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HT(4)											    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 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 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.

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