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Today I saw that the Intel APX-188/186 User's Manual states that you can use a 8087 as coprocessor for the 80188/186 (as the 80187 is only usable with the 186). But I was wondering about the clock frequency. In the figure on page 255 it can be clearly seen that it ran at 6 MHz. And while that's the native clock of the 80188, the 8087 runs at 4 MHz by default and no revision was written. So while I would expect (for example) an Intel N80188-10 and an Intel D8087-1 to run together (both have a 10 MHz clock), I would never use chips with different clocks.

So to the question:
Is Intel there just expecting you to use the correct revisions (I doubt that because, to my knowledge, there was never a 6 MHz version of the 8087)? Or is Intel "overclocking" a 8087 here?
Maybe does the 8087 in fact support that clock and it has never been told?

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  • I don't know the whole answer, but within certain limits it's safe to under-clock, running both parts at 4MHz. (You can't go arbitrarily low unless the part is rated for "static" (stopped clock) operation, but 33% slower should be fine. The original Z80 is only rated for static operation when it encounters a HALT instruction.) Commented Feb 6, 2023 at 22:13

2 Answers 2

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The 8087 datasheet specifies which versions can be used with the 80188/80186. There were three versions of the 8087: the 5MHz 8087, the 8MHz 8087-2, and the 10MHz 8087-1. The recommended NPU for use with the 80188/80186 was the latter.

All variants could function with clocks between 2MHz and their rated speed, so systems could be designed at any of the supported main CPU clock speeds, as long as an equivalent or higher-speed NPU was used. NPU-capable systems had to be designed as such from the outset, notably since some of the 8087 timings were more constrained than the 8086 (see the timings table in the 8087 datasheet).

Because of the way the 8087 and main CPU function together — they both read instructions simultaneously — the 8087 had to run at the same clock speed as the main CPU. The 80287 and later NPUs used a different interface, and it was possible to run 80287 and 80387 NPUs at slower clock speeds than the main CPU.

You might find Everything you always wanted to know about math coprocessors interesting (with a broader perspective than 80188/8087).

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  • Thanks, that helps!
    – juffma
    Commented Feb 5, 2023 at 12:32
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Looking at the mentioned page (3-5) it clearly states that it's a 'Typical System Diagram':

System Overview Drawing

It is not in any way detailed schematic to be built nor suggesting or even listing specific components. It's to show the basic connection/interoperation of CPU/FPU/Bus Controller to help understanding them.

Such overview drawings do not imply any hidden features or secret knowledge. If you want to implement a system, component selection, like picking what 80186 or 8087 speed grade, and full drawings are jobs to be done by you.

The 8087 is available in many speed grades, by Intel up to at least 10 MHz (IIRC), possibly more with second source manufacturers. So pick what suits your need.

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    Thanks so my initial theory of having to pick the right clocks yourself was true after all
    – juffma
    Commented Feb 5, 2023 at 12:17

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