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Retrocomputing

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    How do we know the ‘green’ NOPs are by design? Is there a design document confirming them as such? Commented Sep 28, 2022 at 19:17
  • @user3840170 Well, is there one that tells otherwise? Beside, looking at the decode ROM does show that there are default entries that let them terminate that way - as usual, code is documentation :))= Commented Sep 28, 2022 at 19:23
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    Opcodes of the form xxx0 mm01, including 1000 1001, all behave identically prior to the cleanup cycle, save for the fact that 1000 mm01 turns the last post-decode read, if any are done into a write with the accumulator on the bus. Instructions of the form xxx0 1001, including 1000 1001, all advance the program counter during the operand fetch, and then proceed to the cleanup/start next operation state without doing any post-decode reads, leaving the value that was fetched during the cycle after the opcode fetch in the temporary register. Commented Sep 28, 2022 at 20:41
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    @Raffzahn, it might improve your answer to include a relevant snippet from the microcode, showing STA immediate being "dummied out". Commented Sep 29, 2022 at 11:40
  • I suspect the change in how the 65C02 handles undefined opcodes was motivated by the fact that it's CMOS. In NMOS, if something will pull down a signal in all cases where it needs to be low, and nothing will pull it down in cases where it needs to be high, one can ignore cases where the value doesn't matter. In CMOS, one must identify all cases where nothing will pull down a signal and ensure that it gets pulled up in those cases, even it would have been acceptable for the signal to be pulled down instead. Commented Nov 5, 2023 at 18:04