6510 and 8500 differences

From: Nicolas Welte (welte_at_chemie.uni-konstanz.de)
Date: 2000-07-27 20:52:00

Nicolas Welte wrote:
> This is what I also remembered, but I think recently I tested this and I
> think I was able to write values to bits 6 and 7 of the DDR in the C64 (no
> idea if it was a 6510 or 8500) and when they were set to output, I could
> also write values to the data register and I could read back all of them.

Since I was not sure of that anymore, I did a few quick tests on a 8500
and a 6510 with a few peek and poke commands. The results are as

- both the 6510 and 8500 have a full featured 8bit data direction
register at $00. 
- bits 6 and 7 of $01 will always return 0 on reads if their
corresponding bits are set to 0 (input) in $00
- bits 6 and 7 of $01 will return 1 for a certain time after writing
this value if set to output via $01
- this time varies between the 6510 and 8500 due to their different
nature (NMOS/HMOS). The 6510 will retain the value not as long as the
8500. This is probably a matter of micro to milliseconds (6510) vs
several seconds (8500). 

My guess is that the unconnected bits on the 8502 and the 8501/7501 will
show a similar behaviour as in the 8500. Maybe there will be even a
difference between 7501 and 8501.

Is someone willing to time out the differences exactly? I'm quite sure
that there will be no sharp values, but HMOS and NMOS chips should be
different enough to use this as an extra method to distinguish HMOS and
NMOS CPUs. Since it is internal to the chip, it should be much more
reliable than the $DE00 method.


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