in Forth. Since the Forth compiler itself belongs to those definitions, it is not possible to start the system with the primitives and the Forth source alone. Therefore we provide the Forth code as an image file in nearly executable form. When Gforth starts up, a C routine loads the image file into memory, optionally relocates the addresses, then sets up the memory (stacks etc.) according to information in the image file, and (finally) starts executing Forth code. The image file variants represent different compromises between the goals of making it easy to generate image files and making them portable. @cindex relocation at run-time Win32Forth 3.4 and Mitch Bradley's @code{cforth} use relocation at run-time. This avoids many of the complications discussed below (image files are data relocatable without further ado), but costs performance (one addition per memory access). @cindex relocation at load-time By contrast, the Gforth loader performs relocation at image load time. The loader also has to replace tokens that represent primitive calls with the appropriate code-field addresses (or code addresses in the case of direct threading). There are three kinds of image files, with different degrees of relocatability: non-relocatable, data-relocatable, and fully relocatable image files. @cindex image file loader @cindex relocating loader @cindex loader for image files These image file variants have several restrictions in common; they are caused by the design of the image file loader: @itemize @bullet @item There is only one segment; in particular, this means, that an image file cannot represent @code{ALLOCATE}d memory chunks (and pointers to them). The contents of the stacks are not represented, either. @item The only kinds of relocation supported are: adding the same offset to all cells that represent data addresses; and replacing special tokens with code addresses or with pieces of machine code. If any complex computations involving addresses are performed, the results cannot be represented in the image file. Several applications that use such computations come to mind: @itemize @minus @item Hashing addresses (or data structures which contain addresses) for table lookup. If you use Gforth's @code{table}s or @code{wordlist}s for this purpose, you will have no problem, because the hash tables are recomputed automatically when the system is started. If you use your own hash tables, you will have to do something similar. @item There's a cute implementation of doubly-linked lists that uses @code{XOR}ed addresses. You could represent such lists as singly-linked in the image file, and restore the doubly-linked representation on startup.@footnote{In my opinion, though, you should think thrice before using a doubly-linked list (whatever implementation).} @item The code addresses of run-time routines like @code{docol:} cannot be represented in the image file (because their tokens would be replaced by machine code in direct threaded implementations). As a workaround, compute these addresses at run-time with @code{>code-address} from the executions tokens of appropriate words (see the definitions of @code{docol:} and friends in @file{kernel.fs}). @item On many architectures addresses are represented in machine code in some shifted or mangled form. You cannot put @code{CODE} words that contain absolute addresses in this form in a relocatable image file. Workarounds are representing the address in some relative form (e.g., relative to the CFA, which is present in some register), or loading the address from a place where it is stored in a non-mangled form. @end itemize @end itemize @node Non-Relocatable Image Files, Data-Relocatable Image Files, Image File Background, Image Files @section Non-Relocatable Image Files @cindex non-relocatable image files @cindex image file, non-relocatable These files are simple memory dumps of the dictionary. They are specific to the executable (i.e., @file{gforth} file) they were created with. What's worse, they are specific to the place on which the dictionary resided when the image was created. Now, there is no guarantee that the dictionary will reside at the same place the next time you start Gforth, so there's no guarantee that a non-relocatable image will work the next time (Gforth will complain instead of crashing, though). You can create a non-relocatable image file with doc-savesystem @node Data-Relocatable Image Files, Fully Relocatable Image Files, Non-Relocatable Image Files, Image Files @section Data-Relocatable Image Files @cindex data-relocatable image files @cindex image file, data-relocatable These files contain relocatable data addresses, but fixed code addresses (instead of tokens). They are specific to the executable (i.e., @file{gforth} file) they were created with. For direct threading on some architectures (e.g., the i386), data-relocatable images do not work. You get a data-relocatable image, if you use @file{gforthmi} with a Gforth binary that is not doubly indirect threaded (@pxref{Fully Relocatable