real-time systems design and analysis, 4th edition

Real - time software designers must be familiar with computer architecture and organization, ating systems and related services, programming languages, systems and soft-ware engineering

DESIGN AND ANALYSIS

445 Hoes LanePiscataway, NJ 08854

IEEE Press Editorial Board

Lajos Hanzo, Editor in Chief

Kenneth Moore, Director of IEEE Book and Information Services (BIS)

Technical Reviewers

Larry Bernstein, Stevens Institute of Technology

Bernard Sick, University of Kassel Olli Vainio, Tampere University of Technology

REAL-TIME SYSTEMS DESIGN AND ANALYSIS Tools for the Practitioner

Copyright © 2012 by the Institute of Electrical and Electronics Engineers, Inc.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey All rights reserved.

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

To Nancy, Chris and Charlotte, with all my love

Seppo:

To Helena, Sami and Samu — my everything

CONTENTS

Preface xv Acknowledgments xxi

1 Fundamentals of Real-Time Systems 1

1.1 Concepts and Misconceptions, 2

1.1.1 Defi nitions for Real-Time Systems, 2

1.1.2 Usual Misconceptions, 14

1.2 Multidisciplinary Design Challenges, 15

1.2.1 Infl uencing Disciplines, 16

1.3 Birth and Evolution of Real-Time Systems, 16

2 Hardware for Real-Time Systems 27

2.1 Basic Processor Architecture, 28

2.1.1 Von Neumann Architecture, 29

2.1.2 Instruction Processing, 30

2.1.3 Input/Output and Interrupt Considerations, 33

2.2 Memory Technologies, 36

2.2.1 Different Classes of Memory, 36

2.2.2 Memory Access and Layout Issues, 38

2.2.3 Hierarchical Memory Organization, 41

2.3 Architectural Advancements, 43

2.3.1 Pipelined Instruction Processing, 45

2.3.2 Superscalar and Very Long Instruction

2.4.2 Direct Memory Access, 56

2.4.3 Analog and Digital Input/Output, 58

2.5 Microprocessor versus Microcontroller, 62

3 Real-Time Operating Systems 79

3.1 From Pseudokernels to Operating Systems, 80

3.1.1 Miscellaneous Pseudokernels, 82

3.1.2 Interrupt-Only Systems, 87

3.1.3 Preemptive Priority Systems, 90

3.1.4 Hybrid Scheduling Systems, 90

3.1.5 The Task Control Block Model, 95

3.2 Theoretical Foundations of Scheduling, 97

3.2.1 Scheduling Framework, 98

3.2.2 Round-Robin Scheduling, 99

3.2.3 Cyclic Code Scheduling, 100

3.2.4 Fixed-Priority Scheduling: Rate-Monotonic Approach, 1023.2.5 Dynamic Priority Scheduling: Earliest Deadline

3.3.5 Deadlock and Starvation Problems, 114

3.3.6 Priority Inversion Problem, 118

3.3.7 Timer and Clock Services, 122

3.3.8 Application Study: A Real-Time Structure, 123

3.4 Memory Management Issues, 127

3.4.1 Stack and Task Control Block Management, 127

3.4.2 Multiple-Stack Arrangement, 128

3.4.3 Memory Management in the Task Control

Block Model, 129

3.4.4 Swapping, Overlaying, and Paging, 130

3.5 Selecting Real-Time Operating Systems, 133

3.5.1 Buying versus Building, 134

3.5.2 Selection Criteria and a Metric for Commercial Real-Time Operating Systems, 135

3.5.3 Case Study: Selecting a Commercial Real-Time Operating System, 138

3.5.4 Supplementary Criteria for Multi-Core and Energy-Aware Support, 140

3.6 Summary, 142

3.7 Exercises, 143

References, 146

4 Programming Languages for Real-Time Systems 149

4.1 Coding of Real-Time Software, 150

4.1.1 Fitness of a Programming Language for Real-Time

Applications, 151

4.1.2 Coding Standards for Real-Time Software, 152

4.2 Assembly Language, 154

4.3 Procedural Languages, 156

4.3.1 Modularity and Typing Issues, 156

4.3.2 Parameter Passing and Dynamic

Memory Allocation, 157

4.3.3 Exception Handling, 159

4.3.4 Cardelli’s Metrics and Procedural Languages, 161

4.4 Object-Oriented Languages, 162

4.4.1 Synchronizing Objects and Garbage Collection, 162

4.4.2 Cardelli’s Metrics and Object-Oriented Languages, 164

4.4.3 Object-Oriented versus Procedural Languages, 165

4.5 Overview of Programming Languages, 167

4.6 Automatic Code Generation, 178

4.6.1 Toward Production-Quality Code, 178

4.6.2 Remaining Challenges, 180

4.7 Compiler Optimizations of Code, 181

4.7.1 Standard Optimization Techniques, 182

4.7.2 Additional Optimization Considerations, 188

4.8 Summary, 192

4.9 Exercises, 193

References, 195

5 Requirements Engineering Methodologies 197

5.1 Requirements Engineering for Real-Time Systems, 198

5.1.1 Requirements Engineering as a Process, 198

5.1.2 Standard Requirement Classes, 199

5.1.3 Specifi cation of Real-Time Software, 201

5.2 Formal Methods in System Specifi cation, 202

5.2.1 Limitations of Formal Methods, 205

5.2.2 Finite State Machines, 205

5.2.3 Statecharts, 210

5.2.4 Petri Nets, 213

5.3 Semiformal Methods in System Specifi cation, 217

5.3.1 Structured Analysis and Structured Design, 218

5.3.2 Object-Oriented Analysis and the Unifi ed

Modeling Language, 221

5.3.3 Recommendations on Specifi cation Approach, 224

5.4 The Requirements Document, 225

5.4.1 Structuring and Composing Requirements, 226

6 Software Design Approaches 267

6.1 Qualities of Real-Time Software, 268

6.1.1 Eight Qualities from Reliability to Verifi ability, 269

6.2 Software Engineering Principles, 275

6.2.1 Seven Principles from Rigor and Formality

to Traceability, 275

6.2.2 The Design Activity, 281

6.3 Procedural Design Approach, 284

6.3.1 Parnas Partitioning, 284

6.3.2 Structured Design, 286

6.3.3 Design in Procedural Form Using Finite

State Machines, 292

6.4 Object-Oriented Design Approach, 293

6.4.1 Advantages of Object Orientation, 293

6.4.2 Design Patterns, 295

6.4.3 Design Using the Unifi ed Modeling Language, 298

6.4.4 Object-Oriented versus Procedural Approaches, 301

6.5 Life Cycle Models, 302

7 Performance Analysis Techniques 379

7.1 Real-Time Performance Analysis, 380

7.1.1 Theoretical Preliminaries, 380

7.1.2 Arguments Related to Parallelization, 382

7.1.3 Execution Time Estimation from

Program Code, 385

7.1.4 Analysis of Polled-Loop and Coroutine Systems, 391

7.1.5 Analysis of Round-Robin Systems, 392

7.1.6 Analysis of Fixed-Period Systems, 394

7.1.7 Analysis of Nonperiodic Systems, 396

7.2 Applications of Queuing Theory, 398

7.2.1 Single-Server Queue Model, 398

7.2.2 Arrival and Processing Rates, 400

7.2.3 Buffer Size Calculation, 401

7.2.4 Response Time Modeling, 402

7.2.5 Other Results from Queuing Theory, 403

7.3 Input/Output Performance, 405

7.3.1 Buffer Size Calculation for Time-Invariant Bursts, 405

7.3.2 Buffer Size Calculation for Time-Variant Bursts, 406

7.4 Analysis of Memory Requirements, 408

7.4.1 Memory Utilization Analysis, 408

7.4.2 Optimizing Memory Usage, 410

7.5 Summary, 411

7.6 Exercises, 413

References, 415

8 Additional Considerations for the Practitioner 417

8.1 Metrics in Software Engineering, 418

8.1.1 Lines of Source Code, 419

8.1.2 Cyclomatic Complexity, 420

8.1.3 Halstead’s Metrics, 421

8.1.4 Function Points, 423

8.1.5 Feature Points, 427

8.1.6 Metrics for Object-Oriented Software, 428

8.1.7 Criticism against Software Metrics, 428

8.2 Predictive Cost Modeling, 429

8.2.1 Basic COCOMO 81, 429

8.2.2 Intermediate and Detailed COCOMO 81, 431

8.2.3 COCOMO II, 433

8.3 Uncertainty in Real-Time Systems, 433

8.3.1 The Three Dimensions of Uncertainty, 434

8.3.2 Sources of Uncertainty, 435

8.3.3 Identifying Uncertainty, 437

8.3.4 Dealing with Uncertainty, 438

8.4 Design for Fault Tolerance, 438

8.4.1 Spatial Fault-Tolerance, 440

8.4.2 Software Black Boxes, 443

8.4.3 N-Version Programming, 443

8.4.4 Built-in-Test Software, 444

8.4.5 Spurious and Missed Interrupts, 447

8.5 Software Testing and Systems Integration, 447

8.5.1 Testing Techniques, 448

8.5.2 Debugging Approaches, 454

8.5.3 System-Level Testing, 456

8.5.4 Systems Integration, 458

8.5.5 Testing Patterns and Exploratory Testing, 462

8.6 Performance Optimization Techniques, 465

8.6.1 Scaled Numbers for Faster Execution, 465

8.6.2 Look-Up Tables for Functions, 467

8.6.3 Real-Time Device Drivers, 468

8.7 Summary, 470

8.8 Exercises, 471

References, 473

9 Future Visions on Real-Time Systems 477

9.1 Vision: Real-Time Hardware, 479

9.1.1 Heterogeneous Soft Multi-Cores, 481

9.1.2 Architectural Issues with Individual Soft Cores, 483

9.1.3 More Advanced Fieldbus Networks and Simpler

Distributed Nodes, 484

9.2 Vision: Real-Time Operating Systems, 485

9.2.1 One Coordinating System Task and Multiple Isolated

Application Tasks, 486

9.2.2 Small, Platform Independent Virtual Machines, 487

9.3 Vision: Real-Time Programming Languages, 488

9.3.1 The UML++ as a Future “Programming Language”, 4899.4 Vision: Real-Time Systems Engineering, 491

9.4.1 Automatic Verifi cation of Software, 491

9.4.2 Conservative Requirements Engineering, 492

9.4.3 Distance Collaboration in Software Projects, 492

9.4.4 Drag-and-Drop Systems, 493

9.5 Vision: Real-Time Applications, 493

9.5.1 Local Networks of Collaborating Real-Time Systems, 4949.5.2 Wide Networks of Collaborating Real-Time Systems, 4959.5.3 Biometric Identifi cation Device with Remote Access, 4959.5.4 Are There Any Threats behind High-Speed Wireless

PREFACE

xv

This book is an introductory text about real - time systems — systems where

timeliness is a crucial part of the correctness of the system Real - time software

designers must be familiar with computer architecture and organization, ating systems and related services, programming languages, systems and soft-ware engineering, as well as performance analysis and optimization techniques The text provides a pragmatic discussion of these subjects from the perspective

oper-of the real - time systems designer Because this is a staggering task, depth is occasionally sacrifi ced for breadth Nevertheless, thoughtful suggestions for additional literature are provided where depth has been sacrifi ced due to the available page budget or other reasons

This book is intended for junior – senior level and graduate computer science, computer engineering and electrical engineering students, as well as practicing software, systems and computer engineers It can be used as a graduate level text if it is supplemented with an advanced reader or a focused selection of scholarly articles on a specifi c topic (which could be gathered from the up - to - date bibliographies of this edition) Our book is especially useful in an indus-trial setting for new real - time systems designers who need to get “ up to speed ” very quickly Earlier editions of this book have been used in this way to teach short courses for several industrial clients Finally, we intend for the book to

be a desk reference of long - lasting value, even for experienced real - time systems designers and project managers

The reader is assumed to have basic knowledge in programming in one of the more popular languages, but other than this, the prerequisites for this text are minimal Some familiarity with discrete mathematics is helpful in under-standing some of the formalizations, but it is not essential

Since there are several preferred languages for real - time systems design, such as Ada, C, C ++ , C#, and increasingly, Java, it would be unjust to focus this book on one language, say C, when the theory and framework should be lan-guage independent However, for uniformity of discussion, certain points are illustrated, as appropriate, in generic assembly language and C

While the provided program codes are not intended to be ready - to - use, they can be easily adapted with a little tweaking for use in a real system

This book is organized into nine chapters that are largely self - contained Thus, the material can be rearranged or omitted depending on the background and interests of the instructor or reader It is advised, however, that Chapter

1 would be explored fi rst, because it contains an introduction to real - time systems as well as the necessary terminology

Each of the chapters contains both easy and more challenging exercises that stimulate the reader to confront actual problems The exercises, however, cannot serve as a substitute for carefully planned laboratory work or practical experience

The fi rst chapter provides an overview of the nature of real - time systems Much of the basic vocabulary relating to real - time systems is developed along with a discussion of the main challenges facing the real - time system designer Besides, a brief historical review is given The purpose of this chapter is to foreshadow the rest of the book as well as quickly acquaint the reader with pertinent terminology

The second chapter presents a detailed review of central computer tecture concepts from the perspective of the real - time systems designer Specifi cally, the impact of advanced architectural features on real - time perfor-mance is discussed The remainder of the chapter outlines different memory technologies, input/output techniques, and peripheral support for embedded systems The intent here is to increase the reader ’ s awareness of the impact of the computer architecture on various design considerations

Chapter 3 provides the core of the text for those who are building practical real - time systems This comprehensive chapter describes the three principal real - time kernel services: scheduling/dispatching, intertask communication/synchronization, and memory management It also covers special problems inherent in these designs, such as deadlock and priority inversion

Chapter 4 begins with a discussion of specifi c language features desirable

in good software engineering practice in general and real - time systems design

in particular An evaluative review of several widely used programming guages in real - time systems design, with respect to these features, follows Our intent is to provide explicit criteria for rating a language ’ s ability to support real - time systems and to alert the user to the possible drawbacks of using each language in real - time applications

In Chapter 5 , the nature of requirements engineering is fi rst discussed Then

a collection of rigorous techniques in real - time system specifi cation is sented with illustrative examples Such rigorous methods are particularly useful when automatic design and code - generation approaches are to be used

pre-later in the development life cycle Next, structured and object - oriented odologies are discussed as alternative paradigms for requirements writing At the end of this chapter, an extensive case study is provided

meth-Chapter 6 surveys several commonly applied design specifi cation niques used in both structured and object - oriented design An emphasis on their applicability to real - time systems is made throughout No single tech-nique is a silver bullet, and the reader is encouraged to adopt his or her own formulation of specifi cation techniques for the given application A compre-hensive design case study is also provided

Chapter 7 discusses performance analysis techniques based on diverse mation approaches The proposed toolset is fully usable even before it is pos-sible to perform any direct measurements Moreover, a pragmatic discussion

esti-on the use of classical queuing theory for analyzing real - time systems is vided Input/output performance issues are considered with an emphasis on buffer - size calculation Finally, a focused analysis of memory utilization in real - time systems is presented

Chapter 8 discusses additional software engineering considerations, ing the use of software metrics and techniques for improving the fault - tolerance and overall reliability of real - time systems Later in the chapter, different techniques for improving reliability through rigorous testing are discussed Systems integration and performance optimization issues are also considered

In Chapter 9 , we look to the future of real - time systems hardware, software, and applications Much of this chapter is speculative, and we had great fun imagining things yet to come and the way things ought to be with respect to real - time systems technology This chapter forms a fruitful basis for class dis-cussions, debates, and student projects

When our book is used in a university course, typically students are asked

to build a real - time multitasking system of their choice Usually, it is a game

on a PC, but some students can be expected to build embedded hardware controllers of moderate complexity The authors ’ assignment to the reader would be to build such a game or simulation, using at least the coroutine model The application should be useful or at least pleasing, so some sort of a game is a good choice The mini - project should take no more than 20 hours and cover all phases of the software life cycle model discussed in the text Hence, those readers who have never built a real - time system will have the benefi t of the instructive experience

Real - time systems engineering is based on more than 50 years of experience and global contributions by numerous individuals and organizations Rather than clutter the text with endless citations for the origin of each idea, the authors chose to cite only the key ideas where the reader would want to seek out the source for further reading Some of the text is adapted from two other books written by the fi rst author on software engineering and computer archi-tecture, Laplante ( 2003 ) and Gilreath and Laplante ( 2003 ), respectively Where this has been done, it is so noted

Many solid theoretical treatments of real - time systems exist, and where applicable, they are noted Nonetheless, these books or journal articles are sometimes too theoretical for practicing software engineers and students who are often impatient to wade through the derivations for the resultant payoff They want results that they can use now in the trenches, and they want to see

how they can be used, not just know that they exist In this text, an attempt is

made to distill the most valuable of the theoretical results, combined with practical experience and insight to provide a toolkit for the practitioner This book contains extensive bibliographies at the end of each chapter Where verbatim phrases were used, and where a fi gure came from another source, the authors tried to cite it appropriately However, if any were inad-vertently overlooked, the authors wish to correct the unfortunate error Please notify the authors if you fi nd any errors of omission, commission, citation, and

so forth by e - mail, at plaplante@psu.edu or seppo.ovaska@aalto.fi , and they will be corrected at the next possible opportunity

Since 1992, thousands of copies of the fi rst three editions of this book have been sold to the college text and professional markets throughout the world The only thing more gratifying than its adoption at such prestigious universi-ties as Carnegie Mellon University, the University of Illinois at Urbana -Champaign, Princeton University, the United States Air Force Academy, Polytechnic University, and many others around the world, has been the enthu-siastic feedback received from numerous individuals thankful for the infl uence that the book has had on them The continuing international success of the

fi rst three editions along with recent technological advancements demanded that a fourth edition be produced

The most fundamental change in the fourth edition is a new co - author, Dr Seppo Ovaska, whose vast experience greatly complements that of the fi rst author and adds a strong and timely international perspective

The fourth edition addresses the important changes that have occurred in the theory and practice in the construction of real - time systems since the publishing of the third edition in 2004 Chapters 1 – 8 have been carefully revised to incorporate new material, correction of errors, and elimination of outdated material Moreover, Chapter 9 is a brand - new chapter devoted to future visions on real - time systems Totally new or substantially revised discus-sions include:

• Multidisciplinary design challenges

• Birth and evolution of real - time systems

• Memory technologies

• Architectural advancements

• Peripheral interfacing

• Distributed real - time architectures

• System services for application programs

• Supplementary criteria for multi - core and energy - aware support

• Automatic code generation

• Life cycle models

• Arguments related to parallelization

• Uncertainty in real - time systems

• Testing patterns and exploratory testing

• Real - time device drivers

• Future visions on real - time systems

While approximately 30% of previous material has been discarded, another 40% has been added, resulting in a unique and modern text In addition, several new examples have been included to illustrate various important points Hence, it is with pride and a sense of accomplishment that we are pre-senting this timely and carefully composed book to students and practicing engineers

REFERENCES

W F Gilreath and P A Laplante , Computer Architecture: A Minimalist Approach

Norwell, MA : Kluwer Academic Publishers , 2003

P A Laplante , Software Engineering for Image Processing Boca Raton, FL : CRC Press ,

ACKNOWLEDGMENTS

xxi

Phil Laplante wishes to thank his dear friend Dr Seppo Ovaska for being the perfect collaborator Easy to work with, Seppo ’ s industriousness, experience, insight, patience, and attention to detail perfectly complemented Phil ’ s strengths and weaknesses The vast majority of differences between the third and fourth editions are due to Seppo ’ s hard work As a result of Seppo ’ s con-tributions, the fourth edition is far superior to any previous edition of this book And this book is now as much his vision and legacy, as the fi rst three editions were mine

Phil also wishes to thank his wife Nancy and his children Christopher and Charlotte for putting up with the seemingly endless work on this manuscript and too many other projects to mention over these many years

Seppo: I am grateful to my wife Helena and my sons Sami and Samu for everything we have experienced together Although it is a tiny gesture com-pared with all that you have given to me, I humbly dedicate this book to you And fi nally, Phil, it was a true pleasure to work with you in this exciting and rewarding book project

P.A.L S.J.O

1

FUNDAMENTALS OF

REAL- TIME SYSTEMS

1

Real-Time Systems Design and Analysis: Tools for the Practitioner, Fourth Edition

Phillip A Laplante and Seppo J Ovaska.

© 2012 the Institute of Electrical and Electronics Engineers, Inc Published 2012 by John Wiley

& Sons, Inc.

The term “ real time ” is used widely in many contexts, both technical and ventional Most people would probably understand “ in real time ” to mean “ at

con-once ” or “ instantaneously ” The Random House Dictionary of the English Language (2nd unabridged edition, 1987), however, defi nes “ realtime ” as per- taining to applications in which the computer must respond as rapidly as required by the user or necessitated by the process being controlled These defi -

nitions, and others that are available, are quite different, and their differences are often the cause of misunderstanding between computer, software and systems engineers, and the users of real - time systems On a more pedantic level, there is the issue of the appropriate writing of the term “ real - time ” Across technical and pedestrian literature, various forms of the term, such as

real time , real - time , and realtime may appear But to computer, software, and systems engineers the preferred form is real - time , and this is the convention

that we will follow throughout this text

Consider a computer system in which data need to be processed at a regular rate For example, an aircraft uses a sequence of accelerometer pulses to determine its position Systems other than avionic ones may also require a rapid response to events that occur at nonregular rates, such as handling an overtemperature failure in a nuclear power plant Even without defi ning the term “ real - time, ” it is probably understood that those events demand timely

or “ real - time ” processing

Now consider a situation in which a passenger approaches an airline check

in counter to pick up his boarding pass for a certain fl ight from New York to Boston, which is leaving in fi ve minutes The reservation clerk enters appropri-ate information into the computer, and a few seconds later a boarding pass is printed Is this a real - time system?

Indeed, all three systems — aircraft, nuclear power plant, and airline reservations — are real - time, because they must process information within a specifi ed interval or risk system failure Although these examples may provide

an intuitive defi nition of a real - time system, it is necessary to clearly hend when a system is real - time and when it is not

To form a solid basis for the coming chapters, we fi rst defi ne a number of central terms and correct common misunderstandings in Section 1.1 These defi nitions are targeted for practitioners, and thus they have a strong practical point - of - view Section 1.2 presents the multidisciplinary design challenges related to real - time systems It is shown that although real - time systems design and analysis are subdisciplines of computer systems engineering, they have essential connections to various other fi elds, such as computer science and electrical engineering — even to applied statistics It is rather straightforward

to present different approaches, methods, techniques, or tools for readers, but much more diffi cult to convey the authors ’ insight on real - time systems to the audience Nevertheless, our intention is to provide some insight in parallel with specifi c tools for the practitioner Such insight is built on practical experiences and adequate understanding of the key milestones in the fi eld The birth of real - time systems, in general, as well as a selective evolution path related to relevant technological innovations, is discussed in Section 1.3 Section 1.4 sum-marizes the preceding sections on fundamentals of real - time systems Finally, Section 1.5 provides exercises that help the reader to gain basic understanding

on real - time systems and associated concepts

1.1 CONCEPTS AND MISCONCEPTIONS

The fundamental defi nitions of real - time systems engineering can vary ing on the resource consulted Our pragmatic defi nitions have been collected and refi ned to the smallest common subset of agreement to form the vocabu-lary of this particular text These defi nitions are presented in a form that is intended to be most useful to the practicing engineer, as opposed to the aca-demic theorist

1.1.1 Defi nitions for Real - Time Systems

The hardware of a computer solves problems by repeated execution of machine - language instructions, collectively known as software Software, on the other hand, is traditionally divided into system programs and application programs

System programs consist of software that interfaces with the underlying computer hardware, such as device drivers, interrupt handlers, task schedulers, and various programs that act as tools for the development or analysis of application programs These software tools include compilers, which translate high - level language programs into assembly code; assemblers, which convert the assembly code into a special binary format called object or machine code; and linkers/locators, which prepare the object code for execution in a specifi c hardware environment An operating system is a specialized collection of system programs that manage the physical resources of the computer As such,

a real - time operating system is a truly important system program (Anh and Tan, 2009 )

Application programs are programs written to solve specifi c problems, such

as optimal hall - call allocation of an elevator bank in a high - rise building, inertial navigation of an aircraft, and payroll preparation for some industrial company Certain design considerations play a role in the design of system programs and application software intended to run in real - time environments

The notion of a “ system ” is central to software engineering, and indeed to all engineering, and warrants formalization

Defi nition: System

A system is a mapping of a set of inputs into a set of outputs

When the internal details of the system are not of particular interest, the mapping function between input and output spaces can be considered as a black box with one or more inputs entering and one or more outputs exiting the system (see Fig 1.1 ) Moreover, Vernon lists fi ve general properties that belong to any “ system ” (Vernon, 1989 ):

1 A system is an assembly of components connected together in an nized way

2 A system is fundamentally altered if a component joins or leaves it

3 It has a purpose

4 It has a degree of permanence

5 It has been defi ned as being of particular interest

Figure 1.2 A real - time control system including inputs from a camera and multiple

sensors, as well as outputs to a display and multiple actuators

Real-Time Control System

[Job 3, Job 1, Job n, ]

Every real - world entity, whether organic or synthetic, can be modeled as a system In computing systems, the inputs represent digital data from hardware devices or other software systems The inputs are often associated with sensors, cameras, and other devices that provide analog inputs, which are converted to digital data, or provide direct digital inputs The digital outputs of computer systems, on the other hand, can be converted to analog outputs to control external hardware devices, such as actuators and displays, or used directly without any conversion (Fig 1.2 )

Modeling a real - time (control) system, as in Figure 1.2 , is somewhat ent from the more traditional model of the real - time system as a sequence of jobs to be scheduled and performance to be predicted, which is comparable with that shown in Figure 1.3 The latter view is simplistic in that it ignores the usual fact that the input sources and hardware under control may be highly complex In addition, there are other, “ sweeping ” software engineering con-siderations that are hidden by the model shown in Figure 1.3

Look again at the model of a real - time system shown in Figure 1.2 In its realization, there is some inherent delay between presentation of the inputs (excitation) and appearance of the outputs (response) This fact can be formal-ized as follows:

Defi nition: Response Time

The time between the presentation of a set of inputs to a system and the realization of the required behavior, including the availability of all associ-ated outputs, is called the response time of the system

How fast and punctual the response time needs to be depends on the teristics and purpose of the specifi c system

The previous defi nitions set the stage for a practical defi nition of a real - time system

Defi nition: Real - Time System ( II )

A real - time system is one whose logical correctness is based on both the correctness of the outputs and their timeliness

Defi nition: Failed System

A failed system is a system that cannot satisfy one or more of the ments stipulated in the system requirements specifi cation

Defi nition: Real - Time System ( I )

A real time system is a computer system that must satisfy bounded response time constraints or risk severe consequences, including failure

-But what is a “ failed ” system? In the case of the space shuttle or a nuclear power plant, for example, it is painfully obvious when a failure has occurred For other systems, such as an automatic bank teller machine, the notion of failure is less obvious For now, failure will be defi ned as the “ inability of the system to perform according to system specifi cation, ” or, more precisely:

Because of this defi nition of failure, rigorous specifi cation of the system ating criteria, including timing constraints, is necessary This matter is discussed later in Chapter 5

Various other defi nitions exist for “ real - time, ” depending on which source

is consulted Nonetheless, the common theme among all defi nitions is that the system must satisfy deadline constraints in order to be correct For instance,

an alternative defi nition might be:

In any case, by making unnecessary the notion of timeliness, every system becomes a real - time system

Real - time systems are often reactive or embedded systems Reactive systems are those in which task scheduling is driven by ongoing interaction with their environment; for example, a fi re - control system reacts to certain buttons pressed by a pilot Embedded systems can be defi ned informally as follows:

For example, a modern automobile contains many embedded processors that control airbag deployment, antilock braking, air conditioning, fuel injection, and so forth Today, numerous household items, such as microwave ovens, rice cookers, stereos, televisions, washing machines, even toys, contain embedded computers It is obvious that sophisticated systems, such as aircraft, elevator banks, and paper machines, do contain several embedded computer systems The three systems mentioned at the beginning of this chapter satisfy the criteria for a real - time system An aircraft must process accelerometer data within a certain period that depends on the specifi cations of the aircraft; for example, every 10 ms Failure to do so could result in a false position or veloc-ity indication and cause the aircraft to go off - course at best or crash at worst For a nuclear reactor thermal problem, failure to respond swiftly could result

in a meltdown Finally, an airline reservation system must be able to handle a surge of passenger requests within the passenger ’ s perception of a reasonable time (or before the fl ights leave the gate) In short, a system does not have to process data at once or instantaneously to be considered real - time; it must simply have response times that are constrained appropriately

