FT MEADE
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`ssen ~ s o
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`N A N C Y A. F L O Y D
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`Essentials of Data Processing
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`The Times Mirror/Mosby Data Processing and Information
`Systems Series
`Cohen/Alger/Boyd Business BASIC for the IBM PC with Cases
`Dravillas/Stilwell/Williams Power Pack for the IBM PC
`Floyd Essentials of Data Processing
`Spence/Windsor Using Microcomputers: Applications for Business
`Spence/Windsor Microcomputers in Business: WordStar, dBase II
`and III, and Lotus 1-2-3
`Lientz/Rea Data Communications for Business
`Whitten/Bentley/Ho Systems Analysis and Design Methods
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`Essenti sal o
`Data Processi
`
`N ANCY A. FLOYD
`r~
`
`Times Mirror/Mosby College Publishing
`
`S T. LOUIS 'TORONTO'SANTA CLARA 1987
`
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`~~ ~~
`;~57r~ ~.?'
`
`To David Eaton Arthur and Mildred Porst Arthur
`
`Acquiring Editor: Susan Solomon
`Developmental Editor: Rebecca A. Reece
`Editorial Assistant: Pamela Lanam
`Production: Stacey C. Sawyer, Montaxa, Calif.
`Text and Cover Design: Al Burkhardt
`Text Illustration: Mary Burkhardt
`Copy Editor: Roger McWilliams
`Typesetting: Maryland Composition
`
`FIRST EDITION
`Copyright O 1987 by Times Mirror/Mosby College Publishing
`A Division of the C. V. Mosby Company
`11830 Westline Industrial Drive, St. Louis, MO 63146
`All rights reserved. No part of this publication may be reproduced, stored
`in a retrieval system, or transmitted in any form or by any means,
`electronic, mechanical, photocopying, recording, or otherwise, without prior
`written permission from the publisher.
`
`Printed in the United States of America
`
`Library of Congress Cataloging-in-Publication Data
`
`Floyd, Nancy A. (Nancy Arthur)
`Essentials of data processing.
`
`Bibliography: p.
`Includes index.
`1. Electronic data processing. I. Title.
`004
`86-14574
`~A16:~5~-`I~l~C
`ISBN 0-8016-1660-3
`
`MC/VH/VH
`
`9 8 7 6 5 4 3 2 1
`
`02/A/243
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`Brief Contents
`
`Detailed Contents
`Preface
`xvii
`
`vii
`
`Introduction
`1
`Hardware: The Peripheral Devices
`18
`The Central Processing Unit and the Operating
`System
`44
`
`124
`
`Software
`60
`The Programming Process
`Microcomputers
`104
`File Organization
`Database
`137
`Microcomputer Applications
`Telecommunications
`170
`Office Automation
`Trends
`203
`
`186
`
`81
`
`150
`
`BASIC
`215
`Data Representation
`Glossary
`240
`
`Index
`
`250
`
`229
`
`v
`
`Chapter 1
`Chapter 2
`Chapter 3
`
`Chapter 4
`
`Chapter 5
`
`Chapter 6
`Chapter 7
`Chapter 8
`
`Chapter 9
`Chapter 10
`Chapter 11
`
`Chapter 12
`Appendix A
`Appendix B
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`Detailed Contents
`
`Each Chapter begins with "At XFields, Inc. "—a short business
`scenario—and an Overview. Each Chapter concludes with a
`summary, "At XFields: Conclusion," and Exercises and Projects.
`
`CHAPTER ~I
`
`Introduction
`
`~~
`
`4
`
`What Is a Computer?
`3
`From Calculating Machines to Computers
`The First "Computer"
`5
`Punched Cards and the 1890 Census
`6
`The Electromechanical Computer Arrives
`7
`The Electronic Computer: Who Gets the Credit?
