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Useful notes of storage devices...

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IT (9626) Theory Notes  

DataStorage Whenwetalkabout‘storing’data,wemeanputtingthedatainaknownplace.Wecanlater comebacktothatplaceandgetourdatabackagain.  ‘Writing’dataor‘saving’dataareotherwaysofsaying‘storing’data.  ‘Reading’data,‘retrieving’dataor‘opening’afilearewaysofsayingthatwearegettingour databackfromitsstoragelocation.

MainMemory Mainmemory(sometimesknownasinternalmemoryorprimarystorage)isanothername forRAM(andROM).  Mainmemoryisusuallyusedtostoredatatemporarily.InthecaseofRAM,itisvolatile(this meansthatwhenpowerisswitchedoffallofthedatainthememorydisappears).  MainmemoryisusedtostoredatawhilstitisbeingprocessedbytheCPU.Datacanbeputinto memory,andreadbackfromit,veryquickly.

BackingStorage Backingstorage(sometimesknownassecondarystorage)isthenameforallotherdatastorage devicesinacomputer,hard‐driveetc.  Backingstorageisusuallynon‐volatile,soitisgenerallyusedtostoredataforalongtime.        

IT (9626) Theory Notes 

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StorageMediaandDevices Thedevicethatactuallyholdsthedataisknownasthestoragemedium(‘media’istheplural).  Thedevicethatsavesdataontothestoragemedium,orreadsdatafromit,isknownas thestoragedevice.  Sometimesthestoragemediumisafixed(permanent)partofthestoragedevice,e.g.the magneticcoateddiscsbuiltintoaharddrive  Sometimesthestoragemediumisremovablefromthedevice,e.g.aCD‐ROMcanbetakenout ofaCDdrive.

Serial/SequentialAccess Aserial(orsequential)accessstoragedeviceisonethatstoresfilesone‐by‐oneinasequence.  Anon‐computerserialaccessdevicethatwillbefamiliartoyouisaVHSvideotape.Because videoisstoredonalongpieceoftape,whenTVshowsarerecordedontothetape,theygo onone‐by‐one,inorder.

 Ifyouwanttowatchashowthatyourecordedearlier,youhavetorewind/fast‐ forwardthroughallothershowsuntilyoufindit.  Theshowsareonlyaccessibleinthesameorderthatyourecordedthem.Thistypeofone‐by‐ onestorageandaccessiscalledserialaccess.

Direct/RandomAccess Adirect(or‘random’)accessstoragedeviceisonethatstoresfilessothattheycanbeinstantly accessed‐thereisnoneedtosearchthroughotherfilestogettotheoneyouwant.  AnexampleofadirectaccessdevicewouldbeaDVDmovie.UnliketheVHSvideotapemovie, youcanjumptoanysceneonaDVD.

IT (9626) Theory Notes 

  AllpartsoftheDVDaredirectlyaccessible.Thistypeoffilestorageiscalleddirectaccess.

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DataStorageCapacity Somestoragemediacanonlystoreaverylimitedamountofdata,whilstotherscanstorevast amounts.

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IT (9626) Theory Notes 

 DataAccessSpeeds Somestoragedevicescanaccessdataveryquickly,whilstothersareextremelyslow.

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MagneticStorageMedia/Devices WhyMagnetic? Magneticstoragemediaanddevicesstoredataintheformoftinymagnetiseddots.Thesedotsare created,readanderasedusingmagneticfieldscreatedbyverytinyelectromagnets.  Inthecaseofmagnetictapethedotsarearrangedalongthelengthofalongplasticstripwhichhas beencoatedwithamagnetisablelayer(audioandvideotapesuseasimilartechnology).  Inthecaseofmagneticdiscs(e.g.floppydiscorhard‐drive),thedotsarearrangedincirclesonthe surfaceofaplastic,metalorglassdiscthathasamagnetisablecoating.

HardDrives Hard‐driveshaveaverylargestoragecapacity(upto4TB).Theycanbeusedtostorevast amountsofdata.Hard‐drivesarerandomaccessdevicesandcanbeusedtostorealltypesof files,includinghugefilessuchasmovies.Dataaccessspeedsareveryfast.  Dataisstoredinsideahard‐driveonrotatingmetalorglassdiscs(called‘platters’).

IT (9626) Theory Notes 

 FixedHardDrive Ahard‐drivebuiltintothecaseofacomputerisknownas‘fixed’.Almosteverycomputerhasa fixedhard‐drive.  Fixedhard‐drivesactasthemainbackingstoragedeviceforalmostallcomputerssincethey providealmostinstantaccesstofiles(randomaccessandhighaccessspeeds).

