TSO/E provides commands that you can use to perform a wide variety of tasks. For example, you can use TSO/E commands to define and maintain data sets, and write and test programs.

You can write command processors to replace or add to this set of commands. By writing your own command processors, your installation can add to or modify TSO/E to better suit the needs of its users. For TSO/E, the IBM® Compiler and Library for REXX/370 contains features to assist you in writing simple command processors.

A command processor is a program written in assembler language that receives control when a user at a terminal enters a command name. It receives control from the terminal monitor program (TMP), a program that provides an interface between terminal users and command processors, and has access to many system services.

The main difference between command processors and other programs is that when a command processor is invoked, it is passed a command processor parameter list (CPPL) that gives the program access to information about the caller and to system services.

Command processors must be able to communicate with the user at the terminal, as well as respond to abnormal terminations and attention interruptions. Command processors can recognize subcommand names entered by the terminal user and then load and pass control to the appropriate subcommand processor.

Command processors can use many of the TSO/E programming services described in the following section to perform required functions. For guidelines on how to write a command processor, what TSO/E services to use, and how to test and install the command processor, see z/OS TSO/E Programming Guide.