Memory and storage

Memory and storage

1. MEMORY AND STORAGE

2. STORAGE DEVICE
Astoragedeviceisanycomputing
hardware,thatisusedforstoringand
managingdataforlateruse.

3. Computer data storage
•Computer data storage, often called
storage or memory, refers to
computer components and recording
media that retain digital data used
for computing for some interval of
time.

4. Purpose of storage
Manydifferentformsofstorage,basedon
variousnaturalphenomena,havebeen
invented.Sofar,nopracticaluniversalstorage
mediumexists,andallformsofstoragehave
somedrawbacks.Thereforeacomputersystem
usuallycontainsseveralkindsofstorage,each
withanindividualpurpose.

5. History
1928-magnetictape - fritzpfleumer
1932-magneticdrum - GTaushek
1946-Williamstube - prof.TredrickCWillam
memorytechnologymostlypermittedacapacityofafewbytes.
Thefirstelectronicprogrammabledigitalcomputer,
theENIAC,usingthousandsofoctal-baseradiovacuum
tubes,couldperformsimplecalculationsinvolving20numbers
oftendecimaldigitswhichwereheldinthevacuum
tubeaccumulators.
1956-Harddisk-
1963-Musictape- philips
1966-DRAM - RobertHDennard
1980-CD - JamesTRussel

6. Fundamental storage technologies
•Semiconductor (dynamic random access
memory ,ROM,Cache Memory,Registers,Flash
memory)
•Magnetic (Floppy disk , Hard disk drive,
Magnetic tape data storage )
•Optical (CD, DVD)
•Paper (Paper tape, punch card)

7. Introduction to Memory
Incomputing,memoryreferstothephysical
devicesusedtostoreprograms(sequencesof
instructions)ordata(e.g.programstateinformation)
onatemporaryorpermanentbasisforusein
acomputerorotherdigitalelectronicdevice.
Thetermprimarymemoryisusedforthe
informationinphysicalsystemswhicharefast
(i.e.RAM),asadistinctionfromsecondarymemory,
whicharephysicaldevicesforprogramanddata
storagewhichareslowtoaccessbutofferhigher
memorycapacity.
Primarymemorystoredonsecondarymemoryis
called"virtualmemory".

8. Memory is of two types:-
1.Primary memory (RAM,ROM,CACHE,FLASH )
2.Secondary memory (FLOPPY DISK,MAGNETIC
DISK,MAGNETICTAPES,OPTICAL DISKS)
MEMORY
PRIMARY SECONDARY

9. PRIMARY MEMORY
Primarymemoryisinternalmemoryofthe
computer.RAMandROMbothformpartof
primarymemory.Theprimarymemoryprovides
mainworkingspacetothecomputer.
RAM:-Theprimarystorageisreferredtoas
randomaccessmemorybecauseitispossibleto
randomlyselectanduseanylocationofthe
memorydirectlystoreandretrievedata.Itisalso
calledread/writememory.Itisavolatile
memory.

10. READ ONLY MEMORY (ROM)
•The acronym for ROM is Read Only Memory
•when the data is burnt on a ROM, it cannot be removed
nor it cannot be altered it can only read .
•ROM is known as nonvolatile.
•ROM stores programs such as the program that boots
the computer.
•ROMs are widely used in calculators and other devices
such as laser printers.

11. TYPES OF ROM
There are five basic ROM types:-
1.Masked ROM (READ ONLY MEMORY )
2.PROM (PROGRAMMABLE ROM )
3.EPROM (Erasable Programmable
ROM)
4.EEPROM (Electrically erasable programmable
ROM)
5.FLASH MEMORY( ONE KINDS OF EEPROM)

12. IMPROTANT POINTS OF
ROM
Masked ROM:
•In this type of ROM bits are stored permanently by
marking and metallization process. This is done by
manufacturers. This type of ROM can be programmed
only one-by the manufacture

13. PROM:
•Itisananothertypeofprimarymemoryincomputer,
whichiscalledProgrammableReadOnlyMemory
(PROM).Itsinotpossibletomodifyoreraseprograms
storedinROM,butispossibleforyoutostoreyour
programinPROM.Oncetheprogrammers,arewrittenit
cannotbechangedandremainintactevenifpoweris
switchedoff.

