[Coco] Determining OS-9 Floppy Type

[Robert Gault]

Andrew wrote:
><snip>
> Since under OS-9 there are only 35 track SSDD device descriptors and 40 
> track DSDD descriptors (but no 40 track SSDD descriptors), if there was 
> readable OS-9 data on one side of a floppy and the other was DECB 
> formatted (and there were two index holes punched in the floppy jacket), 
> then it could only be a 35 track SSDD floppy side.
 ><snip>

This is not correct. An OS-9 floppy can be almost anything and certainly 
a single sided 40 track disk is possible. It all depends on the 
descriptor used to format the disk.

One thing normally true about Coco OS-9 disks is that they use 256 byte 
sectors. They also follow a typical structure for LSN0 where some of the 
bytes as shown below.

Address     Size      Function
$00          3         # of sectors on the disk
$03          1         # of sectors per track
$10          1         disk format

The format DD.FMT has bit0=sides and bit1=density. Coco disks are double 
density.

There is no way to determine if a disk is an OS-9 disk as OS-9 does not 
place any special ID on the disk. You just have to assume a disk is 
OS-9, try to read it, and if what you see makes sense the disk is OS-9. 
The same comment is true for Disk Basic.