11th IT Unit -1 Chapter -1.pdf

INTRODUCTION TO COMPUTER SYSTEMS CHAPTER - 1 Hardware Concepts CHAPTER - 2 Software Concepts CHAPTER - 3 Open Source Concepts  UNIT - 1 INTRODUCTION TO COMPUTER SYSTEMS CHAPTER - 1 HARDWARE CONCEPTS 1. A HARDWARE CONCEPTS : Introducti...

INTRODUCTION TO COMPUTER SYSTEMS CHAPTER - 1 Hardware Concepts CHAPTER - 2 Software Concepts CHAPTER - 3 Open Source Concepts  UNIT - 1 INTRODUCTION TO COMPUTER SYSTEMS CHAPTER - 1 HARDWARE CONCEPTS 1. A HARDWARE CONCEPTS : Introduction The term ‗information‘ refers to the facts and opinions provided and received during the course of our daily life. We gather information, while interacting with living beings surrounding us, mass media, electronic media and all sorts of observable phenomena. Interest and necessity of information system has long been a part of the mankind since pre-historic era. The ancient man used to communicate with each other through symbolic and verbal actions. Time has passed and the system of communication has developed from action to alphabets. Modern day information system has grown to such a proportion that it has become an object of study in a number of disciplines, including philosophy, physics, biology, linguistics, information and computer science, electronic and communications engineering, management science, and the social sciences. On the commercial side, the information service industry has become one of the newer industries worldwide. Manufacturing sectors, Service sectors, & others are considering the Information handling as an economic resource, somewhat on par with other resources like labour, material, and capital. This view stems from evidence that the possession, manipulation, and use of information can increase the cost-effectiveness of many physical and cognitive processes. The rise in information-processing activities in industrial manufacturing as well as in human problem solving has been remarkable. As a societal resource, information has some interesting characteristics that separate it from the traditional notions of economic resources. 1.2 +2, Information Technology y Part - I (i) It is expansive, with limits apparently imposed only by time and human cognitive capabilities. (ii) It reproduces rather than being consumed through use (iii) It can be shared only, not exchanged during transactions. (iv) It is transportable at very high speed Application of scientific knowledge to change and manipulate the human environment is often called as technology. Thus information technology (IT) embodies scientific acquisition, recording, organization, retrieval, display, and dissemination of information for the betterment of the human society. It involves four basic operations- gathering, storing, processing and providing the output information to the outside world. This process has been there since ancient times. Before the advent of modern digital computer, the information was stored in analog form through printing, photography and telephony. Modern information technology greatly facilitates the storing and manipulation of information as a result of its representation in digital form. IT literacy includes knowledge of technical tools, skill and comfort in the use of technical hardware and software packages, understanding of the ethical issues related to technology, and an understanding of technology‘s impact on society. In our day to day life, we are gradually being driven towards automation in various mundane activities like; washing clothes by washing machines, cooking by microwave ovens, conventional banking transactions through Automated Teller Machines (ATM), etc. In all these machines, a click of a button starts a process inside the machine which follows a predefined work flow based on which button has been pressed. Broadly speaking, the machine receives certain data from the user, processes it and gives the output (washed clothes, cooked food or currency notes). This is exactly a computer system does. 1.A.1 Basic Computer Organisation: A computer is an electronic device (hardware) that processes input data and produces result (output) according to a set of instructions called program (software). In this section we shall try to outline the basic structure and function of a modern digital computer. Digital computer systems came in to being during 1940s with the design of ENIAC, EDVAC and Harvard Mark I. except ENIAC; Introduction to Computer Systems y Hardware Concepts 1.3 all other systems are based upon a common technology of ―stored program‖ proposed by John von Neumann. All modern digital computer systems till date are based upon that von Neumann technique. Every system performs the following five basic operations to accomplish a job. Though the internal architecture of different computers varies from one system to another, the basic organization remains the same. The block diagram given here gives a basic layout of a modern digital computer system. 1. Input: It accepts data and instructions by way of inputs. 2. Store: It stores data and instructions for the use of the system as and when required. 3. Processing: It processes (carries out arithmetic or logical operations on) data and convert them to useful information. 4. Output: It presents the final result for the user in form of output (printed or visual display format). 5. Control: It controls (co-ordinate and synchronize) all four operations described above. Figure 1.A.1 : Block diagram of functional units of a computer Introduction to Computer System 1.4 +2, Information Technology y Part - I 1.A.2 Hardware Concepts : We shall now discuss briefly the structure and working of different sections of a digital computer as shown in the previous diagram. Hardware represents the physical and tangible components of the computer itself, i.e. components which can be seen and touched. It includes 1. Input devices 2. Central Processing Unit 3. Output devices 4. Memory / Storage Unit Central Processing Unit : The Central Processing Unit (CPU), or simply the processor, is a component in a digital computer which is capable of executing a program. It is considered to be the ‗brain‘ of a computer system. It interprets program instructions and processes data. All calculations, comparisons are carried out inside the CPU and CPU sends instructions to other parts of the computer for their smooth and synchronized operations.CPUs provide the digital computer, trait of programmability. It is one of the necessary components found in computers of any era, along with primary storage and input/output facilities. A CPU that is manufactured as a single integrated circuit is usually known as a microprocessor ( A microprocessor is a programmable device, that takes in numbers, performs arithmetic and logical operations on them according to the instructions stored in the memory and produces other numbers as a result). Beginning in the mid- 1970s, microprocessors of ever-increasing complexity and power gradually supplanted other designs, and today the term ―CPU‖ is usually applied to some type of microprocessor. Some of the processors, presently in use are shown here. Figure 1.A.2 : Microprocessors Introduction to Computer Systems y Hardware Concepts 1.5 As has been shown previously, the CPU contains the Control Unit and the Arithmetic and Logic Unit. (i) Control Unit : This unit manages and co ordinates the functions of the entire computer system. It obtains instructions from the program stored in main memory, decodes and interprets them and issues signals to other components to execute them (Remember, this unit does not process any data!). To carry out all these operations the Control Unit has some special purpose registers (A register is nothing but a collection of special memory units to hold the information on a temporary basis; they facilitate speedy transfer of information between different parts of a CPU) and a decoder. The special purpose registers are: Instruction Register & Program Counter Register. The first register holds the current instruction and later one holds the address of the next instruction in the main memory. The decoder consists of the necessary circuitry to decode and interpret the meaning of every instruction supported by the CPU. Each instruction is accompanied by an op-code, which suggests the nature of its execution. (ii) Arithmetic Logic Unit : This unit performs all arithmetic (addition, subtraction, multiplication and division) and logic (less than, greater than, equal to etc.) operations as and when instructed by the Control Unit. Like Control Unit, this unit has also some special purpose registers like Accumulator Register, Data Register and some general purpose Temporary Registers. The size of a register equals the number of bits required by a CPU to store a character or a number. Thus, the registers are often called as 8-bit, 16-bit, 32- bit or 64-bit registers. The bigger the word size, the faster the computer can process a set of data. Different CPUs possess registers of different sizes and their number also varies from processor to processor. Most recent microprocessors are using 64-bit registers. Each register is assigned to hold a particular type of information. Working of a CPU : The fundamental operation of most