Setting Up Your Subsystem

This chapter describes planning considerations for setting up and writing your own subsystem. When a directed request is made for a specific subsystem, the SSI searches for the subsystem requested. If the SSI finds that the named subsystem handles the requested function, the SSI passes control to the function routine. When a broadcast request is made, the SSI checks every subsystem to see if the subsystem handles the requested function. This search is done in the same order that the subsystems are defined to MVS™, with the exception that the primary job entry subsystem (JES) is first. If the SSI finds that a subsystem handles the requested broadcast function, the SSI passes control to the function routine. This process is repeated for each subsystem that handles the requested function.

When you want to write your own subsystem, you must:

Provide the routines to support the request for a function. These function routines get control from the SSI. They may actually perform the function or may pass control to other routines that you provide.
Provide a subsystem address space (if required).
Let MVS know that the subsystem exists (define the subsystem).
Provide the information to the SSI that it will need to find your function routines (initialize the subsystem).
Provide accounting information parameters to your subsystem (if required).

Note: When writing your own subsystem you must also provide any control blocks or resources that the subsystem requires for its own operation, which MVS does not provide.