2010年度 春学期

System Software / Operating Systems

2010年度春学期 火曜日2時限
科目コード: 60730
担当: Rodney Van Meter
E-mail: rdv@sfc.keio.ac.jp

第2回 4月20日 システムコール
Lecture 2, April 20: System Calls

Today's Picture

Fast food!


Get Google to translate this page. このページをグーグルに翻訳をまかせよう!

Why Study Operating Systems?

Why am I bring this up now? Well, some industry pundits have asserted that the operating system no longer matters.

The OS no longer matters Browser market
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Review of last week: definition of an operating system, history of operating systems

The four key roles of an operating system are:

Let's go back to last week and look at some of the history we didn't have time to finish.

Recall that mainframes went from bare metal, to batch systems, to unprotected multiprogramming, to protected multi-user systems. PC operating systems followed the same pattern, and embedded OSes for devices such as PDAs are following the same pattern.

最初にメインフレームとして利用されていたプログラマブルコンピューターは、 金属自身で(つまり部品を切り替えて)プログラムをつくっていました. 次にバッチシステムが開発されました。 そのあと、保護されないマルチプログラミング用のOSが出現し, 次に保護のあるOSをできました。 PC用と組み込み用のOSは同じ道をたどってきました.

Finally, remember that we discussed the light cone of information and its impact on systems, where we must consider that information is always distributed (and therefore out of date), and that systems are definitely concurrent.

最後に 光円錐の影響で、今の知っていることが古いでしょう。そのふたつの概念:全 ての情報は分散である、とすべてのことは同時で行う可能性がある。

Thinking Like a Researcher

I asked you to read the Levin and Redell paper. That paper will help you learn how to understand a computer system, as well as the larger issue of learning how to both analyze existing research, as well as how to present your own research. Thinking about this issue will help with writing your master's thesis.

前回, Levin and Redell paperを読むという宿題を出しました. この文章はコンピュータシステムへの理解だけではなく, どのように既存研究を分析するか, どのように自分の研究を提供するかといった知識を与えてくれるでしょう. これらの問題に取り組むことはあなたの修士論文の助けになるでしょう.

Everyone always wants to describe what they did, rather than why they did it. A good paper must describe why, and, ultimately, whether or not what you did worked, and how you know.

誰もが何をしたのかを話したがり, 何故それが必要かを説明しません. そして,どういう結果が出て,どうしてそれを知ったのかを説明しないのです.

When you present a project, or an idea, to me, I expect the following:

皆さんはプロジェクトあるいはアイデアを私に提案するとき, 以下のことを考えます.

You will find a slightly updated version of the questions from Levin and Redell here.

また,皆さんはLevin と Redellからの若干のアップデートされた質問を ここ.


Types of Operating Systems

Very briefly, let us note that there are a number of types of operating systems. During this class, we will generally focus on multi-purpose PC operating systems, but many of the principles in today's systems inherit from older systems, and many small operating systems will eventually include many of these features.

非常に簡単にではありますが,いくつかのOSの種類を説明します. 本抗議では,多目的PCのOS(訳注:皆さんが普段使うようなOSです) について解決しますが,世の中には多くのOSが存在します. それらの多くの特徴はほとんど変わらず, 新しいOSは従来のOSから大きく変わるものではないです.

Fast food!
Fast food!

The two photos above demonstrate two critical concepts in operating systems:

We will see these two concepts again in later lectures.

The Basic Idea: Kernel and Processes

Modern general-purpose operating systems user a structure involving a kernel and user processes. The kernel executes the functions we described above, managing the system on behalf of all its users. The application programs that the users want to execute are run in processes. The kernel is protected from interference by applications, so that applications cannot destroy the disk file system, for example. Applications are also protected from one another, so that they can keep secrets, and so that each application can pretend that it owns the entire computer, which dramatically simplifies creation of application programs.

System Calls

System calls provide the defined interface between the operating system and user application programs. The system calls generally provide several types of services:

システムコールはOSとユーザランドプログラムのインタフェースを 定義します. 基本的なシステムコールは以下のように分類されます.

In Unix and operating systems influenced by Unix, the last category is usually implemented using the file I/O interface, but there are often additional system calls that must be made to, for example, correctly open a network connection.

Unix,あるいはその影響を強く受けているOS上で, 最後のカテゴリは通常,ファイルI/Oです. しかし,通常いくつかのシステムコールがファイルI/Oには必要となります. たとえば,ネットワーク接続が正常に開いているのか,などです.

We discussed resource management, data movement and naming as critical functions of an OS. These system calls are an application's means of requesting that the OS perform one of these functions.

前回の授業では, リソース管理データ転送名前空間 という重要なOSの要素について扱ってきました. これらのシステムコースは,アプリケーションの要求する OSの機能を提供する要素です.

