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From: farren@gethen.UUCP (Michael J. Farren)
Newsgroups: comp.sys.ibm.pc,comp.arch
Subject: Re: Cache on a 286 board ?
Message-ID: <256@gethen.UUCP>
Date: Tue, 27-Oct-87 10:17:39 EST
Article-I.D.: gethen.256
Posted: Tue Oct 27 10:17:39 1987
Date-Received: Sat, 31-Oct-87 08:08:47 EST
References: <2467@umn-cs.UUCP>
Reply-To: farren@gethen.UUCP (Michael J. Farren)
Organization: Sci-Fido - Unix in Oakland
Lines: 32
Xref: mnetor comp.sys.ibm.pc:9501 comp.arch:2741

In article <2467@umn-cs.UUCP> amit@umn-cs.UUCP (Neta Amit) writes:
>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.

This is only true if memory cycles are required to complete in one clock
cycle.  I don't have my 80286 data book here, so can't quote chapter and
verse, but I'm almost dead certain that this isn't the case with that
processor.

>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?  

The '386 has the logic to handle cache built in, the '286 does not.

>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?

The actual speed of the machine will be faster given a faster clock,
even if you have wait-stated memory.  All of the Intel processors are
built with an instruction pipeline, which is filled as required with
memory accesses.  All instructions, therefore, execute internally at the
full processor speed.  Any time an instruction takes more time to
execute than the time required to fetch itself from memory, you have a
net gain.  Determining what clock speed/wait state combination will
provide the best net gain is fairly complex.  Perhaps someone could post
some numbers?

-- 
----------------
Michael J. Farren      "... if the church put in half the time on covetousness
unisoft!gethen!farren   that it does on lust, this would be a better world ..."
gethen!farren@lll-winken.arpa             Garrison Keillor, "Lake Wobegon Days"