The value field of the DATASUM keyword shall be a character string containing the unsigned integer value of the checksum (not the complement of the checksum in this case) of the data records of the HDU. This number is not ASCII encoded, but is formatted as a string since FITS support for unsigned integer keyword values is problematic. Any software that needs to read this value should perform normal input conversions to allow handling the value as an unsigned integer as allowed by the host system. For dataless HDU's, this keyword may either be omitted, or the value field shall contain the string value '0', which is preferred. A missing DATASUM keyword asserts no knowledge of the checksum of the data records.