1/2" tape cleaners?

From: Paul Koning <pkoning_at_equallogic.com>
Date: Mon Oct 25 08:12:51 2004

>>>>> "Fred" == Fred N van Kempen <waltje_at_pdp11.nl> writes:

>> If you are using real half-inch tapes, detecting EOT is hard.
>> I've run into tapes with a dozen consecutive tape marks and data
>> after them; with some hardware, if you keep reading you'll
>> eventually run off the *physical* end of the tape - I've done it,
>> and on the less friendly drives it's a mess to re-thread the tape
>> enough to rewind it onto the original reel.
 Fred> True... half-inch (and most other fixed-block) drives didnt do
 Fred> the tapemark thing.

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

Sometimes these things were enforced by narrowminded operating
systems. With OS/360 you had to tell the OS (via JCL) which specific
file -- by position, not name -- you wanted; to read foreign formats,
never mind past tape marks, you'd need EXCP I/O. But at least you
could and it was documented.

Burroughs MCP was just about as bad, except there there wasn't a
documented way to get around it. It *appeared* that you could do so
by writing in unusual languages (DCALGOL or ESPOL) but certainly the
latter was strictly prohibited to ordinary users...

So I did manage to read DEC format magtapes on a 360 -- but never on a
6700...

        paul
Received on Mon Oct 25 2004 - 08:12:51 BST