Time Sharing Option

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Time Sharing Option (TSO) is an interactive environment for IBM mainframe operating systems such as MVS/ESA, OS/390 and z/OS. It fills the same purpose as the login sessions used by users on Unix or Windows. TSO is normally used by mainframe system administrators and programmers. For more information see the book MVS TSO by Doug Lowe.

The name derives from the fact that, when it was originally introduced in the 1960s, time-sharing was considered an "optional feature", and hence TSO was offered as an optional feature of OS/360. It became a standard part of the system with the introduction of MVS in 1974.

The native TSO environment is rarely used in production shops alone. Most mainframe interaction is actually via the ISPF facility which allows for customized menu-driven interaction; the combination is referred to as TSO/ISPF. TSO also can be used to provide a Unix-style environment on OS/390 and z/OS via the UNIX System Services command shell, with or without ISPF.