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