Many z/OS® system functions run in their own address spaces. The master scheduler subsystem, for example, runs in the address space called *MASTER* and is used to establish communication between z/OS and its own address spaces.

When you start z/OS, master initialization routines initialize system services, such as the system log and communication task, and start the master scheduler address space. Then, the master scheduler may start the job entry subsystem (JES2 or JES3). JES is the primary job entry subsystem. On many production systems JES is not started immediately; instead, the automation package starts all tasks in a controlled sequence. Then other subsystems are started. Subsystems are defined in a special file of system settings called a parameter library or PARMLIB. These subsystems are secondary subsystems.

Each address space created has a number associated with it, called the address space ID (or ASID). Because the master scheduler is the first address space created in the system, it becomes address space number 1 (ASID=1). Other system address spaces are then started during the initialization process of z/OS.

At this point, you need only understand that z/OS and its related subsystems require address spaces of their own to provide a functioning operating system. A short description of each type of address space follows:

System

z/OS system address spaces are started after initialization of the master scheduler. These address spaces perform functions for all the other types of address spaces that start in z/OS.

Subsystem

z/OS requires the use of various subsystems, such as a primary job entry subsystem or JES. Also, there are address spaces for middleware products such as DB2®, CICS®, and IMS™.

Besides system address spaces, there are, of course, typically many address spaces for users and separately running programs; for example:

• TSO/E address spaces are created for every user who logs on to z/OS.
• An address space is created for every batch job that runs on z/OS. Batch job address spaces are started by JES.