A data processing system includes: a processor; a data interface for communication with a control unit, the processor being on one side of the data interface; internal storage accessible by the processor, the internal storage being on the same side of the data interface as the processor; and a register array accessible by the processor and comprising a plurality of registers, each register having a plurality of vector lanes. The storage is arranged to store control data indicating an ordered selection of vector lanes of one or more of the registers. The processor is arranged to, in response to receiving instruction data from a control unit, perform a swizzle operation in which data is selected from one or more source registers in the register array, and transferred to a destination register. The data is selected from vector lanes in accordance with control data stored in the internal storage.