For mux control wires, 1 is the most significant bit and 0 is the least. Each data input to a mux is labeled from 0 up to 3, or 2 or 1 if there are fewer inputs. For those muxes with more than two data inputs (A-MUX, MAR-MUX and MBR-MUX), the input is selected according to the binary number that MUX1/MUX0 form. To illustrate, suppose that we desired the MAR to get the output of S. The wire from S-OUT to MAR-MUX is labeled 2, which is 10 in binary. Thus, MAR-MUX1 must be 1 and MAR-MUX0 must be 0. We will call the several inputs to a MUX its ports. Thus, MARMUX has ports 0, 1 and 2.