计算机专业英语教程第5版翻译

2021年2月19日发(作者：shirt是什么意思)

1

We have learned that all computers have similar capabilities and perform essentially the same functions,

although some might be faster than others. We have also learned that a computer system has input, output,

storage, and processing components; that th

e processor is the “intelligence” of a computer system; and

that

a

single

computer

system

may

have

several

processors.

We

have

discussed

how

data

are

represented

inside

a computer system in electronic states called bits. We are now ready to expose the inner workings of the

nucleus of the computer system

—

the processor.

我们已经知道，所有的计算机都具有相似的能力，并且在本质 上执行相同的功能，尽管一些可能会比另一些快一点。

我们也知道，一个计算机系统具有 输入，输出，存储和处理部件；处理器是一个计算机系统智能核心，并且一个计算

机系统 可以有许多个处理器。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 mystery to it. The mystery is

in

the

minds

of

those

who

listen

to

hearsay

and

believe

science-fiction

writer.

The

computer

is

a

nonthinking

electronic device that has to be plugged into an electrical power 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可言。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

of

manufacturers

[1].

The

complexity

of each type may vary considerably, but in the end each processor, sometimes called the central processing

unit

or

CPU,

has

only

two

fundamental

sections:

the

control

unit

and

the

arithmetic

and

logic

unit.

Primary

storage

also

plays

an

integral

part

in

the

internal

operation

of

a

processor.

These

three

—

primary

storage,

the control unit, and the arithmetic and logic unit

—

work together. Let’s look at their functions and

the relationships between them.

不加夸张地讲，市场上有几百种不同类型的计算机在销售

.

每 种电脑在复杂性上可能有很大区别，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

--

一起工作

.

然我们看看它们

(

各自的

)

功能和它们之

间的联系。

Unlike

magnetic

secondary

storage

devices,

such

as

tape

and

disk,

primary

storage

has

no

moving

parts.

With

no mechanical movement, data can be accessed from primary storage at electronic speeds, or close to the

speed of light. Most of today’s computers use DRAM (Dynamic Random

-Access Memory) technology for primary

storage. A state-of-the-art DRAM chip about one eighth the size of a postage stamp

can store about

256,000,000 bits, or over 25,600,000 characters 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

数据访问可以达到电子的速度，或接近于光速。当今计算机的主存大多数使用

DRAM

（动态随机存取存储器）技术。目

前最新的工艺水平是 ：一块只有大约

1/8

张邮票大小的

D RAM

芯片却可以存储大约

256,000,000

< p>
位，约

25,6000,000

个

字符的数据。

Primary storage, or main memory, provides the processor with temporary storage for programs and data. All

programs

and

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 or data can be processed. Primary storage space

is always at a premium; therefore, after a program has been executed, the storage space it occupied is

reallocated to another program awaiting execution.

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。所有的程序和数据在被操作之前必须从输入设备（如

VDT< /p>

）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相当有限的，因此，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。< /p>

Figure 1-

1 illustrates how all input/output (I/O) is “read to” or “written from” primary storage. In

the

figure,

an

inquiry

(input)

is

made

on

a

VDT.

The

inquiry,

in

the

form

of

a

message,

is

routed

to

primary

storage over a channel (such as a coaxial cable). The message is interpreted, and the processor initiates

action to retrieve the appropriate program and data from secondary storage [3].The program and data are

“loaded”, or moves, to primary storage from secondary storage. This is a nondestructive read process.

That

is,

the

program

and

data

that

are

read

reside

in

both

primary

storage

(temporarily)

and

secondary

storage

(permanently). The data are manipulated according to program instructions, and a report is written from

primary storage to a printer.

图

1-1

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

发出一个输入请求，请求是以消息模式 通过通

道（如同轴电缆）发送到主存储器。这个查询被解释，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中，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

data

is

stored

in

a

specific

primary

storage

location

called

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 are executed and new data are processed.

程序指 令和数据是存储在主存中一个特殊的位置，称为地址空间。通过地址空间可以实现计算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

of

primary

storage, called

read-

only

memory

(ROM),

cannot

be

altered

by

the

programmer.

The

contents

of

ROM

are

“hard

-

wired”

(designed

into the logic of the memory chip) by the manufactur

er and can be “read only”. When you turn on a

microcomputer

system,

a

program

in

ROM

automatically

readies

the

computer

system

for

use.

Then

the

ROM

program

produces the initial display screen prompt.

主存储器也称为随机存取存储器，或

RAM

。还有一种特殊的主存储器，称为只读存储器（

ROM

）

，这种存储器不能被程序

更改存放的内容。

ROM

的内容是被生产商通过硬件电路写 入的，并且不能被重写。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。

A variation of ROM is programmable read-only memory (PROM). PROM is ROM into which you, the user, can load

“read

-on

ly”

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,

there

is

EPROM,

erasable

PROM.

Before

a

write

operation,

all the storage cells must be erased to the same initial state.

可编程只读存储器（

PROM

）是另一种

ROM

，它可以载入只读的程序和 数据，一旦载入，将不再改变。然而，若果你需要

去修正

PRO M

的内容，可以使用可擦可编程只读存储器。

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

memory

is

electrically

erasable

programmable

read-only

memory

(EEPROM).

It can be written into at any time without 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 of nonvolatility with the flexibility of being updatable

in 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、需要更新的地方的灵活性结合起来，修改时使用普通的总线控制线、

地址线和数据线。

Another form of semiconductor memory is flash memory (so named because of the speed). Flash memory is

intermediate between EPROM and EEPROM in both cost and functionality. Like EEPROM, flash memory uses an

electrical erasing technology. An entire flash memory can be erased in one or a few seconds, which is much

faster than EPROM. In addition, it is possible to erase just blocks of memory rather than an entire chip.

However, flash memory does not provide byte-level erasure [7]. Like EPROM, flash memory uses only one

transistor per bit, and so achieves the high density of EPROM.

另一种半 导体记忆体是闪存（意味着速度快）

。闪存在性价比上处于

EP ROM

和

EEPROM

之间，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每比特，因此可以实现高密度的

EPROM

。

Cache Memory

Program and data are loaded to RAM from secondary storage because the time required to access a program

instruction or piece of data from RAM is significantly less than from secondary storage. Thousands of

instructions or pieces of data can be accessed from RAM in the time it would take to access a single piece

of

data

from

disk

storage

[8].

RAM

is

essentially

a

high-speed

holding

area

for

data

and

programs.

In

fact,

nothing

really

happens

in

a

computer

system

until

the

program

instructions

and

data

are

moved

to

the

processor.

This

transfer

of

instructions

and

data

to

the

processor

can

be

time-consuming,

even

at

microsecond

speeds.

To

facilitate

an

even

faster

transfer

of

instructions

and

data

to

the

processor,

most

computers

are

designed

with

cache

memory.

Cache

memory

is

employed

by

computer

designers

to

increase

the

computer

system

throughput

(the rate at which work is performed).

程 序和数据从辅助存储器装载到

RAM

中是因为对

RAM

中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。

数以千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

是数据和程序的重要的高速存放区，

事实

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来计算了。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

计者使用高速缓存来提高计算机系统的总处理能 力（工作效率）

。

Like RAM, cache is a high-speed holding area for program instructions and data. However, cache memory uses

SRAM (Static RAM) technology that is about 10 times faster than RAM and about 100 times more expensive.

With only a fraction of the capacity of RAM, cache memory holds only those instructions and data that are

likely to be needed next by the processor. Two types of cache memory appear widely in computers. The first

is referred to as internal cache and is built into the CPU chip. The second, external cache, is located

on chips placed close to the CPU chip. A computer can have several different levels of cache memory. Level

1

cache

is

virtually

always

built

into

the

chip.

Level

2

cache

used

to

be

external

cache

but

is

now

typically

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。

< p>
cache

只保存内存中那一小部分最有可能被处理器执行的指令和数据。 两种类

型的

cache

广泛应用于计算 机，第一种被植入

cpu

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

cpu

的芯片中。

< br>一台计算机可以拥有几个不同级别的高速缓存。

一级缓存实际上总是植入芯片中，

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

cpu

内部。

2.1

Optical laser disk technology eventually may make magnetic-disk and magnetic tape storage obsolete. With

this technology, the read/write head used in magnetic storage is replaced by two lasers. One laser beam

writes to the recording surface by scoring microscopic pits in the disk, and another laser reads the data

from the light-sensitive recording surface. A light beam is easily deflected to the desired place on the

optical disk, so an access arm is not needed.

光盘技术最终可能使磁盘和磁带存储淘汰。用这种技术，磁存储器所用的读

/

写头被两束激光代替。一束激光通过在光

盘上刻制微小的凹点，对记录 表面进行写；而另一束激光用来从光敏感的记录表面读取数据。由于光束容易被偏转到

光 盘上所需要的位置，所以不需要存取臂。

Optical

laser

disks

are

becoming

a

very

inviting

option

for

users.

They

are

less

sensitive

to

environmental

fluctuations,

and

they

provide

more

direct-access

storage

at

a

cost

that

is

much

less

per

megabyte

of

storage

than

the

magnetic-disk

alternative.

Optical

laser

disk

technology

is

still

emerging

and

has

yet

to

stabilize;

however, at present there are three main categories of optical laser disks. They are CD-ROM, recordable

CD-R and rewritable CD-RW, and magneto-optical disk.

