Hello, I am familiar with the programming of the 6502; However, my latest project is reverse engineering a 6503 application. I have the pin out for the 6503 and thought the project was going to be easy, but.......
The application uses a Harris HM6543-5 Prom which only has a nibble (4 bits) of data which is tied into the lowest 4 bits of the 6502 data bus. The upper 4 bits are not connected to the 6543. They merely go to a PIA (along with the lower 4-bits) for I/O processing.
OK, my question. On power up, the hardware addressing is set to give FFFEh to enable the 6543; However, how can valid 4 bit commands be passed to the 6502 from the 6543? I am used to the 8-bit command set of the 6502.