lower most decoders 58 60 are connected directly to the dispatch unit 62 the decoders 50 to 60 are always u201c on u201d that is on each machine cycle they receive the bits on the connection paths cp input to the decoders perform a decode operation and supply an output it can readily be seen therefore that of the top four decoders the outputs of only two of them in each case are of interest for example in gp 32 mode it is the outputs of the decoders 50 54 which are required u2014 the outputs of the decoders 52 56 being redundant a mode signal mode supplied to the multiplexors mux 6 mux 7 selects the appropriate outputs in dependence on the instruction mode of the machine although the decoders are normally u201c on u201d they are responsive only to change in state of the bits supplied to them from the output devices ff 0 to ff 3 if between two machine cycles there is no change in state in the outputs of the output devices then the connected decoder will not need to change any of its internal state thus no power will be consumed by that decoder for as long as the outputs of the output device ff 0 to ff 3 connected to.