Modern.processor.design fundamentals.of.superscalar.processors

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Modern.processor.design fundamentals.of.superscalar.processors

  1. 1. MODERN PROCESSOR DESIGN Fundamentals of Superscalar ProcessorsJOHN PAUL SHEN ● MIKKO H.LIPASTI 高等計算機結構 教師:蔡智強-教授 【2010.09】
  2. 2. John Paul Shen J o h n Paul Shen is the Director of Intels Microarchitecture Research Lab (MRL), providing leadership to about two- dozen highly skilled researchers located in Santa Clara, C A; Hillsboro, OR; and Austin. TX. MRL is responsible for de- veloping innovative microarchitecture techniques that can I potentially be used in future microprocessor products from Modern Processor Design Intel. MRL researchers collaborate closely with microarchi- I tects from product teams in joint advanced-development I efforts. MRL frequently hosts visiting faculty and Ph.D. Fundamentals of Superscalar Processors I interns and conducts joint research projects with academic I research groups. Prior to joining Intel in 2000, John was a professor in the electrical and computer engineering department of CarnegieMellon University, where he headed up the CMU Microarchitecture Research Team John Paul Shen(CMuART). He has supervised a total of 16 Ph.D. students during his years at CMU. Intel CorporationSeven are currently with Intel, and five have faculty positions in academia. He wonmultiple teaching awards at CMU. He was an NSF Presidential Young Investigator. Mikko H. LipastiHe is an IEEE Fellow and has served on the program committees of ISCA, MICRO, University of WisconsinHPCA, ASPLOS, PACT, ICCD, ITC, and FTCS. He has published over 100 research papers in diverse areas, including fault-tolerant computing, built-in self-test, process defect and fault analysis, concurrenterror detection, application-specific processors, performance evaluation, compila-tion for instruction-level parallelism, value locality and prediction, analytical mod-eling of superscalar processors, systematic microarchitecture test generation, per-formance simulator validation, precomputation-based prefetching, database workloadanalysis, and user-level helper threads. John received his M.S. and Ph.D. degrees from the University of SouthernCalifornia, and his B.S. degree from the University of Michigan, all in electricalengineering. He attended Kimball High School in Royal Oak, Michigan. He ishappily married and has three daughters. His family enjoys camping, road trips, andreading The Lord of the Rings. Tata McGraw-Hill Publishing Company Limited N E W DELHI (continued on back inside cover) McGraw-Hill Offices New Delhi N e w York St Louis S a n Francisco Auckland Bogota Caracas Kuala Lumpur Lisbon London Madrid M e x i c o City Milan Montreal S a n Juan Santiago Singapore Sydney Tokyo Toronto
  3. 3. Our parents: Table of Contents Paul and Sue Shen Tarja and Simo Lipasti Our spouses: Amy C. Shen Table of Contents Erica Ann Lipasti Additional Resources Preface Our children: Priscilla S. Shen, Rachael S. Shen, and Valentia C. Shen 1 Processor Design Emma Kristiina Lipasti and Elias Joel Lipasti 1.1 The Evolution of Microprocessors 1.2 Instruction Set Processor Design 1.2.1 Digital Systems Design 1.2.2 Architecture, Implementation, and Realization Tata McGraw-Hiil 1.2.3 Instruction Set Architecture 1.2.4 Dynamic-Static Interface 1.3 Principles of Processor PerformanceMODERN PROCESSOR DESIGN: FUNDAMENTALS OF 1.3.1 Processor Performance EquationSUPERSCALAR PROCESSORS 1.3.2 Processor Performance Optimizations 1.3.3 Performance Evaluation MethodCopyright © 2005 by The McGraw-Hill Companies, Inc., 1.4 Instruction-Level Parallel ProcessingAll rights reserved. No part of this publication may be reproduced or distributed 1.4.1 From Scalar to Superscalarin any form or by any means, or stored in a data base or retrieval system, without the 1.4.2 Limits of Instruction-Level Parallelismprior written consent of The McGraw-Hill Companies, Inc., including, but not 1.4.3 Machines for Instruction-Level Parallelismlimited to, in any network or other electronic storage or transmission, or broadcast 1.5 Summaryfor distance learningSome ancillaries, including electronic and print components, may not be available 2 Pipelined Processorsto customers outside the United States 2.1 Pipelining Fundamentals 2.1.1 Pipelined DesignTata McGraw-Hill Edition 2.1.2 Arithmetic Pipeline Example 2.1.3 Pipelining IdealismRZXQCRBIRQQDD 2.1.4 Instruction Pipelining 2.2 Pipelined Processor DesignReprinted in India by arrangement with The McGraw-Hill Companies, Inc., 2.2.1 Balancing Pipeline StagesNew York 2.2.2 Unifying Instruction Types 2.2.3 Minimizing Pipeline StallsSales territories: India, Pakistan, Nepal, Bangladesh, Sri Lanka and Bhutan 2.2.4 Commercial Pipelined Processors 2.3 Deeply Pipelined ProcessorsISBN 0-07-059033-8 2.4 Summary 3 Memory and I/O SystemsPublished by Tata McGraw-Hill Publishing Company Limited,7 West Patel Nagar, New Delhi 110 008, and printed at 3.1 IntroductionShivam Printers, Delhi 110 032 3.2 Computer System Overview 3.3 Key Concepts: Latency and Bandwidth The McGraw-Hill Companies