CPUs, regardless of the physical form they take, is to execute a sequence of stored instructions called a program. The program is represented by a series of numbers that are kept in some kind of computer memory. There are four basic steps that nearly all von Neumann CPUs use in their operation: fetch, decode, execute, and write back. 1.6 +2, Information Technology y Part - I (1) Fetch: The instruction is retrieved(by the control unit)from the memory and fed in to the Instruction Register where the operation and address parts are separated. (2) Decode: The control unit then sends the operation part to the decoder and the address part to the MAR. (3) Execute: The decoder interprets the instruction,and accordingly the control unit sends signals to the appropriate units to carry out the specified task. For example , for any arithmetic or logic operation the signal is sent to the ALU. Besides instructing ALU to carry out a particular operation, the control unit ensures that the address part of the instruction is loaded to a suitable register in the ALU from where it can read the data. The ALU carries out the necessary operation and sends a signal to the control unit as soon as the operation is finished. (4) Write back: The result is written back either to a register in the CPU for further processing or a slower but cheaper and larger main memory of the computer. As each instruction is executed, the address of the next instruction is automatically loaded in to the PC register and step 1 to 4 is repeated. Clock Speed : The above operations take place inside a computer with incredible speed. In order to synchronize each operation every CPU has a built-in electronic clock inside it. This clock emits millions of electric pulses at precise and regular intervals and is called as a system clock. The periodic pulses (signals) are termed as clock cycles. One cycle is the time it takes toperform one operation, such as moving a byte of data from one memory location to another. Therefore, several clock cycles are required to completely execute a single program instruction following the above 4 steps. The shorter the clock cycle, the faster is the processor. Hence, the speed with which an instruction is executed is directly related to the clock speed which is measured as the number of pulses emitted per second (hertz). Presently, most of the microprocessor clocks possess a speed of 1- 2 Giga hertz (109 Hz). A CPU having faster clock speed can process data faster. Main Memory : The main memory of a computer system is otherwise called as Primary Storage Unit. When we feed data and instructions to a computer for processing, Introduction to Computer Systems y Hardware Concepts 1.7 it gets stored in its main memory. The main memory is outside the CPU. It possesses the following characteristics;  The storage capacity of this memory is much larger compared to the registers in the CPU (which barely stores a byte of information!), so that it can store a complete program and other information.  The storage mechanism is electronic in nature, in such a way, that data retrieval does not require any physical movement of the components. Thus the access time is much faster in comparison to the magnetic disk storage units.  Most part of this memory is volatile in nature, i.e. the stored information gets wiped out, the moment power supply is off. Some portions are non volatile – which retain the stored information (all are in binary form) even if the power supply is off.  The most important characteristic of this memory is that, each site (location) is equally accessible. The data retrieval time is independent of its storage position. Such a facility is often called as Random Access; because one can randomly access data from any location of the memory circuitry with equal ease.  Because of this random access property, the main memory is often called as Random Access Memory or RAM or simply the memory of a computer.  When information is read from the memory locations, the contents of the locations remain intact-called as memory read operation. When some new information is written to these locations the previous data is over written-called as memory write operation. The capacity of memory is defined as the number of bytes it can store. It is expressed in terms of kilobytes (kB = 210 bytes = 1024 bytes), megabytes (MB = 210 kB =1048576 bytes) or gigabytes (GB = 210 MB = 1073741824 bytes). Remember kilo; mega and giga are decimal suffixes for 1000, 1000000 and 1000000000 times. Here we are using these suffixes, just because of the closeness of the corresponding binary values (1000:1024; 1000000:1048576; 1000000000:1073741824)! Memory Partition- RAM, ROM, PROM, EPROM : The key benefit of RAM over other types of storage is its short retrieval time and consistency. Short because no physical movement is necessary, and 1.8 +2, Information Technology y Part - I consistent because the time taken to retrieve a piece of data does not depend on its current distance from a physical head; it requires practically the same amount of time to access any piece of data stored in a RAM chip. Because of this speed and consistency, RAM is used as main memory‗ or primary storage. In most personal computers, the RAM is not an integral part of the motherboard or CPU—it comes in the easily upgraded Figure 1.A.3 : RAM sticks form of modules called memory sticks or RAM sticks. These can quickly be removed and replaced should they become damaged or too small for current purposes. Empty memory slots are available for this purpose in motherboards. A RAM basically stores information until it is needed by different parts of the system. As of today, there are two types of RAM architectures being used in different computer systems- Static RAM (SRAM) and Dynamic RAM (DRAM). These two technologies are different from each other in their memory cell architecture, function and power consumption. Let us briefly outline the features of both these RAMs. Static RAM: The word static‗ indicates that the memory retains its contents as long as power is on. Each memory location of SRAM consists of six transistors. These are used in personal computers, workstations, routers and peripheral equipment: internal CPU caches, hard disk buffers, router buffers, etc. LCD screens and printers also employ static RAM to hold the image displayed (or to be printed). Small SRAM buffers are also found in CDROM and CDRW drives; usually 256 KB or more are used to buffer track data, which is transferred in blocks instead of as single values. The same applies to cable modems and similar equipment connected to computers. The so called CMOS RAM on PC motherboards was originally a battery-powered SRAM chip. Dynamic RAM : It is a type of random access memory that stores each bit of data in aseparate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed Introduction to Computer Systems y Hardware Concepts 1.9 periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory. Its advantage over SRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to six transistors in SRAM. This allows DRAM to reach very high density. Like SRAM, it is in the class of volatile memory devices, since it loses its data when the power supply is removed. A type of solidstate DRAM is called as synchronous DRAM is called as SDRAM are also being used. It is called synchronous in a sense that it waits for the clock signal before responding to its control input. The available modules of packaged DRAMs are SIMM-30pin, SIMM-72pin, DIMM-168pin etc. Software can partition a portion of a computer‗s RAM, allowing it to act as a much faster hard drive that is called a RAM disk. Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery. If a computer becomes low on RAM during intensive application cycles, the computer can resort to so-called virtual memory. In this case, the computer temporarily uses hard drive space as additional memory. Constantly relying on this type of backup memory it is called thrashing, which is generally undesirable, as virtual memory lacks the advantages of RAM. In order to reduce the dependency on virtual memory, more RAM can be installed. No matter, every location of the main memory is randomly accessible; all of them are not volatile like RAM chips. Non volatile and random access memory do constitute a part of the main memory; the information stored in these locations is permanent, which can be read, whenever required, but cannot be changed. These are called as Read Only Memory (ROM). The data is stored in ROM chips by using fuse links and the process is called as burning in the data. ROMs are mainly used to store programs and data which do not change and are frequently used by the computer. Micro programs (special programs written by the manufacturer to substitute some complex electronic circuitry to carryout complex and repetitive tasks) are generally stored Figure 1.A.4 : ROM chip in these ROMs. These micro programs 1.10 +2, Information Technology y Part - I which essentially substitute a hardware component of a computer are called as firmware. An example of this micro program is system boot program which make the computer system ready for use when the power is switched on. It contains a set of start-up instructions