When a user issues a PC instruction, the system transfers control to a PC routine. An entry in the service provider's entry table defines the PC routine that is to receive control. Data in this entry also determines the type of linkage the system will use to invoke the PC routine. The types of linkages are stacking and basic.

The stacking and basic PC linkages share some common capabilities. The stacking linkage, however, offers more capability and provides better performance than does the basic linkage. The service provider uses the PC parameter on the ETDEF macro to define the type of linkage that will be used. Stacking is the default. IBM® recommends the use of the stacking PC linkage. This document refers to a PC routine as either a stacking PC routine or a basic PC routine depending on the linkage used to invoke the routine.