CICS dynamic storage areas

The dynamic storage areas (DSAs) supply CICS® tasks with storage to run transactions and are essential for CICS operation. The DSAs in 24-bit storage are the CDSA, UDSA, SDSA, and RDSA. The DSAs in 31-bit storage are the ECDSA, EUDSA, ESDSA, ERDSA, and ETDSA. The DSAs in 64-bit storage are the GCDSA, GUDSA, and GSDSA.

The dynamic storage areas are made from virtual storage pages taken from MVS storage subpools. In the dynamic storage areas, CICS arranges the storage in CICS subpools. The subpools are dynamically acquired as needed, a page at a time, from within the dynamic storage area. The storage that individual subpools use is shown in the domain subpool statistics in the CICS storage manager statistics.

CICS manages DSA storage in extents. An individual DSA consists of one or more extents.
  • 24-bit storage extents are usually allocated in multiples of 256 KB. However, when transaction isolation is in operation, the UDSA is allocated in 1 MB extents.
  • 31-bit storage extents are allocated in multiples of 1 MB.
  • 64-bit storage extents are allocated in multiples of 1 GB.
Only the owning DSA can use an allocated extent, and a given extent cannot be shared between more than one DSA simultaneously.
The storage for the DSAs can be allocated from CICS-key storage, user-key storage, or read-only key-0 protected storage. The type of storage that is allocated for each DSA can depend on the settings for the STGPROT and RENTPGM system initialization parameters for the CICS region.
  • The storage for the CDSA, ECDSA, ETDSA and GCDSA is always allocated from CICS-key storage.
  • The STGPROT system initialization parameter specifies whether you want storage protection to operate in the CICS region.

    When you specify STGPROT=YES, or allow the system initialization parameter to default, the storage for the CICS dynamic storage areas for user applications is allocated from user-key storage. These DSAs are the UDSA, SDSA, EUDSA, ESDSA, GUDSA, and GSDSA. If you specify STGPROT=NO, the storage for these DSAs is allocated from CICS-key storage.

  • The RENTPGM parameter specifies whether CICS allocates the read-only DSAs from read-only key-0 protected storage.

    When you specify RENTPGM=PROTECT, the read-only DSAs are allocated from read-only key-0 protected storage. These DSAs are the RDSA and the ERDSA. If you specify RENTPGM=NOPROTECT, the storage for these DSAs is allocated from CICS-key storage.

A dynamic storage area that is too small results in increased program compression or, more seriously, short-on-storage (SOS) conditions. You can examine the pressure on virtual storage by using the CICS storage manager statistics, which report the number of times that CICS went short on storage.