[Coco] 80 track disk access under RSDOS

[John Kowalski]

At 12:03 AM 01/05/2004 -0700, CoCoList for Color Computer Enthusiasts wrote:
>I was surprised to find that after a disk has been formatted to 360K, the first
>program(s) you save on it (presumably up to 156K worth?) can still be loaded by
>an un-patched CoCo expecting only 35 tracks. I always knew that 35 and 40 track
>disks were basically interchangeable, but for some reason expected an 80 track
>format to be unreadable.
>
>So is all the extra data simply interleaved among the existing tracks or what?
>I was thinking it was actually more densely stored, but this obviously can't be
>the case.
>
>BTW, 720K on a single RSDOS floppy can hold a whole-damn bunch of games (or
>music or whatever)! The first thing I save to the disk is a program named
>"#.BAS" which pokes in the 6ms step-rate, 80 track access and some other
>patches. Then, when I power up the CoCo, simply RUN"#   from the 360K disk.
>
>cheers,
>Bob

The disk format for 35,40 or 80 tracks in RSDOS is identical except for the
fact that more tracks are formatted and assigned in the granule allocation
table.

So, the 68 granules that correspond to tracks 0-34 are compatible with a 35
track DOS, even though the disk itself may have been formatted 80 tracks.

>From what I recall, RSDOS assigns granules in a particular order on blank
disks - starting from track 18, going up to the max, then starting again at
track 0 and going up to track 16.  (Track 17 is the directory track).

Going by that information, the first 34 granules worth of data you save to
an 80 track disk will work perfectly fine on a 35 track system, and
everything will be fine - AS LONG AS THE 35 TRACK DOS DOES NOT WRITE TO THE
DISK.
If it writes (saves, deletes or modifies files), it will neglect to write
the full size of the granule table to the disk - effectively wiping out
allocation for any files from tracks 35-79.

                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/