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