The implementation of a computer has two components: organization and hard-ware. The term organization includes the high-level aspects of a computer’s design, such as the memory system, the memory interconnect, and the design of the internal processor or CPU (central processing unit—where arithmetic, logic, branching, and data transfer are implemented). The term microarchitecture is also used instead of organization. For example, two processors with the same instruc-tion set architectures but different organizainstruc-tions are the AMD Opteron and the Intel Core i7. Both processors implement the 80x86 instruction set, but they have very different pipeline and cache organizations.
The switch to multiple processors per microprocessor led to the term core also being used for processors. Instead of saying multiprocessor microprocessor, the term multicore caught on. Given that virtually all chips have multiple processors, the term central processing unit, or CPU, is fading in popularity.
Hardware refers to the specifics of a computer, including the detailed logic design and the packaging technology of the computer. Often a line of computers contains computers with identical instruction set architectures and very similar organizations, but they differ in the detailed hardware implementation. For exam-ple, the Intel Core i7 (seeChapter 3) and the Intel Xeon E7 (seeChapter 5) are nearly identical but offer different clock rates and different memory systems, mak-ing the Xeon E7 more effective for server computers.
In this book, the word architecture covers all three aspects of computer design—instruction set architecture, organization or microarchitecture, and hardware.
Computer architects must design a computer to meet functional requirements as well as price, power, performance, and availability goals.Figure 1.8summarizes requirements to consider in designing a new computer. Often, architects also must determine what the functional requirements are, which can be a major task. The requirements may be specific features inspired by the market. Application software typically drives the choice of certain functional requirements by determining how the computer will be used. If a large body of software exists for a particular instruc-tion set architecture, the architect may decide that a new computer should imple-ment an existing instruction set. The presence of a large market for a particular class of applications might encourage the designers to incorporate requirements that would make the computer competitive in that market. Later chapters examine many of these requirements and features in depth.
1.3 Defining Computer Architecture ■ 17
Architects must also be aware of important trends in both the technology and the use of computers because such trends affect not only the future cost but also the longevity of an architecture.
1.4 Trends in Technology
If an instruction set architecture is to prevail, it must be designed to survive rapid changes in computer technology. After all, a successful new instruction set Functional requirements Typical features required or supported
Application area Target of computer
Personal mobile device Real-time performance for a range of tasks, including interactive performance for graphics, video, and audio; energy efficiency (Chapters 2–5and7;Appendix A) General-purpose desktop Balanced performance for a range of tasks, including interactive performance for
graphics, video, and audio (Chapters 2–5;Appendix A)
Servers Support for databases and transaction processing; enhancements for reliability and availability; support for scalability (Chapters 2,5, and7; Appendices A, D, and F) Clusters/warehouse-scale
computers Throughput performance for many independent tasks; error correction for memory;
energy proportionality (Chapters 2,6, and7; Appendix F) Internet of things/embedded
computing Often requires special support for graphics or video (or other application-specific extension); power limitations and power control may be required; real-time constraints (Chapters 2,3,5, and7; Appendices A and E)
Level of software compatibility Determines amount of existing software for computer
At programming language Most flexible for designer; need new compiler (Chapters 3,5, and7;Appendix A) Object code or binary
compatible Instruction set architecture is completely defined—little flexibility—but no investment needed in software or porting programs (Appendix A)
Operating system requirements Necessary features to support chosen OS (Chapter 2;Appendix B) Size of address space Very important feature (Chapter 2); may limit applications Memory management Required for modern OS; may be paged or segmented (Chapter 2)
Protection Different OS and application needs: page versus segment; virtual machines (Chapter 2) Standards Certain standards may be required by marketplace
Floating point Format and arithmetic: IEEE 754 standard (Appendix J), special arithmetic for graphics or signal processing
I/O interfaces For I/O devices: Serial ATA, Serial Attached SCSI, PCI Express (Appendices D and F) Operating systems UNIX, Windows, Linux, CISCO IOS
Networks Support required for different networks: Ethernet, Infiniband (Appendix F) Programming languages Languages (ANSI C, C++, Java, Fortran) affect instruction set (Appendix A) Figure 1.8 Summary of some of the most important functional requirements an architect faces. The left-hand column describes the class of requirement, while the right-hand column gives specific examples. The right-hand col-umn also contains references to chapters and appendices that deal with the specific issues.