< p>
对用户而言，光盘正成为最有吸引力的选择。它们

(

光盘

)

对环境变化不太敏感，并且它们以每兆字节比磁盘低得 多的

存储器价格提供更多的直接存取存储器。光盘技术仍在出现，并且还需要稳定；然而 ，目前有三种主要类型的光盘。

它们是

CD-ROM

< p>
、

WORM

盘和磁光盘。

CD-ROM

Introduced in 1980, the extraordinarily successful CD, or compact disk, is an optical laser disk designed

to enhance the recorded reproduction of music. To make a CD recording, the analog sounds of music are

translated

into

their

digital

equivalents

and

stored

on

a

4.72-inch

optical

laser

disk.

Seventy-four

minutes

of

music

can

be

recorded

on

each

disk

in

digital

format

by

2

billion

digital

bits.

With

its

tremendous

storage

capacity,

computer-industry

entrepreneurs

immediately

recognized

the

potential

of

optical

laser

disk

technology.

In

effect,

anything

that

can

be

digitized

can

be

stored

on

optical

laser

disk:

data,

text,

voice,

still pictures, music, graphics, and video.

1980

年引入的，

非常成功的

CD

，

或紧密盘是设计来提高音乐的录音重放质量的光盘。

为了制作一张

CD

，

把 音乐的模拟

声音转换成等价的数字声音，并且存储在一张

4.7 2

英寸的光盘上。在每张光盘上可以用数字格式（用

20

亿数字位）

记录

74

分钟的音乐。因为它的巨大存储容量，计算机工业的企业家们立刻认识到光盘技术的潜力。事实上，任何可以

被数字化的东西都能存储在光盘上：数据、正文、声音、静止图象、音乐、图形和视频。

CD-ROM(pronounced

cee-dee-ROM)

is

a

spinoff

of

audio

CD

technology.

CD-ROM

stand

for

compact

disk-read

only

memory. The name implies its application. CD-ROM disks, like long-

playing record albums, are “pressed”

at

the

factory

and

distributed

with

their

prerecorded

contents(for

example,

the

complete

works

of

Shakespeare

or

the

first

30

minutes

of Gone

with

the

wind).

Once

inserted

into

the

disk

drive,

the

text,

video

images,

and so on can be read into primary storage for processing or display; however, the data on the disk are

fixed

—

they cannot be altered. This is in contrast, of course, to the read/write capability of magnetic

disks.

CD-ROM

（读作

C

—

D

—

ROM

）是音频

C D

技术的副产品。

CD-ROM

代表紧 密盘—只读存储器。该名字隐含了它的应用。只读

光盘与

(

能长期播放的

)

唱片一样，在工厂里“压制”并带着预先录好的内容

(

如莎士比亚全集，或电影“飘”的前

30

分钟 部分

)

，分发出去。一旦光盘插入光盘驱动器，正文、视频图像 等等就能读入主存进行处理或显示；然而，在只

读光盘上的数据是固定的—它们不能被改 变。当然，这是与磁盘的读

/

写能力不同的。

< br>

The tremendous amount of low-cost dir-access storage made possible by optical laser disks has opened the

door to many new applications.

大 量的、低成本直接存取存储器（是光盘使之成为可能）已经打开了通向许多新应用的大门。

CD-R AND CD-RW

CD-R

and

CD-RW

both

allow

user

to

store

data

on

compact

discs,

but

only

data

on

rewriteable

CDs

can

be

erased

and overwritten. CD-R discs can only be written to once. Both types of discs look very similar to CD-ROM

discs. Recordable CDs are commonly used to store music files, allowing home users to make high-quality

personalized

music

CDs.

CD-RW

discs

can

be

written

to

and

erased

similar

to

floppy

disk.

Consequently,

they

are a good alternative for large file storage, as well as for creating a

“master” disc before burning

a CD-R discs(most CD-RW drives can write to both CD-RW and CD- R discs, but some CD and CD-R drives

cannot

read CD-RW discs). CD-R and CD-RW drives also read CD- ROM discs. However, CDs will be eclipsed BY DVDs,

once that technology becomes improved and standardized.

CD-R

和

CD-RW

都允许用户存储数据在光盘上，但是只有那些在可复写

cd

中的数据能被擦除和重写。

Cd-r

光盘只能被

< p>
写入一次。这俩种光盘看起来与只读

cd

光盘差不 多。可记录

cd

管片通常用于存储音乐文件，这样可以允许家庭 用户

制作高质量的个人音乐唱片。

Cd-rw

< br>光盘就像软盘那样能被重写和擦除。

因此，

他们是大容量 文件存储的另一种不错选

择，同时也作为主盘在对

cd-r

光盘进行刻入之前（大多数的可擦写光盘驱动器可以读取

cd-rw

和

cd-r

光盘，但是也

有一些

cd

驱动器和

cd-r

驱动器不能读

cd-rw

光盘）

。

Cd-r

和< /p>

cd-rw

驱动器也可以读

cd-rom

光盘，

然而，

cd

将会被

dvd

所替代，一旦

dvd

的技术完全成熟和标准化。

Magneto-Optical Disk

Magneto-Optical

disk

offers

promise

that

Optical

laser

disks

will

become

commercially

viable

as

a

read-and-write

storage

technology.

The

5and1/4inch

disks

can

store

up

to

1,000Mb.

At

present,

magneto-optical

disks

are

too

expensive

and

do

not

offer

anywhere

near

the

kind

of

reliability

that

users

have

come

to

expect

of

magnetic

media.

In

addition,

the

access

times

are

relatively

slow,

about

the

same

as

a

low-end

Winchester

disk.

磁光盘有希望使得激光盘成为商业上可行的读

/

写存储技术。

5*1/4

英寸的磁光盘可以存储高达< /p>

1000Mb

。目前磁光盘太

贵且根本未 达到用户对磁介质所期望的那种可靠性。此外，存取时间相对而言比较慢，大约与低档温彻斯特盘相同。

As optical laser disk technology matures to reliable, cost- effective, read/write operation, it eventually

may dominate secondary storage in the future as magnetic disks and type do today.

随着光盘技术成熟到可靠，性能价格合算，可读

/

可写，将来它最终会象现在磁盘、磁带那样统治二级存储器。

DVDs

The

acronym

DVD(for

digital

versatile

disc,

or

digital

video

disc)

refer

to

a

relatively

new

high-capacity

optical storage format that can hold from 4.7GB to 17GB, depending on the number of recording layers and

discs sides being used. A standard single layered, single-sided DVD can store 4.7GB of data; a two-layered

standard enhances the single-side layer to 8.5GB. DVD can be double-sided with a maximum storage of 17GB

per disc. The DVD was initially developed to store the full contents of a standard two-hour movie, but is

now also used to store computer data and software. DVD-ROM technology is seen by many as the successor to

music CDs, computer CD- ROMs, and prerecorded VHS videotape that people buy and rent for home viewing-in

other words, read-only products. DVD-R is DVD rewritable(similar to CD-W). The user can write to the disk

only once. DVD-RW is DVD recordable(similar to CD-RW). The user can erase and rewrite to the disk multiple

times. Only one-sided disks can be used for both DVD-R and dvd-rw. Most DVD drives can play both computer

and audio CDs, but you can’t play DVDs in a CD drive.

这个缩写影碟

（数字化视频光盘或数字视频光盘）

，

指的是相对较新的高容量光存储格式取决于光盘记录层和边数，

< p>
可以从

4.7

GB

的持有以

17

< br>GB

，被使用。一个标准的单层单面的

DVD

< p>
可以存储

4.7

GB

的 数据，一两个层次的标准，增

强了单面层到

8.5

GB

的。

DVD

可双面的每件

17

GB

的最大存储。

该

DVD

最初是存储标准的全部内容的两个小时的电影，

但现在还用于存储计算机数据和软件。

DVD - ROM

技术是被许多人视为对音乐

CD

，电脑光盘和人们购买和租赁的预先

录制的家庭录 影带的后续，换句话说，只读的产品。

DVD-R

是可记录的< /p>

DVD

（就像

CD-R

< br>那样）

。用户可以对其进行一次

写入。

< br>DVD-RW

是可擦写的（类似的

CD - RW

）

，用户可擦除和重写磁盘多次。

DVD-R

和

DVD-RW

都是单面光盘。所有的

DVD

驱动器可以同时播放电脑和音频

CD

。但是，不能在

CD

播放机上应用

DVD

光盘。

DVD Forum Receives Top Information Technology Industry Award for Creation of Unified Specification for

Next Multimedia Era

DVD

论坛因建立下一代数字 多媒体的统一规范而获得最高信息技术工业奖

The DVD Forum today announced that it has received the 1997 PC Magazine Award for Technical Excellence in

the

category

of

“Standards”,

in

recognition

of

the

Forum’s

successful

development

of

the

dvd

-rom

specification.

D VD

论坛今天宣布，因该论坛成功制定了

DVD-ROM

规范而获得

1997

年

PC

杂志的

(

制定标准的

)

卓越技术奖。

“

New

standards

are

particularly

important,

since

they

promise

to

bring

higher

levels

of

technology

innovation

and

market

compatibility

to

today’s

technology

users,”

said

by

Michael

J.

Miller,

editor- in-

chief

of

PC

Magazine.”

DVD

-

ROM

is

a

compelling

technology

that

was

chosen

because

it’s

a

familiar

format that brings a wealth of new computing, educational, gaming and entertainment possibilities to the

user.”

“新的标准特别重要，

因为它们承诺把更高级的技术革新和市场兼容性带给今天的技术用户，

< br>”

PC

杂志主编

Michael

J.

Miller

说，

“

DVD-ROM

是被选 出的激发兴趣的技术，因为它是一种熟悉的格式，它把大量的新计算、新教育、新游戏和

新娱乐的可能性带给用户。

”

In the Award citation to DVD-

ROM technology, PC Magazine referred to DVD as the format that will ”replace

the CD-

ROM as the primary means of PC content distribution. ”R

epresentatives of three companies involved

in development of the specification, Hitachi Ltd. , Matsushita Electric Industrial Company(Panasonic) and

Toshiba Corporation, accepted the award on behalf of the DVD Forum in a ceremony held on November 17 at

COM

DEX’ 97 in Las Vegas.

在对

DVD- ROM

技术的颁奖词中

, PC

杂志称 ，

DVD

将会“代替

CD-ROM

作为

PC

内容

(

即各种软件和资料

)

传播的主要手

段。

”参加规范开发的三个公司的代表（日立，松下，东芝）以

DVD

论坛的名义在

11

月

7

日，

Las

Vegas

COMDEX

'97

举

行的庆祝典礼上接受该项奖。

“

Products based on specifications defined by the DVD Forum are now shipping in volume to the worldwide

computer an

d consumer electronics markets, and the Award for Technical Excellence adds to the market’s

validation of the success of the standards process, ”said Koji Hase, General Manager of the DVD Products

Division at Toshiba Corporation and a founding member of th

e DVD Forum. ”We are extremely pleased to see

the

work

of

the

DVD

Forum

recognized

as

one

of

the

key

technical

achievements

in

the

personal

computer

industry,

particularly as the Forum expands the scope of its work with a large, global membership in 1998.

”

< p>
“基于

DVD

论坛定义的规范的产品现在正在大量 运往世界各地的计算机和电子产品市场，并且卓越技术奖增加了市场

对成功制定标准的认 可，

”东芝公司

DVD

产品部总经理和

DVD

论坛创办成员

Koji Has e

说，

“我们非常高兴地看到

DVD< /p>

论坛的工作被公认为个人计算机工业中重要的技术成就之一，特别是在

1998

年与更大的，全球的会员扩大了它的工作

范围。< /p>

”

“

The

members

of the

DVD

Forum

developed

the

DVD-ROM

specification

as

the

best

technical

approach

and

also

the best approach for customers in the marketplace, ”said Sakon Nagasaki, director of the DVD Business

Development

Office

of

Matsushita

Electric

Industrial

Co.

Ltd(Panasonic).

“Acceptance

of

the format

illustrates how standards

–

m

aking efforts advance the goals of the entire electronics industry. ”

“

DVD

论坛的成员把

DVD-ROM

规范开发作为最好的技术方法和对市场上客户的最好方法”

松下有限公司的

DVD

商业开发

办公室主任

Sakon Nagasaki

说，

< p>
“该格式的接受说明了制定标准如何加快整个电子工业的目标的实现。

”< /p>

In

addition

to

its

role

in

development

of

DVD-ROM

and

DVD- Video

standards,

the

Forum

has

proposed

the

format

for

recordable

DVD,

known

as

DVD-R,

and

rewritable

DVD,

known

as

DVD-RAM,

to

international

standards

bodies.

Work is also continuing on definition of a DVD Audio specification.

除了论坛在开发

DVD- ROM

和

DVD-Video

标准的作 用外，论坛也提出了可记录

DVD

的格式，称为

DVD-R

和可重写的

DVD

，

称为

DVD- RAM

，并提交国际标准组织。定义

DVD- Audio

规范的工作也正在继续着。

“The

mi

sson

of

the

DVD

Forum

is

to

define

a

smooth

migration

from

CD

to

DVD

technology

by

working

with

the

widest

possible

representative

group

of

manufacturers

and

technology

end-users

in

the

converging

industries

of

computers

and

cusumer

electronics,

”said

Dr.

Yoshita

Tsunoda,

a

member

of

the

Exective

Staff

of Hitachi, Ltd. And Chairman of the DVD Forum’s DVD

-

RAM Working Group. “The different working groups

have already completed definition of three separate DVD technology standards, and we have begun work on

de

veloping

next

generation

specifications

that

will

provide

compatible

products

well

into

the

next

century.”

“

DVD

论 坛的任务是，

在汇合计算机工业和消费电子产品工业中，

通过与 最广泛代表性的制造商与技术最终用户小组一

起工作，确定从

C D

技术向

DVD

技术的平滑过渡的途径 。

”日立有限公司执行官之一和

DVD

论坛

DVD- RAM

工作组主席

Yoshita Tsunoda

< p>
博士说，

“不同工作组已完成三个不同的

DVD< /p>

技术标准的定义，并且我们已开始开发下一代规范的工

作，这些规 范将很好地提供可兼容产品到下一世纪。

”

Recipients of the PC Magazine Awards for Technical Excellence are named by a team of editors,

senior

contributors and PC Labs personnel after months of evaluation and discussion. PC Magazine, the sponsor of

the Technical Excellence Awards, is a 1.175 million circulation magazine published by Ziff-Davis Inc. Pc

Magazine is published 22 times a year in print, quarterly on CD, and continuously on the World Wide Web

(<>).

PC

杂志的卓越技术奖的获得者是由 编辑、捐助者和

PC

实验室全体成员组成的小组在几个月的评价 和讨论后提名。

PC

杂志，

卓越技术奖 的主办者，

是由

Ziff-Davis

公司出版、

有

1.175

百万冊发行量 的杂志。

PC

杂志一年印刷出版

22< /p>

次，

每季度出

CD

，并且连续地在环球网上。

3.3

Java is designed to meet the challenges of application development in the context of heterogeneous,

network- wide

distributed

environments.

Paramount

among

these

challenges

is

secure

delivery

of

applications

that consume the minimum of system resources, can run on any hardware and software platform, and can be

extended dynamically.

Java

是为了解决在复杂的、大规模分布式网络环境下开发应用程序的挑战而设计的。其中最大的一个挑战是确保交付

的程序：占用最少的系统资源

,

并且能 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

variety

of

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

of

choice.

But

over

time

the

difficulties

encountered with C++ grew to the point where the problems could best be addressed by creating an entirely

new language platform. Design an architecture decisions drew from a variety of language such as Eiffel,

SmallTalk, Objective, C, and Cedar/Mesa. The result is a language platform that has proven ideal for

developing

secure,

distributed,

network

based

end-user

applications

in

environments

ranging

from

network-embedded devices to the World-Wide-Web an the desktop.

Java

最初只是一个为各种网络和 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面 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

environments

in

which

software

must be deployed.

Java

的设计要求是由软件所部署的计算环境的本质决定的

.

The massive growth of the Internet and the World-Wide Web leads us to a completely new way of looking at

development and distribution of software. To live in the world of electronic commerce and distribution,

Java technology must enable the development of secure, high performance, and highly robust applications

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

必须具有开发安全

性、高效性、健壮的、大众的、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 schemes of binary

distribution , release, upgrade, patch, and so on. To survive in this jungle, java must 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

by

most

developers;

familiar, so that current developers can easily learn java; object oriented ,to take advantage of modern

software development methodologies and to fit into distributed client-server application; multithreaded,

for

high

performance

in

applications

that

need

to

perform

multiple

concurrent

activities

,

such

as

multimedia;

and interpreted ,for maximum portability and dynamic capabilities.

Java

系统的出现使得这些需求变得简单，所以可以很容易的 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，为了最大的可移植性和动态能力。

Together, the above requirements comprise quite a collection of buzzwords, so let’s examine some of them

and their respective benefits before going on.

总的来说，上述包括相当多的专业术语 ，所以在继续之前先让我们来认识一下它们以及它们所带来的的各种好处。

What's completely new is the manner in which java and its run-time system have combined them to 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大的编译设计系统是一种全新的方式。

Developing

your

applications

using

Java

results

in

software

that

is

portable

across

multiple

machine

architectures, operating systems, and graphical user interfaces, secure, and high performance. With Java,

your job as a software developer is much easier--you focus your full attention on the end goal of shipping

innovative products on time, based on the solid foundation of Java. The better way to develop software is

here, now, brought to you by the Java language platform.

使用

Java

开发您的应用软件所带来 的是可在多种机器总体结构、

操作系统和图形用户借口间的移植，

安全性和高性能。

使用

Java

，< /p>

你作为软件开发人员的工作室非常容易的

--

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，现在，给您带来

J ava

语言平台。

Very dynamic languages like lisp, TCL, and SmallTalk are often used for prototyping. one of the

reasons for their success at this is that they are very robust---you don't have to worry about freeing or

corrupting memory.

非常具有生命力的语言如

< br>lisp

、

tcl

和

< p>
SmallTalk

常常用于充当原型构造技术。他们在这取得成功的其中 一个原因是：

他们非常强大，您不需要内存的释放或者损坏。

Similarly ,programmers can be relatively fearless about dealing with memory when programming in java .the

garbage collection system makes the programmer's job vastly easier with the burden of memory management

taken off the programmer's shoulders , storage allocation errors go away.

同样的，程序员在使用

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

下了程序员肩上存储管理的负担，存储分配错误不再发生。

Another reason commonly given that like lisp , TCl, and smalltalk are good for prototyping is that

they don't require you to pin down decision early on these languages are semantically rich.

一般认为 像

LISP

、

TCL

< br>和

SmallTack

这样一些语言十分适合于原型法的 另一理由，是因为它们不要求你受早期决定的

约束

---

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 . along with these

choices come a lot of assistance you can write method invocations and ,if you get something wrong, you get

told about it at compile time . you don't have to worry about method invocations error.

Java

具有完全相 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

---

如：

你可以写一些方法的调 用，

如果有错误，你会在编译时被告知。您也不需要担心方法调用错误。

4.1

An

operating

system

is

the

software

which

acts

as

an

interface

between

a

user

of

a

computer

and

the

computer

hardware.

操作系统是计算机用户和

< br>硬件的软件接口。

The purpose of an operating system is 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 operating system is thus to make the computer system convenient 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

an

operating

system

as

a

resource

allocator.

我们可以把操作系统看做是一个资源分配器。

A

computer

system has many resources which may be required to solve a problem: CPU time, memory space, file storage,

input/output(I/O) devices, and so on.

一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，文件存储，输入输出设备等。

The

operating

system

acts

as

the

manager

of

these

resources

and

allocates

them to specific programs and users as necessary for their tasks. < /p>

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。

Since

there

may

be

many,

possibly

conflicting,

requests

for

resources,

the o/s must decide which requests are allocated resources to operate the computer system fairly and

efficiently.

因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 co nsole.

早期的计算机是从控制台运行的体

积非常大的机器 。

The

programmer

would

write

a

program

and

then

operate

the

program

directly

from

the

operator

’

s

console.

程序员要写一条程序然后直接从操作者的控 制台操作程序。

Software

such

as

assemblers,

loaders,

and

compilers

improved

on

the

convenience

of

programming

the

system,

but

also

required

substantial

set-up

time.

象汇编程 序、装入程序及编译程序这样的软件使系统软件编程更方便，但同时也需要大量的启动时间。

To reduce the

setup time, operators were hired and similar jobs were batched together. [1]

为了能减少这些准备时间，操作

者就被雇佣，相似的作业 成批排在一起。

Batch systems allowed automatic job sequencing by a resident monitor and improved the overall utilization

of

the

computer

greatly.

批处理系统通过一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,

because of the slow speed of the I/O devices relative to the CPU. Offline operation of slow devices was

tried.

然而，< /p>

CPU

使用率仍然不高，这是因为

I/O

设备的处理速度较

CPU

慢导致的。< /p>

Buffering was another approach to improving system performance by overlapping the input, output, and

computation 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、输出和计算工作重叠

交 错进行。

Finally,

spooling

allowed

the

CPU

to

overlap

the

input

of

one

job

with

the

computation

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.

伪脱机也为那些已

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 concept of multiprogramming.

这

个作业池支持一种叫做多程序设计的概念。

With

multiprogramming,

several

jobs

are

kept

in

memory

at

one

time;

the CPU is switched back and forth between them in order to increase CPU utilization and to decrease the

total real time needed to execute a job.

在这种多程序设计技术中，多个作业可以同时存放在内存中；

CPU

在这

些作业中来回切换控制，以至于增加

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。

Time-shared operating systems allow many users (from one to several hundred) to use a computer system

interactive

at

the

same

time.

分时操作系统允许多个用户

（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 given the impression that he has his own

computer.

由于系统迅速地从一个用户切换到下一个用户，每一个用户被给予一种他拥有自己的计算机的印象。

Other

operating systems types include real-time systems and multiprocessor systems.

其他操作系统类型包括实时

系统和多处理机 系统。

A real-time system is often used as control de-vice in a dedicated application.

实时系统通常被用作专用

的应用的 控制设备。

Sensors bring data to the computer.

传感器把数据带给计算机。

The computer must analyze the data and possibly adjust controls to modify the sensor inputs.

计算机必须

分析数据，也可能调整控制去修改 传感器的输入。

Systems, which control scientific experiments, medical computer systems, industrial control systems, and

some display systems are real-time systems.

控制科学的实验，医学的计算机系统，工业的控制系统和一些显示< /p>

系统等系统都是实时系统。

A real-time operating system has well-defined fixed time constraints

．实时操作系统有定义明确的固定的时

间约束。

Processing must be done within the defined constraints, or the system will fail.

处理必须在定义的约束中

完成，否则系 统将失败。

A multiprocessor system has more than one CPU.

< p>
多处理机系统有不只一个

CPU

。

The obvious advantages would appear to be greater computing power and reliability.

显著的优势好像是强大

的计算能力 和可靠性。

There are various types of operating systems for multiprocessors and multicomputers.

有各种各样的操作系

统用于 多处理机和多计算机。

It

is

more

or

less

possible

to

distinguish

two

kinds

of

operating

systems

for

multiple

CPU

systems:

Loosely

coupled, such as network operating system and distributed o/s, and tightly coupled, such as parallel o/s.

这或多或少可能区分两种多

< p>
CPU

系统的操作系统：松耦合，例如网络操作系统和分布式操作系统，和 紧耦合，如并行

操作系统。

As we shall see, loosely and tightly-coupled s/w is rought analogous to loosely and tight-coupled h/w.

我

们应该看到，松紧耦合软件大概类似于松紧耦合硬件。

The operating system must ensure correct operation of the computer system.

操作系统必须确保改正计算机系

< br>统的操作。

To prevent user programs from interfering with the proper operation of the system, the h/w was modified

to create two modes: user mode and monitor mode[4].

为了阻止用户程序干扰系统适当的操作，硬件被修 改成两

种模式：用户模式和监控模式。

Various

instructions(such

as

I/O

instructions

and

halt

instructions)

are

priviledged

and

can

only

be

executed

in monitor mode.

许多指令（例如

I/O

指令和中断指令）是有特权的，只能在监控模式下被执行。

The memory in which the monitor resides must also be protected from modification by the user.

监控程序

所在的内存也必须保护起 来以防用户修改。

A

time

prevents

infinite

loops.

定

时

< br>器

可

以

防

止

死

循

环

。

Once

these

changes

(dual

mode,

privileged

instructions, memory protection, timer interrupt) have been made to the basic computer architecture, it

is possible to write a correct operating system.

一旦这些对基本计算机体系结 构的改变（双模，权限指令，内

存保护，定时器中断）被做到，这可能写成一个改进的操 作系统。

《计算机专业英语教程》

（第

4

版）

5.1 Internet

因特网

@ Many networks exist in the world, often with different hardware and software.

许多网络通常会以不同的硬件和软件的形式存在于世上。

People connected to one network often want to communicate with people attached to a different one.

attached to

附属于

连接了一个网络的人常常想与 附属于一个不相同的网络的人交流。

This

desire

requires

connecting

together

different,

and

frequently

incompatible

networks,

sometimes

by

using

machines called gateways to make the connection and provide the necessary translation, both in terms of

hardware and software.

Desire

心愿

frequently

频繁地

incompatible

不兼容的

in terms of

根据，在

???

方面

这个要求把不同的和频繁不兼容的网络连接在一起的心愿，有时通过使用叫做网关的机器去构成连接和 提供必要的转

换，都依据硬件和软件来实现。

A collection of interconnected networks is called an internetwork or just Internet.

A collection of ...

的集合

interconnected

互连

internetwork

互连网络

Internet

因特网

互连网络的集合被称为互联网络或只是因特网。

@ A common form of Internet is a collection of LANs connected by a WAN.

一种常见的形式的网络是通过一个广域网连接的局域网的集合。

The

Internet(note

uppercase

I)

means

a

specific

worldwide

internet

that

is

widely

used

to

connect

universities,

government offices, companies, and of late, private individuals.

Worldwide

全世界的

note

注意

uppercase

大写字母

internet

互联网

of late

近来

因特网（注意大写字母

I

）是一个特殊的全世界的互联网，被广泛的用来连接大学，政府部门， 公司和近来是私人。

@ By 1995, there were multiple backbones, hundreds of mid-level (i.e. regional) networks, tens of

thousands of LANs, millions of hosts, and tens of millons of users.

Backbones

主干网

到

1 995

年，已经有多个主干网，数以百计的中级网络，数以万计的局域网，数以百万计的 主机和数以千万计的用户。

The size doubles approximately every year.

Approximately

大约

这个大小大约每年翻倍。

Much of the growth comes from connecting existing networks to the Internet.

大部分的增长来自现有网络连接到因特网。

@ The glue that holds the Internet together is the TCP/IP reference model and TCP/IP protocol stack.

Gule

胶水

使因特网在一起的胶水是

TCP/IP

参考模型和

TCP/IP

协议栈。

TCP/IP

makes

universal

service

possible

and

can

be

compared

to

the

telephone

system

or

the

adoption

of

standard

gauge by the railroads in the 19th century.

compared to

比喻为

adoption

采用

gauge

轨迹

railroads

铁路

TCP/IP

< br>使通用服务成为可能和它可以比喻为电话系统或者在

19

世纪采用的标准的铁路轨迹。

@ The

Internet

is

a

vast

global

community

of

real

people

who

constantly

generate

more

unadulterated

stuff

on more topics than you could ever read in your lifetime.

