D Is For Digital

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"D Is For Digital" 是一本由 Brian Kernighan 撰写的关于计算机与通信基础知识的书籍。这本书旨在让知情者了解计算机和通信领域的关键概念。在书的描述中，我们可以看出 Brian Kernighan 是一位知名的计算机科学家和作家，他与多位著名专家合作撰写了多本经典的编程书籍，包括《编程风格的要素》、《Software Tools》系列、《C程序设计语言》（与D.M.Ritchie合著）、《AWK编程语言》（与A.V.Aho和P.J.Weinberger合著）、《Unix编程环境》（与R.Pike合著）以及《编程实践》（与R.Pike合著）。这些书籍对计算机科学教育和实践有着深远的影响。 "D Is For Digital" 这本书可能涵盖了计算机科学的基础，如数据表示、计算原理、网络通信、编程基础等主题。考虑到作者的专业背景，书中可能使用了简洁明了的语言和实际示例来解释复杂的概念。此外，书的排版使用了 groff、ghostscript 和其他开源 Unix 工具，这反映了作者对开放源代码工具的偏好和支持。这本书的出版由 DisforDigital.net 负责，读者可以通过电子邮件联系 info@disfordigital.net 获取更多信息。它由 CreateSpace 打印，并有 ISBN-13:978-1463733896 和 ISBN-10:1463733895 作为身份标识。版权归 Brian W. Kernighan 所有，未经许可，任何部分不得复制或以任何形式传播。总结知识点： 1. 作者 Brian Kernighan 是计算机科学领域的权威，尤其在编程语言和软件开发方面有深厚造诣。 2. "D Is For Digital" 是一本面向对计算机和通信感兴趣的读者的基础知识书籍。 3. 书中可能涵盖的主题包括计算机基础、数据表示、网络通信、编程语言和实践应用。 4. 作者支持并使用开源 Unix 工具进行排版工作，体现出对开源社区的贡献。 5. 出版和信息查询途径通过 DisforDigital.net 提供，同时由 CreateSpace 打印发行。 6. 该书受到版权保护，未经授权，禁止复制和传播。

Introduction

“Any sufﬁciently advanced technology is indistinguishable from magic.”

Arthur C. Clarke, Report on Planet 3, Technology and the Future,1972

Here are twostories about technology,one of worldwide interest, the other entirely

personal and close to home.

In January 2011, popular revolutions in Tunisia and Egypt overthrewlong-time

dictators and led to a breath of freedom in countries that had suffered repression for

manyyears. In both cases, huge numbers of ordinary citizens broadcast news of

ev e nts and rallying places by Twitter,theytook cell phone pictures and movies of

protests and police brutality,and theyposted them on Facebook and YouTube to

inspire their fellowcitizens and inform the world. In Tunisia, during the most dra-

matic periods citizens posted nearly 20,000 tweets per hour,which were posted to

Facebook and blogs, and re-tweeted. ForEgypt, Google quickly wrote an application

called speak2tweet that let anyone send a tweet by making a phone call and speaking

the message; the messages were automatically converted into text and posted. One

could read the messages in their original languages (often French and Arabic) or have

them spoken aloud, or translated into other languages through Google Translate. In

both countries, government efforts to control communications during the events were

unsuccessful, though whether the revolutions will lead to long-term change is

unclear.

Shortly before Christmas in 2010, I wondered brieﬂy about buying a bigger televi-

sion set. Iturned on my Android phone, and spokethe words “big screen TV” into a

search engine. Back came a list of a dozen or more sets with 45-inch or larger

screens. Most were way too expensive but there was one at $650 that looked promis-

ing. I clicked the link and was offered a choice of buying online or checking the

DISFOR DIGITAL

situation at local stores. That latter path led to a list of more than half a dozen stores,

listed by increasing distance from my home, where I was sitting at the time. There

were customer reviews of both the set and the stores themselves. I could order it

right awaytobepicked up, and there were driving instructions to the selected store,

with time, distance, and turn-by-turn directions for the fastest route; current trafﬁc

wassuperimposed on images that gav e aperspective viewofthe terrain and street

levelviews of the roads. In the end I decided to wait—my current television set is

big enough for the modest amount that I watch it—but I was tempted.

Do these stories seem likemagic, or do you just takethe technical wizardry for

granted? Either way, this is today’stechnology,not evenadvanced, and certainly not

magic. If there’sany magic at all, it’sinthe degree to which the technological world

has changed so much and so quickly in the past ten or twenty years, and in its

unimaginable effect on people’sliv e s.

Let’slook brieﬂy at some of today’stechnologies as theywere twenty years ago,

say in the early 1990s. If you’re a college student or recent graduate right now, this is

what the world was likewhen you were born. Dinosaurs no longer walked the earth

(your parents excepted) but most of what I described above didn’texist at all.

