31 Oct
2016
31 Oct
'16
12:40 p.m.
On 10/31/16 2:58 PM, jim stephens wrote:
On 10/30/2016 4:24 PM, Don North wrote:
Big difference between hard disk and floppy.
Floppy the track 0 is generally used for "system level" things like
microcode load or boot block.
DEC varied on hardware (system) and OS and drive(media) as to its use.
On 10/30/2016 5:47 AM, Noel Chiappa wrote:
Track 0 is not used by standard DEC software, block zero of the
device (boot block)
starts at track 1 sector 1. Track 0 is not even accessible thru the
standard drivers.
Applies to both PDP-11 (eg, XXDP, RT11) and PDP-8 (OS8).
Maybe specific software that reads/writes disks in IBM exchange mode
accesses
track 0, but I've never used such s/w and am only guessing
If you cared about
not erasing the drive manufacture's data on sealed
media Winchester and the like you have to avoid any writes to cylinder
0 at all.
From: Don
North <north at alum.mit.edu>
.. the hardware bootstrap reads track 1 sectors
1, 3, 5, 7
Ah, thanks for that. Starting to look at the code, I had missed the
interleave.
So does DEC do anything with track 0, or is it always just empty?
Noel
The drive formatting software could read that cylinder
track 0 for a
defect map. Nothing to stop you from overwriting it, but you would
then need to do a local media certification that is more complicated
than just formatting the drive, and mapping out defective tracks /
sectors.
I never worked with a system that had a controller or software that
could read the defect track, so don't know how that was used. Later
drives with more intelligence in the drive are another matter, but in
those cases, the hiding of the defect data can be a task assigned to
that processor, and don't need magic handling of the addressing.
Every system
that had a MFM drive could format all tracks and even
either enter the printed bad block list or recover it before format.
Most all could discover new bad blocks as well. The RQDX1/2/3 ca with
XXDP software and the controller in the Microvax2000
can as well. THe higher level interfaces like SCSI can if the drive
permits it or its terminated with a ADAPTEC or Xybec SCSI to
MFM or RLL controller. All pre-IDE PCs could as well (WD1002/3/4/5/6
controller with MFM or RLL drive).
Allison
Thanks
Jim