Vast

广阔的

constantly

不断的

generate

生成

unadulterated

纯粹的

stuff

材料

因特网是现实世界中人们的巨大 的全球性社区。在这个网上，人们在众多主题上不断推出的有价值资料是你一辈子也

读不 完的。

If you haven’t tapped into the yet, here’s what you’re missing:

tapped into

接近

挖掘

如果你还没有挖掘到，那么下面就是你所缺少的：

E-mail. Let you send messages to Russia, Japan and so on.

E-mail.

< p>
可以让你发送信息到

Russia

，

Japan

等等。

File

Transfer

Protocol

(FTP).

Download

files

free

of

charge

from

thousands

of

computers

around

the globe.

free of charge

免费

文件 传输协议（

FTP

）

。从全球数以千计 的计算机那里免费下载文件。

Usenet

newsgroups.

More

banter,

blather,

and

nuggets

of

wisdom

than

you’ll

ever

be

able

to

read

on

everything

from archery to stock.

Usenet newsgroups

新闻群组

banter

开玩笑

blather

胡说

nuggets

珍闻

archery

箭术

stock

股票

新闻群组。

你将可以读到关于从箭术 到股票的任何方面的玩笑，胡说和智慧的珍闻。

The World Wide Web(WWW). A fast-growing global network of graphical electronic documents you can browse,

interact with, and even create yourself.

fast-growing

高速发展的

万维网（

WWW

）

。一个你可以浏览，交互甚至创建你自己的图 像电子文档的高速发展的全球的网络。

New technologies. Global chat, video conferencing, fax, free international phone calls and more.

International

国际的

新技术。

全球聊天，视频会议，传真，自由的国际电话和更多的。

The World Wide Web

万维网

@ The

World

Wide

Web

is

an

architectural

framework

for

accessing

linked

documents

spread

out

over

thousands

of machines all over the Internet.

spread out

传播

万维网是一个用来访问传播在全 部因特网中的数以千计的机器中链接的文档的体系结构框架。

Since

the

Web

is

basically

a

client-server

system,

we

discuss

both

the

client

(i.e.

user)

side

and

the

server

side.

Basically

基本上

the client side

客户端

the server side

服务器端

由于

Web

基本上是一个客户

-

服务器系统，客户端和服务器端我们都讨论。

The Client Side

客户端

@ From the users' point of view, the Web consists of a vast, worldwide collection of documents, usually

just called pages for short.

从用户的角度来看，

Web

由巨大的全世界范围收集的通常简 称为页的文件组成。

Each page may contain links (pointers) to other, related pages, anywhere in the world.

每一页可能包含链接到一些在世界任 何地方的其他的相关的页面的链接。

Users can follow a link (e.g., by clicking on it), which then takes them to the page pointed to.

用户可以跟踪一个链接（例如，通过点击它）

，然后带他们到所指向的页面。

This process can be repeated indefinitely, possibly traversing hundreds of linked pages while doing so.

Indefinitely

无限期地

traversing

经过

这个处 理可以无限重复，当我们那么做，可能经过数以百计的链接页面。

Pages that point to other pages are said to use hypertext.

Hypertext

超文本

指向其他一些页面的页面被称为使用超文本。

Web pages can combine color desktop publishing, hypertext linking, interactive scripting, sound, video,

and even virtual reality.

Combine

组合

publishing

出版

网页可 以组合彩色桌面出版，超文本链接，交互脚本，声音，视频甚至虚拟现实。

When hypertext pages are mixed with other media, the result is called hypermedia.

mixed with

与。

。

。混合

hypermedia

多媒体

当超文本与其他媒体混合，其结果就称为多媒体。

Pages are viewed with a browser.

使用浏览器浏览页面。

The Server Sider

服务器端

@ Every Web site has a server process listening to TCP port 80 for incoming connections from clients

(normally browsers).

Incoming

引入的

每个站点都有一个服务进程监听从客户端（通常是浏览器）引入的连接的

TCP

的

80

端口。

< br>

After a connection has been established, the client sends one request and the server sends one reply.

Established

建立

在一个连接已建立，客户端发送 一个请求和服务器发送一个回复。

Then the connection is released.

Released

释放

然后连接被释放了。

The protocol that defines the legal requests and replies is called HTTP.

Legal

合法的

那个定义合法的请求和回复的 协议叫做

HTTP

。

A simple example using it may provide a reasonable idea of how Web servers work.

一个使用

HTTP

的简单例子也许可以提供一个

< p>
Web

服务是如何工作的合理的想法。

Figure 5-1 shows how the various parts of the Web model fit together.

fit together

组装在一起

图< /p>

5-1

显示了

Web

模式的各个部分是如何组装在一起的。

@ For this example, we can imagine that the user has just clicked on some piece of text or perhaps on

an

icon

that

points

to

the

page

whose

name(URL-Uniform

Resource

Locator

)is

/hypertext/WWW/.

对于这个例子，我们可以 想象用户已经点击了指向名为

xxx

的页面的一些文本或者或许 是一个图标。

A URL has three parts:the name of the protocol (http), the name of the machine where the page is located

() , and the name of the file containing the page (hypertext/WWW/).

一 个

URL

有三个部分：协议的名称（

H TTP

）

，页面所在的机器的名称，和包含了页面的文件名。< /p>

The steps that occur between the user's click and the page being displayed are as follows:

Occur

发生

发生在用户点击和页面被显示间的步骤如下：

1The browser determines the URL (by seeing what was selected ).

Deterimines

决定

浏览器决定

URL

（通过查看那个被选择了）

。

2The browser asks DNS for the IP address .

浏览器向域名服务器要

< br>的

IP

地址。

3DNS replies with 18.23.0.23.

域名服务器返回

18

4The browser makes a TCP connection to port 80 on 18.23.0.23.

浏览器创建一个到

18

的

80

端口的

TCP

连接。

5It then sends a GET /hypertext/WWW/ command.

然后它发送一个

xx

命令。

ver sends the file .

7The TCP connection is released.

8The browser displays all the text in

9The browser fetches and displays all images in .

Fetches

获取

Surfing the Web

网络冲浪

@ Once your Web browser is open and you are connected to the Internet, the page currently designated

as your browser's starting page or home page will be displayed within the browser window.

Designate

指定

一旦你的网络浏览器打开然后你连接到了因特网，那个当前指定为你浏览器起始页或者首页的页面 将会在浏览器窗口

中显示。

(Usually this page is the home page for your browser's, school's , or ISP's Web site, but it can usually

be changed to any page using your browser's Options or Preferences dialog box.)

（通常这页是你的浏览器的，学校的，因特网服务提供商的网络站点，但是 你可以使用你的浏览器的选项或者偏爱对

话框来改变成任何页面。

）

@ All browsers have navigational tools to help you move forward or backward through the pages viewed

in your current Internet session, as well as buttons or menu options to print Web pages when necessary.

Navigational

引导的

所有的浏览器都有导航工具来 帮助你在你当前的因特网会话中向前或者向后查看页面，同样也有用来打印当需要时的

网 络页面的按钮和菜单选项。

Figure

5-2

illustrates

the

most

common

components

of

the

Microsoft

Internet

Explorer

and

Netscape

Navigator

browsers.

图

5

–

2

举例说明微软

Internet Explorer

和网景航海家浏览器的最常见的组成部分。

Using URLs and Hyperlinks

使用

URL

和超链接

< br>

@ To

change

from

the

starting

Web

page

to

a

new

Web

page,

you

can

type

the

appropriate

URL

in

the

browser's

address bar or location bar and press Enter.

Type

输入

为了从起始页转到一个新的页面，你可以在浏览器的地址栏和本地栏中输入一个合适的

URL

，然后按下回车键。

You can either edit the existing URL or delete it and type a new one.

你既可以编辑存在的

URL

也可以删除它然后输入一个新的。

Be sure to match the spelling, capitalization, and punctuation exactly.

Spelling

拼写

capitalization

大写

punctuation

标点

一定要准确的匹配拼写，大写和标点。

If

you

don’t

know

the

appropriate

URL

to

type,

you

can

search

for

an

appropriate

page,

as

discussed

shortly.

如果你不知道合适 的

URL

要输入，你可以搜索一个合适的页面，如不久前讨论的 。

@ If there is a hyperlink displayed for the page you would like to go to, simply click on the link.

如果这里有一个显示了你想要去的页面的链接，简单点击链接即可。

Remember, hyperlinks can be either text or image-based.

记住，超链接既可以是文本也可以是基于图像的。

If you are not sure if something on a page is a link or not, rest the mouse pointer on it for a moment.

如果你不确定在页面上的东西是链接或者不是，让 鼠标指向它一段时间。

If it is a hyperlink, the pointer should change to indicate that it is a link.

Indicate

表明

如果它是一个链接，那么指针会改变，这表明它是一个链接。

The URL for the new page is also displayed on the browser's status bar.

新页面的

URL

也会显示在浏览器的状态栏。

< br>

Once you click the hyperlink, the appropriate page is displayed.

一旦你点击了超链接，合适的页面被显示。

To return to a previous page, click the Back button on your browser's toolbar.

为了回到先前的页面，点击在浏览器上的工具条中的返回按钮。

To print the current Web page, use browser's Print button or select Print from the browser's File menu.

为了打印当前的网页，使用浏览器的打印 按钮或者从浏览器的文件菜单中选择打印。

Things You May Encounter on a Web Page

在网页上你将遇到的东西

@ You will encounter a variety of different objects on Web pages as you explore the World Wide Web.

