z/OS® is known for its ability to serve thousands of users concurrently and for processing very large workloads in a secure, reliable, and expedient manner. Its use of multiprogramming and multiprocessing, and its ability to access and manage enormous amounts of virtual and physical storage as well as I/O operations, makes it ideally suited for running mainframe workloads.

The concept of virtual storage is central to z/OS. Virtual storage is an illusion created by the architecture, in that the system seems to have more storage than it really has. Virtual storage is created through the use of tables to map virtual storage pages to frames in central storage or slots in auxiliary storage. Only those portions of a program that are needed are actually loaded into central storage. z/OS keeps the inactive pieces of address spaces in auxiliary storage.

z/OS is structured around address spaces, which are ranges of addresses in virtual storage. Each user of z/OS gets an address space containing the same range of storage addresses. The use of address spaces in z/OS allows for isolation of private areas in different address spaces for system security, yet also allows for inter-address space sharing of programs and data through a common area accessible to every address space.

In common usage, the terms central storage, real storage, real memory, and main storage are used interchangeably. Likewise, virtual memory and virtual storage are synonymous. The amount of central storage needed to support the virtual storage in an address space depends on the working set of the application being used, and this varies over time. A user does not automatically have access to all the virtual storage in the address space. Requests to use a range of virtual storage are checked for size limitations and then the necessary paging table entries are constructed to create the requested virtual storage. Programs running on z/OS and System z® mainframes can run with 24-, 31-, or 64-bit addressing (and can switch between these modes if needed). Programs can use a mixture of instructions with 16-bit, 32-bit, or 64-bit operands, and can switch between these if needed.

Mainframe operating systems seldom provide complete operational environments. They depend on licensed programs for middleware and other functions. Many vendors, including IBM®, provide middleware and various utility products. Middleware is a relatively recent term that can embody several concepts at the same time. A common characteristic of middleware is that it provides a programming interface, and applications are written (or partially written) to this interface.