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What is computer virus

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1. ? A virus is a piece of sof ware desig ed a d wri e o adversely affec your compu er by al eri g he way i works wi hou your k owledge or permissio . I more ech ical erms, a virus is a segme of program code ha impla s i self o o e of your execu able files a d spreads sys ema ically from o e file o a o her. Compu er viruses do o spo a eously ge era e: hey mus be wri e a d have a specific purpose. Usually a virus has wo dis i c fu c io s: Spreads i self from o e file o a o her wi hou your i pu or k owledge. ech ically, his is k ow as self-replica io a d propaga io . Impleme s he symp om or damage pla ed by he perpe ra or. his could i clude erasi g a disk, corrup i g your programs or jus crea i g havoc o your compu er. ech ically, his is k ow as he virus payload, which ca be be ig or malig a a he whim of he virus crea or. A be ig virus is o e ha is desig ed o do o real damage o your compu er. For example, a virus ha co ceals i self u il some prede ermi ed da e or ime a d he does o hi g more ha display some sor of message is co sidered be ig . A malig a virus is o e ha a emp s o i flic malicious damage o your compu er, al hough he damage may o be i e io al. here are a sig ifica umber of viruses ha cause damage due o poor programmi g a d ou righ bugs i he viral code. A malicious virus migh al er o e or more of your programs so ha i does o work, as i should. he i fec ed program migh ermi a e ab ormally, wri e i correc i forma io i o your docume s. Or, he virus migh al er he direc ory i forma io o o e of your sys em area. his migh preve he par i io from mou i g, or you migh o be able o lau ch o e or more programs, or programs migh o be able o loca e he docume s you wa o ope . Some of he viruses ide ified are be ig ; however, a high perce age of hem are very malig a . Some of he more malig a viruses will erase your e ire hard disk, or dele e files. Wha Viruses Do Some viruses are programmed specifically o damage he da a o your compu er by corrup i g programs, dele i g files, or erasi g your e ire hard disk. Ma y of he curre ly k ow Maci osh viruses are o desig ed o do a y damage. However, because of bugs (programmi g errors) wi hi he virus, a i fec ed sys em may behave erra ically. Wha Viruses Do ' Do Compu er viruses do ' i fec files o wri e-pro ec ed disks a d do ' i fec docume s, excep i he case of Word macro viruses, which i fec o ly docume s a d empla es wri e i Word 6.0 or higher. hey do ' i fec compressed files ei her. However, applica io s wi hi a compressed file could have bee i fec ed before hey were compressed. Viruses also do ' i fec compu er hardware, such as mo i ors or compu er chips; hey o ly i fec sof ware. I addi io , Maci osh viruses do ' i fec DOS-based compu er sof ware a d vice versa. For example, he i famous Michela gelo virus does o i fec Maci osh applica io s. Agai , excep io s o his rule are he Word a d Excel macro viruses, which i fec spreadshee s, docume s a d empla es, which ca be ope ed by ei her Wi dows or Maci osh compu ers. Fi ally, viruses do ' ecessarily le you k ow ha hey are here - eve af er hey do some hi g des ruc ive.

1 2. ypes of Compu er Viruses owadays umber of viruses is abou 55000. I i creases co s a ly. ew u k ow ypes of viruses appear. o classify hem becomes more a d more difficul . I commo hey ca be divided by hree basic sig s: a place of si ua i g, used opera io sys em a d work algori hms. For example accordi g hese hree classifica io s virus Cher obyl ca be classified as file i fec or a d reside Wi dows virus. Fur her i will be explai ed wha i mea s. 2.1 A place of exis e ce 2.1.1File I fec ors hese are viruses ha a ach hemselves o (or replace) .COM a d .EXE files, al hough i some cases hey ca i fec files wi h ex e sio s .SYS, .DRV, .BI , .OVL a d .OVY. Wi h his ype of virus, u i fec ed programs usually become i fec ed whe hey are execu ed wi h he virus i memory. I o her cases hey are i fec ed whe hey are ope ed (such as usi g he DOS DIR comma d) or he virus simply i fec s all of he files i he direc ory is ru from (a direc i fec or). here are hree groups of file i fec ors. Viruses of he firs group are called overwri i g viruses because hey overwri e heir code i o i fec ed file erasi g co e s. Bu hese viruses are primi ive a d hey ca be fou d very quickly. O her group is called parasi ic or cavi y viruses. I fec ed file is capable of work fully or par ly bu co e s of las o e are cha ged. Viruses ca copy i self i o begi , middle or e d of a file. hey record heir code i da a k ow o o be used. hird group is called compa io viruses. hey do ’ cha ge files. hey make double of i fec ed file so whe i fec ed file is bei g s ar ed a double file becomes ma agi g, i mea s virus. For example compa io viruses worki g wi h DOS use ha DOS firs ly ru s COM. file a d af er if his file is o fou d ru s EXE. file. Viruses make double file wi h a same ame a d wi h ex e sio COM a d copies i self i his file. Duri g s ar of i fec ed file DOS ru s a COM. file wi h a virus firs ly a d he a virus s ar s a EXE. file. Some ime compa io viruses re ame file will be i fec ed a d record heir code i a double file wi h old ame. For example he file XCOPY.EXE is re amed i o XCOPY.EXD a d virus record i self i file XCOPY.EXE. Whe his file is s ar ed compu er ru s a virus code firs ly a d af er virus s ar s origi al XCOPY, saved as XCOPY.EXD. Viruses like his were fou d o o ly i DOS. hey were fou d i Wi dows a d OS/2. I is o o ly o e way o make double files. For example here is subgroup of compa io viruses called pa h-compa io viruses. hey use special fea ure of DOS - PA H: hierarchical record of file loca io . Virus copies i self i file wi h he same ame bu si ua ed o e level higher. I his case DOS will fi d file wi h virus. 2 2.1.2Boo viruses Boo Sec or I fec ors Every logical drive, bo h hard disk a d floppy, co ai s a boo sec or. his is rue eve of disks ha are o boo able. his boo sec or co ai s specific i forma io rela i g o he forma i g of he disk, he da a s ored here a d also co ai s a small program called he boo program (which loads he DOS sys em files). he boo program displays he familiar &quo ; o -sys em Disk or Disk Error&quo ; message if he DOS sys em files are o prese .

