Ken,

Way back (circa 1988ish timeframe) I remember a digital logic professor giving a little aside on the 8087 and remarking at the time that it (the 8087) used some three value logic circuits (or maybe four value logic). That instead of it being all binary, some parts used base 3 (or 4) to squeeze more onto the chip.

From your microscopic investigations, have you seen any evidence that any part of the chip uses anything other than base 2 logic?

Do you know what other prior systems did for co-processor instructions? The 8086 and 8087 must have been designed together for this approach to work, so presumably there is a reason they didn't choose what other systems did.

It is notable that ARM designed explicit co-processor instructions, allowing for 16 co-processors. They must have taken the 8086/8087 approach into account when doing that.