(完整word版)计算机专业英语教程(第5版)翻译完整版

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1、1. 1 A Closer Look at the Processor and PrimaryStorage仔细看看处理器和主存储器We have learned that all computers have similar capabilitiesand perform essentiallythe same functions, although some might be faster than others. We have also learned that acomputer system has input, output, storage, and processing co

2、mponents; that the processor istheuintelligenceMof a computer system; and that a single computer system may have severalprocessors We have discussed how data are represented inside a computer system in electronicstates called bits We are now ready to expose the inner workings of the nucleus of the c

3、omputersystem the processor 我们已经知道，所有的计算机都具有相似的能力，并Ft在本质匕执行柑同的功能，尽管 一些可能会比另一些快一点。我们也知道，一个计算机系统只有输入，输出，存储和处理部 件：处理器是-个计算机系统智能核心，并且一个计算机系统对以有许多个处理器。我们已 经讨论过如何在计算机系统内部，用被称作“位”的电子状态来表现数据，现在我们要弄明 白计算机系统的核心，即处理器，的内在的工作方式。The internal operation of a computer is interesting,but there really is no mysteryto

4、it. The mystery is in the minds of those who listen to hearsay and believe sciencefictionwriter. The computer is a nonthinking electronic device that has to be plugged into an electricalpower source, just like a toaster or a lamp.计算机的内部操作很有意思，但确实没有什么神秘町言。所谓的神秘只是存在于那些听信 传闻和相信科幻小说作家的人的意识中。计算机就是一种没有思想的

5、需要接通电源的电子设 备而已，与烤面包机和台灯差不多。Literally hundreds of different types of computers are marketed by scores ofmanufacturers 1The complexity of each type may vary considerably, but in the end eachprocessor, some times called the central processing unit or CPU, has only two fundamentalsections： the control u

6、nit and the arithmetic and logic unit Primary storage also playsan integral part in the internal operation ofaprocessor. These three primary storage, thecontrol unit, and the arithmetic and logic unit work together Let s look at their functionsand the relationships between them 不加夸张地讲，市场上有几百种不同类型的计算

7、机在销售每种电脑在复杂性匕町能有很人区别，但归根结底,每种处理器，有时称为中央处理器即CPU,只有两个基本部分:控制单 元和计算逻辑单元主内存在处理器内部操作中也是一个不可缺少的部分这三个部件-主 内存，控制单元和计算逻辑单元一一起工作.然我们看看它们（齐自的）功能和它们之间的联 系。Unlike magnetic secondary storage devices,such as tape and disk, primary storage has nomoving parts With no mechanical movement, data can be accessed from pr

8、imary storage atelectronic speeds, or close to the speed of light Most of todayfs computers use DRAM (DynamicRandom-Access Memory) technology for primary storage A state-of-th亡一art DRAM chip aboutone eighth the size of a postage stamp can store about 256 000,000 bits, or over 25, 600, 000characters

9、of data!主存与其他的辅助存储器(如：磁带、硬盘)不一样的是，主存不含有运转部件由于 没有机械运转的需要，主存种数据访问町以达到电子的速度，或接近于光速。当今计算机的 上存人第数使用DRAM(动态随机取存储器)技术。目前最新的匸艺水平是:块只仃大 约1/8张邮票大小的DRAM芯片却可以存储大约256, 000, 000位,约25, 6000, 000个字符的 数据。Primary storage,or main memory, provides the processor with temporary storage for programsand data All programs a

10、nd data must be transferred to primary storage from an input device(such as a VDT) or from secondary storage (such as a disk) before programs can be executed ordata can be processed Primary storage space is always at a premium; therefore, after a programhas been executed, the storage space it occupi

11、ed is reallocated to another program awaitingexecution.主存储器，也就是说内存，用于为处理器暂时存放程序和数据。所仃的程序和数据在被操 作之前必须从输入设备(如VDT)或者辅助存储器转存到主存储器中。主存储器存储容量通 常是柑当有限的，I大1此，在一个程序执行结束，它所占用的存储空间必须被重新分配给其它 正在等待执行操作的程序。Figure 1-1 illustrates how all input/output (1/0)is44read to” or written from primarystorage. In the figure,

