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From: jtw@lcs.mit.edu (John Wroclawski)
Newsgroups: misc.forsale,comp.sys.mac
Subject: Re: SIMM speed rating (was Re: 256K SIMMS forsale!)
Message-ID: 
Date: 9 Aug 89 17:23:54 GMT
References: <460@mit-amt.MEDIA.MIT.EDU> <24441@iuvax.cs.indiana.edu>
Distribution: usa
Organization: MIT Home for Wayward Triumphs
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In-reply-to: truel@iuvax.cs.indiana.edu's message of 9 Aug 89 16:12:00 GMT

In article <24441@iuvax.cs.indiana.edu> truel@iuvax.cs.indiana.edu (Bob Truel) writes:

   In article <460@mit-amt.MEDIA.MIT.EDU> adam@mit-amt.MEDIA.MIT.EDU (Adam Glass) writes:
   >You're quite wrong. Mac II series computers will work just fine with 150ns
   >SIMM chips. For all normal use, there is no apparent slow down at...

   I don't know about these new dynamic memory, but it used to be that speed
   rating had nothing to do with how fast the chip did anything.

Um, sorry to disagree, but. The speed rating (access time) of a memory
chip is the guaranteed maximum time between when when you set the
inputs (address, etc) and when the data appears on the output lines.
(Actually, a chip will have different access times for the different
inputs - row address strobe, column address strobe, and chip
enable...) Chips with faster access times make the data available
faster. However, as you point out, this doesn't speed up a Mac II,
which cares only that the data be available in 120ns.

The reason that a 150ns SIMM generally works in a Mac II is that the
150ns is a worst-case time, assured over the chip's entire permissible
range of temperature, power supply voltage, etc. Under normal
conditions an average "150ns" SIMM will produce the data much faster
than that, and things will work fine. However, if the SIMM actually
ever does take the full 150ns, you will lose. Since this will only
happen some of the time, you will probably lose in a particularly
confusing manner.

	-john
	jtw@lcs.mit.edu