In addition to initializing system areas, z/OS® establishes system component address spaces. It establishes an address space for the master scheduler and other system address spaces for various subsystems and system components. Some of the component address spaces are: *MASTER*, ALLOCAS, APPC, CATALOG, and so on.

The master scheduler address space is the first system component address space to be created (ASID=1). Then, the master scheduler may start the primary job entry subsystem (JES2 or JES3). On many production systems, JES is not started immediately; instead, an automation package starts all tasks in a controlled sequence.

Then other defined subsystems are started. All subsystems are defined in the PARMLIB library, in member IEFSSNxx. These subsystems are secondary subsystems.

Figure 1 shows some of the important system component address spaces; for started tasks VTAM®, CICS®, TSO; and for a TSO user and a batch initiator. Each address space has 2 GB of virtual storage by default, whether the system is running in 31-bit or 64-bit mode.

Figure 1. Virtual storage layout for multiple address spaces

The private areas are available only to that address space, but common areas are available to all.

After the system is initialized and the job entry subsystem is active, the installation can submit jobs for processing by using the START, LOGON, or MOUNT command. When a job is activated through START (for batch jobs), LOGON (for time-sharing jobs), or MOUNT, a new address space is allocated.