27 Dec
2010
27 Dec
'10
12:37 p.m.
On 27 Dec 2010 at 19:19, Tony Duell wrote:
The HP150 series 1 (I assume by that you mean the origianl one with the
9" CRT [1]) used the normal HP9121, etc, floppy drive units which
certainly did this.
[1] I could say 'like the one that's in lots of bits on my bench at the
moment, but that wouldn't help identify it really :-).
the original HP 3.5" drive unit -- the 9121
was designed ot be exactly
compatible with the 82901 5.25" drive unit. The latter was a 35
cylinder 2 head deevce, so 70 physcial tracks. The 3.5" drive
therefore only used 70 physicla tracks too. But HP also had this idea
f having 'spare' tracks so that slightly defective disks would appear
perfect to the user. And this was handled by the drive unit. There's
soem kind of infromation stored o nthe inntermost track whih gives the
track replacemtn indromation, disk usage count, etc. No hP maual that
I've seen has any real details of this.
I wonder if it was universally implemented on HP gear. I've seen HP
150 Series I floppies with clusters marked bad the usual MS-DOS way. You'd think that such a thing would be impossible
with an active
"alternate track" scheme.
AFAIK, the alternate tracks were only assifend when the disk was
formated, and it was totally transpararent to the user. You ended uup
witha 66 (or whatever) track disk, the drive unit took care of actaulyl
gettign the head to the right physical track. If you haf no bad tracks
during formating (which happens most, if not all, of the time), then it
used the outside 66 tracks so everything looks normal.
-tony