William Stallings William Stallings Computer Organization Computer Organization

and Architectureand ArchitectureChapter 5

Memori External

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Jenis Memori ExternalJenis Memori External

Magnetic Disk◦RAID◦Removable

Optical◦CD-ROM◦CD-Writable (WORM)◦CD-R/W◦DVD

Magnetic Tape

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Magnetic DiskMagnetic Disk

Metal atau plastic dilapisi dg material yg bersifat magnet (iron oxide)

Jenis kemasan◦Floppy◦Winchester hard disk◦Removable hard disk

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Format dan Organisasi Data Format dan Organisasi Data

Lingkaran konsentris atau track◦Ada Gap antar track◦Gap sempit, kapasitas bertambah◦Jumlah bit per track sama (kerapatan

bervariasi)◦Kecepatan putar tetap

Track dibagi menjadi beberapa sectorUkuran minimum block adalah satu sectorSatu block bisa berisi lebih dari satu

sector

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Fixed/Movable Head DiskFixed/Movable Head Disk

Fixed head◦Ada satu head (r/w) per track◦Head diletakkan pada tangkai yg tetap

Movable head◦Hanya ada satu head per side◦Diletakkan pada tangkai yg dpt bergerak

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Removable / NonremovableRemovable / Nonremovable

Removable disk◦Dapat dilepas dari drive dan diganti dg disk lain ◦Memberikan kapasitas simpanan yg tak

terbatas◦Mudah melakukan transfer data antar sistem

Nonremovable disk◦Terpasanang permanen dalam drive

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Floppy DiskFloppy Disk

8”, 5.25”, 3.5”Kapasitas kecil

◦sampai 1.44Mbyte (ada yg 2.88M)LambatUmum dipakaiMurah

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Winchester Hard Disk (1)Winchester Hard Disk (1)

Dikembangkan oleh IBM di Winchester (USA)

Dikemas dalam satu unitBerisi satu cakram atau lebihHead sangat kecilHandal

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Winchester Hard Disk (2)Winchester Hard Disk (2)

Umum digunakanMurahSbg external storage yg sangat cepatKapasitas semakin besar

◦Dalam orde GB

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Removable Hard DiskRemovable Hard Disk

ZIP◦Murah◦Banyak digunakan◦100MB

JAZ◦Mahal◦1G

L-120 (a: drive)◦Juga dpt untuk membaca 3.5” floppy

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Pencarian SectorPencarian Sector

Harus dapat mengenali awal suatu track dan sector

Format disk◦Menambahkan informasi tambahan◦Memberi tanda awal track dan sector

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ST506 format (old!)ST506 format (old!)12

Gap1 Id Gap2 Data Gap3 Gap1 Id Gap2 Data Gap3

TrackSyncByte

Head Sector CRC SyncByte

Data CRC

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KarakteristikKarakteristik

Fixed head atau movable headRemovable disk atau fixed diskSingle side atau double sideSingle platter atau multiple platterMekanisme head

◦Contact (Floppy)◦Fixed gap◦Flying (Winchester)

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Multiple PlatterMultiple Platter

Satu head per sideSemua head di-join dan di-alignTrack-track yg setiap platter membentuk

cylinderData dipecah berdasarkan cylinder

◦Mengurangi gerakan head◦Meningkatkan kecepatan (transfer rate)

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KecepatanKecepatan

Seek time◦gerakan head ke track yg dituju

(Rotational) latency◦Putar platter sampai posisi data dibawah head

Access time = Seek + LatencyTransfer rate

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RAIDRAID

Redundant Array of Independent Disks Redundant Array of Inexpensive DisksAda 6 level Tidak berhirarkiSejumlah disks (fisik) yg dipandang sbg

satu drive (logical) oleh Sistem OperasiData tersebar diantara disk fisik

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RAID 0RAID 0

No redundancyData striped across all disksRound Robin stripingIncrease speed

◦Multiple data requests probably not on same disk

◦Disks seek in parallel◦A set of data is likely to be striped across

multiple disks

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RAID 1RAID 1

Mirrored DisksData is striped across disks2 copies of each stripe on separate disksRead from eitherWrite to bothRecovery is simple

