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Subject: comp.lang.forth FAQ: General Information (1 of 7)

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comp.lang.forth Frequently Asked Questions (1/6): Gen- eral/Misc M. Anton Ertl, anton@mips.complang.tuwien.ac.at ____________________________________________________________ Table of Contents: 1. Acknowledgements 2. comp.lang.forth FAQs 3. General Questions 3.1. What is Forth? 3.2. Where does the name Forth come from? 3.3. Why and where is Forth used? 3.4. Hang on, isn't Forth out of date now? 3.5. Is Forth faster or smaller than C? 3.6. What language standards exist for Forth? 3.7. What is an RFI? 3.8. Are there Coding Standards for Forth? 3.9. I have trouble managing the stack. Should I use global VARIABLEs? 3.10. What is the Forth Interest Group? 3.11. Who is Chuck Moore and what is he doing? 4. Flame baits 4.1. Commercial vs. free Forth systems 4.2. Free Forth systems are bad for Forth. 4.3. Blocks vs. files 4.4. LOCALS| 5. Miscellaneous 5.1. Where can I find a C-to-Forth compiler? 5.2. Where can I find a Forth-to-C compiler? 5.3. RECORDS in Forth? 5.4. Why does THEN finish an IF structure? 5.5. What is threaded code? What are the differences between the different threading techniques? 5.6. Has anyone written a Forth which compiles to Java bytecode? 5.7. What about translating Java bytecode to Forth? 5.8. How about running Forth without OS? 5.9. How about writing an OS in Forth? ______________________________________________________________________ 1. Acknowledgements This FAQ is based on previous work by Gregory Haverkamp, J. D. Verne, and Bradford J. Rodriguez. 2. comp.lang.forth FAQs The comp.lang.forth FAQ is published in seven parts, corresponding to these seven sections. This part is the General/Misc FAQ, where the questions not covered in the other FAQs are answered. The parts are: o General questions <http://www.complang.tuwien.ac.at/forth/faq/faq- general.html> o Online resources <http://www.faqs.org/faqs/computer-lang/forth- faq/part2/> o Forth vendors <http://www.faqs.org/faqs/computer-lang/forth- faq/part3/> o Forth systems <http://www.faqs.org/faqs/computer-lang/forth- faq/part4/> o Books, periodicals, tutorials <http://www.faqs.org/faqs/computer- lang/forth-faq/part5/> o Forth groups & organizations <http://www.faqs.org/faqs/computer- lang/forth-faq/part6/> o ANS Forth <http://dec.bournemouth.ac.uk/forth/ans/faq.html> You can get the text versions of these FAQs at <ftp://ftp.forth.org/pub/Forth/FAQ/>. These FAQs are intended to be a brief overview of the tools and information available for the new FORTHer. For a historical reference, programming paradigms, and deep technical information try some of the listed references. For general questions on the Usenet, or the methods used to get this information, try these other Usenet groups: o news.announce.newusers o news.newusers.questions o news.announce.important 3. General Questions 3.1. What is Forth? Forth is a stack-based, extensible language without type-checking. It is probably best known for its "reverse Polish" (postfix) arithmetic notation, familiar to users of Hewlett-Packard calculators: to add two numbers in Forth, you would type 3 5 + instead of 3+5. The fundamental program unit in Forth is the "word": a named data item, subroutine, or operator. Programming in Forth consists of defining new words in terms of existing ones. The Forth statement ______________________________________________________________________ : SQUARED DUP * ; ______________________________________________________________________ defines a new word SQUARED whose function is to square a number (mul- tiply it by itself). Since the entire language structure is embodied in words, the application programmer can "extend" Forth to add new operators, program constructs, or data types at will. The Forth "core" includes operators for integers, addresses, characters, and Boolean values; string and floating-point operators may be optionally added. 3.2. Where does the name Forth come from? The name FORTH was intended to suggest software for the fourth (next) generation computers, which Moore saw as being characterized by distributed small computers. The operating system he used at the time restricted file names to five characters, so the "U" was discarded. FORTH was spelled in upper case until the late 70's because of the prevalence of of upper-case-only I/O devices. The name "Forth" was gener- ally adopted when lower case became widely available, because the word was not an acronym. Rather, Colbourn, and Moore: The Evolution of Forth <http://www.forth.com/Content/History/History1.htm>, in: History of Programming Languages (HOPL-II), ACM Press/Addison-Wesley 1996. Note: Forth is not a 4GL (language for programming database applications). 