Introduction › Reviewing the Operating System

Reviewing the Operating System

If you use a computer, you work with an operating system. Even the smallest personal computers or personal data assistants (PDA’s) have operating system programs that control the hardware of the computer. Programs that users write communicate their needs for data to the operating system rather than to the computer’s hardware.

The operating system acts as an interpreter between the programs and the computer, reading the program to find out what data it needs from the hardware and relaying that information to the hardware in machine language. In this way, programming is much simpler. Programs ask the operating system to process a command. Instead of including hundreds or thousands of machine instructions that are necessary for the program to work with an input/output device, the program asks the operating system to process a read or write command.

Operating systems perform many other important functions. For example, large operating systems coordinate the use of the computer’s resources among many users at the same time by rapidly switching one or more central processing units (CPU) from user to user, and by managing a shared catalog and file system facility.

With each new generation of computers, operating systems become more complex. Modern operating systems perform multiprogramming, multitasking, telecommunications, virtual storage processing, and a multitude of other important functions. The programs that make up these operating systems are an integral part of the computer. They control virtually all activity in the system.

A highly developed operating system like z/OS performs all the functions described above; schedules the processing of programs, subsystems, and started tasks; and monitors much of the activity that occurs in the system. z/OS also processes job control language (JCL) statements that tell z/OS which files and programs to use for processing, how to process them, and where to put the resulting output. z/OS also coordinates and controls the activities of the hardware devices attached to the system, such as tape drives, disk drives, terminals, control units, and printers. More than 5,000 programs are at work in the z/OS operating system.