Common service area (CSA) and extended common service area (ECSA)

The CSA and ECSA contain pageable system data areas that are addressable by all active virtual storage address spaces.

These service areas contain, for example, buffers or executable modules for IMS, ACF/SNA, and JES3. CSA and ECSA also contain control blocks that are used to define subsystems and provide working storage for areas such as TSO input/output control (TIOC), event notification facility (ENF), and message processing facility (MPF). When system configuration and activity increases, the storage requirements also increase.

CICS® uses the ECSA for multiregion operation (MRO) to store control blocks only and not for data transfer. If cross-memory facilities are used, the ECSA usage is limited to the following amounts:

40 bytes per session if IRC (interregion communication) is open, irrespective of whether the resource is acquired and inservice, or released
4 KB per address space participating in MRO

In addition, the amount of storage used by CICS MRO for interregion buffers is detailed in the DFHIR3794 message issued to the CSMT destination at termination.

CICS also uses ECSA for IMS and shared data tables.

For static systems, the amount of unallocated CSA should be around 10% of the total allocated CSA; for dynamic systems, a value of 20% is optimal. Unlike the SQA, if CSA is depleted there is no scope for expansion and a re-IPL might be required.