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From: mann@LaBrea.ARPA
Newsgroups: net.arch
Subject: Re: I like segmented architectures
Message-ID: <118@LaBrea.ARPA>
Date: Sat, 8-Jun-85 13:26:59 EDT
Article-I.D.: LaBrea.118
Posted: Sat Jun  8 13:26:59 1985
Date-Received: Tue, 11-Jun-85 02:51:21 EDT
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Organization: Stanford University
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> > By the way, where are the 68XXX fans?  Surely the Motorola chip isn't
> > so bad that no one can find a defense for it.
> 
> It's kind of hard to compare the 68000's MMU, which functions in a very
> familiar, traditional way (the same way MMUs on many "mainframe" machines
> work), with the very strange segmentation facilities of the 286.

I felt J. Eric Roskos's message was a good comparison of 8086-style
segmented architecture with more conventional linear address space models,
and I'm essentially in agreement that linear address spaces are superior
overall.  But I'm curious as to what he's referring to when he talks about
the 68000's MMU above.  The 680X0 doesn't have an on-chip MMU -- which is in
a sense one of its strengths, since the chips do support a large linear
address space, and those who use them are free to build any style of
outboard MMU for such an address space.  Most have chosen to do fairly
conventional paged MMUs (for instance, Sun).  But there is no such MMU in
the 68XXX family (yet).  The 68451 is utterly ridiculous garbage (please
look into how it works before flaming back at me).  The more recent promised
MMUs from Motorola and Signetics have not come out yet.

	--Tim