When is a system real - time? It can be argued that all practical systems are ultimately real - time systems Even a batch - oriented system — for example, grade processing at the end of a semester or a bimonthly payroll run — is real - time Although the system may have response times of days or even weeks (e.g., the time that elapses between submitting the grade or payroll informa-tion and issuance of the report card or paycheck), it must respond within a certain time or there could be an academic or fi nancial disaster Even a word - processing program should respond to commands within a reasonable amount

of time or it will become torturous to use Most of the literature refers to such systems as soft real - time systems

Defi nition: Hard Real - Time System

A hard real - time system is one in which failure to meet even a single line may lead to complete or catastrophic system failure

Defi nition: Soft Real - Time System

A soft real - time system is one in which performance is degraded but not destroyed by failure to meet response - time constraints

Defi nition: Embedded System

An embedded system is a system containing one or more computers (or processors) having a central role in the functionality of the system, but the system is not explicitly called a computer

Conversely, systems where failure to meet response - time constraints leads to complete or catastrophic system failure are called hard real - time systems

Firm real - time systems are those systems with hard deadlines where some arbitrarily small number of missed deadlines can be tolerated

TABLE 1.1 A Sampling of Hard, Firm, and Soft Real - Time Systems

Classifi cation

Explanation

Avionics weapons delivery

system in which pressing

a button launches an

air - to - air missile

Hard Missing the deadline to launch the

missile within a specifi ed time after pressing the button may cause the target to be missed, which will result in catastrophe Navigation controller for

an autonomous weed

killer robot

Firm Missing a few navigation deadlines

causes the robot to veer out from

a planned path and damage some crops

Console hockey game Soft Missing even several deadlines will

only degrade performance

Defi nition: Firm Real - Time System

A fi rm real - time system is one in which a few missed deadlines will not lead

to total failure, but missing more than a few may lead to complete or strophic system failure

As noted, all practical systems minimally represent soft real - time systems Table 1.1 gives an illustrative sampling of hard, fi rm, and soft real - time systems There is a great deal of latitude for interpretation of hard, fi rm, and soft real - time systems For example, in the automated teller machine, missing too many deadlines will lead to signifi cant customer dissatisfaction and potentially even enough loss of business to threaten the existence of the bank This extreme scenario represents the fact that every system can often be character-ized any way — soft, fi rm, or hard — real - time by the construction of a support-ing scenario The careful defi nition of systems requirements (and, hence, expectations) is the key to setting and meeting realistic deadline expectations

In any case, it is a principal goal of real - time systems engineering to fi nd ways

to transform hard deadlines into fi rm ones, and fi rm ones into soft ones Since this text is mostly concerned with hard real - time systems, it will use the term real - time system to mean embedded, hard real - time system, unless otherwise noted

It is typical, in studying real - time systems, to consider the nature of time, because deadlines are instants in time Nevertheless, the question arises,

“ Where do the deadlines come from? ” Generally speaking, deadlines are based on the underlying physical phenomena of the system under control For

example, in animated displays, images must be updated at least 30 frames per second to provide continuous motion, because the human eye can resolve updating at a slower rate In navigation systems, accelerations must be read at

a rate that is a function of the maximum velocity of the vehicle, and so on In some cases, however, real - world systems have deadlines that are imposed on them, and are based on nothing less than guessing or on some forgotten and possibly eliminated requirement The problem in these cases is that undue constraints may be placed on the systems This is a primary maxim of real - time systems design — to understand the basis and nature of the timing constraints

so that they can be relaxed if necessary In cost - effective and robust real - time

systems, a pragmatic rule of thumb could be: process everything as slowly as possible and repeat tasks as seldom as possible

Many real - time systems utilize global clocks and time - stamping for nization, task initiation, and data marking It must be noted, however, that all clocks keep somewhat inaccurate time — even the offi cial U.S atomic clock must

synchro-be adjusted regularly Moreover, there is an associated quantization error with clocks, which may need to be considered when using them for time - stamping

In addition to the degree of “ real - time ” (i.e., hard, fi rm, or soft), also, the punctuality of response times is important in many applications Hence, we defi ne the concept of real - time punctuality:

Example: Where a Response Time Comes From

An elevator door (Pasanen et al., 1991 ) is automatically operated, and it may have a capacitive safety edge for sensing possible passengers between the closing door blades Thus, the door blades can be quickly reopened before they touch the passenger and cause discomfort or even threaten the passenger ’ s safety

What is the required system response time from when it recognizes that

a passenger is between the closing door blades to the instant when it starts

to reopen the door?

Defi nition: Real - Time Punctuality

Real - time punctuality means that every response time has an average value,

tR , with upper and lower bounds of tR + εU and tR − εL , respectively, and

εU , εL → 0 +

In all practical systems, the values of εU and εL are nonzero, though they may

be very small or even negligible The nonzero values are due to cumulative latency and propagation - delay components in real - time hardware and soft-

ware Such response times contain jitter within the interval t ∈ [ −εL , +εU ] Real time punctuality is particularly important in periodically sampled systems with high sampling rates, for example, in video signal processing and software radio

Figure 1.4 A partial program fl owchart showing a conditional branch as a change in

Sensor Response Time : t S_min = 5 ms, t S_max = 15 ms, t S_mean = 9 ms

Hardware Response Time : t HW_min = 1 μ s, t HW_max = 2 μ s, t HW_mean = 1.2 μ s

System Software Response Time : t SS_min = 16 μ s, t SS_max = 48 μ s, t SS_mean = 37 μ s

Application Software Response Time : t AS_min = 0.5 μ s, t AS_max = 0.5 μ s,

t AS_mean = 0.5 μ s

Door Drive Response Time : t DD_min = 300 ms, t DD_max = 500 ms,

t DD_mean = 400 ms

Now, we can calculate the minimum, maximum, and mean values of the

composite response time: t min ≈ 305 ms, t max ≈ 515 ms, and t mean ≈ 409 ms Thus, the overall response time is dominated by the door - drive response time containing the required deceleration time of the moving door blades

In software systems, a change in state results in a change in the fl ow - of - control

of the computer program Consider the fl owchart in Figure 1.4 The decision block represented by the diamond suggests that the stream of program instruc-tions can take one of two alternative paths, depending on the response

in question case , if - then , and while statements in any programming language represent a possible change in fl ow - of - control Invocation of proce-dures in Ada and C represent changes in fl ow - of - control In object - oriented

TABLE 1.2 Taxonomy of Events and Some Typical Examples

Synchronous Cyclic code Conditional branch Divide - by - zero

(trap) interrupt Asynchronous Clock interrupt Regular, but not

fi xed - period interrupt

Power - loss alarm

These items will be discussed further in Chapters 2 and 3

languages, instantiation of an object or the invocation of a method causes the change in sequential fl ow - of - control In general, consider the following defi nition

Defi nition: Event

Any occurrence that causes the program counter to change nonsequentially

is considered a change of fl ow - of - control, and thus an event

In scheduling theory, the release time of a job is similar to an event

Defi nition: Release Time

The release time is the time at which an instance of a scheduled task is ready to run, and is generally associated with an interrupt

Events are slightly different from jobs in that events can be caused by rupts, as well as branches

An event can be either synchronous or asynchronous Synchronous events are those that occur at predictable times in the fl ow - of - control, such as that represented by the decision box in the fl owchart of Figure 1.4 The change in

fl ow - of - control, represented by a conditional branch instruction, or by the occurrence of an internal trap interrupt, can be anticipated