PortableHardDrive Aportablehard‐driveisonethatisplacedintoasmallcasealongwithsomeelectronicsthat allowthehard‐drivetobeaccessedusingaUSBorsimilarconnection.  Portablehard‐drivesallowverylargeamountsofdatatobetransportedfromcomputerto computer.

MagneticTape Magnetictapeisalargecapacity,serialaccessmedium.Becauseitisaserialaccessmedium, accessingindividualfilesonatapeisslow.  Tapesareusedwherelargeamountsofdataneedtobestored,butwherequickaccessto individualfilesisnotrequired.Atypicaluseisfordataback‐up(lotsofdata,butrarelyonly accessedinanemergency)  Tapesarealsousedinsomebatch‐processingapplications(e.g.toholdthelistofdatathatwill beprocessed).

RemovableMediaMagneticDiscs FloppyDisc Aremovable,portable,cheap,low‐capacity(1.44MB)storagemedium.Floppydiscs arerandomaccessdevicesusedfortransfersmallamountsofdatabetweencomputers,orto back‐upsmallfiles,etc.Accesstimesareslow. AlmosteveryPCusedtohaveafloppydiscdrive.Theseareobsoletenow,havingbeenreplaced byhighercapacitytechnologysuchasCD‐ROMs,DVDsandUSBmemorysticks.

ZipDisc Aremovableandportablestoragemedium,similarinappearancetoafloppydisk,butwitha muchhighercapacity(100MB,250MBor750MB).  Zipdiscsarerandomaccessdeviceswhichwereusedfordataback‐upor movinglargefilesbetweencomputers. 

IT (9626) Theory Notes 

 Anotherobsoletestoragedevice,zipdiscswereapopularreplacementforfloppydiscsforafew years,buttheynevercaughtonfullybeforebeingsupersededbycheapermedialikeCD‐ROMs andCD‐Rs.

JazzDisc Aremovableandportablestoragemediumbasedonhard‐drivetechnology,withalarge capacity(1GBor2GB).  Jazzdiscsarerandomaccessdeviceswhichwereusedfordataback‐upormovinglarge filesbetweencomputers. Discswereexpensivetobuyandnotveryreliable.  LiketheZipdisc,thissystemneverreallycaughtonandwassupersededbyfarcheaperand morereliableandcheapertechnology.

OpticalStorageMedia/Devices Why'Optical'? Opticalstoragedevicessavedataaspatternsofdotsthatcanbereadusinglight.Alaser beamistheusuallightsource.  Thedataonthestoragemediumisreadbybouncingthelaserbeamoffthesurfaceofthe medium.Ifthebeamhitsadotitisreflectedbackdifferentlytohowitwouldbeiftherewere nodots.Thisdifferencecanbedetected,sothedatacanberead.  Dotscanbecreatedusingthelaserbeam(formediathatiswritablesuchasCD‐Rs).Thebeamis usedinahigh‐powermodetoactuallymarkthesurfaceofthemedium,makingadot.This processisknownas‘burning’dataontoadisc.

Read‐OnlyOpticalDiscs Read‐onlyopticaldiscshavedatawrittenontothemwhentheyaremanufactured.This datacannotbechanged.

CD‐ROM CompactDisc‐Read‐OnlyMemory(CD‐ROM)discscanholdaround800MBofdata.Thedata cannotbealtered(non‐volatile),socannotbeaccidentlydeleted.CD‐ROMsarerandom‐ accessdevices. CD‐ROMsareusedtodistributeallsortsofdata:software(e.g.officeapplicationsor games),music,electronicbooks(e.g.anencyclopediawithsoundandvideo.)

IT (9626) Theory Notes 

 DVD‐ROM DigitalVersatileDisc‐Read‐OnlyMemory(DVD‐ROM)discscanholdaround4.7GBofdata(a dual‐layerDVDcanholdtwicethat).DVD‐ROMsarerandom‐accessdevices. DVD‐ROMsareusedinthesamewayasCD‐ROMs(seeabove)but,sincetheycanholdmore data,theyarealsousedtostorehigh‐qualityvideo.

HighCapacityOpticalDiscs Blu‐Ray Blu‐RaydisksarearecentreplacementforDVDs.ABlu‐Raydisccanhold25‐50GBofdata(a dual‐layerBlu‐Raydisccanholdtwicethat).Blu‐Raydiscsarerandom‐accessdevices. Blu‐RaydiscsareusedinthesamewayasDVD‐ROMs(seeabove)but,sincetheycanholdmore data,theyarealsousedtostoreveryhigh‐quality,high‐definition(HD)video.

