means for providing clock pulses for {HEAD ~ operation} of {TAIL ~ said shift register} in read/write operations 
means for providing clock pulses for { HEAD ~ operation } of { TAIL ~ said shift register } in read / write operations
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH