1. A processor for executing instructions, comprising; a data store, an instruction store, a combiner for combining instructions and data associated with a respective one of the instructions, processing elements for carrying out the instructions and outputting results, wherein the processing elements and the combiner comprise trusted circuitry, the trusted circuitry comprising circuitry whose design has been proven to operate correctly, and comprising self checking circuitry for checking that it has not operated incorrectly, the processor further comprising circuitry for checking for errors in data and instructions input to the processing elements and to the combiner.