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