14. EPROM:
The Erasable PROM chip allows the stored data to
erased and new data can be reprogrammed.
Information store in EPROM exposing the chip for
some time ultraviolet light and it erases chip is
reprogrammed using a special programming
facility. When the EPROM is in use information
can only be read.

15. EEPROM:
•InanelectricityerasablePROM,the
contentsofcellscanbeerasedbythe
applicationofahighvoltage.Advantages
withEEPROMs are:itneednotbe
physicallyremovedforreprogrammingand
theprocesscanbemadeselectivesince
electricalerasureisused.

16. The advantages of ROM are
•They are non-volatile
•They are cheaper than RAM
•They are static and do not refreshing
•They are more reliable than RAM as their
circuit is simple.
•They are available in longer sizes than
RAM.
•They are easier to interface than RAM.

17. CACHE:-
AsmallmemorychipisattachedbetweenCPU
andmainmemory,whoseaccesstimeisvery
closetotheprocessingspeedofCPU.Itiscalled
cachememory.ItsphysicallocationisinCPUand
ismuchfasterthanRAM.Itssizeisnormallysmall.
WecanseethecachememoryofCPUinBIOS
setup.

19. FLASH MEMORY
•The flash memory is a type of chip that is called as the EEPROM.
•BIOS of computer, the memory stick that is found in the digital
cameras , USB pen drives , the memory cards of the mobiles etc are
the example of flash memory.
•The important properties of the flash memory are that it is much fast
and very much easier in use.
•Flash drive can be efficiently used for the purpose of a hard disk.

20. Advantages of flash memory
when used as a hard disk
•Advantage of the flash memory over the hard disk is that
it is compact and much efficient for the data transfer.
•There are no mobile parts in it As in the hard disk there
are the platters and the read write head moves over it.
•There is no such mechanical component in the flash
memory drive. Hence it is a purely electronic device.
•The flash memory is quite expensive as compared to the
hard disk drive, but it is light in weight

21. SECONDARY MEMORY
Secondarymemoryisexternaland
permanentinnature.Thesecondary
memoryisconcernedwithmagnetic
memory.Secondarymemorycanbe
storedonstoragemedialikeFLOPPY
DISK,MAGNETIC DISK,MAGNETIC
TAPES,OPTICALDISKS,SSDetc.

22. Characteristics of storage
PERFORMANCE
CAPACITY
VOLATILITY
DIFFERENTIATION
MUTABILITY
ACCESSIBILITY

23. Characteristics of storage
1.PERFORMANCE: -
•Throughput:-The rate at which information can be read from or written
to the storage. In computer data storage, throughput is usually expressed in
terms of megabytes per second or MB/s, though bit rate may also be used.
The relevant unit of measurement is typically nanosecond for primary storage,
millisecond for secondary storage, and second for tertiary storage.

24. •2. CAPACITY:-
•Raw capacity
•The total amount of stored information that a storage
device or medium can hold. It is expressed as a quantity
of bits or bytes (e.g. 10.4 megabytes).
Memory storage density
•The compactness of stored information. It is the storage
capacity of a medium divided with a unit of length, area
or volume (e.g. 1.2 megabytes per square inch).

25. VOLATILITY:-
Volatilityisatermusedininformationprocessing.
Itreferstothepercentageofrecordsthatare
addedtoordeletedfromafileduringasingle
processingoperation.Adatafilethathasahigh
percentageofchangewouldbedescribedas
havingahighvolatility.
Volatilereferstostoragedevicesthatwillloseall
theirdataifthepowerisswitchedoff.

26. •Differentiation:-
•DynamicrandomaccessmemoryAformofvolatile
memorywhichalsorequiresthestoredinformationtobe
periodicallyrereadandrewritten,orrefreshed,
otherwiseitwouldvanish.
•Staticmemory
•AformofvolatilememorysimilartoDRAMwiththe
exceptionthatitneverneedstoberefreshedaslongas
powerisapplied.(Itlosesitscontentifpoweris
removed).

27. Mutability:-
•Read/write storage or mutable storage :-Allows information to be
overwritten at any time. A computer without some amount of
read/write storage for primary storage purposes would be useless
for many tasks. Modern computers typically use read/write storage
also for secondary storage.
•Read only storage :-
•Retains the information stored at the time of manufacture, and write
once storage (Write Once Read Many) allows the information to be
written only once at some point after manufacture.
•Slow write, fast read storage:-
•Read/write storage which allows information to be overwritten
multiple times, but with the write operation being much slower than
the read operation. Examples include CDRW and flash memory.

