Understanding the master scheduler job control language

The master scheduler JCL data set (commonly called master JCL) controls system initialization and processing. It contains data definition (DD) statements for all system input and output data sets that are needed to communicate between the operating system and the primary job entry subsystem, which can be JES2 or JES3. An IPL is required to add, remove or change any DD statements in the master JCL.

To change system initialization or processing, you can change the information in the member or use an alternate data set. You might need to change the master JCL at an IPL. For example, if you plan to use jobs as the source JCL for started tasks, IBM® suggests that you modify the master JCL. See Setting up started tasks with the Master JCL for more information.

You can also modify the master scheduler JCL data set to include START commands for other subsystems, along with DD statements necessary to communicate with them. You can also delete DD statements that do not apply to your installation's interactive configuration.