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From: ForthNet@willett.pgh.pa.us (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: Memory Management/PIC
Message-ID: <2862.UUL1.3#5129@willett.pgh.pa.us>
Date: 8 Jun 91 02:20:49 GMT
Organization: (n.) to be organized.  But that's not important right now.
Lines: 30


 Date: 06-03-91 (12:21)              Number: 99 of 102 (Echo)
   To: B.RODRIGUEZ2 [BRAD]           Refer#: 81
 From: RAY DUNCAN                      Read: NO
 Subj: MEMORY MANAGEMENT/PIC         Status: PUBLIC MESSAGE
 Conf: FORTH (58)                 Read Type: GENERAL (+)

   >word access at odd address is flat-out illegal on the 68000

 That's true, but it has to be allowed I think if Forth code is to be
 portable.  In our CP/M-68K Forth systems (long since dead & buried) we
 took a compromise approach to this that didn't affect performance.  We
 word aligned everything in the Forth system, and had the @ and !
 operators use the "obvious" (i.e. word-oriented) MOVE instructions.
 However, we also captured exceptions that were caused by non-aligned
 WORD accesses, figured out whether they originated within the @ and !
 primitives, and if they did we would then decompose the @ or ! into the
 appropriate byte-oriented MOVEs and resume execution.  This is a
 straightforward fix in a native Forth system or a Forth running under a
 dumb environment like CP/M-68K, but it is impractical in more complex
 environments since a lot of them won't let you handle these sorts of
 machine exceptions yourself.

 NET/Mail : LMI Forth Board, Los Angeles, CA (213) 306-3530
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