3.3. Why and where is Forth used? Although invented in 1970, Forth became widely known with the advent of personal computers, where its high performance and economy of memory were attractive. These advantages still make Forth popular in embedded microcontroller systems, in locations ranging from the Space Shuttle to the bar-code reader used by your Federal Express driver. Forth's interactive nature streamlines the test and development of new hardware. Incremental development, a fast program-debug cycle, full interactive access to any level of the program, and the ability to work at a high "level of abstraction," all contribute to Forth's reputation for very high programmer productivity. These, plus the flexibility and malleability of the language, are the reasons most cited for choosing Forth for embedded systems. 3.4. Hang on, isn't Forth out of date now? One of the best answers came from Brad Rodriguez <mailto:bj@forth.org>. You can find the full version at <http://www.complang.tuwien.ac.at/forth/faq/why-forth>. In short, Forth's advantages are that it's comprehensible, small, interactive, fast, extensible, and makes it easy to work at a high level of abstraction. BTW, this question came from someone comparing a 10+ year old Forth system with the latest version of Borland C++. His system was really out of date, but also with respect to current Forth systems. 3.5. Is Forth faster or smaller than C? Not in itself. I.e., if you translate a C program literally into Forth, you will see a slow-down (e.g., a factor 4-8 with Gforth, a threaded-code system; for typical native-code systems you will see a factor of 1-3). Similarly, there is no inherent size advantage in Forth. For details see <http://www.complang.tuwien.ac.at/forth/performance.html>. However, there are many reports of cases where Forth programs beat others in size and/or speed. My guess is that the added flexibility of Forth helps programmers produce faster and/or smaller programs. 3.6. What language standards exist for Forth? An American National Standard for Forth, ANSI X3.215-1994, is accepted worldwide as the definitive Forth standard ("ANS Forth"). This standard also has been blessed as international standard (ISO/IEC 15145:1997). IEEE Standard 1275-1994, the "Open Firmware" standard, is a Forth derivative which has been adopted by Sun Microsystems, HP, Apple, IBM, and others as the official language for writing bootstrap and driver firmware. See <http://playground.sun.com/1275/home.html>. Prior Forth standards include the Forth-83 Standard and the Forth-79 Standard issued by the Forth Standards Team. The earlier FIG-Forth, while never formally offered as such, was a de facto "standard" for some years. "FORTH STANDARDS Published standards since 1978 are Forth 79 and Forth 83 from the Forth Standard Team, and ANS Forth - document X3.215-1994 - by the X3J14 Technical Committee. The most recent standard, ANS Forth, defines a set of core words and some optional extensions and takes care to allow great freedom in how these words are implemented. The range of hardware which can support an ANS Forth Standard System is far wider than any previous Forth standard and probably wider than any programming language standard ever. See web page <ftp://ftp.uu.net/vendor/minerva/uathena.htm> for latest details. Copies of the standard cost $193, but the final draft of ANS Forth is free and available (subject to copyright restrictions) via ftp..." --Chris Jakeman, apvpeter.demon.co.uk The (un)official ANS Forth document is available in various formats at <http://www.taygeta.com/forthlit.html> and at <ftp://ftp.uu.net/vendor/minerva/x3j14/>. The format I like best is the HTML version <http://www.taygeta.com/forth/dpans.html>. To get yourself on the ANS-Forth mailing list, consult the various README files at <ftp://ftp.uu.net/vendor/minerva/x3j14/>. Two unofficial test suites are available for checking conformance to the ANS Standard Forth: o John Hayes has written a test suite to test ANS Standard Systems (available through <http://www.taygeta.com/forth.html>). o JET Thomas has written a test suite to test ANS Standard Programs: <ftp://ftp.forth.org/pub/Forth/ANS/stand4th.zip> There is also an ANS Forth FAQ <http://dec.bournemouth.ac.uk/forth/ans/faq.html> that explains the standardization process. 