[Coco] 32kB carts on Coco 3

Robert Gault robert.gault at att.net
Tue Dec 29 20:23:19 EST 2015

RETRO Innovations wrote:
> Does anyone have a link to how it works?
> I read that it is the work of the GIME, and I assume it's the bank registers for
> task 0 and 1 (1 and 2?) that make it work, but I'm trying to understand what
> shows up on the cart port...
> Specifically, how does the upper 16kB of a 32kB cart get accessed from the cart
> port?  Is the 2nd 16kB mapped to $8000-$bfff?  If not, then where?
> Jim


If you look at the bits for $FF90 either in the Service manual or the Spectra 
Associates series, you will see:
$FF90 - initialization register 0
BIT    meaning     value
  7      coco       1=Coco1&2 compatible
  6      M/P        1=MMU enabled
  5      IEN        1=chip IRQ output enabled
  4      FEN        1=chip FIRQ  "      "
  3      MC3        1=DRAM at $xFExx is constant
  2      MC2        1=standard SCS
  1      MC1        ROM map control
  0      MC0        ROM map control

MC1  MC0          Mapping
  0    x            16k Internal, 16k External
  1    0            32k Internal
  1    1            32k External

Based on the above, the external 32k cart should be addressed from $0000-$FFBF 
or perhaps $0000-$FFEF. I'd think the former.


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