Phones.In1990, telephones were bulky, had a rotary dial or 12 numeric buttons

and no display,were connected by wires to phone outlets in homes and ofﬁces, and

could only be used to talk to people, or,with the help of special equipment, to con-

nect to fax machines and computers. By 2000, cell phones were becoming common

buttheycould only be used for phone calls. Cell phones are nowubiquitous, out-

numbering wired phones nearly everywhere (80 percent of Egypt’spopulation have

cell phones; in Tunisia, it’sabout 75 percent). Smartphones likethe iPhone (released

in mid 2007) have anamazing array of capabilities that can be extended by down-

loading applications from Apple’sApp Store, which opened for business in the mid-

dle of 2008, or the Android Market.

Pictures.In1990, most cameras took pictures on ﬁlm, a roll of plastic coated

with sophisticated dyes that changed properties after the ﬁlm was exposed to light

(by taking a picture) and run through an elaborate sequence of chemical processes

(“developed”). A roll of ﬁlm held at most 36 pictures, and there was no way to see

the results without having the roll developed, a process that took anywhere from an

hour to a week. Once that was done, all you had was a piece of paper; if you wanted

to share a picture with friends, you had to showittothem in person or mail it to them

(in a paper envelope). You probably kept your pictures at home in envelopes or

boxes scattered around the house. Extra copies cost another trip to the store and

more money. Photographywas expensive enough that most people didn’ttakeall

that manypictures. But digital cameras begantosupplant ﬁlm cameras, slowly at

ﬁrst and then rapidly,tothe point where ﬁlm has pretty much disappeared for most

people. Today,photographs are sent by email, uploaded to web sites likeFacebook

and Flickr,and downloaded to digital picture frames. High quality photo printers are

cheap if you do want paper copies, and inexpensive point and shoot cameras are

INTRODUCTION

being superseded by cell phone cameras.

Music.In1990 music was distributed as albums on CDs and audio tapes; long-

playing vinyl records (“LPs”) were on the way out, though still common. If you

wanted to makeacopyofasong to share with a friend or listen to in your car,you

would likely use a cassette recorder,though you might be able to makeacopyofa

CD. There wasn’tmuch if anydownloading of music either.That changed dramati-

cally in 1999 with Napster,which made it possible to easily share music online.

Napster died, or,more accurately,was killed quickly,but online services likeiTunes

(which dates from 2003) rose out of the ashes, selling single songs. On the portable

hardware side, we went from boomboxes and cassette players to CD players to MP3

players likethe iPod before settling on the phone.

Email.Twenty years ago, most people didn’tuse email, or evenknowwhat it

was. The movie “You’ve Got Mail,”starring MegRyan and Tom Hanks, appeared in

1998, when email was just starting to catch on with the general public, for whom

“mail” still meant paper envelopes brought by a letter carrier.Today most people use

online services likeGmail for personal correspondence; their mail exists somewhere

on the Internet, accessed from their phones.

Facebook.In1990 there was no Facebook; it was born in 2004 (and Mark

Zuckerberg, its founder,was born in 1984). If you wanted to keep in touch with

friends, you had to call them on the phone or send them letters. Tenyears ago you

might use email. Today,manypeople prefer Facebook posts and texting.

YouTube.There was no YouTube either; it beganlife in 2005. If you wanted to

share home movies with other people, it was likeﬁlm, only worse; video recorders

were bulkyand awkward, and videotape was expensive.Today most cell phones take

decent digital video, and storage for it is cheap and compact; in mid 2011, people

were uploading nearly 50 hours of video to YouTube every minute.

Twitter.Twitter was launched in the middle of 2006. Blogging was somewhat

popular at that time (the word “blog”, a contraction of “web log”, ﬁrst appeared in

1999), but Twitter introduced the idea of microblogging, and a mechanism for imme-

diate rebroadcasting to very large audiences.

Speechprocessing.In1990, it was barely possible for a machine to understand

spoken language, and then only for a restricted vocabulary and under laboratory con-

ditions. Today if you call some companytoget service or register a complaint, it’s

hard to avoid “talking” to a speech-recognition system; getting to a real person is

nearly impossible. This is not necessarily progress. Machine translation was in

much the same situation; today,useful, if not perfect, translations are freely available

from anyofnearly sixty languages into anyofthe others. Speech synthesis—speech

from text—is routine, though readily distinguishable from a real voice.

Shopping.Twenty years ago there were no online stores; the closest parallel

might be shopping from paper catalogs sent by retailers likeSears or L.L.Bean. If

you wanted to compare features or look for the best price, you might be able to use

the advertising ﬂyers that arrivedatyour physical mailbox, but more likely you’d

have tovisit stores in person to do comparisons. Product reviews generally appeared

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only in magazines like Consumer Reports.Today search engines provide links to

physical and online stores, along with prices and reviews, all accessible from a

phone, sitting at home, or standing in a store where scanning a barcode provides the

information.

