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B.Com (IT) - I Year I Semester
Unit – I
Introduction to Computers : Definition, Characteristics & Limitations of Computers, Functional Block Diagram of Digital Computer, Functions Of Control Unit and ALU in CPU Concepts Of Primary and Secondary Memory, Input / Output Devices. Networks: Definition-Network Types-LAN, MAN,WAN. Network Topologies-Mesh, Bus, Star, Ring, Hybrid
Introduction to Computers:
Meaning of a Computer:
Computer is a machine that can solve problems by accepting data performing certain operations and presenting the results of those operations under the direction of detailed step- by-step instructions. Such a set of sequenced instructions, which cause a computer to perform particular operations, is called a program. The term computer has been derived from the word “COMPUTE” which means calculate.
Definition of a computer
Computer is an electronic device, used for performing calculations and controlling operations that be either expressed in logical or numerical terms. ➢ It stores and process data in binary form according to instructions given to it in a variable program. ➢ Computer need to be told exactly what to do and how to do to accomplish a task ➢ It is a combination of hardware and software ➢ Software is the set of instructions given to the computer to perform a task ➢ The physical parts that make up computer are called hardware, which are made of circuits and transistors The three main operations of computer are
- Takes in data and instructions (input)
- works with the data (processing)
- puts out information (output)
Characteristics of a Computer:
The following are the main characteristics of a computer-
- Speed: Computer is able to process the data and give the output in fractions of seconds such that required information is available to the user on time and hence enables him to take right decisions at the right time.
- Accuracy : Computer is error free. There is no scope for inaccuracy in the results given by the computer. Incorrect calculations, errors, mistake do not take place in a computer system. If there errors they are due to the errors in the instructions given by
the programmer, inaccurate data, inconsistent data or defective programs as well as defective system designs. Since the computer is capable of doing only what it is instructed to do, fault instructions for data processing may lead to faulty results this is known as GIGO (Garbage In Garbage Out)
- Reliable: Today, computer is extensively used because of their reliability. The output generated by the computer is very reliable to the extent that the input is reliable.
- Storage capacity: Another important characteristic of a computer is that it has brain of its own with brilliant memory. It can accept and store any information for a long time. It stores large amount of data and can recall information instantly.The main memory of the computer is relatively small and holds certain amount of information. Therefore the data is stored in secondary memory 5. Versatile: The computer performs three basic operations. It accepts information from the user; secondly it performs the basic arithmetic operations and logical operations and finally generates the desired output in the desired form.
- Automatic (Programmable): Computer is a device, which is more than a calculator. Once instructions are given to the system it works automatically without any human intervention until the completion of program until it meets logical instruction to terminate the job.
- Diligent: A computer is a device, which is indefatigable (untiring) and therefore does not feel tired nor gets distracted like a human being. All the instructions are performed at same speed and accuracy
- Recreation: In recent years, computers were used mainly for entertainment and amusement, like computer games and other programs, computer animation gives life and movement to inanimate objects. Multimedia too helped in popularizing the use of computers.
- Networking capacity: A computer can be connected with other computers. This feature of connectivity with other computers leads to the evolution of computer networking.
- Compactness: Now-a-day’s size of computers has decreased o lot due to increased technology. The laptop and Notebook computers can be taken outside.
Limitations of A Computer :
Like any modern technology computers is also not free from limitations- ➢ Computer is a machine and therefore it is only a device that needs human skill to operate. ➢ It does not work on its own and needs set of instructions to be given. ➢ It does not have natural intelligence and hence has to be instructed about every step in detail. ➢ It is not a decision maker and has to be programmed to take an action if some conditions prevail. ➢ Finally it does not learn by experience unlike intelligent human beings.
Memory:
➢ The memory of the computer can also be categorized under hardware but sometimes it is considered as a separate element of the computer system. ➢ Memory allows the user to store the data and processes performed by the computer. ➢ Memory is divided mainly into RAM, ROM and secondary storage devices. ➢ All these components are essential for the effective functioning of computer system Central Processing Unit (CPU). FUNCTIONAL BLOCK DIAGRAM OF COMPUTER:
- A Computer, like a human brain, receives data and instruction, stores them and processes the data according to the instructions given to it.
- It receives data from input devices, stores them in memory and displays them through on an output device
Central Processing Unit (CPU)
CPU is the component of computer system with the circuitry to control interpretation and execution of instructions. It performs the process in parts of Input-Process-Output cycle. The components of CPU are mounted (Fixed) on the main circuit board, called the “motherboard”. The various components of a Central Processing Unit are:
- Arithmetic Logic Unit (ALU).
