A data processing system includes a processor 46 operable to generate control signals to control one or more further circuits 4, 6 to adopt operational states to support different performance levels of the processor. The one or more further circuits generate current operation signals indicative of their current operation. Examples of further circuits are a clock generator 4 which generates a current operation signal indicative of a currently generated clock signal or possibly currently available clock signals. A voltage controller 6 may be a further circuit which serves to provide a power signal to the system and which generates a current operation signal indicative of the current maximum voltage level which the voltage controller is able to supply.