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William Stallings Computer Organization and Architecture Chapter 5 External Memory.

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Presentation on theme: "William Stallings Computer Organization and Architecture Chapter 5 External Memory."— Presentation transcript:

1 William Stallings Computer Organization and Architecture Chapter 5 External Memory

2 Types of External Memory zMagnetic Disk yRAID yRemovable zOptical yCD-ROM yCD-Writable (WORM) yCD-R/W yDVD zMagnetic Tape

3 Magnetic Disk zMetal or plastic disk coated with magnetizable material (iron oxide…rust) zRange of packaging yFloppy yWinchester hard disk yRemovable hard disk

4 Data Organization and Formatting zConcentric rings or tracks yGaps between tracks yReduce gap to increase capacity ySame number of bits per track (variable packing density) yConstant angular velocity zTracks divided into sectors zMinimum block size is one sector zMay have more than one sector per block

5 Disk Data Layout

6 Fixed/Movable Head Disk zFixed head yOne read write head per track yHeads mounted on fixed ridged arm zMovable head yOne read write head per side yMounted on a movable arm

7 Fixed and Movable Heads

8 Removable or Not zRemovable disk yCan be removed from drive and replaced with another disk yProvides unlimited storage capacity yEasy data transfer between systems zNonremovable disk yPermanently mounted in the drive

9 Floppy Disk z8”, 5.25”, 3.5” zSmall capacity yUp to 1.44Mbyte (2.88M never popular) zSlow zUniversal zCheap

10 Winchester Hard Disk (1) zDeveloped by IBM in Winchester (USA) zSealed unit zOne or more platters (disks) zHeads fly on boundary layer of air as disk spins zVery small head to disk gap zGetting more robust

11 Winchester Hard Disk (2) zUniversal zCheap zFastest external storage zGetting larger all the time yMultiple Gigabyte now usual

12 Removable Hard Disk zZIP yCheap yVery common yOnly 100M zJAZ yNot cheap y1G zL-120 (a: drive) yAlso reads 3.5” floppy yBecoming more popular?

13 Finding Sectors zMust be able to identify start of track and sector zFormat disk yAdditional information not available to user yMarks tracks and sectors

14 Gap1 IdGap2DataGap3 Gap1 IdGap2DataGap3 Track Sync Byte HeadSectorCRC Sync Byte DataCRC ST506 format (old!) zForeground reading yFind others

15 Characteristics zFixed (rare) or movable head zRemovable or fixed zSingle or double (usually) sided zSingle or multiple platter zHead mechanism yContact (Floppy) yFixed gap yFlying (Winchester)

16 Multiple Platter zOne head per side zHeads are joined and aligned zAligned tracks on each platter form cylinders zData is striped by cylinder yreduces head movement yIncreases speed (transfer rate)

17 Speed zSeek time yMoving head to correct track z(Rotational) latency yWaiting for data to rotate under head zAccess time = Seek + Latency zTransfer rate

18 RAID zRedundant Array of Independent Disks zRedundant Array of Inexpensive Disks z6 levels in common use zNot a hierarchy zSet of physical disks viewed as single logical drive by O/S zData distributed across physical drives zCan use redundant capacity to store parity information

19 RAID 0 zNo redundancy zData striped across all disks zRound Robin striping zIncrease speed yMultiple data requests probably not on same disk yDisks seek in parallel yA set of data is likely to be striped across multiple disks

20 RAID 1 zMirrored Disks zData is striped across disks z2 copies of each stripe on separate disks zRead from either zWrite to both zRecovery is simple ySwap faulty disk & re-mirror yNo down time zExpensive

21 RAID 2 zDisks are synchronized zVery small stripes yOften single byte/word zError correction calculated across corresponding bits on disks zMultiple parity disks store Hamming code error correction in corresponding positions zLots of redundancy yExpensive yNot used

22 RAID 3 zSimilar to RAID 2 zOnly one redundant disk, no matter how large the array zSimple parity bit for each set of corresponding bits zData on failed drive can be reconstructed from surviving data and parity info zVery high transfer rates

23 RAID 4 zEach disk operates independently zGood for high I/O request rate zLarge stripes zBit by bit parity calculated across stripes on each disk zParity stored on parity disk

24 RAID 5 zLike RAID 4 zParity striped across all disks zRound robin allocation for parity stripe zAvoids RAID 4 bottleneck at parity disk zCommonly used in network servers zN.B. DOES NOT MEAN 5 DISKS!!!!!

25 Optical Storage CD-ROM zOriginally for audio z650Mbytes giving over 70 minutes audio zPolycarbonate coated with highly reflective coat, usually aluminum zData stored as pits zRead by reflecting laser zConstant packing density zConstant linear velocity

26 CD-ROM Drive Speeds zAudio is single speed yConstant linier velocity y1.2 ms -1 yTrack (spiral) is 5.27km long yGives 4391 seconds = 73.2 minutes zOther speeds are quoted as multiples ze.g. 24x zThe quoted figure is the maximum the drive can achieve

27 Comparison of Disk Layouts

28 00 FF x 10 Min Sec Sector Mode Data Layered ECC 12 byte Sync 4 byte Id 2048 byte288 byte 2352 byte CD-ROM Format zMode 0=blank data field zMode 1=2048 byte data+error correction zMode 2=2336 byte data

29 Random Access on CD-ROM zDifficult zMove head to rough position zSet correct speed zRead address zAdjust to required location z(Yawn!)

30 CD-ROM for & against zLarge capacity (?) zEasy to mass produce zRemovable zRobust zExpensive for small runs zSlow zRead only

31 Other Optical Storage zCD-Writable yWORM yNow affordable yCompatible with CD-ROM drives zCD-RW yErasable yGetting cheaper yMostly CD-ROM drive compatible

32 DVD - what’s in a name? zDigital Video Disk yUsed to indicate a player for movies xOnly plays video disks zDigital Versatile Disk yUsed to indicate a computer drive xWill read computer disks and play video disks zDogs Veritable Dinner zOfficially - nothing!!!

33 DVD - technology zMulti-layer zVery high capacity (4.7G per layer) zFull length movie on single disk yUsing MPEG compression zFinally standardized (honest!) zMovies carry regional coding zPlayers only play correct region films zCan be “fixed”

34 DVD - Writable zLoads of trouble with standards zFirst generation DVD drives may not read first generation DVD-W disks zFirst generation DVD drives may not read CD- RW disks zWait for it to settle down before buying!

35 Foreground Reading zCheck out optical disk storage options zCheck out Mini Disk

36 Magnetic Tape zSerial access zSlow zVery cheap zBackup and archive

37 Digital Audio Tape (DAT) zUses rotating head (like video) zHigh capacity on small tape y4Gbyte uncompressed y8Gbyte compressed zBackup of PC/network servers


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