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Path: utzoo!linus!philabs!cmcl2!gottlieb
From: gottlieb@cmcl2.UUCP (Allan Gottlieb)
Newsgroups: net.micro.68k
Subject: 68K MMU is good for something
Message-ID: <1086@cmcl2.UUCP>
Date: Fri, 30-Dec-83 11:27:09 EST
Article-I.D.: cmcl2.1086
Posted: Fri Dec 30 11:27:09 1983
Date-Received: Sat, 31-Dec-83 11:32:58 EST
Organization: New York University
Lines: 25

I will not challange Spencer's arguments about the demand paging
merits of the 68K and 16K MMUs.  The Mororola MMU is, however,
the best chip for the Buddy System, a good algorithm for segment
allocation on a uniprocessor.  Moreover the NYU "Ultracomputer"
project (large scale shared memory multiprocessor) has developed
highly parallel variants of the Buddy System.

Our current prototype uses the serial Buddy algorithm (and the 68K
MMU) on a Master Slave multiprocessor UNIX (TM) system.  In a few
months we will have a highly parallel UNIX kernel (I/O will still be
serial), which will feature parallel memory management using our
parallel Buddy algorithm (and again the MMU).

One negative comment on the MMU is its translation speed.  We decided
to cache virtual rather than physical addresses primarily so that cache
hits would result in no wait states, which would have been impossible
if the cache had to wait for the mmu to leasurely translate the
addresses.  (Note we do NOT support demand paging -- virtual addr =
addr on PE bus = input to MMU; physical addr = addr in memory = output
from MMU).

Allan Gottlieb
New York University
GOTTLIEB@NYU
floyd!cmcl2!gottlieb	<-- char before 2 is el not one