[Coco] Access to Double Sided Disks with standard Floppy Drives on the CoCo?

[Arthur Flexser]

A problem with the simple 2-poke method of getting double-sided drives is
this:  Disk Basic still maintains a 4-byte table that keeps track of what
track the drive head is on for 4 drives.  Since there are in fact only 2
drives, Disk Basic is unaware that the head position for Drive 2 is always
identical to that for Drive 0 and similarly for Drives 1 and 3.  This
causes the drive head to sometimes bump against the Track 0 stop in trying
to get to a new track, because the system is working with erroneous
information about what track the head was previously on.

Because of a bug in the controller chip's logic, when this happens, both
the current track and (inappropriately) the destination track get set to
zero.  Since the two match, the system thinks the head has reached its
destination and the operation gets performed on Track 0 instead of the
appropriate track.  A typical outcome is that the GAT gets written to Track
0 Sector 2 instead of Track 17 Sector 2, and shortly thereafter, you see a
?FS ERROR from Basic when you try to access a file.

Happens only now and then, but enough to trash a significant number of
diskettes if you're not careful.  (Recovery of the diskette may be possible
by copying the GAT from Track 0 to where it belongs.)

A proper implementation (such as is found in ADOS-3) uses a 2-byte drive
table instead of a 4-byte one.

Art

On Mon, Apr 20, 2015 at 11:57 PM, Bob Devries <devries.bob at gmail.com> wrote:

> Steve,
> there's a drive table in the DECB ROM. It is:
>
> 01 02 04 40 (hexadecimal values here)
>
> Change to:
>
> 01 02 41 42
>
> Using the POKE command
>
> I can't remember the exact address, though. You can POKE the change if you
> have a Coco3 since that works in all-ram mode. It's a different story for
> Coco1 & 2.
>
> Hope that helps
>
> Regards, Bob Devries
> Dalby, QLD, Australia
>
>
>
> On 21/04/2015 1:54 PM, Steve Batson wrote:
>
>>
>> Anyone know of a program that lets you use the other side of a floppy as
>> another CoCo Drive? I used to have custom Disk Basic ROM, but that's long
>> gone when I sold that system. I have data on the other side of some of my
>> disks and don't know how to get to it. These are NOT Flippy Disks so
>> accessing them that way is not going to work.
>>
>>
>>
>>
>>
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