The central processing unit, or simply processor, is the engine that interprets (or executes) instructions stored in main memory. At its core is a word-size storage device (or register) called the program counter (PC). At any point in time, the PC points at (contains the address of) some machine-language instruction in main memory. The CPU also has an instruction register where the bits of the instruction are stored so that the instruction can be executed. Before execution, the bytes in memory pointed to by the PC will be brought into the IR, and then the instruction is executed.