Backups [was Re: Is tape dead?]

[Johnny Billquist]

On 2015-09-24 01:15, Antonio Carlini wrote:
> On 21/09/15 14:15, Noel Chiappa wrote:
>>      > From: tony duell
>>
>>      > In some cases it should be possible to write a machine code
>> program
>>      > that executes on 2 processors with wildly different instruciton
>> sets.
>>
>> I have this bit set that I was told (or something, the memory is _very_
>> vague) that early versions of the KL-10 had this hack; the root block
>> on the
>> disk was the boot block both the PDP-10 and the PDP-11 front end
>> machine, and
>> the first instruction or two was very cleverly construced and sent the
>> two
>> machines different ways. Alas, I looked in the front-end PDP-11 code
>> (in the
>> KLDCP; directory) and saw no signs of this, so maybe it was an urban
>> legend?
>>
>>
>
> I can't find a definitive reference right now, but I *think* that the
> ODS-1 disk format
> was first used on the PDP-11 and then later used in early versions of
> VMS. I *think*
> that it was arranged such that a PDP-11 booting and a VMS system booting
> could
> be done from the same disk by arranging for each to interpret the boot
> block in
> a way that each was happy with.

I think that is incorrect, since early VMS didn't havea  boot block. The 
VAX-11/780 was always booted from the PDP-11, and it started with VMB. 
VMB was gotten from the FE, and VMB in turn understood the file system.

It wasn't until the VAX-11/750 that DEC did a VAX that used boot blocks. 
And then, of course, the boot block is just the first block(s) on the 
disk. Don't matter what file system you might have...

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol