[Coco] SuperIDE and floppy disk

[Darren A]

On Thu, May 9, 2013 at 4:27 PM, Gene Heskett wrote:
> On Thursday 09 May 2013 13:02:06 Luis Antoniosi (CoCoDemus) did opine:
>
>> well this is what is written on cloud-9 site:
>>
>> http://www.frontiernet.net/~mmarlette/Cloud-9/Hardware/MPI%20PAL.html
>>
> Mark, can you explain the theory on this? $FF90 is a (write only?) GIME
> address.  Ahh read is going to return $FF because its write only, which is
> precisely the reason in Nitros9 that the DP=00, offset $90-9F is an image
> of what is in the GIME.
>

An MPI without the CoCo3 mod will activate its data bus buffer outputs
when the CoCo reads from addresses between FF80 and FFBF.  The
modification eliminates that range of addresses from the "readable
hardware" space.  Apparently the buffers in the MPI output a '1' when
nothing is driving the input, and so you get 255 for a PEEK of the
write-only location. With the mod in place, the buffers remain
tri-stated (Z) and you get the usual stale data value of 126.

Darren