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Subsystems have four‑character names that you must define to z/OS. The Subsystem Display (3.1) shows subsystems by an IEFSSNxx member of parmlib or the IEASYSxx IPL directory member. The operator uses the SSN=xx operand to identify the appropriate member at IPL.
When a subsystem is defined by IEFSSNxx, a subsystem initialization routine can be specified. This routine can perform subsystem initialization tasks at IPL.
An APF‑authorized program can establish a subsystem by modifying the control blocks z/OS uses to monitor subsystems. The IBM Information Management System (IMS) and Secondary Operator Facility (SOF) use this method. The Subsystem Display shows you all subsystems and indicates how they were introduced to z/OS.
A subsystem supplies control blocks and programs that monitor z/OS system events. Frequently, a subsystem is also associated with an address space (that is, a long‑running batch job or started task), such as is done with CA ACF2. However, this is not a z/OS requirement. Thus, you cannot rely on the z/OS DISPLAY ACTIVE JOBS (DA) command or any similar facility to identify subsystems.
The SSVT (subsystem vector table) is a control block that is built by each subsystem. The SSVT indicates the functions of a subsystem and the addresses of the routines that perform those functions.
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