`Separation of Computer and Program
`8
`Building on the Foundation
`9
`The First Generation: Computers Reach Private
`Industry
`10
`Transistors: The Second Generation Begins
`The Third Generation: Computers and Their
`Families
`12
`The Fourth Generation: A Population Explosion
`Learning from Experience: The Fifth Generation
`Where Are Computers Today?
`15
`
`11
`
`7
`
`14
`15
`
`CHAPTER Z Hard ware: The Peripheral Devices
`
`T8
`
`Input Devices and Media
`Terminals
`20
`
`20
`
`vii
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`x
`
`Detailed Contents
`
`CHAPTER
`
`5 The Pro rammin Proce$s
`g
`g
`
`ST
`
`I Q
`~j
`1
`
`I
`~ !
`i'~
`I~'~
`,~ ;
`
`~ '
`
`83
`What Is Programming?
`83
`What Is Systems Analysis1
`The Systems Analysis Process
`85
`Programming Logic
`Flowcharts
`86
`Flowcharting Symbols
`Logic Flow
`89
`Loops
`89
`Branches
`90
`Structured Programming
`92
`Structure Charts
`IPO Charts
`93
`Pseudocode
`93
`Structured Code
`Coding the Program
`Testing the Program
`96
`Syntax Errors
`Logic Errors
`97
`Marking the Trail
`99
`Documentation for Maintenance
`Documentation for Operators
`101
`Documentation for Users
`Training Operators and End Users
`
`86
`
`92
`
`93
`
`94
`96
`
`84
`
`99
`100
`
`C H A P T E R 6 Microcomputers
`
`Brief History of Microcomputers
`Microcomputer Characteristics
`Hardware
`107
`Software
`107
`Operating Systems
`Applications Software
`110
`The Users
`Individuals
`110
`Educators
`110
`
`107
`108
`
`i
`
`101
`
`105
`106
`
`104
`
`-
`
`.J
`
`_ _
`
`_
`
`___
`
`_ _ __ -
`
`—
`
`----
`
`__ _
`
`-----
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`Detailed Contents
`
`xi
`
`Small Businesses
`111
`Large Businesses
`112
`Criteria for Selecting and Evaluating a System
`Criteria for Evaluating Software
`116
`Criteria for Evaluating Hardware
`118
`Caring for Youx System
`119
`Trends in Microcomputers
`120
`Trends in Development
`120
`Trends in Use
`120
`Trends in Sales
`121
`
`116
`
`C H A P T E R 7 File 4rganiza tion
`
`124
`
`126
`
`129
`
`Data Structure
`125
`Space Considerations
`126
`Record Types: Fixed versus Variable
`Blocking
`127
`Packing
`128
`File Organization
`129
`Sequential File Organization
`Direct File Organization
`130
`Relative File Organization
`130
`Indexed File Organization
`130
`Processing Methods
`131
`Batch Processing
`132
`Transaction-oriented Processing
`Access Methods
`132
`Accessing Sequential Files
`133
`Accessing Direct and Relative Files
`Accessing Indexed Files
`133
`Access Method Programs
`134
`
`132
`
`133
`
`CHAPTER S Database
`
`137
`
`Why Did Database Evolve?
`What Is a Database1
`139
`Advantages of Database Organization
`
`139
`
`139
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`Aga
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`Detailed Contents
`
`147
`
`141
`
`143
`143
`
`Data Redundancy
`141
`Data Integrity
`141
`Data Security
`141
`Shared Data
`Data Management System
`142
`Purpose
`Data Dictionary
`Query Language
`144
`Shared Files
`144
`Types of Database
`Hierarchical Database
`Network Database
`Relational Database
`Problems
`147
`
`144
`144
`147
`
`C HAPTER 9 Microcomputer Applications
`
`150
`
`152
`152
`
`Symphony
`Spreadsheets
`152
`Layout
`Spreadsheet Windows
`152
`How the Spreadsheet Is Used
`Graphics
`156
`Word Processing
`Database
`163
`Other Applications
`Trends
`165
`
`157
`
`165
`
`153
`
`CHAPTER ~ O Telecommunications
`
`170
`
`172
`Hardware
`Terminal
`172
`174
`Modem
`Communications Channel
`174
`Telephone Lines
`Coaxial Cable
`175
`Fiber Optics
`175
`
`174
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`Detailed Contents xiii
`
`177
`
`179
`
`Microwave
`176
`Satellites
`176
`Computer and Front-end Processor
`Networks
`178
`Point-to-Point Network
`Star Network
`180
`Ring Network
`180
`Line Control
`181
`Distributed Data Processing
`Current Issues
`181
`Privacy
`181
`Security
`182
`Ethics
`182
`Trends
`183
`
`181
`
`CHAPTER 11 Office Automation
`
`186
`
`187
`
`The Office Yesterday and Today
`What the Office Really Is
`188
`How the Office Is Changing
`189
`Some Changing Technologies
`189
`Word Processing
`198
`Electronic Mail
`190
`Facsimile Systems
`191
`Teleconferencing and Telecommuting
`Integrated Voice and Data
`196
`Effect on People
`197
`Trends
`200
`
`191
`
`CHAPTER i~ Trends
`
`2~~
`
`205
`
`Decision Support Systems
`Components
`205
`Users
`206
`The Future
`Robotics
`206
`
`206
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`zcaaa Detailed Contents
`
`The Intelligent Machine
`208
`Artificial Intelligence
`210
`AI and Japan's 10-year Plan
`Expert Systems
`211
`Natural Languages
`212
`
`211
`
`APPENDIX A BASIC
`
`215
`
`216
`216
`
`217
`
`217
`218
`218
`218
`218
`
`Error Correction
`System Commands
`NEW
`217
`LIST and LLIST
`RUN
`217
`SAVE
`217
`LOAD
`217
`Data Representation
`Numeric Variables
`String Variables
`Reserved Words
`Line Numbers
`Spaces
`218
`Line Format
`219
`INPUT/OUTPUT and Miscellaneous
`Statements
`219
`REM Statement
`219
`INPUT Statement
`219
`PRINT and LPRINT Statement
`READ Statement
`221
`DATA Statement
`221
`LET Statement
`222
`END Statement
`223
`Branching
`223
`GOTO Statement
`224
`IF/THEN/ELSE Statement
`FOR NEXT Statement
`225
`Subroutines and the GOSUB Statement
`
`220
`
`224
`
`22G
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`A P P E N D I X B Data Representation
`
`229
`
`Detailed Contents
`
`xv
`
`229
`
`Numbering Systems
`229
`Decimal and Binary Numbers
`Octal Numbers
`230
`Hexadecimal Numbers
`Coding Schemes
`235
`EBCDIC
`236
`ASCII
`238
`
`234
`
`Glossary
`
`240
`
`Index
`
`250
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`Preface
`
`The Intended Audience for This Book
`The growth and development of the data processing field have affected al-
`most every aspect of our lives. Consequently, the student taking an intro-
`ductory data processing course today may be majoring in computer studies,
`may be in a related field such as business, or may use computers only in
`support of his or her area of expertise. This divexse group of students—
`enrolled in 2-and 4-year colleges, nonmajors and majors—has one important
`unifying goal: the need to learn fundamental computer concepts. By pro-
`viding in-depth coverage of essential data processing. concepts, Essentials
`of Data Processing is intended to support a practical first course in data
`processing for nonmajors and for majors enrolled in a combined introductory
`data processing/BASIC or introductory data processing/.microcomputer ap-
`pli.cations course.
`
`Why This Book- Was Written
`Students today are interested in what practical information they need to
`deal with the computer in the workplace. They need to learn key concepts
`and important terminologies without being burdened with unnecessary de-
`tail. This book was written because the available textbooks did not satisfy
`students' needs. Specifically:
`The big, 4-color introductory data processing books are too big for some
`courses.
