{HEAD ~ The barrier region} allows {TAIL ~ the I/O driver} to be designed to primarily meet I/O requirements 
allows
{ HEAD ~ The barrier region } allows { TAIL ~ the I / O driver } to be designed to primarily meet I / O requirements
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH