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From: perkins@bnrmtv.UUCP (Henry Perkins)
Newsgroups: comp.sys.intel,comp.sys.ibm.pc
Subject: Re: Clock speed on those darn 80?87 chips ...
Message-ID: <2545@bnrmtv.UUCP>
Date: Tue, 15-Sep-87 16:59:01 EDT
Article-I.D.: bnrmtv.2545
Posted: Tue Sep 15 16:59:01 1987
Date-Received: Sat, 19-Sep-87 01:16:11 EDT
References: <454@hubcap.UUCP>
Followup-To: comp.sys.ibm.pc
Distribution: na
Organization: BNR Inc., Mountain View, California
Lines: 20
Summary: 80287 runs at 2/3 the 80286 machine in an AT-type machine.
Xref: mnetor comp.sys.intel:347 comp.sys.ibm.pc:7843

In article <454@hubcap.UUCP>, oolidjr@hubcap.UUCP (Joe Moll) writes:
> What is the deal with the clock speed rating on the INTeL co-processor
> chips???  My brother just bought a 10Mhz, 0 wait state AT clone.  The
> dealer claims (along with a few people at our institution) that the
> coprocessor runs at (2/3) * main processor clock speed.

This is correct, but is a "feature" of IBM's AT architecture and
not of the Intel chips used.  IBM used a 12 MHz crystal in the
original AT, with a divide-by-2 for the 80286 and a divide-by-3
for the 80287.  This is possible on the 80286 machines because the
CPU and FPU are asynchronous, whereas they weren't on the 8086
machines.  IBM did this to allowed their ATs to use cheaper
lower-speed 80287 chips.  Consequently my original Compaq DeskPro
(with 8086 and 8087 at 7.14 MHz) is a better number cruncher than
most AT-type machines (with 80287 running at 4 or 5.33 MHz).
-- 
{hplabs,amdahl,ames}!bnrmtv!perkins         --Henry Perkins

It is better never to have been born.  But who among us has such luck?
One in a million, perhaps.