This invention relates to a data processing apparatus comprising: a processor (10); a first memory location, the processor (10) being adapted to control whether the operation of the processor may be interrupted in dependence on a value stored in the first memory location; and a second memory location, the processor (10) being adapted a) to store a value in the second memory location following an event that is related to the value stored in the first memory location, and b) otherwise to store a value in the second memory location that is not related to the value stored in the first memory location. This invention also relates to a method of data processing.