18 ■ Chapter One Fundamentals of Quantitative Design and Analysis
architecture may last decades—for example, the core of the IBM mainframe has been in use for more than 50 years. An architect must plan for technology changes that can increase the lifetime of a successful computer.
To plan for the evolution of a computer, the designer must be aware of rapid changes in implementation technology. Five implementation technologies, which change at a dramatic pace, are critical to modern implementations:
■ Integrated circuit logic technology—Historically, transistor density increased by about 35% per year, quadrupling somewhat over four years. Increases in die size are less predictable and slower, ranging from 10% to 20% per year.
The combined effect was a traditional growth rate in transistor count on a chip of about 40%–55% per year, or doubling every 18–24 months. This trend is popularly known as Moore’s Law. Device speed scales more slowly, as we discuss below. Shockingly, Moore’s Law is no more. The number of devices per chip is still increasing, but at a decelerating rate. Unlike in the Moore’s Law era, we expect the doubling time to be stretched with each new technol-ogy generation.
■ Semiconductor DRAM (dynamic random-access memory)—This technology is the foundation of main memory, and we discuss it inChapter 2. The growth of DRAM has slowed dramatically, from quadrupling every three years as in the past. The 8-gigabit DRAM was shipping in 2014, but the 16-gigabit DRAM won’t reach that state until 2019, and it looks like there will be no 32-gigabit DRAM (Kim, 2005).Chapter 2mentions several other technologies that may replace DRAM when it hits its capacity wall.
■ Semiconductor Flash (electrically erasable programmable read-only mem-ory)—This nonvolatile semiconductor memory is the standard storage device in PMDs, and its rapidly increasing popularity has fueled its rapid growth rate in capacity. In recent years, the capacity per Flash chip increased by about 50%–60% per year, doubling roughly every 2 years. Currently, Flash memory is 8–10 times cheaper per bit than DRAM.Chapter 2describes Flash memory.
■ Magnetic disk technology—Prior to 1990, density increased by about 30% per year, doubling in three years. It rose to 60% per year thereafter, and increased to 100% per year in 1996. Between 2004 and 2011, it dropped back to about 40%
per year, or doubled every two years. Recently, disk improvement has slowed to less than 5% per year. One way to increase disk capacity is to add more plat-ters at the same areal density, but there are already seven platplat-ters within the one-inch depth of the 3.5-inch form factor disks. There is room for at most one or two more platters. The last hope for real density increase is to use a small laser on each disk read-write head to heat a 30 nm spot to 400°C so that it can be written magnetically before it cools. It is unclear whether Heat Assisted Magnetic Recording can be manufactured economically and reliably, although Seagate announced plans to ship HAMR in limited production in 2018. HAMR is the last chance for continued improvement in areal density of hard disk 1.4 Trends in Technology ■ 19
drives, which are now 8–10 times cheaper per bit than Flash and 200–300 times cheaper per bit than DRAM. This technology is central to server- and warehouse-scale storage, and we discuss the trends in detail in Appendix D.
■ Network technology—Network performance depends both on the performance of switches and on the performance of the transmission system. We discuss the trends in networking in Appendix F.
These rapidly changing technologies shape the design of a computer that, with speed and technology enhancements, may have a lifetime of 3–5 years. Key tech-nologies such as Flash change sufficiently that the designer must plan for these changes. Indeed, designers often design for the next technology, knowing that, when a product begins shipping in volume, the following technology may be the most cost-effective or may have performance advantages. Traditionally, cost has decreased at about the rate at which density increases.
Although technology improves continuously, the impact of these increases can be in discrete leaps, as a threshold that allows a new capability is reached. For example, when MOS technology reached a point in the early 1980s where between 25,000 and 50,000 transistors could fit on a single chip, it became possible to build a single-chip, 32-bit microprocessor. By the late 1980s, first-level caches could go on a chip. By eliminating chip crossings within the processor and between the pro-cessor and the cache, a dramatic improvement in cost-performance and energy-performance was possible. This design was simply unfeasible until the technology reached a certain point. With multicore microprocessors and increasing numbers of cores each generation, even server computers are increasingly headed toward a sin-gle chip for all processors. Such technology thresholds are not rare and have a sig-nificant impact on a wide variety of design decisions.