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From: amit@umn-cs.UUCP (Neta Amit)
Newsgroups: comp.sys.ibm.pc,comp.arch
Subject: Cache on a 286 board ?
Message-ID: <2467@umn-cs.UUCP>
Date: Mon, 26-Oct-87 00:59:01 EST
Article-I.D.: umn-cs.2467
Posted: Mon Oct 26 00:59:01 1987
Date-Received: Wed, 28-Oct-87 05:21:08 EST
Reply-To: amit@umn-cs.UUCP (Neta Amit)
Organization: University of Minnesota
Lines: 21
Xref: mnetor comp.sys.ibm.pc:9429 comp.arch:2711

To support 0 wait-state on a 10 MHz CPU motherboard, one must use
100 nsec RAM chips. For 12 MHz, it's 80 nsec. For 15.5 MHz -- 65 nsec.
Unfortunately, the 65 nsec chips are not exactly found in abundance.

So what do they do? They make 286 boards running at 15.5 MHz with 1 wait
state, and populate the board with 80 nsec chips, effectively slowing the
CPU to 12.5 MHz. And what about 15.5 MHz 386 boards? Same problem, but 
THERE they use fast (50 nsec?) 64K static RAM chips, that serve as fast
memory ``cache'' . In most cases, they claim (justifiably), this will
make the 15.5 MHz machine run at close to 0 wait state. Now two questions:

1. Why don't they do the same trick with 286 boards (as far as I know)?
   Is there something more complex about the logic there or what?  
2. Since most 15.5 MHz/1ws machines cannot select 12.5 MHz, are you better
   off (for whatever reason) with a 15.5/1, or with a 12.5/0 machine?


-- 
  Neta Amit 
  U of Minnesota CSci
  Arpanet: amit@umn-cs.cs.umn.edu