Re: The ROR bug - seems it wasn't a bug after all.

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.

Now, one can even use another word instead of "bug", the early 6502
worked fine, but it had a missing feature that has been indeed planned
and intended to be present from the start. Pick another word if you
like, but PLEASE don't rewrite history. That video in my humble
opinion is just a brilliant clickbait, but still just another
clickbait.
Frank IZ8DWF