Computer Organization and Architecture

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William Stallings Computer Organization and Architecture Chapter 6 External Memory 

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Types of External Memory I Magnetic Disk I RAID 0 Magnetic Tape 0 Optical | CD-ROM 1 CD-R ۱ | DvD 

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Magnetic Disk 0 Metal or plastic disk coated, on one or both sides, with magnetizable material 0 Data read and written through a magnetic head (coil) by means of induction 

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Disk Data Layout 

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Data Organization and Formatting I Concentric rings or tracks | Gaps between tracks | Reduce gap to increase capacity | Same number of bits per track (variable density) ! Constant angular velocity 0 Tracks divided into sectors 0 Data read/written in blocks ! Minimum block size is one sector I May have more than one sector per block 

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Comparison of variable/fixed density (a) Constant angular velocity (b) Constant linear velocity Rev. by Lurene Bucks (2D0O) 6-9 

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Finding Sectors 0 Must be able to identify start of track and sector 0 Format disk I Additional information not available to user | Marks tracks and sectors (E000) 

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CRC — a Gap1| Id lsap4Datacaps Sync IByte An example format | oS / 0 [rack Head ‏ا‎ CRC Track: | Ga p$ecto Gap$ecto Sync Byte 

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Characteristics of magnetic disks Single or double (usually) sided Removable or fixed Fixed or movable head Single or multiple platter Head mechanism Speed ee et Se te a ‏جد‎ 

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Removable or Not 0 Removable disk | Can be removed from drive and replaced with another disk I Provides unlimited storage capacity (by changing disk) | Easy data transfer between systems 0 Nonremovable disk | Permanently mounted in the drive 

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Fixed/Movable Head Disk U Fixed head ! One read/write head per track | Heads mounted on a fixed arm 0 Movable head I One read/write head per side | Mounted on a movable arm xd0O) 

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Multiple Platters One head per side { Heads are joined and aligned " Aligned tracks on each platter form cylinders " Data is striped by cylinder = ۱ reduces head movement ' increases speed ‏سس‎ ‎(transfer rate) 

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Head mechanism 0 Contact | Floppy 0 Fixed gap 0 Aerodynamic gap or flying head | Winchester 

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Winchester Hard Disk (1) Developed by IBM in Winchester (USA) Sealed unit One or more platters (disks) Heads fly on boundary layer of air as disk spins Very small head-to-disk gap Getting more robust ee a: ‏كد‎ = 

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Winchester Hard Disk (2) 0 Universal 0 Cheap U Fastest external storage 0 Getting larger all the time I Multiple Gigabyte now usual 

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Speed 0 Seek time | Moving head to the right track 0 (Rotational) latency I Waiting for data to rotate under head 0 Access time = Seek + Latency 0 Transfer rate: speed of copying bytes from disk Seek Rotational Data Delay Transfer بت تسس مس ————— Total time ‏يل‎ 8 :)006( 

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RAID Redundant Array of Independent Disks At least 7 different versions in common use (Not a hierarchy) Set of physical disks viewed as single logical drive by the operating system Data distributed (striped) across physical drives Can use redundant capacity to store parity information and provide fault tolerance ۳ سر oa a oa 

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Magnetic Tape 0 Only sequential access 0 Slower than magnetic and optical disks 0 Very very cheap 0 Backup and archive 

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Optical Storage: CD-ROM I Originally for audio 1 650 Mbytes giving over 70 minutes audio 1 Polycarbonate coated with highly reflective coat, usually aluminum 1 Data stored as pits I Read by reflecting laser 1 Audio is single speed ! Constant linear velocity ۱ 1.2 ۸5 1 Track (spiral) is 5.27km long Gives 4391 seconds = 73.2 minutes Rev. by Lurene Bucks (2D0O) 6-09 

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Random Access on CD-ROM oa Difficult, due to constant density and single track Move head to rough position Set correct speed Read address Adjust to required location ات اه تسج 

