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