z/OS® uses several types of supervisor controls to enable multiprogramming.

These supervisor controls include:

Interrupt processing

Multiprogramming requires that there be some technique for switching control from one routine to another so that, for example, when routine A must wait for an I/O request to be satisfied, routine B can execute. In z/OS, this switch is achieved by interrupts, which are events that alter the sequence in which the processor executes instructions. When an interrupt occurs, the system saves the execution status of the interrupted routine and analyzes and processes the interrupt.

Creating dispatchable units of work

To identify and keep track of its work, the z/OS operating system represents each unit of work with a control block. Two types of control blocks represent dispatchable units of work: Task control blocks or TCBs represent tasks executing within an address space; service request blocks or SRBs represent higher priority system services.

Dispatching work

After interrupts are processed, the operating system determines which unit of work (of all the units of work in the system) is ready to run and has the highest priority, and passes control to that unit of work.

Serializing the use of resources

In a multiprogramming system, almost any sequence of instructions can be interrupted, to be resumed later. If that set of instructions manipulates or modifies a resource (for example, a control block or a data file), the operating system must prevent other programs from using the resource until the interrupted program has completed its processing of the resource.

Several techniques exist for serializing the use of resources; enqueuing and locking are the most common (a third technique is called latching). All users can use enqueuing, but only authorized routines can use locking to serialize the use of resources.