Explore

浏览

当你在浏览万维网时，你将在网页上遇到各种各样的不同的对象。

Though we can’s go into an in

-depth discussion on the various possible Web-page components here, it is

good to be a familiar with the most common ones so you'll know how to deal with them as you encounter them.

在这里，尽管 我们不能对各种可能的网页组件进行深入的讨论，最好是对大部分的组件都熟悉，这样你就能知道当你

< p>
遇到它们时如何处理它们了。

Some common things you may run into are illustrated in Figure 5-3.

在图

5-3

中举例说明了一些你也许会遇到的常见的东西。

< p>
《计算机专业英语教程》

（第

4

< br>版）

5.2 Type of Connection

连接类型

@ As discussed in the previous chapter, there are a variety of wired and wireless ways to connect to

the Internet.

wired

有线的

wireless

无线的

正如前面章节讨论的，有各种 各样的的有线的和无线的方式来连接到因特网。

In virtually all setups, the computer to which you connect is usually connected to a large network called

a regional network; regional networks are connected to the major high-speed networks within each country

called backbone networks.

virtually

事实上

setups

设置

事实上所有的设置，你所连接的 计算机通常连接到一个叫做区域网的巨大的网络；区域网又连接到叫做主干网的在每

个国 家内部的主要高速网络。

Backbone

networks

within

a

country

are

connected

to

the

major

high-speed

networks

within

ecah

country

called

backbone networks.

一个国家中多个主干网连接到每个国家中的主要高速网络称为主干网。

Backbone networks within a country are connected together and to backbone networks in other countries to

form the Internet. < /p>

一个国家中的多个主干网的连接及与其他国家中的主干网的连接就形成了因特网。

@ The following sections discuss the most common types of Internet connections.

下面部分讨论了大部分常用的因特网连接类型。

-Up Connections

拨号连接

@ Dial-up connections usually work over regular telephone lines.

regular

正规的

拨号连接通常工作在正规的电话线。

To connect to the Internet, your modem (or other appropriate interface device ) dials up and connects to

a modem attached to your ISP's computer..

modem

调制解调器

为了连接到因特网，你的 调制解调器（或其他适当的接口设备）拨号然后连接到附属于你的因特网服务提供商的计算

机的调制解调器。

While you are connected, your PC is assigned a temporary IP address for the current session.

当你连接上了 ，你的电脑被指派一个用于当前会话的临时的

IP

地址。

At

the

end

of

each

Internet

session,

you

disconnect

from

your

ISP's

computer

to

allow

another

user

to

connect

in your place.

disconnect

断开一个网络连接

在每一次因特网会 话结束时，你从你的因特网服务提供商的计算机上断开一个网络连接以允许处于你位置的其他用户

去连接。

One advantage to a dial-up connection is security.

拨号连接的一个优势是安全。

Since

you

are

not

continually

connected,

it

is

much

less

likely

that

anyone

would

gain

access

to

your

computer

via the Internet, to either obtain information or mess with the data on your PC or, more commonly, to user

your computer in some type of illegal or unethical manner.

via

经由

mess with

搞糟

打扰

illegal

非法的

因为你 不持续地连接，所以不大可能有人能经由因特网获得你计算机的访问，或者获得你的

PC

上的信息或者搞糟电脑

的数据，或者更常见的是以某种不合法或 者不道德的方式使用你的计算机。

These types of hacking activities are discussed later.

hacking

黑客行为

这些不同类型的黑客行为活动后面将讨论。

@ With

a

dial-up

connection

your

telephone

will

be

tied

up

while

you

are

accessing

the

Internet,

unlesss

a second phone line is used.

tied up

被占用

使用拨号连接，当你正在访问 因特网时你的电话将被占用，除非第二条电话线被使用。

Some Internet call-waiting or call- forwarding services allow you to be notified when you get a telephone

call while you are connected to the Internet.

call-waiting

呼叫等待

call-forwarding

自动转接

在你连接了因特网期间，当 来了一个电话呼叫时，一些因特网呼叫等待或者自动转接服务会通知你。

They are generally set up to allow the person to leave a short message;

它们通常设置允许人们留下一个简答的信息。

some

newer

systems

give

you

a

short

window

of

time

to

disconnect

from

the

Internet

and

pick

up

the

telephone

call, if desired.

一些新的系统给你一个较短的时间从因特网上断开然后拿起电话，如果你愿意。

Newer dial-up modems are expected to facilitate some type of call-waiting service, as well.

facilitate

促进

新的拨号调制解调器被期望来促进某种类型的呼叫等待服务。

Standard Dial-up

标准拨号

@ Standard dial-up Internet service uses a conventional dial-up modem rated at a maximum data transfer

rate of 56 Kbps.

标准 拨号因特网服务使用一个最大数据交换数度是

56Kbps

的传 统的拨号调制解调器。

These modems are commonly used with PCs and Internet appplicances.

这些调制解调器通常用于

PCs

和因特 网应用。

Portable devices may also use a conventional dial-up modem; if so, they need to be connected to standard

telephone jacks to access the Internet.

Portable

便携的

jacks

插座

便携的设备也使用 传统的拨号调制解调器，假如这样，他们为了访问因特网需要连接到标准的电话插座上。

Standard dial-up Internet service range from free to about $$ 25 per month.

标准的拨 号因特网服务从免费到大约每个月

25

的范围内。

ISDN

综合业务数字网

@ Another dial-up option is ISDN.

另外一个拨号选择是综合业务数字网。

ISDN(integrated

services

digital

network)

allows

digital

transmission

of

data

over

ordinary

telephone

lines.

transmission

传输

综合业务数字网允许通过不同的电话线进行数据的数字传输。

It requires a special ISDN adapter or modem and can transfer data up to 128 Kpbs.

它需要一个特殊的

ISDN

适配器或者调制解调器， 可以传输数据达到

128Kbps

。

2. Direct Connections

直接相连

@ Unlike

dial-up

connections

that

only

connect

to

your

ISP's

computer

when

you

need

to

access

the

Internet,

direct connections keep you continually connected to the Internet.

不像拨号连接那样，当你需 要访问因特网时，只能连接到

ISP

的计算机上，直连使得你可 以持续连接到因特网。

With a direct (always-on) connection, your PC is typically issued a static (nonchanging) IP address to be

used to transfer data back and forth via the Internet.

issued

发布

通过直连，你的

PC

典型的发布一个用于经由因特网来回交换数据的静态

IP

地址。

PCs connected to the network always have an Internet connection available-Internet access requires only

opening a browser.

电脑连接到网络总是 有一个因特网连接可能的网络访问只需要打开一个浏览器。

@ Types of direct Internet connections include connecting through a school or office LAN, as well as

DSL, cable, satellite, and fixed wireless connections.

Cable

电缆

satellite

卫星

直接的因 特网连接类型包括通过一个学校或者政府机关的局域网以及数字用户线，电缆，卫星和固定无线连接进行连

接。

Though,

in

theory,

broadband

Internet

access

can

be

up

to

100

times

as

fast

as

a

dial-up

connection, actual

speeds at the present time are more like 25 to 50 times as fast.

Theory

理论

broadband Internet access

宽带上网

虽然

,

在理论上

,

宽带上 网可以快速到拨号连接的

100

倍

,< /p>

实际现在的速度则更像是

25

到

50

倍。

Typical download speeds for DSL and cable are about 1.5 Mbps;

数字用户线和电缆的典型地下 载速度大约是

1.5Mbps

。

satellite and fixed wireless usually download data between 500 Kbps to 1 Mbps.

< br>卫星和固定无线通常下载数据在

500Kbps

到

1Mbps

之间。

Virtually all of these services use slower upload speeds.

几乎所有这些服务使用较慢的上传速度。

The

most

significant

characteristics

of

each

of

these

types

of

broadband

Internet

access

are

discussed

next.

每一种这些宽带上网的类型的最重要的特点将在后面讨论。

School or Office LAN

@ Most

school

or

office

LANs

that

offer

a

direct

connection

to

the

Internet

lease

a

high- speed

dedicated

line (such as an ISDN or T1 line) from the telephone company or an Internet provider.

lease

租约

dedicated

专用的

大多数学校或者政府机关的局 域网，提供一个与因特网直接相连的从电话公司或者因特网提供者那里租约一条高速专

用 线。

The

speed

of

this

type

of

Internet

access

depends

on

the

speed

of

the

connection

between

the

school

or

offices

and their Internet provider, as well as the speed of the LAN itself.

这种类型的因特网接入 速度依赖学校或者政府机关和它们的因特网提供者间的连接速度，同样也依赖于局域网本身的

速度。

DSL

@ DSL (digital subscriber line) is currently available only to users who are relatively close (three

miles or less ) to a telephone switching station with telephone lines capable of handling DSL, and long

waits for installation are not uncommon.

subscriber

用户

capable

有能力的

DSL

目前只对那些较靠近电话交换站（

3

英里或小于

< p>
3

英里）

，有能力处理

D SL

电话线的用户可用，并且常要等很长时

间安装。

< p>

Nevertheless, DSL is a popular broadband option.

Nevertheless

然而

然而

, DSL

是一项很受欢迎的宽带的选择。

DSL

is

sometimes

offered

at

different

speeds

to

fit

different

needs

and

budgets,

such

as

a

slower,

less- expensive personal version and a faster business option.

