Time Sharing Option

In computing, the Time Sharing Option (TSO) is an interactive time-sharing environment for the lineage of IBM mainframe operating systems running from OS/MVT through MVS and OS/390 to the current z/OS. It fills the same purpose as the login sessions used by users on Unix. "Time-sharing" means that many persons can access MVS concurrently but each is unaware that the operating system is being accessed by others - so it appears to each TSO user that he or she is the only user on the system.

TSO is most commonly used by mainframe system administrators and programmers because it provides: a text editor; the ability to submit batch jobs, be notified of their completion and view the results without waiting for printed reports; debuggers for some of the programming languages used on System/360 and later IBM mainframes; other vendors' end-user applications, for example for querying IMS and DB/2 databases.

The name "Time Sharing "Option" derives from the fact that, when it was originally introduced in the 1960s, time-sharing was considered an "optional feature", as compared to standard batch processing, and hence TSO was offered as an optional feature of OS/MVT. It became a standard part of IBM's top-end mainframe operating system with the introduction of MVS in 1974. TSO/E (which stands for "Time Sharing Option / Extensions") is a set of extensions to the original TSO. All z/OS installations usually have both TSO and TSO/E functions installed, so it is normal to refer to both TSO and TSO/E as "TSO".

TSO interacts with users in either a line-by-line mode or in a full screen, menu-driven mode. In the line-by-line mode, the user enters commands by typing them in at the keyboard; in turn, the system interprets the commands, and then displays responses on the terminal screen. But most mainframe interaction is actually via ISPF which allows for customized menu-driven interaction; the combination is referred to as TSO/ISPF. TSO can also 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.

TSO in batch and under REXX

It is common to run TSO in batch (as opposed to interactively): all the usual TSO line-mode interactive commands can be also executed via Job Control Language (JCL) by running the program IKJEFT1B and supplying the line commands in a file pointed to by the SYSTSIN DD.

TSO commands can be embedded in REXX execs which can be run either interactively or in batch. Because the TSO command set is function rich it is easy to create and run sophisticated scripts this way.

See also

* CLIST
* ISPF
* Michigan Terminal System
* Multics
* MUSIC/SP
* Rexx

Further reading

The book "MVS TSO" by Doug Lowe.