In z/OS®, cross-memory communication allows a program in one address space to communicate with a program in another address space.

With proper page table management by the operating system, users and applications in different address spaces are completely isolated from each other. One exception to this isolation is the common area. Another exception is cross-memory communication.

A number of cross-memory capabilities are possible, but two are commonly used:

• Ability to call a program that resides in a different address space
• Ability to access (fetch, store) virtual memory in another address space.

The first case uses the program call (PC) instruction. Here, only a single hardware instruction is needed to call a program in another address space. A common example of this involves DB2®, an IBM® database management product. Various parts of DB2 occupy up to four address spaces. Users of DB2 can be TSO users, batch jobs, and middleware, such as a Web server. When these users issue SQL instructions for DB2, the SQL interface in the application uses a program call to obtain services from the DB2 address spaces.

Cross-memory programming must be coordinated through z/OS security controls. In practice, almost all cross-memory usage is in major middleware products and is rarely directly used by typical application programs.