I hadn't noticed that characteristic of 8F. Looking at the microcode, the subopc... | Hacker News

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kens on July 17, 2023 | parent | context | favorite | on: Undocumented 8086 instructions, explained by the m...

I hadn't noticed that characteristic of 8F. Looking at the microcode, the subopcode bits are ignored, so I think your analysis is correct.

I agree that it's very weird that PUSH rm and POP rm are arranged completely differently. The obvious place to put POP would be in FF/7, especially since that spot is unused. I can't think of any reason why they wouldn't do that.

peterfirefly on July 17, 2023 | [–]

> Looking at the microcode, the subopcode bits are ignored, so I think your analysis is correct.

That's what Andrew Jenner's emulator code does.

https://github.com/reenigne/reenigne/blob/master/8088/xtce/x...

Line 3049.

rep_lodsb on July 17, 2023 | [–]

The reason is that opcode FE/FF with a memory operand will always read it into the 'M' register, which would be unwanted for POP.

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