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

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  comp.lang.forth Frequently Asked Questions (1/6): Gen-
  M. Anton Ertl,


  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    Hang on, isn't Forth out of date now?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

  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 <

  o  Online resources <

  o  Forth vendors <

  o  Forth systems <

  o  Books, periodicals, tutorials <

  o  Forth groups & organizations <

  o  ANS Forth <>

     You can get the text versions of these FAQs at

  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  For a historical
  reference, programming paradigms, and deep technical information try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

  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

  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
  <>, in: History of
  Programming Languages (HOPL-II), ACM Press/Addison-Wesley 1996.

  Note: Forth is not a 4GL (language for programming database

  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
  <>. You can find the full version at
  <>. 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

  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

  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

  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  <>.

  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  <> 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,

  The (un)official ANS Forth document is available in various formats at
  <> and at
  <>. The format I like best is
  the HTML version <>.

  To get yourself on the ANS-Forth mailing list, consult the various
  README files at  <>.

  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  <>).

  o  JET Thomas has written a test suite to test ANS Standard Programs:

  There is also an ANS Forth FAQ
  <> 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
  and labeling it as RFI. The answers to earlier RFIs are available at They are also
  integrated in the HTML version of the standard

  3.8.  Are there Coding Standards for Forth?

  Leo Brodie's book Thinking Forth gives some advice; a short excerpt is
  now available online <>. Forth
  shops have rules for their coding. Paul Bennet has published those of
  his company; you can find them on

  3.9.  I have trouble managing the stack. Should I use global VARI-

  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
     <> 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$q3c$>:

       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

  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  <>, 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
  <>).  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

  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

  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

  5.  Miscellaneous

  5.1.  Where can I find a C-to-Forth compiler?

  Parag Patel <> writes:

       We, (CodeGen, Inc. <>) 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 <> 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
  <>. It is
  described in the EuroForth'95 paper
  Another Forth-to-C compiler is supplied with Rob Chapman's
  <> Timbre
  <> 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 ( <>) or
  the structures supplied with Gforth

  5.4.  Why does THEN finish an IF structure?

  Some people find the way THEN is used in Forth unnatural, others do

  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:


  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

  5.6.  Has anyone written a Forth which compiles to Java bytecode?

  Paul Curtis <> 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

  o  You can't manufacture objects out of raw bytes.  This is a security

  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

  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.

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