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From: authorplaceholder@gorgo.UUCP.UUCP
Newsgroups: comp.sys.m68k
Subject: Re: Question:  on-chip or off-chip MMU?
Message-ID: <58500002@gorgo.UUCP>
Date: Sun, 26-Apr-87 18:40:00 EDT
Article-I.D.: gorgo.58500002
Posted: Sun Apr 26 18:40:00 1987
Date-Received: Wed, 29-Apr-87 06:05:43 EDT
References: <5635@shemp.UCLA.EDU>
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Nf-From: gorgo.UUCP!bsteve    Apr 26 17:40:00 1987


This is (of course) a very fuzzy question. I would tend to go (for now) with
an off-chip mmu for several reasons:

	1) On-board MMUs require micro-bus cycles just like separate MMUs,
	   and depending upon the uP architecture may take the same number
	   of cycles.

	2) We can't add virtual cache with an on-board MMU. The advantage of
	   virtual over physical cache is that it operates in parallel with
	   the MMU cycles and returns in nearly half the time on a hit,
	   whereas the physical cache always requires a complete MMU cycle.

	3) Some applications (small signal processing, etc.) don't really
	   require the MMU, so why should one drive up the cost of the uP
	   by adding one on-board?

	4) Some architectures support stackable multiple MMUs that operate
	   parallel. One obviously cannot do this if the MMU is on-board.

I am sure that there are numerous other reasons why off-chip MMUs are more
desirable.

	Steve Blasingame (Oklahoma City)
	ihnp4!gorgo!bsteve