`
`xvii
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`xviii Preface
`
`■ They contain too much material to cover in the time available. Some
`professors attempt to make the big books "fit" their courses by omitting
`chapters and thereby also eliminating coverage of valuable concepts.
`Some professors try to solve this problem by using only selected readings
`from within each chapter, thereby creating gaps that cause problems in
`student comprehension. And even more elaborate "text surgery" is nec-
`essary if the professor wants also to cover BASIC.
`■ Material may be deleted, but the price remains the same, Professors pay
`the price in time spent tailoring the book to meet their course needs, and
`students pay the price for an expensive, 4-color book of which they use
`only a portion.
`The smaller introductory data processing books miss the mark in satisfying
`course needs.
`■ Some texts have shallow coverage of too few concepts. These texts, in
`attempting to be "shorter" than the bigger books, end up containing ma-
`terial that is not technical enough. The coverage lacks depth and fails to
`include important topics.
`■ Many texts do not have a practical orientation. Some texts contain ma-
`terial that is too technical for nonmajors, for example, focusing primarily
`on programming. Some texts focus on the societal implications of the
`computer. Some texts don't devote enough coverage to microcomputers.
`These texts don't address what the average person needs to know about
`dealing with computers in career and family activities.
`
`Why You Should Consider This Book
`I really believe Essentials of Data Processing will better suit your course
`needs than any other book because:
`■ This book is designed so that all the material can be covered in one
`term, with or without a lab, This permits maximum flexibility for those
`instructors who want to cover BASIC programming or microcomputer
`applications, as well as essential data processing concepts. The decision
`to include topics was based on whether that information was essential
`to both majors and nonmajors and whether the material was usually cov-
`ered in another course. For example, Chapter 5, The Programming Pro-
`cess, includes brief coverage of systems analysis—only those concepts
`pertinent to the average nonmajor. Majors will go on to take cone-term
`and sometimes atwo-term Systems Analysis and Design Course.
`■ This book offers a practical, microcomputer-oriented approach to data
`processing concepts. Content is presented so that students can identify
`with it and immediately transfer it into their own lives. Students are
`taught what they need to know to interact with computers and the per-
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`Preface xix
`sons who program computers. Also, the book covers practical informa-
`tion about the type of computer students are most likely to encounter—
`the microcomputer. The microcomputer orientation throughout the text
`is additionally bolstered by two chapters that are microcomputer specific.
`Chapter 4, Software, includes guidelines for evaluating and purchasing
`software and for storing and handling diskettes. Chapter 6, Microcom-
`puters, includes a checklist for purchasing hardware.
`a~ This book is up-to-date. Topics covered include office automation, ro-
`botics, decision support systems, fifth-generation computers, expert sys-
`tems, and artificial intelligence.
`■ Caref ally developed in-text learning aids ensure student comprehension:
`❑ At XFields, Inc. All chapters begin with a short business-oriented
`scenario that sets the scene for the topics to be covered in the chapter.
`Students will identify with the characters' real-life job concerns and
`anxieties about using computers and be motivated to study the chapter.
`❑ Chapter Objectives. After studying the chapter, students can use the
`list of objectives to test their understanding of the chapter contents.
`❑ Overview. Following the Chapter Objectives is a brief introduction
`that previews chapter content.
`❑ On the job Boxes. Each chapter contains this feature, which highlights
`how computers are used in the business world and reinforces chapter
`concepts.
`❑ Summary. All chapters end with a brief section summarizing the topics
`covered. Students can use this feature for review and study purposes.
`❑ At XFields; Conclusion. Each chapter ends by returning to the XFields
`characters introduced at the beginning of the chapter. Because the char-
`acters' learning parallels what students have learned in the chapter,
`students will again identify with the characters.
`❑ Definitions. Terms are boldfaced and defined on first mention within
`chapters. All boldfaced terms appear in the end-of-text glossary.
