{TAIL ~ A multiplexer}, selectively couples data from {HEAD ~ either the main memory device} or the shadow memory device to an output terminal in response to a control signal 
{ TAIL ~ A multiplexer } , selectively couples data from { HEAD ~ either the main memory device } or the shadow memory device to an output terminal in response to a control signal
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH