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Hi Ken, I’m interested in the acknowledgements section: > Thanks to David of Usagi Electric for driving the MC14500B analysis project and thanks to John McMast

Hi Ken, I’m interested in the acknowledgements section: > Thanks to David of Usagi Electric for driving the MC14500B analysis project and thanks to John McMaster for decapping the chips and creating the MC14500B images How/where are projects like this initially organized and then driven to completion? I’d like to answer the question “what went wrong with the first batch of 6502’s ROR instruction,” but don’t have the resources to do that entire project myself.

kens 6y ago
"organized" is probably an overstatement for this project :-) David wanted to find out about the internals of the MC14500B, so he got a couple of them on eBay and sent them to John for decapping (https://siliconpr0n.org/archive/doku.php?id=mcmaster:start#commissioning https://siliconpr0n.org/archive/doku.php?id=mcmaster:start#c...). Since I've been interested in this chip for a while, I studied the die photos and figured out how it works and wrote this blog post. The 6502 ROR bug sounds like an interesting puzzle. As far as I know, there's nothing conclusive on it. The last I heard is https://www.pagetable.com/?p=406 https://www.pagetable.com/?p=406
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 ago
  It’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 ago
    In 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.

kens 6y ago

"organized" is probably an overstatement for this project :-) David wanted to find out about the internals of the MC14500B, so he got a couple of them on eBay and sent them to John for decapping (https://siliconpr0n.org/archive/doku.php?id=mcmaster:start#commissioning https://siliconpr0n.org/archive/doku.php?id=mcmaster:start#c...). Since I've been interested in this chip for a while, I studied the die photos and figured out how it works and wrote this blog post. The 6502 ROR bug sounds like an interesting puzzle. As far as I know, there's nothing conclusive on it. The last I heard is https://www.pagetable.com/?p=406 https://www.pagetable.com/?p=406

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 ago
It’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 ago
  In 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.