[Coco] hybrid CoCo-CP/M disk

[Willard Goosey]

In my continuing work towards getting the CoCo 3 to read & write
TRS-80 Model 4 Montezuma Micro CP/M disks, today I created a hybrid
disk with both filesystems.  I formatted the disk under CP/M and then
used DED under CoCo BASIC to build the directory information by hand.

I can get away with this for the same reason I'll be able to make the
CoCo handle the CP/M disks.  Both formats are SSDD, 256 byte sector,
18 sectors/track.  Disk BASIC uses 35 tracks, CP/M uses 40.

Tracks 0-16 are for CP/M.
Tracks 17-34 are for CoCo Disk BASIC.
Tracks 35-39 are CP/M.

>From CP/M's point of view, there is a 3 extent long file occupying
groups 0x21-0x4a.  On the CoCo side, granules 0-0x21 are allocated.

There is some wasted space, as group 0x4a starts on track 17, sector
17, and continues for six sectors into track 18, but that's not my
fault, 8 sector groups just don't come out even on 18 sector/track
disks.

This is a very slow CoCo disk, as CP/M formats with a 1-1 interleave
and then uses a sector translation table, rather than the CoCo's
native interleave of 4.

One interesting thing is that on the CoCo directory, due to the
DOS-like FAT, I didn't actually have to create a file entry in the
directory, just set up a granule chain that marked the CP/M
"partition" as used.

I'm sending this to the COCO list and to some CP/M people, which is
why I'm explaining about both file systems.

Willard
-- 
Willard Goosey  goosey at sdc.org
Socorro, New Mexico, USA
"I've never been to Contempt!  Isn't that somewhere in New Mexico?"
   --- Yacko