CICS Region

A CICS region is one running copy of the CICS system. It runs as its own MVS address space, started as a task named DFHSIP, and it is known to the network by its APPLID.

What is a CICS region

• A CICS region is a started task that runs the program DFHSIP.
• Each region has an APPLID (VTAM application id), for example CICSA. Users and other systems find the region by this name.
• One region can serve thousands of terminals and process thousands of transactions per second.
• When the region is up, users can sign on and run transactions. When it is down, nobody can use it.
• A single region can do everything - own terminals, run programs and own files. Large shops split the work across several regions.

Types of CICS regions (TOR, AOR, FOR)

• TOR - Terminal Owning Region: owns the terminals. It receives all terminal input from the users.
• AOR - Application Owning Region: runs the application programs. The TOR routes the transaction to the AOR.
• FOR - File Owning Region: owns the VSAM files. The AOR asks the FOR to read or update records (this is called function shipping).
• MOR - Message Owning Region: owns message and queue resources in bigger setups.
• DOR - Data Owning Region: owns shared data tables used by many regions.
• Splitting work across regions improves performance, and a failure in one region does not stop the others.

How a CICS region starts and stops

• The region is started with JCL or a START command that runs DFHSIP.
• The SYSIN stream holds SIT (System Initialization Table) overrides such as the APPLID and the start type.
• Start types: COLD starts completely fresh, WARM reuses the previous shutdown state, AUTO lets CICS decide.
• PLTPI programs run during startup - for example to open files or write a startup message.
• Operators stop a region with CEMT PERFORM SHUTDOWN or with a MODIFY command from the console.

Example - JCL to start a CICS region

• The sample below starts the region with APPLID CICSA. Change the dataset names to match your shop standards.
• //CICSREG EXEC PGM=DFHSIP,REGION=0M,TIME=1440 //STEPLIB DD DSN=CICS.SDFHLOAD,DISP=SHR //DFHRPL DD DSN=CICS.SDFHLOAD,DISP=SHR // DD DSN=MYSHOP.CICS.LOADLIB,DISP=SHR //SYSIN DD * APPLID=CICSA,START=AUTO /*
• STEPLIB points to the CICS load library that holds DFHSIP and the nucleus.
• DFHRPL is the library search order CICS uses to load your application programs and maps.