[Coco] [Color Computer] Is this likley to work ?

[John R. Hogerhuis]

On Tue, 2005-09-06 at 21:16 +0100, Phill Harvey-Smith wrote:
> Hi,
> 
> I am toying with the idea of developing some sort of expansion RAM for 
> the CoCo 1/2 & Dragons, basically the non-gime machines. I'm just trying 
> to bounce the idea here to see if it is likley to work.

Most of the existing "big memory" upgrades for COCO 1/2 are bank
switching deals.

I suggest adding an MMU as the GIME does, but make a better one than the
GIME has.

You might look around at opencores.org for a soft MMU you can put into
CPLD or FPGA.  Steve Adolph created one for the Remem project (for
TRS-80 Model 100) : http://bitchin100.com/remem_project.htm which he
would share. However our design is for an Intel processor w/ IO space,
you could reuse a lot, but you would need to make significant changes.
Better if you find a design for an 8-bit cpu with memory mapped I/O.

With an MMU you can make multiple "virtual" coco environments that can
be switched between. Say with OS-9 in one environment, and Extended
BASIC, or a cartridge in another. Or just multiple extended basic
environments each with different programs. Etc.

I'd suggest a more granular block size than 8K. We chose 1K. If you're
worried about time to switch maps, you could always create multiple maps
in memory (maybe 8 maps?).

Also, my suggestion is to make blocks of the map markable as read-only,
so that you can simulate ROM. Some ROM cartridges write into their own
address space for speed reasons, as well as copy protection, so you want
to make a good emulation possible. The purpose of being able to emulate
ROM is in case you want to, say, load several cartidges into extended
memory and switch between them. This would be even cooler if you had
flash on board, then you could store all games for the coco1/2
internally.

I guess for operating system you could use NitrOS-9 Level II. Then for
software you could multitask the OS-9 Level I applications, or full
context switch to Extended BASIC or ROM cartridges.

-- John.