12、 an inquiry (input) is made on a VDT The inquiry, in the form of amessage, is :routed to primary storage over a channel (such as a coaxial cable). The messageis interpreted, and the processor initiates action to retrieve the appropriate program and datafrom secondary storage L3 The program and data

13、areuloadedM, or moves, to primary storagefrom secondary storage. This is a nondestructive read process That is, the program and datathat are read reside in both primary storage (temporarily) and secondary storage (permanently).The data are manipulated according to program instructions, andareport is

14、 written from primarystorage to a printer.图IT描述了输入/输出设备与主存储器间的读和写过程。在图中，VDT发出个输入请 求，请求是以消息模式通过通道(如同轴电缆)发送到主存储器。这个杳询被解释，处理器 发起操作从辅助存储器中调用合适的程序和数据。程序和数据从辅助存储器传送到主存储器 屮，这是一个非破坏性的读取过程，也就是说，程序和数据同时存在于主存(临时保存)和 辅助存储器(永久保存)中。根据程序指令的指示，处理器对数据进行操作，并从主存传送 一份报告到打印机。A program instruction or a piece of datais st

15、ored in a specific primary storage locationcalled an address Addresses permit program instructions and data to be located, accessed,and processed The content of each address is constantly changing as different programs areexecuted and new data are processed.程序指令和数据是存储在主存屮-个特殊的位豐，称为地址空I叭通过地址空间町以实现 计算

16、机对程序指令和数据的定位、访问和处理。地址空间的内容是经常变化的，这是由于计 算机一直在执行不同的程序和数据。Another name for primary storage is random-access memory,or RAM. A special type ofprimary storage, called read-only memory (ROM), cannot be altered by the programmer Thecontents of ROM are hard-wired (designed into the logic of the memory chip)

17、by the manufacturerand can be read onlyH. When you turn on a microcomputer system, a program in ROM automaticallyreadies the computer system for use. Then the ROM program produces the initial display screenprompt 主存储器也称为随机存取存储器或RAM。还仃 种特殊的主存储器,称为只读存储器(ROM),这种存储器不能被程序臾改存放的内容。R0H的内容是被生产商通过硕件电路写入的，并且 不

18、能被重写。当你启动计算机，ROM中的一个程序会自动就绪等待计算机系统的调用，然后 在显示器中显示开机提示。bleread-only memory (PROM). PROM is ROM into which you,the user, can load read-only programs and data. Once a program is loaded to PROM, it is seldom,if ever, changed 4 However, if you need to be able to revise the contents of PROM, thereis EPROM,

19、 erasable PROM Before a write operation, all the storage cells must be erased tothe same initial state.可编程只读存储器(PROM)是另一种ROM它町以我入只读的程序和数据，一口我入，将 不再改变。然而，若果你需要去修正PROM的内容，可以使用可擦町编程只读存储器。EPROM在进行一次写操作之前，所有的存储单元必须被还原为同一初始状态。A more attractive form of read-mostly iiemoryis electrically erasable programmab

20、leread-only memory (EEPROM). It can be writ ten into at any time wit hout erasing prior contents;only the byte or bytes addressed are updated.一种更吸引人的可改写只读存储器是电可擦除可编程只读存储器(EEPROM).它可以在 任何时候写入，而且不会擦除以前的内容：只会更新被寻址的字节。The EEPROM combines the advantage ofnonvolatility with the flexibility of being updata

21、blein place 6, using ordinary bus control, address, and data lines.电可擦可编程存储器把非易失性优点和可更新、的地方的必话性结合起来，修改 时使用普通的总线控制线、地址线和数据线。Another form of semiconductor memoryis flash memory (so named because of the speed)Flash memory is intermediate between EPROM and EEPROM in both cost and functionality LikeEEPRO

