1. A parallel processor for processing a plurality of different processing instruction streams in parallel, the processor comprising: a plurality of data processing units; and a plurality of SIMD (Single Instruction Multiple Data) controllers, each connectable to a group of data processing units of the plurality of data processing units, and each SIMD controller arranged to handle an individual processing task with a subgroup of actively connected data processing units selected from the group of data processing units; wherein the parallel processor is arranged to vary dynamically the size of the subgroup of data processing units to which each SIMD controller is actively connected under control of received processing instruction streams, thereby permitting each SIMD controller to be actively connected to a different number of processing units for different processing tasks.