[Dr_Jefyll]

> when an ESCAPE instruction is encountered, the 8086 processor starts executing the instruction, even though it is intended for the 8087. The 8086 computes the memory address that the instruction references and reads that memory address, but ignores the result.

Intriguingly, the 65C02 also shows this behaviour. Several of the C02's undefined opcodes cause an address to be computed and a fetch to occur (whose data is then discarded). The spurious address computation wasn't intended as a feature, but an opportunity nevertheless exists. My 1988 KK Computer uses it as part of a co-processor scheme in which microcoded external logic gives the 65C02 six new registers and 44 new instructions, including ITC NEXT (as used by Forth).

Repeatedly posted to HN. :-) If anyone's interested and hasn't yet seen it... https://news.ycombinator.com/item?id=26432154