🚀 go-pugleaf

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

UUCP:  {voder,nsc,dual,sun,ucbvax!mtxinu}!apple!north
CSNET: north@apple.CSNET

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