A processor core includes a front end, and first and second back ends, the front end including a fetch engine configured to retrieve the sequence of data processing instructions for both the first back end and the second back end from a memory, and the first and second back ends are each configured to execute the sequence of program instructions. The core operates in a first mode in which the first back end is active and receives the sequence of data processing instructions from the fetch engine and the second back end is inactive, and a second mode in which the first back end is inactive and the second back end is active and receives the sequence of data processing instructions from the fetch engine, where the cycles-per-instruction rate is lower and energy consumption is higher for the first mode than the second mode.