to check the status of memory, input output devices and other part of the computer and looks for an operating system and loads its core part in the volatile RAM to produce the initial display-screen prompt. There are two types of ROMs-one manufacturer programmed and other to be programmed by the user. The later one which is left to be programmed by the user permanently is called as Programmable Read Only Memory (PROM). In order to customize a system a user may convert his/her program in to micro programs and store it in the PROM chip. PROMs are programmed to record information using a special device, known as PROM-programmer. However, once the chip has been programmed, the recorded information can not be changed. A PROM is a non-volatile storage part of the main memory. Other than ROM and PROM another type of memory chip used in the main memory is called as EPROM (Erasable and Programmable Read Only Memory). This is a non-volatile memory and stores information permanently. However, in worst cases, the information can be altered either by exposing the chip for some time to Ultra Violet light or by using high voltage electric pulses. The EPROMs in which the data is erased by UV radiation are known as UVEPROM and the EPROMs which employ the second technique to erase the data are called as EEPROMs. Generally the EPROMs are used by R&D personnel while configuring new system hardware. Due to the ease with which the stored programs can be erased by using electric pulses, the EEPROMs are also called as Flash Memories. Cache Memory : As discussed earlier, information stored in main memory (RAM) can be accessed by the processor in very short time- around 60 nanoseconds (ns). But the processor executes that information in just 2 ns! This means, the processor seats idle for 58ns during one operation! It makes the processing slower and decreases the efficiency of a computer. The need to overcome this limitation gave rise to the concept of Cache memory. ( Cache‗ is a French word means to hide‗). Cache memory or Cache is a small but faster (access time: 10ns to 30ns) supplementary memory system, that temporarily stores frequently used Introduction to Computer Systems y Hardware Concepts 1.11 instructions and data during a program execution for quicker processing. It is actually an extension of computer main memory and random access in nature. Secondary Storage Devices : The main memory has two distinct limitations- short storage capacity and volatility. The maximum storage capacity of a RAM chip (main memory) is around 4GB, whereas the modern computers require a storage capacity in excess of 10GB. Because of volatility, the main memory loses the stored information once the power is off. These two limitations of the main memory are taken care of by the so called auxiliary memory or secondary storage. These are non volatile, high capacity, cheap but slower memory devices. They are used to store a large volume of data on a permanent basis. As and when required, main memory extracts data from these secondary storage devices for processing. Normally hard disk of computer is used as secondary memory but this is not portable so there are many other secondary storage media in use. Secondary Storage Device Sequential Direct Access Device Access Device Magnetic Magnetic Optical Memory Tape Disk Disk Storage Hard Floppy CD- DVD Flash Memory Disk Disk ROM Drive Card Figure 1.A.5 : Classification of Secondary Storage Devices Hard Disk Drive : It consists of a stack of magnetic disks and read/write assembly. The disks are made up of a non-magnetic material, usually aluminum or glass, and are coated with a very thin layer of magnetic material. Older disks used iron-oxide as the magnetic material, but current disks use a cobalt-based alloy. The magnetic surface of each platter is divided into many small sub- micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. The disks are mounted on a spindle, with spacers in 1.12 +2, Information Technology y Part - I between, and a motor on the bottom end of the spindle for spinning of the disks. To read and write to the surface of the disks, the drive uses a small electro- magnet assembly, referred to as a head, located on the end of an actuator arm. There are as many heads as the number of platter surfaces. The read/write head moves back and forth in a radial pattern with the help of a voice coil (the same technique used to move the cone of a speaker on your stereo). The read element of the head is magneto resistive, whereas the write head is typically thin film inductive. Hard disk drives are sealed to prevent dust and other sources of contamination, from interfering with the operation of the hard disks heads. Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. These drives are fitted in the system‗s cabinet. This is a high capacity storage device ranging from 1GB to few Tera Bytes nowadays. HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras, and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia. Figure 1.A.6 : Hard Disk Drive & Solid State Disk(showing flash memory units) Solid State Disk: An SSD does much the same job functionally (e.g., saving data while the system is off, booting the system, etc.) as an HDD, but instead of a magnetic coating on top of platters, the data is stored on interconnected flash memory chips that retain the data even when there‘s no power present. Introduction to Computer Systems y Hardware Concepts 1.13 Flash memory is a type of EEPROM chip, which stands for Electronically Erasable Programmable Read Only Memory. It has a grid of columns and rows with a cell that has two transistors at each intersection. It has no moving parts, so it’s noiseless. It allows faster access and smaller in size. This memory transfers data at a faster rate of 30Mega Bytes per second! Figure 1.A.7 : Solid State Disk The chips can either be permanently installed on the system‘s motherboard (like on some small laptops and ultrabooks), on a PCI/PCIe card (in some high-end workstations), or in a box that‘s sized, shaped, and wired to slot in for a laptop or desktop‘s hard drive (common on everything else). These flash memory chips differ from the flash memory in USB thumb drives in the type and speed of the memory. The capacity ranges from 100 GB to 4 TB. Floppy Disk: A floppy disk consists of a special and flexible plastic disk, coated on either side, with a magnetic material. The disk is encased in a square plastic or vinyl jacket cover. Because the disk is flexible, it is referred to as floppy disks or simply floppies‗. Floppy disks were first introduced in 1972 by IBM Inc. USA. At present, most commonly used floppies are of 3.5‗‗ size. These are double sided disks and store data on both surfaces. The storage capacity of these floppies varies from 720kB to 2.88MB. These are the cheapest off line secondary storage devices for small computers. Figure 1.A.8 : Floppy Disk Optical Disks: The secondary storage disk, which stores binary data in form of lands and pits to produce constructive and destructive interference of laser light are termed as Optical Disks. Most of these disks are read only memories, because the lands and pits formed once on the disk become permanent. The basic difference between a magnetic disk and the optical disk is their storage pattern. A magnetic disk has concentric circular tracks whereas optical disks possess a spiral track from centre towards the outer edge. Due to the similarity 1.14 +2, Information Technology y Part - I with audio CD, the optical disks are called as CD-ROMs (compact disk- read only memory).Capacity of standard 120mm CD is 700MB. It is a thin optical disk which is commonly used to store audio and video data. Transfer speed is mentioned as muliple of 150 KB/s. 4x means 600 KB/s. In this magnetic coatings are stored as data on a thin tape. Earlier this medium was used for archive purposes. Magnetic Tape: It is sequential storage medium used for data collection, backup and archiving. Like videotape, computer tape is made of flexible plastic with one side coated with a ferromagnetic material. Tapes were originally open reels, but were superseded by cartridges and cassettes of many sizes and shapes. DVD: Digital Versatile Disc or Figure 1.A.9 : Magnetic Tape Cartridge Digital Video Disc. This is an optical disc storage device. Each writable layer of a DVD has a spiral track of data. On single- layer DVDs, the track always circles from the inside of the disc to the outside. The data track and the pits are incredibly tiny. The tracks of a DVD are separated by just 740 nanometers while that of a CD-ROM, the separation is around 1600nm. Similarly the pits are also smaller in case of DVD as well. The minimum pit length is 400nm for a DVD in comparison to 830nm for a CD-ROM. The width of a DVD track is 320nm and the height of bumps is 120nm. Figure 1.A.10 : Digital Versatile Disc A DVD has the same dimensions as a standard CD but cannot be read by a standard CD player; although a DVD player can read standard CDs. DVD players use a higher-power red laser (0.65 micrometer of wavelength) that enables smaller pits and separation tracks to be used. It can be recorded on single side or on double side. Its capacity may range from 4.7 GB to 8.5 GB Introduction to Computer Systems y Hardware Concepts 1.15 USB Drive: This is a class of flash memory, which is small, portable and can be plugged into a computer with USB Port. They have capacity lesser than hard disk but much larger than a floppy or CD. They are more reliable also. They are also called as pen drives. Memory Cards: A memory card (sometimes called a flash memory card or a storage card) is a small storage Figure 1.A.11 : USB Pen Drive medium used to store data such as text, pictures, audio, and video, for use digital cameras, computers, mobile phones, music players, video game console etc. Most of the current products use flash memory, although other technologies are being developed. There are a number of memory cards on the market, including the SD card (secure digital card), the CF card (CompactFlash card), the SmartMedia card, the Memory Stick, and the MultiMediaCard (MMC). These cards are of varying sizes, and each is available in a range of storage capacities. The CompactFlash card is about the size of a matchbook, while the MultiMediaCard and Secure Digital card are each about Figure 1.A.12 : Micro SD Card the size of a postage stamp. Most available cards have constantly powered nonvolatile memory, which means that data is stable on the card, is not threatened by a loss of power source, and does not need to be periodically refreshed. Because memory cards are solid state media, they have no moving parts, and therefore, are unlikely to suffer mechanical difficulties. Earlier removable storage media, such as the PC card, the smart card, and similar cards used for gaming systems can also be considered to be memory cards. However, the newer cards are smaller, require less power, have higher storage capacity, and are portable among a greater number of devices. Memory cards offer a number of advantages over the hard disk drive: they‘re much smaller and lighter, extremely portable, completely silent, allow more immediate access, and are less prone to mechanical damage. 1.16 +2, Information Technology y Part - I Communication Bus: It is a cable, (also called as System Bus) which provides parallel data communication path over which information is transferred between the functional areas of the computer, such as CPU, Memory, I/O devices and other computer systems. The information is transferred byte by byte. The CPU logically controls the transfer of data, instructions, and commands between them. The direction of signal flow for the different buses is indicated on figures in the computer‗s technical manuals. The direction may be unidirectional or bidirectional depending on the type of bus and type of computer. The name of the bus or its operation usually implies the type of signal it carries or method of operation. (i) Control Bus: The control bus is used by the CPU to direct and monitor the actions of the other functional areas of the computer. It is used to transmit a variety of individual signals (read, write, interrupt, acknowledge, etc.) necessary to control and coordinate the operations of the computer. (ii) Address Bus: The address bus contains the connections between the microprocessor and memory that carry the signals relating to the addresses which the CPU is processing at that time, such as the locations that the CPU is reading from or writing to. The width of the bus determines the number of memory locations that can be addressed. For example a system with 64-bit address bus can address 264 memory locations. An address is defined as a label, symbol, or other set of characters used to designate a location or register where information is stored. Before data or instructions can be written into or read from memory by the CPU or I/O sections, an address must be transmitted to memory over the address bus. (iii) Data Bus: This is used for the exchange of data between the processor, memory and peripherals, and is bi-directional in nature. Sometimes called as the memory bus, it handles the transfer of all data and instructions between functional areas of the computer. It transmits data in one direction at a time. The data bus is used to transfer instructions from memory to the CPU for execution. It carries data (operands) to and from the CPU and memory as required by instruction translation. The data bus is also used to transfer data between memory and the I/O section during input/output operations. Introduction to Computer Systems y Hardware Concepts 1.17 The number of wires used in the data bus (sometimes known as the ‗width‘) can differ. Each wire is used for the transfer of signals corresponding to a single bit of binary data. Therefore, a greater width allows greater amounts of data to be transferred at the same time. Figure 1.A.13 : System Bus (Address Bus, Control Bus & Data Bus) Instructions to be executed by a CPU are retrieved from main memory, interpreted by it and executed. CPU is connected to main memory by a set of parallel wires called Address Bus, which carries address bits to the MAR (Memory Address Register) and the Data Bus, which carries data/instructions from CPU to MDR (Memory Data Register) of main memory. The control bus carries instructions to carry out operations such as Read/Write from or to memory and also input/output operations. Communication Ports: A motherboard has a set of connection points called ports to connect units such as disk, VDU, keyboard etc. In a parallel port data-bits are transmitted in parallel (16 or 32 bits simultaneously) to peripherals via a set of parallel wires (called ribbon cables). Serial ports transmit single bits serially, one after another. Faster peripherals such as hard disk are connected to parallel ports. Slower devices such as keyboard are connected to serial port. A standard serial port is known as Universal Serial Bus (USB). A communication port is mounted in a slot on the computer for easy plugging/unplugging of a peripheral device. 1.18 +2, Information Technology y Part - I (i) Serial ports: Through this port the information travels in and out one bit at a time. Serial ports come in the form of 9-pin or 25-pin male connector. These ports are often known as communication (COM) port. Mouse, modem etc. are connected using serial port though now mostly they are been replaced by USB port. Figure 1.A.14 : Serial & Parallel Ports (ii) Parallel Port: Through this port the several data signals are sent simultaneously over several parallel channels. Parallel ports come in the form of of 25 holes and commonly connect the devices to system. Performance capability of these ports is much faster as compared to serial ports and other types of ports. These ports are built in mother board and consist of 25 wires, 8 of those wires are responsible to transfer data and control complete circuit. These ports are used to connect printer, scanner etc. (iii) RJ45 Port: This port is used for Ethernet connections and can be used between computer and any networked device, such as a cable modem or a network hub. It is an eight wire connector. (iv) RJ11 Port: This port is for connecting to a telephone line. It has six-wire connnector in it and is smaller than RJ45 Introduction to Computer Systems y Hardware Concepts 1.19 (v) USB Port: USB stands for Universal Serial Bus, used for short distance digital data communications. This port allows data transfer between devices with little electric power. Flash drives are biggest examples for these Figure 1.A.15 : USB 3.0 & USB 2.0 Ports ports, which are connected on these ports only. These ports are also helping Smartphone users, as they can now transfer data very easily from computer to their phones. (vi) PS/2 Port: These are most simple and oldest type of ports, and are still working in market with systems. These are 6-pin, low-speed serial connection ports which are used with mouse and keyboards mostly. In system Figure 1.A.16 : PS/2 Port two ports of this kind are found, although they look similar but they are not interchangeable. It is important to be extremely careful to attach the keyboard and mouse to their respective PS/2 port. (vii) VGA Monitor Port: It is used to display Video Graphics Array in a system. This port uses an analog monitor which forward signals towards display adapter. All monitors and LCDs accept these signals, Figure 1.A.17 : VGA Monitor Port but some flat displays prefer to use digital signal interface. VGA resolution typically refers to the original resolution of 640×480 pixels and 16 colors, but it is not preferred to use for small monitors or displays. 