CHAPTER 7 Storage2.ppt

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1、CHAPTER7StorageFloppyDiskFloppyDiskDriveTwoTypesofFloppyDiskette1.51/4”FloppyDiskette(1.25MB)2.31/2”FloppyDiskette(1.44MBFloppyDiskDriveRecordingformatTwotypesofrecordingformatsusedforstoringdataonthedisks1.FrequencyModulation(FM)2.ModifiedFrequencyModulation(MFM)FrequencyModulationInthismetho

2、deverydatabitcellstartswithafluxtransition.Aclockisrecordedatthebeginningdatabitcell.Ifadatabit“1”hastobewritten,afluxtransitionisrecordedinthemiddleofabitcellModifiedFrequencyModulationInMFM,noclock(pulse)isrecordedatthebeginningofabitcell.SimilartoFM,afluxtransitionisrecordedinthemiddleofabi

3、t,ifthedatabittoberecordedis“1”.Ifadatabit“0”hastobewrittennexttoadatabit“1”,thennofluxtransitionisrecordedatall.Ifacurrentandaprecedingdatabitsarezeroonlyonepulseisrecordedatthebeginningofabitcell.Fromthefigureitisclearthatwewillbeabletostoredoublethedatainthesamearea,ifweuseMFMrecordingforma

4、tinsteadofF.M.Therecordingformatisdecidedbytheoperatingsystem.InMFMthereisonlyonefluxreversalperbitcellregardlessofthedatabeing“1”or“0”.ForthisreasontwiceasmanybitscanberecordedusingtheMFMmethodthantheF.M.method.HencethetermDoubleDensityisused,forMFMformatofrecording.ClustersClustersDOSrarelyd

5、ealswithindividualsectors.Butinstead,datastorageisdoneingroupofsectors,calledsectors.Dependingonthediskettetype,itsformat,andDOSversion,theclustervaries.FloppyDiskDriveFloppyDiskDriveRead/Writehead:Afloppydiskdrivenormallyhastwo-read/writeheadsmakingModernfloppydiskdrivesasdouble-sideddrives.A

6、headexistsforeachsideofdiskandBothheadsareusedforreadingandwritingontherespectivediskside.Head0andHead1:Manypeopledonotrealizethatthefirsthead(head0)isbottomoneandtopheadishead1.Thetopheadislocatedeitherfouroreighttracksinwardfromthebottomheaddependinguponthedrivetype.HeadMovement:Amotorcalled

7、headactuatormovestheheadmechanism.Theheadscanmoveinandoutoverthesurfaceofthediskinastraightlinetopositionthemselvesovervarioustracks.Theheadsmoveinandouttangentiallytothetracksthattheyrecordonthedisk.Headsaremountedonthesametrack,theymo