Asynchronous events occur at unpredictable points in the fl ow - of - control and are usually caused by external sources A real - time clock that pulses regu-larly at 5 ms is not a synchronous event While it represents a periodic event, even if the clock were able to tick at a perfect 5 ms without drift, the point where the tick occurs with the fl ow - of - control is subject to many factors These factors include the time at which the clock starts relative to the program and propagation delays in the computer system itself An engineer can never count

on a clock ticking exactly at the rate specifi ed, and so any clock - driven event must be treated as asynchronous

Events that do not occur at regular periods are called aperiodic Furthermore, aperiodic events that tend to occur very infrequently are called sporadic Table 1.2 characterizes a sampling of events

For example, an interrupt generated by a periodic external clock represents

a periodic but asynchronous event A periodic but synchronous event is one

represented by a sequence of invocation of software tasks in a repeated, cular fashion A typical branch instruction that is not part of a code block and that runs repeatedly at a regular rate represents a synchronous but aperiodic event A branch instruction that happens infrequently, say, on the detection of some exceptional condition, is both sporadic and synchronous Finally, inter-rupts that are generated irregularly by an external device are classifi ed as either asynchronous aperiodic or sporadic, depending on whether the inter-rupt is generated frequently or not with respect to the system clock

In every system, and particularly in an embedded real - time system, taining overall control is extremely important For any physical system, certain states exist under which the system is considered to be out of control; the software controlling such a system must therefore avoid these states For example, in certain aircraft guidance systems, rapid rotation through a 180 ° pitch angle can cause loss of gyroscopic control Hence, the software must be able to anticipate and avert all such scenarios

Another characteristic of a software - controlled system is that the processor continues to fetch, decode, and execute instructions correctly from the program area of memory, rather than from data or other unwanted memory regions The latter scenario can occur in poorly tested systems and is a catastrophe from which there is almost no hope of recovery

Software control of any real - time system and associated hardware is tained when the next state of the system, given the current state and a set of inputs, is predictable In other words, the goal is to anticipate how a system will behave in all possible circumstances

Defi nition: Deterministic System

A system is deterministic, if for each possible state and each set of inputs,

a unique set of outputs and next state of the system can be determined

Event determinism means the next states and outputs of a system are known for each set of inputs that trigger events Thus, a system that is deterministic

is also event deterministic Although it would be diffi cult for a system to be deterministic only for those inputs that trigger events, this is plausible, and so event determinism may not imply determinism

It is interesting to note that while it is a signifi cant challenge to design systems that are completely event deterministic, and as mentioned, it is pos-sible to inadvertently end up with a system that is nondeterministic, it is defi -nitely hard to design systems that are deliberately nondeterministic This situation arises from the utmost diffi culties in designing perfect random number generators Such deliberately nondeterministic systems would be desirable, for example, as casino gaming machines

Finally, if in a deterministic system the response time for each set of outputs

is known, then the system also exhibits temporal determinism

TABLE 1.3 CPU Utilization (%) Zones

Utilization (%) Zone Type Typical Application

< 26 Unnecessarily safe Various

A side benefi t of designing deterministic systems is that guarantees can be given that the system will be able to respond at any time, and in the case of temporally deterministic systems, when they will respond This fact reinforces the association of “ control ” with real - time systems

The fi nal and truly important term to be defi ned is a critical measure of real - time system performance Because the central processing unit ( CPU ) continues to fetch, decode, and execute instructions as long as power is applied, the CPU will more or less frequently execute either no - ops or instructions that are not related to the fulfi llment of a specifi c deadline (e.g., noncritical “ house-keeping ” ) The measure of the relative time spent doing nonidle processing indicates how much real - time processing is occurring

Defi nition: CPU Utilization Factor

The CPU utilization or time - loading factor, U , is a relative measure of the

nonidle processing taking place

A system is said to be time - overloaded if U > 100% Systems that are too highly utilized are problematic, because additions, changes, or corrections cannot be made to the system without risk of time - overloading On the other hand, systems that are not suffi ciently utilized are not necessarily cost - effective, because this implies that the system was overengineered and that costs could likely be reduced with less expensive hardware While a utilization of 50% is common for new products, 80% might be acceptable for systems that do not

expect growth However, 70% as a target for U is one of the most celebrated

and potentially useful results in the theory of real - time systems where tasks are periodic and independent — a result that will be examined in Chapter 3 Table 1.3 gives a summary of certain CPU utilizations and typical situations

in which they are associated

U is calculated by summing the contribution of utilization factors for each (periodic or aperiodic) task Suppose a system has n ≥ 1 periodic tasks, each

with an execution period of p i , and hence, execution frequency, f i = 1/ p i If task

i is known to have (or has been estimated to have) a worst- case execution time

of e i , then the utilization factor, u i , for task i is

u i =e p i i (1.1) Furthermore, the overall system utilization factor is

Note that the deadline for a periodic task i , d i , is a critical design factor that

is constrained by e i The determination of e i , either prior to, or after the code

has been written, can be extremely diffi cult, and often impossible, in which case estimation or measuring must be used For aperiodic and sporadic tasks,

u i is calculated by assuming a worst - case execution period, usually the minimum

possible time between corresponding event occurrences Such approximations can infl ate the utilization factor unnecessarily or lead to overconfi dence because of the tendency to “ not worry ” about its excessive contribution The danger is to discover later that a higher frequency of occurrence than budgeted has led to a time - overload and system failure

The utilization factor differs from CPU throughput, which is a measure

of the number of machine - language instructions per second that can

be processed based on some predetermined instruction mix This type of surement is typically used to compare CPU throughput for a particular application

Example: Calculation of the CPU Utilization Factor

An individual elevator controller in a bank of high - rise elevators has the

following software tasks with execution periods of p i and worst - case tion times of e i , i ∈ {1, 2, 3, 4}:

Task 1 : Communicate with the group dispatcher (19.2 K bit/s data rate and a proprietary communications protocol); p 1 = 500 ms, e 1 = 17 ms

Task 2 : Update the car position information and manage fl oor - to - fl oor runs, as well as door control; p 2 = 25 ms, e 2 = 4 ms

Task 3 : Register and cancel car calls; p 3 = 75 ms, e 3 = 1 ms

Task 4 : Miscellaneous system supervisions; p 4 = 200 ms, e 4 = 20 ms What is the overall CPU utilization factor?

