Computer Organization and Architecture
اسلاید 1: Rev. by Luciano Gualà (2008)1William Stallings Computer Organization and ArchitectureChapter 6External Memory
اسلاید 2: Rev. by Luciano Gualà (2008)2Types of External MemoryMagnetic DiskRAIDMagnetic TapeOpticalCD-ROMCD-RCD-RWDVD
اسلاید 3: Rev. by Luciano Gualà (2008)3Magnetic DiskMetal or plastic disk coated, on one or both sides, with magnetizable materialData read and written through a magnetic head (coil) by means of induction
اسلاید 4: Rev. by Luciano Gualà (2008)4Disk Data Layout
اسلاید 5: Rev. by Luciano Gualà (2008)5Data Organization and FormattingConcentric rings or tracksGaps between tracksReduce gap to increase capacitySame number of bits per track (variable density)Constant angular velocityTracks divided into sectorsData read/written in blocksMinimum block size is one sectorMay have more than one sector per block
اسلاید 6: Rev. by Luciano Gualà (2008)6Comparison of variable/fixed density
اسلاید 7: Rev. by Luciano Gualà (2008)7Finding SectorsMust be able to identify start of track and sectorFormat diskAdditional information not available to userMarks tracks and sectors
اسلاید 8: Rev. by Luciano Gualà (2008)8Gap1IdGap2DataGap3Track #Head #Sector#SyncByteCRCSyncByteDataCRCAn example formatGapSectorGapSector…Track:
اسلاید 9: Rev. by Luciano Gualà (2008)9Characteristics of magnetic disksSingle or double (usually) sidedRemovable or fixedFixed or movable headSingle or multiple platterHead mechanismSpeed
اسلاید 10: Rev. by Luciano Gualà (2008)10Removable or NotRemovable diskCan be removed from drive and replaced with another diskProvides unlimited storage capacity (by changing disk)Easy data transfer between systemsNonremovable diskPermanently mounted in the drive
اسلاید 11: Rev. by Luciano Gualà (2008)11Fixed/Movable Head DiskFixed headOne read/write head per trackHeads mounted on a fixed armMovable headOne read/write head per sideMounted on a movable arm
اسلاید 12: Rev. by Luciano Gualà (2008)12Multiple PlattersOne head per sideHeads are joined and alignedAligned tracks on each platter form cylindersData is striped by cylinderreduces head movementincreases speed (transfer rate)
اسلاید 13: Rev. by Luciano Gualà (2008)13Head mechanismContact FloppyFixed gapAerodynamic gap or flying head Winchester
اسلاید 14: Rev. by Luciano Gualà (2008)14Winchester Hard Disk (1)Developed by IBM in Winchester (USA)Sealed unitOne or more platters (disks)Heads fly on boundary layer of air as disk spinsVery small head-to-disk gapGetting more robust
اسلاید 15: Rev. by Luciano Gualà (2008)15Winchester Hard Disk (2)UniversalCheapFastest external storageGetting larger all the timeMultiple Gigabyte now usual
اسلاید 16: Rev. by Luciano Gualà (2008)16SpeedSeek timeMoving head to the right track(Rotational) latencyWaiting for data to rotate under headAccess time = Seek + LatencyTransfer rate: speed of copying bytes from diskTotal time
اسلاید 17: Rev. by Luciano Gualà (2008)17RAIDRedundant Array of Independent DisksAt least 7 different versions in common use (Not a hierarchy)Set of physical disks viewed as single logical drive by the operating systemData distributed (striped) across physical drivesCan use redundant capacity to store parity information and provide fault tolerance
اسلاید 18: Rev. by Luciano Gualà (2008)18Magnetic TapeOnly sequential accessSlower than magnetic and optical disksVery very cheapBackup and archive
اسلاید 19: Rev. by Luciano Gualà (2008)19Optical Storage: CD-ROMOriginally for audio650 Mbytes giving over 70 minutes audioPolycarbonate coated with highly reflective coat, usually aluminumData stored as pitsRead by reflecting laserAudio is single speedConstant linear velocity1.2 m/sTrack (spiral) is 5.27km longGives 4391 seconds = 73.2 minutes
اسلاید 20: Rev. by Luciano Gualà (2008)20Random Access on CD-ROMDifficult, due to constant density and single trackMove head to rough positionSet correct speedRead addressAdjust to required location
اسلاید 21: Rev. by Luciano Gualà (2008)21CD-ROM for & againstLarge capacityEasy to mass produceRemovableRobustExpensive for small runsSlower than magnetic diskRead only
اسلاید 22: Rev. by Luciano Gualà (2008)22Other Optical StorageCD-R (for Recordable)Writable, but ... Write Once Read Many (WORM)Now affordableCompatible with CD-ROM drivesCD-RW (for ReWritable)Erasable, hence writable many times (~1000)Different technology (phase change vs pit)Getting cheaperMostly, but not always, CD-ROM drive compatible
اسلاید 23: Rev. by Luciano Gualà (2008)23DVD - Digital Video/Versatile DiskOptical (CD-sized) disk with a very high capacity:4.7 GB per layer (smaller pits and closer tracks)Up to 2 layers on each of the 2 sides (total 17 GB)Drives are CD-ROM compatibleAlso writable (DVD-R, DVD-RW), but not yet fully standardized
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