28. Accessibility:-
•Randomaccess:-Anylocationinstoragecanbe
accessedatanymomentinapproximatelythesame
amountoftime.Suchcharacteristiciswellsuitedfor
primaryandsecondarystorage.
•Sequentialaccess:-
Theaccessingofpiecesofinformationwillbeinaserial
order,oneaftertheother;thereforethetimetoaccessa
particularpieceofinformationdependsuponwhichpiece
ofinformationwaslastaccessed.Suchcharacteristicis
typicalofofflinestorage.

29. Memory management
Memorymanagementistheprocessof
controlling and coordinating
computermemory,assigningportions
calledblockstovariousrunningprogramsto
optimizeoverallsystemperformance.
Memorymanagementresidesinhardware,
intheOS(operatingsystem),andin
programsandapplications.

30. Inhardware,memorymanagementinvolves
componentsthatphysicallystoredata,suchas
RAM(randomaccessmemory)chips,memorycaches,
andflash-basedSSDs(solid-statedrives).
IntheOS,memorymanagementinvolvesthe
allocation(andconstantreallocation)ofspecific
memoryblockstoindividualprogramsasuserdemands
change.
Attheapplicationlevel,memorymanagement
ensurestheavailabilityofadequatememoryfor
theobjectsanddatastructuresofeachrunning
programatalltimes.

31. Virtual Memory
Virtualmemoryisastratagemtoutilizetheharddrivememorytoachieve
enhancedperformancefromtherandomaccessmemory.
virtualmemoryisamemorymanagementtechniquethatisimplemented
usingbothhardwareandsoftware.Itmapsmemoryaddressesusedbya
program,calledvirtualaddresses,intophysicaladdressesincomputer
memory.
Themainmemoryisdividedintoequalsizechunkscalledpageframes
andeachpageframehasauniquephysicaladdress.Sowhenevera
pageneedstobeaccessed,theoperatingsystemhastotranslatethe
virtualaddressintoaphysicaladdressandthisisdonebyaMemory
ManagementUnit(MMU)withthehelpofmappinginapagetable.
automaticallytranslatesvirtualaddressestophysicaladdresses.

32. Software within the operating system may extend these capabilities to provide a
virtual address space that can exceed the capacity of real memory and
thus reference more memory than is physically present in the computer.
The concept of virtual memory was first
developed by German physicist FritzRudolf
Güntsch at the
Technische Universität Berlin in 1956

36. MEMORY ALLOCATION
Memoryallocationisaprocessbywhichcomputer
programsandservicesareassignedwithphysical
orvirtualmemoryspace.
Memoryallocationistheprocessofreservinga
partialorcompleteportionofcomputermemoryfor
theexecutionofprogramsandprocesses.Memory
allocationisachievedthroughaprocessknownas
memorymanagement.

37. MEMORY PROTECTION
Memory protectionisawayto
controlmemoryaccessrightsona
computer,andisapartofmostmodern
operatingsystems.Themainpurpose
ofmemoryprotectionistopreventa
processfromaccessingmemorythathas
notbeenallocatedtoit.
This prevents a bug or malware within a
process from affecting other processes, or
the operating system itself.

38. PROCESSOR REGISTERS
Incomputerarchitecture,aprocessorregisteris
asmallamountofstorageavailableaspartofa
Digitalprocessor,suchasacentralprocessing
unit(CPU).Suchregistersaretypically
addressedbymechanismsotherthanmain
memoryandcanbeaccessedfaster.

39. Almostallcomputers,loadstorearchitectureornot,
loaddatafromalargermemoryintoregisterswhere
itisusedforarithmetic,manipulatedortestedby
machineinstructions.Manipulateddataisthenoften
storedbackintomainmemory,eitherbythesame
instructionorasubsequentone.Modernprocessors
useeitherstaticordynamicRAMasmainmemory,
withthelatterusuallyaccessedviaoneormore
cachelevels.

