OperatingSystems:ThreeEasyPieces国外著名大学研究生操作系统课本(0.90版本)_OperatingSystems:ThreeEasyPieces

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A Dialogue on the Book

Professor: Welcome to this book! It’s called Operating Systems in Three Easy

Pieces, and I am here to teach you the things you need to know about operating

systems. I am called “Professor”; who are you?

Student: Hi Professor! I am called “Student”, as you might have guesse d. And

I am here and ready to learn!

Professor: Sounds good. Any questions?

Student: Sure! Why is it called “Three Easy Pieces”?

Professor: That’s an easy one. Well, you see, there are these great lectures on

Physics by Richard Feynman...

Student: Oh! The guy who wrote “Surely You’re Joking, Mr. Feynman”, right?

Great book! Is this going to be hilarious like that book was?

Professor: Um... well, no. That book was great, and I’m glad you’ve read it.

Hopefully this book is more like his notes on Physics. Some of the basics were

summed u p in a book called “Six Easy Pieces”. He was talking about Physics;

we’re going to do Three Easy Pieces on the ﬁne topic of Operating Syste ms. This

is appropriate, as Operating Systems are about half as hard as Physics.

Student: Well, I liked physics, so that is probably good. What are those pieces?

Professor: They are the three key ideas we’re going to learn about: virtualiza-

tion, concurrency, and persistence. In learning about these ideas, we’ll learn

all about how an operating system works, including how it decides what program

to run next on a CPU, how it handles memory overload in a virtual memory sys-

tem, how virtual machine monitors work, how to manage information on disks,

and even a little about how to build a distributed system that works when parts

have failed. That sort of stuff.

Student: I have no idea what you’re talking about, really.

Professor: Good! That means you are in the right class.

Student: I have another question: what’s the best way to learn this stuff?

Professor: Excellent query! Well, each person needs to ﬁgure this out on their

2 A DIALOGUE ON THE BOOK

own, of course, but here is what I would do: go to class, to hear the professor

introduce the material. Then, at the end of every week, read these notes, to help

the ideas sink into your head a bit better. Of course, some time later (hint: before

the exam!), read the notes again to ﬁrm up your knowledge. Of course, your pro-

fessor will no doubt assign some homeworks and projects, so you should do t hose;

in particular, doing projects where you write real code to solve real problems is

the best way to put the ideas within these notes into action. As Confucius said...

Student: Oh, I know! ’I hear and I forget. I see and I remember. I do and I

understand.’ Or something like that.

Professor: (surprised) How did you know what I was going to say?!

Student: It seemed to follow. Also, I am a big fan of Confucius, and an even

bigger fan of Xunzi, who actually is a better source for this quote

Professor: (stunned) Well, I think we are going to get along just ﬁne! Just ﬁne

indeed.

Student: Professor – just one more question, if I may. What are these dialogues

for? I mean, isn’t this just supposed to be a book? Why not present the material

directly?

Professor: Ah, good question, good question! Well, I think it is sometimes

useful to pull yourself outside of a narrative and think a bit; these dialogues are

those times. So you and I are going to work together to make sense of all of these

pretty complex ideas. Are you up for it?

Student: So we have to think? Well, I’m up for that. I mean, what else do I have

to do anyhow? It’s not like I have much of a life outside of this book.

Professor: Me neither, sadly. So let’s get to work!

According to this website (http://www.barrypopik.com/index.php/new

york city/

entry/tell

me and i forget teach me and i may remember involve me and i will lear/),

Confucian philosopher Xunzi said “Not having heard something is not as good as having

heard it; having heard it is not as good as having seen it; having seen it is not as good as

knowing it; knowing it is not as good as putting it into practice.” Later on, the wisdom got

attached to Confucius for some reason. Thanks to Jiao Dong (Rutgers) for telling us!

OPERATING

SYSTEMS

[VERSION 0.90] WWW.OSTEP.ORG

Introduction to Operating Systems

If you are taking an undergraduate operating systems course, you should

already have some idea of what a computer program does when it runs.

