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From: mash@mips.UUCP
Newsgroups: comp.sys.m68k
Subject: Re: Question:  on-chip or off-chip MMU?
Message-ID: <353@winchester.UUCP>
Date: Wed, 29-Apr-87 04:55:35 EDT
Article-I.D.: winchest.353
Posted: Wed Apr 29 04:55:35 1987
Date-Received: Thu, 30-Apr-87 07:12:58 EDT
References: <5635@shemp.UCLA.EDU> <58500002@gorgo.UUCP>
Reply-To: mash@winchester.UUCP (John Mashey)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 37

In article <58500002@gorgo.UUCP> bsteve@gorgo.UUCP writes:
>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.
Or may not; on-chip is often easier to overlap.

>	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.
This is 100% not true if the chip has the cache control on it.
Recall also that there can be substantial hit-rate losses from virtual
caches, as well as serious complexifications [nothing that can't be beaten,
but some things get more complex in both hardware and software.  There's
an interesting paper on the VM for the SUn3/260 in the next USENIX.]

>	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? ....
This can be true; but only if the chip is intended for controller applications
as a first priority, because it may cost $$ and cycle time to then add
an MMU.

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

On the other hand, at least several vendors [such as Motorola] who used
to have the MMU off-chip are now putting it on.  Finally, our MIPS R2000s
use on-chip TLBs, and they're not exactly slow: if you know a
micro with a separate MMU that's faster, and that you can actually buy,
please post some info.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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