Re: Differences between 8501R1 and 8501R4

From: Gerrit Heitsch <*gerrit_at_laosinh.s.bawue.de*> Date: Sat, 7 Dec 2019 15:44:07 +0100 Message-ID: <de5cc32a-4ec5-140f-89e3-ca85ba1f8b06_at_laosinh.s.bawue.de> · gerrit_at_laosinh.s.bawue.de

On 12/7/19 3:23 PM, smf wrote:
> Interesting, my poor addled brain thought the extra two bits couldn't be
> read back.
> 
> The 6510-1 & 6510-2 do actually have all 8 data bits.

Same for the 8500. Otherwise it would be very easy to determine the type 
of CPU in your C64. Some people proposed using the data retention (*) of 
the unused bits to measure CPU temp or determine CPU type since that 
differs between 6510 and 8500. Doesn't look like it can be done though.

(*) Write out a '1', switch to input, read until you no longer get a '1'.

> I still think it's strange they couldn't fit the extra bits in the 8500,
> when they were able to on the 8501 & 8502

Bit 5 is missing on the 8501, I think you mixed up numbers here.

  Gerrit