  4. 4. MODERN PROCESSOR DESIGN TABLE OF CONTENTS 3.4 Memory Hierarchy 110 5.2.4 The Classic Tomasulo Algorithm 246 3.4.1 Components of a Modem Memory Hierarchy 111 5.2.5 Dynamic Execution Core 254 3.4.2 Temporal and Spatial Locality 113 5.2.6 Reservation Stations and Reorder Buffer 256 3.4.3 Caching and Cache Memories 115 5.2.7 Dynamic Instruction Scheduler 260 3.4.4 Main Memory 127 5.2.8 Other Register Data Flow Techniques 261 3.5 Virtual Memory Systems 136 5.3 Memory Data Flow Techniques 262 3.5.1 Demand Paging 138 5.3.1 Memory Accessing Instructions 263 3.5.2 Memory Protection 141 5.3.2 Ordering of Memory Accesses 266 3.5.3 Page Table Architectures 142 5.3.3 Load Bypassing and Load Forwarding 267 3.6 Memory Hierarchy Implementation 145 5.3.4 Other Memory Data Flow Techniques 273 3.7 Input/Output Systems 153 5.4 Summary 279 3.7.1 Types of I/O Devices 154 6 The PowerPC 620 301 3.7.2 Computer System Busses 161 6.1 Introduction 302 3.7.3 Communication with I/O Devices 165 6.2 Experimental Framework 305 3.7.4 Interaction of I/O Devices and Memory Hierarchy 168 6.3 Instruction Fetching 307 3.8 Summary 170 6.3.1 Branch Prediction 307 4 Superscalar Organization 177 6.3.2 Fetching and Speculation 309 4.1 Limitations of Scalar Pipelines 178 6.4 Instruction Dispatching 311 4.1.1 Upper Bound on Scalar Pipeline Throughput 178 6.4.1 Instruction Buffer 311 4.1.2 Inefficient Unification into a Single Pipeline 179 6.4.2 Dispatch Stalls 311 4.1.3 Performance Lost Due to a Rigid Pipeline 179 6.4.3 Dispatch Effectiveness 313 4.2 From Scalar to Superscalar Pipelines 181 6.5 Instruction Execution 316 4.2.1 Parallel Pipelines 181 6.5.1 Issue Stalls 316 4.2.2 Diversified Pipelines 184 6.5.2 Execution Parallelism 317 4.2.3 Dynamic Pipelines 186 6.5.3 Execution Latency 317 4.3 Superscalar Pipeline Overview 190 6.6 Instruction Completion 318 4.3.1 Instruction Fetching 191 6.6.1 Completion Parallelism 318 4.3.2 Instruction Decoding 195 6.6.2 Cache Effects 318 4.3.3 Instruction Dispatching 199 6.7 Conclusions and Observations 320 4.3.4 Instruction Execution 203 6.8 Bridging to the IBM POWER3 and POWER4 > 322 4.3.5 Instruction Completion and Retiring 206 6.9 Summary 324 4.4 Summary 209 7 Intels P6 Microarchitecture 329 5 Superscalar Techniques 217 7.1 Introduction 330 5.1 Instruction Flow Techniques 218 7.1.1 Basics of the P6 Microarchitecture 332 5.1.1 Program Control Flow and Control Dependences 218 7.2 Pipelining 334 5.1.2 Performance Degradation Due to Branches 219 7.2.1 In-Order Front-End Pipeline 334 5.1.3 Branch Prediction Techniques 223 7.2.2 Out-of-Order Core Pipeline 336 5.1.4 Branch Misprediction Recovery 228 7.2.3 Retirement Pipeline 337 5.1.5 Advanced Branch Prediction Techniques 231 7.3 The In-Order Front End 338 5.1.6 Other Instruction Flow Techniques 236 7.3.1 Instruction Cache and ITLB 338 5.2 Register Data Flow Techniques 237 7.3.2 Branch Prediction 341 5.2.1 Register Reuse and False Data Dependences 237 7.3.3 Instruction Decoder 343 5.2.2 Register Renaming Techniques 239 7.3.4 Register Alias Table 346 5.2.3 True Data Dependences and the Data Flow Limit 244 7.3.5 Allocator 353
  5. 5. Vi M O D E R N PROCESSOR DESIGN TABLE OF C O N T E N T S 7.4 The Out-of-Order Core 355 8.4 Verification of Superscalar Processors 439 7.4.1 Reservation Station 355 8.5 Acknowledgments 440 7.5 Retirement 357 7.5.1 The Reorder Buffer 357 9 Advanced Instruction Flow Techniques 453 7.6 Memory Subsystem 361 9.1 Introduction 453 7.6.1 Memory Access Ordering 362 9.2 Static Branch Prediction Techniques 454 7.6.2 Load Memory Operations 363 9.2.1 Single-Direction Prediction 455 7.6.3 Basic Store Memory Operations 363 9.2.2 Backwards Taken/Forwards Not-Taken 456 7.6.4 Deferring Memory Operations 363 9.2.3 Ball/Larus Heuristics 456 7.6.5 Page Faults 364 9.2.4 Profiling 457 7.7 Summary 364 9.3 Dynamic Branch Prediction Techniques 458 7.8 Acknowledgments 365 9.3.1 Basic Algorithms 459 9.3.2 Interference-Reducing Predictors 472 Surrey of Superscalar Processors 369 9.3.3 Predicting with Alternative Contexts 482 8.1 Development of Superscalar Processors 369 9.4 Hybrid Branch Predictors 491 8.1.1 Early Advances in Uniprocessor Parallelism: 9.4.1 The Tournament Predictor 491 The IBM Stretch 369 9.4.2 Static Predictor Selection 493 8.1.2 First Superscalar Design: The IBM Advanced 9.4.3 Branch Classification 494 Computer System 372 9.4.4 The Multihybrid Predictor 495 8.1.3 Instruction-Level Parallelism Studies 377 9.4.5 Prediction Fusion 496 8.1.4 By-Products of DAE: The First 9.5 Other Instruction Flow Issues and Techniques 497 Multiple-Decoding Implementations 378 9.5.1 Target Prediction 