3.7. What is an RFI? A Request For Interpretation. If you find something in the standard document ambiguous or unclear, you can make an RFI, and the TC (technical committee), that produced the standard, will work out a clarification. You can make an RFI by mailing it to greg@minerva.com and labeling it as RFI. The answers to earlier RFIs are available at ftp://ftp.uu.net/vendor/minerva/x3j14/queries/. They are also integrated in the HTML version of the standard <http://www.taygeta.com/forth/dpans.html>. 3.8. Are there Coding Standards for Forth? Leo Brodie's book Thinking Forth gives some advice; a short excerpt is now available online <http://www.forth.org/forth_style.html>. Forth shops have rules for their coding. Paul Bennet has published those of his company; you can find them on <http://www.forth.org/forth_coding.html>. 3.9. I have trouble managing the stack. Should I use global VARI- ABLEs? No. There are better alternatives: o Keep on trying to use the stack. Reorganize (refactor) your words. One day you will get the knack for it. Elizabeth Rather <mailto:erather@forth.com> writes: The basic skill required for comfortable, efficient Forth programming is good stack management. It's hard for newcom- ers to learn, since it isn't a skill required in other lan- guages, which all require the use of variables for practi- cally everything. Having taught literally hundreds of courses over the last 25 years, I've seen newcomers wrestle with this, and have developed exercises (similar to those in Starting Forth) to help. It seems to be a skill rather like riding a bicycle: wobbly & scary at first, then suddenly a "switch is thrown" in the brain and it seems comfortable and natural ever after. Andrew Haley writes in Message-ID: <7k8lln$q3c$1@korai.cygnus.co.uk>: Try writing all of your code using definitions one, or at most two lines long. Produce a stack digram for each word showing its inputs and its outputs. If you ever need an "intermediate" stack diagram to see what's going on, split your word at that point into two words. By doing this, you may test each half of the word on the command line, checking the stack each time. Do not use PICK and ROLL. Once you get the hang of writing code in this way you can relax these rules, but it's much better to get used to this style first. o Use the return stack. o Use locals. o Use data structures in memory, and pass pointers to it on the stack. o One area that has been mentioned often as troublemaker is graphics programming. Take a look at how Postscript handles this: They do indeed have a global state to avoid stack management problems, but you can access this state only through certain words. 3.10. What is the Forth Interest Group? The Forth Interest Group "FIG" was formed in 1978 to disseminate information and popularize the Forth language, and it remains the premier organization for professional Forth programmers. FIG maintains a Web page at <http://www.forth.org/forth.html>, with a more complete introduction to the Forth language, and links to the Web pages of many Forth vendors. 3.11. Who is Chuck Moore and what is he doing? Chuck Moore discovered (as he puts it) Forth (for historical information read The Evolution of Forth <http://www.forth.com/Content/History/History1.htm>). He later went on to apply his design philosophy to hardware design and designed a number of processors well-suited for executing Forth: Novix 4016, Shboom, uP20, uP21, F21, i21, ... He also explored new ideas and refined his earlier ideas on software and Forth: his cmForth for the Novix has been quite influential. His latest developments are Color Forth and Machine Forth. Machine Forth is a simple virtual machine consisting of 27 instructions. It is implemented in hardare in uP21 and the following chips, but has also been implemented in software on the 386 as simple native-code system. Some of the differences from ANS Forth are that each stack entry contains an extra carry bit, and that there is register A for accessing memory (instead of addressing through the top of stack). 4. Flame baits Some statements spawn long and heated discussions where the participants repeat their positions and ignore the arguments of the other side (flame wars). You may want to avoid such statements. Here, I present some regularly appearing flame baits and the positions you will read (so you don't have to start a flame war to learn them). 