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With regard to the ROR 'bug', Bill Mensch (who did circuit design on the 6502) relates that it wasn't so much a 'bug' as Chuck Peddle left it off the specificat
by djmips 6y ago
With regard to the ROR 'bug', Bill Mensch (who did circuit design on the 6502) relates that it wasn't so much a 'bug' as Chuck Peddle left it off the specification so the early 6502s were missing ROR as designed. However they found a way to add it soon after the first version was released.
https://youtu.be/sPaAnbkhgAE?t=1049 https://youtu.be/sPaAnbkhgAE?t=1049
With that in mind the early ROR behavior should probably be considered like other illegal opcodes as just a side effect of how the decode worked.
Info on what happened when the 'ROR' instruction was used on an early 6502.
https://www.pagetable.com/?p=406 https://www.pagetable.com/?p=406
It would still be fun to look at the early 6502 and contrast it with the later version to see the changes that were made and how much work was involved.
- krallja 6y agoIt’s very suspicious to me that it was “not specified” but the opcode list has holes exactly where you would expect the ROR instructions to go. I want to see the ALU and decode ROM PLA to figure out the truth about what was taped out. Did a team of dozens really forget to add ROR but included SHR, ASL, and ROL? Or did they make a mistake in the mask?
- kens 6y agoIn any case, if someone gets die photos of a 6502 that's missing the ROR instruction, I can analyze it and explain what's going on.