Maps.In1990, if you didn’tknowhow toget somewhere, you’dhav e to ﬁnd the

address from a phone book, look on a paper map to see where it was, then navigate

while driving and holding the map. If you got lost, you’dhav e to ﬁgure out for your-

self where you were and howtoget back on track. Yo u’dhav e to guess about trafﬁc,

or hope to ﬁnd a report on the radio. If you wanted to see what the route looked like

from the air,you’dneed to rent a plane! GPS navigation devices for cars appeared

about ten years ago, solving manyofthe navigation problems, and today theyare

being supplanted by similar systems running on phones. And with street viewmaps,

you can see pictures of your destination before you arrive.

The purpose of this book is to remove any vestige of magic, so you understand

howsuch systems operate. Howcan pictures, music, and movies be sent around the

world in no time at all? Howdoes email work? Howprivate is your email? Whyis

spam so easy to send and so hard to get rid of? Do cell phones really knowwhere

you are? HowdoiPhones and Android phones differ and whyare theyfundamen-

tally exactly the same? By the end, you should have a pretty decent idea of how

computer and communications systems work and howtheyaffect you.

Ahandful of important underlying ideas, which I will discuss in much more detail

in the rest of the book, cut across the hardware, software and communications organi-

zation mentioned above.

First is the universal digital representation of information.Complexand sophisti-

cated mechanical systems likethose for storing pictures and music have been

replaced by a single uniform mechanism. This is possible because information is

represented in digital form (as numbers) rather than in specialized forms (colored

dyes embedded in plastic ﬁlm or magnetic patterns on audio tape, for example).

Printed mail is replaced by digital mail. Paper maps are replaced by electronic ones.

Different analog representations of information are replaced by a single digital repre-

sentation.

Second is the universal digital processor.All of this information is being pro-

cessed by a single device, the digital computer.Digital computers that process the

uniform digital representation of information have replaced complicated physical

machines that process specialized analog representations. I’ve mostly avoided the

word “computer” so far,but in fact a cell phone is a computer of considerable sophis-

tication and with as much computing power as a laptop that you might have used half

adozen years ago. As we’ll see, computers are all equally capable in what theycan

compute, differing only in howfast theyoperate and howmuch data theycan store.

It’sinevitable that more and more of what might have been limited to desktop or lap-

top computers will ﬁnd its way onto cell phones. If anything, this convergence

process is accelerating.

INTRODUCTION

Third is the universal digital network.The Internet connects the digital comput-

ers that process the digital representation; it connects computers and cell phones to

mail, search, social networks, shopping, banking, and everything else. Youcan

exchange email with anyone, regardless of where theymight be or howtheychoose

to access their mail. Youcan search, comparison shop and purchase from your

phone, laptop or tablet. Social networks keep you in touch with friends and family,

again from phone or computer.Ofcourse there is great deal of infrastructure that

makes all these computing services work.

An immense amount of digital data is also continuously being collected and

updated. Maps, aerial and street-levelphotographs of the whole world are freely

available. Search engines tirelessly scan the entire Internet so theycan answer

queries efﬁciently.Millions of books are available in digital form. Social networks

and sharing sites maintain enormous amounts of data for and about us. Online stores

and services provide access to their wares while quietly recording everything we do

when we visit them online, aided and abetted by search engines and social networks.

Forall of our online interactions, Internet service providers log the connections we

makeand perhaps more.

All of this is changing rapidly because digital systems are getting smaller,faster,

and cheaper at an exponential rate: every year or two, things are twice as powerful for

the same price, or cost half as much as theydid. Newphones with fancier features,

better screens, and more interesting applications arrive continuously.New gadgets

appear all the time; the most useful ones often ﬁnd their functionality subsumed into

phones overtime. This is a natural by-product of digital technology,inwhich any

technological development leads to improvement across the board for digital devices:

if some change makes it possible to handle data cheaper,faster or in larger quantity,

all devices will beneﬁt. As a result, digital systems are pervasive,anintegral part of

our livesboth visibly and behind the scenes.

This progress must surely seem wonderful, and indeed in most ways it is. But

there are occasional clouds around the silver lining. One of the most obvious and

perhaps the most worrying to individuals is the impact of technology on personal pri-

vacy.When I searched for a TV,then visited store web sites, all parties kept records

of what I visited and what I clicked on. Howdid theyknowwho I was? Well, I was

using a cell phone, which identiﬁes me uniquely.How did theyknowwhere I was so

theycould give methe list of stores? That’seasy.Our cell phones report our loca-

tions all the time.Mycell phone has GPS so the phone companyhad me located to

within about ﬁvemeters in my house; evenwithout GPS, theyknowmylocation to

within a hundred meters or so. The tracking that we permit without eventhinking

about it makes the monitoring in Orwell’s 1984 look casual and superﬁcial.

The records of what we do and where we do it are also likely to live forever. Dig-

ital storage is so cheap and the value of data is high enough that information is rarely

discarded. If you post something embarrassing online or send mail that you subse-

quently regret, it’stoo late. Information about you can be combined from multiple

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