497 8.1.5 IBM Cheetah, Panther, and America 380 9.5.2 Branch Confidence Prediction 501 8.1.6 Decoupled Microarchitectures 380 9.5.3 High-Bandwidth Fetch Mechanisms 504 8.1.7 Other Efforts in the 1980s 382 9.5.4 High-Frequency Fetch Mechanisms 509 8.1.8 Wide Acceptance of Superscalar 382 9.6 Summary 512 8.2 A Classification of Recent Designs 384 RISC and CISC Retrofits 384 10 Advanced Register Data Flow Techniques 519 8.2.1 8.2.2 Speed Demons: Emphasis on Clock Cycle Time 386 10.1 Introduction 519 8.2.3 Brainiacs: Emphasis on IPC 386 10.2 Value Locality and Redundant Execution 523 8.3 Processor Descriptions 387 10.2.1 Causes of Value Locality 523 8.3.1 Compaq/DEC Alpha 387 10.2.2 Quantifying Value Locality 525 8.3.2 Hewlett-Packard PA-RISC Version 1.0 392 10.3 Exploiting Value Locality without Speculation 527 8.3.3 Hewlett-Packard PA-RISC Version 2.0 395 10.3.1 Memoization 527 8.3.4 IBM POWER 397 10.3.2 Instruction Reuse 529 8.3.5 Intel i960 402 10.3.3 Basic Block and Trace Reuse 533 8.3.6 Intel IA32—Native Approaches 405 10.3.4 Data Flow Region Reuse 534 8.3.7 Intel IA32—Decoupled Approaches 409 10.3.5 Concluding Remarks 535 8.3.8 x86-64 417 10.4 Exploiting Value Locality with Speculation 535 8.3.9 MIPS 417 10.4.1 The Weak Dependence Model 535 8.3.10 Motorola 422 10.4.2 Value Prediction 536 8.3.11 PowerPC—32-bit Architecture 424 10.4.3 The Value Prediction Unit 537 8.3.12 PowerPC—64-bit Architecture 429 10.4.4 Speculative Execution Using Predicted Values 542 8.3.13 PowerPC-AS 431 10.4.5 Performance of Value Prediction 551 8.3.14 SPARC Version 8 432 10.4.6 Concluding Remarks 553 8.345 SPARC Version 9 435 . 10.5 Summary 554
  6. 6. Viii M O D E R N PROCESSOR DESIGN 11 Executing Multiple Threads 559 Additional Resources 11.1 Introduction 559 11.2 Synchronizing Shared-Memory Threads 562 11.3 Introduction to Multiprocessor Systems 565 11.3.1 Fully Shared Memory, Unit Latency, In addition to the comprehensive coverage within the book, a number of additional and Lack of Contention 566 resources are available with Shen/Lipastis MODERN PROCESSOR DESIGN 11.3.2 Instantaneous Propagation of Writes 567 through the books website at www.mhhe.coin/shen. 11.3.3 Coherent Shared Memory 567 11.3.4 Implementing Cache Coherence 571 11.3.5 Multilevel Caches, Inclusion, and Virtual Memory 574 11.3.6 Memory Consistency 576 ! 11.3.7 The Coherent Memory Interface 581 ^HPW. p*Ur - Mfc* 1 11.3.8 Concluding Remarks 583 I Website 11.4 Explicitly Multithreaded Processors 584 11.4.1 Chip Multiprocessors 584 tatah ef Superscalar r SahaSftu! CaroMi" M a t * * Unhwilty 11.4.2 Fine-Grained Multithreading 588 Mtkk- Upaso, UnWtrelty ef Wlsceaski-Meelf 11.4.3 Coarse-Grained Multithreading 589 I walcai.^tatrwwabntafarMbdwnAn9M*vCW4..,A2003, ISBN O-07-26296i-0. This is an BMcMng raw beta edition from 11.4.4 Simultaneous Multithreading 592 I John Shan of Carnegie W "Ion Untvan » ft. Intal and Hikko I Lipase of the Umwrtitt of Wtscorwin-Madison. Dm ba_k tu <QI 11.5 Implicitly Multithreaded Processors 600 I I togeffierthenumarouirwcroartWtacturalttoV— lesfor h<rvistjng mora mstrucrxin-kva! parallelism (ILP) to ad iy» I batter processor parrormante that nave been proposed ano 11.5.1 Resolving Control Dependences 601 I I irnptamantad m rsal mad—ie*. T*~tm techniques, as wall at trie foundational prinaptes behind tham, art organized and I present-J w M n a dear framework that a^-Mvs for aasa of 11.5.2 Resolving Register Data Dependences 605 I comprehension. 11.5.3 Resolving Memory Data Dependences 607 I Tht» text is intended for an advanced compUEar arcncaccura course or a court* m tuparscelai I processor design. It is written at a (aval appropriate for sennr or first year graduata leva! 11.5.4 Concluding Remarks 610 I studertt, and can alto be used by proressii Is I WaeraL.rap^youtDa>(p)enib^5s«^rbr*>elplJresou/p^ 11.6 Executing the Same Thread 610 11.6.1 Fault Detection 611 11.6.2 Prefetching 613 11.6.3 Branch Resolution 614 11.6.4 Concluding Remarks " 615 Instructor Resources 11.7 Summary 616 • Solutions Manual—A complete set of solutions for the chapter-ending Index 623 homework problems are provided. • PowerPoint Slides—Two sets of MS PowerPoint slides, from Carnegie Mellon University and the University of Wisconsin-Madison, can be down- loaded to supplement your lecture presentations. • Figures—A complete set of figures from the book are available in eps format. These figures can be used to create your own presentations. • Sample Homework Files—A set of homework assignments with answers from Carnegie Mellon University are provided to supplement your own assignments. • Sample Exams—A set of exams with answers from Carnegie Mellon Uni- versity are also provided to supplement your own exams. • Links to www.simplescalar.com—We provide several links to the Simple- Scalar tool set, which are available free for non-commercial academic use.
