计算机英文文献和翻译Word文件下载.doc
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计算机英文文献和翻译Word文件下载.doc
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计算机病毒
指导教师:
李正民职称:
副教授
学生姓名:
张永星学号:
20052430239
专业:
计算机科学与技术
院(系):
信息工程学院
完成时间:
2009年5月25日
2009年05月25日
COMPUTERVIRUSES
Whatarecomputerviruses?
AccordingtoFredCohen’swell-knowndefinition,acomputervirusisacomputerprogramthatcaninfectothercomputerprogramsbymodifyingtheminsuchawayastoincludea(possiblyevolved)copyofitself.Notethataprogramdoesnothavetoperformoutrightdamage(suchasdeletingorcorruptingfiles)inordertobecalleda“virus”.However,Cohenusesthetermswithinhisdefinition(e.g.“program”and“modify”)abitdifferentlyfromthewaymostanti-virusresearchersusethem,andclassifiesasvirusessomethingswhichmostofuswouldnotconsiderviruses.
Computervirusesarebitsofcodethatdamageoreraseinformation,files,orsoftwareprogramsinyourcomputer,muchlikevirusesthatinfecthumans,computervirusescanspread,andyourcomputercancatchaviruswhenyoudownloadaninfectedfilefromtheInternetorcopyaninfectedfilefromadiskette.Oncethevirusesisembeddedintoyourcomputer’sfiles,itcanimmediatelystarttodamageordestroyinformation,oritcanwaitforaparticulardateoreventtotriggeritsactivity.
Whatarethemaintypesofviruses?
Generally,therearetwomainclassesofviruses.ThefirstclassconsistsofthefileInfectorswhichattachthemselvestoordinaryprogramfiles.Theseusuallyinfectarbitrary.COMand/or.EXEprograms,thoughsomecaninfectanyprogramforwhichexecutionisrequested,suchas.SYS,.OVL,.PRG,&
.MNUfiles.
Fileinfectorscanbeeitherdirectactionorresident.Adirect-actionvirusselectsoneormoreotherprogramstoinfecteachothertimetheprogramwhichcontainsitisexecuted,andthereafterinfectsotherprogramswhen“they”areexecuted(asinthecaseoftheJerusalem)orwhencertainotherconditionsarefulfilled.TheViennaisanexampleofadirect-actionvirus.Mostothervirusesareresident.
Thesecondclassissystemorboot-recordinfectors:
thoseviruses,whichinfectexecutablecode,foundincertainsystemareasonadiskthatarenotordinaryfiles.OnDOSsystems,thereareordinaryboot-sectorviruses,whichinfectonlytheDOSbootsectorondiskettes.ExamplesincludeBrain,Stoned,Empire,Azusa,andMichelangelo.Suchvirusesarealwaysresidentviruses.
Finally,afewvirusesareabletoinfectboth(theTequilavirusisoneexample).Thereareoftencalled“multipartite”viruses,thoughtherehasbeencriticismofthisname;
anothernameis“boot-and-file”virus.
Filesystemorclusterviruses(e.g.Dir-II)arethosethatmodifydirectorytableentriessothatthevirusisloadedandexecutedbeforethedesiredprogramis.Notethattheprogramitselfisnotphysicallyaltered;
onlythedirectoryentryis.Someconsidertheseinfectorstobeathirdcategoryofviruses,whileothersconsiderthemtobeasub-categoryofthefileinfectors.
Whataremacroviruses?
Manyapplicationsprovidethefunctionalitytocreatemacros.Amacroisaseriesofcommandstoperformsomeapplication-specifictask.Macrosaredesignedtomakelifeeasier,forexample,toperformsomeeverydaytasksliketext-formattingorspreadsheetcalculations.
Macroscanbesavedasaseriesofkeystrokes(theapplicationrecordwhatkeysyoupress);
ortheycanbewritteninspecialmacrolanguages(usuallybasedonrealprogramminglanguageslikeCandBASIC).Modernapplicationscombinebothapproaches;
andtheiradvancedmacrolanguagesareascomplexasgeneralpurposeprogramminglanguages.Whenthemacrolanguageallowsfilestobemodified,itbecomespossibletocreatemacrosthatcopythemselvesfromonefiletoanother.Suchself-replicatingmacrosarecalledmacroviruses.
MostmacrovirusesrununderWordforWindows.Sincethisisaverypopularwordprocessor,itprovidesaneffectivemeansforvirusestospread.MostmacrovirusesarewrittenusingthemacrolanguageWordBasic.WordBasicisbasedonthegoodoldBASICprogramminglanguage.However,ithasmany(hundredsof)extensions(forexample,todealwithdocuments:
edit,replacestring,obtainthenameofthecurrentdocument,opennewwindow,movecursor,etc.).
WhatisaTrojanhorseprogram?
Atypeofprogramthatisoftenconfusedwithvirusesisa‘Trojanhorse’program.Thisisnotavirus,butsimplyaprogram(oftenharmful)thatpretendstobesomethingelse.
Forexample,youmightdownloadwhatyouthinkisanewgame;
butwhenyourunit,itdeletesfilesonyourharddrive.Orthethirdtimeyoustartthegame,theprogramE-mailyoursavedpasswordstoanotherperson.
Note:
simplydownloadafiletoyourcomputerwon’tactivateaviru
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