李亮附录1.docx

李亮附录1.docx

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李亮附录1

MassStorage

Duetothevolatilityandlimitedsizeofacomputer’smainmemory,mostcomputershaveadditionalmemorydevicescalledmassstoragesystems,whichincludemagneticdisks,CDs,andmagnetictapes.Theadvantagesofmassstoragesystemsovermainmemoryincludelessvolatility,largestoragecapacities,lowcost,andinmanycases,theabilitytoremovethestoragemediumfromthemachineforarchivalpurposes.

Thetermson-lineand0ff-lineareoftenusedtodescribedevicesthatcanbeeitherattachedtoordetachedfromamachine.On-linemeansthatthedeviceorinformationisconnectedandreadilyavailabletothemachinewithouthumanintervention.Off-linemeansthathumaninterventionisrequiredbeforethedeviceorinformationcanbeaccessedbythemachine-perhapsbecausethedevicemustbeturnedon,orthemediumholdingtheinformationmustbeinsertedintosomemechanism.

Amajordisadvantageofmassstoragesystemsisthattheytypicallyrequiremechanicalmotionandthereforerequiresignificantlymoretimetostoreandretrievedatathanamachine’smainmemory,whereallactivitiesareperformedelectronically.

MagneticDisks

Oneofthemostcommonformsofmassstorageinusetodayisthemagneticdisk,inwhichathinspinningdiskwithmagneticcoatingisusedtoholddata.Read/writeheadsareplacedaboveand/orbelowthedisksothatasthediskspins,eachheadtraversesacircle,calledatrack,aroundthedisk’supperorlowersurface.Byrepositioningtheread/writeheads,differentconcentrictrackscanbeaccessed.Inmanycases,adiskstoragesystemconsistsofseveraldiskmountedonacommonspindle,oneontopoftheother,withenoughspacefortheread/writeheadstoslipbetweentheplattersInsuchcases,theread/writeheadsmoveinunison.Eachtimetheread/writeheadsarerepositioned,anewsetoftracks-whichiscalledacylinderbecomesaccessible.

Sinceatrackcancontainmoreinformationthanwewouldnormallywanttomanipulateatanyontime,eachtrackisdividedintoarcscalledsectorsonwhichinformationisrecordedastrackisdividedintoarcscalledsectorsonwhichinformationisrecordasacontinuousstringofbits.Eachtrackonatraditionaldiskcontainsthesamenumberofsectors,andeachsectorcontainsthesamenumberthecenterofbits.（Thusthebitswithinasectoraremorecompactlystoredonthetracknearerthecenterofthediskthanthoseonthetracksneartheouteredge.）

Thus,adiskstoragesystemconsistsofmanyindividualsectors,eachofwhichcanbesectorspertrackvarygrealyfromonedisksystemtoanother.SectorsizestendtobenomorethanafewKB;sectorsof512bytesor1024bytesarecommon.

Thelocationoftracksandsectorsisnotapermanentpartofadisk’sphysicalstructure.Instead,theyaremarkedmagneticallythroughaprocesscalledformatting（orinitializing）thedisk.Thisprocessisusuallyperformedbythedisk’smanufacturer,resultinginwhatareknownasformatteddisks.Mostcomputersystemscanalsoperformthistask.Thus,iftheformatinformationonadiskisdamaged,thediskcanbereformatted,althoughthisprocessdestroysalltheinformationthatwaspreviouslyrecordedonthedisk.

Thecapacityofadiskstoragesystemdependonthenumberofnumberofdisksusedandthedensityinwhichthetracksandsectorsareplaced.Lower-capacitysystemsconsistofasingleplasticdiskknownasadisketteor,inthosecasesinwhichthediskisflexible,bythelessprestigioustitleoffloppydisk.（today’s31／2-inchdiameterfloppydisksarehousedinrigidplasticcases,whichdonotconstituteasflexibleapackageastheirolder51／4-inchdiametercousinsthatwerehousedinpapersleeves.）Diskettesareeasilyinsertedandremovedfromtheircorrespondingread/writeunitsandareeasilystored.Asaconsequence,diskettesareoftenusedforoff-linestorageofinformation.Thegeneric31／2-inchdisketteiscapableofholding1.44MBofdatabutnongenericdiskettesareavailablewithmuchhighercapacities.AnexampleistheZipdisksystemfromIomegaCorporation,whichprovidesstoragecapacitiesuptoseveralhundredMBonasinglerigiddiskette.

High-capacitydisksystems,capableofholdingmanygigabytes,consistofperhapsfivetotenrigiddisksmountedonacommonspindle.Thefactthatthedisksusedinthesesystemsarerigidleadsthemtobeknownashard-disksystems,incontrasttotheirfloppycounterparts.Toallowforfasterrotationspeeds,theread/writeheadsinthesesystemsdonottouchthediskbutinstead“float”justoffthesurface.Thespacingissoclosethatevenasingleparticleofdustcouldbecomejammedbetweentheheadanddisksurface,destroyingboth（aphenomenonknownasaheadcrash）,Thushard-disksystemsarehousedincasesthataresealedatthefactory.

