[Coco] MC6829 Memory Management Unit

[Dave Philipsen]

> On Oct 11, 2022, at 3:55 AM, gene heskett via Coco <coco at maltedmedia.com> wrote:
> 
> On 10/11/22 00:37, RETRO Innovations via Coco wrote:
>>> On 10/10/2022 10:29 PM, Walter ZAMBOTTI via Coco wrote:
>>> Would that technique allow the video circuitry to access all the 2meg of mem as well?
>> 
>> GIME video uses a "flat" memory model, as I recall, where the data is placed in the physical memory space 0-512kB regardless of task mappings.  So, to extend the GIME to allow 2MB of memory access, you just create a register with 2 bits, and those 2 bits are used as a video "bank" register.  The GIME can only access 512kB at a time, but it can access it anywhere within the 2MB memory address space, with the requirement that memory access cannot span 2 512kB banks.
>> 
>> Jim
>> 
>> 
> Somehow, in running myram set for a 1.5 meg ramdisk, I never ran into that limit. Probably saved by the size limits imposed by the c compiler.
> It got distinctly unhappy with src files exceeding 24k. Even cprep-19 puked over that.
> 
> Cheers, Gene Heskett.
> -- 
> 

That’s because myram accesses memory through the MMU as any other process running under OS9 would. The above mentioned limitations are in reference to the video subsystem of the GIME.