40. Processorregistersarenormallyatthetopof
thememoryhierarchy,andprovidethefastest
waytoaccessdata.
Registersarenormallymeasuredbythe
numberofbitstheycanhold,forexample,an
"8bitregister"ora"32bitregister".Aprocessor
oftencontainsseveralkindsofregisters,that
canbeclassifiedaccordingtotheircontentor
instructionsthatoperateonthem:

41. Cache memory
Cache memory is a small amount of fast memory
∗Placed between two levels of memory hierarchy
» To bridge the gap in access times
–Between processor and main memory (our focus)
–Between main memory and disk (disk cache)
∗Expected to behave like a large amount of fast memory

43. TYPES OF CACHE
Separate instruction and data caches
» Initial cache designs used unified caches
» Current trend is to use separate caches (for level 1)

44. Several reasons for preferring separate caches
∗Locality tends to be stronger
∗Can use different designs for data and instruction caches
» Instruction caches
–Read only, dominant sequential access
–No need for write policies
–Can use a simple direct mapped cache implementation
» Data caches
–Can use a set-associative cache
–Appropriate write policy can be implemented
∗Disadvantage
» Rigid boundaries between data and instruction caches

45. Hierarchy of storage
•Primary storage
•Secondary storage or external memory
•Tertiary storage
•Off-line storage
•Network Storage

46. Secondary storage or external
memory
•it is not directly accessible by the CPU.
•it is non-volatile
•hard disk drive, SSD, CD , DVD,Blue Ray,
floppy disks, magnetic tape, paper tape,
punched cards are the example of this.
•It need to be formatted before they can
store data.

47. Tertiary storage
•Provides a third level of storage .
•This is not a popular storage device
•Its main use is for storing data at a very large
scale.
•It require a database to organize the data that
are stored in them, and the computer needs to
go through the database to access those data.
•tape libraries , optical jukeboxes are the
example of this

48. Large tape library

49. Off-line storage
Itsanothernameisdisconnectedstorage.
Itisnotdirectlyconnectedtothecomputerandisused
asatransfermediumonly.
Offlinestoragedevicesareremotelylocatedand
accessedasperneedonly.
Thisstoragedevicesalsoneedshumaninterventionto
bereadproperlybythemaincomputersystem.
Itservesasagoodbackupdevicesinceitisremotely
located.
Italsoprovidesgoodsecurityfordatasinceyoucannot
easilyaccessitfromacomputer.

50. NETWORK STORAGE
•ANASunitisacomputerconnectedtoanetworkthat
providesonlyfilebaseddatastorageservicestoother
devicesonthenetwork.Althoughitmaytechnicallybe
possibletorunothersoftwareonaNASunit,itisnot
designedtobeageneralpurposeserver.Forexample,NAS
unitsusuallydonothaveakeyboardordisplay,andare
controlledandconfiguredoverthenetwork,oftenusinga
browser.

51. Magnetic storage
Magneticstorageormagneticrecordingisthe
storageofdataonamagnetisedmedium.Magnetic
storageusesdifferentpatternsofmagnetisationina
magnetisablematerialtostoredataandisaform
Ofnonvolatilememory.Theinformationisaccessed
usingoneormoreread/writeheads.
Example-harddisk,Solidstatedisk,Floopydisk,
Magnetictape.

52. Floppy Disk
Afloppydisk,alsocalledadiskette,isadiskstoragemedium
composedofadiskofthinandflexiblemagneticstorage
medium,sealedinarectangularplasticcarrierlinedwith
fabricthatremovesdustparticles.Floppydisksarereadand
writtenbyafloppydiskdrive(FDD).
Floppy disks, initially as 8inch (200 mm) media and later in
5¼inch (133 mm) and 3½inch (90 mm) sizes

54. Disk format Year
introduced
Formatted storage
capacity
Marketed
capacity
3½inch HD 1987 1440 kB (1760 kB on
Amiga)
1.44 MB (2.0
MB
unformatted)
3½inch ED 1987 2880 kB 2.88 MB
3½inch Floptical
(LS)
1991 20385 kB 21 MB
3½inch Superdisk(LS120)1996 120.375 MB 120 MB
3½inch Superdisk(LS240)1997 240.75 MB 240 MB
3½inch HiFD 1998/99 1998/99 150/200 MB
HD = High Density; ED = Extended Density;
LS = Laser Servo; HiFD = High capacity Floppy Disk;
SS = Single Sided;
DS = Double Sided

55. Hard disk
Aharddiskdrive(oftenshortenedasharddisk,harddrive,
orHDD)isanon-volatilestoragedevice.
Aharddiskdrive(HDD),harddisk,harddriveorfixeddisk
isadatastoragedeviceusedforstoringandretrieving
digitalinformationusingoneormorerigid("hard")rapidly
rotatingdisks(platters)coatedwithmagneticmaterial.
Theplattersarepairedwithmagneticheadsarrangedona
movingactuatorarm,whichreadandwritedatatothe
plattersurfaces.Dataisaccessedinarandomaccess
manner,meaningthatindividualblocksofdatacanbe
storedorretrievedinanyorderratherthansequentially.
HDDsretainstoreddataevenwhenpoweredoff.

