COMPUTER VIRUS
It’s March 14，11：55 PM．A group of business partners are putting the finishing touches on an important report．After celebrating the completion of their efforts they identify a previously unnoticed typo．At 12：05 AM they turn the computer back on only to be greeted with a message saying“Beware the Ides of March．”[1] The hard drive spins furiously and the report is deleted by a computer virus．
Computer viruses are just one example of what is commonly referred to as malicious code or malicious programs．Malicious programs are created to perform a series of harmful actions on a computer system．Examples of some actions include file deletion，file corruption，data theft，and the less harmful but equally annoying practical joke．These programs often remain dormant and hidden until an activation event occurs．Examples of activation events are program execution and specific access dates such as March 15，system reboot，and file access．When the predetermined activation event occurs，the malicious program begins its task．In the example above，this task was the deletion of all files in the computer system．
Theory behind Viruses
To better understand what a virus can do，it is helpful to understand how a virus performs its task．Fig. 18-4 contains a flow chart describing the basic actions of a virus．Each action identified in the figure is numerically labeled for explanatory purpose only．The order in which these actions are performed is indicated both pictorially，in Fig. 18-4 and in the discussion which follows．While the order of these actions may vary with each virus，the general process remains unchanged—perform an undesirable task and infect other programs and operating systems．
For this discussion two terms are defined：virus activation and virus execution．Virus activation will refer to the initiation of the virus．Virus execution，however，will refer to the initiation of the portion of the virus that performs the possibly harmful activity the code not directly concerned with infecting a system．[2] Thus，virus execution must be preceded by activation，but activation may not necessarily lead to execution．
Once an infected program is executed or an infected operating system performs a task，the virus is activated．The virus will first determine whether it should be executed．In the event that it should not be executed，it will attempt to locate and identify other susceptible programs，disks，or systems．Any such item will then be infected．After determining that all susceptible items carry a copy of the virus，the virus will stop and allow normal operations to proceed．If the virus meets all of its internal conditions to execute，it will do so．Upon completion of execution，the virus may either reset its conditions or remove itself．The virus will complete and allow normal computation to continue．While this process may seem time consuming and obvious to the user，it is not[3]．Computers operate so quickly nowadays that this process may go easily unnoticed，and often does．
NOTES
[1]Ides of March指刺殺古羅馬獨裁者愷撒（Julius Caesar）的預示日（3月15日），即一個不吉利的日子。
[2]句中，由that引出的定語從句修飾它前面的portion；the code...是介詞賓語to the initiation的補足語，中間被定語從句隔開。
[3]While引出的是讓步狀語從句。
KEYWORDS
typo 非正式的打印錯
malicious code 惡意代碼
malicious program 惡意程序
activation event 激活事件
system reboot 系統重新引導
EXERCISES
1．Multiple choices．
（1）Computer viruses are ．
a．useful programs b．malicious code
c．malicious programs d．harmful programs
（2）Virus program consists of ．
a．two parts b．three parts
c．four parts d．five parts
（3）The harmful actions performed by viruses include ．
a．data theft b．practical joke
c．file corruption d．file deletion
（4）The virus execution is ．
a．the virus activation
b．the code not directly concerned with infecting a system
c．the initiation of the portion of the virus
d．the code directly concerned with infecting a system
（5）The virus is activated when ．
a．an infected program is executed
b．a computer is accessing a mailbox through the Internet
c．a computer is in shutdown status
d．an infected operating system performs a task
（6）Objects that are susceptible from the viruses are ．
a．CPU b．disks
c．systems d．programs
（7）A virus program often remains ．
a．dormant b．open
c．hidden d．awaked
（8）If a virus does not begin its activity，it ．
a．stops its activity
b．performs its activity
c．updates its execution condition
d．finds the infect programs after updating its execution condition
2．True／False．
（1） Viruses can cause programs crashing or entire hard disk deleting．
（2） Some one died as the result of a computer virus．
（3） Before performing any actions you’d better treat all files and programs with virus check software．
（4） An infected computer may lose its data．
（5） In reality the viruses and their destructive capabilities have been grossly exaggerated by people．
答案：
1．
（1）b，c，d （2）a （3）a，b，c，d （4）b，c
（5）a，b，d （6）b，c，d （7）a，c （8）c，d
2．
（1）t （2）f �。�3）t
（4）t �。�5）t
翻譯：
計算機病毒
3月14日晚11時55分，一群商業伙伴正在對一個重要報告進行最后的修飾，在慶祝他們完成使命之后，他們認出一個以前未注意到的打印錯誤。上午12時5分，他們再把計算機打開，不料面前呈現一條消息“當心不吉利的日子”，硬盤飛快地轉動，隨即該報告被計算機病毒刪除了。
計算機病毒正是通常所說的惡意代碼或惡意程序的一個例子。惡意程序的編寫是對計算機系統實施一系列傷害動作的。例如文件的刪除，文件的毀壞，數據的竊取以及傷害不大但完全令人討厭的玩笑。這些程序往往處于休眠和隱藏狀態，直到一個激活事件產生。激活事件的例子是程序的執行和專門發作日期，如3月15日，系統自舉和文件讀寫。當預先確定的激活事件發生時，惡意程序便開始工作。在上面例子中，這一工作就是刪除計算機系統中的所有文件。
病毒背后的理論
為了更好地知道病毒能做什么，了解病毒如何完成它的任務是有幫助的。圖18-4所示是描述病毒基本動作的流程圖。為了說明圖中每個動作，都用數字標出。這些動作完成的順序可用圖18-4所示的圖示方式指明，也可在下面討論。雖然這些動作的順序可能對每種病毒都不同，但一般過程保持不變一一完成一個不希望的任務并且傳染給其他程序和操作系統。
根據這種討論定義了兩個術語：病毒激活和病毒發作。病毒激活是指病毒啟動，而病毒發作是指可能造成危害動作的病毒部分的啟動，該代碼不直接去感染一個系統。因此，病毒發作之前必須激活，但激活不一定導致發作。
一旦受感染的程序被執行或者受感染的操作系統完成一項任務，病毒就會被激活。該病毒首先確定它應該在什么地方被執行，萬一它不被執行，它將力圖尋找并確認其他易受感染的程序、磁盤或系統。任何類似的對象將被感染。在確定易受感染的對象得到一份病毒的復制品之后，該病毒將停止并且讓正常操作繼續進行。如果滿足了所有的內部執行條件，病毒就發作。發作完成以后，該病毒要么恢復其狀態，要么自行刪除。該病毒將結束并允許繼續正常運算。這一過程可能看起來是費時的并且對用戶來說是很明顯的，其實不然。當今計算機的運算如此之快，以至這一過程很容易不被注意并經常如此。
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