HT(4) HT(4)
NAME
ht - RH-11/TU-16 magtape interface
DESCRIPTION
The files mt0, mt1, ... refer to the DEC RH/TM/TU16 magtape.
When opened for reading or writing, the tape is not rewound.
When closed, it is rewound (unless the 0200 bit is on, see
below). If the tape was open for writing, a double end-of-
file is written. If the tape is not to be rewound the tape
is backspaced to just between the two tapemarks.
A standard tape consists of a series of 512 byte records
terminated by a double end-of-file. To the extent possible,
the system makes it possible, if inefficient, to treat the
tape like any other file. Seeks have their usual meaning
and it is possible to read or write a byte at a time. Writ-
ing in very small units is inadvisable, however, because it
tends to create monstrous record gaps.
The last octal digit of the minor device number selects the
drive. The middle digit selects a controller. The initial
digit is even to select 800 BPI, odd to select 1600 BPI. If
the 0200 bit is on (initial digit 2 or 3), the tape is not
rewound on close. Note that the minor device number has no
necessary connection with the file name, and in fact tp(1)
turns the short name x into `/dev/mtx'.
The mt files discussed above are useful when it is desired
to access the tape in a way compatible with ordinary files.
When foreign tapes are to be dealt with, and especially when
long records are to be read or written, the `raw' interface
is appropriate. The associated files may be named rmt0,
..., rmt7, but the same minor-device considerations as for
the regular files still apply.
Each read or write call reads or writes the next record on
the tape. In the write case the record has the same length
as the buffer given. During a read, the record size is
passed back as the number of bytes read, provided it is no
greater than the buffer size; if the record is long, an
error is indicated. In raw tape I/O, the buffer must begin
on a word boundary and the count must be even. Seeks are
ignored. A zero count is returned when a tape mark is read;
another read will fetch the first record of the next tape
file.
FILES
/dev/mt?, /dev/rmt?
SEE ALSO
HT(4) HT(4)
tp(1)
BUGS
The magtape system is supposed to be able to take 64 drives.
Such addressing has never been tried.
Taking a drive off line, or running off the end of tape,
while writing have been known to hang the system.
If any non-data error is encountered, it refuses to do any-
thing more until closed. In raw I/O, there should be a way
to perform forward and backward record and file spacing and
to write an EOF mark explicitly.