[Coco] MC6829 Memory Management Unit

[Jeff Teunissen]

On Mon, Oct 10, 2022 at 12:19 AM Walter ZAMBOTTI via Coco
<coco at maltedmedia.com> wrote:
>
>  From what I can tell from the my brief reading of the manual the device
> works in a similar way to the GIME MMU except
>
> it supports up to 2 megabytes,  2K block sizes and 4 tasks.
>
>            Max Mem  Block Size Tasks
>
> GIME       512MB     8K           2
>
> 6829     2048MB     2K           4
>
>
> The smaller 2K block size makes that quite attractive.
>
> However since OS9 is written for 8x8k pages the MMU register structure
> would need to be enlarged to cater for 32x2k, if it is not already reserved.

OS-9 Level II was not only written for 8x8K pages, it ran on many
machines -- most of which had 2K (like GIMIX) or 4K (like CMS) page
sizes. The CoCo is the only oddball machine using the giant 8K pages.

And while those machines above did not use the 6829, they implemented
a lot of the interfaces introduced by it -- such as the "Fuse
register".

On the Discord server over 6 months, I and a few others put together a
new MMU design (addressing 4MB in 2K blocks, with 256 'tasks' and
protected memory) influenced by the 6829, as well as the GIMIX CPU III
and CMS 9829 CPU boards, to be implemented in FPGA. And when the FPGA
hobbyist chip drought is over, we hope to get the designs implemented
in a real device.