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 follows: - 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. Nicolas - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
Archive generated by hypermail 2.1.1.