HOW TO RECOVER DATA FROM AN INITIALIZED 9-TRACK TAPE

               Short Title:  Recover Data from INIT'd Tape

                                 by
                          Charles M. Taylor
                 McDonnell Douglas Missile Systems Company


   PROBLEM

     -  A tape containing non-repeatable data was accidentally reinitialized.

     -  Tape owner wanted to retrieve data

    
   FAILED ATTEMPTS

     -  Mounted tape on TU78 tape drive

     -  Parity error encountered at end of label

     -  Tried to change parity, no effect

     -  Tried combinations of mount qualifiers and QIO routines to
        read virtual, logical, and physical blocks - none worked

     -  Tried to manually forward the tape past the place where the 
        parity error occurred - result was vacuum loss in tape drive

   SUCCESS

     -  Mounted tape on TU81 PLUS tape drive

     -  Used DUMP command to read past label

     -  No parity error occurred

     -  Data was dumped from tape

   COMMANDS

     -  $ MOUNT/FOREIGN mub0:

     -  $ DUMP mub0:

     -  Determined block size was 2048, stated by DUMP output

   COMMANDS . . .

     -  Copied the files

         -  $ MOUNT/FOREIGN/BLOCK=2048/RECORD=128  mub0:

         -  $ COPY mub0:

               -  Repeat COPY command until no more data
               -  Ignore errors while copying
               -  Files named .;1  .;2  etc.
               -  Loses some data at beginning of tape

   COMMANDS . . .

     -  Copied file labels
          -  $ DISMOUNT/NOUNLOAD mub0:
          -  $ MOUNT/FOREIGN/BLOCK=80/RECORD=80 mub0:
          -  $ COPY mub0:*.*  *

   RECORD FORMAT ON TAPE

     -  File format:  variable-length records

     -  Individual Records:

         -  First 4 bytes contained record length
         -  Byte count in ASCII

     -  Blocks:

         -  Contained only full records
         -  Padded with circumflex characters (^)

   Wrote FORTRAN program to read 128-byte records and extract 
   variable-length records.  See CONVERT_RECORDS.FOR


   PRIME TAPE

     -  File from 7-bit tape from Prime computer looked
        like gibberish in the editor

     -  High-order bit of every byte was 1 instead of 0

     -  Program CONVERT_PRIME was used to clear bits

     -  Then file could be used