From - Mon Jul 28 12:06:54 1997
From: "Marc de Groot - Replace username with my first name for all e-mail -"
Subject: Re: CMFORTH
Date: Mon, 28 Jul 1997 08:22:52 -0700
Organization: Immersive Systems, Inc.
X-Newsreader: Microsoft Outlook Express 4.71.1008.3
X-MimeOle: Produced By Microsoft MimeOLE Engine V4.71.1008.3
Xref: uunet comp.lang.forth:35173
> Around five years ago I thought cmForth was so old-fashioned
>that it would be replaced with something better. But I still see
>favorable references about it. So now I can't decide. Should
>cmForth be studied and documented to make a new and better Forth for
>current computers, or should it just die a natural death from old age
>and lack of interest?
My opinion is that the weekend or so that one would take to
learn cmForth would be time very well invested.
I first encountered cmForth and the Novix chip in the early
eighties. I bought one of the evaluation kits with cmForth in
ROM, and (amongst other things) wrote a Forth-83 standard
interpreter for the system which became a commercial product.
I regret that the metacompiler in my Forth system, which was
built directly on top of cmForth, used older, better known, and
far less elegant techniques than Chuck's code. :-) By the time
I understood cmForth's metacompilation (which is far simpler
than one's intuition would suggest) I had invested a fair
amount of time and energy in writing my own metacompiler.
Chuck has a unique way of looking at and resolving software
engineering problems. One thing about Chuck that's unusual is
his patience with his own thought processes--patience is not a
prevalent trait among computer people! Chuck seems to derive
great satisfaction from contemplating relatively small problems
for periods as long as years, and he feels like he has really
done something interesting when he has shortened the program by
40% and has somehow managed to add some functionality that
makes the software more useful.
Any programmer with some passion for the craft and a supply of
caffeine can implement bells and whistles at record speed,
writing code that gets the job done--and gets it done
*today*--but few of us have approached the design (and
redesign) of software the way Chuck does.
Mike Perry relates a story about Chuck's CAD system which ran
on the board whose design it had been written for. Chuck
showed Mike that he had written the core of the CAD application
in about five lines of code. When Mike asked him how long it
had taken, he replied, "Oh, about two years."
cmForth's metacompiler is the result of one of Chuck's long
contemplations. It reduces the largest headaches in
metacompilation to fewer than twenty lines of code, all told.
To make that possible, the structure of cmForth departs
somewhat from the familiar.
The fundamental issue in metacompilation is this: Forth
systems, in most cases, are not designed so that compilation
outside the running system's dictionary is as straightforward
as compiling into its dictionary. The biggest reason for this
is that the programmer now has two dictionaries that must be
considered; the host (or running system's) dictionary, and the
target (or metacompiled system's) dictionary.
Think about what happens when a colon definition is compiled
into the target system. The next symbol in the input stream
has to be parsed and compared against the name fields in the
dictionary so we can find the address to compile. Which
dictionary gets searched? Why, the target dictionary, of
course; we need the address of the Forth word in the target,
*not* the one in the host, if we want the metacompiled system
to run once the host has terminated! Okay, so we search the
target dictionary for any name we encounter in the input
stream? Well, not quite. What if the word is IMMEDIATE, that
is, what if the word executes at compilation time? In that
case, we can't look up the address of the word in the target
and pass that address to EXECUTE. For all we know, we are
cross-compiling for another processor, and the copy of BEGIN or
UNTIL that's in the target dictionary will crash the system if
we try to execute it.
The above example describes the behavior during
compilation--what about during execution? Well, if we're
looking up words to execute them, they had better be in the
host system, or we had better have a strong reason for
believing that code in the target can be safely executed during
the meta- compilation. Okay, so outside of colon definitions,
we always look up words in the host, right? Well, no. It
depends. Let's say we're using "tick" to get the execution
address of a word. Why are we doing that? One possibility is
we want to get the execution address of a word and store it in
a variable, so it can be fetched and passed to EXECUTE at some
future time. If we're patching a variable in the target system,
"tick" needs to search the target system for the address of the
word in the target. If we're trying to find information about
the current state of the running Forth environment, we will
likely want to look in the host, where the active Forth
system's data is stored.
These considerations often lead the metacompiler designer to
write separate versions of words such as , C, ALLOT CREATE
"tick" -- and all of the words that depend on these
definitions. In some systems, the new set of words is given a
distinct set of names indicating it acts on the target
system...names such as ,-T C,-T ALLOT-T etc. are common. In
other systems, the designer keeps the names the same as the
host counterparts, but puts the new set of words in its own
vocabulary, and makes use of the vocabulary search order to
enforce proper dictionary searching.
Chuck's careful approach to the design of cmForth, his patient
contemplation of the issues in metacompilation, and an
unwillingness to "settle for less" led to a design and
implementation that are cleaner and more elegant than any other
that I have seen.
cmForth is roughly ten pages long, written the old-fashioned
way, three 16-line disk "screens" to a page. Chuck was proud
of the fact that he could recompile cmForth with itself,
reading the source off of a 1.44-MB floppy, and the system
would completely recompile and prepare a memory image of the
new Forth system *before the floppy had spun all the way up to
speed*. There's, like, nothin' there, man!
I believe the biggest hurdle to understanding cmForth is
Chuck's very terse (read: undocumented) coding style.
Certainly, Ting has made efforts to present the material in a
somewhat more palatable form. I remember hearing that Jay
Melvin, one of Chuck's neighbors (now since passed away?) had
annotated and expanded the source code for cmForth. I wonder
if anyone has a copy of that... I'm assuming that the legal
status of cmForth hasn't changed, and that it is still freely
available at no charge?
Thanks much to Frank Sergeant for Pygmy, which kept me
entertained for a while this past winter. I've been using Pygmy
to build a tiny DOS TSR program. I have been using it to
analyze network traffic for multi-user video games. I need
something that is very flexible (so a command prompt at
run-time is just the ticket!) and the memory footprint has to
be as small as possible, since these 32-bit DOS video games
tend to be big consumers of low memory on a PC. Aside from its
small size and decent features, it *screams*! A very
efficient little execution engine, with direct threading. It
might be instructive to implement a few of Java's primitives as
code words in Pygmy, and compare the execution times...
-Marc de Groot
^^^^^^^^^^ Replace this with my first name or mail won't get to me.