1.20 +2, Information Technology y Part - I INPUT DEVICES : The devices which are used to enter data and program instructions in to a computer for storage or processing are called as input devices. Punched Card and paper-tape readers were extensively used for inputting data in earlier systems. Now a days, those have been supplanted by type writer like key boards, interactive graphic devices like mouse, light pen, and joystick and touch screens. Other direct entry devices include optical laser scanners (bar code reader, OCR, OMR), smart cards, voice recognition devices capable of understanding simple verbal commands, digital camera and some auxiliary storage devices like floppy disks, CD-ROMs and Pen drives. We shall discuss few of these input devices, which are widely used not only for scientific purposes but also for business, industry, education, etc. (i) Keyboard: This is the most common input device which uses an arrangement of buttons or keys. Keyboards can have different keys depending on the manufacturer, the operating system, and whether they are attached to a desktop computer or part of a laptop. In a keyboard each press of a key typically corresponds to a single written symbol. However some symbols require pressing and holding several keys simultaneously or in sequence. While most keyboard keys produce letters, numbers or characters; other keys or simultaneous key presses can produce actions (Do /Undo; Copy / Paste, etc.) or computer commands. In normal usage, the keyboards are used to type text and numbers while in a modern computer; the interpretation of key press is generally left to the software. Keyboards are also used for computer gaming, either with regular keyboards or by using keyboards with special gaming features. Apart from the Alphabet Key Pad (Type writer keys), there are several other keys for various purposes such as 1. Number keys – There are ten keys (0-9) on each keyboard. Sometimes, there are two sets of these keys; for laptops & others, they are in one line just above the Alphabet Key Pad & in detached key boards, they are arranged in an additional Numeric Key Pads to the right of the Alphabet Key Pad. This is generally found in adding machines or Desktop Calculators. 2. Direction keys - There are four arrow keys [left (¨), right (Æ), up ( ) and down (Ø)] arranged in an inverted T formation between the Introduction to Computer Systems y Hardware Concepts 1.21 typing keys and numeric keypad. They move the cursor on the screen in small increments along the direction of keys. Unlike alphabet and number keys, these keys do not display anything. 3. Function keys - There are generally twelve functions keys F1-F12. Application programs and operating systems can assign specific tasks to the function keys. Just like direction keys, these too do not print anything. Control keys - These keys like, Home, End, Insert, Delete, Page Up, Page Down, Ctrl, Alt and Esc provide screen control. These keys have special tasks and do not print anything. 4. Other keys - There are several other non-printable keys for various different purposes. These include caps lock, tab, pause, backspace, spacebar, shift; enter etc which are used for special purposes. Figure 1.A.18 : Key Board showing different keys (ii) Mouse: Interaction with computers was initially restricted to text mode only. The development of Graphical User Interface (GUI), in which a user interacts with the computer programs through graphic icons and menus, demanded an input device, which can be used directly to point and select the icons / menus on the computer screen. A host of such pointing devices were developed subsequently; they are-mouse, joystick, trackball etc. Invented by Douglas Engelbart of Stanford Research Center in 1963, and pioneered by Xerox in the 1970s, the mouse is one of the great breakthroughs in computer ergonomics. It is a small object which can roll along a hard, flat 1.22 +2, Information Technology y Part - I surface. Its name is derived from its shape, which looks a bit like a mouse, its connecting wire that one can imagine to be the mouse‗s tail, and the fact that one must make it scurry along a surface. It controls the movement of the cursor or pointer on a display screen. A mouse contains at least one button and sometimes as many as three, which have different functions depending on what program is running. The scroll wheel of a mouse helps in scrolling through long documents. The applications which run by the clicking of a mouse button are often called point-and-click programs. The mouse is also useful for graphics programs that allow you to draw pictures by using the mouse like a pen, pencil, or paintbrush. There are three basic types of mice: 1. Mechanical Mouse: Has a rubber or metal ball on its underside that can roll in all directions. Mechanical sensors within the mouse detect the direction the ball is rolling and move the screen pointer accordingly. 2. Opto-mechanical Mouse: Same as a mechanical mouse, but uses optical sensors to detect motion of the ball. 3. Optical Mouse: An optical mouse uses a tiny camera to take 1,500 pictures every second. It works on almost any surface. It has a small, red light-emitting diode (LED) that bounces light off that surface onto a complimentary metal-oxide semiconductor (CMOS) sensor. The CMOS sensor Figure 1.A.19 : Optical Mouse sends each image to a digital signal processor (DSP) for analysis. The DSP, operating at 18 MIPS (million instructions per second), is able to detect patterns in the images and see how those patterns have moved since the previous image. Based on the change in patterns over a sequence of images, the DSP determines how far the mouse has moved and sends the corresponding coordinates to the computer. The computer moves the cursor on the screen, based on the coordinates received from the Introduction to Computer Systems y Hardware Concepts 1.23 mouse. This happens hundreds of times each second, making the cursor appear to move very smoothly. By default, a mouse is configured to work for the right hand. If you are left-handed, the settings can be changed to suit your needs. Laptops are equipped with a small flat surface or sometimes with a very short stick for performing same job as mouse. Using left button of mouse different operations like selection, dragging, moving and pasting can be done. With the right button we can open a context menu for an item, if it is applicable. Mouse connects to PCs in one of several ways: Serial mouse connects directly to an RS-232C serial port or a PS/2 port. This is the simplest type of connection. A PS/2 mouse connects to a PS/2 port, whereas an USB mouse connects to an USB port. A Cordless mouse uses infrared or radio waves to communicate with the computer. (iii) Joystick: A joystick is a personal computer peripheral or general control device consisting of a handheld stick that pivots about one end and transmits its angle in two or three dimensions to a computer. It works on the same principle as a mouse. A joystick is generally configured so that moving the stick left or right signals movement along the X axis, and moving it forward (up) or back (down) signals movement along the Y axis. In joysticks that are configured for three- dimensional movement, twisting the stick left (counter-clockwise) or right (clockwise) signals movement along the Z axis. Joysticks are often used to control video games, and usually have one or more push-buttons whose state can also be read by the computer. The term joystick has become a synonym for game controllers that can be connected to the computer since the computer defines the input as a ―joystick input‖. Apart from controlling games, joysticks are also used for controlling machines such as aircraft, cranes, trucks, powered wheelchairs and some zero turning radius lawn mowers. More recently miniature joysticks have been adopted as navigational devices for smaller electronic equipment such as mobile phones. Additionally joysticks often have one or more fire buttons, used to trigger some kind of action. These are simple on/off switches. 1.24 +2, Information Technology y Part - I Joystick elements: 1. Stick 2. Base 3. Trigger 4. Extra buttons 5. Auto fire switch 6. Throttle 7. Hat Switch (POV Hat) 8. Suction Cup Most I/O interface cards for PCs have a joystick (game control) port. Modern joysticks use a USB interface for Figure 1.A.20 : Joystick connection to the PC. (iv) Light pen: A light pen is an input pointing device shaped like a pen and is connected to a Display Unit (CRT only). The tip of the light pen contains a light-sensitive element (photo-electric cell) which, when placed against the screen, detects the light from the screen enabling the Figure 1.A.21: Light Pen computer to identify the location of the pen on the screen. This device is just a plastic cylinder housing a phototransistor, a light- activated switch..When the pen is held to the screen, the beam of electrons which light up the screen, triggers the photo-electric cell, causing it send a signal to the computer that the beam has been detected. Since the computer knew exactly where the electron beam was at that instant, it could figure out the location, where the pen was pointing The user can point to displayed objects or draw on the screen in a similar way to a touch screen but with greater positional accuracy. It is commonly used to select or otherwise modify text or data on a screen. It is also used to draw pictures or to select menu options. Introduction to Computer Systems y Hardware Concepts 1.25 (v) Graphics Tablet A graphics tablet is a hardware input device that enables an artist to draw or sketch digitally using a pen or stylus. They are helpful because they provide a more natural and precise feel than a standard computer mouse. In a graphics tablet, thin sheets of a special film are used in place of the variable resistors. When you press down Figure 1.A.22 : Graphics Tablet Pen at a point