56. IntroducedbyIBMin1956,HDDsbecamethedominant
secondarystoragedeviceforgeneralpurposecomputers
bytheearly1960s.Continuouslyimproved,HDDshave
maintainedthispositionintothemoderneraofserversand
personalcomputers.Morethan200companieshave
producedHDDunits,thoughmostcurrentunitsare
manufacturedbySeagate,ToshibaandWesternDigital.
The primary characteristics of an HDD are its capacity and
performance. Capacity is specified in unit prefixes
corresponding to powers of 1000: a 1terabyte (TB) drive
has a capacity of 1,000 gigabytes (GB; where 1 gigabyte =
1 billion bytes).
Capacity–80GB,160GB,250GB,320GB,500GB,1TB

57. The two most common form factors for modern HDDs are
3.5inch, for desktop computers, and 2.5inch, primarily for
laptops. HDDs are connected to systems by standard
interface cables such as SATA (Serial ATA), USB cables.

59. MECHANISM
HDDsrecorddatabymagnetizingferromagneticmaterial
directionally,torepresenteithera0ora1binarydigit.They
readthedatabackbydetectingthemagnetizationofthe
material.AtypicalHDDdesignconsistsofaspindlethatholds
oneormoreflatcirculardiskscalledplatters,ontowhichthe
dataisrecorded.Theplattersaremadefromanon-magnetic
material,usuallyaluminumalloyorglass,andarecoatedwith
athinlayerofmagneticmaterial,typically10–20nmin
thicknesswithanouterlayerofcarbonforprotection.Older
disksusediron(III)oxideasthemagneticmaterial,butcurrent
disksuseacobalt-basedalloy.

60. Magnetic tape
Magnetictapeisamediumformagneticrecording,
madeofathinmagnetizablecoatingonalong,
narrowstripofplasticfilm.Itwasdevelopedin
Germany,basedonmagneticwirerecording.
Devicesthatrecordandplaybackaudioandvideo
usingmagnetictapearetaperecordersandvideo
taperecorders.Adevicethatstorescomputerdata
onmagnetictapeisatapedrive(tapeunit,
streamer).

61. Data storage
Magnetic tape was first used to record computer data
in 1951. The recording medium was a thin strip of one
half inch (12.65 mm) wide metal, consisting of nickel
plated bronze (called Vicalloy). Recording density was
128 characters per inch (198 micrometre/character) on
eight tracks.
Modern cartridge formats include LTO(LINEAR TAPE
OPEN), DLT(DIGITAL LINEAR TAPE), and
DAT(DIGITAL AUDIO TAPE).

63. RANDOM ACCESS MEMORY
RAM(randomaccessmemory)istheplaceina
computerwheretheoperatingsystem,application
programs,anddataincurrentusearekeptsothat
theycanbequicklyreachedbythecomputer's
processor.
RAMismuchfastertoreadfromandwritetothan
theotherkindsofstorageinacomputer,thehard
disk,floppydisk,andCD-ROM.However,thedata
inRAMstaysthereonlyaslongasyourcomputeris
running.Whenyouturnthecomputeroff,RAM
losesitsdata.

64. When you turn your computer on again, your operating
system and other files are once again loaded into RAM,
usually from your hard disk.
The two main forms of modern RAM are static RAM
(SRAM) and dynamic RAM (DRAM).

65. Dynamic Random Access Memory (DRAM)
Amemorychipthatdependsuponanappliedvoltageto
keepthestoreddata.
Dynamicrandomaccessmemory(DRAM)isatypeof
randomaccessmemorythatstoreseachbitofdataina
separatecapacitorwithinanintegratedcircuit.Thecapacitor
canbeeitherchargedordischarged;thesetwostatesare
takentorepresentthetwovaluesofabit,conventionally
called0and1.
Sinceeven"nonconducting"transistorsalwaysleakasmall
amount,thecapacitorswillslowlydischarge,andthe
informationeventuallyfadesunlessthecapacitorchargeis
refreshedperiodically.Becauseofthisrefreshrequirement,it
isadynamicmemoryasopposedtostaticrandomaccess
memory(SRAM)andotherstatictypesofmemory.

