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From: edler@cmcl2.UUCP
Newsgroups: net.arch
Subject: Re: MMUs and OSs
Message-ID: <304@cmcl2.UUCP>
Date: Mon, 10-Jun-85 19:52:22 EDT
Article-I.D.: cmcl2.304
Posted: Mon Jun 10 19:52:22 1985
Date-Received: Thu, 13-Jun-85 01:17:41 EDT
References: <276@spar.UUCP> <5653@utzoo.UUCP> <291@spar.UUCP> <1031@peora.UUCP> <118@LaBrea.ARPA> <7983@ucbvax.ARPA>
Reply-To: edler@cmcl2.UUCP (Jan Edler)
Organization: New York University
Lines: 37
Summary: There is nothing wrong with the M68451

In article <7983@ucbvax.ARPA> mwm@ucbvax.UUCP (Mike Meyer) writes:
>In article <118@LaBrea.ARPA> mann@LaBrea.ARPA writes:
>>The 68451 is utterly ridiculous garbage (please
>>look into how it works before flaming back at me).
>
>I've seen this kind of claim a lot recently, and would like someone to
>explain why the 68451 is "utterly ridiculous garbage" (one of the nicer
>things I've heard said about it).

I agree, I'm tired of hearing general condemnation of the 68451.
It works.  We have built a multiprocessor prototype here with eight
68010's and 68451's, and I don't think we've found any bugs in the chip.
We have developed a parallel version of UNIX for it, including the
68451-related code.

It doesn't do a very good job of supporting a large number of relatively
small fixed-size pages.  But it can still support large virtual memory,
fast memory allocation is easy, large page tables are not needed, and
it is only one chip.

There are a few disadvantages to it; it is kind of slow, internal
fragmentation can increase memory waste, and there aren't very many
segment descriptors.  Does anyone use multiple 68451's ganged together
to provide more descriptors?  I think the cost (in board space) would
tend to make such a configuration un-attractive.

In the years ahead, with more programs requiring very large address spaces,
I think variable-sized segments might be particularly advantageous
because a new address space can be created very quickly -- it isn't
necessary to initialize all those page table entries.

In any case, not all mmu's need to be demand-paged mmu's.

	Jan Edler
	New York University
	cmcl2!edler
	edler@nyu