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HT(4)                      Kernel Interfaces Manual                      HT(4)

       ht - RH-11/TU-16 magtape interface

       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 pos-
       sible,  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 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.

       /dev/mt?, /dev/rmt?


       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 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 explicitly.