Synchronous cross memory communication enables one program to provide services synchronously to other programs. This document calls the program that provides the services, the service provider, and the programs that use the services, users. The service provider can provide services to one user or to many.

Synchronous cross memory communication takes place between the user and the service provider when the user issues a program call (PC) instruction. If the service provider has previously established the necessary environment, the PC instruction transfers control to a service provider program called a PC routine. The PC routine provides the requested service and then returns control to the user.

The user program and the PC routine can execute in the same address space or in different address spaces. In either case, the PC routine executes under the same TCB as the user. Thus, the PC routine provides the service synchronously.

A PC routine can access (fetch or store) data in the user's address space by using access registers (ARs) and the full set of assembler instructions. If the PC routine has the proper authority, it can also access data in other address spaces or in data spaces. For information about using access registers, see Using access registers.

The rest of this section discusses when you should consider using synchronous cross memory communication and explains the environment the service provider must create and how to create it.