Image Files}). @node Fully Relocatable Image Files, Stack and Dictionary Sizes, Data-Relocatable Image Files, Image Files @section Fully Relocatable Image Files @cindex fully relocatable image files @cindex image file, fully relocatable @cindex @file{kern*.fi}, relocatability @cindex @file{gforth.fi}, relocatability These image files have relocatable data addresses, and tokens for code addresses. They can be used with different binaries (e.g., with and without debugging) on the same machine, and even across machines with the same data formats (byte order, cell size, floating point format). However, they are usually specific to the version of Gforth they were created with. The files @file{gforth.fi} and @file{kernl*.fi} are fully relocatable. There are two ways to create a fully relocatable image file: @menu * gforthmi:: The normal way * cross.fs:: The hard way @end menu @node gforthmi, cross.fs, Fully Relocatable Image Files, Fully Relocatable Image Files @subsection @file{gforthmi} @cindex @file{comp-i.fs} @cindex @file{gforthmi} You will usually use @file{gforthmi}. If you want to create an image @i{file} that contains everything you would load by invoking Gforth with @code{gforth @i{options}}, you simply say: @example gforthmi @i{file} @i{options} @end example E.g., if you want to create an image @file{asm.fi} that has the file @file{asm.fs} loaded in addition to the usual stuff, you could do it like this: @example gforthmi asm.fi asm.fs @end example @file{gforthmi} is implemented as a sh script and works like this: It produces two non-relocatable images for different addresses and then compares them. Its output reflects this: first you see the output (if any) of the two Gforth invocations that produce the nonrelocatable image files, then you see the output of the comparing program: It displays the offset used for data addresses and the offset used for code addresses; moreover, for each cell that cannot be represented correctly in the image files, it displays a line like the following one: @example 78DC BFFFFA50 BFFFFA40 @end example This means that at offset $78dc from @code{forthstart}, one input image contains $bffffa50, and the other contains $bffffa40. Since these cells cannot be represented correctly in the output image, you should examine these places in the dictionary and verify that these cells are dead (i.e., not read before they are written). If you type @file{gforthmi} with no arguments, it prints some usage instructions. @cindex @code{savesystem} during @file{gforthmi} @cindex @code{bye} during @file{gforthmi} @cindex doubly indirect threaded code @cindex environment variable @code{GFORTHD} @cindex @code{GFORTHD} environment variable @cindex @code{gforth-ditc} There are a few wrinkles: After processing the passed @i{options}, the words @code{savesystem} and @code{bye} must be visible. A special doubly indirect threaded version of the @file{gforth} executable is used for creating the nonrelocatable images; you can pass the exact filename of this executable through the environment variable @code{GFORTHD} (default: @file{gforth-ditc}); if you pass a version that is not doubly indirect threaded, you will not get a fully relocatable image, but a data-relocatable image (because there is no code address offset). The normal @file{gforth} executable is used for creating the relocatable image; you can pass the exact filename of this executable through the environment variable @code{GFORTH}. @node cross.fs, , gforthmi, Fully Relocatable Image Files @subsection @file{cross.fs} @cindex @file{cross.fs} @cindex cross-compiler @cindex metacompiler You can also use @code{cross}, a batch compiler that accepts a Forth-like programming language. This @code{cross} language has to be documented yet. @cindex target compiler @code{cross} also allows you to create image files for machines with different data sizes and data formats than the one used for generating the image file. You can also use it to create an application image that does not contain a Forth compiler. These features are bought with restrictions and inconveniences in programming. E.g., addresses have to be stored in memory with special words (@code{A!}, @code{A,}, etc.) in order to make the code relocatable. @node Stack and Dictionary Sizes, Running Image Files, Fully Relocatable Image Files, Image Files @section Stack and Dictionary Sizes @cindex image file, stack and dictionary sizes @cindex dictionary size default @cindex stack size default If you invoke Gforth with a command line flag for the size (@pxref{Invoking Gforth}), the size you specify is stored in the dictionary. If you save the dictionary with @code{savesystem} or create an image with @file{gforthmi}, this size will become the default for the resulting image file. E.g., the following will create a fully relocatable version of @file{gforth.fi} with a 1MB dictionary: @example gforthmi gforth.fi -m 1M @end example In other words, if you want to set the default size for the dictionary and the stacks of an image, just