z/OS® uses both types of physical storage (central and auxiliary) to enable another kind of storage called virtual storage. In z/OS, each user has access to virtual storage, rather than physical storage. This use of virtual storage is central to the unique ability of z/OS to interact with large numbers of users concurrently, while processing the largest workloads.

Virtual storage means that each running program can assume it has access to all of the storage defined by the architecture's addressing scheme. The only limit is the number of bits in a storage address. This ability to use a large number of storage locations is important because a program may be long and complex, and both the program's code and the data it requires must be in central storage for the processor to access them.

z/OS supports 64-bit long addresses, which allows a program to address up to 18,446,744,073,709,600,000 bytes (16 exabytes) of storage locations. In reality, the mainframe might have much less central storage installed. How much less depends on the model of the computer and the system configuration.

To allow each user to act as though this much storage really exists in the computer system, z/OS keeps only the active portions of each program in central storage. It keeps the rest of the code and data in files called page data sets on auxiliary storage, which usually consists of a number of high-speed direct access storage devices (DASD).

Virtual storage, then, is this combination of real and auxiliary storage. z/OS uses a series of tables and indexes to relate locations on auxiliary storage to locations in central storage. It uses special settings (bit settings) to keep track of the identity and authority of each user or program. z/OS uses a variety of storage manager components to manage virtual storage.

Mainframe workers use the terms central storage, real memory, real storage, and main storage interchangeably. Likewise, they use the terms virtual memory and virtual storage synonymously.