Each of the smaller rectangles are registers, composed of 16 flip-flops, except for the MAR, which is 12 flip-flops. They have 16 input wires and 16 output wires, one per flip-flop. In addition, there is a load wire which is connected to the CK wires of each flip-flop. The register stores a new value only when this load wire goes high. Fig. 7.1.2 shows a block diagram of a 16-bit register.