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Computer Organization and Design: The Hardware/Software Interface 1 Civilization advances by extending the number of important operations which we can perform without thinking about them. Alfred North Whitehead, An Introduction to Mathematics, 1911 Computer Abstractions and Technology 1.1 Introduction 3 1.2 Eight Great Ideas in Computer Architecture 11 1.3 Below Your Program 13 1.4 Under the Covers 16 1.5 Technologies for Building Processors and Memory 24 Computer Organization and Design. DOI: © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/B978-0-12-407726-3.00001-1 2013 1.6 Performance 28 1.7 The Power Wall 40 1.8 The Sea Change: The Switch from Uniprocessors to Multiprocessors 43 1.9 Real Stuff: Benchmarking the Intel Core i7 46 1.10 Fallacies and Pitfalls 49 1.11 Concluding Remarks 52 1.12 Historical Perspective and Further Reading 54 1.13 Exercises 54 1.1 Introduction Welcome to this book! We’re delighted to have this opportunity to convey the excitement of the world of computer systems. Th is is not a dry and dreary fi eld, where progress is glacial and where new ideas atrophy from neglect. No! Computers are the product of the incredibly vibrant information technology industry, all aspects of which are responsible for almost 10% of the gross national product of the United States, and whose economy has become dependent in part on the rapid improvements in information technology promised by Moore’s Law. Th is unusual industry embraces innovation at a breath-taking rate. In the last 30 years, there have been a number of new computers whose introduction appeared to revolutionize the computing industry; these revolutions were cut short only because someone else built an even better computer. Th is race to innovate has led to unprecedented progress since the inception of electronic computing in the late 1940s. Had the transportation industry kept pace with the computer industry, for example, today we could travel from New York to London in a second for a penny. Take just a moment to contemplate how such an improvement would change society—living in Tahiti while working in San Francisco, going to Moscow for an evening at the Bolshoi Ballet—and you can appreciate the implications of such a change. 4 Chapter 1 Computer Abstractions and Technology Computers have led to a third revolution for civilization, with the information revolution taking its place alongside the agricultural and the industrial revolutions. Th e resulting multiplication of humankind’s intellectual strength and reach naturally has aff ected our everyday lives profoundly and changed the ways in which the search for new knowledge is carried out. Th ere is now a new vein of scientifi c investigation, with computational scientists joining theoretical and experimental scientists in the exploration of new frontiers in astronomy, biology, chemistry, and physics, among others. Th e computer revolution continues. Each time the cost of computing improves by another factor of 10, the opportunities for computers multiply. Applications that were economically infeasible suddenly become practical. In the recent past, the following applications were “computer science fi ction.” ■ Computers in automobiles: Until microprocessors improved dramatically in price and performance in the early 1980s, computer control of cars was ludicrous. Today, computers reduce pollution, improve fuel effi ciency via engine controls, and increase safety through blind spot warnings, lane departure warnings, moving object detection, and air bag infl ation to protect occupants in a crash. ■ Cell phones: Who would have dreamed that advances in computer systems would lead to more than half of the planet having mobile phones, allowing person-to-person communication to almost anyone anywhere in the world? ■ Human genome project: Th e cost of computer equipment to map and analyze human DNA sequences was hundreds of millions of dollars. It’s unlikely that anyone would have considered this project had the computer costs been 10 to 100 times higher, as they would have been 15 to 25 years earlier. Moreover, costs continue to drop; you will soon be able to acquire your own genome, allowing medical care to be tailored to you. ■ World Wide Web: Not in existence at the time of the fi rst edition of this book, the web has transformed our society. For many, the web has replaced libraries and newspapers. ■ Search engines: As the content of the web grew in size and in value, fi nding relevant information became increasingly important. Today, many people rely on search engines for such a large part of their lives that it would be a hardship to go without them. Clearly, advances in this technology now aff ect almost every aspect of our society. Hardware advances have allowed programmers to create wonderfully useful soft ware, which explains why computers are omnipresent. Today’s science fi ction suggests tomorrow’s killer applications: already on their way are glasses that augment reality, the cashless society, and cars that can drive themselves. 