425

175

20

200 0 31. Hence, the utilization percentage is 31%, which belongs to the “ very safe ” zone of Table 1.3

The choice of task deadlines, estimation and reduction of execution times, and other factors that infl uence CPU utilization will be discussed in Chapter 7

1.1.2 Usual Misconceptions

As a part of truly understanding the nature of real - time systems, it is important

to address a number of frequently cited misconceptions These are summarized

as follows:

1 Real - time systems are synonymous with “ fast ” systems

2 Rate - monotonic analysis has solved “ the real - time problem ”

3 There are universal, widely accepted methodologies for real - time systems specifi cation and design

4 There is no more a need to build a real - time operating system, because many commercial products exist

5 The study of real - time systems is mostly about scheduling theory

The fi rst misconception, that real - time systems must be fast, arises from the fact that many hard real - time systems indeed deal with deadlines in the tens

of milliseconds, such as the aircraft navigation system In a typical food industry application, however, pasta - sauce jars can move along the conveyor belt past a fi lling point at a rate of one every fi ve seconds Furthermore, the airline reservation system could have a deadline of 15 seconds These latter deadlines are not particularly fast, but satisfying them determines the success

-or failure of the system

The second misconception is that rate - monotonic systems provide a simple recipe for building real - time systems Rate - monotonic systems — a periodic system in which interrupt (or software task) priorities are assigned such that the faster the rate of execution, the higher the priority — have received a lot

of attention since the 1970s While they provide valuable guidance in the design of real - time systems, and while there is abundant theory surrounding them, they are not a panacea Rate - monotonic systems will be discussed in great detail in Chapter 3

What about the third misconception? Unfortunately, there are no sally accepted and infallible methods for the specifi cation and design of real - time systems This is not a failure of researchers or the software industry, but

univer-is because of the diffi culty of duniver-iscovering universal solutions for thuniver-is ing fi eld After nearly 40 years of research and development, there is still no methodology available that answers all of the challenges of real - time specifi ca-tion and design all the time and for all applications

The fourth misconception is that there is no more a need to build a real time operating system from scratch While there are a number of cost - effective, popular, and viable commercial real - time operating systems, these, too, are not

-a p-an-ace-a Commerci-al solutions h-ave cert-ainly their pl-ace, but choosing when

to use an off - the - shelf solution and choosing the right one are challenges that will be considered in Chapter 3

Finally, while it is scholarly to study scheduling theory, from an engineering standpoint, most published results require impractical simplifi cations and clairvoyance in order to make the theory work Because this is a textbook for practicing engineers, it avoids any theoretical results that resort to these measures

1.2 MULTIDISCIPLINARY DESIGN CHALLENGES

The study of real - time systems is a truly multidimensional subdiscipline of computer systems engineering that is strongly infl uenced by control theory, operations research, and, naturally, software engineering Figure 1.5 depicts some of the disciplines of computer science, electrical engineering, systems engineering, and applied statistics that affect the design and analysis of real - time systems Nevertheless, those representative disciplines are not the only ones having a relationship with real - time systems Because real - time systems engineering is so multidisciplinary, it stands out as a fascinating study area with a rich set of design challenges Although the fundamentals of real - time systems are well established and have considerable permanence, real - time systems is a lively developing area due to evolving CPU architectures, distributed system structures, versatile wireless networks, and novel applica-tions, for instance

Figure 1.5 A variety of disciplines that affect real - time systems engineering

Real-Time Systems

Software Engineering

Systems Theory

1.2.1 Infl uencing Disciplines

The design and implementation of real - time systems requires attention to numerous practical issues These include:

• The selection of hardware and system software, and evaluation of the trade - off needed for a competitive solution, including dealing with distributed computing systems and the issues of concurrency and synchronization

• Specifi cation and design of real - time systems, as well as correct and sive representation of temporal behavior

• Understanding the nuances of the high - level programming language(s) and the real - time implications resulting from their optimized compilation into machine - language code

• Optimizing (with application - specifi c objectives) of system fault tolerance and reliability through careful design and analysis

• The design and administration of adequate tests at different levels of hierarchy, and the selection of appropriate development tools and test equipment

• Taking advantage of open systems technology and interoperability An open system is an extensible collection of independently written applica-tions that cooperate to function as an integrated system For example, several versions of the open operating system, Linux, have emerged for use in various real - time applications (Abbott, 2006 ) Interoperability can

be measured in terms of compliance with open system standards, such as the real - time CORBA ( common object request broker architecture ) stan-dard (Fay - Wolfe et al., 2000 )

• Finally, estimating and measuring response times and (if needed) reducing them Performing a schedulability analysis, that is, determining and guar-

anteeing deadline satisfaction, a priori

Obviously, the engineering techniques used for hard real - time systems can be used in the engineering of all other types of systems as well, with an accom-panying improvement of performance and robustness This alone is a signifi -cant reason to study the engineering of real - time systems

1.3 BIRTH AND EVOLUTION OF REAL - TIME SYSTEMS

The history of real - time systems, as characterized by important developments

in the United States, is tied inherently to the evolution of the computer Modern real - time systems, such as those that control nuclear power plants, military weapons systems, or medical monitoring equipment, are sophisticated, yet many still exhibit characteristics of those pioneering systems developed in the 1940s through the 1960s

In the introductory paragraphs of this chapter, some real - time systems were mentioned The following descriptions provide more details for each system, while others provide additional examples Clearly, these descriptions are not rigorous specifi cations The process of specifying real - time systems unambigu-ously but concisely is discussed in Chapter 5

Consider the inertial measurement system for an aircraft The software

specifi cation states that the software will receive x , y , and z accelerometer

pulses at a 10 ms rate from special hardware The software will determine the acceleration components in each direction, and the corresponding roll, pitch, and yaw of the aircraft

The software will also collect other information, such as temperature at a

1 - second rate The task of the application software is to compute the actual velocity vector based on the current orientation, accelerometer readings, and various compensation factors (such as for temperature effects) at a 40 ms rate The system is to output true acceleration, velocity, and position vectors to a pilot ’ s display every 40 ms, but using a different clock

TABLE 1.4 Typical Real - Time Domains

and Diverse Applications

Aerospace Flight control

Navigation Pilot interface Civilian Automotive systems

Elevator control Traffi c light control Industrial Automated inspection

Robotic assembly line Welding control Medical Intensive care monitors

Magnetic resonance imaging Remote surgery

Multimedia Console games

Home theaters Simulators

These tasks execute at four different rates in the inertial measurement system, and need to communicate and synchronize The accelerometer read-

ings must be time - relative or correlated; that is, it is not allowed to mix an x accelerometer pulse of discrete time instant k with y and z pulses of instant

k + 1 These are critical design issues for this system

Next, consider a monitoring system for a nuclear power plant that will be handling three events signaled by interrupts The fi rst event is triggered by any

of several signals at various security points, which will indicate a security breach The system must respond to this signal within one second The second and most important event indicates that the reactor core has reached an over-temperature This signal must be dealt with within 1 millisecond (1 ms) Finally,

an operator ’ s display is to be updated at approximately 30 times per second The nuclear - power - plant system requires a reliable mechanism to ensure that the “ meltdown imminent ” indicator can interrupt any other processing with minimal latency

As another example, recall the airline reservation system mentioned earlier Management has decided that to prevent long lines and customer dissatisfac-tion, turnaround time for any transaction must be less than 15 seconds, and no overbooking will be permitted At any time, several travel agents may try to access the reservations database and perhaps book the same fl ight simultane-ously Here, effective record - locking and secure communications mechanisms

Figure 1.6 Mars Exploration Rover; a solar - powered, autonomous real - time system

with radio - communications links and a variety of sensors and actuators Photo courtesy

of NASA

are needed to protect against the alteration of the database containing the reservation information by more than one clerk at a time

Now, consider a real - time system that controls all phases of the bottling of jars of pasta sauce as they travel along a conveyor belt The empty jars are

fi rst microwaved to disinfect them A mechanism fi lls each jar with a precise serving of specifi c sauce as it passes beneath Another station caps the fi lled bottles In addition, there is an operator ’ s display that provides an animated rendering of the production line activities There are numerous events trig-gered by exceptional conditions, such as the conveyor belt jamming and a bottle overfl owing or breaking If the conveyor belt travels too fast, the bottle will move past its designated station prematurely Therefore, there is a wide range of events, both synchronous and asynchronous, to be dealt with

