Which routine receives control after an interrupt is processed depends on whether the interrupted unit of work was preemptable.

If so, the operating system determines which unit of work should be performed next. That is, the system determines which unit or work, of all the work in the system, has the highest priority, and passes control to that unit of work.

A non-preemptable unit of work can be interrupted, but must receive control after the interrupt is processed. For example, SRBs are often non-preemptable. Thus, if a routine represented by a non-preemptable SRB is interrupted, it will receive control after the interrupt has been processed. In contrast, a routine represented by a TCB, such as a user program, is usually preemptable. If it is interrupted, control returns to the operating system when the interrupt handling completes. z/OS® then determines which task, of all the ready tasks, will execute next.