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