The present invention provides a data processing apparatus and method for managing processor configuration data. The data processing apparatus comprises a processor operable in a plurality of modes and a plurality of domains, said plurality of domains comprising a secure domain and a non-secure domain, said plurality of modes including at least one non-secure mode being a mode in the non-secure domain, at least one secure mode being a mode in the secure domain, and a monitor mode. The processor is operable such that when executing a program in a secure mode the program has access to secure data which is not accessible when said processor is operating in a non-secure mode. A storage unit is used to store processor configuration data, and the processor is operable at least partially in the monitor mode to execute a monitor program to manage switching between the secure domain and the non-secure domain, the switching including switching the processor configuration data in the storage unit between secure processor configuration data and non-secure processor configuration data. When in the monitor mode, the monitor program is operable to use monitor mode specific processor configuration data, thereby ensuring that operation of the processor in the monitor mode is unaffected by the switching of the processor configuration data.