In order to carry out the operations  computer allocates the task between its various functional units.The computer system is divided into three separate units for its operation. These are 1) arithmetic logical unit, 2) control unit, and 3) central processing unit.

(a) Arithmetic Logical Unit (ALU)

After you enter data through the input device it is stored in the primary storage unit. Arithmetic Logical Unit performs the actual processing of data and instruction. The major operations performed by the ALU are addition, subtraction, multiplication, division, logic and comparison. Data is transferred to ALU from storage unit when required. After processing, the output is returned back to storage unit for further processing or getting stored.

(b)Control Unit (CU)

The next component of computer is the Control Unit, which acts like the supervisor seeing whether things are done in proper fashion. The control unit determines the sequence in which computer programs and instructions are executed. Things like processing of programs stored in the main memory, interpretation of the instructions and issuing of signals for other units of the computer to execute them. It also acts as a switch board operator when several users access the computer simultaneously. Thereby it coordinates the activities of computer’s peripheral equipment as they perform the input and output. Therefore it is the manager of all operations mentioned in the previous section.

(c) Central Processing Unit (CPU)

The ALU and the CU of a computer system are jointly known as the central processing unit. You may call CPU as the brain of any computer system. It is just like a human brain that takes all major decisions, makes all sorts of calculations and directs different part of the computer by activating and controlling the operations.