As a fi nal example, consider a system used to control a set of traffi c lights

at a four - way traffi c intersection (north - , south - , east - , and west - bound traffi c) This system controls the lights for vehicle and pedestrian traffi c at a four - way intersection in a busy city like Philadelphia Input may be taken from cameras, emergency - vehicle transponders, push buttons, sensors under the ground, and

so on The traffi c lights need to operate in a synchronized fashion, and yet react to asynchronous events — such as a pedestrian pressing a button at a crosswalk Failure to operate in a proper fashion can result in automobile accidents and even fatalities

The challenge presented by each of these systems is to determine the priate design approach with respect to the multidisciplinary issues discussed

appro-in Section 1.2

1.3.2 Advancements behind Modern Real - Time Systems

Much of the theory of real - time systems is derived from the surrounding ciplines shown in Figure 1.5 In particular, certain aspects of operations research (i.e., scheduling), which emerged in the late 1940s, and queuing theory

dis-in the early 1950s, have dis-infl uenced most of the more theoretical results Martin published one of the fi rst and certainly the most infl uential early book on real - time systems (Martin, 1967 ) Martin ’ s book was soon followed

by several others (e.g., Stimler, 1969 ), and the infl uence of operations research and queuing theory can be seen in these works It is also educational to study these texts in the context of the great limitations of the hardware of the time

In 1973, Liu and Layland published their seminal work on rate - monotonic theory (Liu and Layland, 1973 ) Over the last nearly 40 years, signifi cant refi ne-ment of this theory has made it a practical theory for use in designing real - time systems

The 1980s and 1990s saw a proliferation of theoretical work on improving predictability and reliability of real - time systems, and on solving problems related to multitasking systems Today, a rather small group of experts contin-ues to study pure issues of scheduling and performance analysis, while a larger group of generalist systems engineers tackles broader issues relating to the

implementation of practical systems An important paper by Stankovic et al (Stankovic et al., 1995 ) described some of the diffi culties in conducting research

on real - time systems — even with signifi cant restriction of the system, most problems relating to scheduling are too diffi cult to solve by analytic techniques

Instead of any single “ groundbreaking ” technology, the new millennium saw a number of important advancements in hardware, viable open - source software for real - time systems, powerful commercial design and implementa-tion tools, and expanded programming language support These advancements have in some ways simplifi ed the construction and analysis of real - time systems but on the other hand introduced new problems because of the complexities

of systems interactions and the masking of many of the underlying subtleties

of time constraints

The origin of the term real- time computing is unclear It was probably fi rst

used either with project Whirlwind, a fl ight simulator developed by IBM for the U.S Navy in 1947, or with SAGE, the Semiautomatic Ground Environment air defense system developed for the U.S Air Force in the late 1950s Both of these projects qualify as real - time systems even by today ’ s defi nitions In addi-tion to its real - time contributions, the Whirlwind project included the fi rst use

of ferrite core memory ( “ fast ” ) and a form of high - level language compiler that predated Fortran

Other early real - time systems were used for airline reservations, such as SABRE (developed for American Airlines in 1959), as well as for process control, but the advent of the national space program provided even greater opportunities for the development of more advanced real - time systems for spacecraft control and telemetry It was not until the 1960s that rapid develop-ment of such systems took place, and then only as signifi cant nonmilitary interest in real - time systems become coupled with the availability of equip-ment adapted to real - time processing

Low - performance processors and particularly slow and small memories handicapped many of the earliest systems In the early 1950s, the asynchronous interrupt was introduced and later incorporated as a standard feature in the Univac Scientifi c 1103A The middle 1950s saw a distinct increase in the speed and complexity of large - scale computers designed for scientifi c computation, without an increase in physical size These developments made it possible to apply real - time computation in the fi eld of control systems Such hardware improvements were particularly noticeable in IBM ’ s development of SAGE

In the 1960s and 1970s, advances in integration levels and processing speeds enhanced the spectrum of real - time problems that could be solved In 1965 alone, it was estimated that more than 350 real - time process control systems existed (Martin, 1967 )

The 1980s and 1990s have seen, for instance, distributed systems and non von Neumann architectures utilized in real - time applications

Finally, the late 1990s and early 2000s have set new trends in real - time embedded systems in consumer products and Web - enabled devices The avail-

ability of compact processors with limited memory and functionality has venated some of the challenges faced by early real - time systems designers Fortunately, around 60 years of experience is now available to draw upon Early real - time systems were written directly in microcode or assembly language, and later in higher - level languages As previously noted, Whirlwind used an early form of high - level language called an algebraic compiler to simplify coding Later systems employed Fortran, CMS - 2, and JOVIAL, the preferred languages in the U.S Army, Navy, and Air Force, respectively

In the 1970s, the Department of Defense ( DoD ) mandated the ment of a single language that all military services could use, and that provided high - level language constructs for real - time programming After a careful selection and refi nement process, the Ada language appeared as a standard in

develop-1983 Shortfalls in the language were identifi ed, and a new, improved version

of the language, Ada 95, appeared in 1995

Today, however, only a small number of systems are developed in Ada Most embedded systems are written in C or C ++ In the last 10 years, there has been

a remarkable increase in the use of object - oriented methodologies, and guages like C ++ and Java in embedded real - time systems The real - time aspects

lan-of programming languages are discussed later in Chapter 4

The fi rst commercial operating systems were designed for the early frame computers IBM developed the fi rst real - time executive, the Basic Executive, in 1962, which provided diverse real - time scheduling By 1963, the Basic Executive II had disk - resident system and user programs

By the mid - 1970s, more affordable minicomputer systems could be found

in many engineering environments In response, a number of important real time operating systems were developed by the minicomputer manufacturers Notable among these were the Digital Equipment Corporation ( DEC ) family

-of real - time multitasking executives ( RSX ) for the PDP - 11, and Hewlett Packard ’ s Real - Time Executive ( RTE ) series of operating systems for its HP

A selective summary of landmark events in the fi eld of real - time systems

in the United States is given in Table 1.5

1.4 SUMMARY

The deep - going roots of real - time systems were formed during the historical years of computers and computing — before the microprocessor era However, the fi rst “ boom ” of real - time systems took place around the beginning of 1980s, when appropriate microprocessors and real - time operating systems became

TABLE 1.5 Landmarks in Real - Time Systems History in the United States

Year Landmark Developer Development Innovations

1947 Whirlwind IBM Flight simulator Ferrite core

memory ( “ fast ” ), high - level language

1957 SAGE IBM Air defense Designed for

operating systems

Hosted by minicomputers

1973 Rate

monotonic

system

Liu and Layland

Fundamental theory

Upper bound on utilization for schedulable systems 1970s

Hosted by microprocessors

1983 Ada 83 U.S DoD Programming

language

For mission critical embedded systems

1995 Ada 95 Community Programming

language

Improved version

of Ada 83 2000s – – Various advances

in hardware, open - source, and commercial system software and tools

A continuously growing range

of innovative applications that can be “ real - time ”

available (to be used in embedded systems) for an enormous number of trical, systems, as well as mechanical and aerospace engineers These practicing engineers did not have much software or even computer education, and, thus, the initial learning path was laborious in most fi elds of industry In those early times, the majority of real - time operating systems and communications proto-