1.1 Introduction 5 Classes of Computing Applications and Their Characteristics Although a common set of hardware technologies (see Sections 1.4 and 1.5) is used in computers ranging from smart home appliances to cell phones to the largest supercomputers, these diff erent applications have diff erent design requirements and employ the core hardware technologies in diff erent ways. Broadly speaking, computers are used in three diff erent classes of applications. Personal computers (PCs) are possibly the best known form of computing, which readers of this book have likely used extensively. Personal computers emphasize delivery of good performance to single users at low cost and usually execute third-party soft ware. Th is class of computing drove the evolution of many computing technologies, which is only about 35 years old! Servers are the modern form of what were once much larger computers, and are usually accessed only via a network. Servers are oriented to carrying large workloads, which may consist of either single complex applications—usually a scientifi c or engineering application—or handling many small jobs, such as would occur in building a large web server. Th ese applications are usually based on soft ware from another source (such as a database or simulation system), but are oft en modifi ed or customized for a particular function. Servers are built from the same basic technology as desktop computers, but provide for greater computing, storage, and input/output capacity. In general, servers also place a greater emphasis on dependability, since a crash is usually more costly than it would be on a single- user PC. Servers span the widest range in cost and capability. At the low end, a server may be little more than a desktop computer without a screen or keyboard and cost a thousand dollars. Th ese low-end servers are typically used for fi le storage, small business applications, or simple web serving (see Section 6.10). At the other extreme are supercomputers, which at the present consist of tens of thousands of processors and many terabytes of memory, and cost tens to hundreds of millions of dollars. Supercomputers are usually used for high-end scientifi c and engineering calculations, such as weather forecasting, oil exploration, protein structure determination, and other large-scale problems. Although such supercomputers represent the peak of computing capability, they represent a relatively small fraction of the servers and a relatively small fraction of the overall computer market in terms of total revenue. Embedded computers are the largest class of computers and span the widest range of applications and performance. Embedded computers include the microprocessors found in your car, the computers in a television set, and the networks of processors that control a modern airplane or cargo ship. Embedded computing systems are designed to run one application or one set of related applications that are normally integrated with the hardware and delivered as a single system; thus, despite the large number of embedded computers, most users never really see that they are using a computer! personal computer (PC) A computer designed for use by an individual, usually incorporating a graphics display, a keyboard, and a mouse. server A computer used for running larger programs for multiple users, oft en simultaneously, and typically accessed only via a network. supercomputer A class of computers with the highest performance and cost; they are confi gured as servers and typically cost tens to hundreds of millions of dollars. terabyte (TB) Originally 1,099,511,627,776 (240) bytes, although communications and secondary storage systems developers started using the term to mean 1,000,000,000,000 (1012) bytes. To reduce confusion, we now use the term tebibyte (TiB) for 240 bytes, defi ning terabyte (TB) to mean 1012 bytes. Figure 1.1 shows the full range of decimal and binary values and names. embedded computer A computer inside another device used for running one predetermined application or collection of soft ware. 6 Chapter 1 Computer Abstractions and Technology Embedded applications oft en have unique application requirements that combine a minimum performance with stringent limitations on cost or power. For example, consider a music player: the processor need only be as fast as necessary to handle its limited function, and beyond that, minimizing cost and power are the most important objectives. Despite their low cost, embedded computers oft en have lower tolerance for failure, since the results can vary from upsetting (when your new television crashes) to devastating (such as might occur when the computer in a plane or cargo ship crashes). In consumer-oriented embedded applications, such as a digital home appliance, dependability is achieved primarily through simplicity— the emphasis is on doing one function as perfectly as possible. In large embedded systems, techniques of redundancy from the server world are oft en employed. Although this book focuses on general-purpose computers, most concepts apply directly, or with slight modifi cations, to embedded computers. Elaboration: Elaborations are short sections used throughout the text to provide more detail on a particular subject that may be of interest. Disinterested readers may skip over an elaboration, since the subsequent material will never depend on the contents of the elaboration. Many embedded processors are designed using processor cores, a version of a processor written in a hardware description language, such as Verilog or VHDL (see Chapter 4). The core allows a designer to integrate other application-specifi c hardware