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

Paul Koning paulkoning at comcast.net
Mon Oct 31 09:19:27 CDT 2016

> On Oct 31, 2016, at 9:55 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>> From: Don North
>> Track 0 is not used by standard DEC software
> I wonder why DEC did't use track 0. The thing is small enough (256KB in the
> original single-density) that even 1% is a good chunk to throw away. Does
> anyone know? (I had a look online, but couldn't turn anything up.)
> If I had to _guess_, one possibility would be that track 0 is the innermost
> track, where the media is moving the slowest, and as a result it's more
> error-prone. Another is that IBM used track 0 for something special, and DEC
> tried to conform with that. But those are pure guesses, I would love to know
> for sure.

I don't know either.  But for what it's worth, this odd addressing carries over to the RX50.  Not exactly, though.  Logical block 0 is the first sector on track 1, sectors are 2:1 interleaved, and there's a 3 sector skew from track to track.  The difference here is that track 0 does get used: it holds the last 10 sectors of the logical address space.  In other words, physical track 0 follows physical track 79.


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