CIT 611 Computer Fundamentals PDF

CIT 611 Computer Fundamentals Course Developer: Professor R. 0. Ayeni Ladoke Akintola University of Technology, Ogbomoso. Programme Leaders: Dr Sam. I. Ogunrinde Dr S. A. Reju Course Co-ordinators: A. Balogun C. Okonkwo (Mrs.) Funded by C1T 611: Computer Fundamentals National Open University of Nigeria Headquarters National Open University of Nigeria 14/16 Ahmadu Bello Way Victoria Island Lagos Abuja Annex Office 245 Samuel Adesujo Ademulegun Street Central Business District Opposite Arewa Suites Abuja E-mail: [email protected] URL: www.nou.edu.ng National Open University of Nigeria, 2004 First published 2004 ISBN 978-058-216-9 I All Rights Reserved. I Published by Macmillan Nigeria Publishers Limited for National Open University of Nigeria Table of Contents CourseGuide ------------------------------------------------------------ 1 Module 1 Computer Fundamentals: Hardware -------------------------------------------- 3 Unit! What is a Computer? ------ ----- ----- ----- ------ ------------ ----------- -----. 4 1.0 Introduction ---------------------------------------------------------------------------------------------------- 4 1.1 Objectives --------------------------------------------------------------------------------------------------- 4 1.2 What is Computer? ------------------------------------------------------------------------------------------- 4 1.2.1 The Computer and Integrated Circuit Technology ---------------------------------------- 7 1.2.2 Classification of Computers ------------------------------------------------------------- 10 1.3 Conclusion ------------------------------------------------------------------------------------------------- 11 1.4 Summary --------------------------------------------------------------------------------------------------- 11 1.5 Tutor-Marked Assignment (TMA) ------------------------------------------------------------------- 11 1.6 Further Reading. ----------------------------------------------------------------------------------------------11 Unit 2 Memory System 12 2.0 Introduction ------------------------------------------------------------------------------------------------ 12 2.1 Objectives -------------------------------------------------------------------------------------------------- 12 2.2 Memory System ------------------------------------------------------------------------------------------- 12 2.3 Characteristics Terms for Various Memory Devices ----------------------------------------------- 13 2.4 Main Memory or Primary Storage --------------------------------------------------------------------- 15 2.5 Conclusion -------------------------------------------------------------------------------------------------- 16 2.6 Summary ---------------------------------------------------------------------------------------------------- 16 2.7 Tutor-Marked Assignment (TMA) -------------------------------------------------------------------- 16 2.8 Further Reading. ---------------------------------------------------------------------------------------------------16 Unit 3 External/Auxiliary Memory ----------------------------------------------------------------------17 3.0 Introduction --------------------------------------------------------------------------------------------------- 17 3.1 Objectives -------------------------------------------------------------------------------------------------- 17 3.2 External/Auxiliary Memory ----------------------------------------------------------------------------- 17 3.2.1 Magnetic Disk --------------------------------------------------------------------------- 17 3.2.2 Winchester Disk -------------------------------------------------------------------------- 18 3.2.3 Magnetic Tape ---------------------------------------------------------------------------- 19 3.2.4 Optical Memories ------------------------------------------------------------------------ 19 3.3 High Speed Memories --------------------------------------------------------------------------------------- 20 3.4 Conclusion -------------------------------------------------------------------------------------------------------------- 22 3.5 Summary -------------------------------------------------------------------------------------------------- 22 3.6 Tutor-Marked Assignment (TMA) --------------------------------------------------------------------- 22 3.7 Further Reading ------------------------------------------------------------------------------------------- 22 Unit 4 Input/Output Organization and New Technologies -------------------- 23 4.0 Introduction ------------------------------------------------------------------------------------------------- 23 4.1 Objectives ------------------------------------------------------------------------------------------------ 23 4.2 Input /Output (Peripherals) ------------------------------------------------------------------------------- 23 4.2.1 Input Devices ----------------------------------------------------------------------------- 23 4 2.2 Output Device ---------------------------------------------------------------------------- 25 4.3 Conclusion -------------------------------------------------------------------------------------------------------------28 4.4 Summary --------------------------------------------------------------------------------------------------- 28 4.5 Tutor-Marked Assignment (TMA) ----------------------------------------------------------------------- 28 4.6 Further Readings ------------------------------------------------------------------------------------------------- 28 Unit 5 Interfaces, Parallel Processidg, and Vector Processing, Pipelining and Introduction to RISC ------------------------------------------------------------------------------------ 29 5.0 Introduction ------------------------------------------------------------------------------------------------------ 29 5.1 Objectives ------------------------------------------------------------------------------------------------- 29 5.2 Input/Output Module Interface --------------------------------------------------------------------------- 29 5.3 External Interfaces --------------------------------------------------------------------------------------------30 5.4 What is Parallel Processing? ------------------------------------------------------------------------------ 31 5.5 Pipelining --------------------------------------------------------------------------------------------------------------- 32 5.6 Vector Processing ------------------------------------------------------------------------------------------- 33 5.7 Introduction to RISC ----------------------------------------------------------------------------------------34 5.7.1 Reasons for Increase Complexity ----------------------------------------------------------- 34 5.7.2 Principles of RISC -------------------------------------------------------------------------- 35 5.8 Conclusion ------------------------------------------------------------------------------------------------------------- 36 5.9 Summary --------------------------------------------------------------------------------------------------- 36 5.10 Tutor-Marked Assignment (TMA) ----------------------------------------------------------------- 36 5.11 Further Reading ----------------------------------------------------------------------------------------------36 Module 2 Computer Fundamentals: Software -------------------------------------------------------- 37 Unit 1 Software Concepts and Terminology ------------------------------------------------------------- 38 1.0 Introduction -------------------------------------------------------------------------------------------------------38 1.1 Objectives ------------------------------------------------------------------------------------------------- 39 1.2 Computer Software -----------------------------------------------------------------------------------------------39 1.2.1 System Software ---------------------------------------------------------------------------------------- 40 1.2.2 Application Software ---------------------------------------------------------------------- 40 1.3 Conclusion -------------------------------------------------------------------------------------------------------------- 42 1.4 Summary --------------------------------------------------------------------------------------------------- 42 1.5 Tutor-Marked Assignment (TMA) ------------------------------------------------------------------------42 1.6 Further Read i ng ----------------------------------------------------------------------------------------------------- 42 Unit 2 Categories of Languages -------------------------------------------------------------------------------- 43 2.0 Introduction -------------------------------------------------------------------------------------------------------43 2.1 Objectives ----------------------------------------------------------------------------------------------------------43 2.2 Categories of Languages ------------------------------------------------------------------------------------43 2.2.1 Machine Language --------------------------------------------------------------------- 43 2.2.2 Assembly Language ------------------------------------------------------------------------ 44 2.2.3 High Level Language ---------------------------------------------------------------------- 45 2.2.4 Fourth Generation Language -------------------------------------------------------------- 46 2.3 Conclusion -------------------------------------------------------------------------------------------------------------- 47 2.4 Summary --------------------------------------------------------------------------------------------------- 47 2.5 Tutor-Marked Assignments (TMA) ---------------------------------------------------------------------- 47 2.6 Further Reading ---------------------------------------------------------------------------------------------- 47 iv Unit 3 Elements of Programming Language 48 3.0 Introduction -------------------------------------------------------------------------------------------------48 3.1 Objectives ---------------------------------------------------------------------------------------------------48 3.2 Elements of a Programming Language ----------------------------------------------------------- 48 3.2.1 Variables, Constants, Data Type, Array and Expressions ----------------------------- 48 3.2.2 Input and Output Statement ------------------------------------------------------------- 51 3.2.3 Conditional and Looping Statement ------------------------------------------------- 52 3.2.4 Subroutine and Functions ------------------------------------------------------------ 53 3.3 Conclusion -------------------------------------------------------------------------------------------------- 53 3.4 Summary ----------------------------------------------------------------------------------------------------- 54 3.5 Tutor-Marked Assignment (TMA) -------------------------------------------------------------- 54 3.6 Further Reading --------------------------------------------------------------------------------------------- 54 Unit 4 Operating System Concepts -- 55 4.0 Introduction ------------------------------------------------------------------------------------------------- 55 4.1 Objectives --------------------------------------------------------------------------------------------------- 55 4.2 What is an Operating System? ------------------------------------------------------------------------ 56 4.3 Evolution of Operating Systems -------------------------------------------------------------------57 4.3.1 Serial Processing ------------------------------------------------------------------------------ 57 4.3.2 Batch Processing --------------------------------------------------------------------- 57 4.3.3 Multiprogramming ---------------------------------------------------------------- 59 4.4 Conclusion --------------------------------------------------------------------------------------------------- 61 4.5 Summary ----------------------------------------------------------------------------------------------------- 61 4.6 Tutor-Marked Assignment (TMA) -------------------------------------------------------------- 61 4.7 Further Reading ---------------------------------------------------------------------------------------- 61 Unit 5 Types of Operating System ----------------------------------------------------------------------- 62 5.0 Introduction ------------------------------------------------------------------------------------------------- 62 5.1 Objectives --------------------------------------------------------------------------------------------------- 62 5.2 Types of Operating System ---------------------------------------------------------------------------- 62 5.2.1 Batch Operating System ---------------------------------------------------------------- 62 5.2.2 Multiprogramming Operating System ------------------------------------------------ 63 5.2.3 Network Operating System ------------------------------------------------------------- 64 5.2.4-1 Distributed Operating System ------------------------------------------------------------------ 64 5.3 Conclusion --------------------------------------------------------------------------------------------------- 65 5.4 Summary ----------------------------------------------------------------------------------------------------- 65 5.5 Tutor-Marked Assignment (TMA) --------------------------------------------------------------- 65 5.6 Further Reading ---------------------------------------------------------------------------------------- 65 Module 3 Communication and Networking -------------------------------------------------------- 66 Unit 1 Fundamentals of Data Communication ------------------------------------------------------- 67 1.0 Introduction --------------------------------------------------------------------------------------------------67 1.1 Objectives ----------------------------------------------------------------------------------------------------67 1.2 Definitions ----------------------------------------------------------------------------------------------------67 1.3 Concept of Data Communication ------------------------------------------------------------------- 67 1.3.1 Data Communication Codes (Data Encoding) ----------------------------------------- 68 1.3.2 Communication Speed or Rate ---------------------------------------------------------- 68 1.3.3 Private vs. Switched Channels ----------------------------------------------------------- 68 -'1.3.4 Analog and Digital Transmission --------------------------------------------------- 68 1.3.5 Parallel and Serial Transmission ----------------------------------------------------------- 69 1.3.6 Tariff --------------------------------------------------------------------------------------------- 69 1.3.7 Communication Protocols/Standard ------------------------------------------------------ 69 1.4 Conclusion ------------------------------------------------------------------------------------------------ 70 1.5 Summary --------------------------------------------------------------------------------------------------- 70 1.6 Tutor-Marked Assignment (TMA) -----------------------------------------------------------------------70 1.7 Further Reading ---------------------------------------------------------------------------------------------70 Unit 2 Data Communication and Hardware --------------------------------------------------------- 71 2.0 Introduction -------------------------------------------------------------------------------------------------- 71 2.1 Objectives ------------------------------------------------------------------------------------------------71 2.2 Data Communication --------------------------------------------------------------------------------------- 71 2.2.1 Synchronous and Asynchronous Transmission ----------------------------------------- 71 2.2.2 Simplex, Half Duplex and Full Duplex Communication ------------------------------72 2.3 Communications Hardware ------------------------------------------------------------------------------- 73 2.3.1 Sender and Receiver Hardware --------------------------------------------------------- 73 2.3.2 Communication Devices --------------------------------------------------------------- 74 2.3.3 Communication Channels ------------------------------------------------------------------76 2.4 Conclusion ------------------------------------------------------------------------------------------------------------ 77 2.5 Summary --------------------------------------------------------------------------------------------------- 77 2.6 Tutor-Marked Assignment (TMA) ---------------------------------------------------------------------- 77 2.7 Further Reading -------------------------------------------------------------------------------------------- 77 Unit 3 Introduction to Computer Networks and Emerging Trends ----------------------------- 78 3.0 Introduction -------------------------------------------------------------------------------------------------- 78 3.1 Objectives ----------------------------------------------------------------------------------------------- 78 3.2 Network Concept and Classification -------------------------------------------------------------------- 78 3.3 Local Area Network (LAN) ------------------------------------------------------------------------------ 79 3.3.1 LAN Topology -------------------------------------------------------------------------- 79 3.3.2 LAN Access Method ------------------------------------------------------------------- 81 3.3.3 Communication Architecture for Networks ----------------------------------------------- 82 3.3.4 LAN Hardware and Software ------------------------------------------------------------- 84 3.3.5 LAN Software/Operating System ------------------------------------------------------- 85 3.4 Conclusion ----------------------------------------------------------------------------------------------------------- 85 3.5 Summary -------------------------------------------------------------------------------------------------- 85 3.6 Tutor-Marked Assignment (TMA) ----------------------------------------------------------------------85 3.7 Further Reading --------------------------------------------------------------------------------------------85 Unit 4 Wide Area Network --------------------------------------------------------------------------------- 86 4.0 Introduction --------------------------------------------------------------------------------------------------86 4.1 Objectives ----------------------------------------------------------------------------------------------- 86 4.2 Wide Area Network ----------------------------------------------------------------------------------- 86 4.2.1 Communication Switching Techniques ------------------------------------------------- 86 4.2.2 WAN Devices/Hardware ------------------------------------------------------------------- 87 4.2.3 Types of Wide Area Networks ------------------------------------------------------------ 88 4.2.4 Public Networks ------------------------------------------------------------------------ 88 4.3 Conclusion ------------------------------------------------------------------------------------------------------------- 89 4.4 Summary --------------------------------------------------------------------------------------------------- 89 4.5 Tutor-Marked Assignment (TMA) ---------------------------------------------------------------------- 89 vi 4.6 Further Reading 90 Unit 5 Networking Applications 5.0 Introduction -------------------------- 91 91 5.1 Objectives - 91 5.2 Networking Applications --------- 91 5.2.1 E-mail (Electronic Mail) - 91 5.2.2 EDI ------------------------ 92 5.3 Networking Scenario --------------- 5.3.1 Internet ---------------------- 94 5.3.2 BITNET ----------------- 94 5.3.3 Compuserve --------------- 94 5.3.4 ISDN ----------------------- 95 5.3.5 NICNET ----------------- 95 5.4 Conclusion ---------------------------- 95 5.5 Summary ------------------------------ 95 5.6 Tutor-Marked Assignment (TMA) 95 5.7 Further Reading ---------------------- 95 95 Module 4 The Management of Computer Security and