  7. 7. PREFACEPreface software and run-time hardware is defined and discussed. The goal of this chapter is not to revisit in depth the traditional issues regarding ISA design, but to erect the proper framework for understanding modem processor design. Chapter 2: Pipelined ProcessorsThis book emerged from the course Superscalar Processor Design, which has beentaught at Carnegie Mellon University since 1995. Superscalar Processor Design is a This chapter focuses on the concept of pipelining, discusses instruction pipelinemezzanine course targeting seniors and first-year graduate students. Quite a few of design, and presents the performance benefits of pipelining. Pipelining is usually in-the more aggressive juniors have taken the course in the spring semester of their jun- troduced in the first computer architecture course. Pipelining provides the foundationior year. The prerequisite to this course is the Introduction to Computer Architecture for modem superscalar techniques and is presented in this chapter m a fresh andcourse. The objectives for the Superscalar Processor Design course include: (1) to unique way. We intentionally avoid the massive dumping of bar charts and graphs;teach modem processor design skills at the microarchitecture level of abstraction; instead, we focus on distilling the foundational principles of instruction pipelining.(2) to cover current microarchitecture techniques for achieving high performance viathe exploitation of instruction-level parallelism (ILP); and (3) to impart insights and Chapter 3: Memory and I/O Systemshands-on experience for the effective design of contemporary high-performance This chapter provides a larger context for the remainder of the book by including amicroprocessors for mobile, desktop, and server markets. In addition to covering the thorough grounding in the principles and mechanisms of modem memory and I/Ocontents of this book, the course contains a project component that involves the systems. Topics covered include memory hierarchies, caching, main memory de-microarchitectural design of a future-generation superscalar microprocessor. sign, virtual memory architecture, common input/output devices, processor-I/O in- During the decade of the 1990s many microarchitectural techniques for increas- teraction, and bus design and organization.ing clock frequency and harvesting more ILP to achieve better processor perfor-mance have been proposed and implemented in real machines. This book is an Chapter 4: Superscalar Organizationattempt to codify this large body of knowledge in a systematic way. These techniques This chapter introduces the main concepts and the overall organization of superscalarinclude deep pipelining, aggressive branch prediction, dynamic register renaming, processors. It provides a "big picture" view for the reader that leads smoothly into the multiple instruction dispatching and issuing, out-of-order execution, and speculative detailed discussions in the next chapters on specific superscalar techniques for achiev-load/store processing. Hundreds of research papers have been published since the ing performance. This chapter highlights only the key features of superscalar processorearly 1990s, and many of the research ideas have become reality in commercial organizations. Chapter 7 provides a detailed survey of features found in real machines.superscalar microprocessors. In this book, the numerous techniques are organized and presented within a clear framework that facilitates ease of comprehension. The Chapter 5: Superscalar Techniquesfoundational principles that underlie the plethora of techniques are highlighted. This chapter is the heart of this book and presents all the major microarchitecture tech- While the contents of this book would generally be viewed as graduate-level niques for designing contemporary superscalar processors for achieving high perfor- material, the book is intentionally written in a way that would be very accessible to mance. It classifies and presents specific techniques for enhancing instruction flow,undergraduate students. Significant effort has been spent in making seemingly register data flow, and memory data flow. This chapter attempts to organize a plethora complex techniques to appear as quite straightforward through appropriate abstrac- of techniques into a systematic framework that facilitates ease of comprehension. tion and hiding of details. The priority is to convey clearly the key concepts andfundamental principles, giving just enough details to ensure understanding of im- Chapter 6: The PowerPC 620plementation issues without massive dumping of information and quantitative data. This chapter presents a detailed analysis of the PowerPC 620 microarchitecture and The hope is that this body of knowledge can become widely possessed by not just uses it as a case study to examine many of the issues and design tradeoffs intro- microarchitects and processor designers but by most B.S. and M.S. students with duced in the previous chapters. This chapter contains extensive performance data interests in computer systems and microprocessor design. of an aggressive out-of-order design. Here is a brief summary of the chapters. Chapter 7: Intels P6 MicroarchitectureChapter 1: Processor Design This is a case study chapter on probably the most commercially successful contempo-This chapter introduces the art of processor design, the instruction set architecture rary superscalar microarchitecture. It is written by the Intel P6 design team led by Bob(ISA) as the specification of the processor, and the microarchitecture as the imple- Colwell and presents in depth the P6 microarchitecture that facilitated the implemen-mentation of the processor. The dynamic/static interface that separates compile-time tation of the Pentium Pro, Pentium n, and Pentium in microprocessors. This chapter offers the readers an opportunity to peek into the mindset of a top-notch design team.
  8. 8. xii M O D E R N PROCESSOR DESIGN PREFAC Chapter 8: Survey of Superscalar Processors Simplescalar simulation suite available from the Simplescalar website at http://www -simplescalar.com. A companion website for the book contains additional support mate- This chapter, compiled by Prof. Mark Smotherman of Clemson University, pro- rial for the instructor, including a complete set of lecture slides (www.mhhe.com/shen). vides a historical chronicle on the development of superscalar machines and a survey of existing superscalar microprocessors. The chapter was first completed in Acknowledgments 1998 and has been continuously revised and updated since then. It contains fasci- Many people have generously contributed their time, energy, and support toward nating information that cant be found elsewhere. the completion of this book. In particular, we are grateful to Bob Colwell, who is Chapter 9: Advanced Instruction Flow Techniques the lead author of Chapter 7, Intels P6 Microarchitecture. We also acknowledge his coauthors, Dave Papworth, Glenn Hinton, Mike Fetterman, and Andy Glew, This chapter provides a thorough overview of issues related to high-performance who were all key members of the historic P6 team. This chapter helps ground this instruction fetching. The topics covered include historical, currently used, and pro- textbook in practical, real-world considerations. We are also grateful to Professor posed advanced future techniques for branch prediction, as well as high-bandwidth Mark Smotherman of Clemson University, who meticulously compiled and au- and high-frequency fetch architectures like trace caches. Though not all such tech- thored Chapter 8, Survey of Superscalar Processors. This chapter documents the rich niques have yet been adopted in real machines, future designs are likely to incorpo- and varied history of superscalar processor design over the last 40 years. The guest rate at least some form of them. authors of these two chapters added a certain radiance