22、M, flash memory uses an electrical erasing technology An entire flash memory can be erasedin one or a few seconds, which is much faster than EPROM In addition, it is possible to eraseA variation of ROM is prograjust blocks of memory rat her t han an entire chip However, flash memory does not provide

23、bytelevel erasure 7 Like EPROM, flash memory uses only one transistor per bit, and so achievesthe high density of EPROM 另一种半导体记忆体是闪存(直:味着速度快)。闪存在性价比上处I* EPROM和EEPROM Z间， 它使用电擦写技术。整个闪存的内容可以在一到几秒内被清除，这是远快于EPROM的。另外， 它还可以对部分记忆块而不是整个存储器进行清除。然而，闪存并不提供字节级的擦除。像EPROM,闪存只使用一个晶体管毎比特，因此町以实现高密度的EPROMoCache Memo

24、ryProgram and data areloaded to RAM from secondary storage because the time required to accessa program instruction or piece of data from RAM is significantly less than from secondary storageThousands of instructions or pieces of data can be accessed from RAM in the time it would taketo access a sin

25、gle piece of data from disk storage 8 RAM is essentially a high-speed holdingarea for data and programs In fact, nothing really happens in a computer system until theprogram instructions and data are moved to the processor This transfer of instructions anddata to the processor can be tim己一consuming,

26、 even at microsecond speeds To facilitatean even faster transfer of instructions and data to the processor, most computers are designedwith cache memory Cache memory is employed by computer designers to increase the computersystem throughput (the rate at which work is performed) 程序和数据从辅助存储器装我到RAM中是因

27、为对RAH屮的程序指令和数据的访问时间要明 显的少于从辅助存储器访问。数以T条的指令和数据能被访问而只需花费从硬盘访问一次数 据的时间。RAM是数据和程序的航要的高速存放区，專实上，计算机系统不会进行任何操作 直到程序指令被传入处理器进行执行，这种指令和数据的传输是需要花费时间的，即使传输 速度已经是以纳秒來计算了。为了能更快速的传输指令和数据，大多数计算机设计使用高速 缓存。计算机设计若使用高速缓存来提高计算机系统的总处理能力(工作效率)。LikeRAM,cache is ahigh-speed holding area for program instructions and data H

28、owever,cache memory uses SRAM (Static KAM) technology that is about 10 times faster than RAM and about100 times more expensive With only a fraction of the capacity of RAM, cache memory holds onlythose ins true tions and data that are likely to be needed next by the processor Two typesof cache memory

29、 appear widely in computers The first is referred to as internal cache andis built into the CPU chip The second, external cache, is located on chips placed close tothe CPU chip A computer can have several different levels of cache memory Level 1 cacheis virtually always built into the chip Level 2 c

30、ache used to be external cache but is nowtypically also built into the CPU like level 1 cache 和RAM一样，高速缓存也是程序指令和数据的高速存储区。 然而， 高速缓存使用静态RAM技术，这种技术比RAM在速度上要高出10倍，价格上高出100倍。cache只保存内存中那 一小部分最右可能被处理器执行的指令和数据。两种类型的cache T泛应用于计算机，第一 种被植入epu中的叫做内部高速存储，第二种是外部高速存储，它位于那些靠近epu的芯片 中。一台计算机M以拥有几个不同级别的高速缓存。一级缓存实际上总

31、是植入芯片中，二级 缓存过去常常作为外部高速缓存，但是现在也像一级缓存植入epu内部。3.2C+和面向对象的程序设计一些面向对象的程序设计概念在语言间渗透。例如微软Quick Pascal是允许使用对象的 第一批语言中的一个。C+有什么使得它是一种适合干开发面向对象程序的语言？如同先前 所提到的，答案是类（class）数据类型。给该语言建立对彖之能力的是建立在C结构类型 之上的C卄类（class）类型。还有，C卄把另外几个特性引入面向对象的程序设计，这些特 性并不包含在简单地利用对彖的其他一些语言中。C卄的优点包扌舌强类型、运算符重我和较 少地强调预处理。的确你能使用其他一些产品和采用其他一些