If not, this book (and the corresponding cour se) is going to be difﬁcult

— so you should probably stop reading this book, or run to the nearest

bookstore and quickly consume the necessary background material be-

fore continuing (both Patt/Patel [PP03] and particularly Bryant/O’Hallaron

[BOH10] are pretty great books).

So what happens when a program runs?

Well, a running program does one very simple thing: it executes in-

structions. Many milli ons (and these days, even billions) of times ev-

ery second, the processor fetches an instruction from memory, decodes

it (i.e., ﬁgures out which instruction this is), and executes it (i.e., it does

the thing that it is supposed to do, like add two numbers together, access

memory, check a condition, jump to a function, and so forth). After it is

done with this instruction, the processor moves on to the next instruction,

and so on, and so on, until the program ﬁnally completes

Thus, we have just described the basics of the Von Neumann model of

computing

. Sounds simple, right? But in this class, we will be learning

that while a program runs, a lot of other wild things are going on with

the primary goal of making the system easy to use.

There is a body of software, in fact, that is responsible for making it

easy to run programs (even allowing you to seemingly run many at the

same time), allowing programs t o share memory, enabling programs to

interact with devices, and other fun stuff like that. That body of software

Of course, modern processors do many bizarre and frightening things underneath the

hood to make programs run faster, e.g., executing multiple instructions at once, and even issu-

ing and completing them out of order! But that is not our concern here; we are just concerned

with the simple model most programs assume: that instructions seemingly execute one at a

time, in an orderly and sequential fashion.

Von Neumann was one of the early pioneers of computing systems. He also did pioneer-

ing work on game theory and atomic bombs, and played in the NBA for six years. OK, one of

those things isn’t true.

2 INTRODUCTION TO OPERATING SYSTEMS

THE CRUX OF THE PROBLEM:

HOW TO VIRTUALIZE RESOURCES

One central question we will answer in this book is q uite simple: how

does the operating system virtualize resources? This is the crux of our

problem. Why the OS does this is not the main question, as the answer

should be obvious: it makes the system easier to use. Thus, we focus on

the how: what mechanisms and policies are implemented by the OS to

attain virtualization? How does the OS do so efﬁciently? What hardware

support is needed?

We will use the “crux of the problem”, in shaded boxes such as this one,

as a way to call out speciﬁc problems we are trying to solve in building

an operating system. Thus, within a note on a particular topic, you may

ﬁnd one or more cruces (yes, this is the proper plural) which highlight the

problem. The details within the chapter, of course, present the solution,

or at least the basic parameters of a solution.

is called the operating system (OS)

, as it is in charge of making sure the

system operates correctly and efﬁciently in an easy-to-use m anner.

The primary way the OS does this is through a general technique that

we call virtualization. That is, the OS takes a physical resource (such as

the processor, or memory, or a disk) and transforms it into a more gen-

eral, powerful, and easy-to-use virtual form of itself. Thus, we sometimes

refer to the operating system as a virtual machine.

Of course, in order to allow users to tell the OS what to do and thus

make use of t he features of the virtual machine (such as running a pro-

gram, or allocating memory, or accessing a ﬁle), the OS also provides

some interfaces (APIs) that you can call. A typical OS, in fact, exports

a few hundred system calls that are available to applications. Because

the OS provides these calls to run programs, access memory and devices,

and other related actions, we also sometimes say that the OS provides a

standard library to applications.

Finally, because virtualization allows many programs to run (thus shar-

ing the CPU), and many programs to concurrently access their own in-

structions and data (thus sharing memory), and many programs t o access

devices (thus sharing disks and so forth), the OS is sometimes known as

a resource manager. Each of the CPU, memory, and disk is a resource

of the system; it is thus the operating system’s role to manage those re-

sources, doing so efﬁciently or fairly or indeed with many other possible

goals in mind. To understand the role of the OS a little bit better, let’s take

a look at some examples.

Another early name for the OS was the supervisor or even the master control program.

Apparently, the latter sounded a little overzealous (see the movie Tron for details) and thus,

thankfully, “operating system” caught on instead.

OPERATING

SYSTEMS

[VERSION 0.92] WWW.OSTEP.ORG

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