to this textbook that we could not possibly have produced on our own. The PowerPC 620 case study in Chapter 6 Chapter 10: Advanced Register Data Flow Techniques is based on Trung Dieps Ph.D. thesis at Carnegie Mellon University. Finally, the This chapter highlights emerging microarchitectural techniques for increasing per- thorough survey of advanced instruction flow techniques in Chapter 9 was authored formance by exploiting the program characteristic of value locality. This program by Gabriel Loh, largely based on his Ph.D. thesis at Yale University. characteristic was discovered recently, and techniques ranging from software In addition, we want to thank the following professors for their detailed, in- memoization, instruction reuse, and various forms of value prediction are described sightful, and thorough review of the original manuscript The inputs from these in this chapter. Though such techniques have not yet been adopted in real machines, reviews have significantly improved the first edition of this book. future designs are likely to incorporate at least some form of them. Chapter 11: Executing Multiple Threads • David Andrews, University of Arkansas • Walid Najjar, University of California • Angelos Bilas, University of Toronto Riverside This chapter provides an introduction to thread-level parallelism (TLP), and pro- • Fred H. Carlin, University of California at • Vojin G. Oklabdzija. University ofCali vides a basic introduction to multiprocessing, cache coherence, and high-perfor- Santa Barbara at Davis mance implementations that guarantee either sequential or relaxed memory • Yinong Chen, Arizona State University • Soner Onder, Michigan Technological ordering across multiple processors. It discusses single-chip techniques like multi- • Lynn Choi, University of California at Irvine University threading and on-chip multiprocessing that also exploit thread-level parallelism. • Dan Connors. University of Colorado • Parimal Patel, University of Texas at S Finally, it visits two emerging technologies—implicit multithreading and Antonio • Karel Driesen, McGill University preexecution—that attempt to extract thread-level parallelism automatically from • Jih-Kwon Peir, University of Florida • Alan D. George, University of Florida single-threaded programs. • Gregory D. Peterson, University of • Arthur Glaser, New Jersey Institute of In summary, Chapters 1 through 5 cover fundamental concepts and foundation- Technology Tennessee al techniques. Chapters 6 through 8 present case studies and an extensive survey of • Rajiv Gupta, University of Arizona • Amir Roth, University of Pennsylvanii actual commercial superscalar processors. Chapter 9 provides a thorough overview • Vincent Hayward, McGill University • Kevin Skadron, University of Virginia of advanced instruction flow techniques, including recent developments in ad- • James Hoe, Carnegie Mellon University • Mark Smotherman, Clemson Universii vanced branch predictors. Chapters 10 and 11 should be viewed as advanced topics • Lizy Kurian John, University of Texas at Austin • Miroslav N. Velev, Georgia Institute c chapters that highlight some emerging techniques and provide an introduction to • Peter M. Kogge, University of Notre Dame Technology multiprocessor systems. • Angkul Kongmunvattana, University of • Bin Wei, Rutgers University This is the first edition of the book; An earlier beta edition was published in 2002 Nevada at Reno • Anthony S. Wojcik, Michigan State Uni with the intent of collecting feedback to help shape and hone the contents and presen- • Israel Koren, University of Massachusetts at • Ali Zaringhalam, Stevens Institute of tation of this first edition. Through the course of the development of the book, a large Amherst Technology set of homework and exam problems have been created. A subset of these problems • Ben Lee, Oregon State University • Xiaobo Zhou, University of Colorado are included at the end of each chapter. Several problems suggest the use of the • Francis Leung, Illinois Institute of Technology Colorado Springs
  9. 9. civ M O D E R N PROCESSOR DESIGN This book grew out of the course Superscalar Processor Design at Carnegie Mellon University. This course has been taught at CMU since 1995. Many teaching assis- CHAPT tants of this course have left their indelible touch in the contents of this book. They include Bryan Black, Scott Cape, Yuan Chou, Alex Dean, Trung Diep, John Faistl, Andrew Huang, Deepak Limaye, Chris Nelson, Chris Newbum, Derek Noonburg, 1 Kyle Oppenheim, Ryan Rakvic, and Bob Rychlik. Hundreds of students have taken this course at CMU; many of them provided inputs that also helped shape this book. Since 2000, Professor James Hoe at CMU has taken this course even further. We both are indebted to the nurturing we experienced while at CMU, and we hope that Processor Design this book will help perpetuate C M U s historical reputation of producing some of the best computer architects and processor designers. A draft version of this textbook has also been used at the University of Wisconsin since 2000. Some of the problems at the end of each chapter were actu- ally contributed by students at the University of Wisconsin. We appreciate their test CHAPTER OUTLINE driving of this book. 1.1 The Evolution of Microprocessors John Paul Shen, Director, 1.2 Instruction Set Processor Desigrv Microarchitecture Research, Intel Labs, Adjunct Professor, 13 Principles of Processor Performance ECE Department, Carnegie Mellon University 1.4" thstructtorf Level Paraltel Processing Mikko H. Lipasti, Assistant Professor, 15 Summary ECE Department, University of Wisconsin June 2004 References Soli Deo Gloria Homework Problems Welcome to contemporary microprocessor design. In its relatively brief lifetime of 30+ years, the microprocessor has tmdergone phenomenal advances. Its performance has improved at the astounding rate of doubling every 18 months. In the past three decades, microprocessors have been responsible for inspiring and facilitating some of the major innovations in computer systems. These innovations include embedded microcontrollers, personal computers, advanced workstations, handheld and mobile devices, application and file servers, web servers for the Internet, low-cost super- computers, and large-scale computing clusters. Currently more than 100 million microprocessors are sold each year for the mobile, desktop, and server markets. Including embedded microprocessors and microcontrollers, the total number of microprocessors shipped each year is well over one billion units. Microprocessors are instruction set processors (ISPs). An ISP executes h v ^ structions from a predefined instruction set. A microprocessors functionality is fully characterized by the instruction set that it is capable of executing. All the pro- grams that run on a microprocessor are encoded in that instruction set. This pre- defined instruction set is also called the instruction set architecture (ISA). An ISA serves as an interface between software and hardware, or between programs and processors. In terms of processor design methodology, an ISA is the specification