Principles of Cryptography------ 96 Unit 1 The Management of Computer 97 1.0 Introduction ----------------------------------------------------------------------------------------------------97 1.1 Objectives ------------------------------------------------------------------------------------------------- 97 1.2 Definitions -------------------------------------------------------------------------------------------------------------- 97 1.3 Security Status on PC ----------------------------------------------------------------------------------------97 1.4 Breaches of Security ------------------------------------------------------------------------------------------98 1.5 Conclusion --------------------------------------------------------------------------------------------------99 1.6 Summary ----------------------------------------------------------------------------------------------------99 1.7 Tutor-Marked Assignment ---------------------------------------------------------------------------------- 99 1.8 Further Reading --------------------------------------------------------------------------------------------------- 99 Unit 2 100 2.0 Introduction ---------------------------------------------------------------------------------------------- 100 2.1 Objectives ------------------------------------------------------------------------------------------------ 100 2.2 Security Measures -------------------------------------------------------------------------------------- 100 2.2.1 Physical Security --------------------------------------------------------------------------------- 101 2.2.2 Software Security ----------------------------------------------------------------------- 102 2.2.3 Network Security ---------------------------------------------------------------------- 102 2.2.4 Password Security --------------------------------------------------------------------- 103 2.3 Conclusion ------------------------------------------------------------------------------------------------ 103 2.4 Summary ------------------------------------------------------------------------------------------------- 1 0 4 2.5 Tutor-Marked Assignment (TMA)------------------------------------------------------------------ 1 0 4 2.6 Further Reading --------------------------------------------------------------------------------------------- 104 Unit 3 Crytography 105 3.0 Introduction ---------------------------------------------------------------------------------------------- 105 3.1 Objectives ---------- -------- -------- ------ -------- ------------ -------- -- 105 3.2 Cryptography: A Brief History ----------------------------------------------------------------------- 105 3.3 Cryptography ----- -------- -------------- -------------------------- 106 vii 3.3.1 Cipher Systems --------------------------------------------------------------------------- 106 3.3.2 Data Encryption Standard (DES) ----------------------------------------------------- 107 3.3.3 RSA Approach to Encryption --------------------------------------------------------- 107 3.4 Cryptonalysis ---------------------------------------------------------------------------------------- 108 3.5 Conclusion-------------------------------------------------------------------------------------------- 108 3.6 Summary ---------------------------------------------------------------------------------------------- 109 3.7 Tutor-Marked Assignment (TMA) -------------------------------------------------------------- 109 3.8 Further Reading ------------------------------------------------------------------------------------- 109 Unit 4 Computer Virus ----------------------------------------------------------------------------- 110 4.0 Introduction ------------------------------------------------------------------------------------------ 110 4.1 Objectives -------------------------------------------------------------------------------------------- 110 4.2 The Evolution of Virus----------------------------------------------------------------------------- 110 4.3 The Menace ----------------------------------------------------------------------------------------- 111 4.4 The Process of Infection -------------------------------------------------------------------------- 111 4.5 Classification of Viruses --------------------------------------------------------------------------- 112 4.5.1 BOOT Infectors -------------------------------------------------------------------------- 112 4.5.2 SYSTEM Infectors ---------------------------------------------------------------------- 112 4.5.3 GENERAL.COM or EXE Infectors -------------------------------------------------- 112 4.6 Conclusion-------------------------------------------------------------------------------------------- 113 4.7 Summary ---------------------------------------------------------------------------------------------- 113 4.8 Tutor-Marked Assignment (TMA) ------------------------------------------------------------- 113 4.9 Further Reading ------------------------------------------------------------------------------------- 113 Unit 5 Viruses and Prevention --------------------------------------------------------------------- 114 5.0 Introduction ------------------------------------------------------------------------------------------ 114 5.1 Objectives -------------------------------------------------------------------------------------------- 114 5.2 Some Viruses ---------------------------------------------------------------------------------------- 114 5.3 Prevention ------------------------------------------------------------------------------------------- 116 5.4 The Cure --------------------------------------------------------------------------------------------- 116 5.5 Conclusion------------------------------------------------------------------------------------------- 117 5.6 Summary ---------------------------------------------------------------------------------------------- 117 5.7 Tutor-Marked Assignment (TMA) ------------------------------------------------------------- 117 5.8 Further Reading ------------------------------------------------------------------------------------ 117 viii Course Guide Introduction Computer Fundamental is a one semester and one credit foundation level course for first year students in Postgraduate Diploma in Information Technology and Postgraduate Diploma in Information Technology for Teachers. The course will consist of four modules. The course has been written in such a way that you can work at your own pace subject to the requirement of National Open University of Nigeria. Course Introduction This course presents an overview of the technology relating to the computer systems and exposes you to operating systems and application packages. The course covers aspects on computer hardware, computer software, data communication and computer security. The first and second modules discuss hardware and software of computer system. It includes discussions on data presentation, memory system of computers, input/output devices, parallel organisation, pipelining and reduced instruction set computers. It also covers aspects relating to the computer software such as the components of programming languages and operating systsem concepts. The third and fourth modules present an introduction to data communication, networking and related technology. The field of data communication has progressed tremendously during the past decade. Therefore, the concepts relating to it are of utmost importance for a computer user. What you will learn in this Course In this course you will be exposed to Operating Systems and Application packages. The course covers aspects of computer hardware and computer software. It includes discussions on data presentation, memory system of computers, input/output devices, parallel organisation, pipelining and reduced instruction set computers. This course also covers data communication, computer security and virus. Course Aims The aim of the course can be summarized as follows: to give you an overview of the technology relating to computer systems, operation systems and application packages. Course Objectives On successful completion of the course you should be able to:  explain hardware and software of computer system.  explain data communication, networking and related technology. Working through this Course To complete this course you are required to study the course materials carefully and attempt all self assessment - items and then answer all Tutor Marked Assignments. Materials on further reading are not compulsory. - The materials could be of added advantage but you may do without them. At the end of the course is a final examination. Course materials The major components of the course are: 1. Cour se Gui de 2. Modules 3. Assignment File 4. Presentation Schedule Assessment There are two aspects to the assessment of the course. First are the Tutor-Marked Assignments. The second is the final examination. How to get the most from this Course In distance learning, the study units replace the university lecturer. This is one of the great advantages of distance learning; you can read and work through specially designed study materials at your own pace, and at a time and place that suit you best. Think of it as reading the lecture instead of listening to the lecturer. Just as a lecturer might give you an in-class exercise, your study units provide exercises for you to do at appropriate points. Each of the study units follows a common format. The first item is an introduction to the subject matter of the unit. Next is a set of learning objectives. These objectives let you know what you should be able to do by the time you have completed the unit. When you have finished the unit you must go back and check whether you have achieved the objectives. This habit will significantly improve your chances of passing the course. The main body of the unit guides you through the required reading materials. The following is a practical strategy for working through the course. 1. Read this course guide thoroughly. 2. Organize a study schedule. Note how the assignments relate to the units. 