Sent from my iPhone
Thanks, Noel,
The archive has the card:
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:
I think I understand- the bytes that we have on hand are not device faithful representations, but rather are failthful representations of what is presented to the OS. That is, back in the day, a tape would be stored in various formats as would disks, but unix would show these devices as streams of bytes, and those are the streams of bytes are what have been preserved.
Some formats have header info, file lists, and such placed in predetermined locations, and some have different block sizes, file/tape marks, but no track, sector type or device specific info is retained? Does this sound reasonably accurate?
Will