◦Swap faulty disk & re-mirror◦No down time

Expensive

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RAID 2RAID 2

Disks are synchronizedVery small stripes

◦Often single byte/wordError correction calculated across

corresponding bits on disksMultiple parity disks store Hamming code

error correction in corresponding positionsLots of redundancy

◦Expensive◦Not used

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RAID 3RAID 3

Similar to RAID 2Only one redundant disk, no matter how

large the arraySimple parity bit for each set of

corresponding bitsData on failed drive can be reconstructed

from surviving data and parity infoVery high transfer rates

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RAID 4RAID 4

Each disk operates independentlyGood for high I/O request rateLarge stripesBit by bit parity calculated across stripes

on each diskParity stored on parity disk

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RAID 5RAID 5

Like RAID 4Parity striped across all disksRound robin allocation for parity stripeAvoids RAID 4 bottleneck at parity diskCommonly used in network servers

N.B. DOES NOT MEAN 5 DISKS!!!!!

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Optical Storage CD-ROMOptical Storage CD-ROM

Originally for audio650Mbytes giving over 70 minutes audioPolycarbonate coated with highly

reflective coat, usually aluminumData stored as pitsRead by reflecting laserConstant packing densityConstant linear velocity

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CD-ROM Drive SpeedsCD-ROM Drive Speeds

Audio is single speed◦Constant linier velocity◦1.2 ms-1

◦Track (spiral) is 5.27km long◦Gives 4391 seconds = 73.2 minutes

Other speeds are quoted as multiplese.g. 24xThe quoted figure is the maximum the

drive can achieve

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CD-ROM FormatCD-ROM FormatMode 0=blank data fieldMode 1=2048 byte data+error correctionMode 2=2336 byte data

25

00 00FF x 10 M

in

Sec

Sec

tor

Mod

e

DataLayeredECC

12 byteSync

4 byteId

2048 byte 288 byte

2352 byte

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Random Access on Random Access on CD-ROMCD-ROM

DifficultMove head to rough positionSet correct speedRead addressAdjust to required location(Yawn!)

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CD-ROM for & againstCD-ROM for & against

Large capacity (?)Easy to mass produceRemovableRobust

Expensive for small runsSlowRead only

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Other Optical StorageOther Optical Storage

CD-Writable◦WORM◦Now affordable◦Compatible with CD-ROM drives

CD-RW◦Erasable◦Getting cheaper◦Mostly CD-ROM drive compatible

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DVD - what’s in a name?DVD - what’s in a name?

Digital Video Disk◦Used to indicate a player for movies

Only plays video disksDigital Versatile Disk

◦Used to indicate a computer drive Will read computer disks and play video disks

Dogs Veritable DinnerOfficially - nothing!!!

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DVD - technologyDVD - technology

Multi-layerVery high capacity (4.7G per layer)Full length movie on single disk

◦Using MPEG compressionFinally standardized (honest!)Movies carry regional codingPlayers only play correct region filmsCan be “fixed”

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DVD - WritableDVD - Writable

Loads of trouble with standardsFirst generation DVD drives may not read

first generation DVD-W disksFirst generation DVD drives may not read

CD-RW disksWait for it to settle down before buying!

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Foreground ReadingForeground Reading

Check out optical disk storage optionsCheck out Mini Disk

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Magnetic TapeMagnetic Tape

Serial accessSlowVery cheapBackup and archive

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Digital Audio Tape (DAT)Digital Audio Tape (DAT)

Uses rotating head (like video)High capacity on small tape

◦4Gbyte uncompressed◦8Gbyte compressed

Backup of PC/network servers

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