The Prodigy #2 - identify the ROM type

Once we have some ROM code to work with it is time to take a look at it in action. I do that by writing a minimal skeleton driver for the MAME emulator:

The driver is just assuming that the CPU is a 6502 and that the 2KB ROM is mapped at he highest possible adresses as the reset vector must be in the ROM area. No RAM or other devices are assumed at this point as they can be easily added later when we know more.

This allows me to open up the ROM:s in the MAME debugger and start looking around. First I look at the reset vector to see where the entry point is:

Disappointingly the reset vector at adress 0xFFFC and 0xFFFD is very wrong! :(

It points at 0x0111 which is a quite unlikely remapping of the ROM. Either ROM is trashed, I didn't read it correctly or both. Even if it is mapped somewhere else the reset vector must be mapped to these addresses at power up reset. We can check this by review the disassembly view

The first observation scrolling through the code is that is seems ok, no strange trashed areas or so.

Now we look for absolute jumps which to some degree of certainty will reveal some addresses of ROM code. It does, but now the second important finding is that the jumps are more than 2KB apart, eg JSR $6580 on address $FB7A and JMP $749D on address $FB7F.

This tells us two things, the ROM executes in the $6000 range upwards probably towards the $7FFF mark which should be an 8KB ROM! I errornously assumed that 8KB didn't fit in a 24 pin DIP package but I vaguely remembered a device called 2364 and indeed a search away I found a data sheet and an instructable how to make some 2364-27xx adapters.

So I think I'll go for the 'cheap-ass' adapter and wish the Prodigy ROM is not clocked, hiya! Next-->