Abstract: An arithmetic and logic unit carries out arithmetic or logic operations on long operands. The unit comprises: an operation unit having a processing location, and configured for carrying out processing on bits at the processing location, the processing comprising any of a plurality of pre-defined arithmetic or logical operations, the processes being defined for a first number of bits determined by the operand word length; a fetch and write unit comprising direct memory access circuitry for fetching a second number of bits of operand data by direct access from an external memory and for writing results to memory, the second number being set by a predetermined memory access width; the second number being smaller than said operand word length, and the direct memory access circuitry being configured to deliver said second number of bits directly to the processing location without aggregation prior to processing.