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From: coffee@aero.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Serious math-coprocessor on the 80386 (80387 ?)
Message-ID: <11502@aero.ARPA>
Date: Mon, 11-May-87 16:24:07 EDT
Article-I.D.: aero.11502
Posted: Mon May 11 16:24:07 1987
Date-Received: Thu, 14-May-87 05:48:44 EDT
References: <3477@jade.BERKELEY.EDU> <579@neoucom.UUCP>
Reply-To: coffee@aero.UUCP (Peter C. Coffee)
Organization: The Aerospace Corporation, El Segundo, CA
Lines: 36
Keywords: 80287 8087 Floating Point Numeric Co-Processor
Summary: Real clock rate is different for 286 and 287

In article <579@neoucom.UUCP> ...(Bill Mayhew) writes:
>In article <3477@jade.BERKELEY.EDU>, ..(Michel Bruneau) writes:
>> ..the 8086-8087 combination beats easily the 80286-80287 setup...
>
>I was surprised when I...disovered that the AT&T with the V-30...
>performed at about 1.4 times the level of the '286 machine!

There have been a lot of messages on this one, but so far as I know
no one has mentioned an awfully important fact. The 80286 takes the main
clock signal and divides its frequency by two, so that a 16 MHz crystal
drives the 286 at 8 MHz. The 80287, on the other hand, does a divide by
three, and so normally runs at 2/3 the speed of the co-processing 286.
The 8086 and 8087, on the other hand, both use the incoming clock signal
directly. According to Intel documents, there is no other functional
difference between an 8087 and an 80287: they are merely different packages
for the same basic stack machine. I believe this, because way back
when the first AT came out we ran a CPU-intensive benchmark and discovered
that a standard PC ran faster than a "6 MHz" AT by a ratio of almost
exactly 4.77 to 4 (i.e., the effective clock rates of the _numeric_ chips).

The reason that this is important is that the 8087 and 80287 are internally
divided into an execution unit and, if I remember correctly, a control
unit: the first does the work, the second is an appointments secretary that
handles bus interface and such. This means that the FP work can take
place asynchronously; clever assembly programmers can decide for
themselves when they want to synchronize the two chips instead of accepting
the automatic FWAIT instructions generated by (I believe) most FP-supporting
compilers and assemblers. I believe there's at least one add-on tiny-board
that has an 8087 with its own crystal on a board that plugs into
the FP socket to speed up FP work without affecting other aspects of the
machine's behavior.

After normalizing for clock rate (_real_ clock rate) differences, I have
found the 8087 and 80287 to be effectively identical.

							- Peter C.