  10. 10. PROCESSOR DESIGI2 M O D E R N PROCESSOR DESIGN { of a design while a microprocessor or ISP is the implementation of a design. As 8-bit microprocessor. These narrow bit-width microprocessors evolved into self- with all forms of engineering design, microprocessor design is inherently a creative contained microcontrollers that were produced in huge volumes and deployed in process that involves subtle tradeoffs and requires good intuition and clever numerous embedded applications ranging from washing machines, to elevators, to insights. jet engines. The 8-bit microprocessor also became the heart of a new popular com- This book focuses on contemporary superscalar microprocessor design at the puting platform called the personal computer (PC) and ushered in the PC era of microarchitecture level. It presents existing and proposed microarchitecture tech- computing. niques in a systematic way and imparts foundational principles and insights, with The decade of the 1980s witnessed major advances in the architecture and the hope of training new microarchitects who can contribute to the effective design microarchitecture of 32-bit microprocessors. Instruction set design issues became of future-generation microprocessors. the focus of both academic and industrial researchers. The importance of having an instruction set architecture that facilitates efficient hardware implementation and that can leverage compiler optimizations was recognized. Instruction pipelin- 1.1 The Evolution of Microprocessors ing and fast cache memories became standard microarchitecture techniques. Pow- The first microprocessor, the Intel 4004, was introduced in 1971. The 4004 was a erful scientific and engineering workstations based on 32-bit microprocessors 4-bit processor consisting of approximately 2300 transistors with a clock fre- were introduced. These workstations in turn became the workhorses for the design quency of just over 100 kilohertz (kHz). Its primary application was for building of subsequent generations of even more powerful microprocessors. calculators. The year 2001 marks the thirtieth anniversary of the birth of micropro- During the decade of the 1990s, microprocessors became the most powerful cessors. High-end microprocessors, containing up to 100 million transistors with and most popular form of computers. The clock frequency of the fastest micropro- a clock frequency reaching 2 gigahertz (GHz), are now the building blocks for cessors exceeded that of the fastest supercomputers. Personal computers and work- supercomputer systems and powerful client and server systems that populate the stations became ubiquitous and essential tools for productivity and communication. Internet. Within a few years microprocessors will be clocked at close to 10 GHz Extremely aggressive microarchitecture techniques were devised to achieve un- and each will contain several hundred million transistors. precedented levels of microprocessor performance. Deeply pipelined machines The three decades of the history of microprocessors tell a truly remarkable capable of achieving extremely high clock frequencies and sustaining multiple story of technological advances in the computer industry; see Table 1.1. The evo- instructions executed per cycle became popular. Out-of-order execution of instruc- lution of the microprocessor has pretty much followed the famed Moores law, tions and aggressive branch prediction techniques were introduced to avoid or observed by Gordon Moore in 1965, that the number of devices that can be inte- reduce the number of pipeline stalls. By the end of the third decade of microproces- grated on a single piece of silicon will double roughly every 18 to 24 months. In a sors, almost all forms of computing platforms ranging from personal handheld little more than 30 years, the number of transistors in a microprocessor chip has devices to mainstream desktop and server computers to the most powerful parallel increased by more than four orders of magnitude. In that same period, micropro- and clustered computers are based on the building blocks of microprocessors. cessor performance has increased by more than five orders of magnitude. In the We are now heading into the fourth decade of microprocessors, and the past two decades, microprocessor performance has been doubling every 18 months, momentum shows no sign of abating. Most technologists agree that Moores law or an increase by a factor of 100 in each decade. Such phenomenal performance will continue to rule for at least 10 to 15 years more. By 2010, we can expect improvement is unmatched by that in any other industry. microprocessors to contain more than 1 billion transistors with clocking frequen- In each of the three decades of its existence, the microprocessor has played cies greater than 10 GHz. We can also expect new innovations in a number of major roles in the most critical advances in the computer industry. During the first areas. The current focus on instruction-level parallelism (ILP) will expand to decade, the advent of the 4-bit microprocessor quickly led to the introduction of the include thread-level parallelism (TLP) as well as memory-level parallelism (MLP). Architectural features that historically belong to large systems, for exam- ple, multiprocessors and memory hierarchies, will be implemented on a singleTable 1.1 chip. Many traditional "macroarchitecture" issues will now become microarchi-The amazing d e c a d e s of t h e evolution of microprocessors tecture issues. Power consumption will become a dominant performance impedi- ment and will require new solutions at all levels of the design hierarchy, including 1970-1980 1980-1990 1990-2000 2000-2010 fabrication process, circuit design, logic design, microarchitecture design, and Transistor count 2K-100K 100K-1M 1M-100M 100M-2B software run-time environment, in order to sustain the same rate of performance improvements that we have witnessed in the past three decades. Clock frequency 0.1-3 M H z 3-30 MHz 30MHz-lGHz 1-15 G H z The objective of this book is to introduce the fundamental principles of micro- Instructions/cycle 0.1 0.1-0.9 0.9-1.9 1.9-2.9 processor design at the microarchitecture level. Major techniques that have been
  11. 11. M O D E R N PROCESSOR DESIGN PROCESSOR DESIGN developed and deployed in the past three decades are presented in a comprehensive between input and output variables. The implementation is typically an optimized way. This book attempts to codify a large body of knowledge into a systematic two-level AND-OR design or a multilevel network of logic gates. The optimiza- framework. Concepts and techniques that may appear quite complex and difficult tion attempts to reduce the number of logic gates and the number of levels of logic to decipher are distilled into a format that is intuitive and insightful. A number of used in the d e s i g n j | p r sequential circuit design, the specification is in the form of~3 innovative techniques recently proposed by researchers are also highlighted. We state machine descriptions that include the specification of the state variables as hope this book will play a role in producing a new generation of microprocessor well as the output and next state functions. Optimization objectives include the designers who will help write the history for the fourth decade of microprocessors. reduction of the number of states and the complexity of the associated combina- tional logic circuits. Logic minimization and state minimization software tools are, • % essential. Logic and state machine simulation tools are used to assist the analysis 1.2 Instruction Set Processor Design task. These tools can verify the logic correctness of a design and determine the The focus of this book is on designing instruction set processors. Critical to an critical delay path and hence the maximum clocking rate of the state machine. instruction set processor is the instruction set architecture, which specifies the The design process for a microprocessor is more complex and less straightfor- _ functionality that must be implemented by the instruction set processor. The ISA ward. The specification of a microprocessor design is the instruction set architec-J^ plays several crucial roles in instruction set processor design. ture, which specifies a set of instructions that the microprocessor must be able to executeT^he implementation is the actual hardware design described using a hard-1 1.2.1 Digital Systems Design ware description language (HDL^The primitives of an HDL can range from l o g i c " r Any engineering design starts with a specification with the objective of obtaining a gates and flip-flops, to more complex modules, such as decoders and multiplexers, { good design or an