A p code frame or simply frame represents a unit of {HEAD ~ action} or operation in {TAIL ~ the system} 
in
A p code frame or simply frame represents a unit of { HEAD ~ action } or operation in { TAIL ~ the system }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL