26 Dec
2013
26 Dec
'13
8:33 p.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
IIRC, LDOS on the model 1 used the normal 'deleted data marker' on the
directory track (as it did on the model 3, and indeed model 4 TRS-DOS
6.x, which is LDOS). A mdoel 3 can write a correct model 1 _LDOS_ single
densiy disk.
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 would make sesne. The directoy was naar the middle of the recorded
area, on a track of about half the number of total tracks, on both
TRS-DOS and LDOS. So the nromal model 1 TRS-DOS disk (35 tracks) has the
directory on track 17. IIRC, with LDOS you can specify where you want the
directory when you format the disk, but the defuult is 'halfway along'.
So track 39 would be normal for an 80 track disk.
-tony