How does the ALU handle the fact that INC/DEC do not update the carry flag? Diff... | Hacker News

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		bonzini on Feb 12, 2023 \| parent \| context \| favorite \| on: Silicon reverse-engineering: the Intel 8086 proces... How does the ALU handle the fact that INC/DEC do not update the carry flag? Different control signals depending on the ALU operation being performed? Also, does LOOP use a different ALU operation to decrement CX, since it doesn't update the flags at all?

kens on Feb 12, 2023 [–]

The INC/DEC instructions block the control signal that updates the carry flag. This happens in the carry flag circuit.(See footnote 10.)

For LOOP, the microcode doesn't have the F bit set, so the flags aren't updated from the ALU. Same with NOT.

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