3. Once you have created your own study schedule, do everything you can to stick to it. 4. Turn to a unit and read the introduction and the objectives for the unit. 5. Work through the unit. 6. Review the objectives for each study unit to confirm that you have achieved them. 7. When you are confident that you have achieved a unit objectives, do the Tutor-Marked Assignments. 8. Proceed unit by unit. Tutors and Tutorials You will be notified of the date, times and location of where to submit the tutorials. Your tutor will mark and comment on your assignment, keep a close watch on your progress and on any difficulties you might encounter and provide assistance to you during the course. Summary Computer Fundamentals— intends to introduce you to the fundamentals of computer science. Upon completing the course, you will be equipped with basic knowledge of computer hardware and software. 2 Module 1 Computer Fundamentals: Hardware Introduction This is the first module of the course. In this module many terms and concepts related to computers have been introduced. The module is divided into five units. In unit 1, we define a computer and concepts relating to Von Ne);rian Architecture. In unit 2, you are introduced to memory system and characteristics terms for varrous memory devices. Unit 3 defines external /auxiliary memory and explains high speed memories. Unit 4 deals with input/output organization and new technologies. Unit 5 explains important concepts of interfaces, parallel processing, vector processing and pipelining. 3 Unit 1 What is a Computer? 1.0 Introduction The use of Information Technology is well recognised. It has become a must for the survival of business houses with growing information needs. Computer is one of the major components of an information technology network. Today, computer technology has permeated every sphere of existence of modern man.From railway reservations to medical diagnosis, from TV programmes to satellite launching, from match-making to criminal catching— everywhere, we witness the elegance, sophistication and efficiency possible only with the help of computers. In this unit, we will introduce you to the computer hardware technology, how does it work and what it is. In addition we will als., discuss some of the terminology closely linke, with information technology and computers. More details on these terms can be obtained from further readings. In this unit, we shall discuss about basic computer structure. 1.1 Objectives This unit being the first unit of the block introduces you to the world of computers. At the end of the unit, you will be able to:  define the term computer  define Von Neumann architecture  describe key characteristics of memory system. 1.2 What is a Computer? Let us first define the term "Computer". In Oxford Dictionapt,.Computer is defined as "An automatic electronic apparatus for making calculations or controlling operations that are expressible in numerical or logical terms". The basic function performed by a computer is the execution of a program. A program is a sequence of instructions, which operate on data to perform a certain task. In modern digital computers, data is represented in binary form by using two symbols 0 and I which are called binary digits or bits. Computers use eight bits to. representachatacterinternally. This allows up to 2 - 256 different items to be represented uniquely. This _— collection of eight bits is called a byte. Thus, one byte is used to represent one character internally. One of the most common codes to represent characters in computers is ASCII (American Standard Code for Information Interchange). Most computers use two bytes or four bytes to represent numbers (positive and negative) internally. Another term that is commonly used in computer is a Word, A word may be defined as a unit of 1 ; information that a computer can process or transfer at a time. A word must be equal to the number of bits transferred between the central processing unit and the main memory in a single step or it may be defined as A 1 - 2 c Y t , „ 4 the basic unit of storage of integer data in a computer. Normally, a word may be equal to 8, 16,32 or 64 bits. The terms 32 bit computer, 64 bit computers etc basically point to the word size of the computer. One of the key aspects in program execution is the execution of an instruction. The key questions that can. be asked in this respect are (a) how are instructions supplied to the computer? and (b) how are they interpreted and executed? We will answer thesequestions along with the discussion on the basic structure of the computer system. " Most of today's computer designs are based on concepts developed by John Von Neumann referred to as a Architecture". Von Neumann proposed that there should be a unit performing arithmetic V the "Von Neum _nn — __— — - am_lilirperations on the data. This unit is termed Arithmetic Logic Unit (ALU). A control unit directs the ALU to perform specific arithmetic and logical functions on the data. Therefore, in such a system by changing the control signal, the desired operation can be performed on data. But, how can these control signals be supplied? Let us try to answer this from the definition of a program. A trus_sontmi 41 _ists of a sequence of steps. Each of_tljeleSsmiu. ire certain or logical or input/. _ _ output operations to be performed on data. Therefore, each step may require a set of control signal. Is it possible for us to provide_a unique code for each set alcontrol signals? Well, the answer is Yes. But what do we do with these codes? What about adding a hardware segment that accepts the code and generates control signals? T uhg_i_iit that interprets a code to generate respective control signal is termed as Control Unit (CU). Thus, a program now consists ofa sequence of codes. This machine is quite flexible, as we only need to provide a new sequence of codes for a new program. Each code is, in effect, an instruction, for the computer. lbe hardware interprets instructions and produces the respective control signals. The Arithmetic Logic,Un it (ALLIj and the Control Unit CU) together are termed as the Central Processing Unit.(CPU). The CPU is the most iiii-Portant component of a computer's hardware. The ALU performs the arithmetic operations suche, subtraction multiplication and division, and the logical operations such as: "Is A = B? (where A and B are both numeric or alphanumeric data). "I- a given character equal to M (for male) or F (for female)?" The control unit interprets instructions and produces the respective control signals. ) ? All — d lo ical operations are erforn_thear.1 in the CPU in vedallAorarre areas called reat ,The,ziadiliersgatisone of the important considerations in determining the processing capabilities of the ' CPU. Register size refers 6 The amount of information that can be held in a register at a time for ng processi. 1 _....re_.... __. , Tlt - - r -' 'El - Taster manie,_The. speed — Of procestlig. A — CPU's processing power 'measured in Million Instructions Per Second (MIPS). - How can instructions and data be put into computers? The instruction and data are to be supplied by external environment. Therefore, an input module is needed. The main responsibility of input module will be to put the data in the form of signals that can be recognized by the system. Similarly, we need another component that will report the results in proper format and form. This component is called outpui'module. These components are referred together as input/output (I/O) components. Are these two coin, onents sufficient for a working computer? No, because, input devices can bring instructions or data only sequentially and a program may not be executed sequentially as jump instructions are normally encountered in programming. In addition, more than one data element may be required at a time. Therefore, a temporary storage area is needed in a computer to store instructions and data temporarily. This component is referred to as memory. It was pointed out by Von Neumann that the same memory can be used for storingdata and instructions. In such case, the data can be treated as data on which processing can be performed, while instructions can be treated as data, which can be used for the generation of control signals. The memory unit stores all the information in a group of memory cells, also called memory locations, as binary digits (bits). Each memory location has a unique address and can be addressed independently. The contents of the desired memory locations are provided to the central processing unit by referring to the address of memory location. The amount of information that can be held in the main memory is known as memory capacity. The capacity of the main memory is measured in Kilobytes (KB) or Megabytes (MB). One kilobyte stands for 2 bytes, which is approximately little over one million bytes. In addition, to transfer the information, the computer5 system internally needs interconnections. The most common interconnection structure is the Bus Structure. A bus is a set of Wires (Lines) which you can visualize on the motherboard of a computer. It is a shared media4_121.a_corinectiritt,U memory and I/O components is called a system bus. A system bus may consist of 50 to 100 separate lines. Let us summarise the key features of a Von Neumann machine. The hardware of the Von Neumann machine consist of:  A CPU which includes a ALU and CU  A main memory system  An input/output system  The Von Neumann machine uses stored program concept, i.e. the program and data are stored in the same memory unit. The computers, prior to this idea, used to store programs and data on separate memories. Entering and modifying these programs. is very difficult as they were entered manually by setting switches and plugging and unplugging.  