[TUHS] Determining what was on a tape back in the day
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).
More information about the TUHS