System calls are different from library functions. Many OS environments (whether the library is or is not a part of the OS itself is arguable) provide libraries of functions, often standardized, that application programmers may wish to use. What's the difference? Library functions start by running in user space, though they may also make system calls on behalf of the user process. Library functions perform actions like string formatting, calculating math functions, etc. System calls generally involve access to things that must be protected: disk drives, files on those disk drives, process control structures, etc.

システムコールは通常のライブラリとは違う機能を持ちます. 一般的な多くのOS環境はアプリケーションプログラマが使用したがる 標準化されているライブラリの機能を提供します. (ライブラリがOSの機能かどうかについては議論の余地があるけれども) では,システムコールとライブラリは何が違うのでしょうか? ライブラリはシステムコールを呼ぶものであってもユーザランドからスタートします. ライブラリファンクションは, 文字列の成型や,数式の計算などを処理します. システムコールは一般的に,ディスクドライバ,その上のファイル,保護された領域など にアクセスするために呼ばれます.

Last week we discussed naming as a critical function of an OS. Humans use a readable form of the name of a system call, such as write(). However, the operating system itself does not actually use the human-readable names. In this case, the C compiler uses header files as a means to translate the human-readable name into a machine-readable one.

前回,我々はネーミングというOSの重要な機能について議論しました. 通常,人間は可読なシステムコールの名前(たとえば,writeのような)を用います. しかし,OS自身は人間が可読な名前を持ちません. つまり,Cコンパイラのヘッダファイルが,マシンが用いる名前と人間の用いる名前を 変換しているのです.

Here is part of the list in a Linux 2.6.19 kernel:


[rdv@2 ~]$ more /usr/include/asm/unistd.h 
#ifndef _ASM_I386_UNISTD_H_
#define _ASM_I386_UNISTD_H_

 * This file contains the system call numbers.

#define __NR_restart_syscall      0
#define __NR_exit                 1
#define __NR_fork                 2
#define __NR_read                 3
#define __NR_write                4
#define __NR_open                 5
#define __NR_close                6
#define __NR_move_pages         317
#define __NR_getcpu             318
#define __NR_epoll_pwait        319

#endif /* _ASM_I386_UNISTD_H_ */
...that's it. In Linux, there are 319 system calls that do everything.


The execution of a system call occurs in several phases:


The application side may or may not wrap the system call in a library routine. The compiler will take care of most of that for you.

アプリケーション側では,ライブラリの中でシステムコールに飛ぶかもしれませんし,飛ばないかもしれません. コンパイルではこのことを非常に注意深く扱います.

Note that this same essential structure is followed for making calls to remote servers, as well as to local system services. Again, our principles of distributed and concurrent actions and information (the light cone) applies.

このような本質的な構造が, リモートサーバとの接続もローカルサーバのように扱われているのです. 繰り返しますが,我々の原則である分散かつ並列に処理可能である, ということが実現されているのです.

Most system calls are synchronous; your application program stops until the OS completes the call and returns (or decides that it cannot complete, in which case an error is returned).

ほとんどのシステムコールは同期型です. 同期型とは,アプリケーションがシステムコールの動作を完了するまで 停止するということです. (もちろん場合によってはエラーを返すこともあります)

Looking in a little more detail at the setuid system call:


which will expand to:

  subl $4,%exp
  pushl %ebx
  movzwl 12(%esp),%eax
  movl %eax,4(%esp)
  movl $23,%eax
  movl 4(%esp),%ebx
  int $0x80
  movl %eax,%edx
  testl %edx,%edx
  jge L2
  negl %edx
  movl %edx,_errno
  movl $-1,%eax
  popl %ebx
  addl $4,%esp
  movl %edx,%eax
  popl %ebx
  addl $4,%esp
(This code is a little old, but illustrates the necessary points.)


So, How Big is an Operating System?

Number of lines of C in the Linux kernel distribution, 2.6.19:


Printing that out would require a hundred thousand pages! The original Unix kernel was 5,000 lines; the current Linux kernel is more than 17,000 files! How is Linux following the general dictum to KISS: keep it simple, stupid? Well, partly because the breakdown is actually like this (C files only):

なんということだ!これを印刷したら10万ページにもなるじゃないか! 最初のUnixカーネルは5000行だったというのに今のLinuxカーネルは17,000ファイルもある! LinuxはKISS: keep it simple, stupidという基礎的な概念を守っていてもいいのにね.

More than half of the total volume, and a third of the total number of files, is in drivers for various types of devices, most of which are not used on any given system.

全体量の半分以上と3分の一ものだいるが, さまざまなデバイスのために用意されています. そして,そのほとんどが実際にシステムでは使われないのです.

The Linux kernel hackers are generally reasonable about maintaining comments, so consider these numbers to be high by almost a factor of two. Note also that this does not include any of the following:

Linux Kernelのハッカーたちは,メンテナンスコメントの中で, この大きな2つのサイズのコードについて話します. ただし,以下のものは含まれていないことに注意してください.

