IBM System/360 System Programmer's Guide

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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IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English