[Coco] MC6829 Memory Management Unit

[Walter ZAMBOTTI]

Would that technique allow the video circuitry to access all the 2meg of 
mem as well?

On 11/10/22 11:20, RETRO Innovations via Coco wrote:
> 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
>