[Coco] CoCo Progression...

[James Dessart]

On Mon, 27 Sep 2004, David Gacke wrote:

> The new CPU would be backward compatible with the old CPU. Also, it's
> much easier to implement some of the new features in the list of items
> you mentioned if they are attached to a new cpu.
> 
> A possible idea would be to leave the addresses from 0-ffff as is for
> legacy support, including the bank switching, gime register maps, etc.
> And build the new features in above that address.
> 
> Also, it's easy to emulate a CPU, it's much harder to emulate the
> idiosyncrasies (timing, etc) of a graphics chip. 

I think that's essentially what they did with that C64 processor
replacement. They created a new CPU that emulates the 6502, added some
features, used a high clock for the processor, and there you go, easy to
add onto a C64, using the same peripherals and software.

Any modern CPU architecture could be used to emulate the 6809, in fact,
you could probably get decent performance out of a cheap PIC or AVR, and
have modern compiler support for new development. Somewhat like the 6309
software out there. So you'd still have the GIME, but you could write
software that uses the native instruction set of the procesor used.

I can definitely see this working well, whereas on the other hand, I'm not
sure what making a new GIME would get us, except headaches for the
developer. The 6809 is well documented, the GIME is not. It's much easier
to work with well documented hardware.

James