An apparatus has processing circuitry to perform data processing in response to instructions; at least one control storage element to store internal state for controlling operation of the processing circuitry; and checksum generating circuitry to generate a checksum based on at least one item of internal state stored in the at least one control storage element. The checksum is stored in a diagnostic storage location from which information is accessible to a diagnostic control agent (e.g. software executing on the processing circuitry and/or an external device). This makes design of software test libraries for detecting hardware faults much more efficient.