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From: gillies@p.cs.uiuc.edu
Newsgroups: comp.sys.mac
Subject: Re: When will MacOS get virtual memory?
Message-ID: <76000293@p.cs.uiuc.edu>
Date: 5 Oct 88 16:26:00 GMT
References: <5624@zodiac.UUCP>
Lines: 28
Nf-ID: #R:zodiac.UUCP:5624:p.cs.uiuc.edu:76000293:000:1382
Nf-From: p.cs.uiuc.edu!gillies    Oct  5 11:26:00 1988


/* Written  1:11 pm  Oct  4, 1988 by jellinghaus-robe@CS.YALE.EDU */
>In article <76000290@p.cs.uiuc.edu> gillies@p.cs.uiuc.edu writes:
>>The sad thing is, the vast majority of Macintoshes in this world
>>cannot support virtual memory.  This is because the 68000 chip has a
>>design flaw that doesn't allow virtual memory to be implemented.
>
>Someone really should tell Sun or Apollo.  They've produced many workstations
>with 68000's running UNIX, which DOES support virtual memory and page 
>swapping.  Gosh, wonder how they do it?

I believe that a microcode bug leaves the CPU in a corrupt state if
you take a page fault on a certain memory reference by the 68000 CPU.
All the paging Sun's I know of use the 68010 or 68020, not the 68000
CPU.  I think this is the main reason why Motorola designed the 68010
-- to fix this microcode bug.

I once heard that someone implemented VM on the 68000 by using two
CPU's, one executing a few cycles behind the other.  This way, if
one takes a page fault & is corrupted, the other can be stopped early,
and the corrupted CPU can be reinitialized from the "trailing" CPU's 
registers.

I wasn't trying to malign the 68000, which I have a lot of respect for.

Don Gillies, Dept. of Computer Science, University of Illinois
1304 W. Springfield, Urbana, Ill 61801      
ARPA: gillies@cs.uiuc.edu   UUCP: {uunet,ihnp4,harvard}!uiucdcs!gillies