IBM System/360 System Programmer's Guide

• Work areas The GETMAIN macro-instruction must be used to obtain
main storage for all of your work areas, including those areas used
to read in or create a label. The FREEMAIN macro-instruction must
be used to release this main storage.
Program Functions
In processing nonstandard labels, you must perform many of the
functions that the control program performs in processing standard
labels. All I/O operations, as, for example, those required to read
labels, to write labels, and to position volumes, must be performed by
using the execute channel program (EXCP) macro-instruction. The use of
EXCP normally requires that you build several control blocks in your
work area. However, you can save much coding effort by using existing
control blocks built by the control program.
When your routine receives control from the OPEN or CLOSE routine,
the status of control information and pointers is as shown in Figure 1.
When your routine receives control from the EOV routine, register 2
contains the address of a DCB, and register 4 contains the address of a
combined work and control block area. The format of this area is shown
in Figure 2.
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IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English