`o Exercises. Each chapter ends with discussion questions that test stu-
`dent comprehension of chapter content.
`❑ Projects. Following the Exercises are Projects to expand students'
`learning experience beyond the text.
`■ This book was written for students. The book presents essential concepts
`in a concise, clear, easy-to-understand manner so that students are in-
`terested and encouraged, not intimidated by computers. Discussions are
`not overly technical. Concepts are explained through the use of objects
`and events with which students can relate. For example, in the expla-
`nation of how matrix construction creates letters, the matrix pins of a
`printer are compared to the individual bulbs that light up to show the
`score on a basketball scoreboard (Chapter 2~. The functional use of a
`second color also enhances student interest.
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`xx
`
`Preface
`
`How to Use This Book
`Essentials of Data Processing is designed so that all material may be com-
`pleted in one term. Specific suggestions on how to use the book in the
`various course configurations can be found in the Instructor's Guide.
`Professors have the flexibility to decide to:
`Use the book by itself.
`This book is self-contained; it covers all essential computer concepts. If the
`class is taught without a lab, the Projects at the end of each chapter will
`offer students some exposure to computer usage. If the course is taught with
`a programming lab, the BASIC appendix can be used.
`Use this book. with a language supplement.
`This book is available with an accompanying BASIC supplement Student
`Study Guide) that builds on the in-text BASIC appendix. Times Mirror/
`Mosby's Cohen-Alger-Boyd Business BASIC for the IBM PC with Cases
`for a machine-specific, business approach) might be suitable, or another
`language supplement can be used.
`Use this book with a microcomputer supplement.
`This text may be used with Times Mirror's texts Dravillas-Stilwell-Williams
`Power Pack for the IBM PC, which offers generic explanation of packages
`and comes with software; Spence-Windsor Using Microcomputers:. Appli-
`cations for Business, which offers generic concepts and hands-on coverage
`of 15 popular business paclzages and comes with software; or Spence-Wind-
`sor Microcomputers in Business; WordStar, dBASE II and III, and Lotus
`1-2-3. Or professors can use another microcomputer text or supplement of
`their choosing.
`Use this book with a language and a microcomputer supplement.
`Many instructors today want to teach an introductory course that covers
`data processing concepts, programming, and microcomputer applications.
`Again, this book can be used as the ideal compliment to other required texts.
`
`Supplements
`A solid package of instructional supplements has been created with the
`needs of the large number of adjunct faculty, part-time faculty, and teaching
`assistants foremost in mind.
`Instructor's Guide
`This supplement was written by Nancy A. Floyd and contains:
`Answers to all exercises in the book and the Student Study Guide.
`Chapter outlines to use as lecture guides.
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`Preface xxi
`100 transparency masters of key figures in the text, modifications of text
`figures, and totally new illustrations.
`Conversion notes to help you convert from your present text.
`Additional lab exercises and lecture materials to supplement text.
`Accompaniment notes that address how to use this book (1 ~ with a BASIC
`text, (2) with a microcomputer text, and (3) with a BASIC and a microcom-
`puter text.
`Teaching tips for the instructor.
`Annotated journal references.
`Topics for discussion relevant to chapter content.
`Student Study Guide
`This supplement was written by Paul W. Ross of Millersville University,
`Pennsylvania.
`The first part of the Student Study Guide supports the chapters and contains:
`Learning objectives
`Overview Chapter summary
`Problem set to define key terms
`Discussion questions
`Multiple choice, true/false, matching, and short-answer questions
`Additional projects
`Case studies with problem/activity sets
`The second part supports and builds on the BASIC appendix, by covering:
`■ The mechanics of using BASIC on a microcomputer
`■ Numeric and string variables
`■ Reserved words and operators
`■ Simple input and output
`■ Assignment statements
`■ Looping and decision malting
`■ Arrays, tables, and strings
`~ Subroutines
`■ File processing
`Also included are:
`Self-test questions
`Programming problerris
`
`Test bank
`The test questions were also written by Nancy
`The printed version of the test items includes
`using the following formats:
`■ Multiple choice
`■Short answer
`■ Fill-in-the blank
`■True/false
`■ Matching
`
`A. Floyd.
