[Coco] 80 track disk access under RSDOS

Bob theother_bob at yahoo.com
Wed May 5 04:16:38 EDT 2004

--- John Kowalski <sock at axess.com> wrote:
> 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 inter-
> >changeable, 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.

> 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
> 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/

Thanks for the info John. I'm still doing what I consider "ongoing testing" and
seem to have some issues reading from 35 track disks when in 80 track mode, or
maybe I have a typo in my patch program.

FWIW, I'm using HD disks with tape over the HD hole and playing various games
from that disk regularly to see how reliable it's going to be. So far I've had
no data loss (that I can detect) after about two weeks of mobile use.


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