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.

So there's not a second ROM to fill in the other 4 bits?

There is only one ROM IC! Strange huh?

Thanks for asking!

There's no sort of pulse input to get a lower and upper set of 4 bits? Unfortunately, I don't have a datasheet for that chip in front of me....

It sounds like it would be an addressing nightmare, as it would take 2 addresses to store one instruction.

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-Tony
KG4WFX