Trailing-Edge - PDP-10 Archives - bb-jr93h-bb - 7,6/ap020/backup.nte
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         BACKUP %6(700) and DIRECT %7A(504) implement tape
         blocking.  A single switch controls the use of blocking factors
         and its meaning in both programs is identical.  The new switch
         is /BLOCKINGFACTOR:n, where "n" is an optional decimal number.

         The /BLOCKINGFACTOR switch sets the number of disk blocks per
         tape block read or written on tape.  The default value is 4 for
         compatibility with tapes written by BACKUP without blocking
         factor capability.  The specified value must be in the range of
         4 to 96, and be a multiple of 4 disk blocks.  Due to KS10
         UNIBUS adapter limitations, the maximum blocking factor on a
         KS10 is 56.

         Tape blocking is defined as writing a tape block that contains
         more than one disk block.  Versions of BACKUP prior to %6(700)
         write 4 disk blocks per tape block.  Thus, older versions of
         BACKUP may be said to use a blocking factor of 4.  The
         advantage of using higher blocking factors is to permit BACKUP
         to fit more information on a tape than it could writing a tape
         with the standard blocking factor of 4.  This is possible
         because the number of gaps between tape blocks is reduced.
         Note that INTERCHANGE mode forces a blocking factor of 4 for
         compatibility with other programs such as TOPS-20 DUMPER.