66. Themainmemory(the"RAM")inpersonalcomputers
isdynamicRAM(DRAM).ItistheRAMindesktops,
laptopsandworkstationcomputersaswellassome
oftheRAMofvideogameconsoles.
TheadvantageofDRAMisitsstructuralsimplicity:
onlyonetransistorandacapacitorarerequiredper
bit,comparedtofourorsixtransistorsinSRAM.This
allowsDRAMtoreachveryhighdensities.

67. GeneralDRAMformatsDynamicrandomaccessmemoryis
producedasintegratedcircuits(ICs)bondedandmountedinto
plasticpackageswithmetalpinsforconnectiontocontrolsignals
andbuses.InearlyuseindividualDRAMICswereusuallyeither
installeddirectlytothemotherboardoronISAexpansioncards;
latertheywereassembledintomultichipPluginmodules(DIMMs,
SIMMs,etc.).Somestandardmoduletypesare:DRAMchip
(IntegratedCircuitorIC)DualinlinePackage(DIP)DRAM
(memory)modulesSingleInlinePinPackage(SIPP)SingleInline
MemoryModule(SIMM)DualInlineMemoryModule(DIMM)
RambusInlineMemoryModule(RIMM),technicallyDIMMsbut
calledRIMMsduetotheirproprietaryslot.SmalloutlineDIMM
(SODIMM),abouthalfthesizeofregularDIMMs,aremostlyused
innotebooks,smallfootprintPCs(suchasMiniITXmotherboards),
upgradableofficeprintersandnetworkinghardwarelikerouters.
SmalloutlineRIMM(SORIMM).SmallerversionoftheRIMM,
usedinlaptops.TechnicallySODIMMsbutcalledSORIMMsdueto
theirproprietaryslot.

68. Common DRAM modules
Common DRAM packages as illustrated to the right, from top to bottom (last three
types are not present in the group picture, and the last type is available in a
separate picture):
DIP 16pin (DRAM chip, usually prefast page mode DRAM (FPRAM))
SIPP 30pin (usually FPRAM)
SIMM 30pin (usually FPRAM)
SIMM 72pin (often extended data out DRAM (EDO DRAM) but FPRAM is not uncommon)
DIMM 168pin (most SDRAM but were some extended data out DRAM (EDO DRAM))
DIMM 184pin (DDR SDRAM)
RIMM 184pin (RDRAM)
DIMM 240pin (DDR2 SDRAM and DDR3 SDRAM)
DIMM 288pin (DDR4 SDRAM)
SIMM-single in-line memory module
DIMM-dual in-line memory module
RIMM-rambus in-line memory module

69. COMMON SODIMM DRAM modules
72pin (32bit)
144pin (64bit) used for SODIMM SDRAM
200pin (72bit) used for SODIMM DDR SDRAM and
SODIMM DDR2 SDRAM
204pin (64bit) used for SODIMM DDR3 SDRAM
260pinused for SODIMM DDR4 SDRAM

71. STSTIC RANDOM ACCESS MEMORY
Staticrandomaccessmemory(SRAMorstaticRAM)isa
typeofsemiconductormemorythatusesbistablelatching
circuitry(Flipflop)tostoreeachbit.Thetermstatic
differentiatesitfromdynamicRAM(Dynamicrandomaccess
memory)whichmustbeperiodicallyrefreshed.SRAM
exhibitsdataremanence,butitisstillvolatileinthe
conventionalsensethatdataiseventuallylostwhenthe
memoryisnotpowered.
SRAM is more expensive and less dense than DRAM and is
therefore not used for highcapacity, lowcost applications
such as the main memory in personal computers.

72. SOLID STATE DISK
Solidstatestoragedevicesstorecomputerdataonnon-
volatile"flash"memorychipsratherthanbychangingthe
surfacepropertiesofamagneticoropticalspinningdisk.
Withnomovingpartssolidstatedrives(SSDs)--arealso
verymuchthefutureforalmostallformsofcomputer
storage.
Asof2014,mostSSDsuseNANDbasedflashmemory,
whichretainsdatawithoutpower.Forapplicationsrequiring
fastaccess,butnotnecessarilydatapersistenceafter
powerloss,SSDsmaybeconstructedfromrandomaccess
memory(RAM).Suchdevicesmayemployseparatepower
sources,suchasbatteries,tomaintaindataafterpower
loss.

