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