Computer is an electronic machine that is used to solve different kinds of problems according to a set of instructions given to it. Computer consists of different units that perform various functions. All the operations of the computer are controlled by the program instructions. These program instructions are known as software. A system is a group of related components that make up a body to perform a specific function. Therefore computer system is defined as:

The computer along with various units and software that performs different activities in data processing are collectively known a computer system. A computer system is divided into two sub-systems:

1. Computer Software
2. Computer Hardware

Computer Software

A set of instructions given to the computer in machine code that tells the computer what to do and how to perform the given task of the user is known as computer software.

 OR

A set of instructions given to the computer in machine code to solve problems or to control different operations of the computer is known as computer software.

the sofware is developed in computer programming languages. You can not feel, touch or see software inside the computer memory.

Computer Hardware

The physical parts of a computer are known as computer hardware. You can touch, see and feel the hardware. The hardware consists of electronic circuits and mechanical equipment etc. used to perform various functions in the computer. The hardware components are:

1. Input devices
2. Output devices
3. Central Processing Unit (CPU)
4. Memory unit
5. Storage devices

Components of Computer

The internal architechtural design of computers differs from one system model to anohter. However the basic components and their functions are same for all computer systems. The construction of a typical comupter (PC) is very simple. It contains different components. Each components performs a specific function in the computer.

Input Devices

The devices that are used to enter data and instructions into the computer are called input devices or units. In old computers the punched card readers, paper tape readers were used as input devices. Now-a-days the most commonly used input devices are Keyboard and Mouse.

Output Devices

The computer processes the given input data and gives the output. The devices that are used to get output from a computer in readable from are called output devices. A number of output devices are available. Some commonly used output devices are video display units (VDU) and printers.

Central Processing Unit

Central Processing Unit is simply called as CPU. It is the main components of the computer and it is also called the Processor. The processor used in microcomputer is referred to as Microprocessor. CPU is considered the brain of the computer. It performs all operations of data according to program’s instructions. It executes the program instructions and tells other parts of the computer what to do. The CPU fetches instructions of program from main memory and executes them one by one. The CPU also has the ability to decide which instruction will be executed next. The speed of the CPU is measured in Mega Hertz (MHz) or Giga (or more).

The CPU is divided into two main prart of sub-units. These are:

1. Control Unit (CU)
2. Arithmetic Logic Unit (ALU)

1. Control Unit (CU)

The control unit is the most important component of the CPU. It controls and coordinates the activities of all the other units of the computer. The control unit is considered as the logical hub of the computer. It acts as a central nervous system for the other components of the computer.

For example the control unit receives the data and instructions from the input device and stores them in the main memory. It performs all these functions by issuing commands to the relevant unit of the computer.

The control unit also controls the execution of instructions given to the computer. It fetches the instructions and data from the memory unit. It decodes and executes the instructions one by one.

2. Arithmetic Logic Unit (ALU)

Arithmetic Logic Unit or simply ALU is another important component of CPU. It performs the arithmetic and logical operations on the data. When the control unit encounters and instruction related to perform arithmetic or logical operation on data, it passes that instruction to ALU.

• In arithmetic operations ALU performs the addition, subtraction, multiplication and division.
• In logical operations ALU compare the numerical data as well as alphabetic data. For example, it checks whether first number is greater than second, less than second or equal to second etc. It must be noted that some of the logical operations can be done on test data. For example when you want to search a word in a document., the CPU matches each word of the docum ent with the given word. Actually the ALU compares each word of document with the given word through relational equal operator (=).

The ALU consists of a number of registers and adder circuits. A register is a temporary storage device which holds data and instruction as long as it is being interpreted (decoded) and executed. When the data and instructions are fetched from main memory for processing, these have to be stored in one of the registers of the CPU.

The motherboard is the main circuit board of the system unit. It is also referred to as system board. Many electronic components are connected to the motherboard and some components are built into it. The processor chip and memory chips are plugged (installed) into the motherboard. The motherboard contains sockets or slots in which electronic components are installed. You can say that motherboard is the master circuit board in a computer.

CMOS

CMOS stands for Complementary Metal Oxide Semiconductor. It is similar to RAM and is a high-speed but it is non-volatile memory. It uses batter to reatin information even when the power to the computer is off. The contents of CMOS can be changed very easily.

CMOS is a special type of memory, which stores the configuration information about the computer such as information about type of disk drives, keyboard, monitorm system current date and time, password, system startup information etc.

Cache Memory

Data and program instructions are moved from RAM to CPU’s registers during data processing. It is most time consuming method and CPU has to waste a lot of time ot access data from memory.