The original Unix system also did not include:


In a system of this size, well-defined, high-performance interfaces with minimal hidden assumptions are critical. However, in keeping with Lampson's command to throw the first implementation away, almost all of Linux, including the virtual memory subsystem, has been rewritten frequently (as has Windows!). We will talk more about how to engineer such systems (including "The Cathedral and the Bazaar") later in the term.

このレベルのシステムでは,十分な定義がされた最低限の過程を隠した 高機能なインタフェースが極めて重要です. しかしながら,最初の実装を捨てるためにLampsonのコマンドを守り, ほぼすべてバーチャルメモリサブシステムを持つLinuxは, よくに(windowsのように)書き換えられている. 本抗議では,これらのシステムをどのようにエンジニアリングしていくか を話す予定です.

Term Projects

The goal of your term project is to learn how one particular part of an operating system works, without having to hack the kernel and threaten the stability of your system. You should be able to conduct your term project on your own PC safely.

Schedule and Grading

The schedule is as follows:

That gives you seven to eight weeks to actually implement your project. I would expect that your project will take 30-40 hours total, including writing and debugging code, taking data, analyzing the data, and writing up a report of the results. This is actually not very much time for a project, so they must be sized appropriately.

Your grade on your project will be 40% of your total grade, split 10% for the mid-term progress review June 15, and 30% for the final evaluation. The things I will look for are those detailed in the Levin and Redell paper. Because many of your projects will involve performance measurements, I also expect data with error bars and carefully designed experiments. One great book on the topic is Jain, The Art of Computer Systems Performance Analysis, but there are probably also good books available in Japanese.


Some people have asked what language they must write their program(s) in. I don't care what language you use; I care what you learn about the operating system. In order to learn about the OS, a low-overhead, predictable, compiled language is probably preferable. C would be the obvious choice. Interpreted scripting languages are probably bad choices.

Likewise, there is no requirement to perform your project on a particular operating system. Class lectures will focus on principles highlighted by Unix and Linux examples, as above. The importance of Unix in the history of operating systems cannot be overstated, and Linux and MacOS are (arguably) its most vibrant current implementations; students must have some familiarity with the basic ideas of Unix. However, if your OS of choice is Windows, you will learn a great deal by comparing concepts from lectures and the book with what you see on your Windows machine. One obvious advantage of Linux is the easy availability of source code, turning "black box" experiments into "white box" ones.

The first step in either research or development is to identify a problem. Most of these projects will help you carefully characterize a system problem that you might want to attack more thoroughly in research later.

Project Suggestions

The ideal project for this class is probably a performance measurement of the system. Examples include:

Some of these projects will be more difficult on Windows systems, due to the lack of source code. Some of the measurements will also be difficult if you cannot flush the file system cache, either by unmounting the file system or rebooting the machine.

I will ask you not just what happened, but why it happened. In most cases, you should be able to point to some kernel source code, or a conference paper, design document, book, or web site that supports your understanding of why the system behaves the way it does. For most of these projects, I will also ask you to predict what will happen as technology continues to improve: modest improvements in clock speed and disk speed, significant increases in number of processors and disk capacity.

Previous Years' Projects

Some of the projects from previous years:


The OS class at Wisconsin works on a project basis. Here are links to some prior years' projects:


There will be readings from the textbook assigned almost every week. Those are for your own benefit; you are not required to keep notes on them. There will also be five papers (in English) that are assigned reading, for which you are required to post a summary and comments on your blog. They will be assigned through the semester, but FYI, a summary:


This week's homework. You probably want to do the second problem first.

  1. This week we have talked about system calls. Take your "Hello, world" program from last week and produce the assembler output from the compiler. Post the assembler file on your blog.
    1. Identify the instruction that calls the string output function. Is it a library function or a system call? How can you tell?
    2. Identify the arguments to the function. How many are there?
    3. If your string output function is a library function rather than system call, can you find the function and instruction that actually does the trap into the kernel?
  2. Unfortunately, even "Hello, world" is a little bit complicated. Take the following even shorter program and repeat the above exercise.
    #include <unistd.h>
    int main() {
      char *buf = "123";
            write(1, buf, 3);
            return 0;
    (Hint: if you are using Linux, some of the information you need to complete this exercise is above in the lecture notes.)
  3. Find a list of all of the system calls on the OS of your choice, and post it (or a link) on your blog. How many are there?
  4. Report on the "Hints for Computer System Design" paper on your blog.
  5. How long did it take you to complete this homework?

Next Lecture

Next lecture:

第3回 4月27日 プロセスとスレッド
Lecture 3, April 27: Processes and Threads

Readings for Next Week and Followup for This Week

Follow-up readings for this week:

Readings for next week:

その他 Additional Information