32、语言来进行面向对象的程序设 计，但是采用C+的众务好处是显著的。这是为面向对象的程序设计而设计的语言，并非（现 有语言的）式样翻新。面向对象的程序设计是一种程序设计技术，使得你能把一些概念看作各种齐样的对象。 通过使用对彖，你能表示要被执行的任务、它们之间的相互作用和必须观察的某些给定的条 件。一种数据结构经常形成某个对象的基础；因此，在C或C卄中，结构类型能形成某种 基本对象。与对象的通信，如前提到的，能通过使用消息來完成。消息的使用类似于在面向 过程的程序中对函数的调用。当某对象收到 个消息时，包含在该对象内的一些方法作出响 应。方渺似于面向过程程序设计的函数。然而，方法是对象的一部分。C

33、卄的类是对C和C卄结构类型的扩充，并且形成了面向对彖程序设计所需要的抽彖数 据类型。类能包含紧密相关的一些条目，它们共享一些属性。更正式地说，对象只不过是类 的实例。鼓终，应该出现包含很多对象类型的类库，你能使用这些对彖类型的实例去拼合程序代 码。在你更详细地考察这些术语Z前，一个好的主意是熟悉与C卄和面向对级程序设计相关 的另外几个概念，如同下面几节所述的。封装超裝指的是每个对象把它的成员数据和成员函数（方法）组合成单个结构的方式。图3-1举例说明了你如何能组合数据域和方法以建立对彖。数据域方法数据成员函数成员函数成员函数数据成员函数成员函数数据成员函数图3-1为建立对彖而组介的数据域和方法

34、典型地，一个对象的描述是一个C卄类的一部分，且包括对该对象内部结构的描述、 该对象如何打其他对彖相关，以及把该对象的功能细节和该类的外部相隔离的某种形式的保 护。C卄类结构做到了所有这些。在一个C卄类中，你使用私有的、公共的和/或受保护的描述符来控制对彖的功能细节。 在面向対象的程序设计中，公與（public）部分一般用于接门信息（方法），使得该类町在各 应用中重用。如果数据或方法被包含在公共部分，它们在该类外部也町用。类的祕部分把 数拯或方法的可用性局限于该类本身。包含数据或方法的受保护部分被局限于该类和任何派 生子类。类层次结构C卄类实际上用作创建对彖的模板或模式。从类描述形成的对象都是该

35、类的实网 开发 类层次结构是可能的，其中有一个主类和几个子类。在C+中，做这爭的基础是瀝生类。 父类表示更一般化的任务，而派生子类执行一些特定的任务。例如，早先讨论的林肯类也许 包含整个林肯系列公共的数据和方法， 诸如引擎、检测仪表、电池、制动能力和操纵。从父 类派生的子类，诸如TourCar、MarkVID和Continental nJ能包含该类专用的些款项。例如，1995 Continental是该系列中唯一具有主动悬架系统的汽车。继承面向对象程序设计中的邂承使得一个类能继承某対象类的一些性质。父类用作派生类的 模式，且能以几种方式被改变（在下一章中你将了解成员函数能被重栽、新的成员函数能

36、被 添加，并且成员存取特权能被改变）。如果某个对彖从单个父类继承其属性，称为单继承。如果某个对象从聂个父类继承属性，便称为多継承。继承是一个重要概念，因为它使得无须 对代码做人的改变就能巫用类定义。继承鼓励重用代码，因为子类是对父类的扩充。多态性与类层次结构相关的另一个甫要的面向对象概念是公共消息能被发送到诸父类对彖和 所有派生子类对象。按正式的术语，这称为多态性。多态性使每个子类对象能以一种对其定义來说适当的方式对消息格式作出响应。试设想 收集数据的一个类层次结构。父类可能负资收集某个个体的姓名、社会安全兮、职业和 雇佣年数，那末你能使用子类来决定根据职业将添加什么附加信息。一种情况，一个管

