[Coco] MC6829 Memory Management Unit

[RETRO Innovations]

On 10/10/2022 9:48 PM, Walter ZAMBOTTI via Coco wrote:
> If the GIME can only access 512K how do memory kits allow upto 2meg? 
> How is this accomplished?

As someone who has implemented, here's the simple answer:

The GIME mapping registers are 6 bits in length, with the upper 2 bits 
not implemented (and I think they read back as 1 all the time).

You make a piece of HW that watches for writes to those mapping 
registers and you save off the upper 2 bits of each write.

Then, when that chunk of memory is accessed (A15:13 matches the register 
number, and the task number matches, it's a CPU portion of the cycle, 
and a few other pre-reqs), you feed those 2 extra bits to a companion 
memory expansion designed to accept those 2 bits.

Tada, 4 times the memory, or 2MB.

There's a bit of hand waving in that answer (you have to take into 
account shared memory, etc.), but the above is the gist of it.

Jim