- Control Unit. Arithmetic and Logic Unit (ALU) ➢ It is that part of CPU where actual data processing occurs.
➢ All mathematical operations (+, - , *, /) and all comparisons (>, <, =) take place in this unit. ➢ In addition to arithmetic functions, the ALU also performs logic functions. ➢ A logic function is one where numbers or conditions are compared to each other. ➢ Circuits in the ALU are generally used to compare two numbers by subtracting one from the other. ➢ The sign (negative or positive) and the value of the difference tell the processor that the first number is equal to, less than, or greater than the second number. ➢ Alphabetic data may also be compared according to an assigned order sequence. Control Unit ➢ It is a component of CPU that control and coordinates the flow of data between different components of computer ➢ It interprets instructions it receives from memory and directs the sequence of events necessary to execute the instruction. ➢ Control Unit uses a system clock, which synchronizes (manage) all tasks by sending out electrical pulses. ➢ Clock speed (the number of pulses or clocks per second) is measured in Megahertz (MHz) and is the main element in determining the speed of the processor. ➢ Computer speed is also measured in the number of instructions completed per second, or Millions of instructions per second (MIPS). MEMORY The memory is that part of the computer where programs and data are stored. There are two types of memory:
- Primary Memory
- Secondary Memory MEMORY PRIMARY MEMORY SECONDARY MEMORY
- PRIMARY MEMORY (MAIN MEMORY): Primary memory holds only those data and instructions on which computer is currently working. It has limited capacity and data is lost when power is switched off. It is generally made up of semiconductor device. These memories are not as fast as registers. The data and instruction required to be processed reside in main memory. RAM ROM CACHE REGISTER R MAGNETIC DISK MAGNETIC TAPES OPTICAL DISK
any input data on instructions from user. The actual contents are set by the manufacturers and they are unchanged and permanent. ROM chips are not only used in the computer but also in
other electronic items like washing machine and microwave oven. ROM is mainly of three
types:
- Programmable read only memory (PROM) allows a chip to be programmed by the user for converting critical and lengthy operations into micro programs that are fused into a chip. The user buys a blank PROM and enters the desired contents using a PROM program. Inside the PROM chip there are small fuses which are burnt open
during programming. It can be programmed only once and is not erasable. Once they
are in a hardware form, they can be executed at a very high speed and can’t be altered.
- Erasable PROM (EPROM ) can be erased and reprogrammed. The EPROM chip has a small window on top allowing it to be erased by exposing it to ultra-violet light for duration of up to 40 minutes. After reprogramming the window is covered to prevent new contents being erased
- Electrically Erasable PROM (EEPROM) is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (milli second). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow. 3. Cache Memory: Cache memory is typically integrated directly with the CPU chip or placed on a separate chip. Many processors also have built-in specialized storage elements that are used for specific processing and control purposes. A computer microprocessor can access more quickly than it can access regular RAM. The basic purpose of cache memory is to store program instructions that are frequently re-referenced by software during operation. Fast access to these instructions increases the overall speed of the software program. One element used during processing operations is a high speed Buffer Memory (cache memory), which is both fast and expensive, as compared to primary storage. It is a small amount of memory typically of 256 or 512 kilobytes. L1 and L2 are levels of cache memory in a computer. L1 is "level-1" cache memory, usually built onto the microprocessor chip itself. L2 (that is, level-2) cache memory is on a separate chip that can be accessed more quickly than the larger "main" memory. 4. Registers Registers are a special high-speed storage area within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied,
both numbers must be in registers, and the result is also placed in a register. (The register can contain the address of a memory location where data is stored rather than the actual data itself.) The number of registers that a CPU has and the size of each (number of bits) help determine the power and speed of a CPU. For example a 32 - bit CPU is one in which each register is 32 bits wide. Therefore, each CPU instruction can manipulate 32 bits of data. The CPU contains a number of special-purpose registers ➢ Instruction Register (IR): The instruction register holds the instruction currently being executed. ➢ Memory Data Register (MDR) : The memory data register (also known as the memory buffer register or data buffer) holds the piece of data that has been fetched from memory. ➢ Memory Address Register (MAR) : The memory address register holds the address of the next piece of memory to be fetched. ➢ Program Counter Register (PC): The program counter holds the location of the next instruction to be fetched from memory. It is automatically incremented between supplying the address of the next instruction and the instruction being executed. ➢ Accumulator Register : The accumulator is an internal CPU register used as the default location to store any calculations performed by the arithmetic and logic unit.