Each location of the main memory of Von Neumann machine is carried out in a sequential fashion (unless explicitly altered by the program itself) from one instruction to the next. Figure 1 shows the basic structure of a conventional Von Neumann machine. Address Main Memory Data and Instruction Interconnection Interconnection Central Processing Unit (CPU) Operational Registers Arithmetic and Logic Unit Contml Unit Data and Control Information Interconnection Mput/Output System Fig. 1: Structure of a Computer A Von Neumann machine has only a single path between main memory and control unit (CU). This feature/ constraint is referred to as Von Neumann bottleneck. Several other architectures have been suggested for modern computers. Exercise 1.1 State True or False I. A byte is equ to 8 bits and can represent a character internally. True False Goo c c oohed,/ 2. A word of PC386 is equal to one byte tr33-Cc etaicalenis - a- a n St ef t 1/0 tb-,— True False / 3 C I A - Z e 3 A i t c. 4 1 R P r i m R 942-c>, wmkr. 11-- q t-r SA-n/1 re-P\ 0-eg- g; Gect-7 iNcied-- 6.N I.> CArncLA 2.3 a s eivvi co n - AAA r frt!..4t.0 - +A 4-412,21 im faie-,7_osci tt 1 1 i oc -etcaarib, c/Jrr 3. Von Neumann architecture specifies separate memory for data and instructions. The memory which stores data is called data memory and the one that stores instructions is called instruction memory. True False 4. In Von Neumann architecture each bit of memory can be accessed independently. True False 5. A program is a sequence of instructions designed for achieving a task/goal. True False 7. One MB is equal to 1024 KB. True False 1.2.1 The Computer and Integrated Circuit Technology Before we discuss the relation of computers to integrated circuit technology, let us explore more about the term Integrated Circuit (IC). In an integrated circuit, the components such as transistors, resistors and conductors are fabricated on semi-conductor material such as silicon; Thus, a desired circuit can be fabricated in a tiny piece of silicon rather than assembling several discrete components into the same circuit. Hundreds or even thousands of transistors could be fabricated on a single wafer of silicon. In addition, these fabricated transistors can be connected with a process of metalizaW,Lia_foluloacii_tuits on the ,same chip they have been produced. Chip A Gate Gate Fig. 2: Wafer, chip and Gate An integtated circuit is constructed on a thin wafer of silicon that is divided into a matrix of small areas (size Ofthe order of a few millimeter-ia-cftiareT. An identical circuit pattern is fabricated on each of these areas and', the wafer is then broken into chips (Refer to figure 2). Each of these chips, consist of several gates, a useful logic component, and a number of input and output connection points. Each of these chips. then, can be packaged separately in a housing to protect it. In addition, this housing piovides a number of pins for connecting this chip with other devices or circuits. The pins on these packages can be provided in two ways:  In two parallel rows with 0.1 inch spacing between two adjacent pins in each row. This package is called dual in line package (DIP) (Refer Figure 3a.)  In case, more than hundred pins are required. thensingljd arrays of rows and columns, with spacing between two adjacent pin of 0.1 inch (Refer to Figure 3b5 C1-/ rt fx -s e „/. ' it., r 2.5-01-1 7 f'? t I, c/ '3 CS A P 72 nte-c - x 4 ecjer to - firstni ityvAL e t WK./ e r C A //0 Ce silfrie t 0 P Chia_ 4 cf calk cell, q n 9 n — eiteziecc8 catsf-es 1 0 obese& - 4 4 - - ‘ Integrated 0 1 inch Circuit Chip —111" 4— efe Yesfrikriple , , 0,1t  ; , , 0, ook  too eon :owes" ap'erwinmeskafimes efien el gee!) (B) 144 Pin Grid Array (PGA) Package Integrated 0.1 inch Circuit C ip Pin (A) A 24 pin dual in-line package (DI) (Contains dual rows which are in-line) Fig. 4: Integrated Circuit Packages Different circuits can be constructed on different wafers. All these packaged circuit chips then can be 2 - interconnected on a printed-circuit board to produce several complex electronic circuits such as computers. Initially, only a few gates were integrated reliably on a chip and then packaged. This initial integration was I referred to as small-scale integration (SSI). Later, with the advances in micro-electronics technologies, the SSI gave way to Medium Scale Integration where 100s of gates were fabricated on a chip. Then came Large Scale Integration (1,000 gates) and Very Large Scale Integration (VLSI 100,000,000 components) are expected to be 10,000,000 components will be fabricated on a single chip. What are the advantages of having densely packed Integrated Circuits? These are: Low cost: The cost of a chip has remained almost constant while the chip density (number of gates per chip) is ever increasing. It implies that the cost of computer logic and memory circuit has been reducing rapidly. Greater Operation Speed: More is the density, and closer are the logic or memory elements, which implies shorter electrical paths and hence higher operating speed. Smaller computers: Better portability, reduction in power and cooling requirements. Reliability: The integrated circuit interconnections are much more reliable than soldered connections. In addition, densely packed integrated circuits enable fewer inter-chip connections. Thus, the computers are more reliable. 8 One of the major milestones in this technology was the Very Large Scale Integration (VLSI) where thousands of transistors can be integrated in, a single chip. Themaimirnpactof VLSI was that, it was possible to produce ass&pin ete CPU or main memory or other similar devices on a single IC chip. Let us discuss some of the important breakthroughs of VLSI technologies. Semiconductor Memories Initially, the IC teehnology was used for constructing processor, but soon it was realized that same technology can be used integrated on a single chip. The first memory chip was constructed in 1970 and could hold 256 bits. Although the cost of this chip was high, but gradually the cost of semiconductor memory ring down. The memory capacity per chip has increased e.g. 1K, 4K,16K, 64K, 256K and 1M bits. Microprocessors Keeping pace with electronics as more and more component were fabricated on a single chip, fewer chips were needed to construct a single processor. hal in 1971 achieved the breakthrough of putting all the components on a single chip. The siacjiip processor is known as a microprocessor. The InteL400.4 waste first microprocessor. It wasa_p'rimitive microprocessor designed for a specific application. Intel 8080 that came in 1974 wn first general purpose microprocessor. It was an 8 bits microprocessor. Motorola is another manufacturer in this area. At present 32 and 64 bit general purpose microprocessors are already in the market. For example, Intel Pentium is a 32 bit processor, similarly Motorola's 68000 is a 32 bit microprocessor. P6 that is announced by Intel 1995 can process 64 bit data at a time. Figure 4 shows the Intel family of microprocessors. INTEC 8 bit I 8080 I 8085 16 bit CPU I 8086 16 bit data path 8186 32 bit CPU 8088 80286 16 bit data path (PC/AT) 16 bit CPU 8 bit data path 80386 32 bit CPU (IBM PC) AT/386 80486 2 bit CP Pentium 2.2 bi CPUI Fig. 4: Intel Microprocessor Families The VLSI technology is still evolving and more and more powerful microprocessor and more storage space is being put in a single chip. 9 1.2.2 Classification of Computers One question which we have still not answered is, is there any classification of computers? Well for quite sometime computers have been classified under three main classes. These are:  Mi cr ocomput er s  Mi ni comput er s  Mainframes Although with development in technology the distinction between them is becoming blurred, yet it is important to classify them, as it is sometimes useful to differentiate the key elements and architecture among the classes. Microcomputers A microcomputer's CPU is a microprocessor. The first microcomputers were built around 8-bit microprocessor chips. What do we mean by an 8-bit chip? It means that the chip can retrieve instructions/data from storage, manipulate, andirocess,an 8-bit data at a time or we can say that the chip has a built in 8-bit data transfer path. An improvement on 8-bit chip technology was in early 1980s, when a series of 16-bit chips namely 8086 and 8088 were introduced by Intel Corporation, each one with an advancement over the other. 8088 is a 8/16 bit chip i.e. an 8-bit path is used to move data between chip and primary storage (external path), at a time, but processing is done within the chip using a 16 bit path (internal path) at a time. 8086 is a 16/16 bit chip i.e. the internal and external paths both are 16 bit wide. Both these chips can support a primary storage capacity of up to 1-megabyte (MB). Most of the popular microcomputers are developed around Intel's chips, while most of the minis and super -minis are built around Motorola's 68000 series chips. With the advancement of display and VLSI technology now a microcomputer is available in very small size. Some of these are laptops, notebook computers etc. Most of these are of the size of a small note but equivalent capacity of an older mainframe. Minicomputer The term minicomputer originated in 1960s when it was realized that many computing tasks do not require an expensive contemporary mainframe computers but can be solved by a small, inexpensive computer. Initial minicomputers were 8 bit and 12 bit machines but by 1970s almost all minicomputers were 16 bit machines. The 16 bit minicomputers have the advantage of large instruction set and address field; and efficient storage and handling of text, in comparison to lower bit machines. Thus, 16 bit minicomputer was more powerful machine which could be used in variety of applications and could support business applications along with the scientific applications. With the advancement in technology the speed, memory size and other characteristic developed and the minicomputer was then used for yarious stand alone or dedicated applications. The minicomputer was then used as a multi-user system, which can be used by various users at the same time. Gradually the architectural requirement of minicomputers grew and a 32-bit minicomputer, which was called super mini, was introduced. ; The supermini had more peripheral devices, large memory and could support more users working simultaneously on the computer in comparison to previous minicomputers. Mainframes Mainframe computers are generally 32-bit machines or on the higher side. These are suited to big organizations, to mariage high volume applications. Few of the popular mainframe series are IBM, HP, etc. Mainframes are also used as central host computers in distributed systems. Libraries of applications programs developed for mainframe computers are much large than those of the micro or minicomputers because of their evolutiodover several decades as families of coMputing. All these factors and many more 10 make the mainframe computers indispensable even with the popularity of microcomputers. Supercomputer The upper ends of the state of the art mainframe machine are the supercomputer. These are among the fastest machines in terms of processing speed, and use multi-processing techniques, where a number of processors are used to solvei problem. Lately ranges of parallel computing products, which are multiprocessors sharing common buses, have been in use in combination with th6 mainframe supercomputers. The supercomputers are reaching up to speeds as well over 25000 million arithmetic operations per second. India also has its indigenous supercomputer. Supercomputers are mainly being used for weather forecasting, computational fluid dynamics, remote sensing, image processing, biomedical applications, etc. In India, we have one such mainframe supercomputer system — CRAY XMIP — 14, which is at present, being used by Meteorological Department. 