第11章计算机学科基础

Chapter11,FileSystemsandDirectories,11-2,ChapterGoals,Describethepurposeoffiles,filesystems,anddirectoriesDistinguishbetweentextandbinaryfilesIdentifyvariousfiletypesbytheirextensionsExplainhowfiletypesimprovefileusageDefinethebasicoperationsonafile,11-3,ChapterGoals,CompareandcontrastsequentialanddirectfileaccessDiscusstheissuesrelatedtofileprotectionDescribeadirectorytreeCreateabsoluteandrelativepathsforadirectorytreeDescribeseveraldisk-schedulingalgorithms,11-4,FileSystems,FileAnamedcollectionofrelateddataFilesystemThelogicalviewthatanoperatingsystemprovidessothatuserscanmanageinformationasacollectionoffilesDirectoryAnamedgroupoffiles,11-5,TextandBinaryFiles,TextfileAfileinwhichthebytesofdataareorganizedascharactersfromtheASCIIorUnicodecharactersetsBinaryfileAfilethatcontainsdatainaspecificformat,requiringinterpretation,11-6,TextandBinaryFiles,ThetermstextfileandbinaryfilearesomewhatmisleadingTheyseemtoimplythattheinformationinatextfileisnotstoredasbinarydataUltimately,allinformationonacomputerisstoredasbinarydigitsThesetermsrefertohowthosebitsareformatted:aschunksof8or16bits,interpretedascharacters,orinsomeotherspecialformat,11-7,FileTypes,Mostfiles,whethertheyareintextorbinaryformat,containaspecifictypeofinformationForexample,afilemaycontainaJavaprogram,aJPEGimage,oranMP3audioclipThekindofinformationcontainedinadocumentiscalledthefiletypeMostoperatingsystemsrecognizealistofspecificfiletypes,11-8,FileTypes,Filenamesareoftenseparated,usuallybyaperiod,intotwopartsMainnameFileextensionThefileextensionindicatesthetypeofthefile,Figure11.1Somecommonfiletypesandtheirextensions,11-9,FileOperations,CreateafileDeleteafileOpenafileCloseafileReaddatafromafileWritedatatoafileRepositionthecurrentfilepointerinafile,AppenddatatotheendofafileTruncateafile(deleteitscontents)RenameafileCopyafile,11-10,FileAccess,Figure11.2Sequentialfileaccess,11-11,FileAccess,SequentialaccessInformationinthefileisprocessedinorder,andreadandwriteoperationsmovethecurrentfilepointerasfarasneededtoreadorwritethedataThemostcommonfileaccesstechnique,andthesimplesttoimplement,11-12,FileAccess,DirectaccessFilesareconceptuallydividedintonumberedlogicalrecordsandeachlogicalrecordcanbeaccesseddirectlybynumber,11-13,FileAccess,Figure11.3Directfileaccess,11-14,FileProtection,Inmultiusersystems,fileprotectionisofprimaryimportanceWedontwantoneusertobeabletoaccessanotherusersfilesunlesstheaccessisspecificallyallowedAfileprotectionmechanismdetermineswhocanuseafileandforwhatgeneralpurpose,11-15,FileProtection,AfilesprotectionsettingsintheUnixoperatingsystemisdividedintothreecategoriesOwnerGroupWorld,Page356,11-16,DirectoryTrees,AdirectoryoffilescanbecontainedwithinanotherdirectoryThedirectorycontaininganotherisusuallycalledtheparentdirectory,andtheoneinsideiscalledasubdirectoryDirectorytreeAlogicalviewofafilesystem;astructureshowingthenesteddirectoryorganizationofafilesystemRootdirectoryThedirectoryatthehighestlevel,11-17,DirectoryTrees,Figure11.4AWindowsdirectorytree,11-18,DirectoryTrees,Atanypointintime,youcanbethoughtofasworkinginaparticularlocation(thatis,aparticularsubdirectory)WorkingdirectoryThesubdirectoryinwhichyouareworking,11-19,Figure11.5AUnixDirectoryTree,11-20,PathNames,PathAtextdesignationofthelocationofafileorsubdirectoryinafilesystem,consistingoftheseriesofdirectoriesthroughwhichyoumustgotofindthefileAbsolutepathApaththatbeginsattherootandspecifieseachstepdownthetreeuntilitreachesthedesiredfileordirectoryRelativepathApathnamethatbeginsatthecurrentworkingdirectory,11-21,PathNames,ExamplesofabsolutepathC:ProgramFilesMSOfficeWinWord.exeC:MyDocumentslettersapplicationsvaTech.docC:WindowsSystemQuickTimeSupposethecurrentworkingdirectoryisC:MyDocumentslettersThenthefollowingrelativepathnamescouldbeusedcancelMag.docapplicationscalState.doc,11-22,DiskScheduling,FilesystemsmustbeaccessedinanefficientmannerAsacomputerdealswithmultipleprocessesoveraperiodoftime,alistofrequeststoaccessthediskbuildsupDiskschedulingThetechniquethattheoperatingsystemusestodeterminewhichrequeststosatisfyfirst,DiskScheduling,Figure11.5UnixDirectoryTree,11-23,11-24,DiskScheduling,First-Come,First-ServedRequestsareservicedintheordertheyarrive,withoutregardtothecurrentpositionoftheheadsShortest-seek-time-first(SSTF)DiskheadsaremovedtheminimumamountpossibletosatisfyapendingrequestScanDiskheadscontinuouslymoveinandoutservicingrequestsastheyareencountered,11-25,DiskScheduling,SCANDiskSchedulingworkslikeanelevatorAnelevatorisdesignedtovisitfloorsthathavepeoplewaiting.Ingeneral,anelevatormovesfromoneextremetotheother(say,thetopofthe