Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!dali.cs.montana.edu!uakari.primate.wisc.edu!aplcen!aplcomm!uunet!mcsun!ukc!acorn!melbourne!AWilliams
From: AWilliams@melbourne.uucp (Alan Williams)
Newsgroups: comp.sys.acorn
Subject: Re: Mac emulators.
Message-ID: <125@melbourne.uucp>
Date: 17 Jun 91 12:20:16 GMT
References: <4549@stl.stc.co.uk>
Sender: alan@melbourne.uucp
Distribution: comp
Organization: Acorn Computers Ltd, Cambridge, England
Lines: 22

In article <4549@stl.stc.co.uk> ajdh@stl.stc.co.uk (Andrew J D Hurley) writes:

Lots of stuff deleted, about lots of previous items about Mac disc readers, MS 
DOS and other stuff.

I comment on the item previous to this one.  It raised the point that a Mac 
emulator, rather like the PC Emulator would be a nice idea.  I don't know a lot 
about these sort of things but is strikes me as if there is a single rather serious 
difficulty involved in writing an emulator for a 680x0 processor: it has the 
opposit byte sex to an ARM.  That is to say if an ARM stores a 32 bit register to 
RAM it goes in low byte first, but if a 680x0 was to do this it would go in high 
byte first.  Thus an emulator would have to do byte reversals on all multi byte or 
word access.  This I guess might cause a performance reduction of maybe 30 or 
40 % relative to an emulator such as the 80188 one.

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.  

Alan.

The opinions expressed here are my own and not necessarily those of Acorn.