`approximately 1300 items
`
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`Xxii Preface
`
`A computerized testing package, Microtest, is available from the publisher
`to all adopters.
`
`Acknowledgments
`I am indebted to the many people who contributed to the development of
`this book: my family, who supported my decision to write and accepted the
`many other things that did not get done this year, and the reviewers, whose
`feedback and suggestions were invaluable.
`Reviewers
`A. I. Ali
`Texas Univerity
`William Bonney
`Hudson Valley Community
`College
`Robert Carmichael
`California State University,
`Northridge
`Barbara Comfort
`J. Sargent Reynolds Commmunity
`College
`Glen Emerson
`Central State University
`Joseph Franklin
`Asheville-Buncombe Technical
`College
`Douglas Frantz
`Catonsville Community College
`Jim Dillard
`Ferris State College
`Carla Hall
`St. Louis Community College,
`Florrisant Valley
`Bob Harrison
`Southwestern Technical College
`Mark Hogan
`Trident Technical College
`Linda Knight
`Northern Illinois University
`M~ thanks are also due to the data processing students at Rockingham
`Community College, Wentworth, North Carolina. They read various drafts
`
`Guy Langsford
`California State University,
`Northridge
`E. Gladys Norman
`Linn Benton Community College
`William O'Hare
`Prince Georges Community
`College
`Vicki Olson
`Indiana State University
`Janet Portzer
`North Carolina State University
`Carol Refiner
`University of Wisconsin,
`Oshkosh
`Linda Rice
`Saddleback Community College
`Tim Sylvester
`College of DuPage
`Ralph Szweda
`Monroe Community College
`Beverly White
`Lackawanna Junior College
`Dean Whittaker
`Ball State University
`
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`Preface 7[xU~
`of the manuscript, endured countless practice runs of the material and ex-
`amples, and humored me through the agony of deadlines. Their interest in
`and enthusiasm for data processing have fueled my own and made teaching
`a most joyous profession.
`Special thanlcs are due to the staff of Times Mirror/Mosby: Executive
`Editor Susan Solomon, N~.arketing Manager Jim Donohue, Developmental
`Editor Rebecca Reece, Editorial Assistant Pam Lanam, and Production Co-
`ordinator Stacey Sawyer. I've learned so much from them all.
`I assume full responsibility for any errors or inaccuracies. Any com-
`ments, criticisms, suggestions, or improvements are welcome. Write to me
`in care of Times Mirror/Mosby 4633 Old Ironsides Drive, Suite 410, Santa
`Clara, CA 95054.
`
`Nancy Arthur Floyd
`
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`C H A P T B R
`
`The Central . Process~ing
`Unit and the
`Operating System
`
`A T XFIELDS, INC.
`
`■ Anne MacDermott watched Sam settle in at his desk.
`Five years ago they had both been computer operators; after
`that time and two upgrades in the system, she doubted that
`she would be much of an operator anymore.
`She spent most of her time these days scheduling jobs
`and then explaining why schedules weren't met.
`Last Wednesday had been one of those days that took
`a lot of explanation. The field service engineer had come in
`to make a minor repair and ended up crashing the entire
`system. Forty users suddenly stopped in the middle of
`whatever they were doing. Being able to run several
`programs at once might be great for getting jobs through
`when the system ran properly, Anne thought, but it sure
`kept the telephone lines busy when the system failed.
