> On Jun 23, 2020, at 9:57 PM, Chuck Guzis via cctalk <cctalk@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
