majid@shamu.WV.TEK.COM (Majid Hajari) (04/14/90)
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? Thanx in advance. Majid
alanf@bruce.OZ (Alan Grant Finlay) (04/14/90)
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.
news@wrgate.WR.TEK.COM (-Network News daemon) (04/15/90)
Path: copper!michaelk That "rule" is NOT the pushing of speed specifications. The "normal" way of connecting a 287 to a 286 is to have the same clock source tied to BOTH, however: 1. A 286 uses a clock source twice what the chip runs at. 2. A 287 uses a clock source three times what the chip runs at. So, with a 12-Mhz 286 system, the clock source is 24 Mhz. The 286 divides by two internally, yielding the 12 Mhz that it runs at. The 287 divides internally by three, yielding the 8 Mhz it runs at. Thus "the rule". I personally run a 5 Mhz 287 in my 12-Mhz 286 machine, but I doctored the circuitry so that the 287 actually runs at 5 Mhz. And it works just fine thankyou. -- Mike Kersenbrock Tektronix Microprocessor Development Products michaelk@copper.WR.TEK.COM Aloha, Oregon