Path: utzoo!attcan!uunet!snorkelwacker!spdcc!esegue!johnl
From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Speed of coprocessor vs. speed of main processor
Message-ID: <1990Jun11.000613.6589@esegue.segue.boston.ma.us>
Date: 11 Jun 90 00:06:13 GMT
References: <3076@crash.cts.com> <459@accucx.cc.ruu.nl>
Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine)
Organization: Segue Software, Cambridge MA
Lines: 20

You can clock the 287 in two different ways.  The default is to use the same
clock as the 286.  The 286 divides the external clock rate by 2, and the 287
divides it by 3, so the 287 runs at an effective clock rate 2/3 of the 286.
Alternatively, you can provide a dedicated clock for the 287, in which case
the 287 runs at the full rate of its own clock.  There is an input on the 287
called CKM which tells it which way it's being run.  This is all from Intel's
286 hardware reference.

The 287 is quite slow compared to a 286: a 286 fixed point multiply takes 21
cycles, but a 287 floating multiply takes about 100.  Intel wanted to make it
possible to attach a faster 287 to a 286 in order to have a more balanced
system.

Different PC clones clock their 287s differently.  The IBM PS/2 series clock
the 287 directly so that the 286 and 287 both run at 10 MHz.

-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650
johnl@esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl
Marlon Brando and Doris Day were born on the same day.