invoke @file{gforthmi} with the appropriate options when creating the image. @cindex stack size, cache-friendly Note: For cache-friendly behaviour (i.e., good performance), you should make the sizes of the stacks modulo, say, 2K, somewhat different. E.g., the default stack sizes are: data: 16k (mod 2k=0); fp: 15.5k (mod 2k=1.5k); return: 15k(mod 2k=1k); locals: 14.5k (mod 2k=0.5k). @node Running Image Files, Modifying the Startup Sequence, Stack and Dictionary Sizes, Image Files @section Running Image Files @cindex running image files @cindex invoking image files @cindex image file invocation @cindex -i, invoke image file @cindex --image file, invoke image file You can invoke Gforth with an image file @i{image} instead of the default @file{gforth.fi} with the @code{-i} flag (@pxref{Invoking Gforth}): @example gforth -i @i{image} @end example @cindex executable image file @cindex image file, executable If your operating system supports starting scripts with a line of the form @code{#! ...}, you just have to type the image file name to start Gforth with this image file (note that the file extension @code{.fi} is just a convention). I.e., to run Gforth with the image file @i{image}, you can just type @i{image} instead of @code{gforth -i @i{image}}. This works because every @code{.fi} file starts with a line of this format: @example #! /usr/local/bin/gforth-0.4.0 -i @end example The file and pathname for the Gforth engine specified on this line is the specific Gforth executable that it was built against; i.e. the value of the environment variable @code{GFORTH} at the time that @file{gforthmi} was executed. You can make use of the same shell capability to make a Forth source file into an executable. For example, if you place this text in a file: @example #! /usr/local/bin/gforth ." Hello, world" CR bye @end example @noindent and then make the file executable (chmod +x in Unix), you can run it directly from the command line. The sequence @code{#!} is used in two ways; firstly, it is recognised as a ``magic sequence'' by the operating system@footnote{The Unix kernel actually recognises two types of files: executable files and files of data, where the data is processed by an interpreter that is specified on the ``interpreter line'' -- the first line of the file, starting with the sequence #!. There may be a small limit (e.g., 32) on the number of characters that may be specified on the interpreter line.} secondly it is treated as a comment character by Gforth. Because of the second usage, a space is required between @code{#!} and the path to the executable. The disadvantage of this latter technique, compared with using @file{gforthmi}, is that it is slower; the Forth source code is compiled on-the-fly, each time the program is invoked. @comment TODO describe the #! magic with reference to the Power Tools book. doc-#! @node Modifying the Startup Sequence, , Running Image Files, Image Files @section Modifying the Startup Sequence @cindex startup sequence for image file @cindex image file initialization sequence @cindex initialization sequence of image file You can add your own initialization to the startup sequence through the deferred word @code{'cold}. @code{'cold} is invoked just before the image-specific command line processing (by default, loading files and evaluating (@code{-e}) strings) starts. A sequence for adding your initialization usually looks like this: @example :noname Defers 'cold \ do other initialization stuff (e.g., rehashing wordlists) ... \ your stuff ; IS 'cold @end example @cindex turnkey image files @cindex image file, turnkey applications You can make a turnkey image by letting @code{'cold} execute a word (your turnkey application) that never returns; instead, it exits Gforth via @code{bye} or @code{throw}. @cindex command-line arguments, access @cindex arguments on the command line, access You can access the (image-specific) command-line arguments through the variables @code{argc} and @code{argv}. @code{arg} provides convenient access to @code{argv}. If @code{'cold} exits normally, Gforth processes the command-line arguments as files to be loaded and strings to be evaluated. Therefore, @code{'cold} should remove the arguments it has used in this case. doc-'cold doc-argc doc-argv doc-arg @c ****************************************************************** @node Engine, Binding to System Library, Image Files, Top @chapter Engine @cindex engine @cindex virtual machine Reading this chapter is not necessary for programming with Gforth. It may 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 @*@url{http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z}. @menu * Portability:: * Threading:: * Primitives:: * Performance:: @end menu @node Portability, Threading, Engine, Engine @section Portability @cindex engine portability An important goal of the Gforth Project 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. @cindex C, using C for the engine 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 is very cumbersome