There is a provided a data processing system comprising: a processor operable in a plurality of modes and either a secure domain or a non-secure domain including: at least one secure mode being a mode in said secure domain; and at least one non-secure mode being a mode in said non-secure domain; wherein when said processor is executing a program in a secure mode said program has access to secure data which is not accessible when said processor is operating in a non-secure mode; and wherein said processor is responsive to a switching request to initiate a switch between a secure mode and a non-secure mode under control of a mode switching program starting at a location specified by an exception vector associated with said switching request.