[Coco] MPI

[Robert Gault]

Chester A Patterson wrote:
><snip>
> So either OS9 or the Burke and Burke software play with the CTS to get
> to the appropriate slot as required. Does this make sense? Comments?
> Thanks.
> 

Not having a Burke and Burke hard drive interface, I can't be sure how 
it works. The Cloud-9 and Ken-Ton interfaces don't access the hard drive 
via $FF40-$FF4B so it does not matter what the MPI slot select switch is 
set to for hard drive access. That probably is what the Burke and Burke 
does as well.

The real issue is where the ROM for DOS is located. That must run when 
the Coco is turned on. From your description, you boot from the ROM in 
the Burke and Burke in slot #3. You might have a special OS-9 module 
from Burke and Burke which alters the MPI to slot #4 for floppy I/O but 
leaves the MPI set for a ROM at slot #3. That is possible because the 
low nibble at $FF7F controls the MPI slot for $FF40-$FF5F while the high 
nibble controls the slot for the ROM.

When you changed to the Cloud-9 unit, you also seem to have a ROM in the 
hard drive interface. However, you should not expect it to behave as the 
B&B unit did. NitrOS-9 software does not switch the MPI byte for floppy 
access which is why you need to do that manually.

If your hardware permits, you may now find the preferred arraignment 
will be to have the ROM in the floppy controller with the MPI set to the 
slot where the floppy controller is located. You will be able to access 
the hard drive without changing the MPI slot. If you are using HDBDOS, 
you will also be able to boot NitrOS-9 from the hard drive keeping the 
MPI set to the floppy slot.

Mark most likely has already told you what the optimal settings are for 
the hardware you purchased. If they don't match what I suggest, you 
should certainly use what Cloud-9 has suggested.