PDP-11 tape question

[Paul Koning]

> On Jun 23, 2020, at 9:57 PM, Chuck Guzis via cctalk <cctalk at classiccmp.org> wrote:
> 
> I've been processing some PDP-11 9 track (800 NRZI) tapes and run across
> something that I don't recognize.
> 
> Every file on the tape consists of a number of 512 byte blocks (okay,
> that's normal) but at the head of each file, there's a short block of 14
> bytes.
> 
> Usually, a short record like this is discarded as "noise" on many
> mainframe tape systems, but here it's consistently present. Here's what
> one of the records looks like:
> 
> 15 34 fe 51 fe 76 01 01 00 00 01 80 10 00
> 
> Doesn't seem like a file name in RAD50 format, so I'm puzzled.
> 
> Inquiring minds want to know...
> 
> Thanks,
> Chuck

As others have answered, that's a DOS file label, used in PDP-11 magtapes since the beginning.  Most PDP-11 operating systems also support ANSI labels, but DOS labels are the most common on RSTS (except for V9 and later backup tapes which are always ANSI).  Bootable tapes are DOS labeled.

I remember that OS/360 documentation stated < 18 bytes would be treated as noise but that's not precisely true.  The accurate description is that a record encountering a read error but < 18 bytes long would be silently discarded as noise rather than being reported as a real block with a read error.  But error free 14 byte blocks can be read just fine.  I wrote an OS/360 utility (now long lost unfortunately) that would read DOS-11 format tapes so we could print large files (our RSTS/E system didn't have a line printer but the 360/44 did).  No problem there.  It used EXCP to allow reading across tape marks, and possibly that made handling the 14 byte records easier but I don't remember that it was required for that purpose.

	paul