19 Nov
2017
19 Nov
'17
3:55 p.m.
Sent from my iPhone
On Nov 19, 2017, at 7:41 AM, Noel Chiappa <jnc(a)mercury.lcs.mit.edu> wrote:
Thanks, Noel,
The archive has the card:
https://archive.org/details/bitsavers_decpdp11PDul75_1582192
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