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Interesting side effect of the direction bit and the MOD-REG-R/M byte organization: some instructions can have two different opcodes, and both are legal!
For example, encoding of
add cl, al
could be 00 C1 (if d=0), or 02 C8, if d bit is set to 1.
The possibility of opcode duality issue here applies to all instructions with two register operands.
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