The job of an operating system is to share a computer among multiple programs and to provide a more useful set of services than the hardware alone supports. The operating system manages and abstracts the low-level hardware, so that, for example, a word processor need not concern itself with which type of disk hardware is being used. It also shares the hardware among multiple programs so that they run (or appear to run) at the same time. Finally, operating systems provide controlled ways for programs to interact, so that they can share data or work together.
操作系统的任务是在多个程序之间共享一台计算机，并提供比硬件本身支持的更有用的服务。操作系统管理和抽象底层硬件，例如文字处理器不需要关心使用哪种类型的磁盘硬件。一个操作系统在多个程序之间共享硬件，这样它们就可以(或者看起来可以)同时运行。最后，操作系统为程序提供了可控的交互方式，这样它们就可以共享数据或者一起工作。
An operating system provides services to user programs through an interface.Designing a good interface turns out to be difficult. On the one hand, we would like the interface to be simple and narrow because that makes it easier to get the implementation right. On the other hand, we may be tempted to offer many sophisticated features to applications. The trick in resolving this tension is to design interfaces that rely on a few mechanisms that can be combined to provide much generality.
操作系统通过接口向用户程序提供服务。设计良好的接口是很困难的。一方面，我们希望接口简单明了，因为这样更利于正确使用。另一方面，我们可能倾向于为应用程序提供许多复杂的特性。解决这个问题的诀窍在于设计接口时，依赖一些可结合的机制，以此来提供更好的通用性。
This book uses a single operating system as a concrete example to illustrate operating system concepts. That operating system, xv6, provides the basic interfaces introduced by Ken Thompson and Dennis Ritchie’s Unix operating system, as well as mimicking Unix’s internal design. Unix provides a narrow interface whose mechanisms combine well, offering a surprising degree of generality. This interface has been so successful that modern operating systems—BSD, Linux, Mac OS X, Solaris, and even, to a lesser extent, Microsoft Windows—have Unix-like interfaces. Understanding xv6 is a good start toward understanding any of these systems and many others.
本书使用单一的操作系统作为具体的例子来说明操作系统的概念。xv6这个操作系统提供了Ken Thompson和Dennis Ritchie的Unix 介绍的基本接口，并且模仿了Unix的内部设计。Unix提供了一个窄接口，其机制表现突出，提供了令人惊讶的通用程度。这个接口非常成功，甚至现代操作系统BSD、Linux、Mac OSX、Solaris，甚至在一定程度上，Microsoft windows都有类Unix的接口。理解xv6是理解这些系统和其他系统的一个良好开端。
As shown in Figure 0-1, xv6 takes the traditional form of a kernel, a special program that provides services to running programs. Each running program, called a process, has memory containing instructions, data, and a stack. The instructions implement the program’s computation. The data are the variables on which the computation acts. The stack organizes the program’s procedure calls.
如下图0-1所示，xv6采用传统的内核形式，内核是一个特殊的程序，为正在运行的程序提供服务。每个正在运行的程序，称为进程，都有包含指令、数据和堆栈的内存。指令实现了程序的运算，数据是计算所依赖的变量，堆栈组织程序的过程调用。一台给定的计算机通常有许多进程，但只有一个内核。
Figure 0-1 kernel and process
When a process needs to invoke a kernel service, it invokes a procedure call in the operating system interface. Such a procedure is called a system call. The system call enters the kernel; the kernel performs the service and returns. Thus a process alternates between executing in user space and kernel space.
当一个进程需要调用一个内核服务时，它会调用一个系统调用，这是操作系统接口中的一个调用。系统调用进入内核；内核执行服务并返回。因此，一个进程在用户空间和内核空间之间交替执行。
The kernel uses the CPU’s hardware protection mechanisms to ensure that each process executing in user space can access only its own memory. The kernel executes with the hardware privileges required to implement these protections; user programs execute without those privileges. When a user program invokes a system call, the hardware raises the privilege level and starts executing a prearranged function in the kernel.
内核使用CPU提供的硬件保护机制来确保每个在用户空间执行的进程只能访问它自己的内存。内核程序的执行拥有操控硬件的权限，它需要实现这些保护；而用户程序执行时没有这些特权。当用户程序调用系统调用时，硬件会提升权限级别，并开始执行内核中预先安排好的函数。
The collection of system calls that a kernel provides is the interface that user programs see. The xv6 kernel provides a subset of the services and system calls that Unix kernels traditionally offer. Figure 0-2 lists all of xv6’s system calls.
xv6的所有系统调用
内核提供的系统调用集合是用户程序看到的接口。Xv6内核提供了Unix内核传统上提供的服务和系统调用的子集。表0-2列出了xv6的所有系统调用。
Figure 0-2 xv6’s system calls
The shell is an ordinary program that reads commands from the user and executes them. The fact that the shell is a user program, not part of the kernel, illustrates the power of the system call interface: there is nothing special about the shell. It also means that the shell is easy to replace; as a result, modern Unix systems have a variety of shells to choose from, each with its own user interface and scripting features. The xv6 shell is a simple implementation of the essence of the Unix Bourne shell. Its implementation can be found at line (8550) .
Shell是一个普通的程序，它从用户那里读取命令并执行它们。Shell是一个用户程序，而不是内核的一部分，这一事实说明了系统调用接口的强大之处：shell没有什么特别之处。这也意味着shell很容易替换；因此，现代Unix系统有多种shell可供选择，每种shell都有自己的用户界面和脚本特性。Xv6 Shell是Unix Bourne shell本质的简单实现。它的实现可以在(xv6/sh.c:1)中找到.