Budgets

预算

DSL

有时提供不同的速度以满足不同的需要和预算，例如慢速低价的个人版本和快速的商业选择。

The monthly cost of standard DSL service is about $$50.

标准

DSL< /p>

的每月消费大约是

50

美元。

< p>

Cable

电缆

@ A third type of direct Internet connection-cable-is the most widely used home broadband alternative.

Alternative

供选择的

第三种因特网直连类型是电缆，它是最广泛使用的家庭宽度选择。

Cable

connections

are

very

fast

and

are

available

to

anyone

with

cable

access

whose

cable

provider

has

upgraded

the system to support Internet access.

upgraded

升级

电缆连接是相当快的，可以让任 何人通过电缆访问，电缆提供商升级了支持因特网访问的系统。

Just about all cable Internet access today is two-way (full-duplex), with the cable being used to transfer

data in both directions.

今天几乎所有的电缆因特网访问是双向的（全双工）

，通过在两个方向上用来交换数据的电缆。

One disadvantage of cable is that all users in an immediate geographical area share the bandwidth of their

local cable.

电缆的一个劣势是在邻近地区的所有用户共同使用他们本地电缆的带宽。

Though

this

may

not

prove

to

be

a

problem

all

the

time,

during

high-use

times

of

day-- such

as

early

evening--the

speed of cable Internet service can slow down dramatically as your neighbors go online.

dramatically

戏剧的

尽管这并不能证明是一个问题，在一天的高使用时间电缆因特网服务的速度会变低，戏剧 的你的邻居上线了。

Cost is about $$40 per month, in addition to regular cable fees.

in addition to

除

...

之外

fee

费用

费用大概是每月

40

美元，除了常规的电缆费用外。

Satellite

卫星

@ Satellite Internet access is often the only broadband option in rural areas.

rural

农村的

卫星因特网访问是在农村地区唯一的宽带选择。

It requires a satellite modem and a transceiver satellite dish mounted outside the home or building.

transceiver

收发器

dish

蝶形天线

mounted

安装

它需要人造卫星调制解调器和安 装在室外或建筑物外的收发卫星蝶形天线。

The two- way satellite services available today, such as StarBand, are relatively reliable, though an

unobstructed view of the southern sky is requited and access may degrade very heavy rainstorms.

虽然需要在蝶形天线收发方向的上空没有任何障碍，并且在大 暴风雨期间访问性可能降低，但目前可用的两路人造卫

星服务，如

StarBand

，是相当可靠的。

Typical cost is about $$70 per month.

典型的费用大约是每月

70

美元。

Fixed Wireless

固定无线

@ One last type of broadband access that is relatively new for consumer use but shows great promise is

fixed wireless.

最后一种宽度访问是固定无 线，这种访问方式对消费者使用是相当新的，但显示出很大的前途。

This type of service, such as Sprint Broadband Direct, is similar to satellite service in that it requires

a modem and an outside-mounted transceiver, but is typically available only in larger metropolitan

areas.

metropolitan

大都市的

这种服务的类型，例如

Sprint

Broadband

Direct

， 像卫星服务一样需要一个调制解调器和一个外部安装的收发器，但

通常只有在大的大城市 地区。

A

clear

line-or-sight

is

required

between

the

transceiver

and

the

provider's

radio

transmission

tower,

which

relays information between the user's transceiver and the Internet.

radio

无线电广播

tower

塔

relays

中继

转播

在收发器和提供商的无线传输塔之间需要有一条无障碍的视线通路，传输塔在用户的收发 器和

Internet

间转播信息。

Cost for service is about $$50 per month.

这种服务的费用大约是每月

50

美元。

@ Regardless of the type of connection used, a very important consideration for all users with a

direct

Internet connection is protecting the computer from unauthorized access or hackers.

Regardless of

不管

unauthorized

未经授权的

不管那种类型的连接被使 用，对所有直接连接到因特网的用户一个非常重要的考虑是保护计算机不受未经授权的访问

或者黑客的攻击。

While most schools and offices are protected by some type of firewall, many home direct connections are

not.

然而大多数学校和政府机关通过一些防火墙来保护， 许多直接连接的家庭则不是。

Though it is unlikely that someone would access your home PC in this manner, the possibility will continue

to increase as direct home connections become more common.

尽管不大可能有人访问你家的电脑用这种方 式，直接连接回家的可能性将持续增长也变得更加普遍。

To

protect

yourself,

all

computers

using

a

direct

connection

should

use

a

personal

firewall

program--especially those on a home network that allows file sharing.

especially

尤其

为了保护你自己，

所有使用了直连的计算机可以使用个人防火墙程序

--

尤其是那些允许文件共享的家庭网络的计算机。

< p>

Firewall

programs

typically

block

access

to

your

PC

from

outside

computers,

as

well

as

allow

you

to

specify

which programs on your PC can access the Internet.

block

阻止

specify

指定

防火墙程序典型地阻止从外部来 的一些计算机去访问你的电脑，同样也允许你指定在你计算机上的那些程序可以访问

因特 网。

Some firewall software, such as the Zone Alarm software, are available free for home use.

一些防火墙软件，例如

Zone Alarm

软件，对家庭可以免费使用。

Some firewall protection is beginning to be incorporated into other programs, such as Windows XP.

incorporated

包含

一些防火墙保护开始纳入其他程 序

,

如

Windows XP

。

3. Mobile Wireless Connections

移动无线连接

@ Unlike satellite and fixed wireless connections, which use a cable to connect the modem to some type

of fixed transceiver, mobile wireless connections allow the device to be moved from place to place.

不像卫星和固定的无线连接，使用电缆去 连接调制解调器或一些固定收发器，移动无线连接允许设备从一个地方移动

到另一个地方 。

Consequently,

most

handheld

PCs

and

other

mobile

devices

(like

Web-enabled

cell

phones)

use

a

mobile

wireless

connection

and

access

the

Internet

through

the

same

wireless

network

as

cell

phones

and

messaging

devices.

Consequently

因此

cell

电池

细胞

因此，大部分手提电脑和其他移 动设备（像能用

Web

服务的手机）使用移动无线连接然后通过 同样的无线网络如手机

和通信设备访问因特网。

Most mobile phones today capable of Internet access come

used to retrieve Web content, provided wireless Web service is included in the phone's service plan.

retrieve

检索

< p>
现在大多数能访问

Internet

的移动电话都 处于无线

Internet

就绪状态并且能直接用来检索

Web

内容，只要提供的无

线

Web

服务包括在该手机的服务计划中。

Many handheld PCs also have Internet connectivity built in; if not, a wireless modem can be used.

许多手提电脑也可以建立有互联网的连接

< br>;

如果不能

,

可以使用一个无线 调制解调器。

Alternately, a modem or other type of adapter can connect the device to a cell phone.

Alternately

交替地

交替地，一个调制解调器或者 其他类型的适配器可以连接设备到手机。

Notebook

PCs

can

connect

to

the

Internet

or

a

corporate

network

using

either

a

wireless

modem

or

cell

modem.

corporate

公司的

cell modem

细胞调制解调器

笔记本电脑使用无线 调制解调器或者细胞调制解调器可以连接到因特网或者一个公司的网络。

Some notebook computers today come with wireless capability built into the unit.

come with

伴随

built into

固定于

现在的一些笔记本电脑伴随有固定了无线能力的单元。

《计算机专业英语教程》

（第

4

版）

6.2 Carrier Frequencies and Multiplexing

载波频率和多路复用

@ Computer

networks

that

use

a

modulated

carrier

wave

to

transmit

data

are

similar

to

television

stations

that use a modulated carrier wave to broadcast video.

modulated

已调制的

使用调制载波发送数据的计 算机网络和利用调制载波广播视频信息的电视台相类似。

The similarities provide the intuition needed to understand a fundamental principle:

similarity

相似性

intuition

直观

这一相似性给理解下述基本原理提供了启示：

Two or more signals that use different carrier frequencies can be transmitted over a single medium

simultaneously without interference.

signal

信号

simultaneously

同时地

interference

干扰

两个或多个使用不同载波频率的信号可以在单一介质上同时传输而互不干扰。

@ To understand the principle, consider how television transmission works.

为理解这一原理，考虑（有线）电视传输是如何工作的。

Each television station is assigned a channel number on which it broadcasts a signal.

channel

频道

每个电视台都分配一个频道号，它在该频道上广播信号。

In fact, a channel number is merely shorthand for the frequency at which the station's carrier oscillates.

shorthand

速记

oscillate

振荡

事实上

,

频道号就是电视台载波用的振荡频率的速记。

To receive a transmission, a television receiver must be tuned to the same frequency as the transmitter.

tune to

调谐到

为接收一个频道传输，电视接 收器必须调谐到与发送器相同的频率。

More

important,

a

given

city

can

contain

many

television

stations

that

all

broadcast

on

separate

frequencies

simultaneously.

更重要的是，一个城市可以 有许多电视台，它们彼此在不同的频率上同时广播。

A receiver selects one to receive at any time.

一个接收器在任一时间选择接收其中一个。

@ Cable television illustrates that the principle applies to many signals traveling across a wire.

有线电视这一 例子说明了以上原理应用于许多信号在一根导线上同时传输的情况。

Although

a

cable

subscriber

may

have

only

one

physical

wire

that

connects

to

the

cable

company,

the

subscriber

receives many channels of information simultaneously.