on the film, a current flows, with the amount of resistance depending on where on the film you press. Similar to an artist drawing with a pencil and paper, a user draws on the graphics tablet with a stylus. The computer will convert the drawing strokes into digital form, displaying them on the computer screen. The graphics tablet can also be used to capture users‘ signatures. This use is similar to the signature pads found at many retail stores, where you would write your signature after using a credit/debit card to make a purchase. (vi) Touchscreen A touch screen sensor is a clear glass panel with a touch responsive surface. The touch sensor/panel is placed over a display screen so that the responsive area of the panel covers the viewable area of the video screen. There are several different touch sensor technologies on the market today, each using a different method to detect touch input. The sensor generally has an electrical current or signal going through it and touching the screen causes a voltage or signal change. This voltage change is used to determine the location of the touch to the screen. Some touchscreens also use beams across the screen to create a grid that senses the presence of a finger. A touchscreen allows the user to interact with a device without a mouse or keyboard and is used with smart phones, tablets, computer screens, Bank Figure 1.A.23 : Touch Screen of a Tablet PC 1.26 +2, Information Technology y Part - I ATMs and information kiosks (at Railway stations, Urban Local Body Offices & others). There are three basic touch sensor technologies those are used to recognize a person‘s touch; Resistive, Capacitive & Surface Acoustic Wave. Other technologies such as Infrared, Near Field Imaging are also being implemented Resistive touchscreens work a bit like ―transparent keyboards‖ overlaid on top of the screen. There‘s a flexible upper layer of conducting polyester plastic bonded to a rigid lower layer of conducting glass and separated Figure 1.A.24 : BankATM Touch Screen by an insulating membrane. When you press on the screen, you force the polyester to touch the glass and complete a circuit— just like pressing the key on a keyboard. A chip inside the screen figures out the coordinates of the place you touched. In the capacitive system, a layer that stores electrical charge is placed on the glass panel of the monitor. The inner layer conducts electricity and so does the outer layer, so effectively the screen behaves like a capacitor. When a user touches the monitor with his or her finger, some of the charge is transferred to the user, so the charge on the capacitive layer decreases. This decrease is measured in circuits located at each corner of the monitor. The computer calculates, from the relative differences in charge at each corner, exactly where the touch event took place and then relays that information to the touch-screen driver software. One advantage that the capacitive system has over the resistive system is that it transmits almost 90 percent of the light from the monitor, whereas the resistive system only transmits about 75 percent. This gives the capacitive system a much clearer picture than the resistive system. Figure 1.A.25 : Touch Screen of a Smartphone Introduction to Computer Systems y Hardware Concepts 1.27 In case of Surface Acoustic Wave technology, the screen detects your fingers using sound instead of light. Ultrasonic sound waves are generated at the edges of the screen and reflected back and forth across its surface. When you touch the screen, you interrupt the sound beams and absorb some of their energy. The screen‘s microchip controller figures out from this, where exactly you touched the screen. Note : Not all touchscreens act the same, and any of the below actions may react differently on your device. Tap - A gentle touch or tap of the screen with a finger once to open an app or select an object. Double-tap - Depending on where you are a double-tap can have different functions. For example, in a browser double-tapping the screen zooms into where you double-tapped. Double-tapping text in a text editor selects a word or section of words. Touch and hold - Gently touching and holding your finger over an object selects or highlights the object. For example, you could touch and hold an icon and then drag it somewhere else on the screen or highlight text. Drag - After you have touched and held anything on the screen while keeping the your finger on the screen drag that object or highlight in any direction and then lift your finger off of the screen to drop it in place or stop highlighting. Swipe - Swiping your finger across the screen scrolls the screen in that direction. For example, pressing the finger at the bottom of the screen and quickly moving it up (swiping) scrolls the screen down. Pinch - Placing two fingers on the screen and then pinching them together zooms out and pinching your fingers together and then moving them away from each other zooms into where you started. (vii) Microphone Sometimes abbreviated as mic, it is an input device that allows the users to input audio into the computers. Most microphones connect to the computer using the ―mic‖ port on the computer sound card. Higher quality Figure 1.A.26 : Microphone 1.28 +2, Information Technology y Part - I microphones or microphones with additional features can connect to the USB port in the computer system. It is used to convert sound into electrical signal. The analog electrical signal is digitized and fed to the computer; Sound recording software does the necessary editing. (viii)Scanners Optical scanning involves, illuminating an object by an incident light source and then converting the reflected light into electrical signal by suitable electronic circuitry( Charge Coupled Device / Photo Multiplier Tube). A scanning device optically scans an image and converts it to a digital image. The digital image is then, suitably manipulated by the application software for the specific use in the computer systems. Since these are direct data entry devices, they require a very good quality of input documents for proper processing. The accurate dimension and ink specification are other two requirements for the purpose. However, image scanners may not require the same restrictions as they do not need any data processing. Scanning devices are used for different purposes as outlined below (1) Image Scanner (2) OCR scanner (3) OMR scanner (4) MICR scanner (5) Bar Code Scanner Image Scanner: An image scanner translates a paper document in to an electronic format for storing in a computer system. It helps in preserving the paper document in electronic form for years without any physical damage. With the help of available software the images can be manipulated in desired manner. There are different types of image scanners-such as, flat bed scanner, sheet fed scanner, hand held scanner and drum scanner. OCR Scanner Optical Character Recognition (OCR) is the electronic conversion of images of typed, handwritten or printed text into machine-editable text. Introduction to Computer Systems y Hardware Concepts 1.29 The OCR scanner is equipped with character recognition software which converts the images of characters to equivalent ASCII codes (machine encoded text). The scanner first creates the image of the document, and the software (OCR software) translates the array of grid points in to ASCII text, which the computer can interpret as letters, numbers or special characters. The scanned document is stored in the system as a bit-mapped file in TIF format. It is widely used as a form of data entry from printed paper data records, whether passport documents, invoices, bank statements, computerized receipts, business cards, mail, printouts of static-data, or any suitable documentation. It is a common method of digitizing printed texts so that it can be electronically edited, searched, stored more compactly, displayed on-line, and used in machine processes such as machine translation, text-to-speech, key data and text mining. It is widely used to convert books and documents into electronic files, to computerize a record-keeping system in an office, or to publish the text on a website. OCR is a field of research in pattern recognition, artificial intelligence and computer vision. OMR Scanner Optical mark recognition is the process of capturing data by contrasting reflectivity (low reflection: high reflection) at predetermined positions on a page. By shining a beam of light onto the document, the scanner is able to detect a marked area because it reflects less light than an unmarked surface. It is generally distinguished from optical character recognition by the fact that a recognition mechanism is not required. That is, the marks are constructed in such a way that there is little chance of not reading the marks correctly. One of the most familiar applications of optical mark recognition is the use of HB pencil bubble optical answer sheets in multiple choice question examinations. Students mark their answers, or other information, by darkening circles or ovals or squares on a pre-printed sheet. Afterwards the sheet is automatically graded by a scanning machine. 