1.3 Conclusion In this unit you have learned a number of important issues that relate to a computer. In particular you learned the main units of a computer. You also learned the relationship between a computer and integrated circuit technology. 1.4 Summary What you learned in this unit concerns general definitions of a computer and what Von Neumann Architecture is. What you learned here will be useful in the next units. 1.5 Tutor-marked Assignment (TMA) 1. What is a general purpose machine? 3. Define the following terms briefly r (i) Microprocessor 11c (ii) L a p t o p ]. (iii) Supercomputer 1.6 Further Reading Milan Milenkovic, Operating Systems Concepts and Designs, Second Edition. Tata Mcgraw Hill. 11 Unit 2 Memory System 2.0 Introduction In this unit we will introduce you to the memory system and characteristics of various memories. We will also define terms such as the main memory, cache memory, secondary storage and optical memories. 2.1 Objectives This unit introduces you to memory of a computer. At the end of the unit, you will be able to:  describe key characteristics of a memory system  distinguish various types of memories 2.2 Memory System Memory in a computer system is required for storage and subsequent retrieval of instructions and data. A computer system uses variety of devices for storing the instructions and data, which are required for its operations. Normally we classify the information to be stored on computer in two basic categories, data and the instructions. The storage device along with the algorithm or information on how to control and manage these storage devices constitute the memory system of a computer. A memory system is a very simple system yet it exhibits a wide range of technology and types. But unfortunately, faster memory technology is more expensive. In addition, fast memories require power supply till the information need to be stored. Both these things are not very convenient, but on the other hand the memories with less cost have very high access time, that is the time taken by CPU to access a location in the memory is high, which will result in slower operation of the \CPU. Thus, the cost versus access time anomaly has led to a hierarchy of memory where we supplement fast memories with larger, cheaper and slower memories. These memory units may have very different physical and operational characteristics, therefore, making the memory system very diverse in type, cost, organization, technology and performance. This memory hierarchy will be fruitful if the frequencies of access to slower memories are significantly less than the faster memories. 12 CPU Register Ti Cache Memory Ti Main Memory Ti Secondary Memory Thus, a memory system can be considered to consist of three groups of memories. These are: (a) Internal Processor Memories: These consist of the small set of high speed registers which are internal to a processor and are used as temporary locations where actual processing is done. (b) Primary Memory or Main Memory: It is a large memory, which is fast but not as fast as internal processor memory. This memory is accessed directly by the processor. It is mainly based on integrated circuits. (c) Secondary Memory/Auxiliary Memory/Backing Store: Auxiliairy memory in fact is much larger in size than main memory but is slower than main memory. It normally stores system programs (programs which are used by system to perform various functions), other instructions, programs and data files. Secondary memory can also be used as an overflow memory in case the main memory capacity has been exceeded. (How? the answer is not supplied in the block. You need to refer to further readings to get this answer). Secondary mem_orieacanot be accessed directly by a processor. First the information of these memories_ is transferred to the main memory and then the information can be accessed as the information of main memory. There is another kind of memory, which is increasingly being used in modem computers and this is called Sacittuemery. It is logically positioned between the internal memory (registers) and main memory. It stores or catches some of the content of the main memory, which is currently in use of the processor. Before discussing more about these memories let us first discuss the technological terms commonly used in defining and accessing the memory. 2.3 Characteristics Terms for Various Memory Devices The following terms are most commonly used for identifying comparative behaviour of various memory devices and technologies: Storage Capacity: It is a representative of the size of the memory. The capacity of internal memory and main memory can be expressed in terms of number of words or bytes. The storage capacity of external memory is normally measured in terms of bytes. Access Modes: A memory is considered to consist of various memory locations. The information from these memory locations can be accessed in the following ways: Random Access Memory (RAM): It is the mode in which any memory location can be accessed 13 _ in any order in the same amount of time. Ferrite and Semiconductor memories, which generally constitute main memory, are of this nature. The storage locations cap be accessed independently and there exist separate access mechanism for each location.  Sequential Access: On the other hand we have memories which can be accessed in a pre-defined sequence. For example, the songs stored on a cassette can be accessed only one by one. The example ofaquential_ access memory is Magnetic Tape. Here ;Le access mechanism need to be shared among different locations. Thus, either the location or the location or the read/write head or both should be moved to access the desired location.  „..Direct Access: In certain cases the information is neither accessed randomly nor in sequence but something in between. In direct access, a separate read/write head exist for a track and on a track - \ the Information be accessed serially:this semi-random mode of operations exists in magnetic disks. Access Time: The access time is the time required between the request made for a read or write operation till the time the data is made available or written at the requested location. Normally it is measured for read operation. The access time depends on the physical characteristics and access mode used for that device. Permanence of Storage: Some memories loose information over a period of time. For example, there can be some memories where the stored data bit value 1 looses its strength to become 0 over a period of time. These kinds of memories require refreshing. The memories, which require refreshing. are termed as dynamic memories. In contrast, the memories, which do not require refreshing are called static memories. Another factor, which can destroy the contents, is the presence and absence of electricity The memories which looses their content on failure of power are termed as volatile memories those which do not are called non- volatile. Magnetic memories are non-volatile and semircatiLluctor main memories are volatile in nature. Physical Characteristics: In this respect, the memory devices can be categorized into four main categories viz, electronic, magnetic, mechanical and optical. One of the requirements for a storage device is that it should exhibit two well-defined physical states, such that 0 and 1 can be represented in those two states. The Data transfer rate of the memory depends on the how quickly the state can be recognized and altered. The following table lists some of the memory technologies along with their physical and other important characteristics. Physical Average Technology Access time Access Mode Performance nature of cost(Rs/bit) (in seconds) of Storage storage (Approx.) medium Semiconductor 10-8 Random Volatile Electronic 10-2 Magnetic disk 10-2 Direct Non-volatile Magnetic 10-s Magnetic tape 10-' Sequential Non-volatile Magnetic 10-s Compact disk Approx. Direct Non-volatile Optical 10-' ROM 10-' Table I: Characteristics of Some Memory Technologies. The physical size of memories should be small and it must consume less power. Higher power consumption may result in more costly equipment for internal cooling of computer. The storage devices, which require mechanical motion e.g. hard disks are more prone to failure rather than the semiconductor memories which Wit luw ri 3141 c mem...net' menani dr 440 ao.fr ref- 9 yNest cc 4 4 e---'a tel. /vie "-arty AL ter, r e f r e s e o:rt Iv-npuo __ -ey Arts," 1/0641/Co r mewenezros- ;—ivienay s Looses --7,4442 as-mekt ernAtz).-t. cift are totally electronic in nature. Very high speed semiconductor memories are also prone to failure as technology is moving towards its limits. 