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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.