[Coco] Diagnostic ROM

[Alex Evans]

There is a major shortcoming I should address. The data in ROMS that are
overlaid by the diagnostic pak. For the CoCo3, this is relatively easy to
accomplish. I can copy some code into RAM and switch the ROM mode to 32k
internal. Run the code and switch back. For the CoCo 1/2 and Dragon 32,
this isn't an issue as the program pak doesn't overlay any of the ROMS.
However for the Dragon 64, this is actually an issue. As I understand it
the Dragon 64 has two BASIC ROMs, one in the Usual location that is for
running in 32k RAM mode and another that is copied to upper RAM to allow
for more space in BASIC in all RAM mode. This ROM is (IIRC) mapped from
$C000 to $FFEF and is normally disabled when there is a program pak
inserted.