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This book (CS:APP) is for computer scientists, computer engineers, and others who want to be able to write better programs by learning what is going on "under the hood" of a computer system.
Out aim is to explain the enduring concepts underlying all computer systems, and to show you the concrete ways that these ideas affect the correctness, performance, and utility of your application programs.Other systems books, are written primarily from a builder's perspective, describing how to implement the hardware or the systems software, including the operating system, compiler, and networking code. This book is written from a programmer's perspective, describing how application programmers can use their knowledge of a system to write better programs. Of course, learning what a system is supposed to do provides a good first step in learning how to build one, and so this book also serves as a valuable introduction to those who go on to implement systems hardware and software.
If you study and learn the concepts in this book, you will be on your way to becoming the rare "power programmer" who knows how things work and how to fix them when they break. You will also be prepared to study specific systems topics such as compilers, computer architecture, operating systems, embedded systems, and networking.
Assumptions About the Reader's Background
The presentation in the book are based on two related machine code formats supported by Intel and its competitors, colloquially known as "x86." IA32 is the machine code that has become the de facto standard for a wide range of systems. x86-64 is an extension of IA32 to enable programs to operate on larger data and to reference a wider range of memory address. Since x86-64 systems are able to run IA32 code, both of these forms of machine code will see widespread use for the foreseeable future. We will consider how these machines execute C programs on Unix or Unix-like(such as Linux) operating systems. (To simplify our presentation, we will use the term "Unix" as an umbrella term for systems having Unix as their heritage. including Solaris, MacOS, and Linux) The text contains numerous programming examples that have been compiled and run on Linux systems. We assume that you have access to such a machine, and are able to log in and do simple things such as changing directories.
If your computer runs Microsoft Windows. you have two choices. First, you can get a copy of Linux(see www.linux.org or www.redhat.com) and install it as a "dual boot" option, so that your machine can run either operating system. Alternatively, by installing a copy of the Cygwin tools (www.cygwin.com), you can have up a Unix-like shell under Windows and have an environment very close to that provided by Linux. Not all features of Linux are available under Cygwin, however.
We also assume that you have some familiarity with C or C++. If you only prior experience is with Java. the transition will require more effort on your part, but we will help you. Java and C share similar syntax and control statements. However, there are aspects of C, particularly pointers, explicit dynamic memory allocation, and formatted I/O, that do not exist in Java. Fortunately, C is a small language, and it is clearly and beautifully described in the classic "K&R" text by Brian Kernighan and Dennis Ritchie[58]. Regardless of your programming background, consider K&R an essential part of your personal systems library.
Several of the early chapters in the book explore the interactions between C programs and their machine language counterparts. The machine language examples were all generated by the GNU GCC compile running on IA32 and X86-64 processors. We do not assume any prior experience with hardware, machine language, or assembly-language programming.
New to C? Advice on the C Programming Language
To help readers whose background in C programming is weak (or nonexistent), we have also included these special notes to highlight features that are especially important in C. We assume you are familiar with C++ or Java, End.
How to Read the Book
Learning how computer systems work from a programmer's perspective is great fun, mainly because it can be done so actively. Whenever you learn some new thing, you can try it out right away and see the result first hand. In fact, we believe that the only way to learn systems is to do systems, either working concrete problems, or writing and running programs on real systems.
This theme pervades the entire book. When a new concept is introduced, it is followed in the text by one or more practice problems that you should work immediately to test your understanding. Solutions to the practice problems are at the end of each chapter. As you read, try to solve each problem on your own, and then check the solution to make sure you are on the right track. Each chapter is followed by a set of homework problems of varying difficulty. Your instructor has the solutions to the homework problems in an Instructor's Manual. For each homework problem, we show a rating of the amount of effort we feel it will require:
Should require just a few minutes. Little or no programming required.
Might require up to 20 minutes. Often involves writing and testing some code. Many of these are derived from problems we have given on exams.
Requires a significant effort, perhaps 1-2 hours. Generally involves writing and testing a significant amount of code.
A lab assignment, requiring up to 10 hours of effort.
Each code example in the text was formatted directly, without any manual intervention, from a C program compiled with GCC and tested on a Linux system. Of course, your system may have a different version of GCC, or a different compiler altogether, and so your compiler might generate different machine code, but the overall behavior should be the same. All of the source code is available from the CS:APP Web page at csapp.csNaNu.edu. In the text, the file names of the source programs are documented in horizontal bars that surround the formatted code. For example, the program in Figure 1can be found in the file hello.c in directory code/intro/. We encourage you to try running the example programs on your system as you encounter them.
To avoid having a book that is overwhelming, both in bulk and in content, we have created a number of web asides containing material that supplements the main presentation of the book. These asides are referenced within the book with a notation of the form CHAP:TOP, where CHAP is a short encoding of the chapter subject, and TOP is short code for the topic that is covered. For example, Web Aside DATA:BOOL contains supplementary material on Boolean algebra for the presentation on data representations in Chapter 2, while Web Aside ARCH:VLOG contains material on describing processor designs using the Verilog hardware description language, supplementing the presentation of processor design in Chapter 4. All of these web asides are available from the CS:APP web page.
Aside: What is an aside?
You will encounter asides of this form throughout the text. Asides are parenthetical remarks that give you some additional insight into the current topic. Asides serve a number of purpose. Some are little history lessons. For example, where did C, Linux, and the Internet come from? Other asides are meant to clarify ideas that students often find confusing. For example, what is the difference between a cache line, set, and block? Other asides give real-world examples. For example, how a floating-point error crashed a French rocket, or what the geometry of a real IBM disk drive looks like. Finally, some asides are just fun stuff. For example, what is a "hoinky"? End Aside.
Origins of the Book
The book stems from an introductory course that we developed at Carnegie Mellon University in the Fall of 1998, called 15-213: Introduction to Computer Systems (ICS)[14]. The ICS course has been taught every semester since then, each time to about 150-250 students, ranging from sophomores to masters degree students and with a wide variety of majors. It is a required course for all undergraduates in the CS and ECE departments at Carnegie Mellon, and it has become a prerequisite for most upper-level systems courses.
The idea with ICS was to introduce students to computers in a different way. Few of our students would have the opportunity to build a computer system. On the other hand, most students, including all computer scientists and computer engineers, will be required to use and program computers on a daily basis. So we decided to teach about systems from the point of view of the programmer, using the following filter: We would cover a topic only if it affected the performance, correctness, or utility of user-level C programs.
For example, topics such as hardware adder and bus designs were out. Topics such as machine language were in, but instead of focusing on how to write assembly language by hand, we would look at how C constructs such as pointers, loops, procedure calls and returns, and switch statements are translated by the compiler. Further, we would take a broader and more wholistic view of the system as both hardware and systems software, covering such topics as linking, loading, processes, signals, performance optimization, I/O, and network and concurrent programming.
This approach allowed us to teach the ICS course in a way that is practical, concrete, hands-on, and exciting for the students. The response from our students and faculty colleagues was immediate and overwhelmingly positive, and we realized that others outside of CMU might benefit from using our approach. Hence this book, which we developed from the ICS lecture notes, and which we have now revised to reflect changes in technology and how computer systems are implemented.
Overview of the Book
The CS:APP book consists of 12 chapter designed to capture the core ideas in computer systems:
Chapter 1: A Tour of Computer Systems. This chapter introduces the major ideas and themes in computer systems by tracing the life cycle of a simple "hello world" program.
Chapter 2: Representing and Manipulating Information. We cover computer arithmetic, emphasizing the properties of unsigned and two's complement number representations that affect programmers. We consider how numbers are represented and therefore what range of values can be encoded for a given word size. We consider the effect of casting between signed and unsigned numbers. We cover the mathematical properties of arithmetic operations. Students are surprised to learn that the (two's complement) sum or product of two positive numbers can be negative. On the other hand, two's complement arithmetic satisfies the algebraic properties of a ring, and hence a compiler can safely transform multiplication by a constant into a sequence of shifts and adds. We use the bit-level operations of C to demonstrate the principles and applications of Boolean algebra. We cover the IEEE floating point format in terms of how it represents values and the mathematical properties of floating point operations.Having a solid understanding of computer arithmetic is critical to writing reliable programs. For example, programmers and compilers cannot replace the expression (x < y) with (x - y < 0) due to the possibility of overflow. They cannot even replace it with the expression (-y < -x) due to the asymmetric range of negative and positive numbers in the two's complement representation. Arithmetic overflow is a common source of programming errors and security vulnerabilities, yet few other books cover the properties of computer arithmetic from a programmer's perspective..