I is also he program ha ge s i fec ed by viruses. You ge a boo sec or virus by leavi g a i fec ed diske e i a drive a d reboo i g he machi e. Whe he boo sec or program is read a d execu ed, he virus goes i o memory a d i fec s your hard drive. Remember, because every disk has a boo sec or, i is possible (a d commo ) o i fec a machi e from a da a disk. O E: Bo h floppy diske es a d hard drives co ai boo sec ors. Mas er Boo Record I fec ors he firs physical sec or of every hard disk (Side Ш, rack Ш, Sec or 1) co ai s he disk's Mas er Boo Record a d Par i io able. he Mas er Boo Record has a small program wi hi i called he Mas er Boo Program, which looks up he values i he par i io able for he s ar i g loca io of he boo able par i io , a d he ells he sys em o go here a d execu e a y code i fi ds. Assumi g your disk is se up properly, wha i fi ds i ha loca io (Side 1, rack Ш, Sec or 1) is a valid boo sec or. O floppy disks, hese same viruses i fec he boo sec ors. You ge a Mas er Boo Record virus i exac ly he same ma er you ge a boo sec or virus -- by leavi g a i fec ed diske e i a drive a d reboo i g he machi e. Whe he boo sec or program is read a d execu ed, he virus goes i o memory a d i fec s he MBR of your hard drive. Agai , because every disk has a boo sec or, i is possible (a d commo ) o i fec a machi e from a da a disk. 3 2.1.3 Mul i-par i e Viruses Mul i-par i e viruses are a combi a io of he viruses lis ed above. hey will i fec bo h files a d MBRs or bo h files a d boo sec ors. hese ypes of viruses are curre ly rare, bu he umber of cases is growi g s eadily. 2.1.4 Macro Viruses U il rece ly, he macro la guages i cluded wi h mos applica io s were o powerful or robus e ough o suppor wri i g a effec ive virus. However, ma y of he more adva ced applica io s ha are bei g developed oday i clude buil -i programmi g capabili ies ha rival some of he larger developme packages. his has rece ly bee demo s ra ed by he various s rai s of Microsof Word viruses, i cludi g he so-called Word Co cep a d Word uclear viruses. hese viruses ra spor hemselves hrough Microsof Word docume s. Whe ope ed i Word, hey perform various ac io s, i cludi g spreadi g hemselves i o he user's i s alla io of Word, hus prepari g o i fec all fu ure docume s o he sys em. A addi io al co cer is ha macro viruses ca be cross-pla form. he Word Co cep virus has he claim o fame of bei g he firs promi e cross-pla form virus, because i ca i fec bo h Wi dows a d Maci osh sys ems. Because mos applica io macro la guages suppor passi g execu io o a ex er al shell, such as COMMA D.COM or CMD.EXE, he power of he macro virus is o limi ed o he co s rai s of he macro la guage i self4. 2.2 Used opera io sys em. A y compu er or e virus ca i fec files of o e or more opera io sys ems: DOS, Wi dows, OS/2, Li ux, MacOS a d o hers. I is a base of his way of classifica io . For example virus BOZA worki g wi h Wi dows o ly is classified as Wi dows virus, virus BLISS – as Li ux virus. 2.3 Work algori hms. Viruses ca be differed by used algori hms maki g hem da ger a d hard for ca chi g.

As for Paperbacks are cheaper than hardcovers, most of us would accept the sentence as true as a general rule of thumb, even if we knew that lots of individual paperbacks (say, imports) are more expensive than many individual hardcovers (such as discounted bestsellers printed in large quantities). We agree with the statement Mosquitoes carry the West Nile vi *Is that good or bad? That depends on your point of view. The logic of partial matches is what makes languages sloppy, and, for better or worse, keeps poets, stand-up comedians, and linguistic curmudgeons gainfully employed. ("Didja ever notice that a near-miss isn't a miss at all?") rus, even if only (say) 1 percent of mosquitoes carry the virus, yet we wouldn't accept the statement Dogs have spots even if all the dalmatians in the world did. Computer-programming languages admit no such imprecision; they have ways of representing formal quantifiers ([DO THIS THING REPEATEDLY UNTIL EVERY DATABASE RECORD HAS BEEN EXAMINED ] ) but no way of expressing generics at all

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