37、 理职位会包括年薪，而另一种情况，销售员职位会包括小时工资和回扣信息。因此，父 类收集一切子类公共的通用信息，而子类收集与特定工作描述相关的附加信息。多态性 使得公共的数据收集消息能被发送到每个类。父类和子类两若都以对该消息是恰当的方 式作出响应。多态性促进现有代码的可扩充性。虚函数多态性赋予对象以当对象的精确类型还未知时响应来自例行程序之消息的能力。在C+中这能力是蜩龙的结果。使用迟绑定，地址在运行时刻动态地确定，而不是如同传统的编 译型语言在编译时刻挣态地确定。这静态的（固定的）方法往往称为军级疗。函数名被替换 为存储地址。你使用於两数来完成迟绑定。在随后的派生类将通过觅定义函数之实现而旋

38、载 该函数时，在父类中定义虚函数。当你使用虚函数时.消息不是直接传给对象，而是作为指 向对象的指针传送。虚函数利用了地址信息表，该表在运行时刻使用构造符而被初始化。一个构造符每当 创建它的类的一个对象时被调用。这里构造符的工作是把虚函数与地址信息表链接，在编译 运转期间虎函数的地址是未知的；相反，给出的是（在运行时刻确定的）地址表中将包含该 函数（入口）地址的位豐。3. 4 JavaScript教程Java is designed to meet the challenges of application development in the context ofheterogeneous,

39、network-wide distributed environments. Paramount among these challenges issecure delivery of applications that consume the minimum of system resources, can run on anyhardware and software platform, and can be extended dynamical1y.Java是为了解决在复杂的、大规模分布式网络环境卞开发应用程序的挑战而设计的。其中 最人的一个挑战是确保交付的程序：占用最少的系统资源，并且

40、能在任何硬件、软件平台下 运行，能被动态扩展。Java originated as part of a research project to develop advanced software for a wide varietyof network devices and embedded systems. The goal was to develop a small, reliable, portable,distributed, real-time operating platform When the project started, C+ was the language ofc

41、hoice But over time the difficulties encountered with C+ grew to the point where the problemscould best be addressed by creating an entirely new language platform. Design an architecturedecisions drew from a variety of language such as Eiffel, SmallTalk, Objective, C, and Cedar/Mesa The result is a

42、language platform that has proven ideal for developing secure, distributed,network based end-user applications in environments ranging from network-embedded devices toth亡Wor1d-WideWeb an the desktop.Java最初只是一个为齐种网络和嵌入式设备而研究的软件系统的一部分，这个系统的目标 是一个小的、可信赖的、可移植的，分布式的、实时的平台。最开始C+语言是这个平台的 首选。但随着时间的推移，C+出现了很

43、多问题， 有必要开发一个全新的语言平台。 我们根 据Eiffel, SmallTalk,Objective, C, and Cedar/Mesa等语言来做出决策,结果是开发出 了一个理想的、能在从嵌入式网络设备到万维网到桌而等各种环境下方便的开发安全的、分 布的、基于网络的应用程序的平台。The design requirements of Java are driven by the nature of the computing environmentsin which software must be deployed Java的设计要求是由软件所部署的计算坏境的本质决定的.The ma

44、ssive growth of the Internet and the World-Wide Web leads us to a completely new way oflooking at development and distribution of software To live in the world of electronic commerceand distribution, Java technology must enable the development of secure, high performance, andhighly robust applicatio

45、ns on multiple platforms in heterogeneous, distributed networks 互联网和万维网的快速发展带领我们用新的眼光去看待开发和发布软件。在电子商务的时代,Java必须具尙开发安全性、高效性、健壮的、人众的、能在多平台和多样化、分布式网络 运行的程序。Operating on multiple platforms in heterogeneous networks invalidates the traditional schemesof binary distribution , release, upgrade, patch, and