`
`~~
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`Central Processing Unit ~~
`
`CHAPTER OBJECTIVES
`
`After completing this chapter, you will be able to:
`1• Identify the components of the central processing unit
`2• Describe what occurs during a machine cycle
`3. Explain how main storage differs from secondary storage and
`the purpose of each
`4. Differentiate between multiprogramming and multiprocessing
`5. Describe the purpose of an operating system and compare. sev-
`eral current ones
`
`O VERVIEW
`
`In the Overview in Chapter 2, an inventory clerk updated the
`inventory manually. You saw how he input and output the
`data, but how does he process it?
`The clerk compares the item number of an entry on the
`receipt list to the, item number of an entry on the current in-
`ventory. If the inventory's item number is not the same as the
`receipt's item number, the clerk does not update it but con-
`tinues to look for the. matching inventory item. When the item
`numbers match, he adds the number received to the current
`inventory balance. Thus he performs the two functions ex-
`pected of a computer's processing unit: logical decisions and
`mathematical calculations. How the computer performs these
`two functions is the subject of this chapter.
`
`Central Processing Unit
`
`The central processing unit, also called the CPU, is the hardware that per-
`forms the processing functions. It has two parts, the arithmetic/logic unit
`and the control unit. Housed within the same physical device is the internal
`storage unit, also called main memory, or primary storage (Figure 3.1~.
`Arithmetic/Logic Unit
`The arithmetic/logic unit (ALU), as the name implies, contains all the cir-
`cuitry necessary to perform the computer's arithmetic and comparison op-
`
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`CHAPTER 3 The Central Processing Unit and the Operating System
`
`CENTRAL PROCESSING UNIT
`
`_-~ , 1
`
`CONTROL
`UNIT
`
`ARITHMETIC
`LOGIC UNIT
`
`Figure 3.1
`
`The central processing unit consists of the arithmetic/logic unit
`and the control unit.
`
`erations. It can calculate or round off numbers, compare algebraically or
`logically, and so on. For example, if you write the instruction "add A to B,"
`the ALU performs the addition.
`
`Control Uni t
`The control unit directs the activities of the computer. It also controls the
`activities of the input/output units as instructed by the programs. The con-
`trol unit transfers data and instructions between the ALU and main mem-
`ory. In general, it controls the flow of data within the system.
`
`Internal Storage
`Internal storage, also called main memory or primary storage, contains the
`data and instructions being processed after they have been received from
`the input units. Data and instructions must be stored within internal storage
`
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`r
`
`t'
`Central Processing Unit
`'~<-~
`before being processed, since this is where the control unit will look for
`them (Figure 3.2~.
`Several types of internal storage are available. The two most common
`are random access memory (RAM) and read-only memory (ROME.
`Random Access Memory. The part of storage that holds programs and the
`data they use is called random access memory. RAM's contents are easily
`changed; in fact, they change constantly during the execution of a program
`as new data is read and processed.
`Read-Only Memory. A second type of internal storage is called read-only
`memory because its contents can only be read, not altered. These contents
`are loaded once, usually by the computer's manufacturer. ROM is used to
`hold these instructions meant to remain in memory storage permanently.
`
`CENTRAL PROCESSING UNIT
`
`1 . ' 1
`
`Figure 3.Z
`
`Data and instructions must be located in internal storage before
`they can be processed. Registers serve as high-speed access areas
`between internal storage and the CPU, where the instructions are
`processed.
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`cxAPTEx 3 The Central Processing Unit and the Operating System
`
`This would include parts of the operating system, such as those programs
`that control computer operations, regardless of which user program is run-
`ning. It might also include a compiler, a program used to translate a program
`written by the user into the computer's language so that it can be executed
`(more on languages later).
`
`Registers
`Linking the CPU and internal memory are several storage locations called
`registers. They are used to hold data and instructions temporarily during a
`program's execution. Since the contents of registers can be quickly located
`and retrieved, data and instructions are normally moved from main memory
`to a register just before they are processed.
`
`CENTRAL PROCESSING UNIT
`
`1
`
`Figure 3.3
`
`Although the registers link internal storage and the CPU, they are
`just holding areas. The data and the instructions are transferred
`within the computer via channels that carry electrical signals.
