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

Arthur Krewat krewat at kilonet.net
Mon Nov 20 04:35:58 AEST 2017

When reading magtapes, or any tapes for that matter, I usually dd the 
"files" on the tape into separate files on disk. This doesn't preserve 
the actual blocking factor on the tape, but I used other methods to 
determine that - usually experimenting with dd and small block sizes 
until I got to the point where it no longer errors.

One of my favorite pastimes was to read until the soft EOT (usually a 
double EOF), and then do a "mt fsf". Depending on the tape drive, this 
could advance the tape beyond the soft EOT where more data might be 
saved. The idea being that a tape had been overwritten from the 
beginning, but with less data overall.

So it would look like this: |data|EOF|data|EOF|data|EOT|short 

I used this method to rescue a TOPS-10 6.03A install monitor from a 
backup set that was past the soft EOT on one of my own personal tapes. 
I've rescued a lot of other data that way too.

For anyone reading old tapes, I implore you to attempt to read data past 
the soft EOT ;)

On 11/19/2017 12:49 PM, Noel Chiappa wrote:
>      > 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

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