Circuitry for providing error check values for indicating errors in data portions within a data stream. The circuitry comprises error detecting code generation circuitry configured to apply an error detecting code algorithm to the data stream and to thereby generate and periodically update a multi-bit check value as the data stream is processed, each update of the multi-bit check value being indicative of the error detecting code generation circuitry receiving a further item of the data stream. An output for periodically outputting a fragment of the multi-bit check value from the error detecting code generation circuitry during the processing of the data stream, the fragments output each corresponding to a data portion of the data stream. Wherein each of the fragment of the multi-bit check value provides a value indicative of an error occurring either in the corresponding portion of the data stream or in an earlier portion of the data stream.