/sys/doc/ Documentation archive

TM(IV)                       2/21/74                       TM(IV)

     tm - TM-11/TU-10 magtape interface

     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'' inter-
     face 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 be-
     gin  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

     /dev/mt?, /dev/rmt?


     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.

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