[TUHS] Determining what was on a tape back in the day

[Paul Winalski]

On 11/19/17, Clem cole <clemc at ccc.com> wrote:
> Noel is correct. DECtape (aka linctape) was a block oriented technology.
> Traditional 1/2” mag tape which had 5, 7 or 9 tracks is a stream oriented
> technology.

DECtape/LINCtape was unusual as tape technology goes.  It was
block-oriented, meaning that data were formatted as a sequence of
fixed-length blocks (256 or 512 bytes, IIRC).  It was also
block-replaceable--you could seek (fast-forward or rewind) to a
particular block and rewrite it without destroying the data following
the block.  DECtape also had a timing track and could be read at
variable speed.  I heard one story from a PDP-10 operator who had
critical data on a DECtape, but the motor on the drive broke down.  He
was able to use a pencil to wind the tape, and because of the timing
track the drive was able to read the tape successfully.

> DECtape was used liked disk in the late 60s.  It was comparably cheap and
> very reliable.  The joke was you could unroll it and run over it with a car
> and then roll it back up and it would still work.

Because of the block-oriented and block replacement feature, you could
treat a DECtape as if it were a high-capacity disk with a horribly
long seek time.

> Magtape was traditional back up scheme.  Cost per bit was low and good for
> archiving.  But you could only add to the end of a tape.  You can do funny
> things like change recording techniques between files (not recommended as it
> can confuse many tape controllers but is technically allowed and was done).

I never used 5-track 1/2" magtape, and had only a brief acquaintance
with 7-track tape.  9-track tape at 800 bits/inch used non-return to
zero inverted (NRZI) encoding, 1600 bpi using phase encoding (PE), and
6250 bpi using group coded recording (GCR).  Data were written as
variable-length blocks, with a special tape mark indicating
end-of-file.  The first block of a file was typically a file label.
PE- and GCR-encoded tapes had a special "PE burst" or "GCR burst"
record as the first block on the tape.  This allowed the tape drive to
determine automatically the encoding for the tape.

9-track magtape could have some peculiar quirks.  VMS Engineering at
DEC once received a 6250 bpi tape from a customer containing a crash
dump, but when they tried to read it, the tape had a completely
different file on it.  The customer verified that they had sent the
correct tape.  The VMS engineer mounted the tape on a different drive,
and lo and behold, the crash dump was on the tape!  It turned out that
the first tape drive was out of adjustment, missed the GCR burst, and
read the tape as 800 bpi NRZI.  The 6250 bpi crash dump had been
recorded on top of an earlier 800 bpi file, but the old file was still
readable at 800 bpi.

-Paul W.