2. SECONDARY MEMORY: Secondary (Auxiliary) storage is the non-volatile memory that is stored externally to the computer. A secondary medium usually used for the storage of large amount of data for permanent or long-term storage of data or programs. While the secondary media can hold much more data than primary storage, access to the data is slower. The benefits of secondary storage are
The surface of the magnetic disk is divided into number of invisible concentric circles called “tracks”. The tracks are further subdivided into “sectors”, “blocks” etc. each its own unique addresses to facilitate the location of data. Disk moves on a vertical rotating spindle. Data on magnetic disks can be read randomly. The access time for data stored on a magnetic disk is determined by two factors:
- The seek time i.e. the time required for positioning read/write head over the proper track.
- The search time i.e. the time required for spinning the required data under the head. Magnetic disks come in various sizes. They can be portable or permanently mounted in their storage devices, disk drives. They can be made of rigid metal (Hard Disks) or flexible plastic (Floppy Diskettes). Advantages of Magnetic Disk
- Data records can be stored for both sequential and direct-accessing.
- High speed of access and update as no sorting of transactions.
- On-line disk records of several related files can be updated simultaneously by a single input transaction. Limitations of Magnetic Disk
- Risk of loss of data – Records may be lost in case of writing of new records or drive failure. Therefore, special backup procedures are required for disk records protection. This is not so in the case of magnetic tape records. 2. Data insecurity – It is easier to maintain the security of magnetic tape files than that of magnetic disk files.
3. Optical Technology:
Optical technology involves the use of laser beams highly concentrated beams of light. It comes in the form of Optical laser disk, Optical card and Optical tape. Optical Laser Disk: A common version of the optical disk is the CD-ROM (compact disk, read–only memory). While not suited to applications where data changes, they are very convenient for storing data that remains constant. CD-ROM [Compact Disc Read Only Memory] is a Compact Disc contains data accessible by a computer. ➢ The CD-ROM is also known as a laser disc, which is shiny metal like disk. The diameter of the disk is 5.25 inches (or) 12 cm disk. Information of 650 MB can be stored, which is equal to nearly 2, 50,000 pages of printed text. ➢ The data is recorded as deep holes on the disk surface or burning microscopic bits. ➢ CD-ROMs are popularly used to distribute computer software, including games and multimedia applications, though any data can be stored. ➢ The CD-ROM s are pre-recorded disks used for storing a large amount of data (or) information.
Optical Card: The Optical card, or laser card, is the size of a credit card and has an optical laser-encoded strip that can store approximately 2 megabytes of data. These cards have many potential uses, most notably for storing credit records or medical histories. Optical Tape: Optical tape is similar in appearance to magnetic tape, but data are stored by optical laser techniques. Optical tapes, which are in cassette form, can store over 8 gigabytes each providing a total storage of about 1 terabyte (1,000 gigabytes). Like other optical methods of data storage, optical tape is read only.
INPUT AND OUTPUT DEVICES:
The CPU provides the means for the fast and efficient processing of data into usable information. For computer processing, data needs to be entered into the computer (input) and the result of processing needs to be communicated to the user (output). Any hardware item which is attached to the main unit of a computer that houses the CPU is referred to as a Peripheral Device. ➢ INPUT : Input is the process of entering and translating incoming data in machine- readable form. The data to be entered are often referred as input. Input process involves data preparation, processing and accuracy checks. There are two types of input device: manual and automatic. ▪ A Manual Input Device requires a human hand to control i.e. (Mouse, Keyboard, Scanner, Camera, track ball, joystick, and Graphics tablet.) ▪ An Automatic Input Device inputs data without the need for human intervention (once the device has been set up), e.g. MICR, OMR, barcode reader, webcam, microphone. ➢ OUTPUT: The result of processing is also often referred as output. Output is divided into two general categories: Output that can be readily understood and used by humans. Output to secondary storage devices that hold the data to be used as input for further processing by computer.
Every key on the keyboard underneath it consists of a tiny chip called as keyboard controller, when a key is pressed the controller places the code into the part of the memory called keyboard buffer. The buffer temporarily holds the data till it is processed. The signal the keyboard sends to the computer is called as interrupt. The keyboard sends an interrupt request to the system software and the data is processed.
