保护和安全英文文档格式.docx

保护和安全英文文档格式.docx

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devicemanagement,processandresourcemanagement,memorymanagement,filemanagement,andfunctionalorganization.Next,wewilladdressgeneralimplementationmethodologies:

performanceandtrustedsoftware.Softwaremodularizationissusesareintroducedinthischapter,andarediscussedinmoredetailinChapter19.ThischapterconcludeswithadescriptionofthegeneralorganizationofUNIXkernelsandtheWindowsNTkernel.

BASICFUNCTIONS

TherearetwobasicresponsibilitiesoftheOS（seeFigure3.1）:

⏹Createanabstractmachineenvironmentwithmultiple,autonomousabstractcomponents.Manyofthecomponentscanbeinuseconcurrently.Forexample,theOSusesmultiprogrammingtocreateanabstractmachineforeachprocess.

⏹Coordinatetheuseofthecomponentsaccordingtothepoliciesofthemachine’sadministrator.Forexample,theschedulerdecideswhenandwhichdifferentprocessesshouldbeallocatedtheprocesspor.

ThecreationpartoftheOSprovidesthespectrumofabstraction（suchasprocesses,threads,andresources）thatprogrammersuse,andthecoordinationpartmanagestheirconcurrentusesothatthecommunityofprocessesworksinharmony.

◆FIGURE3.1PurposeofanOS

PartofanOScreatesasetofabstractionsthatwillbeusedbyapplicationprogrammerstheseabstractionsincludeprocesses,threads,andfiles.InamultiprogrammingOS,multipleprocessescompetefortheuseoftheabstractresources,sotheOScorrdinatesthewaythattheseprocessesusetheabstractresources.

ThereisnocompleteagreementabouttheexactsetoffunctionsrequiredforanOS.Instead,eachOSprovidesasetdeterminedbyengineeringandmarketingchoices.OurgoalistolearnthegeneralprinciplesbehindOSfunctions,andthentoputthoseprinciplestoworkinOS-specificLaboratoryRxercises.Overtheyears,OSfunctionshavebeencharacterizedassatisfyingoneofthefollowingbasicrequirements:

Devicemanagement

Process,thread,andresourcemanagement

Memorymanagement

Filemanagement

Wewillusethesegeneralcharacterizationsasaframeworkforconsideringdetailedrequirements,designissues,architectures,andimplementation.Let’sfirstconsiderageneraldescriptionofeachofthesecomponentsofanOS.

DEVICEMANAGEMENT

TheOSmanagestheallocation,isolation,andsharingofthedevicesaccordingtopolicieschosenbythedesignerorsystemadminstrator.Evenoperationsystemsthatdonotsupportmultiprogrammingincorporatedevicemanagrment.Mostoperatingsystemstreatalldevicessuchasdisks,tapes,terminals,andprintersinthesamegeneralmanner,buttheyprovidespecialmanagementapproachesfortheprocessorandmemory.Devicemanagementreferstothewaythegenericdevicesarehandled.

Therearedevice-dependentanddevice-independentpartsofadevicemanager（seeFigure3.2）.Thedependentparts,calleddevicedrivers,implementtheaspectsofdevicemanagementthatareuniquetoeachdevicetype.Forexample,adevicedriverforakeyboardisconstructedexplicitlytosensekeystrokesfromakeyboarddeviceasthecorrespondingkeysarepressed.Adevicemanagerforadisplayscreenisconstructedtowritecharactersorgraphicstoabideodisplay.

◆FIGURE3.2DeviceManagement

Thedevicemanageriscomposedofadevice-independentpartandacollectionofdevice-dependentparts,oneforeachtypeofdevice.Thedevice-dependentpartsexportservicestothedeviceindependentpart,whichprovidesaunifiedinterfaceforallthedifferentdevicetypes.

Theindependentpartofthedevicemanagerdfinesageneralsoftwareenvironmentinwhichadevice-dependentdrivercanexecute.Forexample,theindependentpartofthedevicemanager.Mostofthefunctionalityisimplementedinthecollectionofdevicedrivers.

Bypartitioningthedesignintodependentandindependentcomponecnts,thetaskofaddingadevicetoacomputerisgreatlysimplified.First,theOSdesignerdecideswhichaspectsofdevicemanagementaredevice-dependentandwhichpartscanbeindependentofalldevices.Theindependentpartsaretheimplementedidthebaseoperationsystem（theywillworkwithalldevices）.Thedependentpartsareimplementedinthedriverforeachdevicetype.Thismeansthattheimdependentpartofthedevicemanagerhassystemcallstoreadand/orwriteanydevice.Aprinterdevicedrivercontainsallthesoftwarethatisspecifictoaparticulartypeofprinter（suchasaPostscript）.SincetheindependentpartsaregenericforalldevicesandarebulitintotheOS,thedesignercanadddevicedriverstotheOSwheneveradeviceisaddedtothecamputer.

