25 Oct
2004
25 Oct
'04
1:12 a.m.
>>>> "Fred" == Fred N van Kempen
<waltje(a)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