4.1. Commercial vs. free Forth systems "You get what you pay for. With a commercial Forth you get commercial documentation and support. We need commercial Forth systems or Forth will die." "I have had good experiences with free Forths. I cannot afford a commercial Forth system. I want source code (some commercial vendors don't provide it for the whole system). Examples of bad support from commercial software vendors. Without free Forth systems Forth will die." 4.2. Free Forth systems are bad for Forth. "Anyone can write a bad Forth and give it away without documentation or support; after trying such a system, nobody wants to work with Forth anymore. Free Forths give Forth a bad name. Free Forths take business away from the vendors." "Many people learned Forth with fig-Forth. There are good free Forths. Most successful languages started with (and still have) free implementations. Languages without free implementations (like Ada, Eiffel and Miranda) are not very popular [There are free Ada and Eiffel implementations now]." 4.3. Blocks vs. files The discussions on this topic are much cooler since Mike Haas has dropped from comp.lang.forth. "Everyone is using files and all third-party tools are designed for files. Files waste less space. Blocks lead to horizontal, unreadable code. Blocks make Forth ridiculous." "We are not always working under an operating system, so on some machines we don't have files. We have very nice block editors and other tools and coding standards for working with blocks (e.g., shadow screens)." 4.4. LOCALS| Everyone who mentions using LOCALS| gets the following flame from me: LOCALS| is bad because the locals are ordered contrary to the stack comment convention. I.e.: ______________________________________________________________________ : foo ( you can read this -- ... ) LOCALS| this read can you | ... ; ______________________________________________________________________ The following locals syntax is better and widely used: ______________________________________________________________________ : foo { you can read this -- ... } ... ; ______________________________________________________________________ You can find an implementation of this syntax in ANS Forth at <http://www.complang.tuwien.ac.at/forth/anslocal.fs> 5. Miscellaneous 5.1. Where can I find a C-to-Forth compiler? Parag Patel <mailto:parag@pinhead.parag.codegen.com> writes: We, (CodeGen, Inc. <http://www.codegen.com>) sell a C-to- Fcode compiler. Well, it actually generates IEEE-1275 Forth that then must be run through a tokenizer. Really, it generates pretty ugly Forth code. It's easy to generate lousy Forth, but it's very difficult to generate nice clean optimized Forth. C and stack-based languages don't mix too well. I end up faking a C variable stack- frame using a Forth $frame variable for local vars. Stephen Pelc <mailto:sfp@mpeltd.demon.co.uk> writes: MPE has produced a C to stack-machine compiler. This gener- ates tokens for a 2-stack virtual machine. The code quality is such that the token space used by compiled programs is better than that of the commercial C compilers we have tested against. This a consequence of the virtual machine design. However, to achieve this the virtual machine design has local variable support. The tokens can then be back end interpreted, or translated to a Forth system. The translater can be written in high level Forth, and is largely portable, except for the target architecture sections. These are not shareware tools, and were written to support a portable binary system. 5.2. Where can I find a Forth-to-C compiler? An unsupported prototype Forth-to-C compiler is available at <http://www.complang.tuwien.ac.at/forth/forth2c.tar.gz>. It is described in the EuroForth'95 paper <http://www.complang.tuwien.ac.at/papers/ertl&maierhofer95.ps.gz>. Another Forth-to-C compiler is supplied with Rob Chapman's <mailto:rc@compusmart.ab.ca> Timbre <http://www.compusmart.ab.ca/rc/Timbre/timbre.htm> system. 5.3. RECORDS in Forth? Many packages for data structuring facilities like Pascal's RECORDs and C's structs have been posted. E.g., the structures of the Forth Scientific Library ( <http://www.taygeta.com/fsl/fsl_structs.html>) or the structures supplied with Gforth <http://www.complang.tuwien.ac.at/forth/struct.fs>. 5.4. Why does THEN finish an IF structure? Some people find the way THEN is used in Forth unnatural, others do not. According to Webster's New Encyclopedic Dictionary, then" (adv.) has the following meanings: 2b: following next after in order ... 