Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!mcsun!ukc!cam-cl!news
From: bdb@cl.cam.ac.uk (Brian Brunswick)
Newsgroups: comp.sys.acorn
Subject: Re: Mac emulators.
Message-ID: <1991Jun18.075215.19917@cl.cam.ac.uk>
Date: 18 Jun 91 07:52:15 GMT
References: <4549@stl.stc.co.uk> <125@melbourne.uucp> <9106172305.AA05255@ucbvax.Berkeley.EDU>
Reply-To: bdb@cl.cam.ac.uk (Brian Brunswick)
Distribution: comp
Organization: U of Cambridge Comp Lab, UK
Lines: 16

In article <125@melbourne.uucp> AWilliams@melbourne.uucp (Alan Williams) writes:
>The up shot of this is that an emulator for a 680x0 MPU running on an ARM 
>would be far too slow to be useful.  
(Because of the byte sex problem)

The answer to this is to eor all byte addresses with 3 - then word
and half-word accesses go quite nicely, apart from ones that are only
2 byte aligned. These can be fixed up nicely by using the fact that LDR
from a non aligned address rotates the word read.... This actually looked
quite reasonable, and I began to write a 68000 emulator, but I discovered that
STR doesn't rotate on non aligned stores :-( Oh well... perhaps my enthusiasm
will be restored some time in the future... the trouble is I'm always
very annoyed when I use macs because they lack stuff the arc has...
but then again I get annoyed with the arc too... The OS is so yucky inside!
Such is life.
Brian.Brunswick@uk.ac.cam.cl  Disclaimer.  Short sig rules!