`These channels are called bus lines.
`
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`Processing Techniques
`
`~~
`
`Bus Lines
`Bus lines, special electrical cables, are the physical connection linking main
`memory, the registers, and the CPU (Figure 3.3~. The CPU, registers, and
`main storage work together to enable a program to be run. The easiest way
`to see how these various components work together is to walk through one
`machine cycle in detail
`The Machine Cycle
`A machine cycle (Figure 3.4) consists of the series of operations necessary
`to execute a single machine language instruction. This cycle is divided into
`two parts, instruction time (I-time) and execution time (E-time). I-time is
`the time necessary for the control unit to pull an instruction from main
`memory and prepare it for processing. E-time is the period required to locate
`the data needs, execute the instruction, and store the results in main
`memory.
`The time needed to complete one machine cycle depends on the com-
`puter being used.
`
`Processing Techniques
`
`As Figure 3.4 shows, to execute a single machine language instruction, the
`computer must fetch, translate, pass control, execute, and store. How does
`the computer know which of these activities to perform? How does it start?
`What makes it work?
`A computer with all the necessary hardware still needs something else
`to make it work: application programs, which instruct the computer what
`to do. These programs determine whether inventory, payroll, or something
`else is to run, but they do not provide all the details about how to run it.
`Another group of programs serves as a go-between for application programs
`and hardware.
`For example, consider a simple request to read a record. The program-
`mer asks that a record be read. If the record is to be read from a tape, the
`tape must be started, the record copied into memory, and the tape stopped.
`If it is to be read from disk, the access arm mechanism must be moved, the
`correct read/write head switched on, and the record copied into memory.
`The programmer does not give these detailed instructions on how to read
`the record. These instructions are a part of the programs that make up the
`operating system.
`The operating system is a series of programs designed to manage re-
`sources: hardware, other software, and data. (Software comprises the pro-
`
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`~~~•~~
`
`CHAPTER 3 The Central Processing Unit and the Operating System
`
`CENTRAL PROCESSING UNIT
`
`BUS LINES
`
`CONTROL
`UNIT
`
`ARITHMETIC
`LOGIC UNIT
`
`O
`
`REGISTERS
`
`MAIN MEMORY
`
`INSTRUCTIONS
`
`INSTRUCTIONS
`
`In I-time, (1) the control unit fetches the ne~ct instruction
`to be executed from the CPU; (2) it interprets what this
`instruction is to do and where the needed data are
`located; and (3) it passes control to the arithmetic/logic
`unit.
`In E-time, (4) the ALU executes the instruction and
`returns control to the control unit; (5) the control unit
`stores the data in main memory, if necessary; and (6) it
`now returns to step 1 to locate the next instruction.
`
`Figure 3.4
`
`Outline of a machine cycle.
`
`grams that run the computer system.) Its purpose is twofold: to make it
`easier to use the hardware and to allocate the computer's resources effi-
`cieritly among the various users.
`There are two measures of how efficiently the resources are used. The
`first, throughput, is the amount of processing that occurs in a given period.
`The second measure is called turnaround; this is the elapsed time between
`job submission and output receipt.
`The speeds at which data is processed have increased dramatically
`during the past few years. However, improvements of speeds of peripheral
`devices have been much less dramatic. This has led to a significant speed
`disparity between ,the CPU and the peripheral devices. For example, cal-
`culating the amounts on a paycheck is much quicker than printing those
`amounts on the check itself. Buffers, spooling, multiprogramming, and mul-
`tiprocessing are all used to improve efficiency.
`
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`Processing Techniques
`
`~~
`
`Buf fern
`Buffers are storage areas used to collect data as it is transferred into or out
`of memory from the peripheral devices. However, if the computer had only
`one buffer, it would have to wait for the data to be moved into the buffer
`from the peripheral device before it could process the record. The use of
`multiple buffers improves efficiency.
`For e