[Coco] ROM Cart registers

[Darren A]

On Sun, Jan 31, 2016 at 8:46 PM, RETRO Innovations wrote:

> Darren,
>
> Given Mark's explanation for how the sIDE strobes SLENB for SCS
> addresses(I assume only those in the 16 bytes that it requires in the
> memory map), does CocoSDC do the same for $ff4x addresses so that the SDC
> will be available at all times, no matter the state of $ff7f on an
> MPI-based system?
>
> Wondering, trying to determine how many Coco peripherals do this and how
> many addresses in the address space are left for other things.
>
>

The CoCo SDC only responds when SCS* is asserted. This is necessary to
allow the CoCo SDC and a floppy controller to co-exist in an MPI since they
both use the same addresses. The software (SDC-DOS and the Nitros9 driver
for SDC) handle selecting and restoring the active MPI slot.

I am surprised that the SuperIDE would assert SLENB* for addresses that are
in the SCS range. It prevents any other device from using those addresses
in a co-operative fashion as the MPI allows. I can't think of a reason why
that would be needed except to ensure that the device is always active
without having to manage MPI slot selection in software.

If Mark has a different reason, I would be curious to know what it is.

- Darren