On Mon, Feb 12, 2024 at 8:31 AM Francesco Messineo <francesco.messineo_at_gmail.com> wrote: > > On Mon, Feb 12, 2024 at 8:12 AM Julian Perry <jp_at_digitaltapestries.com> wrote: > > > > I'm sure I'm very late to the party, but have learned that the infamous ROR bug in 1975 6502's actually was not a bug - just an unimplemented feature. > > Seems like the builder of the Monster 6502,Eric Schlaepfer spoke with Bill Mensch in 1991 on the topic, and Bill confirmed that the ROR instruction was simply not implemented in the silicon at all - and was only added after demand for it became compelling. In a short youtube video he puts a compelling case for this - (including reverse-engineering, die-shots and circut: diagrams and I think is the definitive word on the subject. > > > > it was definitely a bug, no other manufacturer would add an > instruction on the second revision of a CPU. They couldn't fix the > design in time for the planned first release and that was a brilliant > move to not delay the CPU time to market but obviously they intended > to fix the missing instruction. > Both Chuck Peddle and Bill Mensch in their oral history confirmed that > the ROR instruction was planned but something went wrong during the > first masks layout and they better left it out instead of delaying the > 6502 release. and just to add some other facts: $66 $76 $6E $7E opcodes line perfectly into the ASL/LSR/ROL decoding mask and there're almost no holes in that line of opcodes, so they had left just the right holes in the decoding ROM but without planning for it? Give me a break. >Received on 2024-02-12 09:00:27
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