HT(IV)                       2/9/75                        HT(IV)


     ht - RH-11/TU-16 magtape interface


     The files mt0, ..., mt7 refer to the DEC RH/TM/TU16 magtape.

     When  opened  for  reading  or writing, the tape is rewound.

     When closed, it is rewound; if it was open  for  writing,  a

     double end-of-file is written first.

     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 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.


     /dev/mt?, /dev/rmt?


     tp (I)


     Raw I/O doesnt work yet.  The magtape system is supposed  to

     be  able  to take 64 drives.  Such addressing has never been

     tried.  These bugs will be fixed when we get more experience

     with this device.

     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.