26 Dec
2013
26 Dec
'13
2:02 a.m.
> A long shot:
> Some BIOS's INT13h, when using 250K, and particularly 300K, data transfer
> rate will AUTOMATICALLY enable double stepping.
On Wed, 25 Dec 2013, Steven
Hirsch wrote:
I'll file that one away. Thanks for the
heads-up!
. . . be determined is why the ImageDisk conversion fails.
. . .
When I read the working disk with IMD, it complains about "deleted data
marker" on track 39 but continues to read all 80 cylinders.
Randy Cook (the original author of TRS-DOS) used some oddball data address
marks, specifically including using a different data address mark for
directory sectors than for "normal" sectors! In fact, the 179x
controller, as used in model 2 and "doubler"s was incapable of writing
some of the DAMs used on the model 1 (single density, 1771 FDC)! Because
of that, the model 3 hardware COULD NOT write a "correct" model 1 Single
Density disk. That is the main reason why the "doubler"s included an
extra socket in order to have BOTH 179x AND 1771 chips, instead of just
179x.
Therefore, THAT disk format presumably has the DIRectory on track 39.
That is
probably what's throwing the DMK2IMD tool for a loop. It has a couple of
switches that seem pertinent and I'll give that a try tomorrow. Would be
nice to understand in case this comes up again.
--
Grumpy Ol' Fred cisin at xenosoft.com
http://www.xenosoft.com/FPUIB