Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!dlyons
From: dlyons@Apple.COM (David Lyons)
Newsgroups: comp.lang.forth
Subject: Re: Forth Implementation Question
Keywords: Forth, 68000
Message-ID: <34738@apple.Apple.COM>
Date: 14 Sep 89 23:13:40 GMT
References: <1715@thumper.bellcore.com> <7972@medusa.cs.purdue.edu>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 36

In article <7972@medusa.cs.purdue.edu> bouma@cs.purdue.EDU (William J. Bouma) writes:
>  [...]  are there any other advantages of your dynamic code movement?
>      less error prone than the scheme you have presented.

This may not be directly related to the original posting, but in environments
such as the Macintosh and Apple IIgs, there is a Memory Manager that you have
to request memory from--there's no guarantee that you can get the *same* memory
from one execution of your program to the next, so a traditional Forth
architecture (with lots of pointers) is a problem.

Even if the processor provides some kind of "base register" & you use offsets
rather than absolute pointers, there are still problems.  For example, how
do you "grow" the block of memory allocated for your dictionary?  You have to
allocate all the space you want in the beginning, *or* you have to be prepared
to *not* be able to grow your block, even if the amount of memory you want is
available (probably split up into several pieces).


The closest the 65816 comes to providing a base register is to limit the main
Forth memory area to 64K and allocate a memory block of up to 64K, starting at
a 64K boundary--at least one commercial GS Forth, called GS16Forth, uses this
approach.  It works, and it has the advantage (?) that things are still 16 bits
long.  (I'd rather have 24- or 32-bit pointers/offsets, but not necessarily
24- or 32-bit integers, by default.  I suspect this would break a lot of
existing Forth code.)

Has anybody seen Forth systems using this sort of approach?
-- 

 --Dave Lyons, Apple Computer, Inc.          |   DAL Systems
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