[Coco] MPI question

[Robert Gault]

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?
>>
>>C.
> 
> |--------------------
> 
> 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
> 

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.