1.30 +2, Information Technology y Part - I Figure 1.A.27 : OMR Sheet & Scanner Other examples of OMR are the Magnetic Ink Character Recognition (MICR) & Bar Code Reader. We shall discuss these two very important applications below. MICR Scanner Magnetic Ink Character Recognition (MICR) is the common machine language specification for the paper-based payment transfer system. It consists of magnetic ink printed characters of a special design which can be recognized by high speed magnetic recognition equipment. This process excludes any other markings, like human signatures or bank seals, etc. on the cheques. A series of readable characters (The MICR devices can read only a select group of characters; E-13B and CMC-7 Font) on a cheque provides the receiving party with information needed for processing. These include: the cheque number, bank routing number, cheque account number and in some cases the amount of the cheque. A sample cheque, which employs MICR technology, is shown here. Figure 1.A.28 : A Bank Cheque with MICR Number; MICR Scanner Introduction to Computer Systems y Hardware Concepts 1.31 MICR code consists of 9 numerical digits. First three digits represent the postal code of the city, in which the Bank is situated. For example; for Bhubaneswar the PIN Code starts with 751; so for any Bank having approved MICR facility would have its 1st, 2nd and 3rd digit as 751. The middle three digits represent the Bank Code allotted by the Reserve Bank of India. For example; the State Bank of India has been allotted the code 002. The last three digits represent the Branch Code of the particular Bank Bar Code Scanner When information are encoded in form of parallel lines of varying thickness and spaces between them, they are called as bar codes. These codes are being used for unique identification of different types of goods in retail stores, books, postal packages, tags, etc. A barcode reader (or barcode scanner) is a computer peripheral for reading barcodes printed on various surfaces. It consists of a light source (a laser beam), a lens and a photo conductor translating optical impulses into Figure 1.A.29 : Bar Code Scanner electrical ones. In fact a laser beam after striking a pattern of bars reflects in a particular pattern which is sensed by the photo conductor. The electrical pulse so generated goes to a decoder circuitry which analyzes the barcode‘s image data and sends the barcode‘s content to the scanner‘s output port. (ix) Smart Card Reader It is used to access the microprocessor of a smart card. There are two broad categories of smart cards - Memory cards and Microprocessor cards. Memory cards contain only non-volatile memory storage components, and some specific security logic. Microprocessor Figure 1.A.30 : Smart Card Reader 1.32 +2, Information Technology y Part - I cards contain volatile memory and microprocessor components. The card is made of plastic, generally PVC (Poly Vinyl Chloride). Smart cards are used in large companies and organizations for strong security authentication. (x) Biometric Sensor A biometric sensor is an input device that recognizes individuals based on their physical or behavioral traits (fingerprint, voice, face, iris etc.). It converts any of these traits in to electrical signals by measuring the quantities like; pressure, temperature, light, speed, electrical capacity or other kinds of energies. Different technologies can be applied to achieve this conversion using common digital cameras or more Figure 1.A.31 : Bio-metric Sensor sophisticated combinations or networks of sensors. Biometric sensor is used to mark attendance of employees/students in organizations/institutions. It is also popular as a security device to provide restricted entry for secured areas. (xi) Web Camera: It is a video camera that captures image as data for a computer system and is normally connected directly to a computer through a USB port. Figure 1.A.32 : Web Camera OUTPUT DEVICES : An electromechanical device, which accepts data from a computer and translates them in to a form suitable for the user is called an Output Device. These peripheral devices are classified in to following categories; (i) Monitors (ii) Printers (iii) Plotters Introduction to Computer Systems y Hardware Concepts 1.33 (iv) Screen Image Projectors (v) Voice Response System (Speakers) Output devices generate output in two formats: Soft copy and Hard copy. Soft copy: A soft copy is an output, which is not produced on a paper or some material, which can be touched or carried to another place in physical forms. These are temporary in nature and vanish after the use. Examples of soft copy are output on a monitor or sound produced by a voice response system. Hard copy: It is an output on a paper or any other material which can be touched and carried for being shown to others. These are permanent in nature. Outputs of printer and plotters are hard copy outputs. 1. Computer Monitor : A computer display monitor, usually called a monitor, is a piece of electrical equipment which displays viewable images generated by a computer without producing a permanent record. The monitor displays instructions we send to the computer and the instruction and results from the computer back to us. Several different hardware technologies exist for computer monitors, such as: Cathode ray tube (CRT), Liquid crystal display (LCD), Plasma display, Surface-conduction electron-emitter display (SED), Video projector - implemented using LCD/CRT/ other technologies, Organic light-emitting diode (OLED) display. A monitor comprises the display device, circuitry to generate a picture from electronic signals sent by the computer, and an enclosure or case. A monitor may be within the computer as an integral part (laptops, notebook PC, PDA) or a plugged-in device for desktop or other form of computers. (i) CRT Monitor: The CRT or cathode ray tube is the picture tube of a monitor. The back of the tube has a negatively charged cathode. The electron gun shoots electrons down the tube and onto a charged screen. The screen is coated with a pattern of dots Figure 1.A.33 : CRT Monitor 1.34 +2, Information Technology y Part - I that glow when struck by the electron stream. Each cluster of three dots (for Red, Green & Blue color), constitute a picture element (pixel). The image on the monitor screen is usually made up from at least tens of thousands of such tiny dots glowing on command from the computer. The closer together the pixels are, the sharper the image on screen. The distance between pixels on a computer monitor screen is called its dot pitch and is measured in millimeters. Most monitors have a dot pitch of 0.28 mm or less. (ii) LCD Monitors Liquid crystal display technology works by blocking light. An LCD is made up of two pieces of crossed polarized glass (called as substrates) that contain a liquid crystal material between them. Liquid crystals are liquid chemicals whose molecules can be aligned precisely when subjected to an electric field. When properly aligned, liquid crystals allow light to pass through. A backlight creates light that passes through the first substrate. At the same time, electrical Figure 1.A.34 : LCD Monitor currents cause the liquid crystal molecules to align to allow varying levels of light to pass through to the second substrate and create the colors and images that you see. Most LCD displays use active matrix technology. A thin film transistor (TFT) arranges tiny transistors and capacitors in a matrix on the glass of the display. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. 2. Printers: Printers are most commonly used output devices, producing hard copies of computer output. They are classified in to two categories-Impact Printers and Non Impact Printers. Introduction to Computer Systems y Hardware Concepts 1.35 (i) Impact Printers: These printers form images by a mechanism of making contact with the paper through an ink-coated ribbon. There are two different ways of forming images on a paper. In one case, there are character shaped print hammers, which strike the paper through the ribbon to produce good quality output. In other case there is a print head containing a row of pins that produce a pattern of dots in the form of characters or other images. Drum Printers, Band / Chain Printer, Dot-Matrix Printer are the Impact Printers. (ii) Non Impact Printers: Most non impact printers form images from a matrix of dots; but they employ different techniques. The non- impact printers include-Laser Jet Printer, Inkjet Printer, LED printer, Ion deposition printer etc. Dot-Matrix Printer : A dot matrix printer or impact matrix printer refers to a type of computer printer with a print head that runs back and forth on the page and prints by impact. A vertical column of hammers strike an ink-soaked cloth ribbon against the paper, much like a typewriter. Unlike a typewriter, letters are drawn out of a dot matrix, and thus, varied fonts and arbitrary graphics can be produced. Because the printing involves mechanical pressure, these Figure 1.A.35 : Dot-matrix Printer printers can create carbon copies and carbonless copies. Ink Jet Printer: Any of a class of non impact printers, which form characters and different kind of images by spraying small drops of ink on to the