Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!uwvax!rocky.cs.wisc.edu!g-verbru
From: g-verbru@rocky.cs.wisc.edu. (Rob Verbrugghe)
Newsgroups: comp.sys.mac
Subject: Mac memory speed confusion
Keywords: Memory speed, Cache memory, Wait states, Virtual Memory, Help
Message-ID: <7400@spool.cs.wisc.edu>
Date: 9 Apr 89 20:22:34 GMT
Sender: news@spool.cs.wisc.edu
Reply-To: g-verbru@rocky.cs.wisc.edu (Rob Verbrugghe)
Organization: U of Wisconsin CS Dept
Lines: 49


  Well now I'm a little confused on this whole memory speed issue.  Now the
Mac +/SE runs at 8 Mhz (roughly) and requires 150 ns memory for a no wait
state operation.

  The Mac II runs at 16 Mhz and requires 120 ns memory for no wait state 
operation. But the 68020 takes more cyclces per memory access, so the
memory speed does not have to be doubled.

  The Max IIx / SE/30 also runs at 16 Mhz and requires the same 120 ns memory
for no wait state operation.  But, the 68030 is different than the '020
in the number of memory cycles required to access memory.

  Somewhere in here the Apple Memory Chip vs the 68851 Virtual memory chip
figures in in the rate at which memory is accessed.  I belive that the
68851 was slower than the Apple chip because it might have to do Virtual
Memory translation, and that takes time.  The 68030 has some 68851 
functionality built in, but since its on chip, it may not take as long
as a separate memory unit.

	1/8  Mhz   == 125   ns  2 cycles == 250 ns
	1/16 Mhz   ==  62.5 ns  2 cycles == 125 ns  3 cycles == 187.5 ns

  Now I see a RAM CACHE board being advertised that claims to give a
15% to 30% speed improvement to a Mac II.  If there are no wait states
currently in the Mac II, and we do not adjust the speed of the processor,
then faster memory shouldn't help.  Right?

  Someone is confused.  I suspect that it is me.  Could some kind soul
set me straight on the facts here?  Feel free to get technical, if I
can't figure out what you've said, I can get someone to translate.

Thanks buckets,
Rob Verbrugghe  g-verbru@cs.wisc.edu   {backbone}!uwvax!rocky!g-verbru

"So you think you know great, nail-biting excitement, you think you know truly
abject fear, you think you know total and complete despair, you think you know
the incredibly degenerate underside  of this world we live in, and the ridicu-
lously despicable lengths that your fellow man  can sink to, more rotten, more
putrid than the  lowest form of fungus....   Oh.   You are a sorcerer as well.
Then perhaps you do."
			    -- Further Conversations with Ebenezum, Volume III

"There are  those who claim that  magic is like  the tide; that  it swells and
fades over the surface of the earth, collecting in concentrated pools here and
there, almost disappearing from other  spots, leaving them parched for wonder.
There are also those  who believe that if you stick  your fingers up your nose
and blow, it will increase your intelligence."
				      -- The Teachings of Ebenezum, Volume VII