Chapter 3: Machine-Level Representation of Programs. We teach students how to read the IA32 and x86-64 assembly language generated by a C compiler. We cover the basic instruction patterns generated for different control constructs, such as conditionals, loops, and switch statements. We cover the implementation of procedures, including stack allocation, register usage conventions and parameter passing. We cover the way different data structures such as structures, unions, and arrays are allocated and accessed. We also use the machine-level view of programs as a way to understand how their programs are represented on the machine. One certain benefit is that develop a thorough and concrete understanding of pointers.
Chapter 4: Processor Architecture. This chapter covers basic combinational and sequential logic elements and then shows how these elements can be combined in a datapath that executes a simplified subset of the IA32 instruction set called "Y86". We begin with the design of a single-cycle, nonpipelined datapath, which we extend into a five-stage pipelined design. The control logic for the processor designs are described using a simple hardware description language called HCL. Hardware designs written in HCL can be compiled and linked into simulators provided with the textbook, and they can be used to generate Verilog descriptions suitable for synthesis into working hardware.
Chapter 5: Optimizing Program Performance. In this chapter we introduce a number of techniques for improving code performance, with the idea being that programmers learn to write their C code in such a way that a compiler can then generate efficient machine code. We start with transformations that reduce the work to be done by a program and hence should be standard practice when writing any program for any machine. We then progress to transformations that enhance the degree of instruction-level parallelism in the generated machine code, thereby improving their performance on modern "superscalar" processors. To motivate these transformation, we introduce a simple operational model of how modern out-of-order processors work and show how to measure the potential performance of a program in terms of the critical paths through a graphical representation of a program.
Chapter 6: The Memory Hierarchy. The memory system is one of the most visible parts of a computer system to application programmers. To this point, the students have relied on a conceptual model of the memory system as a linear array with uniform access times. In practice, a memory system is a hierarchy of storage devices with different capacities, costs, and access times. We cover the different types of RAM and ROM memories and the geometry and organization of magnetic-disk and solid state drives. We describe how these storage devices are arranged in a hierarchy. We show how this hierarchy is made possible by locality of reference. We make these ideas concrete by introducing a unique view of a memory system as a "memory mountain" with ridges of temporal locality and slopes of spatial locality. Finally, we show students how to improve the performance of application programs by improving their temporal and spatial locality.
Chapter 7: Linking. This chapter covers both static and dynamic linking, including the ideas of relocatable and executable object files, symbol resolution, relocation, static libraries, shared object libraries, and position-independent code. Linking is not covered in most systems texts, but we cover it for several reasons. First, some of the most confusing errors that students can encounter are related to glitches during linking, especially for large software packages. Second, the object files produced by linkers are tied to concepts such as loading, virtual memory, and memory mapping.
Chapter 8: Exceptional Control Flow: In this part of the course we break the single-program model by introducing the general concept of exceptional control flow (i.e., changes in control flow that are outside the normal branches and procedure calls). We cover examples of exceptional control flow that exist at all levels of the system, from low-level hardware exceptions and interrupts, to context switches between concurrent processes, to abrupt changes in control flow caused by the delivery of Unix signals, to the nonlocal jumps in C that break the stack discipline. This is the part of the book where we introduce students to the fundamental idea of a process. Students learn how processes work and how they can be created and manipulated from application programs. We show them how application programmers can make use of multiple processes via Unix system calls. When students finish this chapter, they are able to write a Unix shell with job control. It is also their first introduction to the nondeterministic behavior that arises with arises with concurrent program execution.
Chapter 9: Virtual Memory: Our presentation of the virtual memory system seeks to give students some understanding of how it works and its characteristics. We want students to know how it is that the different simultaneous processes can each use an identical range of addresses, sharing some pages but having individual copies of others. We also cover issues involved in managing and manipulating virtual memory. In particular, we cover the operation of storage allocators such as the Unix malloc and free operations. Covering this material serves several purposes. It reinforces the concept that the virtual memory space is just an array of bytes that the program can subdivide into different storage units. It helps students understand the effects of programs containing memory referencing errors such as storage leaks and invalid pointer references. Finally, many application programmers write their own storage allocators optimized toward the needs and characteristics of the application. This chapter, more than any other, demonstrates the benefit of covering both the hardware and the software aspects of computer systems in a unified way. Traditional computer architecture and operating systems texts cover only part of the virtual memory story.