paper, are called as Ink Jet Printers. The dots are extremely small (the diameter of these dots varies between 50 and 60 microns; the diameter of a human hair is 70 microns!). The dots are positioned very precisely, with resolutions of up to 1440 dots per inch (dpi). The dots can have different colors combined together to create photo- quality images. The print head of an ink jet printer contains up to 64 tiny nozzles, 1.36 +2, Information Technology y Part - I which can be selectively heated up in a few micro seconds by an IC controlled resistor. When the resistor heats up, the ink near it vaporizes and is ejected through the nozzle on to the paper. This is a character printer, i.e. a selective group of nozzles are heated at a time to print a character on the printer. The selection is made by the computer program. The print head moves horizontally to print a document or image. Laser Printers: Any of a class of non impact printers which create electrostatic image with the help of laser beams and imprint them on a paper or a plastic sheet by hot pressing the toner (black or coloured pigment) are called as Laser Printers. A laser printer prints one page at a time. The first laser printer was produced by Xerox. In 1971, researcher Gary Starkweather at Xerox Inc. modified a copier to produce this printer. The technology of the laser printers has been renovated each passing year. However, some of the basic components and functions remain the same. Few of them are outlined here. A laser printer consists of the following components: (i) A corona wire or a charged roller (ii) A photo conducting drum. (iii) A laser beam, mirror, lens assembly (iv) Toner (v) Fuser Figure 1.A.36 : Laser Printer (vi) Rubber blade Working of a laser printer: A laser printer works in following six steps: (1) Charging: A corona wire (in older printers) or a primary charge roller projects a positive electrostatic charge onto the a revolving photosensitive drum or belt which is capable of holding an electrostatic charge on its surface as long as it hasn‘t been exposed Introduction to Computer Systems y Hardware Concepts 1.37 to certain electromagnetic radiation of a particular wavelength (laser beam) (2) Writing: The laser beam, being controlled by an image processor circuit is aimed at a moving mirror which directs the laser beam through a system of lenses and mirrors onto the revolving, charged drum. Wherever the laser strikes the photo conducting drum the charge is reversed (becomes negative), thus creating a latent electro- photographic image on the drum surface. (3) Developing: The surface containing the latent image is exposed to toner. The toner are very fine particles of dry plastic powder mixed with carbon black or coloring agents. The positively charged toner particles get electro statically attracted to the drum surface where the laser wrote the latent image with negative charges. (4) Transferring: The drum is rolled over the paper (which is given a strong negative charge to attract the toner particles from the drum) to transfer the image on to it. (5) Fusing: The paper then passes through a fuser assembly. Here a roller provides heat and pressure (up to 200 degrees Celsius) and bonds the plastic powder to the paper by melting. (6) Cleaning: When the print is complete an electrically neutral rubber blade cleans any excess toner from the photo conducting drum and deposits it into a waste reservoir. A discharge lamp removes the remaining charge from the drum. Colour Laser Printers: Essentially, color printers work the same way as monochrome printers, except they go through the entire printing process four times - one pass each for cyan (blue), magenta (red), yellow and black. By combining these four colours of toner in varying proportions, the printer generates the full spectrum of colours (VIBGYOR). 3. Plotters: The printers discussed above are capable of producing graphics output, but lag in some cases like, engineering design, architecture plan of a building, design of mechanical components of a car etc. These are the cases where high 1.38 +2, Information Technology y Part - I quality and perfectly proportioned graphics are required on large sheets of paper. A special type of output device used for this purpose is called a Plotter. Two commonly used plotters are drum plotter and flat bed plotter. Figure 1.A.37 : Fiat-bed Plotter 4. Speakers: Speakers are output devices used with computer systems to produce sound. They receive audio input from the computer‘s sound card and produce audio output in the form of sound waves. Most computer speakers are active speakers having an internal amplifier which allows one to increase the volume of the sound. Speakers usually come in pairs, which allow them to produce stereophonic sound from two separate audio channels. Figure 1.A.38 : Computer Speaker 5. Digital projector: A digital projector is a device which connects to a computer and is used to project the video output from the computer onto a wall or screen. In classrooms they are often used with electronic whiteboards. The projector can be portable and placed on a stand or it can be permanently fixed to the ceiling. They are used in: Figure 1.A.39 : Digital Projector  Classroom teaching  Staff training sessions  A presentation to an audience  Home theaters Introduction to Computer Systems y Hardware Concepts 1.39 Memory Units: Register A computer system possesses two types of memory units: - Primary/Main memory and Secondary/auxiliary memory; a brief description of these memory units have already been discussed earlier in this chapter. Here we shall discuss some special type of memory units, which reside at the core of a computer processor: the Registers. A collection of special memory units to hold the information on a temporary basis inside the CPU (not in main memory) constitute a Register. These are electronic memory units and facilitate speedy transfer of information between different parts of a CPU. The size of a register equals the number of bits required by a CPU to store a character or a number. Thus, the registers are often called as 8-bit, 16-bit, 32- bit or 64-bit registers. The number of bits (or numeral places) a CPU uses to represent numbers is often called ―word size‖ or ―bit width‖ or ―data path width‖ or ―integer precision‖. The bigger the word size, the faster the computer can process a set of data. Different CPUs possess registers of different sizes and their number also varies from processor to processor. Most recent microprocessors are using 16-bit registers. Each register is assigned to hold a particular type of information. Notable among them are (i) Memory Address Register: This register holds the address of the memory location, where the current instruction is stored. (ii) Memory Buffer Register: The contents of the memory word are held in this register. A memory word is either an instruction or a data, which is read from or written to a memory location. An instruction word in MBR gets transferred to the instruction register, whereas a data word goes either to the Accumulator register for processing or gets transferred to the I/O register. Any word to be stored in a memory must go through the MBR. (iii) Program Counter Register: This register holds the address of the next instruction to be executed. The instructions of program are generally stored in consecutive memory locations, and read and executed sequentially, unless a branching instruction is encountered. In case of branching, the address of the branch instruction gets transferred to PC register to become address of the next instruction. (iv) Accumulator Register: This register holds the data for any operation. The intermediate results and the final result of a processing are also held in this register. In most of the CPUs there 1.40 +2, Information Technology y Part - I are more than one AC register. The final result of a processing goes from this register via MBR to the memory. (v) Instruction Register: The current instruction, which is being executed, is held here. Every instruction has two parts -an operation part and an address part. These parts are separated in this register; the address part is sent to MAR and the operation part is sent to the control unit. In the Control Unit it is decoded and interpreted to generate the command signals specified in the instruction. (vi) Input-Output Register: All information (instruction and data) coming from an input device or the information before going to any Output device are placed in this register. (vii) Data Register: It holds the memory operands. Summary: ÷ A computer is a machine, which takes data and instructions as input and performs computations based on those instructions to give some output. ÷ The main functional units of a computer system are - input devices, CPU, memory and output devices. ÷ There are various input and output devices for receiving input and for giving output. ÷ CPU is divided into ALU and CU. ÷ Primary memory iare of two types: RAM and ROM. ÷ Communication bus is used to transfer data between computer and peripherals. ÷ Different ports are used to connect peripherals to a computer. ÷ For future use and permanent storage secondary storage devices are used. –––––––