The Prodigy #3 - make a ROM adapter

Well, the cheap ass adapter to get the R2912 ROM read involved a manual step to reassemble the ROM in which case I may have done it wrong and hunting ghost bugs afterwards. In any case when I saw the image to the left below I decided to go for the correct adapter and read out the R2912 as a 2764.

This worked out beautifully and we can now establish that the Rockwell R2912 is pin compatible with a 2364 mask programmable ROM. I wonder about the R2907 still but that's not our problem at the moment. :)

In order to fit the new ROM file into the MAME driver we need the following adjustments to the code:

We start at 0xe000 as rom now is 8KB or 0x2000 bytes in hex so the reset vector is at 0xFFFC. We also need to enlarge the size of "roms" region to 0x2000 and update the size of the rom to 0x2000. The offset where we load the rom within the "roms" section is still 0x0000 though.

Firing up MAME again we see that the reset vector now seems to be 0x60B7 which is within the identified actual ROM area. Since we mapped the "roms" at the top we end up in nowhere:

By setting the PC to 0xE0B7 we can observe and step though the actual code as long as it is position independent:

Now we end up in real code and it does stuff that will tell us more about the system. We can see that the code store things at 0xAB and 0x2003. The latter look very much like a device address which usually is decoded at a nice even address boundary such as 0x2000. This means the RAM is mapped at 0x0000.

So I will map some RAM at 0x0000 and the VIA at 0x2000 next, stay tuned Next -->