Creating and using SCOPE=COMMON data spaces

The SCOPE=COMMON data space provides your programs with virtual storage that is addressable from all address spaces and all programs. In many ways, it is the same as the common service area (CSA) of an address space. You might use a SCOPE=COMMON data space instead of CSA because:

A SCOPE=COMMON data space offers up to two gigabytes of commonly addressable virtual storage for data (but not executable code). The CSA offers a much smaller amount of storage.
The CSA is a limited resource; because it is a part of all address spaces, the use of this virtual storage area reduces the amount of common area available for all programs.

To create this space, use the SCOPE=COMMON parameter on DSPSERV CREATE. You can use any of the parameters on that macro to establish the characteristics of that space.

To gain addressability to the space, issue the ALESERV ADD macro with the AL=PASN parameter. ALESERV ADD then adds an entry for the data space to the caller's PASN-AL and returns the ALET for that entry. Additionally, ALESERV ADD adds the same entry to every PASN-AL in the system. As new address spaces come into the system, their PASN-ALs have this entry on them. All programs use the same ALET to access the data space. In other words, with the entry on all PASN-ALs, programs in other address spaces do not have to issue the ALESERV ADD macro. However, the creating program must pass the ALET for the data space to the other programs.

The use of the virtual storage in the SCOPE=COMMON data space is similar to the use of the CSA. A program wanting to share CSA storage with another program has to pass the address of that area to the other program; the creator of the SCOPE=COMMON data space has to pass the ALET value to the other program. (It might also have to tell the other program the origin of the data space.)

Figure 1 shows an example of a SCOPE=COMMON data space named COMDS that PROG1 created. PROG1 uses ALESERV ADD to add an entry to its PASN-AL. Because COMDS is SCOPE=COMMON,that same entry appears on all PASN-ALs in the system, plus all PASN-ALs that will exist from the time the entry for the SCOPE=COMMON data space is added to the access list until the data space terminates. PROG1 has the ALET for the entry. To give access to COMDS to programs in the other address spaces, PROG1 passes the ALET to the other programs.

Figure 1. Example of Using a SCOPE=COMMON Data Space

Programming Considerations: When you use SCOPE=COMMON data spaces, keep in mind the following advice:

Use the SCOPE=COMMON data space when your program has large amounts of data that it wants to share across multiple address spaces. For example, to share more than 10 megabytes of commonly addressable data, consider using a SCOPE=COMMON data space. To use less than 10 megabytes, consider using CSA.
To make sure problem state programs cannot access the SCOPE=COMMON data space, use the FPROT and KEY parameters to assign fetch protection and a specific storage key.
For example, consider that PROG1 in Figure 1 used the following parameters on DSPSERV when it created COMDS:
```
FPROT=YES,KEY=5
```
In this case, only programs with PSW key 5 or PSW key 0 can access the data in COMDS. A TSO/E user (with PSW key 8) would then be unable to either store into or fetch from the data space.
The system can reuse the ALET associated with a SCOPE=COMMON data space after the space terminates. Therefore, manage the termination and reuse of ALETs for the SCOPE=COMMON data space. This action is described in ALET reuse by the system.
To help solve system problems and error conditions, use the data space dumping services to dump appropriate areas of the SCOPE=COMMON data space. See Dumping storage in a data space for information about dumping data space areas.

Your installation can use the IEASYSxx member of SYS1.PARMLIB to set limits on the total number of SCOPE=COMMON data spaces available to programs. For information about how to set up this member, see z/OS MVS Initialization and Tuning Reference.