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Central Processing Unit

Central processing unit is called the brain of computer system. It is also known as microprocessor when it is constructed on a single chip. Arithmetic logic and control unit along with registers forms a complete central processing unit and central processing unit along with input/output unit and memory unit forms a computer system.

Instruction Execution

The program which is to be executed is a set of instructions which are stored in memory. The CPU executed the instructions of the program to complete task. The major responsibilities of the instruction execution are with the CPU. The instruction execution takes place in the CPU with the help of registers, ALU and CU. When the processor executes instructions, data is temporarily stored in small, local memory locations of 8, 16, 32 or 64 bits called registers. These registers are:-
  • Accumulator Register:- Which stores the intermediate result of arithmetic and logical operations.
  • Memory Address Register (MAR):- Which contain the address of memory location to which data is to be stored.
  • Program Counter (PC):- Which contain the address of the next instruction to process .
  • Instruction Register (IR):- Which contain the current instruction being processed.


Microprocessors processes the instruction. The microprocessor is made of millions of transistors. These are tiny electronic devices that carry an electric charge. The first microprocessor 4004 was invented by INTEL in 1971. It was 4 bit microprocessor designed for specific application. In 1972, first general purpose microprocessor 8008 was introduced. It was 8 bit microprocessor. At present 32/64 bit processor present in the market.

Note:- All CPU’s are known as microprocessor but all processors are not CPU. Because CPU control all the components of the computer system where as processor only process the instruction. Processors can’t control any component.

Control Unit

Control Unit controls the flow of information within the computer system. Control units are in-charge of the computer. Control units decode machine instructions and generate control signal. Control units may also control some external devices.

ALU (Arithmetical Logical Unit)

An arithmetic/logic unit (ALU) performs arithmetic and logic operations. It performs only four types of mathematical operations i.e. +, -, etc..

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