comparing {HEAD ~ the original value} of the data unit with a value of {TAIL ~ the update data}; and 
comparing { HEAD ~ the original value } of the data unit with a value of { TAIL ~ the update data } ; and
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH