Basically, in the series of IIT or inside the BIT we are now going to talk about the computer hardware with it’s benefits. This is the course which we normally read in the unit 2 of IIT subject of BIT 1st sem. So, let’s talk about what is computer hardware with its benefits in detail. Starting with its introduction.


Computer hardware, the three related terms that require introduction are-computer architecture, computer organization and computer design. Computer architecture includes the specifications of the components and techniques for addressing memory.

Computer organization deals with how the hardware components operate. And, Computer design focuses on the hardware to be used and the interconnection of parts.

A computer consists of three main components- (1) Input/ Output (I/O) Unit, (2) Central
Processing Unit (CPU), and (3) Memory Unit. The purpose of I/O unit is to provide data and instructions as input to the computer and to present information as output from the computer.

CPU controls the operations of the computer and processes the received input to generate the relevant output. The memory unit stores the instructions and the data during the input activity.

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Central Processing Unit (CPU) or the processor is also often called the brain of computer.
CPU consists of Arithmetic Logic Unit (ALU) and Control Unit (CU). ALU performs the arithmetic and logic operations on the data that is made available to it.

CU is responsible for organizing the processing of data and instructions. CPU uses the registers to store the data, instructions during processing.CPU executes the stored program instructions, i.e. instructions and data are stored in memory
before execution.

For processing, CPU gets data and instructions from the memory. It interprets
the program instructions and performs the arithmetic and logic operations required for the processing of data. Then, it sends the processed data or result to the memory.

Figure: CPU

2.1 Arithmetic Logic Unit

  • ALU consists of two units—arithmetic unit and logic unit.
  • The arithmetic unit performs arithmetic operations on the data that is made available to it like addition, subtraction, multiplication and division.
  • The logic unit of ALU is responsible for performing logic operations like comparisons of numbers, letters and special characters. Logic operations include testing for greater than, less than or equal to condition.
  • ALU performs arithmetic and logic operations, and uses registers to hold the data that is being processed.

2.2 Registers

  • Registers are high-speed storage areas within the CPU, but have the least storage
  • Registers store data, instructions, addresses and intermediate results of processing.
    Registers are often referred to as the CPU’s working memory.
  • The data and instructions that require processing must be brought in the registers of CPU before they can be processed.

Registers are used for different purposes, with each register serving a specific purpose.
Some of the important registers in CPU are as follows—

1. Accumulator (ACC) stores the result of arithmetic and logic operations.
Instruction Register (IR) contains the current instruction most recently fetched.
Program Counter (PC) contains the address of next instruction to be processed.
Memory Address Register (MAR) contains the address of next location in the
memory to be accessed.
Memory Buffer Register (MBR) temporarily stores data from memory or the data
to be sent to memory.
Data Register (DR) stores the operands and any other data.

The size of register, also called word size, indicates the amount of data with which the
computer can work at any given time. The bigger the size, the more quickly it can process data.

The size of a register may be 8, 16, 32 or 64 bits.

Note :- Nowadays, PCs have 32–bit or 64–bit registers.

2.3 Control Unit

  • It organizes the processing of data and instructions. It acts as a supervisor and, controls and coordinates the activity of the other units of computer.
  • It directs the computer to carry out stored program instructions by communicating with the ALU and the registers.
  • CU uses the instructions in the Instruction Register (IR) to decide which circuit needs to be activated. It also instructs the ALU to perform the arithmetic or logic operations.
  • When a program is run, the Program Counter (PC) register keeps track of the program instruction to be executed next.
  • CU tells when to fetch the data and instructions, what to do, where to store the results, the sequencing of events during processing etc.


A computer program is a set of instructions that describe the steps to be performed for carrying
out a computational task. The program and the data, on which the program operates, are stored in
main memory, waiting to be processed by the processor.

This is also called the stored program concept. An instruction is divided into groups called fields. The common fields of an instruction are— Operation (op) code and Operand code. The remainder of the instruction fields differs from one computer type to other.

The operation code represents action that the processor must execute. The operand code specifies the locations of the data or the operand on which the operation is to be performed.

FIG: Instruction Format

FIG: Instruction format for ADD command


A processor has a set of instructions that it understands, called as instruction set. It relates to
programming, instructions, registers, addressing modes, memory architecture, etc. An instruction in the instruction set involves a series of logical operations that are performed to complete each task. The instruction set is embedded in the processor, which determines the machine language for the processor.


FIG: Examples of some instructions

Two processors are different if they have different instruction sets. A program run on one
computer may not run on another computer having a different processor.

Two processors are compatible if the same machine level program can run on both the processors. Therefore, the system software is developed within the processor’s instruction set.

Microarchitecture is the processor design technique used for implementing the Instruction Set. Computers having different microarchitecture can have a common Instruction Set. Pentium and
Athlon CPU chips implement the x86 instruction set, but have different internal designs.


The primary responsibility of a computer processor is to execute a sequential set of instructions
that constitute a program. CPU executes each instruction in a series of steps, called instruction

A instruction cycle involves four steps:-

1. Fetching: The processor fetches the instruction from the memory. The fetched
instruction is placed in the Instruction Register. Program Counter holds the
address of next instruction to be fetched and is incremented after each fetch.

2. Decoding: The instruction that is fetched is broken down into parts or decoded.
The instruction is translated into commands so that they correspond to those in the
CPU’s instruction set. The instruction set architecture of the CPU defines the way
in which an instruction is decoded.

3. Executing: The decoded instruction or the command is executed. CPU performs
the operation implied by the program instruction. For example, if it is an ADD
instruction, addition is performed.

4. Storing: CPU writes back the results of execution, to the computer’s memory.

FIG: Instruction Cycle

FIG: Steps in instruction cycle

* Instructions are of different categories. Some categories of instructions are:-
1. Memory access or transfer of data between registers.
2. Arithmetic operations like addition and subtraction.
3. Logic operations such as AND, OR and NOT.
4. Control the sequence, conditional connections, etc.

A CPU performance is measured by the number of instructions it executes in a second, i.e., MIPS
(million instructions per second), or BIPS (billion instructions per second).


A processor’s instruction set is a determining factor in its architecture. On the basis of the
instruction set, microprocessors are classified as—Reduced Instruction Set Computer (RISC),
and Complex Instruction Set Computer (CISC). The x86 instruction set of the original Intel 8086
processor is of the CISC type. The PCs are based on the x86 instruction set.


  1. CISC has a large instruction set that includes simple and fast instructions for
    performing basic tasks, as well as complex instructions that correspond to
    statements in the high level language.
  2. An increased number of instructions (200 to 300) results in a much more complex
    processor, requiring millions of transistors.
  3. Instructions are of variable lengths, using 8, 16 or 32 bits for storage.
  4. AMD and Cyrix are based on CISC.


  1. RISC has fewer instructions and requires fewer transistors, which results in the
    reduced manufacturing cost of processor.
  2. The instruction size is fixed (32 bits). The processor need not spend time in
    finding out where each instruction begins and ends.
  3. RISC architecture has a reduced production cost compared to CISC processors.
  4. The instructions, simple in nature, are executed in just one clock cycle, which
    speeds up the program execution when compared to CISC processors.
  5. RISC processors can handle multiple instructions simultaneously by processing
    them in parallel.
  6. Apple Mac G3 and PowerPC are based on RISC.


Pipelining improves instruction execution speed by putting the execution steps into parallel. A
CPU can receive a single instruction, begin executing it, and receive another instruction before it
has completed the first. This allows for more instructions to be performed, about, one instruction
per clock cycle.

Parallel Processing

Parallel Processing is the simultaneous execution of instructions from the same program on
different processors. A program is divided into multiple processes that are handled in parallel in
order to reduce execution time.


  • A bus is a set of wires used for interconnection, where each wire can carry one bit of data.
  • A bus width is defined by the number of wires in the bus.
  • A computer bus can be divided into two types—Internal Bus and External Bus.

Internal Bus

The Internal Bus connects components inside the motherboard like, CPU and system memory. It is also called as System Bus. Figure shows the interaction between processor and memory.

FIG: Interaction between CPU and memory

External Bus

The External Bus connects the different external devices, peripherals, expansion slots, I/O ports and drive connections to the rest of computer. The external bus allows various devices to be attached to the computer. It is generally slower than the system bus. It is also referred to as the Expansion Bus.

NOTE:- A system bus or expansion bus comprise of three kinds of buses- data bus, address bus and control bus.

  • The command to access the memory or the I/O device is carried out by the control bus.
  • The address of I/O device or memory is carried out by the address bus.
  • The data to be transferred is carried by the data bus.

FIG: Interaction between CPU, memory and peripheral devices

7.1 External Ports

The peripheral devices interact with the CPU of the computer via the bus. The connections to the
bus from the peripheral devices are made via the ports and sockets provided at the sides of the

Some of the standard port connections available on the outer sides of the computer
are— port for mouse, keyboard, monitor, network, modem, and, audio port, serial port, parallel
port and USB port.

