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From: maniac@howlin.cs.unlv.edu (Eric J. Schwertfeger)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Assembler Programming - Costs versus Benefits
Message-ID: <1990Dec1.184559.22509@unlv.edu>
Date: 1 Dec 90 18:45:59 GMT
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Reply-To: maniac@jimi.cs.unlv.edu (Eric J. Schwertfeger)
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> Ah, then explain why Atari ST programs don't run on a 68020.

Four reasons I've seen 68000 programs not run on higher processors.
The
ST may have one of these problems

1)  Software timing loops run faster.  On the Amiga, we have hardware
timers
    to handle that.  I don't know if the ST does

2)  Since the 68000 only had 24-bit addressing, some 68000 OS's used
the
    Upper 8 bits of the longword to store tag data.  The Mac OS does
this,
    but doesn't use the upper 8 bits to address memory, which is why
you
    still can't get much memory into the older Mac II's. Minix does it
    too (so I've been told).

3)  The Exception frame is different for the different models of
680x0.
    The Amiga OS handles the different frames.  I'd assume the Mac
does
    this.  Which means that if you throw out the Amiga OS, you need to
    write your own exeption handlers, and if you're lazy, you won't
    bother writing code to handle the different frames.  Again, I
don't know
    if the ST OS handles the different frames at all.

4)  The move.w SR,<ea> is the ONE instruction that changed from the
68000
    to the higher family members.  I've never seen a legitimate use
for
    this instruction outside of the OS, and even if you do need it,
there's
    a functionally identical OS call you can use.  Unless of course,
    you're throwing out the OS.  I don't know if the ST OS uses that
    instruction.



Eric J. Schwertfeger, maniac@jimi.cs.unlv.edu