Apple II Plus 6502 vs. 6502a?

Discussion in 'Apple Collectors' started by tevion5, Jan 19, 2014.

  1. tevion5 macrumors 68000

    tevion5

    Joined:
    Jul 12, 2011
    Location:
    Ireland
    #1
    I've noticed on the system requirement for a very cool looking Ethernet card for the Apple II, it says that it supports the Apple II plus but only if it has "6502a" or higher.

    I understand the Plus has just a regular 6502 processor in it. Therefore was the 6502a an upgrade or expansion card for the II plus, or did later models come with a different processor or something?

    Ref: http://a2retrosystems.com/support.htm#why
     
  2. tdiaz macrumors 6502

    Joined:
    Feb 7, 2006
    #2
    The 6502 is a 1MHz design, and the 6502A is designed for 2MHz, is also more prevalent.

    .. and with that, the unique characteristics of the II design come into play.

    As a programmer, if you are so anally relying on timing to accomplish stuff and your code does not run on something, it's probably because there's a 6502, not A, in there. Or the other way around.

    Mostek produced the 6502A, and it was intended to be 100% compatible with the MOS 6502, though it's designed around a 2MHz clock. Think Apple ///.

    From there on out, further 6502x derivatives were built on that.
     
  3. tyche macrumors 6502

    Joined:
    Jul 30, 2010
    #3
    I believe it's possible to put in 6502A in ][ or ][+ only some software that relies on specific timing would fail. There were accelerators for the pre-//e line that could go ~4MHz but had a fallback key on boot up to run at 1MHz to keep it compatible.

    Maybe the card is compatible with ]['s with accelerator chips as well hence the > 2MHz mention.
     
  4. tdiaz macrumors 6502

    Joined:
    Feb 7, 2006
    #4
    Yes, putting a 6502A in the oldest II's isn't an issue. There are -far more- 6502A's out there, and any compatibility is going to be built on the highest percentage of the marketshare.

    But the Uthernet specs are written because it's been established that it -does not- function without the 6502A.
     
  5. tevion5 thread starter macrumors 68000

    tevion5

    Joined:
    Jul 12, 2011
    Location:
    Ireland
    #5
    Ahh so a run of the mill II plus will not have the "a"?
     
  6. tdiaz macrumors 6502

    Joined:
    Feb 7, 2006
    #6
    It's possible, I've seen them both ways. "Check first" and get one at the same time, if needed..
     

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