The system identifies a DESERV exit by a DST (DESERV Screen Table). The system maintains at most one task level DST for each task, and at most one DST for the system (which represents the global exit). However, multiple DESERV exits can be established. To support multiple exits of a given type (task or global), when a DESERV FUNC=EXIT is issued with EXIT_OPTION=REPLACE, DESERV returns the address of the DST that was replaced (or zero if no DST was replaced). Then it is the responsibility of the newly defined exit to pass control to the previously defined exit.