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

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. 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