FIG: Interaction of serial and parallel port interfaces


8.1 Motherboard

The computer is built up around a motherboard. The motherboard is the most important
component in the PC. It is a large Printed Circuit Board (PCB), having many chips, connectors
and other electronics mounted on it.

The motherboard is the hub, which is used to connect all the
essential components of a computer. The RAM, hard drive, disk drives and optical drives are all
plugged into interfaces on the motherboard.

The motherboard contains the processor, memory chips, interfaces and sockets, etc. The Basic Input Output System (BIOS) and Complementary Metal-Oxide Semiconductor (CMOS) are present on the motherboard.


It is the basic program used as an interface between the operating system and the
motherboard. The BIOS is stored in the ROM and cannot be rewritten. BIOS contain the
instructions for the starting up of the computer.

It checks whether the operating system is present on the hard drive. BIOS can be configured using an interface named BIOS setup, which can be accessed when the computer is booting up (by pressing the
DEL key).


CMOS chip

BIOS ROMs are accompanied by a smaller CMOS (CMOS is a type of
memory technology) memory chip. When the computer is turned off, the power supply
stops providing electricity to the motherboard.

When the computer is turned on again, the
system still displays the correct clock time. This is because the CMOS chip saves some
system information, such as time, system date and essential system settings.

FIG: Battery for CMOS chip

8.2 Ports and Interfaces

Motherboard has a certain number of I/O sockets that are connected to the ports and interfaces
found on the rear side of a computer.

  • Serial Port- to connect old peripherals.
  • Parallel port- to connect old printers.
  • USB Ports—to connect newer peripherals like cameras, scanners and printers to the computer.
  • RJ45 connector (called LAN or Ethernet port) is used to connect the computer to a network.
  • Audio plugs (line-in, line-out and microphone), for connecting sound speakers and the microphone.
  • PS/2 port to connect mouse and keyboard into PC.
  • SCSI port for connecting the hard disk drives and network connections.

FIG: Ports on the rear side of a PC

8.3 Expansion Slots

The expansion slots are located on the motherboard. The expansion cards are
inserted in the expansion slots. There are several types of slots:

  • ISA (Industry Standard Architecture) slot—To connect modem and input devices.
  • PCI (Peripheral Component Interconnect) slot—To connect audio, video and graphics.
  • AGP (Accelerated Graphic Port) slot—A fast port for a graphics card.
  • PCI (Peripheral Component Interconnect) Express slot—Faster bus architecture than AGP and PCI buses.
  • PC Card—It is used in laptop computers. It includes Wi-Fi card, network card and external modem.

FIG: Expansion Slots

8.4 Memory Chips

The RAM consists of chips on a small circuit board. Two types of memory chips—
1. Single In-line Memory Module (SIMM) 2. Dual In-line Memory Module (DIMM)

The CPU can retrieve information from DIMM chip at 64 bits compared to
32 bits or 16 bits transfer with SIMM chips. DIMM chips are used in Pentium 4 onwards to
increase the access speed.

FIG: RAM memory chip

8.5 Processor

The processor or the CPU is the main component of the computer. Select a processor based on
factors like its speed, performance, reliability and motherboard support.

Pentium Pro, Pentium 2 and Pentium 4 are some of the processors.

8.6 Hard Disk

A hard disk (HD) consists of one or more platters divided into concentric tracks and
sectors. It is mounted on a central spindle, like a stack. It can be read by a read/write head
that pivots across the rotating disks. The data is stored on the platters covered with
magnetic coating.

FIG: Parts of hard disk

  • Hard disk can store much more data than floppy disk.
  • Hard disk is a fixed disk. The disk is not removable from the drive, unlike floppy disk.
  • Hard disk can spin at the speed of up to 10,000 revolutions per minute and have an access time of 9—14 ms.
  • Nowadays, hard disks are available that can store up to 500 GB of data. Generally, PCs come with 160 GB hard disk.
  • Hard disk is the key secondary storage device of computer. The operating system is stored on the hard disk.
  • Nowadays, portable external hard disk drive is available which can be attached to the USB drive of the computer. They come in the storage capacities of 80 GB to 500 GB.


In this article I have explained about the topic Computer Hardware by providing full details on it.

I have took the ideas and researched about this topic from different websites and books. I hope this article will be very useful and knowledgeable one. Don’t hesitate to share this article with your friends.

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