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From: north@apple.UUCP (Donald N. North)
Newsgroups: comp.sys.mac
Subject: Re: Virtual Memory with the Mac OS
Message-ID: <6453@apple.UUCP>
Date: Sun, 11-Oct-87 02:00:59 EDT
Article-I.D.: apple.6453
Posted: Sun Oct 11 02:00:59 1987
Date-Received: Mon, 12-Oct-87 21:36:02 EDT
References: <2653@okstate.UUCP> <2542@batcomputer.tn.cornell.edu> <1478@pdn.UUCP>
Reply-To: north@apple.UUCP (Donald N. North)
Organization: Apple Computer, Inc.
Lines: 28
Keywords: virtual memory 68851 HMMU wait states

In article <1478@pdn.UUCP> alan@pdn.UUCP (0000-Alan Lovejoy) writes:
>/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.

For the record, the HMMU (an Apple custom, not a 68461) adds ONE wait state
to each basic memory access; when it is replaced with a PMMU (68851) TWO wait
states are added to each memory access.  Thus it only pays to use the 68851
when you really need it (ie, for A/UX).  It not only costs you $$ but degrades
performance too.
-- 

Don North
Apple Computer, Inc.
Advanced Technology Group

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{{ Facts are facts, but any opinions expressed are my own, and do not     }}
{{ represent any viewpoint, official or otherwise, of Apple Computer, Inc.}}