HDDVD High‐densityDVD(HD‐DVD)discscanholdaround15GBofdata(adual‐layerHD‐DVDcanhold twicethat).HD‐DVDsarerandom‐accessdevices. HD‐DVDdiscsareusedinthesamewayasDVD‐ROMs(seeabove)but,sincetheycanholdmore data,theyarealsousedtostoreveryhigh‐quality,high‐definition(HD)video.

RecordableOpticalDiscs Recordableopticaldiscscanhavedatawrittenontothem(‘burnt’)byacomputeruserusinga specialdiscdrive(adisc‘burner’). CD‐RandDVD‐R CD‐Recordable(CD‐R)andDVD‐recordable(DVD‐R)discscanhavedataburntontothem, butnoterased.Youcankeepaddingdatauntilthediscisfull,butyoucannotremoveanydata orre‐useafulldisc. CD‐RWandDVD‐RW CD‐ReWritable(CD‐RW)andDVD‐ReWritable(DVD‐RW)discs,unlikeCD‐RsandDVD‐Rs,can havedataburntontothemandalsoerasedsothatthediscscanbere‐used. DVD‐RAM DVD‐RandomAccessMemory(DVD‐RAM)discsareatypeofre‐writableDVD.Theyoftencome inafloppy‐discstylecase(toprotectthedisc).  DVD‐RAMdiscshaveasimilarcapacitytoanormalDVD,holding4.7GBofdata.DVD‐RAMdiscs arerandom‐accessdevices. 

IT (9626) Theory Notes 

 DVD‐RAMdiscsareusedinmanycamcorders(videorecordingcameras).  ThediscsaremuchhigherqualitythannormalDVD‐RWsandcanreliablystoredataforupto30 years.Thismeansthattheyareoftenusedforvideoanddataback‐upandarchiving.

Solid‐StateStorageDevices 'Solid‐State'? Theterm‘solid‐state’essentiallymeans‘nomovingparts’.  Solid‐statestoragedevicesarebasedonelectroniccircuitswithnomovingparts(noreelsof tape,nospinningdiscs,nolaserbeams,etc.)  Solid‐statestoragedevicesstoredatausingaspecialtypeofmemorycalledflashmemory...

FlashMemory FlashmemoryisatypeofElectronically‐ErasableProgrammableRead‐OnlyMemory(EEPROM). Flashmemoryisnon‐volatile(likeROM)butthedatastoredinitcanalso beerasedorchanged(likeRAM).  Flashmemorycanbefoundinmanydatastoragedevices...

USBMemorySticks Memorysticks(or‘thumb‐drives’)havemademanyotherformsofportablestoragealmost obsolete(whyburnaCDorDVDwhenyoucanmoreeasilycopyyourfilesontoamemory stick?).  Memorysticksarenon‐volatile,random‐accessstoragedevices.  EachofthesesmalldeviceshassomeflashmemoryconnectedtoaUSBinterface.Plugitinto yourcomputeranditappearsasadrive.Youcanthenaddfiles,erasefiles,etc.Youcanuseit tomoveanytypeoffilebetweencomputers.  Flashmemoryusedtobeveryexpensive,butinrecentyearsithasbecomemuchcheaperand youcannowbuya16GBmemorystickforjustafewdollars.

MemoryCards Manyofourdigitaldevices(cameras,mobilephones,MP3players,etc.)requirecompact,non‐ volatiledatastorage.Flashmemorycardsprovidethisandcomeinavarietyofshapesandsizes.

IT (9626) Theory Notes 

  OneofthemostcommonformatsusedbydigitalcamerasistheSDCard.Thecardsstorethe digitalimagestakenbythecamera. MobilephonescontainaSubscriberIdentityModule(SIM)cardthatcontainsthephone’s number,thephonebooknumbers,textmessages,etc.  Manyphonesalsohaveextramemorycardstostoremusic,video,photos,etc.(e.gTinyMicro‐ SDcards).

SmartCards Manycreditcards(e.g.‘chip‐and‐pin’cards),doorentrycards,satelliteTVcards,etc.have replacedtheverylimitedstorageofthemagneticstrip(thedarkstriponthebackofoldercards) withflashmemory.Thisismorereliableandhasamuchlargerstoragecapacity.  Cardswithflashmemoryarecalledsmartcards. ...