When a user invokes a basic PC routine, the PC routine must save the user's environment. To save the environment, the PC routine can issue the PCLINK macro or can provide code that performs the save function. After completing its function, the PC routine must restore the user's environment. Again, the PC routine can use the PCLINK macro or provide code that performs the restore function. Only PC routines that run in supervisor state can issue the PCLINK macro. PC routines that run in problem state must provide code that performs the same functions as the PCLINK macro.

To return control to the user, the PC routine issues the PT instruction. To execute a PT instruction, the service provider must have previously been granted PT authority by the user. PT authority means that the service provider is authorized to issue the PT instruction with the target being the user's address space. The AXSET macro provides the means to grant PT authority.

The following figure shows how a user invokes a PC routine to obtain a service.

Figure 1. PC routine invocation