Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!bruce!alanf
From: alanf@bruce.OZ (Alan Grant Finlay)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Does 80287-8 will work with 80286-12
Message-ID: <2044@bruce.OZ>
Date: 14 Apr 90 08:16:58 GMT
References: <5975@shamu.WV.TEK.COM>
Organization: Monash Uni. Computer Science, Australia
Lines: 23

In article <5975@shamu.WV.TEK.COM>, majid@shamu.WV.TEK.COM (Majid Hajari) writes:
> I have a Compaq286 (12mhz) nd trying to get a 80287 math co-processor
> for it. Do I have to buy a 80287-12 or can I buy 80287-8 or 80287-10?
>

This question keeps cropping up and I have heard various suggestions such as
"it is safe to use a co-processor rated at 2/3 the speed of the processor". 
I am not familiar with chip manufacturing myself but my intuition tells me the
situation is as follows:
	1. Co-processor chip masks are designed to allow a yield of let us say
	   95%.  In other words (ignoring other fabrication faults) after
	   manufacture the chips are tested and the 5% which are too slow are
	   discarded.
	2. In order to achieve this yield the chips probably on average can
	   run quite a deal faster than the specs.
If you buy a chip at a lower speed rating than required then depending upon
how far you are pushing your luck you may get what you need or you may have
wasted your money.  If you work out the odds it probably pays to take a
risk at about 2/3 the required speed.  If you don't like to gamble (at least
when you can avoid it) then don't - buy the correctly rated chip.

The real situation is probably more complex than this and as I said this is
all based on intuition not facts - please correct me if I am wrong.