All mainline exits receive the address of an exit parameter block (EXPB) passed in register 0. Each exit receives the address of an address list passed in register 1. Each address in the list points to a parameter.
Figure 1 illustrates the contents of register 0 and EXPB for the mainline exits.
Figure 1. EXPB control block for mainline exits
Both the mainline exits and the services exits receive the address of EXPB in register 0. Some of the fields in EXPB are used only by the service exits and are reserved fields for the mainline exits.