In the rest of this chapter, we introduce the basic concepts that have motivated the design of Unix over the past two decades, as well as Linux and other operating systems. While the concepts are probably familiar to you as a Linux user, these sections try to delve into them a bit more deeply than usual to explain the requirements they place on an operating system kernel. These broad considerations refer to virtually all Unix-like systems. The other chapters of this book will hopefully help you understand the Linux kernel internals.
在本章的后续部分，我们介绍过去20多年推动了Unix设计不断发展的基本概念，以及Linux和其他操作系统。虽然作为Linux用户，你已经对这些概念很熟悉了，这些部分试图比通常理解的更加深入，以此来解释它们对操作系统内核是多么重要。这些被广泛提及的概念涉及到所有类Unix系统。
Multiuser Systems
A multiuser system is a computer that is able to concurrently and independently execute several applications belonging to two or more users. Concurrently means that applications can be active at the same time and contend for the various resources such as CPU, memory, hard disks, and so on. Independently means that each application can perform its task with no concern for what the applications of the other users are doing. Switching from one application to another, of course, slows down each of them and affects the response time seen by the users. Many of the complexities of modern operating system kernels, which we will examine in this book, are present to minimize the delays enforced on each program and to provide the user with responses that are as fast as possible.
多用户操作系统
一个多用户操作系统意味着一台计算机可以并发且相互独立为两个或更多用户执行多个程序。并发意味着应用程序可以在相同的时间点处于活跃状态并争取各种资源，比如CPU、内存、硬盘等。独立执行意味着每个程序可以处理自己的任务而不需要关心其他用户程序在做什么。当然，从一个程序切换到另一个程序会使两个程序运行同时放缓，而且会影响两个用户对响应时间的直观感受。在本书中，我们会考察现代操作系统内核的许多复杂设计，这些复杂性设计师为了最大程度降低每个程序的延迟，并为用户提供尽可能快的相应。
Multiuser operating systems must include several features:
• An authentication mechanism for verifying the user’s identity
• A protection mechanism against buggy user programs that could block other applications running in the system
• A protection mechanism against malicious user programs that could interfere with or spy on the activity of other users
• An accounting mechanism that limits the amount of resource units assigned to each user.
To ensure safe protection mechanisms, operating systems must use the hardware protection associated with the CPU privileged mode. Otherwise, a user program would be able to directly access the system circuitry and overcome the imposed bounds. Unix is a multiuser system that enforces the hardware protection of system resources.
多用户操作系统必须包含的加几个特性：
1.用于区别用户身份的身份认证机制；
2.用来阻止程序缺陷导致系统中正在运行的其他程序挂掉的保护机制；
3.用来组织那些干扰甚至窥视其他用户的恶意用户程序的保护机制；
4.用来限制每个用户申请的资源数量的记账机制；
为了使得保护机制足够的安全，操作系统必须使用与CPU特权模式相关的硬件保护；否则，应用程序就可能直接操作系统的电路从而克服强制性限制（从而使系统不安全）；Unix是一个对系统资源进行强制硬件保护的多用户系统；