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comp.lang.forth Frequently Asked Questions (1/6): Gen-
eral/Misc
M. Anton Ertl, [email protected]
____________________________________________________________
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 Hang on, isn't Forth out of date now?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:[email protected]>. 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 [email protected]
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:[email protected]> 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: <[email protected]>:
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:[email protected]> 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:[email protected]> 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:[email protected]> 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:[email protected]> 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.
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