Cross memory communication takes place when a user program issues a PC instruction that specifies a valid PC number. The PC number identifies the PC routine that the system is to invoke. The service provider must have previously defined the PC routine and made the PC number available to the user. The PC routine can provide the requested service or can invoke other programs to provide the service. When the PC routine completes its function, it issues either the PR instruction (for a stacking PC) or a PT instruction (for a basic PC) to return control to the user program.