[Coco] high density floppy controller mods

[Mike Pepe]

Hi folks,

I'm about half way through my hacking on a 26-3029 disk controller to 
make it function with both high and low density 3.5" disks.

I've split out the chip select signals and I have an additional 6-bit 
register at $FF41 that I intend to use to switch between high and low 
density mode. Actually only one bit is needed to do this, but wiring up 
the other 5 was trivial. (not sure what I'm going to do with those bits- 
  if anything- at this point)

I'd like to do something to make this mod portable and maybe a 
"standard" mod, especially for nitros9 where this sort of mod is 
actually useful. (assuming it works, of course)

I don't see anything wrong with using $FF41 as a secondary control 
register, but if anyone can think of a reason why it might not be a good 
idea, let me know.

Also I'm working on making a read register so it would be possible to 
query the drive's density select line, just to make sure that the 
inserted disk and the controller mode agree. It would probably be nice 
to also be able to read the status of the disk change bit too.

Assuming this works I'll post the mods to a web page so everyone can 
make their own.

-Mike