[Coco] 3.5" Floppies on my Coco 3!

[Arthur Flexser]

On Sat, Apr 6, 2013 at 1:41 PM, Luis Antoniosi (CoCoDemus) <
retrocanada76 at gmail.com> wrote:


> on my coco my experience is: format first on your PC as 720K then format on
> your coco either 35 tracks DSKINI or OS9 80 tracks double side as well. I
> use both sides with disk basic as drive 0 and drive and it just work fine.l
>
> One note: saving BASIC files seems to corrupt the disk from time to time
> but this seems to be an old coco issue right ?
>
>

I think you must be patching Basic to use both sides
simply by 2 pokes to the drive mask table.  This indeed causes occasional
corruption of the disk.  The problem is that Disk Basic isn't aware that
when the head moves to a certain track on Drive 0, it is also moving to the
same track on Drive 2 (the back of Drive 0).  So Basic's table of where the
head position is for each drive gets messed up.

Normally, this would merely cause some extra seeking.  But there is also a
problem in the design of the controller chip that causes the disk
corruption you're talking about, usually seen as a ?FS ERROR (bad file
structure) when you try to access the disk.
The problem is that when the head inappropriately hits the Track 0 stop
because Basic has bad data as to head position during a seek, two things
happen.  One is appropriate:  the FDC register for head position gets
zeroed.  The other one is brain damaged:  the FDC register for the
destination track is also zeroed.  So, when Basic is looking to write the
file allocation table to to Track 17, Sector 2 and the confusedly wandering
head hits the Track 0 stop, the FDC now finds the head position and
destination registers are in agreement, with both equal to zero, so,
with the head supposedly having reached its destination, the FAT gets
written to Track 0, Sector 2 instead of Track 17, Sector 2 and the disk is
now seriously messed up.

ADOS-3 fixes this problem, incidentally, if you want to banish the dreaded
?FS ERROR from your CoCo.

Art