At each call, the exit receives the address of the KGUP exit parameter block (KGXP) in register 1. The exit can access any of the data in KGXP. The exit can alter some of the fields in KGXP, while others are simply references. Also, the KGUP exit can alter some fields at some calls but not at other calls.
A field in KGXP gives the calling point of the exit. The exit uses this field to determine when to call the exit to perform appropriate processing. Input gives a more detailed explanation of the KGXP control block, the values it contains, and when an exit can use or change the values.