Draw the functional block diagram of CPU and describe its function.

Subject Introduction to Computer System
NU Year Set: 1.(b) Marks: 6 Year: 2013

A CPU or Central Processing Unit is the heart of a computer and is installed in a socket specified on a motherboard. Since a CPU performs a lot of calculations at a high speed, it gets heat up quickly. To cool down the temperature of a CPU a cooling FAN is installed on it.

Control Unit

The Control Unit is an internal part of a CPU that co-ordinates the instructions and data flow between CPU and other components of the computer. It is the CU that directs the operations of a central processing unit by sending timing and control signals.

Arithmetic Logic Unit

The ALU is an internal electronic circuitry of a CPU that performs all the arithmetic and logical operations in a computer. The ALU receives three types of inputs.

Control signal from CU ( Control Unit )

Data(operands) to be operated

Status information from operations done previously.

When all the instructions have been operated, the output that consists of data is stored in memory and a status information is stored in internal registers of a CPU.

Working of a CPU

All the CPUs regardless of their origin or type performs a basic instruction cycle that consists of three steps named Fetch, decode and Execute


A program consists of a number of instructions. Various programs are stored in memory. During this step, the CPU reads instruction that is to be operated from a particular address in the memory. The program counter of CPU keeps the record of address of the instructions.


A circuitry called instruction decoder decodes all the instructions fetched from the memory. The instructions are decoded to various signals that control other areas of CPU.


In the last step, the CPU executes the instruction. For example, it stores a value in the particular register and the instruction pointer then points to other instruction that is stored in next address location.

Clock Speed           

The speed of processor is measured by the number of clock cycles a CPU can perform in a second. The more the number of clock cycles, the more number of instructions (calculations) it can carry out. The CPU speed is measured in Hertz. Modern Day processors have speed units of GHz. ( 1GHz=1 million thousand cycles per second).

CPU Manufacturers

The 3 major giant manufacturers of CPUs used in desktop or laptops are Intel, AMD and VIA(Embedded devices) while Qualcomm, Samsung and Apple are the top three manufacturers of mobile processors.


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