to express double integer arithmetic. @cindex GNU C for the engine @cindex long long 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 @pxref{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. @cindex explicit register declarations @cindex --enable-force-reg, configuration flag @cindex -DFORCE_REG 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 configuration flag @code{--enable-force-reg} (@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, Engine @section Threading @cindex inner interpreter implementation @cindex threaded code implementation @cindex labels as values 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 @i{label} by writing @code{&&@i{label}}. This address can then be used in a statement like @code{goto *@i{address}}. I.e., @code{goto *&&x} is the same as @code{goto x}. @cindex @code{NEXT}, indirect threaded @cindex indirect threaded inner interpreter @cindex inner interpreter, indirect threaded With this feature an indirect threaded @code{NEXT} looks like: @example cfa = *ip++; ca = *cfa; goto *ca; @end example @cindex instruction pointer 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}. @cindex @code{NEXT}, direct threaded @cindex direct threaded inner interpreter @cindex inner interpreter, direct threaded 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 @code{NEXT} after fetching the cfa). @menu * Scheduling:: * Direct or Indirect Threaded?:: * DOES>:: @end menu @node Scheduling, Direct or Indirect Threaded?, Threading, Threading @subsection Scheduling @cindex inner interpreter optimization 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 @code{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 @code{sp} and @code{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: @code{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? @cindex threading, direct or indirect? @cindex -DDIRECT_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 @code{docol} or @code{dodoes}). These routines are inherently machine-dependent, but they do not amount to many source lines. Therefore, even porting direct threading to a new machine requires little effort. @cindex --enable-indirect-threaded, configuration flag @cindex --enable-direct-threaded, configuration flag The default threading method is machine-dependent. You can enforce a specific threading method when building Gforth with the configuration flag @code{--enable-direct-threaded} or @code{--enable-indirect-threaded}. Note that direct threading is not supported on all machines. @node DOES>, , Direct or Indirect Threaded?, Threading @subsection DOES> @cindex @code{DOES>} implementation @cindex @code{dodoes} routine @cindex @code{DOES>}-code 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 @code{DOES>}-code)? There are two solutions: In fig-Forth the code field points directly to the @code{dodoes} and the @code{DOES>}code address is stored in the cell after the code address (i.e. at @code{@i{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 and for direct threading on some machines. Leaving a cell unused in most words is a bit wasteful, but on the machines we are targeting 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 (direct threaded systems require two-cell code fields on many machines). @cindex @code{DOES>}-handler 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 (the @code{DOES>}-handler). @code{dodoes} can then get the @code{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 @code{DOES>}-code address. Since the two cells available in the code field are used up by the jump to the code address in direct threading on many architectures, we use this approach for direct threading on these architectures. We did not want to add another cell to the code field. @node Primitives, Performance, Threading, Engine @section Primitives @cindex primitives, implementation @cindex virtual machine instructions, implementation @menu * Automatic Generation:: * TOS Optimization:: * Produced code:: @end menu @node Automatic Generation, TOS Optimization, Primitives, Primitives @subsection Automatic Generation @cindex primitives, automatic generation @cindex @file{prims2x.fs} 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 has 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: @cindex primitive source format @format @i{Forth-name} @i{stack-effect} @i{category} [@i{pronounc.}] [@code{""}@i{glossary entry}@code{""}] @i{C code} [@code{:} @i{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 @code{NEXT}. @node TOS Optimization, Produced code, Automatic Generation, Primitives @subsection TOS Optimization @cindex TOS optimization for primitives @cindex primitives, keeping the TOS in a register 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 @i{in1}...@i{inx} @code{--} @i{out1}...@i{outy}, keeping the top @i{n} items in registers @itemize @bullet @item is better than keeping @i{n-1} items, if @i{x>=n} and @i{y>=n}, due to fewer loads from and stores to the stack. @item is slower than keeping @i{n-1} items, if @i{x<>y} and @i{x