TM(IV) 2/21/74 TM(IV)
NAME
tm - TM-11/TU-10 magtape interface
DESCRIPTION
The files mt0, ..., mt7 refer to the DEC TU10/TM11 magtape.
When opened for reading or writing, the tape is rewound.
When closed, it is rewound; if it was open for writing, an
end-of-file is written first.
A standard tape consists of a series of 512 byte records
terminated by an 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. Writing in
very small units is inadvisable, however, because it tends
to create monstrous record gaps.
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 are named
rmt0, ..., rmt7. 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. An error is returned when a tape
mark is read, but another read will fetch the first record
of the new tape file.
FILES
/dev/mt?, /dev/rmt?
SEE ALSO
tp (I)
BUGS
If any non-data error is encountered, it refuses to do
anything 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.