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Did you know about the undocumented POP CS instruction? What would you possibly use it for?
It isn't as simple as you might think. The POP CS instruction was included with the INTEL 8088/8086 processor.
If we look at the opcode makeup, we see that it fits between POP ES and POP SS:
Opcode Bit Pattern of POP segreg = 0000nn111 where:
Instruction nn opcode
POP ES 00 07h
POP CS 01 0Fh
POP SS 10 17h
POP DS 11 1Fh
Maybe INTEL left it in to save microcode chip logic. Unknown. Anyway, we need to look at the opcode of POP CS, which is 0Fh. In the newer processors, INTEL used this opcode to denote a new set of opcodes. So if you try to use POP CS on a 386 or higher, you will get an "unknown instruction" fault and probably crash the machine.
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