From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Sun, 19 Nov 2017 12:49:19 -0500 (EST)
Subject: [TUHS] Determining what was on a tape back in the day
Message-ID: <20171119174919.166D018C0F5@mercury.lcs.mit.edu>

    > From: Will Senn

    > 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.

Yes and no.

To start with, one needs to differentiate three different levels; i) what's
actually on the medium; ii) what the device controller presented to the CPU;
and iii) what the OS (Unix in this case) presented to the users.

With the exception of magtapes (which had some semantics available through
Unix for larger records, and file marks, the details of which escape me - but
try looking at the man page for 'dd' in V6 for a flavour of it), you're correct
about what Unix presented to the users.


As to what is preserved; for disks and DECtapes, I think you are broadly
correct. For magtapes, it depends.

E.g. SIMH apparently can consume files which _represent_ magtape contents (i,
above), and which include 'in band' (i.e. part of the byte stream in the file)
meta-data for things like file marks, etc. At least one of the people who
reads old media for a living, when asked to read an old tape, gives you back
one of these files with meta-data in it. Here:

  http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/tools/rdsmt.c

is a program which reads one of those files and convert the contents to a file
containing just the data bytes. (I had a tape with a 'dd' save of a
file-system on it, and wanted just the file-system image, on which I deployed
a tool I wrote to grok 4.2 filesystems.)


Also, for disks, it should be remembered that i) and ii) were usually quite
different, as what was actually on the disk included thing like preambles,
headers, CRCs, etc, none of which the CPU usually could even see. (See here:

  http://gunkies.org/wiki/RX0x_floppy_drive#Low-level_format

for an example. Each physical drive type would have its own specific low-level
hardware format.) So what's preserved is just an image of what the CPU saw,
which is, for disks and DECtapes, generally the same as what was presented to
the user - i.e. a pile of bytes.

     Noel