[Coco] MPI question
robert.gault at worldnet.att.net
Mon Jul 7 17:21:55 EDT 2008
jdaggett at gate.net wrote:
> On 7 Jul 2008 at 13:54, Chuck Youse wrote:
>>Ok, so I know that multi-paks had some sort of problem with the Coco
>>3, requiring a modification to the PAL - something to do with
>>conflicts with the GIME. As far as I can tell, the MPI only uses
>>address FF7F - did it only partially decode and thus overlap with the
>>GIME? Or was there a different problem?
> The later model sof the MPI uses $FF7F. If I remember correctly the original MPI
> uses $FF9F to do slot switch. This conflicts with the GIME's Horizontal Offset
> register. There is another conflict that I am not familiar with.
> The GIME chip knows when and where the MPU addresses anything within the
> 64K memory map of the 6809. The internal registers to the GIME chip are at
> $FF90 to $FFDF. Also the GIME chip mirrors the PIA registers at $FF00 - $FF03
> AND $FF20 - $FF23.
James, I think there is a minor error above. The MPI always used $FF7F
but ghosted to $FF9F. There is also a ghosting problem with some other
third party hardware PAKs.
The ghosting is removed by the MPI modifications.
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