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