Cache memory is similar to RAM but it is extremely fast than RAM. It is normally used between RAM and CPU. Cache speeds up processing speed of computer because CPU stores frequently used instructions and data in it. When the program is running and the CPU needs a specific data or program instructions, the CPU first checks it in cache memory. If the data is not there, the CPU reads the data from RAM into its registers, but it also loads a copy of the same data or instruction in cache memory. The next time the CPU uses it if required again and saves the time needed to load it from RAM.

Epansion Slot

PC motherboard has two or more expansion slots used to add new components to the computer. The slots hold adapter card also called expansion card. The adapter card provides connections to peripherals. A peripheral is a device that connects to the system unit and is controlled by the CPU. Examples of peripherals are modem, disk drive, printer, scanner, keyboard, mouse etc.

The memory (RAM) chips are also inserted into the expansion slot to expand the size of memory. Similarly network interface card is inserted expansion slot to connect the computer to network and so on.

The types of adapter cards with their functions are given below.

Accelerator Card
It is used to increase the speed of processor.

I/O Card
It is used to connect input/output devices such as printer, mouse keyboard etc.

Game Card
It is used to connect joystick.

Disk Controller Card
It is used to connect disk drives.

Modem Card
It is used to connect other computers through telephone line or cable television line.

Network Card
It is used to connect other computers and peripherals in local area network.

PC-to-TV Connector Card
It is used to connect a television.

Sound Card
It is used to connect a speaker or microphone.

TV Tuner Card
It is used for viewing television channels on the computer monitor.

Video Card
It is used to connect a monitor.

In modem computer, motherboard includes all necessary capabilities and don’t require adapter cards. For example, sound and video functions may be built into the mother board.

In the past, it was very difficult job to install a card. The computer technician had to set switches and other elements on the motherboard. Now-a-days, computers have plug and play capabilities. It means that a computer automatically can configure adapter card when it is installed into it. In these computers, you can easily plug in a device and turn on the computer. The device is ready to use (It means device is ready to play its role).

Computer Buses

We know that data and instructions inside the computer are stored and moved from one unit to another in the form of bits. Actually the bits are moved in the form of electric pulses. Therefore, computer bus is defined as:

The electric paths through which computer (CPU) sends and receives data and instructions (and also sends command signals) to and from different components of computer are called computer buses.

The computer bus is backbone of the computer and computer cannot perform any function without it. The computer bus is like electric wiring used to create circuits. However computer buses are used to connect devices with the CPU.

Typically a bus consists of 40 to 150 electric wires or lines running parallel to each other. One line can carry one bit at a time. The capacity of a computer bus depends on the number of data lines it contains. For example a bus with 16 lines can transfer 16 bits at a time.

In the past the performance of computer buses was measured by the number of bits they could transfer at one time. Hence, the newest 64-bit buses are typically considered the fastest for data transfer. However performance of buses was measured according to their data transfer rates. It was measured in Mbps (Megabits per second).

Today data transfer rate of a computer bus is measured in MHz. Every bus also has a clock speed, just like the processor. The clock speed for a computer bus is measured in hertz (Hz). Most of today’s processor have a computer bus clock speed from 100 MHz to 400 MHz (one Mega Hertz is equal to one million ticks per second). The higher the computer bus clock speed, the faster the transmission of data. It means that due to higher the computer bus clock speed, application running and data accessing speed will be very fast.

A computer has two basic types of buses. These are:

1. Expansion buses
2. System buses

Expansion Buses
Expansion buses are also referred to as External Buses. These buses connect the external devices such as keyboard, mouse, modem, printer etc. to the processor. Expansion buses allow the processor to communicate with the peripherals. We connect a peripheral with the system unit through a port on an adapter card, which is inserted into an expansion slot on the motherboard.

The basic types of expansion buses are:

ISA Bus
ISA stands for Industry Standard Architecture. It is most common and slowest bus used to connect mouse, model card, sound card and low-speed network card.

PCI Bus
PCI stands for Peripheral Component Interconnect. PCI is a high-speed local bus used to connect video card, sound card and high-speed network card. This bus transfers data about four times faster than the ISA bus. Today most of the PCs have a PCI bus as well as ISA bus.

AGP Bus
AGP stands for Accelerated Graphics Port. AGP bus is specially designed by Intel to improve the transmission speed of video and 3-D graphics. The AGP bus provides a faster, dedicated interface between the video card and memory.

USB
USB stands for Universal Serial Bus. In a computer with a USB, USB devices can be connected to each other outside the system unit and then single cable attaches to the USB port. The USB port then connects to the USB, which connects to the PCI bus on the motherboard. This bus eliminates the need to install cards into the expansion slots.

