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

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun Nov 19 23:41:09 AEST 2017

    > From: Will Senn

    > I don't quite no how to investigate this other than to pore through the
    > pdp11/40 instruction manual.

One of these:


is useful; it has a list of all the opcodes in numerical order; something none
of the CPU manuals have, to my recollection. Usually there are a flock of
these "pdp11 Programming Cards" on eBait, but I only see this one at the

If you do any amount of work with PDP-11 binary, you'll soon find yourself
recognizing the common instructions. E.g. MOV is 01msmr (octal), where 'm' is
a mode specifier, and s and r are source and destination register
numbers. (That's why PDP-11 people are big on octal; the instructions are easy
to read in octal.) More here:


So 0127xx is a move of an immediate operand.

    >> You don't need to mount it on DECTape drive - it's just blocks. Mount
    >> it as an RK05 image, or a magtape, or whatever.

    > I thought disk (RK05) and tape (magtape) blocks were different...

Well, you need to differentiate between DECtape and magtape - very different

DECtape on a PDP-11 _only_ supports 256 word (i.e. 512 byte) blocks, the same
as most disks. (Floppies are an exception when it comes to disks - sort
of. The hardware supports 128/256 byte sectors, but the usual driver - not in
V6 or V7 - invisibly makes them look like 512-byte blocks.)

Magtapes are complicated, and I don't remember all the details of how Unix
handles them, but the _hardware_ is prepared to write very long 'blocks', and
there are also separate 'file marks' which the hardware can write, and notice.

But a magtape written in 512-byte blocks, with no file marks, can be treated
like a disk; that's what the V6 distribution tapes look like:


and IIRC 'tp' format magtape tapes are written the same way, hardware-wise (so
they look just like DECtapes).


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