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From: sonny@charybdis.harris-atd.com (Bob Davis)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 8087 Coprocessor
Message-ID: <3952@trantor.harris-atd.com>
Date: 11 Jul 90 01:01:31 GMT
References: <169@evtprp0b.UUCP>
Sender: news@trantor.harris-atd.com
Reply-To: sonny@trantor.harris-atd.com (Bob Davis)
Organization: Advanced Technology Dept., Harris ESS, Melbourne, FL
Lines: 39

In article <169@evtprp0b.UUCP> you write:
>I know there has been a similar discussion about 80287's, but I
>need related info on the 8087:
>
>I have an XT with a V20 processor running at 8/4.77Mtz.  What speed
>coprocessor do I need.  If there are different speed 8087s, can I
>use a 4.77Mtz 8087 and set the system speed to 4.77?  What if I run
>the system at 8Mtz with a 4.77Mtz 8087?  Will it work?  Will it
>overheat?
>
>Thanx in advance!

	8087's run synchronous with the CPU. Therefore, if you
want to run at 8 MHz, you need an 8 MHz 8087. When you are running
a 4.77 MHz 8087, there is no possibility at all of having it run
at 8 MHz. It is not merely a matter of the chip overheating: it
simply will not be able to keep up with the fast clock rate. It
will stumble and drop clock pulses and just royally fail. It will
be totally useless. So, if you want to run at 8MHz, you must have
an 8 MHz 8087.
	80387's share with the 8087 this trait of the coprocessor 
clock operating synchronous with the CPU clock.
	80287's, on the other hand, do not HAVE to be operated
synchronous with the 80286 CPU. Most often, in fact, the 80287
is clocked at 2/3 the CPU rate - but this is more convention
than requirement. As I understand it, one may buy whatever rate
80287 one wishes, and if you can find a clock on the motherboard
to clock it with (or you can provide your own), you can run it at 
that rate.

Good luck...


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