Functional overview

An MVS™ subsystem consists of two control blocks and a set of functional routines, all resident in common memory. The control blocks are:

SSCT
The subsystem communication table, which contains the 4-character name of the subsystem and a pointer to the SSVT.
SSVT
The subsystem vector table, which contains a list of the subsystem function codes that the subsystem supports, and the addresses of the functional routines that support them.

The subsystem is active when the SSCT contains a nonzero pointer to the SSVT, and inactive when the pointer is zero.

Subsystem definition

Each subsystem is defined to MVS by an entry in an IEFSSNxx member of SYS1.PARMLIB. (See the OS/390 MVS Initialization and Tuning Guide, SC28-1751.) Each subsystem can be defined with an initialization routine and some initialization parameters. The CICS® subsystem is defined with an initialization routine of DFHSSIN, and an initialization parameter that specifies the name of an additional member of SYS1.PARMLIB, which contains further CICS-specific subsystem parameters. These parameters specify whether the console message handling facility is required.

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