CICS Address Spaces

Every CICS region runs inside its own MVS address space. Understanding address spaces explains how CICS gets its storage, how it is protected, and how regions talk to each other.

What is an address space

• An address space is a block of virtual storage that MVS gives to one running job or started task.
• Each address space can address up to 2 GB of virtual storage (31-bit addressing).
• A CICS region runs in its own address space, separate from batch jobs, TSO users and other CICS regions.
• Because each region has its own address space, a storage error in one region does not crash the others.
• Real storage (central memory) is shared, but MVS paging keeps each address space's virtual storage separate.

Why CICS needs its own address space

• Isolation: CICS programs, tables and control blocks are protected from other work running on the system.
• Performance: the CICS dispatcher switches between hundreds of tasks inside the region without asking z/OS for help each time.
• Size: a busy CICS region needs a large amount of virtual storage for programs, tables, terminal buffers and user data.
• Availability: the region can be started, stopped and recycled on its own without touching batch or TSO work.

CICS dynamic storage areas (DSAs)

• CICS divides its address space into dynamic storage areas. Each DSA holds a different kind of storage.
• CDSA and UDSA: CICS-key and user-key storage below the 16 MB line (24-bit addressing).
• ECDSA and EUDSA: CICS-key and user-key storage above the 16 MB line (31-bit, extended). Most storage lives here.
• SDSA and ESDSA: shared storage that can be seen across MVS tasks. RDSA and ERDSA: read-only storage for programs.
• EDSALIM sets the maximum size of the extended DSAs. DSALIM sets the limit below the line.
• When a DSA fills up, CICS goes short on storage (SOS). Message DFHSM0131 is written and tasks start waiting.

Other address spaces CICS talks to

• Batch regions: batch jobs that share VSAM files with CICS (in RLS mode) or exchange data through queues.
• TSO address spaces: developers editing, compiling and testing programs.
• VTAM: owns the telecommunication network. Terminals reach the CICS region through VTAM.
• SMSVSAM server address space: used when CICS accesses VSAM files in RLS (record-level sharing) mode.
• Coupling facility: shared high-speed storage used by CICS regions in a sysplex. It is hardware-backed, not an address space itself.

Example - SIT overrides for DSA sizes

• The SYSIN overrides below set the DSA limits for a medium-size region. Tune the numbers for your workload.
• EDSALIM=200M, DSALIM=24M
• EDSALIM=200M allows up to 200 MB of extended dynamic storage. Raise it if the region goes short on storage above the line.
• DSALIM=24M limits 24-bit storage. Keep it small - almost nothing should need below-the-line storage today.