74. Flash memory
Flashmemoryisanon-volatilecomputerstoragethatcan
beelectricallyerasedandreprogrammed.Itisatechnology
thatisprimarilyusedinmemorycardsandUSBflashdrives
forgeneralstorageandtransferofdatabetweencomputers
andotherdigitalproducts.
ItisaspecifictypeofEEPROM (ElectricallyErasable
ProgrammableRead-OnlyMemory)thatiserasedand
programmedinlargeblocks;inearlyflashtheentirechip
hadtobeerasedatonce.

75. USB MEMORY STICKS
USB memory sticks (or USB memory keys, USB memory drives, or whatever
you choose to call them!) are basically a combination of a flash memory card
and a flash memory card reader in one handy and tiny package.
As with other storage devices, there are two key factors to consider when
selecting a USB memory stick: capacity and data transfer speed.
Itisafunctionofthetypeofflashmemorychipsusedtoholdthedata.
Withoutgoingintogreattechnicalities,thesechipscomeintwovarieties
calledsinglelevelcell(SLC)andmultilevelcell(MLC).Basically,MLC
flashchipsstoretwoormorebitsofdataineachmemorycell,whilstSLC
chipsstoreonlyone.MLCsolidstatedisksarethereforecheaperto
producethanSLCdisksatanygivencapacity,butduetostoringmore
thanonebitofinformationineachmemorycelltakelongertowriteand
readdata.IfyouneedafastUSBkey,memorycardorindeedhard-disk
replacementSSDthenyouneedtopaymoretoobtainanSLCdevice.

76. First commercial product
IBM's USB flash drive became available on December 15, 2000, and had a storage
capacity of 8 MB, more than five times the capacity of the then common floppy disks.
In 2000, Lexar introduced a Compact Flash (CF) card with a USB connection, and a
companion card read/writer and USB cable that eliminated the need for a USB hub.
Second generation
By 2013, most USB flash drives had USB 2.0 connectivity, which has 480 Mbit/s as
the transfer rate upper bound; after accounting for the protocol overhead that
translates to a 35 MB/s effective throughput.
Third generation
Like USB 2.0 before it, USB 3.0 dramatically improved data transfer rates compared
to its predecessor. It was announced in late 2008, but consumer devices were not
available until the beginning of 2010. The USB 3.0 interface specifies transfer rates
up to 5 Gbit/s (625 MB/s), compared to USB 2.0's 480 Mbit/s (60 MB/s).

77. Fourth generation
As of March 2015, some manufacturers have announced
USB 3.1 typeC flash drives with read/write speeds of
around 530 MB/s.
Storage capacity
ThefirstUSBflashdriveappearedonthemarketinlate
2000,providingastoragecapacityof8MB.Later,the
maximumavailablestoragecapacitygraduallydoubled
(16MB,32MB,etc.)allthewayuptoreachingcapacities
of512GBand1TBbyJanuary2013.However,asof
May2014flashdriveswithanywherefrom8to128GB
arestillfrequentlysold.

78. USB hub
A USB hub is a device that expands a single Universal Serial Bus (USB)
port into several so that there

79. SD CARDS
solid state storage devices come in two basic forms: flash memory
cards and USB memory sticks.
Flashmemorycardsweredevelopedasastoragemediafordigital
camerasandmobilecomputers.Theyconsistofasmallplasticpackage
withacontactarraythatslotsintoacameraorothermobile
computingdevice,oranappropriatememorycardreader.

80. U3
U3 was a joint venture between SanDisk and M-Systems, producing a
proprietary method of launching Windows applications from special USB flash
drives.
Flash drives adhering to the U3 specification are termed
"U3 smart drives".
U3 smart drives come preinstalled with the U3 Launchpad, which looks similar to
the Windows OS start menu and controls program installation.
Applications that comply with U3 specifications are allowed to write files or
registry information to the host computer, but they must remove this information
when the flash drive is ejected.
Customizations and settings are instead stored with the application on the flash
drive.
Microsoft and SanDisk created a successor called StartKey.