46、so on. To survive in this jungle, javamust be architecture neutral, portable, and dynamically adaptable 为了能在异构网络的多个平台上执行二进制发布、升级、修补等计划的环境中发展，Java必须是总体结构不带偏向性的、可移植的、且能动态地自适应的。The java system that emerged to meet there needs in simple, so it can be easily programmed bymost developers; familiar, so tha

47、t current developers can easily learn java; object oriented ,to take advantage of modern software development methodologies and to fit into distrib utedclient-server application; multi threaded, for high performance in applications that need toperform multiple concurrent activities , such as multime

48、dia; and interpreted , for maximumportability and dynamic capabilities.Java系统的出现使得这些需求变得简单，所以町以很容易的被大多数开发人员所使用：大 众的，所以现在的开发人员可以很容易的学习Java；面对对象的，利用现代软件开发方法 的优势以适应分布式客户端-服务器应用程序;多线程的,用于高性能的执行多个并发活动, 如多媒体；解释型的，为了駛大的可移植性和动态能力。Together, the above requirements comprise quite a collection of buzzwords, so

49、let * s examinesome of them and their respective benefits before going on.总的來说，上述包括相当多的专业术语，所以在继续之前先让我们來认识一下它们以及它们 所带来的的各种好处。Whats completely new is the manner in which java and its run-time system have combined themto produce a flexible and powerful programming system Java和它在运行时的系统能包含有良好的操作性以及强人的编译

50、设计系统是一种全新的方 式。Developing your applications using Java results in software that is portable acrossmultiple machine architectures, operating systems. and graphical user interfaces, secure, andhigh performance With Java, your job as a software developer is much easieryou focus yourfull attention on the

51、 end goal of shipping innovative procuc ts on time, based on the solidfounda tion of Java The bet ter way to develop software is here, now, brought to you by theJava language platform.使用Java开发您的应用软件所带来的是町在姜种机器总体结构、操作系统和图形用户借I 1间的移植，安全性和高性能。使用Java,你作为软件开发人员的工作室非常容易的一基于Java的基础您可以专注发展和及时创新。开发软件的最好方法在这里

52、，现在，给您带來Java语言平台。Very dynamic languages like lisp, TCL. and SmallTalk are often used for prototypingone of the reasons for their success at this is that they are very robustyou don* t haveto worry about freeing or corrupting memory.非常貝有生命力的语言如lisp、tel和SmallTalk常常用于充为原型构造技术。他们在这取 得成功的其中一个原因是：他们非常强大，您

53、不需要内存的释放或考损坏。Similarly programmers can be relatively fearless about dealing with memory when programmingin java the garbage collection system makes the programmer* s job vastly easier ;with theburden of memory management taken off the programmer* s shoulders , storage allocation errorsgo away 同样的，程序

54、员在使用Java编程的时候可以不用过多担心存储问题。无用单元手机系统使 程序员的工作大为容易，卸卞了程序员启上存储管理的负担，存储分配错误不再发生。Another reason commonly given that like lisp , TCltand Smalltalk are good for prototypingis that they dont require you to pin down decision early on these languages are semanticallyrich 一般认为像LISP、TCL和SmallTack这样一些语言十分适合于原型法的另一

55、理由，是因为它 们不要求你受早期决定的约束-一这样的语言的语义是很丰富的。Java has exactly the opposite property ; it forces you to make explicit choices . alongwith these choices come a lot of assistance you can write met hod invoca tions and , if youget something wrong, you get told about it at compile time you dont have to worry abo

56、utmethod invocations error.Java其月完全相反的性质，它强制你做出明确的选择。伴随着这些选择的是许多助理-一如: 你可以写一些方法的调用，如果令错误，你会在编译时被告知。您也不需要担心方法调用错4.1 summary of osAn operating system is the software which acts as an interface between a user of a computerand the computer hardware.操作系统是计算机用户和硬件的软件接口。The purpose of an operatingsystem is

