A diagnostic data generation apparatus for a display controller comprises an underrun detector arranged to monitor, when in use, buffer depletion in order to detect an underrun condition. The underrun condition results from a data feed lag associated with a mismatch between a buffer fill rate and a predetermined output data rate. The underrun detector is arranged to generate diagnostic data in response to detection of the underrun condition, the diagnostic data identifying the underrun condition and a location in an array of pixels associated with the underrun condition.