2. Mouse A mouse is a pointing device. It usually contains one or three buttons: as the user rolls it on a flat surface, the mouse controls cursor movement on the screen. When the user presses one of the buttons, the mouse either marks a place on the screen or makes selections from data on the screen. It can be used for many applications ranging from games to designing products with graphics. It can be used as an alternative to keyboard or it can be used in combination with a keyboard to enhance input operations. Advantages: - Easy to use - Not very expensive - Moves the cursor faster than the arrow keys of keyboard. 3. Joystick A joystick is a most popular input device used to play video games. A joystick uses a level to control the position and speed with which the joystick is moved into digital signals that are sent to the computer to control the cursor movement. 4. Track Ball A track ball uses a hard sphere to control cursor movement. The ball can be rotated by hand in any direction. The track ball translated the sphere’s direction and speed of rotation into a digital signal, used to control the cursor. 5. Touch Screen A touch screen registers input when a finger or other object comes in beams and ultrasonic acoustic waves. Infrared beams crisscross the surface of the screen and when a light beam is broken, that location is recorded. 6. Light Pen A light pen is also a pointing device like mouse. It can be used to choose a displayed menu option. The pen contains a photocell placed in a small tube. As the user moves the tip of the pen over the surface of the screen, it detects the light coming from a limited field of view. The light from the screen causes the photocell to respond and this electric response is transmitted to the process, which can identify the menu option that is
triggering the photocell. Light pen is useful for graphics work, especially for Computer Aided Design (CAD) purposes.
7. Magnetic Ink Character Recognition (MICR) Magnetic Ink Character Recognition (MICR) is an interpretation by computer of a line of character written in special magnetic ink. These characters can be read by human eye as well. There are several advantages associated with the use of MICR: - Checks may be roughly handled, folded, smeared, and stamped. But they can still be read with a high degree of accuracy. - People can easily read the magnetic ink characters. The main limitation of MICR is that only the 10 digits and 4 special characters needed for bank processing are used. No alphabetic characters are available. 8. Magnetic strips They are thin bands of magnetically encoded data that are found at the back of usually, Credit cards, Debit cards etc. the data stored on the card vary from application to another. Data in the form of magnetic strips cannot be seen or interpreted by simply looking at the card and so it can be highly sensitive or personal. 9. Optical Recognition / Scanners Optical Recognition when a device scans a printed surface and translates the image the scanner sees into a machine-readable format that is understandable by the computer. Optical Recognition can be of the following types i. Optical Mark Recognition (OMR) It uses mark sensing to scan and translate, based on its location, which is a series of pen or pencil marks into a computer readable form. For instance, the objective type multiple choice question paper we get in the bank recruitment exam. A computerized optical mark reader scores the tests by identifying the position of the mark ii. Optical Bar Recognition (OBR) This is slightly more sophisticated type of optical recognition. An optical bar reader recognizes and interprets bar codes or product codes which are arranged to represent data, such as the name of the manufacturer, and the type of the product etc. on the basis of the width of the lines. A scanner reads the bar code, and the computer then matches the price and product.
Impact printers :
Impact Printers use a print head containing a number of metal pins which strike an inked ribbon placed between the print head and the paper.
Character Printers : Character printers print character by
character. The different character printers are: Dot Matrix Daisy Wheel Dot Matrix Printer It uses printer heads containing a 9 - 24 pins these pins produced pattern of dots on the paper to form a character. Dot matrix printers are very popular as they are relatively inexpensive and typically print at speed of 100-600 characters per second. These printers can give us draft quality, standard quality and near letter quality prints Daisy Wheel Printer: This printer gives us letter quality print but is slow, typically 25 to 55 characters per second. It is a circular printing element which has a plastic or metal wheel on which the shape of each character stands out in relief. A hammer presses the wheel against a ribbon, which in turn makes an ink stain in the shape of the character on the paper. Daisy-wheel printers produce letter-quality print but cannot print graphics.
Line Printers: The line printer is a form of high speed impact printer in which one line of
type is printed at a time. Print speeds of 600 to 1200 lines-per-minute (approximately 10 to 20 pages per minute. Types of line printers Printers Impact printers Non impact printers Character printer Line printer^ Laser printer Thermal printer Dot matrix printer Daisy wheel printer Drum printer Chain printer Ink jet printer
- Drum Printer
- Chain Printer Typical typeface of a drum printer with staggered characters. A fixed font character set is engraved onto the periphery of a number of print wheels. The wheels, joined to form a large drum (cylinder), spin at high speed and paper and an inked ribbon is stepped (moved) past the print position. As the desired character for each column passes the print position, a hammer strikes the paper from the rear and presses the paper against the ribbon and the drum, causing the desired character to be recorded on the continuous paper.
