Time Sharing Option
|History of IBM mainframe operating systems|
TSO fulfills a similar purpose to Unix login sessions. Time-sharing means that many persons can access the operating system concurrently, while unaware that others are also accessing the operating system. It appears to each TSO user that they are the only user on the system.
TSO is most commonly used by mainframe system administrators and programmers. It provides:
- A text editor
- Batch job support, including completion notification
- Debuggers for some programming languages used on System/360 and later IBM mainframes
- Support for other vendors' end-user applications, for example for querying IMS and DB2 databases
The name "Time Sharing Option" derives from the fact that, when it was originally introduced in 1971, IBM considered time-sharing an "optional feature," as compared to standard batch processing, and hence offered TSO as an option for OS/360 MVT. With the introduction of MVS in 1974, IBM made it a standard component of their top-end mainframe operating system. TSO/E ("Time Sharing Option/Extensions") is a set of extensions to the original TSO. TSO/E is a base element of z/OS. Before z/OS, TSO Extensions (TSO/E) was an element of OS/390 and was a licensed program for the MVS and MVS/ESA System Products. Since all z/OS installations usually have both TSO and TSO/E functions installed, 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. This combination is called TSO/ISPF. TSO can also 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 
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 any of the programs
IKJEFT01, IKJEFT1A, or
IKJEFT1B and supplying the line commands in a file pointed to by the
SYSTSIN DD. The primary difference between the three programs is their handling of return codes from the executed commands.
Batch execution of TSO is one way to allow an IBM mainframe application to access DB2 resources.
See also 
Further reading 
The books: MVS TSO, Part 1: Concepts and ISPF, Second Edition and, MVS TSO, Part 2: Commands And Procedures, Second Edition both by Doug Lowe, and published by Mike Murach & Associates, Inc., 1991
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