Severalmeasurementsareusedtoevaluateadisksystem’sperformance:

（1）seektome（thetimerequiredtomovetheread/writeheadsfromoneracktoanother）;

（2）rotationdelayorlatencytime（halfthetimerequiredforthedisktomakeacompleterotation,whichistheaverageamountoftimerequiredforthedesireddatatorotatearoundtotheread/writeheadoncetheheadhasbeenpositionedoverthedesiredtrack）;（3）accesstime（thesumofseektimeandrotationdelay）;and（4）transferrate（therateatwhichdatacanbetransferredtoorfromthedisk）.

 Hard-disksystemsgenerallyhavesignificantlybettercharacteristicsthanfloppysystems.Sincetheread/writeheadsdonottouchthedisksurfaceinahared-disksystem,onefindsrotationspeedsofseveralthousandrevolutionsperminute,whereasdisksinfloppy-disksystemsrotateontheorderof300revolutionsperminute.Consequently,transferratesforhard-disksystems,usuallymeasuredinmegabytespersecond,aremuchgreaterthanthoseassociatedwithfloppy-disksystems,whichtendtobemeasuredinkilobytespersecond.

Sincedisksystemsrequirephysicalmotionfortheiroperation,bothhardandfloppysystemssufferwhencomparedtospeedswithinelectroniccircuitry.Delaytimeswithinanelectroniccircuitaremeasuredinunitsofnanoseconds（billionthsofasecond）orless,whereasseektimes,latencytimes,andaccesstimes,andaccesstimesofdisksystemsaremeasuredinmilliseconds（thousandthsofasecond）.Thusthetimerequiredtoretrieveinformationfromadisksystemcanseemlikeaneternitytoanelectroniccircuitawaitingaresult.

CompactDisks

Anotherpopulardatastoragetechnologyisthecompactdisk（CD）.Theredisksare12centimeters（approximately5inches）indiameterandconsistofreflectivematerialcoveredwithaclearprotectivecoating.Informationisrecordedonthembycreatingvariationsintheirreflectivesurfaces.ThisinformationcanthenberetrievedbymeansofalaserbeamthatmonitorsirregularitiesonthereflectivesurfaceoftheCDasitspins.

CDtechnologywasoriginallyappliedtoaudiorecordingusingarecordingusingarecordingformatknownasCD-DA（compactdisk-digitalaudio）,andtheCDsusedtodayforcomputerdatastorageuseessentiallythesameformat.Inparticular,informationontheseCDsisstoredonasingletrackthatspiralsaroundtheCDlikeagrooveinanold-fashionedrecord;however,unlikeold-fashionedrecords,thetrackonaCDspiralsfromtheinsideout.Thistrackisdividedintounitscalledsectors,eachwithitsownidentifyingmarkingsandacapacityof2KBofdata,whichequatesto1／75ofasecondofmusicinthecaseofaudiorecording.

InformationisstoredonaCDatauniformlineardensityovertheentirespiraledtrack,whichmeansthatmoreinformationisstoredinalooparoundtheouterportionofthespiralthaninalooparoundtheinnerportion.Inturn,moresectorswillbereadinasinglerevolutionofthediskwhenthelaserbeamisscanningtheouterportionofthespiraledtrackthanwhenthebeamisscanningtheinnerportion.Thus,toobtainauniformrateofdatatransfer,CD-DAplayersaredesignedtovarytherotationspeeddependingonthelocationofthelaserbeam.However,mostCDdrivesusedforcomputerdatastoragespinatafaster,constantspeedandthusmustaccommodatevariationsindatatransferrates.

Asaconsequenceofsuchdesigndecision,CDstoragesystemsperformbestwhendealingwithlong,continuousstringofdata,aswhenreproducingmusic.Incontrast,whenanapplicationrequiresaccesstoitemsofdatainarandommanner,theapproachusedinmagneticdiskstorage（individual,concentrictracks）outperformsthespiralapproachusedinCDs.

TraditionalCDshavecapacitiesintherangeof600to700MB.However.NewerDVD（DigitalVersatileDisks）,whichareconstructedfrommultiple,semi-transparentlayersthatcanbedistinguishedbyapreciselyfocusedlaser,providestoragecapacitiesonorderofseveralGB.Suchdisksarecapableofstoringlengthymultimediapresentations,includingentiremotionpictures.

MagneticTape

Anolderformofmassstoragedeviceusesmagnetictape.Here,informationisrecordedonthemagneticcoatingofathinplastictapethatiswoundonareelforstorage.Toaccessthedata,thetapeismountedinadevicecalledatapedrivethattypicallycanread,write,andrewindtherapeundercontrolofthecomputer.Tapedrivesrangeinsizefromsmallcartridgeunits,calledstreamingtapeunits,whichunits.Althoughthecapacityofthesedevicesdependsontheformatused,mostcanholdmanygigabytes.

Modernstreamingtapesystemsdivideatapeintosegments,eachofwhichismagneticallymarkedbyaformattingprocesssimilartothatofdiskstoragedevices.Eachofthesesegmentscontainsseveraltracksthatrunparalleltooneanotherlengthwiseonthetape.Theretrackscanbeaccessedindependently