3d: as a necessary consequence (if you were there, then you saw them). Forth's THEN has the meaning 2b, whereas THEN in Pascal and other pro- gramming languages has the meaning 3d. If you don't like to use THEN in this way, you can easily define ENDIF as a replacement: ______________________________________________________________________ : ENDIF POSTPONE THEN ; IMMEDIATE ______________________________________________________________________ 5.5. What is threaded code? What are the differences between the dif- ferent threading techniques? Threaded code is a way of implementing virtual machine interpreters. You can find a more in-depth explanation at <http://www.complang.tuwien.ac.at/forth/threaded-code.html>. 5.6. Has anyone written a Forth which compiles to Java bytecode? Paul Curtis <mailto:paul.curtis@ra-ltd.demon.co.uk> writes: The JVM, although a stack machine, can't really be used to compile Forth efficiently. Why? Well, there are a number of reasons: o The maximum stack depth of a called method must be known in advance. JVM Spec, p. 111 o JVM methods can only return a single object to the caller. Thus, a stack effect ( n1 n2 -- n3 n4 ) just isn't possible. o There is no direct support for unsigned quantities. o CATCH and THROW can't be resolved easily; you need to catch exceptions using exception tables. This doesn't match Forth's model too well. JVM Spec, p. 112 o You'd need to extend Forth to generate the attributes required for Java methods. o There is no such thing as pointer arithmetic. o You can't take one thing on the stack and recast it to another type. o You can't manufacture objects out of raw bytes. This is a security issue. o There is no support for the return stack. That said, it is possible to write something Forth-like using JVM bytecodes, but you can't use the JVM stack to implement the Forth stack. ... If you're serious, try getting Jasmin and programming directly on the JVM. 5.7. What about translating Java bytecode to Forth? Some of the non-trivial pieces in translating JavaVM to Forth, that we have identified, are: o garbage collection o threads o control structures (branches->ANS Forth's seven universal control structure words) o exceptions o subroutines (JavaVM does not specify that a subroutine returns to its caller) o JavaVM makes the same mistake as Forth standards up to Forth-83: It specifies type sizes (e.g., a JavaVM int is always 32-bit). A few operators have to be added to support this. o The native libraries (without them JavaVM can do nothing). 5.8. How about running Forth without OS? A Forth system running on the bare hardware is also known as a native system (in contrast to a hosted system, which runs on an OS). Don't confuse this with native-code systems (which means that the system compiles Forth code to machine code); hosted native-code systems exist as well as native threaded-code systems. In the beginning Forth systems were native and performed the functions of an OS (from talking to hardware to multi-user multi-tasking). On embedded controllers Forth systems are usually still native. For servers and desktops most Forth-systems nowadays are hosted, because this avoids the necessity to write drivers for the wide variety of hardware available for these systems, and because it makes it easier for the user to use both Forth and his favourite other software on the host OS. A notable exception to this trend are are the native systems from Athena. 5.9. How about writing an OS in Forth? Native Forth systems can be seen as OSs written in Forth, so it is certainly possible. Several times projects to write an OS in Forth were proposed. Other posters mentioned the following reasons why they do not participate in such a project: If you want to write an OS in Forth for a desktop or server systems, the problems are the same as for native Forth systems (and any other effort to write a new OS): the need to write drivers for a wide variety of hardware, and few applications running on the OS. To get around the application problem, some posters have suggested writing an OS that is API or even ABI compatible with an existing OS like Linux. If the purpose of the project is to provide an exercise, the resulting amount of work seems excessively large; if the purpose is to get an OS, this variant would be pretty pointless, as there is already the other OS. And if the purpose is to show off Forth (e.g., by having smaller code size), there are easier projects for that, the compatibility requirement eliminates some of the potential advantages, and not that many people care about the code size of an OS kernel enough to be impressed.