CICS virtual storage

Each CICS® region operates in its own z/OS® address space. The storage available in a z/OS address space is divided into several different areas.

Figure 1 shows an outline of the storage available in a z/OS address space. Although the theoretical upper limit for this virtual storage is extremely high, there are practical limits to real storage. For this reason, in z/OS, each address space is subject to REGION and MEMLIMIT parameters that limit the amount of storage the address space can use.

Diagram is described in surrounding text. — Figure 1. z/OS Address Space

CICS uses and manages virtual storage in three areas of its z/OS address space:

Storage below the line (0 MB to 16 MB)

The storage in this area is 24-bit storage.

Addresses below the 16 MB address are accessed by 24-bit addressing, and programs can use this storage when they run in AMODE 24 or higher. The 16 MB address is known as the line, so 24-bit storage is also called storage below the line.

Storage above the line (16 MB to 2 GB)

The storage in this area is 31-bit storage.

Addresses above the 16 MB address but below the 2 GB address are accessed by 31-bit addressing, and programs can use this storage when they run in AMODE 31 or higher. The 16 MB address is known as the line, so 31-bit storage is also called storage above the line.

The area that separates the virtual storage area below the 2 GB address from the user private area is known as the bar. 24-bit and 31-bit storage are in storage below 2 GB and can together be referred to as storage below the bar.

Storage above the bar (4 GB to a theoretical 16 exabytes)

The storage in this area is 64-bit storage.

The area that separates the virtual storage area below the 2 GB address from the user private area is known as the bar, and 64-bit storage is also known as storage above the bar.

The storage above the bar comprises a user private area between 4 GB and 2 terabytes, a shared area between 2 terabytes and 512 terabytes, and a user private area between the end of the shared area and 16 exabytes.

Addresses above the bar are accessed by 64-bit addressing, and programs can use this storage when they run in AMODE 64.

In each private area of storage, virtual storage is used for the following purposes:

CICS dynamic storage areas: The dynamic storage areas are used to supply the storage requirements of CICS, access methods, and applications running in CICS. See CICS dynamic storage areas.
MVS™ storage: MVS storage is available to the operating system to perform region-related services. See 64-bit MVS storage and MVS storage below 2 GB.

Note: This information and the following topics refer to other products installed with CICS, and is valid at the time of writing. For other products installed with CICS, always check the information for the versions of those products that you are using.