System Buses
The system buses are also called the Internal Buses. The system buses are the part of motherboard and connect the processor to the main memory as well as other devices that reside on the motherboard. The system buses are classified according to their functions as:

1. Data bus
2. Address bus
3. Control bus

1. Data Bus
The electric path through which the contents of data (or actual data) are transferred from one unit to another is called Data Bus. Actually the data bus is a group of parallel wires. The number of wires in the bus affects the speed at which data can transfer from one component of computer to another. Each wire can transfer one bit of data at a time. A data bus with 8 wires can transfer 8 bits (one byte) of data at a time. The modern PCs have 64-bit data bus and can transfer 8 bytes of data at a time. The data bus connects the CPU, memory unit and other hardware devices on the motherboard.

2. Address Bus
We know that the components of computer are connected to one another through the system bus. So it is important to assign a unique ID to each component. This ID is called the address of that component. Therefore the computer bus, which is used to identify different components of a computer as well as to specify the address of different memory locations is called address bus. For example when a computer component wants to communicate with other component, it specifies the address of the destination component. Similarly when CPU wants to write some data at memory location 932, it places the address of main memory and the address of location i.e., 932, on the address bus. When the main memory sees its address on the address bus, it reads the data from the data bus and writes it to the specified location within the main memory.

The capacity of address bus depends upon the number of its wires. If the address bus has 8 wires then CPU can address 256 bytes of memory, i.e. 2⁸ = 256. Now-a-days most CPUs have 32-bit address buses can address 4 GB of memory.

3. Control Bus
The electric path through which the control unit sends out control signals (commands) to control the sequence of operations that take place while a program is being executed, is called control bus. For example, the control unit directs the transfer of data from the main memory to the ALU for arithmetic or logical operations. Similarly data flow between secondary memory and main memory is controlled by the control unit through this bus.

The control bus directly connects the CPU, main memory and I/O ports. We know that many system devices are connected to the data bus but only one of these is enabled at a time. The CPU provides the enabling signal through the control bus lines. For example if the CPU wants to read data from the port attached to the keyboard, it will perform this activity as follow:

• sends out the address of keyboard port on the address bus.
• enables keyboard port by sending a signal to it on a control bus line.
• reads the data from the data bus. The data is put on it by the port.

Ports

Power Supply

Power supply is an electrical component inside the system unit that converts the A.C power into D.C power. Power Supply supplies the correct voltage or power to all parts of the comptuer. It is a simple electrical system, which takes 240 or 110 A.C voltages and converts them into D.C voltages of 3V, 6V, 12V and 24V. The D.C voltage is required to operate different components of a comptuer.

Some exterals peripherals such as an external modems, speakers etc. have an A.C adapter, which converts the A.C power into D.C power that the peripheral requires.

The input-output or I/O devices provide the means of communication between the computer and the user. The input and output devices are the hardware components that allow the user to input data and instructions into the computer and to receive the processed data. The data and instructions are given to the computer through input device. The computer processes the data according to the given instructions and output is received onto the output device or it is stored permanently on the storage device.

Various types of input/output devices are available used for different types of applications. The I/O devices are slower as compare to the speed of CPU and primary storage. It is because the speed of I/O devices depends upon the mechanical movement of the components of the devices. It is impossible that the I/O devices and the CPU and primary storage have the same speed.

Input and Input Devices

Input is any data or instructions entered into the computer in the form of signals. The input into the computer can be entered:

• Through keyboard (by typing characters).
• By selecting commands (icons) on the screen and then clicking with mouse.
• By pressing finger on a touch screen.
• By speaking into a microphone.
• By sending image through digital camera.
• By scanning data printed on paper through scanner etc.

Therefore, the devices that are used to enter data and instructions or commands into the computer are called input devices or units. The input devices are the eyes and ears of computers. In old computers the punched card readers, paper tape readers were used as input devices. Now-a-days, the commonly used input devices are:

• Keyboard
• Mouse
• Touchpad
• Trackball
• Joystick
• Light Pen
• Digital Scanner
• Digital Camera
• Microphone

It must be noted that floppy drives, hard disks and CD drives are used for both input and output devices. Now-a-days, the most popular input devices are keybaord and mouse. The other input devices are used for special purposes.

Output and Output Devices

The processed input data into a useful form is called output when input data is processed, computer generates several types of output, depending upon the hardware and software used and the requirements of the user. The user use output on a screen, prints it on the printer or hear it through speakers or head sets.

Therefore, output is normally classifieds as:

An output device is a hardware component used to get output from the computer. A number of output devices are available. The commonly used output devices include display device (Monitor), printer, speaker and headset, fax machine etc.