[Coco] Copying over an IO ERROR

[Darren A]

On Tue, Feb 3, 2015 at 8:34 PM, Arthur Flexser wrote:

> I wonder if anyone has ever written a read-track based CoCo disk zapper for
> recovering damaged data.  I would envision something that would read the
> desired track and use some smart algorithms to try to intelligently guess
> where the sector boundaries lie, allowing the user to modify these in some
> user-friendly fashion (e.g., using the arrow keys to adjust the boundaries
> of a 256-byte region and edit bytes within there) and save a copy of the
> modified sector.
> (I seem to remember hearing, from an author of Spit N Image, that the read
> track command is a little twitchy, so that you don't always get exactly the
> same result if you use it twice in a row.)
>
> One problem is that you can't just do a read track, modify a bit of data,
> and save it back out with a write track.  There are a few byte values that
> the write track command uses for things like "generate a CRC here" that
> mess you up if you happen to have one of these bytes in the data that the
> disk contains.  So saving would have to be by means of sector, not track,
> writes.
>


There is also the problem that the WD17xx floppy controllers leave the
address mark detector on during the entire Read Track operation.  There are
certain bit sequences that will trigger a false AM detection. If any of
these sequences occur within legitimate sector data then all data following
it up until the next address mark will be garbage (clock bits instead of
data).

- Darren