In z/OS®, the dispatcher component is responsible for routing control to the highest priority unit of work that is ready to execute.

New work is selected, for example, when a task is interrupted or becomes non-dispatchable, or after an SRB completes or is suspended (that is, an SRB is delayed because a required resource is not available).

The dispatcher processes work in the following order:

1. Special exits

   These are exits to routines that have a high priority because of specific conditions in the system. For example, if one processor in a multiprocessing system fails, alternate CPU recovery is invoked by means of a special exit to recover work that was being executed on the failing processor.

2. SRBs that have a global priority
3. Ready address spaces in order of priority

   An address space is ready to execute if it is swapped in and not waiting for some event to complete. An address spaces's priority is determined by the dispatching priority specified by the user or the installation.

   After selecting the highest priority address space, z/OS (through the dispatcher) first dispatches SRBs with a local priority that are scheduled for that address space and then TCBs in that address space.

If there is no ready work in the system, z/OS assumes a state called an enabled wait until fresh work enters the system.

Different models of the z/Series hardware can have from one to 54 central processors (CPs). Each and every CP can be executing instructions at the same time. Dispatching priorities determine when ready-to-execute address spaces get dispatched.

Figure 1. Dispatching work

An address space can be in any one of four queues:

• IN-READY - In central storage and waiting to be dispatched
• IN-WAIT - In central storage but waiting for some event to complete
• OUT-READY - Ready to execute but swapped out
• OUT-WAIT - Swapped out and waiting for some event to complete

Only IN-READY work can be selected for dispatching.