Five Component Model of a Computer

Computer: A computer system consists of following five components that performs five major operations: 

1. Input Unit:- This accepts data or instructions as input,
2. Storage Unit:- This stores data and instruction.  
3. Arithmetic logic unit(contained in CPU):- It processes data as per the instructions. 
4. Control unit(contained in CPU):- It controls all operations inside a computer.
5. Output Unit:- This gives results in the form of output. 

Central Processing unit(CPU) includes Arithmetic logic unit and control unit, as shown in the figure:

1. Input Devices

Data and instructions must enter the computer system before any computation can be performed on the supplied data. Input Devices link the external environment with the computer system to perform this task. Input Devices are used for entering data and programs into the computer system by the user for processing. Some Examples of Input Devices are: 

• Keyboard: The keyboard is very much like a standard typewriter keyboard with a few additional keys. The additional keys are included to perform certain special functions. These are known as function keys that vary in number from keyboard to keyboard.
• Mouse: A device that controls the movement of the cursor or pointer on a display screen. A mouse is a small object you can roll along a hard and flat surface. As you move the mouse, the pointer on the display screen moves in the same direction.
• Trackball: A trackball is an input device used to enter motion data into computers or other electronic devices. It serves the same purpose as a mouse, but is designed with a moveable ball on the top, which can be rolled in any direction.
• Touchpad: A touch pad is a device for pointing (controlling input positioning) on a computer display screen. It is an alternative to the mouse. Originally incorporated in laptop computers, touch pads are also being made for use with desktop computers. A touch pad works by sensing the user’s finger movement and downward pressure. 
• Touch Screen: It allows the user to operate/make selections by simply touching the display screen. A display screen that is sensitive to the touch of a finger or stylus. Widely used on ATM machines, retail point-of-sale terminals, car navigation systems, medical monitors industrial control panels and smart phones.
• Light Pen: Light pen is an input device that utilizes a light-sensitive detector to select objects on a display screen.
• Magnetic ink character recognition (MICR): MICR can identify character printed with a special ink that contains particles of magnetic material. This device particularly finds applications in banking industry.
• Optical mark recognition (OMR): Optical mark recognition, also called mark sense reader is a technology where an OMR device senses the presence or absence of a mark, such as pencil mark. OMR is widely used in tests such as aptitude test.
• Bar code reader: Bar-code readers are photoelectric scanners that read the bar codes or vertical zebra strips marks, printed on product containers. These devices are generally used in super markets, bookshops etc.

2. Storage Unit:  The data and instructions which are entered into the computer system through input
devices (keyboard, mouse etc.) have to be stored inside the computer before the actual processing starts. Similarly, after processing, the results produced by the computer must be kept somewhere inside the computer system before being passed onto the output devices (monitor/screen or printer). Thus, the storage unit of a computer system is designed to take care of all these needs. Storage unit generally has primary memory and secondary/auxiliary memory for storing data. 

A central processing unit (CPU) is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logical, control and input/output (I/O) operations specified by the instructions. It mainly have two sub units.These are Arithmetic Logic Unit and Control Unit.  

3. Arithmetic Logic Unit: All calculations and comparisons, based on the instructions provided, are carried out within the ALU. It performs arithmetic functions like addition, subtraction, multiplication, division and also logical operations like greater than, less than and equal to etc.

4. Control Unit: Controlling of all operations like input, processing and output are performed by control unit. It takes care of step by step processing of all operations in side the computer.
Memory

5. Output Unit:  Output Unit handles the process of producing results from the data for getting useful information through the screen or printer. The output generated by a computer can be transmitted to the user via some devices or media. These devices are called output devices. The following are some examples of different types of output devices commonly found on a computer.

• Monitor
• Printer
• Sound card & Speakers
• Video card
• Plotter

Five Generations of Computers

• The first generation computers used vacuum tubes for circuitry and magnetic drums for memory, and were often enormous, taking up entire rooms. These computers were very expensive to operate and in addition to using a great deal of electricity, the first computers generated a lot of heat, which was often the cause of malfunctions. First generation computers relied on machine language, the lowest-level programming language understood by computers, to perform operations, and they could only solve one problem at a time. It would take operators days or even weeks to set-up a new problem. Input was based on punched cards and paper tape, and output was displayed on printouts.
• Transistors replaced vacuum tubes in the second generation of computers. The transistor was invented at Bell Labs in 1947 but did not see widespread use in computers until the late 1950s. The transistor was far superior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficient and more reliable than their first-generation predecessors. Though the transistor still generated a great deal of heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Second-generation computers still relied on punched cards for input and printouts for output.Second-generation computers moved from cryptic binary machine language to symbolic, or assembly, languages, which allowed programmers to specify instructions in words.
• The development of the integrated circuit was the hallmark of the third generation of computers. Transistors were miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speed and efficiency of computers. Instead of punched cards and printouts, users interacted with third generation computers through keyboards and monitors and interfaced with an operating system, which allowed the device to run many different applications at one time with a central program that monitored the memory. Computers for the first time became accessible to a mass audience because they were smaller and cheaper than their predecessors.
• The microprocessor brought the fourth generation of computers, as thousands of integrated circuits were built onto a single silicon chip. What in the first generation filled an entire room could now fit in the palm of the hand. The Intel 4004 chip, developed in 1971, located all the components of the computer—from the central processing unit and memory to input/output controls—on a single chip.
• Fifth generation computing devices, based on artificial intelligence, are still in development, though there are some applications, such as voice recognition, that are being used today. The use of parallel processing and superconductors is helping to make artificial intelligence a reality. Quantum computation and molecular and nanotechnology will radically change the face of computers in years to come. The goal of fifth-generation computing is to develop devices that respond to natural language input and are capable of learning and self-organization.

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