1. Apparatus for performing data processing operations specified by program instructions, said apparatus comprising: return logic responsive to a return instruction having an address as an input operand to trigger a return to execution of a program instruction indicated by said address; wherein (i) if said address has a value not matching one or more predetermined address values, then said return logic triggers a first return response comprising one or more first return response operations to yield a first returned state; and (ii) if said address has a value matching said one or more predetermined address values, then said return logic triggers a second return instruction response comprising one or more second return response operations to yield a second returned state, said one or more second return instruction response operations differing from said one or more first return instruction response operations.