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From: alan@pdn.UUCP (Alan Lovejoy)
Newsgroups: comp.sys.mac
Subject: Re: Virtual Memory with the Mac OS
Message-ID: <1478@pdn.UUCP>
Date: Sun, 4-Oct-87 16:50:55 EDT
Article-I.D.: pdn.1478
Posted: Sun Oct  4 16:50:55 1987
Date-Received: Thu, 8-Oct-87 04:40:57 EDT
References: <2653@okstate.UUCP> <2542@batcomputer.tn.cornell.edu>
Reply-To: alan@pdn.UUCP (0000-Alan Lovejoy)
Organization: Paradyne Corporation, Largo, Florida
Lines: 28
Keywords: virtual memory 68851

In article <2542@batcomputer.tn.cornell.edu> chow@tcgould.tn.cornell.edu (Christopher Chow) writes:
/In article <2653@okstate.UUCP> norman@a.cs.okstate.edu (Norman Graham) writes:
/>Does anyone out there know what it would take to have demand paging virtual 
/>memory on the Mac II?  Could it be as simple as plugging in a 68851 and 
/>writing an interrupt handler to take care of swapping the pages?
/>
/
/One thing which you might want to question is whether if you want to add an
/68851 MMU to your Mac II.  I'm not a hardware guru, but from what
/understand, the 68851 chip adds wait state(s) to every memory reference.
/How's that for slowing down your machine.  Note that Suns, etc., all use
/their custom MMU chips to avoid the wait state problems with the Motorola
/chip.
/

Not quite.  The Mac II in its present form uses either the "HMMU" (a 68461)
or the "PMMU" (a 68851), either of which introduces a wait state.
Therefore, taking out your current 68461 and putting in a 68851 should 
have no effect on wait states.

Also, the extra wait state is due more to the memory access protocol and
architecture of the 68000, 68010 and 68020 than to the particular brand or 
model of MMU you use.  Only by designing the MMU with a *large* external
cache is it possible to virtually avoid wait states (or incurr them only on 
cache misses, that is).  Someone should market a 68851 compatible MMU
with a large on-chip cache (Hey! Motorola! Pay attention!).

--alan@pdn