implementation. Specification is a behavioral description of to entire functional modules, such as adders and multipliers. A design is described what is desired and answers the question "What does it do?" while implementation as a schematic, or interconnected organization, of these primitives. is a structural description of the resultant design and answers the question "How is The process of designing a modem high-endjiiicroprocessnrjypically involves ^ it constructed?" Typically the design process involves two fundamental tasks: syn- two major steps: microarchitecture design and logjc_design. Microarchitecture thesis and analysis. Synthesis attempts to find an implementation based on the design involves developing and defining the key techniques for achieving the tar- specification. Analysis examines an implementation to determine whether and geted performance. Usually a performance model is used as an analysis tool to how well it meets the specification. Synthesis is the more creative task that assess the effectiveness of these techniques. The performance model accurately searches for possible solutions and performs various tradeoffs and design optimi- models the behavior of the machine at the clock cycle granularity and is able to zations to arrive at the best solution. The critical task of analysis is essential in quantify the number of machine cycles required to execute a benchmark program. determining the correctness and effectiveness of a design; it frequently employs The end result of microarchitecture design is a high-level description of the orga- , simulation tools to perform design validation and performance evaluation. A typi- nization of the microprocessor. This description typically uses a register transfer 1 cal design process can require the traversing of the analysis-synthesis cycle language (RTL) to specify all the major modules in the machine organization and ^ n u m e r o u s times in order to arrive at the final best design; see Figure 1.1. the interactions between these modules. During the logic desjgnjtep, the RTL In digital systems design, specifications are quite rigorous and design optimi- description is successively refined by the incorporation of implementation details zations rely on the use of powerful software tools. Specification for a combina- to eventually yield the HDL description of the actual hardware design. Both the tional logic circuit takes the form of boolean functions that specify the relationship RTL and the HDL descriptions can potentially use the same description language. For example, Verilog is one such language. The primary focus of this book is on I microarchitecture design. . *. — 1.2.2 Architecture, Implementation, and Realization In a classic textbook on computer architecture by Blaauw and Brooks [1997] the authors definecfthree fundamental and distinct levels of abstraction: architecture," implementation, and realization. Arvtutecture. specifies the functional behavior of a processor. Implementation is the logical structure or organization that performs the architecture. ReaiizgtianAs the physical structure that embodies the implementation. _ Architecture is also referred to as the instruction set architecture. It specifies Figure 1.1 an instruction set that characterizes the functional behavior of an instruction set Engineering Design. processor?" All software must be mapped to or encoded in this instruction set in
  12. 12. » M O D E R N PROCESSOR DESIGN PROCESSOR DESIGN order to be executed by the processor. Every program is compiled into a sequence multiple implementations that provide different levels of cost and performance can AIM of instructions in this instruction set. Examples of some well-known architectures be simultaneously developed. A program only needs to be developed once for that a r e J B M 360, DEC VAX M o t o r o l a ^ g K ^ o w e i P Q ^ ^ r r M T A ^ Attributes ± ISA, and then it can run on all these implementations. Such program portability / associated" with an architecture include the assembly language, instruction format, significantly reduces the cost of software development and increases the longevity addressing modes, and programming model. These attributes are all part of the of software. Ujrfortunately this same benefit also makes migration to a a i e w ISA- ISA and exposed to the software as perceived by the compiler or the programmer. very difficult. Successful ISAs, or more specifically ISAs with a large software An implementation is a specific design of an architecture, and it is also± r. referred to as the microarchitecture. An architecture can have many implementa- instafledbase, tend to stay around for quite a while. Two examples are the IBM 360/370 and the Intel IA32. E X A M P1 E tions in the lifetime of that ISA. All implementations of an architecture can execute any program encoded in that ISA. Examplesof^messellrlcrMfflirurnrilementations Besides serving as a reference targeted by software developers or compilers, ISA serves as the specification for processor designers. Microprocessor design r the above-listed architecture are IBM 360/91^ VAXJ.1/780, Motorola 68040, of starts with the ISA and produces a microarchitecture that meets this specification. PowerPC 604, and~IhteTP6. AhiibmesassocTated with an implementation include Every new microarchitecture must be validated against the ISA to ensure that it per- pipeline design, cache memories, and branch predictors. Implementation or forms the functional requirements specified by the ISA. This is extremely impor- microarchitecture features are generally implemented in hardware and hidden from • tant to ensure that existing software can run correctly on the new microarchitecture. the software. To develop these features is the job of the microprocessor designer or Since the advent of computers, a wide variety of ISAs have been developed the microarchitect. and used. They differ in how operations and operands are specified. Typically an A realization of an implementation is a specific physical embodiment of a ISA defines a set of instructions called assembly instructions. Each instruction design. For a microprocessor, this physical embodiment is usually a chip or a multi- specifies an operation and one or more operands. Each ISA uniquely defines_an_ C chip package?For a given implementation, there can be various realizations of that assemblzJangugge. An assembly language program constitutes a sequence of v implementation. These realizations can vary and differ in terms of the clock fre- assembly instructions. ISAs have beenjhfferentiated according to tie_Jwmber_oi quency, cache memory capacity, bus interface, fabrication technology, packaging, operands that can be explicitly specified in eacli Instruction, for example two- etc. Attributes associated with a realization include die size, physical form factor, add>ess~cFthree^lito use an accumulator as an power, cooling, and reliability. These attributes are the concerns of the chip implicit operand. In an accumulator-based architecture, the accumulator is used as ^designer and the system designer who uses the chip. an implicit source operand and the destination. Other early ISAs assume that oper- f~ The primary focus of this book is on the implementation of modern micropro- ands are stored in a stack [last in, first out (LIFO)] structure and operations are cessors. Issues related to architecture and realization are also important. Architecture performed on the top one or two entries of the stack. Most modemJSAs assume serves as the specification for the implementation. Attributes of an architecture that operands are stored Jn^ajnultifintry xegister_fi!e, and jhjtfjdUuitlunetic and can significantly impact the design complexity and the design effort of an implemen- logical operations are performed on operands stored in the registers. Special tation. Attributes of a realization, such as die size and power, must be considered mstfiictions; sucITasToaTaHd store instmctions^ajre d e v i s e d j o ^ m p j e ^ r ^ a n d s r in the design process and used as part of the design objectives. BefweeiTthe regTstei•fileand the main memory^ Some traditional ISAs allow oper- ands to come directly from both the register file and the main memory. 