[Coco] ROM Cart registers

[RETRO Innovations]

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.