PDP-11 tape question

jim stephens jwsmail at jwsss.com
Tue Jun 23 21:09:14 CDT 2020

We had a 12.5 IPS drive on a Microdata 1600.  Once you gave the "go" 
command to the controller, you could actually drive it with byte / byte 
programmed I/O.  We had to enforce the blocksize on tapes which went 
anywhere else.  But we could write from one byte to an entire tape as a 
single record.  The latter would (at the time we first did it) crash the 
campus 360/50 by the way.

But we enforced 14 bytes as the minimum.

We had a fellow who did his masters project implement a formal library 
to write 100% compatible tapes with different organizations, and all 
labels, etc.  I don't recall any record shorter than 14 bytes if any 
there either.  Of course labels were 80 bytes.

Main comment here is 14 bytes seems okay.

Short story of crashing the 360 MVT system.  We had IBM reels we reused 
which IBM used for patches called DTRs.  they had 50 or 100' max length 
tape.  We had a guy muck up a program to send a file off our system to 
the mainframe, and he forgot to break up the file into records.  So he 
ended up with an entire tape with one big physical record.

what happened was our code didn't  properly terminate, so when the IBM 
job was run with the tape as input it read the entire tape and got a 
tape error.  then printed a spew on the console.  repeat 10 times.

Problem, tape drive and console was on the same channel.  With normal 
size records even up to 64k long the tape didn't busy the channel up for 
very long.

Second part of problem, if you busy out the channel to the console(s) on 
MVT when the buffers fill, the system crashes.  So with the tape 
retrying and spewing messages along with other crap onto the console 
from other operations, cratered the system in about 30 seconds.

Console by the way was a KSR 1052, similar to the 2741 Selectric 
terminals, but faster.

On 6/23/2020 6:57 PM, Chuck Guzis via cctalk 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

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