TM(4) Kernel Interfaces Manual TM(4)
tm - TM-11/TU-10 magtape interface
The files mt0, ..., mt7 refer to the DEC TU10/TM11 magtape. When
closed it can be rewound or not, see below. If it was open for writ‐
ing, two end-of-files are written. If the tape is not to be rewound it
is positioned with the head between the two tapemarks.
If the 0200 bit is on in the minor device number the tape is not
rewound when closed.
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. A zero byte count is returned when a tape
mark is read, but another read will fetch the first record of the new
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.