z/OS®, a widely used mainframe operating system, is designed to offer a stable, secure, and continuously available environment for applications running on the mainframe.

z/OS today is the result of decades of technological advancement. It evolved from an operating system that could process a single program at a time to an operating system that can handle many thousands of programs and interactive users concurrently. To understand how and why z/OS functions as it does, it is important to understand some basic concepts about z/OS and the environment in which it functions.

In most early operating systems, requests for work entered the system one at a time. The operating system processed each request or job as a unit, and did not start the next job until the one being processed had completed. This arrangement worked well when a job could execute continuously from start to completion. But often a job had to wait for information to be read in from, or written out to, a device such as a tape drive or printer. Input and output (I/O) take a long time compared to the electronic speed of the processor. When a job waited for I/O, the processor was idle.

Finding a way to keep the processor working while a job waited would increase the total amount of work the processor could do without requiring additional hardware. z/OS gets work done by dividing it into pieces and giving portions of the job to various system components and subsystems that function interdependently. At any point in time, one component or another gets control of the processor, makes its contribution, and then passes control along to a user program or another component.