OK, so I'm thinking about where gestalt ID could be coming from. Apparently it's not in the ROM since the ROM is identical yet the IDs are different. The ROM must set the gestalt ID based on what hardware it finds on the logic board. So somewhere outside the ROM, on the logic board, there is an actual electronic difference between the TAM and 6500 logic boards.
The gestalt is often varied a bit by hardware configuration. For example, the Q605 and LC475 have the same motherboard (and ROM) but a jumper changes the gestalt between one and the other.
I guess I always thought, assumed even, that the gestalt ID was stored directly in ROM, but this proves that it indeed is in RAM at least for this model. This would explain how a utility like Wish I Were could exist; all it has to do is modify this RAM location.
It could be a bit more difficult than that. The Gestalt may be a register in the memory controller which is accessed during POST by the code in the ROMs. In this case the Gestalt would not be stored in RAM, although the register in the memory controller (or other chip) might be addressable in a fashion similar to RAM.
What would be really cool is to be able to run a debugger and step through just until the gestalt ID is set, then modify it to be the TAM gestalt and see what happens if you resume operation. I bet you'd hear the TAM startup sound on a 6500.
It might be easier to run a debugger and step until the start-up sound is played. The note that address and back up a bit. Now look for a branch statement which compares with the two gestalt values you've identified. Substitute a Jump for that Branch, such that the code always plays the TAM sound. Of course, you'd have to modify the ROMs to do that, but it shouldn't be too hard to find Flash chips which will do the job...
