Central Processing Unit {CPU}

The computer's central processing unit (CPU) is the portion of a computer that retrieves and executes instructions. The CPU is essentially the brain of a CAD system. It consists of an arithmetic and logic unit (ALU), a control unit, and various registers. 

The CPU is often simply referred to as the processor. The ALU performs arithmetic operations, logic operations, and related operations, according to the program instructions. The control unit controls all CPU operations, including ALU operations, the movement of data within the CPU, and the exchange of data and control signals across external interfaces (system bus). 

Registers are high-speed internal memory-storage units within the CPU. Some registers are user-visible; that is, available to the programmer via the machine instruction set. Other registers are dedicated strictly to the CPU for control purposes. An internal clock synchronizes all CPU components. The clock speed (number of clock pulses per second) is measured in megahertz (MHz) or millions of clock pulses per second. The clock speed essentially measures how fast an instruction the CPU processes.