1/2" tape cleaners?

From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Sun Oct 24 17:10:28 2004

>> Some tape drives really do have fixed hardware block sizes.
> Yes, such as the half-inch ones,

Half-inch? Are you perhaps thinking of quarter-inch?

The only half-inch tapes I've ever used are the big reel-to-reel
nine-track tapes, and those most definitely did not have a fixed
hardware block size (except for the contrived way I sketched, looking
at each byte as a record).

QIC - quarter-inch cartridge - tapes did have a fixed block size, or at
least most of the ones I used did.

> What I forgot to mention was: teh tape-marking "standard" was mostly
> a software-based, voluntary standard, to prevent programs from
> reading past EOT. Of course, pretty much nothing prevented people
> from doing a <TM><TM> sequence, and then writing more data :)

Well, it depends on the drive. QIC tapes with their serpentine
recording pattern had an EOT mark on the tape that was physically
different from two tape marks; the driver kept track, and when you did
a second consecutive write-EOF, backed up over the previous EOF and did
a write-EOT instead. Similar things were done when closing the tape,
if it had been written to (or perhaps just if it was open for write).

This is relevant because the drive would not read past an EOT marker,
and (or so I've been told - I have no way of verifying it) the EOT
marker was written not just at the current logical place (in the
serpentine pattern) but at the current physical place, across the
current track and all later tracks, and thus even advancing the tape by
hand wouldn't buy you much. With half-inch nine-track tapes, if you
overwrote the beginning of the tape, you could deliberately read past
the second EOF and get whatever of the original data hadn't been
overwritten (possibly with an error in the first, partial, record).
Can't do that on a QIC. I think 8mm is similar to QIC in this respect,
that EOT is not, on the tape, the same thing as two EOFs, and the drive
will not read past an EOT marker. (Hmm, I wonder if you could "read
past EOT" by overwriting the EOT and then cutting power ungracefully.
I don't think I ever tried it.)

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