In a z/OS® system, CICS® provides a layer of function for managing transactions, while the operating system remains the final interface with the computer hardware. CICS essentially separates a particular kind of application program (namely, online applications) from others in the system, and handles these programs itself.

When an application program accesses a terminal or any device, for example, it doesn't communicate directly with it. The program issues commands to communicate with CICS, which communicates with the needed access methods of the operating system. Finally, the access method communicates with the terminal or device.

Figure 1. Transactional system and the operating system

A z/OS system might have multiple copies of CICS running at one time. Each CICS starts as a separate z/OS address space. CICS provides an option called muti-region operation (MRO), which enables the separation of different CICS functions into different CICS regions (address spaces); so a specific CICS address space (or more) might do the terminal control and will be named terminal owning region (TOR). Other possibilities include application-owning regions (AORs) for applications and file-owning regions (FORs) for files.