The fixed nonlinear function processing units in a given stage implement only the nonlinear function needed for {HEAD ~ the MD5 round} being performed by {TAIL ~ the stage} 
performed by
The fixed nonlinear function processing units in a given stage implement only the nonlinear function needed for { HEAD ~ the MD5 round } being performed by { TAIL ~ the stage }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL