[Coco] I/O address decoding [WAS: co80 and cowprs modules in NitrOS-9]

[Tormod Volden]

On Sun, Jan 20, 2019 at 10:34 PM Gene Heskett  wrote:
> hour to fix that and redo the driver. Killing the IO above $FF7C for
> coco3's memory management, without opening up some lower addresses with
> better address decoding was a money saver I'll never forgive the shack
> for when there is room for 14 more IO slots below the floppy at $FF40,
> forever wasted. But you've read this rant from me in more colorfull
> language many times in the past. what has amazed me is that no one has
> come up with a kit to fix that, taking 11 or 12 pieces of wire wrap wire
> and some trace cuts to install the 74ls138 it would take to fix it.
> Since theres no way to get its output into the mpi, it would take a
> second kit for it, but then even a 16 slot mpi would be practical.
> Shoulda been done in '87.  Now?

I think the idea (maybe already back in '87) is that a cartridge
wanting to use any of these <$FF40 addresses that should have been
free, can actually do that as long as it asserts the /SLENB cartridge
port input line so that PIA0 and PIA1 are not enabled at the same time
as the cartridge.

Of course, this requires that all drivers actually accessing PIA0 and
PIA1 are not abusing the "ghost" addresses but stick to $FF00-FF03 and
$FF20-FF23.

Regards,
Tormod