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Path: utzoo!mnetor!seismo!caip!cbmvax!daveh
From: daveh@cbmvax.cbm.UUCP (Dave Haynie)
Newsgroups: net.micro.atari16,net.micro.amiga,net.micro.68k
Subject: Re: 68000 Memory Managment
Message-ID: <625@cbmvax.cbmvax.cbm.UUCP>
Date: Thu, 14-Aug-86 13:52:48 EDT
Article-I.D.: cbmvax.625
Posted: Thu Aug 14 13:52:48 1986
Date-Received: Thu, 14-Aug-86 22:17:36 EDT
References: <508@elmgate.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 38
Xref: mnetor net.micro.atari16:1604 net.micro.amiga:4263 net.micro.68k:1130

> Keywords: 68000 atari amiga 68k mmu
> Xref: cbmvax net.micro.atari16:1562 net.micro.amiga:2159 net.micro.68k:186
> 
> 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?
>
> BTW the 68010 fixes this.

That basically it.  The 68010 uses instruction continuation to support virtual
memory and/or MMUs.  When an exception occurs, some processor internal
state information is pushed on the exception stack.  This allows a bus error
exception handler to do whatever's necessary to get the desired address
into physical memory, then RTE, at which point the instruction that caused
the bus error will continue, recovering at the cycle that caused the fault.
This is more efficient than the method used by some processors, which 
restart the instruction from its beginning.  The 68000 doesn't save enough
exception stack information to accomplish this.
> 
> -- 
> Jeff Gortatowsky       {allegra,seismo}!rochester!kodak!elmgate!jdg
> Eastman Kodak Company  
> <Kodak won't be responsible for the above comments, only those below>
-- 
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Dave Haynie    {caip,ihnp4,allegra,seismo}!cbmvax!daveh

	"I gained nothing at all from Supreme Enlightenment, and
	 for that very reason it is called Supreme Enlightenment."
							-Gotama Buddha

	These opinions are my own, though for a small fee they be yours too.
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