You can use the SVCUPDTE macro or the IEASVCxx parmlib member to define a user-written SVC routine as non-preemptable for I/O interruptions. If a non-preemptable SVC routine sustains an I/O interrupt, the SVC, rather than the highest priority ready work, gets control when I/O processing is complete. The non-preemptable SVC cannot issue other SVCs and remain non-preemptable. If a non-preemptable SVC issues a STAX DEFER=NO macro, the SVC routine remains non-preemptable until it exits. For a description of the STAX macro and its syntax, see z/OS TSO/E Programming Services.