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

Will Senn will.senn at gmail.com
Mon Nov 20 00:55:51 AEST 2017

Sent from my iPhone

On Nov 19, 2017, at 7:41 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>  https://www.ebay.com/itm/Digital-pdp-Programming-Card-8-Pages/142565890514
> is useful;

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?

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