57、 to provide an environment in which a user may execute program 操作系统的目的是提供用户一个可以执行程序的环境。The primary goal of an opera ting system is thus to makethe computer system convenien t to use 操 作系统的主要目标因此是使计算机系统便于使用。A secondary goal is to use the computer hardware in an efficient way.其次是高效地使用计算机硬件。We can view

58、 an operating system as a resource allocator.我们可以把操作系统看做 是 j 个资源分配器。A computer system has many resources which may be required to solve a problem： CPUtime, memory space, file storage, input/output(I/O) devices, and so on.个计算机系统拥有许多资源，因此需要去解决一些问题，如cpu时间，内存空间，文 件储 输入输出 设备第 uTheoperating system acts as

59、 the manager of these resources and allocates them to specific programsand users as necessary for their tasks.操 作系统就像是资源管理员，把资源分配给特定的程序和用户当它们工作需要时o Since there may be many, possibly conflicting, requests forresources, the o/s must decide which requests are allocated resources to operate the computer

60、system fairly and efficiently.此有可能会有很多有可能产生冲突的资源请求,操作系统必须决定分配资 源给哪个请求能使计算机系统合理和有效的运行。Early computers were (physically) very large machines run from a console Y期的计 算机是从控制台运行的体枳非常人的机器。The programmer would write a program and then operate theprogram directly from the operator* s console 程序!4要写条程序然 后克接从

61、操作者的控制台操作程序Software such as assemblers, loaders, and compilers improved on the convenienceof programming the system. but also required substantial set-up time.象汇编程序、装入程序及编译稈序这样的软件使系统软件编程更方便，但同 时也需要人量的丿【；动时间。To reduce thesetup time, operators were hired and sim订ar jobs were batched together. 1为了能减少这些

62、准备时间，操作者就被庭佣，相似的作业 成批排在一起。Batch systems allowed automatic job sequencing by a resident monitor and improved the overallutilization of the computer greatly.批处理系统通过个常驻内存的监 控程序允许自动的作业定序。The computer no longer had to wait for human operation.这样计算机就不必再等待用户操作。CPU utilization was still low, however, becaus

63、e of the slow speed of the I/O devicesrelative to the CPU Offline operation of slow devices was tried.然而，CPU使用率仍然不高，这是因为I 0设备的处理速度较CPU慢 导致的。Buffering was another approach to improving system performance by overlapping th亡input, output,andcomputation of a single job.缓冲机制是另一种提高系统系能的方 法，它将一条作业的输入、输出和计算

64、工作重叠交错进彳亍。Finally, spooling allowed the CPU to overlap the input of onejob with the coitputation and output of other jobs 最后，伪脱机技术允许CPU将一条作业的输入和其他作业的计算输出交错进行。Spooling also provides a pool of jobs which have been read and are waiting to be run.伪脱机也为那些己经被读入并且还在等待运行的作业提供一个作业池。This job pool supports the

65、 conceptof multiprogramming.这个作业池支持一种叫做多程序设计的概念。With multi programming,several jobs are kept in memory at one time; the CPU is switched back and for th between themin order to increase CPU uti lization and to decrease the total real time needed to executea job.在这种多程序设计技术中，多 个作业可以同时存放任内存中：CPU任这些作业中米回

66、切换控制，以至于览加CPU使用率和 减少执行一条作业的总时间。Multiprogramming, which was developed to improve performance, also allows time sharing.多道程序设计被开发来提高性能，也允许分时。Timeshared opera ting sys tems allow many users (from one to several hundred) to use a computersystem interactive at the same time.分时操作系统允许*个用户(从一个 到几百个)同时交互式地使用计算机系统。As the system switches rapidly from one user to the next, each user is giventhe impression that he has his own computer. it于系统迅速地从一个用户切换到卜一个用户，每一个用户被给予一种他拥有自己的计算机的E卩彖。Other operating systems typ