1.2.3 Instruction Set Architecture £~ ISAs tend to evolve very slowly due to the inertia against recompiling or rede- Instruction set architecture plays a very crucial role and has been defined as a con- veloping software. Typically a twofold performance increase is needed before tract between the software and the hardware, or between the program and the software developers will be willing to pay the overhead to recompile their existing machine. ByTiaxingJlie-ISA as a contract, programs ajia^macbiries_can. be devel- applications. While new extensions to an existing ISA can occur from time to time oped, independently. Programs can be developed that target the ISA without to accommodate new emerging applications, the introduction of a brand new ISA requiring knowledge of the actual machine implementation. Similarly,.machines is a tall order. The development of effective compilers and operating systems for a can be designed that implement the ISA without concern for what programs will new ISA can take on the order of UH- years. The longer an ISA has been in exist- run on them. Any program written for a particular ISA should be able to run on any ence and the larger the installed base of software based on that ISA, the more diffi- machine implementing that same ISA. The notion of maintaining the same ISA cult it is to replace that ISA. One possible exception might be in certain special across multiple implementations of that ISA was first introduced with the IBM application domains where a specialized new ISA might be able to provide signif- S/360 line of computers [Amdahl et al., 1964]. r icant performance boost, such as on the order of 10-fold. C Having the ISA also ensures software portability. A program written for a par- ticular ISA can run on all the implementations of that same ISATTypically given an ISA, many implementations will be developed over the lifetime of that ISA, or r~ Unlike the glacial creep of ISA innovations, significantly new microarchitectures j ^ c a n be and have been developed every 3 to 5 years. During the 1980s, there were widespread interests in ISA design and passionate debates about what constituted the
  13. 13. PROCESSOR DESI8 M O D E R N PROCESSOR DESIGN best ISA features. However, since the 1990s the focus has shifted to the implemen- DEL -CISC -VLIW -RISC HLL Program tation and to innovative microarchitecture techniques that are applicable to most, if not all, ISAs. It is quite likely that the few ISAs that have dominated the micro- processor landscape in the past decades will continue to do so for the coming decade. On the other hand, we can expect to see radically different and innovative microarchitectures for these ISAs in the coming decade. 1.2.4 Dynamic-Static Interface So far we have discussed two critical roles played by the ISA. First, it provides a Figure 1.3 contract between the software and the hardware, which facilitates the independent Conceptual Illustration of Possible Placements of D S I in ISA D e s i g n . development of programs and machines. Second, an ISA serves as the specifica- tion for microprocessor design. All implementations must meet the requirements with the decision of what to place above the DSI and what to place below the DSI. and support the functionality specified in the ISA. In addition to these two critical For example, performance can be achieved through optimizations that are carried roles, each ISA has a third role. Inherent in the definition ofevery ISA is an a s s o c P out above the DSI by the compiler as well as through optimizations that are per- ated definition of an interface that separates what is done statically aTcorfiple time formed below the DSI in the microarchitecture. Ideally the DSI should be placed at versus what is done dynamically at run time. This interface has been called the a level that achieves the best synergy between static techniques and dynamic tech- dynamic-static interface (DSI) by Yale Part and is illustrated in Figure 1.2 [Melvin niques, i.e., leveraging the best combination of compiler complexity and hardware and Part, 1987]. ~~ complexity to obtain the desired performance. This DSI placement becomes a real | The DSI is a direct consequence of having the ISA serve as a contract between challenge because of the constantly evolving hardware technology and compiler the software and the hardware. (Jraditionatly, all the tasks and optimizations done in technology. the static domain at compile time involve the software and the compiler, and are In the history of ISA design, a number of different placements of the DSI have considered above the DSI. Conversely, all the tasks and optimizations done in the been proposed and some have led to commercially successful ISAs. A conceptual dynamic domain at run time involve the hardware and are considered below the illustration of possible placements of the DSI is shown in Figure 1.3. This figure is I D S I T A I I the architecture features are specified in the ISA and are therefore exposed intended not to be highly rigorous but simply to illustrate that the DSI can be to the software above the DSI in the static domain. On the other hand, all the imple- placed at different levels. For example, Mike Flynn has proposed placing the DSI mentation features of the microarchitecture are below the DSI and operate in the very high and doing everything below the DSI, such that a program written in a dynamic domain at run time; usually these are completely hidden from the software high-level language can be directly executed by a directly executable language and the compiler in the static domain. As stated earlier, software development can machine [Flynn and Hoevel, 1983]. A complex instruction set computer (CISC) take place above the DSI independent of the development of the microarchitecture ISA places the DSI at the traditional assembly language, or macrocode, level. In features below the DSI. contrast, a reduced instruction set computer (RISC) ISA lowers the DSI and A key issue in the design of an ISA is the placement of the DSI. In between expects to perform more of the optimizations above the DSI via the compiler. The the application program written in a high-level language at the top and the actual lowering of the DSI effectively exposes and elevates what would have been con- hardware of the machine at the bottom, there can be different levels of abstractions sidered microarchitecture features in a CISC ISA to the ISA level. The purpose of where the DSI can potentially be placed. The placement of the DSI is correlated doing this is to reduce the hardware complexity and thus achieve a much faster machine [Colwell et al„ 1985]. The DSI provides an important separation between architecture and implemen- Program (Software) tation. Violation of this separation can become problematic. As an ISA evolves and 1 1 Compiler Exposed to software "Static" extensions are added, the placement of the DSI is implicitly shifted. The lowering n of the DSI by promoting a former implementation feature to the architecture level V complexity jPSI) I Architecture Hardware Hidden in "Dynamic" effectively exposes part of the original microarchitecture to the software. This can complexity hardware facilitate optimizations by the compiler that lead to reduced hardware complexity. Machine (Hardware) However, hardware technology changes rapidly and implementations must adapt Figure 1.2 and evolve to take advantage of the technological changes. As implementation ^Qje Dynamic-Static Interface. styles and techniques change, some of the older techniques or microarchitecture

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