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

[jim stephens]

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.

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.
Thanks
Jim