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The first 6502 was the 6502 revision A, which famously does not include the ROR instruction (this is sometimes described as a hardware bug, but that's apparently a myth).

Then, some time later, there's a 6502 revision D. This has the ROR instruction, and is the same revision implemented by the Visual6502.

So this naming implies there ought to exist revisions B and C, which presumably were done to fix other problems, or add other features, but I can't seem to find much information about this.

So the question is, what are 6502 revisions B and C, and what are their differences from revision A?

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    Or B and C were planned, even designed and laid out, but never green lighted for production. But I have no insight into the real situation.
    – Jon Custer
    Aug 13, 2023 at 16:10
  • @JonCuster sounds quite plausible. After all, doing a schematic is usually made in stages of layout and check with version numbers incremented inbetween.
    – Raffzahn
    Aug 13, 2023 at 22:23
  • At the risk of igniting an possible flame-war, the wiki page, see en.wikipedia.org/wiki/MOS_Technology_6502, states (my emphasis): The original version of the processor had no rotate right (ROR) capability, so the instruction was omitted from the original documentation. Assuming wiki is accurate, then if it's in neither the chip nor any doco for the chip, then it cannot really be considered a bug. I don't consider it a bug for the x86 family to not have an instruction for polynomial expansion using a table of co-efficients (I believe the VAX CPU had this beast). ...
    – paxdiablo
    Aug 14, 2023 at 0:49
  • ... That's because Intel in no way documented this as a possibility. This even covers violation of the "principle of least astonishment". If it's undocumented, then any behaviour is allowed.
    – paxdiablo
    Aug 14, 2023 at 0:50
  • And, looking at the wiki page, the sequence appears to be 6502, 6502A, 6502B. Or is revision A in the question a different type of moniker? But the wiki doesn't mention 6502D so maybe this is just an off-by-one error.
    – paxdiablo
    Aug 14, 2023 at 0:55

1 Answer 1

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It appears from some comments that the items shown in the bullet list of the final section of this answer bullet list below are probably different from the revisions discussed in the question. On other comment suggestions, I'm leaving that section below to ensure it's made clear.

Hence the likely reason for the missing B and C revisions is simply that they were done without a release to the wild, and only the D revision was considered substantial enough to warrant it.

The business may have already decided that they were going to add ROR so a released revision before that point would have resulted in wasted effort and possible recall.

Granted, that's supposition on my part but only because we haven't yet found a definitive reason.


Final (deprecated) section here:

For what it's worth, the Wikipedia page states:

  • 6502: A 1 MHz chip used in KIM-1 and other single board computers in the mid-1970s.
  • 6502A: A 1.5 MHz chip used in Asteroids Deluxe and at 2 MHz, in the BBC Micro
  • 6502B: Version of the 6502 capable of running at a maximum speed of 3 MHz instead of 2 MHz. The B was used in the Apple III and, clocked at 1.79 MHz, early Atari 8-bit computers.
  • 6502C: The "official" 6502C was a version of the original 6502 able to run at up to 4 MHz. Not to be confused with SALLY, a custom 6502 designed for Atari (and sometimes referred to by them as "6502C") nor with the similarly-named 65C02.

How they relate to your revisions I'm not sure, since the first was the 6502 (not 6502A) and they do not mention a 6502D.

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  • No, does not relate at all. Those A/B/C aren't chip revisions but speed grades of the same chip.
    – Raffzahn
    Aug 14, 2023 at 2:32
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    @Raffzahn: requested CW since it's possibly valuable if only to ensure no-one else makes the same mistake. Or, if preferred, I can just delete this answer if we make that clear in a comment on the question.
    – paxdiablo
    Aug 14, 2023 at 2:46
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    @paxdiablo I'd probably leave the answer here but with a note added to the top.
    – JeremyP
    Aug 14, 2023 at 8:57
  • @Jeremy et al: done...
    – paxdiablo
    Aug 14, 2023 at 9:58
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    @paxdiablo leave it. It is a plausible association. The modified version does a good job in summing up the known (to us) state.
    – Raffzahn
    Aug 14, 2023 at 11:14

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