[spc476]

The original 8086 had a six byte instruction cache (the 8088 in the original IBM PC had a four byte instruction cache). If you modify an instruction less than six bytes away, it won't be seen by the CPU unless you issue a JMP (or CALL) instruction. It was not normally that big of an issue (just make sure you modify the instruction from far enough away). You can use the fact that the 8086 has a six byte instruction cache and the 8088 a four byte instruction cache to determine which CPU the program is executing on.

These days, I think you would need to 1) have memory pages with code with write permissions, 2) possibly flush the instruction cache and 3) hope no other thread is using said routine. With today's security concerns, 1) will not be likely, 2) possibly requires elevated privileges (I don't recall---I've only really done ring-3 level code on x86) and 3) is probably okay in a single-threaded program.

[saagarjha]

You can get around 1 by having the page be mapped multiple times with different permissions. Modern x86 processors don’t need a cache flush.

[stevekemp]

I remember using the instruction-cache as a way to catch debuggers single-stepping through my code. If you rewrote a near instruction to be a jump-to-self single-steppers would fall victim to it..