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开发：长沙通信职业技术学院英语教研组 Computer English New Horizon College English 《计算机英语》电子教案 Lesson 10 Computer Viruses

What are computer viruses? What are computer viruses? • According to Fred Cohen's well-known definition, a computer virus is a computer program that can infect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself. Note that a program does not have to perform outright damage (such as deleting or corrupting files) in order to be called a "virus". However, Cohen uses the terms within his definition (e.g. "program" and "modify") a bit differently from the way most anti-virus researchers use them, and classifies as viruses some things which most of us would not consider viruses. Computer English

What are computer viruses? • Computer viruses are bits of code that damage or erase information, files, or software programs in your computer. Much like viruses that infect humans, computer viruses can spread. Your computer can catch a virus when you download an infected file from the Internet or copy an infected file from a diskette. Once the virus is embedded into your computer's files, it can immediately start to damage or destroy information, or it can wait for a particular date or event to trigger its activity. Computer English

What are the main types of PC viruses? • Generally, there are two main classes of viruses. The first class consists of the file Infectors which attach themselves to ordinary program files. These usually infect arbitrary .COM and/or .EXE programs, though some can infect any program for which execution is requested, such as .SYS, .OVL, .PRG, & .MNU files. • File infectors can be either direct action or resident. A direct-action virus selects one or more other programs to infect each time the program which contains it is executed. A resident virus hides itself somewhere in memory the first time an infected program is executed, and thereafter infects other programs when "they" are executed (as in the case of the Jerusalem) or when certain other conditions are fulfilled. The Vienna is an example of a direct-action virus. Most other viruses are resident. Computer English

What are the main types of PC viruses? • The second class is system or boot-record infectors: those viruses, which infect executable code, found in certain system areas on a disk that are not ordinary files. On DOS systems, there are ordinary boot-sector viruses, which infect only the DOS boot sector, and MBR viruses which infect the Master Boot Record on fixed disks and the DOS boot sector on diskettes. Examples include Brain, Stoned, Empire, Azusa, and Michelangelo. Such viruses are always resident viruses. Computer English

What are the main types of PC viruses? • Finally, a few viruses are able to infect both (the Tequila virus is one example). These are often called "multipartite" viruses, though there has been criticism of this name; another name is "boot-and-file" virus. • File system or cluster viruses (e.g. Dir-II) are those that modify directory table entries so that the virus is loaded and executed before the desired program is. Note that the program itself is not physically altered, only the directory entry is. Some consider these infectors to be a third category of viruses, while others consider them to be a sub-category of the file infectors. Computer English

What do viruses do and how do they work? • To understand what damage viruses can implicit, we need to understand how they work and what they do. The methodology of virus infection was pretty straightforward when first computer viruses such as Lehigh and Jerusalem started appearing. A virus is a small piece of computer code, usually from several bytes to a few tens of bytes, that can do, well, something unexpected. Such viruses attach themselves to executable files-programs, so that the infected program, before proceeding with whatever tasks it is supposed to do, calls the virus code. One of the simplest ways to accomplish that is to append the virus code to the end of the file, and insert a command to the beginning of the program file that would jump right to the beginning of the virus code. After the virus is finished, it jumps back to the point of origination in the program. Such viruses were very popular in the late eighties. The earlier ones only knew how to attach themselves to .COM files, since structure of a .COM file is much simpler than that of an .EXE file-yet another executable file format invented for MS-DOS operating system. The first virus to be closely studied was the Lehigh virus. It attached itself to the file that was loaded by the system at boot time-COMMAND.COM. The virus did a lot of damage to its host, so after three-four replications it was no longer usable. For that reason, the virus never managed to escape the university network. Computer English

What do viruses do and how do they work? • Modern viruses can exist on any system from MS DOS and Windows 3.1 to MacOS, UNIX, OS/2, Windows NT. Some are harmless, though hard to catch. They can play a jingle on Christmas or reboot your computer occasionally. Others are more dangerous. They can delete or corrupt your files, format hard drives, or do something of that sort. There are some deadly ones that can spread over networks with or without a host, transmit sensitive information over the network to a third party, or even mess with financial data on-line. Computer English

What are macro viruses? • Many applications provide the functionality to create macros. A macro is a series of commands to perform some application-specific task. Macros are designed to make life easier; for example, to perform some everyday tasks like text-formatting or spreadsheet calculations. Computer English

What are macro viruses? • Macros can be saved as a series of keystrokes (the application records what keys you press); or they can be written in special macro languages (usually based on real programming languages like C and BASIC). Modern applications combine both approaches; and their advanced macro languages are as complex as general purpose programming languages. When the macro language allows files to be modified, it becomes possible to create macros that copy themselves from one file to another. Such self-replicating macros are called macro viruses. Computer English

What are macro viruses? • Most macro viruses run under Word for Windows. Since this is a very popular word processor, it provides an effective means for viruses to spread. Most macro viruses are written using the macro language WordBasic. WordBasic is based on the good old BASIC programming language. However, it has many (hundreds of) extensions (for example, to deal with documents: edit, replace string, obtain the name of the current document, open new window, move cursor, etc.). Computer English

Words & Expressions • anti-virus 反病毒 • append v. 附加，添加 • attach v. 系上，贴上，依附上 • boot-record 引导区记录 • cluster n. 串，簇 • corrupt a. 腐败的，被破坏的 v. 腐蚀，使恶化 • definition n. 定义，解说 • financial a. 财政的，金融的 • fulfill v. 履行，实现，完成(计划等) • implicit a. 暗示的，含蓄的，固有的，隐含的 • infect v. 传染，感染 Computer English

Words & Expressions • infector n. 传播者，传染者，感染者 • Jerusalem n. 耶路撒冷[巴勒斯坦著名古城] • keystrokes n. 键击，按键 • loosely ad. 宽松地，松散地 • macro n. 宏 • MBR(Master Boot Record) 主引导区记录 • mess n. 混乱，脏乱 v. 弄乱 • methodology n. 方法学，方法论 • modify v. 更改，修改 • multipartite a. 分成多部分的，多国参加的 Computer English

Words & Expressions • occasionally ad. 有时候，偶而 • outright a. 直率的，完全的 • physically ad. 身体上地 • replication n. 复制 • resident n. 居民 a. 常驻的 • straightforward a. 易懂的，直接了当的 • sub-category n. 子类 • term n. 学期，术语 • transmit v. 传输，转送，发射 • trigger v. 引发，引起，触发 • Vienna n. 维也纳[奥地利首都] • virus n. 病毒 Computer English

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