[TUHS] Booting PDP-11's from RX02's

allison ajp166 at verizon.net
Mon Oct 31 14:40:13 CDT 2016

On 10/31/16 2:58 PM, jim stephens wrote:
> On 10/30/2016 4:24 PM, Don North wrote:
>> On 10/30/2016 5:47 AM, Noel Chiappa wrote:
>>>      > 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
>> 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.
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.

> 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).

> Thanks
> Jim

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