z/OS® has several characteristics that distinguish it from other mainframe operating systems.

The defining characteristics of z/OS are summarized as follows:

• The use of address spaces in z/OS holds many advantages: Isolation of private areas in different address spaces provides for system security, yet each address space also provides a common area that is accessible to every address space.
• The system is designed to preserve data integrity, regardless of how large the user population might be. z/OS prevents users from accessing or changing any objects on the system, including user data, except by the system-provided interfaces that enforce authority rules.
• The system is designed to manage a large number of concurrent batch jobs, with no need for the customer to externally manage workload balancing or integrity problems that might otherwise occur due to simultaneous and conflicting use of a given set of data.
• The security design extends to system functions as well as simple files. Security can be incorporated into applications, resources, and user profiles.
• The system allows multiple communications subsystems at the same time, permitting unusual flexibility in running disparate communications-oriented applications (with mixtures of test, production, and fall-back versions of each) at the same time. For example, multiple TCP/IP stacks can be operational at the same time, each with different IP addresses and serving different applications.
• The system provides extensive software recovery levels, making unplanned system restarts very rare in a production environment. System interfaces allow application programs to provide their own layers of recovery. These interfaces are seldom used by simple applications– they are normally used by sophisticated applications.
• The system is designed to routinely manage very disparate workloads, with automatic balancing of resources to meet production requirements established by the system administrator.
• The system is designed to routinely manage large I/O configurations that might extend to thousands of disk drives, multiple automated tape libraries, many large printers, large networks of terminals, and so forth.
• The system is controlled from one or more operator terminals, or from application programming interfaces (APIs) that allow automation of routine operator functions.
• The operator interface is a critical function of z/OS. It provides status information, messages for exception situations, control of job flow, hardware device control, and allows the operator to manage unusual recovery situations.