2.4 Main Memory or Primary Storage Er.imarnetacuy consists of semiconductor memory chips and is used to store the data andtprogram urrently in_use—Each storage element of memory is directly (randomly) accessible and can be examined and thbdffd without affecting other cells and hence primary memory is also called Random Access Memory (RAM). Main memory stores a variety of critical information required for processing by the CPU. How does it store the information? Please answer it yourself. The memory unit stores all the information in memory cells also called memory locations, in binary digits. Each memory location has a unique address. The contents of the desired memory locations are provided to the central processing unit by referring to the address of the memory location. The amount of information that can be held in the main memory is known as memory capacity. The capacity of the main memory is measured in kilobytes (KB) or megabytes (MB). All modem computers have semiconductor memory as its main memory. Semiconductor memory is Icnown i as Random Access Memory (RAM) since any part of the memory can be accessed for reading and writing. ATC6IWEVait of main memory is Read Only Memory (ROM). ROMs (Read Only Memories) are the memories on which it is not possible to write the data when they are on-line to the computer. They can only...12e reid.The ROMs can be used in storing. programs provided by the manufacturer of computer for basic operations. ROMs are non-volatile in nature and need not be loaded in a secondary storage device. ROMs are fabricated in large number in a way where there is no room for even a single error. ROMs can be written only at the time of manufacture. Another similar memory is PROM. PROMs are also non-volatile and can be programmed only once by a special write device hence the name Programmable ROM (PROM). The writing process in PROM can be performed eletrically by the supplier or the customer. Special equipment is needed to perform this writing operation. Therefore, PROMs are more flexible and convenient ROMs. The ROMs/PROMs can be written just once (in ROMs at the time manufacture and PROMs at any time later), but in both the cases once whatever is written on, cannot be changed. But what about a case where you read mostly but write only very few times. This lead to the concept of Read mostly memories and the best examples of these are EPROMs (Erasable PROMS) and EPROMS (Electrically erasable PROMS). The EPROMS can be written electrically. But, the write operation is not simple. It requires erasure of whole storage cells by exposing the chip to ultra violet light, thus bringing them to same initial state. This venture is a time consuming process. Once all the cells have been brought to same initial state, then the EPROM can be written electrically. EPROMs becoming increasingly popular as they do not require prior erasure of previous contents. However, in EPROMs, the writing time is considerably higher than reading time. The biggest advantage of EPROM is that it is non-volatile memory and can be updated easily, while the disadvantages are the high cost and at present they are not completely non-volatile and the write operation takes considerable time. Figure 7 summarize the features of these read only and read mostly memories. Memory Type Write Time Order of Read Time Number of Cycles allowed ROM Once at the time Nano seconds ONE of manufacture PROM Hours Nano seconds ONE EPROM Minutes (including Nano seconds HUNDREDS time of erasure EPROM Milliseconds Nano seconds THOUSANDS 15 Common Features  Non -dest r uct i ve  Long dat a l i f e  Non -vol at i l e 2.5 Conclusion In this unit you have learned about the memory system of a computer. We have also introduced you to the characteristics terms of various memory devices. In particular you learned the functions of the main memory. 2.6 Summary You are now able to describe the characteristics of memory system. You are also in a position to distinguish types of memories. You will need knowledge that you have required in this unit in the subsequent units. 2.7 Tutor Marked Assignment - State True or False I.Memory hierarchy is built in a computer system, as the main memory cannot store very large data True False 2.The secondary memory is slower than that of main memory but has larger capacity. True False 3.In Random access memory any memory location can be assessed independently. True I False 4.What are the difference in: (a) Volatile and Non-volatile memory (b) Static and Non-dynamic memory 2.8 Further Reading Milan Milenkovic, Operating Systems Concepts and Designs, Second Edition, Tata Mcgraw Hill. 16 Unit 3 External / Auxiliary Memory 3.0 Introduction In this unit we will introduce you to External/Auxiliary memory. You will also learn about high speed memories. 3.1 Objectives At the end of this unit, you will be able to:  differentiate various external memories  define the importance of cadre memory. 3.2 External/Auxiliary Memory As discussed earlier the cost of RAM is very high and the semiconductor RAMS are mostly volatile, therefore, it is highly important that a secondary cheap media should be used which should show some sort of permanence of storage and should be relatively inexpensive. The magnetic material was found to be inexpensive and quite long lasting. Therefore, it became an ideal choice. Magnetic _ tape and magnetic disks are commonly used as I storage media. With the advancements in the optical technology now the optical disks are trying to make in roads as one of the major external memory. We will discuss about the characteristics of these memories in this section. 3.2.1 Magnetic Disk A magnetic disk is a circular platter of plastic, which is coated with magnetized material. One of the key components of a magnetic disk is a conducting coil named as Head which performs the job of reading or writing operation. Organization and Format A head of disk is a small coil and reads or writes on the position of the disk rotating below it, therefore, the data is stored in concentric set of rings (refer to figure 4). These are called tracks. The width of a track is equal to the width of the head, the adjacent tracks are separated by inter track gaps. As we go towards the outer tracks the size of a track increase but to simplify electronics same number of bits are stored in each track. 17 Inter Sector gap or Intra track gap Inter track gap Fig. 8: Logical layout of Magnetic Disk The data is transferred from and to the disks in blocks. Block' is a section of disk data and is normally equal to a sector. A track is divided into 10-100 sectors and these sectors should be either fixed or variable length sectors. Two adjacent sectors are separated by intra-track gaps. This helps in reducing The precision requirements of sectors. To identify the sector position normally there may be a starting point on a track or a starting and end point of each sector. Floppy disks A floppy disk is made of a flexible thin sheet of plastic material with a magnetic coating and grooves arranged in concentric circles with tracks. Floppy disk becomes a convenient recording medium to transport information from one location to another. Disk, isremoygkle tom the reading device attached to the computer and therefore provides unlimited storage capacity. The floppy disks of today are available in two sizes 5.25 inches and 3.5 inches and their capacity ranges from 360 KB to 1.44 MB per disk. 3.2. 2 Winchester Disk This is a.sealed rigid magnetic oxide medium disk, which typically holds 101\413 to 10 GB of data. Winchester disks are notcontaminations, removable from. the_drives and since they are sealed, dust and other which are likely in a floppy disk, are minimized. These provide substantially faster data access compared to floppy disk and provide very large data storage for on-line retrieval. Sides: The magnetic coating if applied to both the sides of the platter is called a double sided disks. The data can be recorded on either side of these disks. Some inexpensive disks were initially single sided. Platters: Some disks have single platter e.g. floppy disks while some disks have multiple platters which are stacked vertically, normally at a distance of an inch. This is known as disk pack. In disk pack one additional term cylinder is defined which is the ring of all co-centric tracks (figure 9). A disk pack can contain multiple heads mounted with the same arm. Access Time on Disk Disk operates in semi-random mode of operation and normally is referenced block wise. The data access time on disk consist of two main components. Seek time: Time to position the head on a specific track. On a fixed disks it is the time taken by electronic circuit to select the require head while in movable head disk it is time required to move the head to a particular track. 