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    Whilst that's true, it doesn't address the specific question of what behaviour these opcodes have on the 8086 or 80286. Commented Jun 7, 2021 at 7:21
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    8086 has no illegal-instruction exception; every byte sequence ran as something. But later CPUs, at least starting with 286, do have a #UD exception. But a few special undocumented opcodes like SALC don't trigger that, even in 32-bit mode on modern x86 CPUs. Commented Apr 21, 2022 at 7:44