The invention provides a processor comprising an execution unit for executing multiple threads, each thread comprising a sequence of instructions and each thread being designated to handle activity from at least one specified source. The processor also comprises a thread scheduler for scheduling a plurality of threads to be executed by the execution unit, said scheduling being based on the respective activity handled by the threads; and a plurality of sets of registers connected to the execution unit. Each set of registers is arranged to store information representing a respective one of the plurality of threads, at least a part of the information being accessible by the execution unit for use in executing the respective thread when scheduled.