> Signed multiplication and division use an internal flag called F1 to keep trac... | Hacker News

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		powerpiglet on April 10, 2023 \| parent \| context \| favorite \| on: Reverse-engineering the division microcode in the ... > Signed multiplication and division use an internal flag called F1 to keep track of the sign. The F1 flag is toggled by microcode through the CF1 (Complement F1) micro-instruction. The F1 flag is implemented with a flip-flop, along with a multiplexer to select the value. It is cleared when a new instruction starts, set by a REP prefix, and toggled by the CF1 micro-instruction. Does this mean that one can run 'REP DIV' and get a negated quotient?

kens on April 10, 2023 [–]

Yes, REP IDIV will negate the quotient. Andrew Jenner discusses that: https://www.reenigne.org/blog/8086-microcode-disassembled/

powerpiglet on April 11, 2023 | [–]

That's fascinating. Thanks for the pointer.

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