CPU/FPU running asynchronously \ VOGONS

CPU/FPU running asynchronously

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First post, by sliderider

Posted on 2012-04-23, 10:49

sliderider Offline

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Is this possible? I mean running the CPU at one speed and the FPU at another. I'm looking at building a system around a Harris 286-25 and as far as I can tell there are no FPU's compatible with a 287 that run at that speed. 20mhz is the fastest I've seen. Would you need separate clock crystals for each chip or would the motherboard chipset have to have some sort of divider to keep the speed of the FPU below that of the CPU?

Reply 1 of 2, by DonutKing

Posted on 2012-04-23, 11:40

DonutKing Offline

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Yes, many 286 mainboards ran their co-pro at 1/3 the speed of the motherboard's crystal which is usually twice the CPU speed. eg a 12MHz 286 system would have a 24MHz crystal, and the 287 runs at 8MHz. Some boards apparently did have a seperate crystal for the co-pro but this was apparently not common.

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Reply 2 of 2, by Markk

Posted on 2012-04-23, 12:54

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Yeap. My 2 identical DFI 286 boards, have the option of running a 287 at 8/10/12MHz, which is jumper selectable. But some others require a seperate crystal, equivalent to the fpu speed. And I think some 386 boards work that way ,too.

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