虽然一个有线电视用户可能只有一根线连向有线电视公司，但用户可同时收到许多频道的信息。

The signal for one channel does not interfere with the signal for another.

一个频道中的信号不会与另一频道中的信号相互干扰。

@ Computer networks use the principle of separate channels to permit multiple communications to share

a single, physical connection.

permit

允许

计算机网络使用分离频道的原理使多个通信共享单根物理连线。

Each sender transmits a signal using a particular carrier frequency.

每个发送器使用一个特定载波频率传输信号。

A

receiver

configured

to

accept

a

carrier

at

a

given

frequency

will

not

be

affected

by

signals

sent

at

other

frequencies.

一个接收器被设置成接收给定频率 的载波，它将不会受其它频率信号的影响（或干扰）

。

All carriers can pass over the same wire at the same time without interference.

所有载波可在同一时间通过同一导线而互不干扰。

Frequency Division Multiplexing

频分多路复用

@ Frequency

division

multiplexing(FDM)

is

the

technical

term

applied

to

a

network

system

that

uses

multiple

carrier frequencies to allow independent signals to travel through a medium.

频分多路复用（

FDM

）是使用多个载波频率在一个介质中同时传输多个独立信号的计算机网络系统术语。

FDM technology can be used when sending signals over wire, RF, or optical fiber.

当通过导线、

RF

或光纤发送信号时便可使用

FDM

技术。

Figure 6-1 illustrates the concept, and shows the hardware components needed for FDM.

图

6-1

说明了这一概念并显示了

FDM

所需的硬件部件。

@ In theory, as long as each carrier operates at a different frequency than the others, it remains

independent.

理论上，工作在不同频率上的载波将保持相互独立。

In

practice,

however,

two

carriers

operating

at

almost

the

same

frequency

or

at

exact

multiples

of

a

frequency

can interfere with one another.

但实际上，两个频率相近或频率成整倍的载波会彼此干扰。

To

avoid

problems,

engineers

who

design

FDM

network

systems

choose

a

minimum

separation

between

the

carriers.

为了避 免这些问题，设计

FDM

网络系统的工程师们在各载波之间选择 一个最起码的频率分隔。

The

mandate

for

large

gaps

between

the

frequencies

assigned

to

carriers

means

that

underlying

hardware

used

with FDM can tolerate a wide range of frequencies.

mandate

授权

underlying

相关的

tolerate

容纳

在各载波频率之间要求有较大的 间隔意味着

FDM

所用的相关硬件能容纳很宽的频率范围。

Consequently, FDM is only used on high-bandwidth transmission channels.

因此，

FDM

仅用于高带宽传输通道中。

In summary,

概括地说，

Frequency

division

multiplexing(FDM)

allows

multiple

pairs

of

senders

and

receivers

to

communicate

over

a shared medium simultaneously.

频分多路复用允许多对发送和接收装置在一个共享介质 上同时通信。

The carrier used by each pair operates at a unique frequency that does not interfere with the others.

每对装置使用的载波频率唯一的，并彼此互不干扰的频率运行。

Time Division Multiplexing

时分多路复用

@ The general alternative to FDM is time division multiplexing (TDM), in which sources sharing a

medium

take turn.

与

FDM

不同的另一种复用形式是时分多路复用

(TDM)

，按这种方式，各发送源轮流使用共享的通信介质。

For example, some time-division multiplexing hardware use a round-robin scheme in which the multiplesor

sends a packet from sourcel, then sends a packet from souces2, and so on.

round-robin

循环

例如，某些

TDM

硬件使用循环法方案共享介质，用这种方法，多路复用器先发送源

< p>
1

的一个包，然后发送源

2

的一个

包，如此循环。

Figure 6-2 illustrates the idea.

图

6-2

说明了这种思想。

@ Dividing data into small packets ensures that all sources receive prompt service because it prohibits

one source from gaining exclusive access for an arbitrarily long ary

prompt

立刻地

prohibit

禁止

arbitrarily

任意地

exclusive

独占

把数据分成许多小包保证了所有 源都接受到即时服务，因为它禁止一个源任意长时间独占介质”

。

In particular, if one source has a few packets to send and another has many, allowing both sources to take

turns sending packets guarantees that the source with a small amount of data will finish promptly.

特别 是，如果一个源只有少量的包发送，而另一个源有许多包发送，于是允许两个源轮流发送包就保证只有少量数据< /p>

的源会很快结束。

@ In fact, most computer networks use some form of time division multiplexing. demultiplexor

实际上，大多数计算机网络使用某种形式的

TDM

。

《计算机专业英 语教程》

（第

4

版）

< br>

7.1 An Overview of a Database System

数据库系统概述

@ Let

us

consider

an

enterprise,

such

as

an

airline,

that

has

a

large amount

of

data

kept

for

long

periods

of time in a computer.

让我们考虑航空公司这样的企业，它有大量 的数据长时间保存在计算机中。

This data might include information about passengers, flights, aircraft, and personnel, for example.

这些数据，例如，可能包括关于乘客、航班、飞机与乘务组人员的信息。

Typical relationahips that might be represented include bookings (which passengers have seats on which

flights? ) flight crews (who is to be the pilot, copilot, etc., on which flights?), and service records

(when and by whom was each aircraft last serviced? ).

booking

预订

flight crew

机组人员

pilot

驾驶员

copilot

副驾驶员

可能表示的典型关系包括订 票（哪些乘客在那些航班上订有座位？）

、航班乘务组（在哪些航班上谁是驾驶员，谁是 副

驾驶员，等等）

，以及服务记录（每架飞机最后是何时由谁服 务？）

。

@ Data, such as the above, that is stored more-or-less permanently in a computer we term a database.

term

把

...

叫做

我们把上述那样的不同程度地长久存储在计算机中的数据称为数据库。

< br>

@ The software that allows one or many persons to use and/or modify this data is a database management

system(DBMS).

使得一个人或许多人能使用和< /p>

/

或修改这些数据的软件是数据库管理系统（

DBMS

）

。

The primary goal of a DBMS is to provide an environment that is both convenient and efficient to use in

retrieving information from and storing information into the database.

retrieving

检索

DBMS

的主要目标是提供这样的环境，在从数据库中检索信息和把信息存储在数据库中时使用起来既方便，功效又高。

Data Abstraction

数据抽象

@ It should be obvious that between the computer, dealing with bits, and the ultimate user dealing with

abstracions

such

as

flights

or

assignment

of

personnel

to

aricraft,

where

will

be

many

levels

of

abstraction.

显然，在处理二进位的计算机和处 理像航班或把乘务组人员分派到飞机这样一些抽象的最终用户之间将存在很多层抽

象。

A fairly standard viewpoint regarding levels of abstraction is shown in Figure 7-1.

关于抽象层的相当标准的观点如图

7-1

所示。

There we see a single database, which may be one of many databases using the smae DBMS software, at three

different levels of abstraction.

在那里我们看到

3

个不同抽象层的单个数据库，它可能是使用同一个

DBMS

软件的很多个数据库中的一个数据库。

@ The lowest level, i.e., the physical level has the data stored on hardware devices.

最低层，也即物理层，把数据存储在硬件设备上。

User programs cannot access them directly.

用户程序不能直接存取它们。

They have to go through the logical level to access the data.

它们必须通过逻辑层存取数据。

The

external

level

defines

the

different

views

of

the

database

as

required

by

the

extenal

or

user

programs.

外部层按外部或用户程序的需要定义数据库的不同视图。

One user program may not require all the data in the database.

一个用户程序不可能需要数据库中的所有数据。

Hence the user/application programs view only the required information from the database.

hence

因此

因此用户

/

应用程序仅观察所需的数据库信息。

That means different programs will have different views of the database depending on their requirement of

data.

这意味着取决于 对数据的需求，不同的程度对同一个数据库将有不同的视图。

Such views are external to the database and are specifical at the external level.

这样一些视图是数据库外部的，因而在外部层规定。

Also it is not necessary that different views should contain altogether different data.

不同的视图也不必要包含完全不同的数据。

There can be common information in different views.

在不同的视图中会存在公共的信息。

@ The conceptual level describes the entire database.

概念层描述整个数据库。

It is used by database administrators, who must decide what information is to kept in the database.

它是由数据库管理人员使用的，他们必须决定什么信息要被保存在 数据库中。

Data Models

数据模型

@ A

data

model

is

a

collection

of

conceptual

tools

for

describing

data,

data

relationships,

data

semantics

and data constrains.

semantic

语义的

数据模型是一组概念工具，用 来描述数据、数据关系、数据语义和数据约束。

The data models are divided into three classes, viz., object-based logical models, record-based logical

models and physical data models.

数据模型分成三类，就是基于对象的逻辑模型、基于记录的逻辑模型和物理数据模型。

@ Object-based logical models are used for describing data at the conceptual and view levels.

基于对象的逻辑模型用于描述概念层与视图层的数据。

They are very close to human logic.

它们非常接近于人的逻辑。

Many different models are available to describe object- based logical models.

很多不同的模型可用来描述基于对象的逻辑模型。

The most important among them are semantic data model and entity-relationship model.

其中最重要的是语义数据模型和实体关系模型。

Semantic data model provides a facility for expressing meaning about the data in the database.

语义数据模型提供了表达数据库中数据的意义的设施。

The Entity-Relationship model (E-R model) is based on a perception of a real world which consists of a

collection of objects called entities and relationships among these objects.