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I've got a motherboard with a Am386/DX processor.

I've also got an Intel 387 coprocessor. Is it possible to use the Intel 387 that I've got together with the Am386 or do I need a coprocessor from AMD?

Also, the Intel coprocessor physically fits in the slot for the Am387 but it is a bit smaller which causes a row of socket pinholes to be visible at each side of the coprocessor.

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The Intel 387 should work fine with an AMD 386DX; the latter was a direct clone of the Intel 386. The extra rows of pins are perfectly normal — see this photo for an example. (The extra pins were used for Weitek 3167 co-processors.)

I'm not sure AMD ever produced their own 387; various on-line collections document the AMD 287, but not the 387. See here or here for example. AMD isn't listed in CPU World's list of 387 manufacturers...

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AMD 80386 chips are die-identical to Intel's, as AMD cloned the Intel 386. So, putting an Intel 80387 (or ULSI 80387 or IIT-387) will do fine, as long as their speed is equal or faster than the main CPU..

The row of socket pinholes is, efectively, for the less standard Weitek 3167 coprocessor, which was not binary compatible with the 387.

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    AMD cloned the 386 but they didn't have a license for it (unlike the 8086 and 286). Intel eventually sued AMD over the cloning... – Stephen Kitt Apr 30 '16 at 16:55
  • Updating answer... – mcleod_ideafix Apr 30 '16 at 17:00
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I confirmed this worked as expected, using Am386/DX-40 + Intel 387 co-processor. The trick with the Am386 is having separate clock for CPU/co-processor, as the former runs at 40 MHz and the latter at 33 MHz. The UMC-386 mainboard supports this configuration fine.

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    Asynchronous operation for x86 FPUs is pretty normal. Its probably worth pointing out though that Intel FPUs of that era are well known for running just fine above their rated limits (Motorola ones as well). You could probably get away with running that 33MHz 387 at 40MHz without any problems aside from a bit of extra heat and power consumption. – mnem Jul 31 at 0:24

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