The central processing unit (CPU) is the heart of the computer systems. It is the most complex computer system component, responsible for directing most of the computer system activities based on the instructions provided. The CPU comprisesthe control unit and the arithmetic / logic unit (ALU) . The control unit is responsible for directing and coordinating most of the computer systems activities. The ALU performs all the arithmetic and logical (comparison) functions — that is, it adds , subtracts, multiplies, divides, and does comparisons.