18 Latency time: The time required by a sector to reach below the read/write head. On an average it is half of the time taken for a rotation by the disk. Rotating shaft Cylinder Read write head Access arm can move in both directions Surface 0 Surface I Fig. 8: The Disk Pack In addition to these two times the time taken to read block of word can be considered but normally it is too small in comparison to latency and seek time and in general theaugas.tame.is considered to be sum of sedilime and latency time. Since access time to disks is large, therefore, it is advisable to read a sizeable portion of data in a single go and that is why the disks are referenced block wise. 3.2.3 Magnetic Tape Magnetic tapes are mounted on reels or a cartridge or a cassette of tape to store large volumes or backup data. These are cheaper and since these are removable from the drive, they provide unlimited storage capacity. Information retrieval from tapes is sequential _ — and not random. These are not suitable for on-line retrieval of data, since sequential searching will take long time. These are convenient for archival storage, or for' I- backup. The tapes are one of the earliest storage devices. They are low cost, low speed, portable and are still( widely used because of their low cost. , 3.2.4 Optical Memories Optical memories are alternate mass storage devices with huge capacity. The advent of compact disk digital audio system, a non-erasable optical disk, paved the way for the development of a new low cost storage technology. In optical storage devices the information is wr,it_tea usiag lase,Lbeam. These devices which are memories can store large amount of data. We will discuss here three optical memory devices, which are now becoming increasingly popular in various computer applications. CD-ROM The CD-ROM (Compact Disk Read-Only Memory) is a direct extension of audio CD. CD-ROM players are more rugged and have error-correction facility. This ensures proper data transfer from CD-ROM to the main memory of the computer. CD-ROM is written into during the process of manufacture by a high power laser beam. Information is retrieved from a CD-ROM using a low power laser, which ingenerate in an optical disk drive unit. In CD-ROMs the information is stored evenly across the disk in segments of the same size. 19 Therefore, in CD-ROMs data stored on a track increases as we go towards outer surface of disk. Thus, the CD-ROMs are rotated at variable speeds for the reading process. Sector of equal length Identification offirst address is 0 minute 0 second 0 sector Fig. 7: A CD-ROM's disk layout Figure 7 indicates the layout used for CD-ROMs. As discussed earlier, the data is stored sequentially along a spiral track. In this disk random access becomes more difficult locating a desired address involves first moving the head to the specific area then adjusting the rotation speed and then reading the address, and then to find and access the specific sector. CD-ROMs are very good for distributing large amount of information or data to large number of users. The three main advantages of CD-ROMs are:  large data/information storage capacity  mass replication is inexpensive and fast these are removable disks, thus are suitable for archival storage The disadvantages of CD-ROMs are:  It is read-only, therefore, cannot be updated  Access time is longer than that of a magnetic disk WORM In certain applications only few copies of compact disks are to be made which makes CD-ROMs production economically unviable. For such cases write-once, read-many CD has been developed. WORM disks are prepared in such a way that they can be written only once subsequently by a laser beam of modest intensity. The disk controller of WORM is more expensive than that of CD-ROM. WORM uses sector structures same as that of magnetic disks. High power laser first prepares the WORM disk. A CD writer can write them once. 3.3 High Speed Memories The Need: Why the high speed memories? Is the main memory not a high-speed memory? The answer to second question is definitely "No", but why so, well for this we have to go to the fundamentals of semiconductor technology which is beyond the scope of the unit, if the memories are slower then how slow are they? It has been found that the access of main memories is slower than the speed of the processor. Since each instruction require several memory aicesses therefore, faster memories will be of tremendous help in increasing the overall through out of a computer. 20 1. Magnetic Ink Character Recognition (MICR) devices are generally used by the banking industry to read the account numbers on cheques directly and does the necessary processing. 2. Optical Mark Recognition (OMR) devices can sense marks on computer readable papers. This kind of device is used by academic and testing institutions to grade aptitude tests where candidates mark the correct alternatives on a special sheet of paper. The optical mark recognition devices then directly read these answer sheets and the information sent to a computer for processing. 3. Optical Bar Code Reader (OBR) scans a set of vertical bars of different widths for specific data and are used to read tags and merchandise in stores, medical records, library books, etc. These are available as hand held devices. 4.2.2 Output Devices The output normally can be produced in two ways: either on a display unit/device or on a paper. Other kinds of output such as speech output, mechanical output is also being used in Certain applications. In this section, we will discuss only the display and printing output devices. Display Devices One of the most important peripherals in computer is the graphic display device. Conventional computer display terminals known as alphanumeric terminals, display characters (images) from a multi-dot array (normally 5 x 7 or 7 x9). These are used to read text information displayed on the screen. However, there are increasing demand for display of graphics, diagrams and pictures to make the visual presentation of information more effective for user interaction and decision making. Graphic displax is made qp of a series dslotc railed 'pixels' (picture elements) whose pattern produces the image. Each dot on the screen is defined as a separate unit, which can be directly addressed. Since each dot can be controlled individually there is much greater flexibility in drawing pictures. There are three categories of display screen technology: I. Cathode Ray Tube (CRT) 2. Liquid Crystal Display (LCD) 3. Projection Displays CRT Displays The main components of a CRT terminals are the electron gun, the election beam controlled by an electromagnetic field and a phosphor coated display screen. The electromagnetic field or order to create an image (refer to figure 5). There are two types of CRT displays (a) Vector CRT displays in which the elestraabeatt„Aisslirecte(JaLyill o theffilaseawhere the hits to be created. (b) Raster displays in which the image is projected on to the screen by directing the electron beam across each row of icture eleme 5 5 I ' 1....... This type of display 1 provides a high dynamic capability since image is continuously refreshed and it allows for continual user input and output. It offers full colour display at a relatively low cost and is becoming very popular. Liquid Crystal Displays (LCD) First introduced in watches and clocks in 1970s, LCD is now applied to display terminals. The major advantage ofLCDisthelowenergyconsumption.TheCRT(CathodeRayTube)isreplacedbyliquidcrystaltoproduce theimage.Thisalsohavecolourcbilii r I. oo. These are commonly used iLt2gmble_clevicelliesause of compactness and low energy ts. 25 Ng Phket ine04'd e( fi'duvre e,tep-r-94-e-ces Projection Displays The personal size screen of tlieSs _displays is replacecarge screen upon which images are normally use cnTY—riTge group presentation. These systems can be connected to computer and whatever appears on the computer terminal gets enlarged and projected on a large screen. Another popular method is to connect computer to a LCD flat screen, and to project the LCD image using Overhead Projector. These are popularly used for seminars, classrooms, marketing presentations, etc. Printers Printers are used for producing output on a paper. There are a large variety of printing devices, which can be aiss a c - u. t nd the printing speeds. Current estimates indicate that about 1500 types of printers are commercially available conforming to about 15 different printing technologies. The following categories of-printers are identified. (a) Printing Technology — impact printers vs non-impact printers Impact printen use variations of standards typewriter printing_meehanism where a hammer strikespap(er. Impact printers use variations of standard typewriter printing mechanism through inked ribbon. Non-impact printer twatorelectrical signals to etch or induce symbols on paper. Many of these require special coated or tsated_paper. (b) Character forms — fully formed characters vs dot matrix Frully_forstme characters_arunstnictecifromsulalinesand curves like thectsciar ters offy_pew_rjfer whereas a dot matrix character is made up from a carefull a - ' S ' I s I. ked vaclose_ to each other. Obviously print quality of a dot matrix printer will be poorer compared to that from fully formed characters. (c) Printing sequence — serial vs page This indicated the am t of information ar  - I put within a - 'II SI' Serial printing is done character by character whereas line printing forms an entire line at a time. A page printer outputs a whole page of character and images simultaneously during one cycle. Clearly the speed of output will depend upon the printing sequence incorporated in the printing device. We will now look at three of the most popular printers. Dot Matrix Printers This is one of the most popular printers used for personal computing systems. These printers are relatively (cheaper compared to other technologies. This uses impact technology and a print head containing banks of wires moving at high speeds against inked ribbon and paper. Characters are produced in matrix format. The speeds range from 40 characters per second (CPS) to about 1,000 cps. A disadvantage of this technology is that the print quality is low.    Side view Front view Fig. 8: Dot Matrix Printer 26 Ink Jet Printers These print by spraying a controlled stream of tiny ink droplets-accurately on the paper forming either dot matrix or solid characters. These are non-impact and hence relatively silent and high quality printers. The typical speeds range from 50 cps to above 300 cps and this technology has been used well for production of colour printing and elaborate graphics. Paper Ink Ink droplets 1111 1 (..s ) a /....... nodis en. dQ11 "jail Recycled Ink Fig. 9: Inkjet Printing Laser Printers This is a high quality, high speed, andlgaithumeiechnology, which works in non-impact fashion on plain paper or pre-printed forms. Printing is achieved by deflectingl_a_es Meant ol_____Itp_the,photosensitive surface of a drum and the latent image attracts the toner to the image areas. The toner is then electrostatically transferred to the paper and fixed into a permanent image. Speeds can range from 10 pages a minute to about 200 pages per minute. This technology is relatively expensive but is becoming very popular because of the quality, speed and noiseless operations. These three printers are compared in Table 2. Plotters To produce graphical output on paper the plotters are used. After discussing so much about the I/O devices, let us come back to one of the basic question: How are I/O devices connected to computers? We will try to answer the

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