Non Impact Printers:
Non-impact printers are much quieter than impact printers as their printing heads do not strike the paper. The main types of non-impact printer are:
- Thermal Printer
- Laser Printer
- Ink Jet Printer 1. Thermal Printer Characters are formed by heated elements being placed in contact with special heat sensitive paper forming darkened dots when the elements reach a critical temperature. 2. Laser Printer When speed and quality are required and cost is no factor a laser printer is the solution. They produce images on paper by directing a laser beam at a minor, which bounces the beam on to a drum. The laser leaves a negative charge on the drum to which positively charged black toner powder will stick. As the paper rolls by the drum, the toner is transferred to the paper. Laser printer uses buffer to store an entire page at a time that is why they print very fast at rate of approximately 21,000 lines or 437 pages per minute. 3. Ink-Jet Printer Ink-Jet printer forms character on paper by spraying ink from tiny nozzles through an electrical field that arranges the charged ink particles into characters at the rate of approximately 250 characters per second. The ink is absorbed into the paper and dries instantly. Various colors of ink can be used. Although this might sound like a messy way of printing, ink-jet printers are reliable, but expensive.
3. Plotter
A Plotter reproduces drawings using pen that are attached to movable arms. The pens are directed across the surface of a
Local-Area Network (LAN):
A local area network(LAN) is two or more computers directly linked within a small well- defined area such as a room, office building, or a campus. A LAN can be made up of only microcomputer or any combination of microcomputer and large system. Main benefit of LAN is reduction of hardware and software costs because user can share several computers, peripheral devices such as laser printers, color printers, hard disk, and modems. Another benefit of LAN is the user can share the same data. A LAN generally consists of the following.
- Two or more computers.
- Software to control the operations of the computers.
- Peripheral devices such as modems, printers, plotters etc.
- Coaxial or fiber optic cables are usually used to connect the computers and there devices.
- A plug-in board to handle the data transmissions. Some LAN’s specifically requires all the computers to be of a certain brand while others allow a variety of brands to be connected. The number of computers in LAN varies widely from small LANs that connect two to twenty five computers, to large LANs that may connect more than 10,000 computers. LAN must get the data to its destination, transmit the data correctly, and prevent unauthorized users from gaining access to that data.
Metropolitan-Area Networks (MAN):
A Metropolitan-Area Network (MAN) is a network that is larger than a LAN. It is called metropolitan since it normally covers the area of a city. Different hardware and transmission media are often used in MAN because they must efficiently cover these distances (a few tens to about one hundred kilometers).
Wide-Area Networks (WAN):
A Wide-Area Network (WAN) is two or more geographically dispersed computer, linked by communication facilities such as telecommunication or microwave relays. In other words, one of the most significant aspects of Wide-area network while comparing it with the freedom of a local-area network is the involvement of public telecommunication authority. WAN is usually limited to use by large organizations and government agencies due to high costs involved in building and maintaining them. There are two specific categories of WAN,
- Enterprise: An enterprise network connects all LAN’S of a single organization. The term is normally used for networks connecting extremely large organization, or for those that cross regional boundaries.
- Global: By definition a global network is one that spans the earth. While global networks may not cover the entire globe, they cross multiple national boundaries and may include the networks of several organizations.
NETWORK TOPOLOGY:
Each computer or device in a network is called a node. The geometrical arrangement of computer resources, remote device and communication facilities are known as network topology. A network topology can be made in one of the five different topologies.
- Mesh Topology
- Bus Topology
- Ring Topology
- Star Topology
- Tree Topology
- Hybrid Topology
1.Mesh Topology:
Mesh Topology has point-to-point connection between every device in the network. Each device requires an interface for every other device on the network. Mesh topologies are not usually considered practical, in addition unless each station frequently sends signals to all the other station, an excessive amount of network bandwidth. Advantages :
- Units affected by media failure: Mesh topologies resist media failure better than other topologies. Implementations that include more than two devices will always have multiple paths to send signals from one device to another. If one path fails, the transmission signals can be routed around the failed link.
- Easy for troubleshooting: Mesh network are easy to trouble shoot because, each medium link is independent of all other devices. Disadvantages :
- Difficult to install: Mesh networks are relatively difficult to install because each device must be linked directly to all other devices, as the number of devices increases, the difficulty of installation increases geometrically.
- Difficult to reconfigure: Mesh topologies are difficult to reconfigure for the same reasons that they are difficult to install.
