CICS application programs

Online application programs have certain features and needs in common. Typically, they:

* Serve many online users, apparently simultaneously.
* Require common access to the same data sets and databases.
* Try to give each end user a timely response to each interaction.
* Involve telecommunications access to remote terminals.

The host operating system is in overall charge of the computer and manages resources in whatever way you set up. But the very versatility of a general-purpose operating system means that it often cannot give online programs the sort of priority treatment they need. Instead, CICS may be given "privileged" treatment on behalf of all the online programs that run under it.

To make the best use of the time and system resources that the operating system gives to CICS, CICS takes on itself some of the aspects of an operating system. For example, CICS allows more than one of its programs (tasks) to be in an active state at the same time. But CICS doesn't duplicate all of the services provided by the operating system. Whenever appropriate, CICS goes straight to the operating system to provide what its tasks ask for.