[Coco] MC6829 Memory Management Unit

[RETRO Innovations]

On 10/10/2022 5:58 PM, L. Curtis Boyle via Coco wrote:
> (and Jim Brain was looking into a special 2MB RAM upgrade that would include 32 hardware task sets (which exactly matches the maximum number of software task #’s OS-9 Level 2 on the Coco 3 uses), allowing the faster MMU map switching.

CoCoMEM and CoCoMEM Jr both include the 5 bit task register, allowing 32 
"hard" tasks, as opposed to the 32 "soft" tasks that OS-9 implements 
using the 2 hard tasks available in GIME.  It's a small speedup, saving 
4 ldd and 4 std ops (probably 64 cycles or so), but it was easy to add. 
Ed might even be able to add it into GIME-X with little effort.  It 
takes 30*8=240 bytes of RAM to implement.

CoCoMEM and CoCoMEM Jr also extends the "virtual" portion of the address 
to be 14 bits in length, from 6 of the original GIME and 8 of the 
various 2MB expansion options.  That provides a maximum virtual memory 
size of 64*2MB = 128MB.  Theoretically, the size could be extended to 
512MB with 16 bits of address mapping, but the top two bits were 
reserved to support such additional functions like marking a page as 
read-only and generating an optional IRQ/FIRQ on write access to that page.

Jim