Mainframe hardware consists of processors and a multitude of peripheral devices such as disk drives (called direct access storage devices or DASD), magnetic tape drives, and various types of user consoles.

Tape and DASD are used for system functions and by user programs executed by z/OS®.

Figure 1. Hardware resources used by z/OS

The z/OS operating system executes in a processor and resides in processor storage during execution. z/OS is commonly referred to as the system software.

To fulfill a new order for a z/OS system, IBM® ships the system code to the customer through the Internet or (depending on customer preference) on physical tape cartridges. At the customer site, a person such as the z/OS system programmer receives the order and copies the new system to DASD volumes. After the system is customized and ready for operation, system consoles are required to start and operate the z/OS system.

The z/OS operating system is designed to make full use of the latest IBM mainframe hardware and its many sophisticated peripheral devices. Figure 1 presents a simplified view of these mainframe concepts:

Software

The z/OS operating system consists of load modules or executable code. During the install process, the system programmer copies these load modules to load libraries residing on DASD volumes.

Hardware

The system hardware consists of all the devices, controllers, and processors that constitute a mainframe environment.

Peripheral devices

These include tape drives, DASD, consoles, and many other types of devices.

Processor storage

Often called real or central storage (or memory), this is where the z/OS operating system executes. Also, all user programs share the use of processor storage with the operating system.

As a "Big Picture" of a typical mainframe hardware configuration, Figure 1 is far from complete. Not shown, for example, are the hardware control units that connect the mainframe to the other tape drives, DASD, and consoles.