The main part of this Skeletal Processor is PC, Program Counter, which contains the address of the machine instruction and is capable of incrementing one itself (PC+1->PC) in the second subcycle, SC1. It consists of three units; PC as a general 12 bit register, ADD1 unit which performs the addition, and a 12 2-to-1 multiplexer, each of which chooses either external input line or the output from ADD1 unit. In order to study for details, click each unit in the schematic.