DevicemanagementisanimportantbutrelativelysimplepartoftheoverallOSdesign.Chapter4introducesdevicebehavior（fromasystemprogrammer’sperspective）aspartofthecomputerorganizationdiscussion.Chapter5addressesdevicemanagementindetail.

PROCESS,THREAD,ANDRESOURCEMANAGEMENT

Processesandthreadsarethebasicunitsofcomputationdefinedbyprogrammers,and（abstract）resourcesaretheelementsofthecomputingevironmentneededbyaprocesssothatitsthreadscanexecute.ThispartoftheOSimplementsallpartsoftheabstractmachinethatcreatetheabstractionsofprocesses,threads,andresources（seeFigure3.3）.ThispartoftheOSiscompletelyresponsibleformanagingthehardwareprocessorresourceandvariousabstractresourcessuchasmessages.Itsharestheresponsibilityofmanagingthehardwareprimarymemorywiththememorymanager.

◆FIGURE3.3Process,Thread,andResourceManagement

Theprocess,thread,andresourcemanagerisresponsiblefortheadministrationoftheprocessorandvariousabstractresources.Itcooperateswiththememorymanagertoadministertheprimarymemory.

Processmanagement,threadmanagement,andresourcemanagementcouldbesepatatedintotheirownlogicalunits,butmostoperatingsystemscombinethemintoasinglemodule,sincetogethertheydefinetheessentialpartsoftheabstractmachineevironment.Inthisbook,thispartoftheOSissimpleyreferredtoasthe“processmanager”ratherthanthemoreproper（andlonger）name“process,thread,andresourcemanager.”

InChapter2,theUNIXprocessmodelwasdescribedasoneexampleofhowanOScandefineacomputationalenvironment.AUNIX-styleOSprovidesasetofprocessmanagementfacilitiesforcreating,destroying,blocking,andrunningaprocess.Chapter2alsodesribedthemoremodernthread-basedapproachinwhichthecomputationalelementisdividedintoastaticprocesspartandadynamicthreadpart.Theprocessmanagerforthread-basedsystemsismorecomplexthanoneforprocess-basedsystemssinceitmustmanageprocessesandthreadsasseparateentities.

Theresourcemanagerallocatesresourcestoprocesseswhentheyarerequestedbyathread,andkeepstrackofresourceswhenathreadfinishesusingthem.Logically,adesignercouldseparateresourcemanagementfunctionalityfromprocess/threadmanagement.However,changesisstatustoaresourceareusuallyrelatedtoachangeinstatusofaprocess,soOSdesignerstendtotalkaboutresourcemanagementsasifitwereapartofprocessmanagement.

Theprocessmanagerallowsmultipleusers（orprocessesandthreads）tosharethemachinebyprovidingmultipleexecutioncontextsandschedulingtheprocessorsothateachthreadreceievesanequitablefractionoftheavailabletime.Primaryconsiderationsoftheprocessmanagerarehowitwillenforceisolationofresourceaccessamongtheprocesses（accordingtosomepolicy）andhowitwillallowtheprocessestocircumventtheisolationmechanismwhenthepolicycallsfortheprocessestosharearesource.Chapters6through10describetheissuesrelatedtodesigningaprocessmanager.

MEMORYMANAGEMENT

Thememorymanagercooperateswhittheprocessmanagertoadministertheallocationanduseoftheprimary（alsocalledtheexecutableormain）memoryresource---seeFigure3.4Everyprocessrequestsandusesmemoryaccordingtoitsprogramdefinition.Thememorymanagerallocatesmemorytocompetingprocessesaccordingtoaspecifiedpolicyandenforcesresourceisolation.Astrategythatallowssharingismorecomplextomanagethanonewithnosharing,Hence,thememorymanagerincorporatesmechanismstoallowblockstobesharedinthepresenceoftheisolationmechanism.

◆FIGURE3.4MemoryManagement

Thememorymanagercooperateswiththeprocessmanagertoadministertheprimarymemory.IftheOSsupportsvirtualmemory,thenthatpartofthememorymanagercooperateswiththedeviceand/prthefilemanagerstomanagethememoryandpaginddevices.

Modernmemorymanagersprovidevirtualmemoryextensionssothattheabstractmachine’sprimarymemoryappearstobelargerthanthephysicalmachine’smemory.Thisisaccomplishedbyintegratingthecomputer’sprimarymemoryandthememoryfromitsstoragedevices,allowingprocessestorefernceinformationstoredonastoragedeviceasifitwerestoredintheprimarymemory.Virtualmemoryrequirementsdifferfromconventionalmemorymanagementrequirements.Thisisbecausethesystemismanaginganabstractresource---thevirtualmemoryspace---whichmustbemanagedbycontrollingthewaytheprimarymemoryandthestoragedevicesareusedtogether.Chapters11and12discussmemorymanagementapproaches,issues,anddesigns.

Modernmenorymanagersalseprovideameansforathreadononemachinetoaccessands