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SECTION 1:
WRITING NONSTANDARD LABEL PROCESSING ROUTINES
For
magnetic
tape
voluroes
with nonstandard latels <i.e., input or
output header or trailer labels that do
not
conform
to
the
standard
label
format),
you must write your own label processing routines.
You
must also perform such functions as
volume
identification
and
volurr,e
positioning.
The
appropriate
nonstandard
label
processing routine is selected,
brought into main storage, and executed when the data control
block
is
either
opened
or
closed,
or when an end-of-volume or end-of-data set
condition occurs.
When
a
nonstandard
label
processing
routine
has
completed,
it
must
return control to the OPEN, CLOSE, or EOV routine,
which then continues its norrral processing.
The
following
paragraphs
explain
this
flow of control between the control program and each type
of nonstandard label processing routine.
Information on tape
position-
ing and volume identification is also provided.
Input Header Label Routines
An
input
header
label
routine
is
brought into main storage when
either the OPEN or EOV routine is executed.
Your
routine
~s
entered
after
the
control
program
has
determined
that a tape does not have
standard labels.
When your routine receives control, the tape will have
been positioned at the interrecord gap preceding the nonstandard
label.
If your routine determines that the wrong volume is mounted, you must
place
a
1
in the high-order bit position of the SRTEDMCT field of the
unit control block (UCB), and return control
to
the
control
program.
The
control
program
then
issues a message to the operator requesting
that the correct volume be mounted.
When the
new
volume
is
mounted,
your
routine is entered again after the contrcl program has checked the
initial label.
Before returning control to the
control
program,
you
should,
for
forward
read
operations, position the tape at the interrecord gap that
precedes
the
initial
record
of
the
data
set;
for
backward
read
operations,
the
tape should be positioned after the last record of the
data set.
Input
~railer
Label Routine
An input trailer label routine is brought into main storage when
the
EOV
routine
is
executed.
Your routine is entered when a tape mark is
encountered.
When your routine receives control,
the
tape
will
have
been positioned, for forward read operations, immediately after the tape
mark
at
the
end
of
the
data
set;
for
backward
read operations,
immediately before the tape mark at the beginning of the data set.
You need not reposition the tape
before
returning
control
to
the
control program.
Output Header Label Routines
An
output
header
label
routine
is brought into main storage when
either the OPEN or EOV routine is executed.
Your
routine
is
entered
after
the
control
program
has
determined
that a tape does not have
standard labels.
When your routine receives control, the tape will have
been positioned at the interrecord gap preceding the nonstandard
label.
If your routine determines that the wrong volume is mounted, you must
place
a
1
in the high-order bit position of the SRTEDMCT field of the
unit control block (UCB), and return control
to
the
control
program.
The control program issues a wessage to the operator requesting that the
correct volume be mounted.
When the new volume is mounted, your routine
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