19 Nov
2017
19 Nov
'17
3:55 p.m.
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 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.
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).
Because of the variable nature of things the OS needs a way to support these behaviors.
Unix makes it pretty easy by letting the user do it all in user space and passing info to
the driver. Other OSs do a lot of work when ‘mounting’ a tape. But either way simh needs
to support these type of functions. Hence the idea of the virtual tape format that
includes meta data to describe things like the size of each block written. A ‘file mark’
can be read/written (which is special) besides data blocks
Sent from my PDP-7 Running UNIX V0 expect things to be almost but not quite.
On Nov 19, 2017, at 8:41 AM, Noel Chiappa <jnc(a)mercury.lcs.mit.edu> wrote:
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:
https://www.ebay.com/itm/Digital-pdp-Programming-Card-8-Pages/142565890514
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
moment.
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:
http://gunkies.org/wiki/PDP-11_architecture#Operands
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
beasts.
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:
http://gunkies.org/wiki/Installing_UNIX_Sixth_Edition#Installation_tape_con…
and IIRC 'tp' format magtape tapes are written the same way, hardware-wise (so
they look just like DECtapes).
Noel