A data processing apparatus includes a plurality of processing elements arranged in a single instruction multiple data array. The apparatus includes an instruction controller operable to receive instructions from a plurality of instructions streams, and to transfer instructions from those instructions streams to the processing elements in the array, such that the data processing apparatus is operable to process a plurality of processing threads substantially in parallel with one another. A data transfer controller is provided which is operable to control transfer of data between the internal memory units associated with the processing elements, and memory external to the array.