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From: tmb@mit-prep.ARPA (Thomas M. Breuel)
Newsgroups: net.micro.atari16,net.micro.amiga,net.micro.68k
Subject: Re: 68000 Memory Managment
Message-ID: <64@mit-prep.ARPA>
Date: Thu, 14-Aug-86 16:50:15 EDT
Article-I.D.: mit-prep.64
Posted: Thu Aug 14 16:50:15 1986
Date-Received: Fri, 15-Aug-86 20:16:48 EDT
References: <508@elmgate.UUCP>
Reply-To: tmb@prep.UUCP (Thomas M. Breuel)
Organization: The MIT AI Lab, Cambridge, MA
Lines: 26
Keywords: 68000 atari amiga 68k mmu
Xref: mnetor net.micro.atari16:1609 net.micro.amiga:4275 net.micro.68k:1131

|With all the talk of the new CoCo's memory management system and the
|discussions wondering why a 68000 was not used I have the following
|thoughts (yes I have ONE now and again :^> )
|
|If memory serves me correct the 68000 can not restart/resume a bus faulted
|instruction.  So regardless of what mkind of MMU you try to hang on it, it
|can not overcome this problem.  As I understand it,  the 68000 does NOT
|ensure the address pointed to on the stack, is the address of the fault
|instruction.  Therefore you can't restart the instruction and 68000 does
|not dump it's micro-state, so you can't resume the faulted instruction.  Is
|this about right?   Motorola you out there?  Guy Harris, this about the
|scheme of things?  Henry of UTZOO, this acurate?

The fact that it is difficult to restart an instruction that received
a bus fault on the 68000 makes it difficult to implement paging.
However, an MMU has other uses besides paging, e.g. to protect processes
from one another, to help utilise memory more efficiently, and
to allow swapping and process duplication (a la fork(1); you can't
implement fork(1), in general, on the 68000 without an MMU).

I consider it a grave mistake not to have included even a rudimentary kind
of address remapping on the Amiga; even just an adder on the address lines
of the 68000 would have simplified the system software and allowed some
kind of swapping to be hacked.

						Thomas.