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CD-ROM for & against Large capacity Easy to mass produce Removable Robust ee a: ‏كد‎ oa Expensive for small runs Slower than magnetic disk Read only et ‏نت‎ 

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Other Optical Storage I CD-R (for Recordable) ! writable, but ... Write Once Read Many (WORM) | Now affordable ! Compatible with CD-ROM drives 1 CD-RW (for ReWritable) ! Erasable, hence writable many times (~1000) | Different technology (phase change vs pit) ! Getting cheaper 1١ Mostly, but not always, CD-ROM drive compatible 

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DVD - Digital Video/Versatile Disk 0 Optical (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 compatible U Also writable (DVD-R, DVD-RW), but not yet fully standardized 

William Stallings Computer Organization and Architecture Chapter 6 External Memory Rev. by Luciano Gualà (2008) 5- 1 Types of External Memory  Magnetic Disk  RAID  Magnetic Tape  Optical     CD-ROM CD-R CD-RW DVD Rev. by Luciano Gualà (2008) 5- 2 Magnetic Disk  Metal or plastic disk coated, on one or both sides, with magnetizable material  Data read and written through a magnetic head (coil) by means of induction Rev. by Luciano Gualà (2008) 5- 3 Disk Data Layout Rev. by Luciano Gualà (2008) 5- 4 Data Organization and Formatting  Concentric rings or tracks  Gaps between tracks  Reduce gap to increase capacity  Same number of bits per track (variable density)  Constant angular velocity  Tracks divided into sectors  Data read/written in blocks  Minimum block size is one sector  May have more than one sector per block Rev. by Luciano Gualà (2008) 5- 5 Comparison of variable/fixed density Rev. by Luciano Gualà (2008) 5- 6 Finding Sectors  Must be able to identify start of track and sector  Format disk  Additional information not available to user  Marks tracks and sectors Rev. by Luciano Gualà (2008) 5- 7 An example format Track: GapSectorGapSector … Gap1 Id Gap2DataGap3 Sync Track Head Sector CRC Byte # # # Sync Data CRC Byte Rev. by Luciano Gualà (2008) 5- 8 Characteristics of magnetic disks  Single or double (usually) sided  Removable or fixed  Fixed or movable head  Single or multiple platter  Head mechanism  Speed Rev. by Luciano Gualà (2008) 5- 9 Removable or Not  Removable disk  Can be removed from drive and replaced with another disk  Provides unlimited storage capacity (by changing disk)  Easy data transfer between systems  Nonremovable disk  Permanently mounted in the drive Rev. by Luciano Gualà (2008) 5 - 10 Fixed/Movable Head Disk  Fixed head  One read/write head per track  Heads mounted on a fixed arm  Movable head  One read/write head per side  Mounted on a movable arm Rev. by Luciano Gualà (2008) 5 - 11 Multiple Platters  One head per side  Heads are joined and aligned  Aligned tracks on each platter form cylinders  Data is striped by cylinder  reduces head movement  increases speed (transfer rate) Rev. by Luciano Gualà (2008) 5 - 12 Head mechanism  Contact  Floppy  Fixed gap  Aerodynamic gap or flying head  Winchester Rev. by Luciano Gualà (2008) 5 - 13 Winchester Hard Disk (1)  Developed by IBM in Winchester (USA)  Sealed unit  One or more platters (disks)  Heads fly on boundary layer of air as disk spins  Very small head-to-disk gap  Getting more robust Rev. by Luciano Gualà (2008) 5 - 14 Winchester Hard Disk (2)  Universal  Cheap  Fastest external storage  Getting larger all the time  Multiple Gigabyte now usual Rev. by Luciano Gualà (2008) 5 - 15 Speed  Seek time  Moving head to the right track  (Rotational) latency  Waiting for data to rotate under head  Access time = Seek + Latency  Transfer rate: speed of copying bytes from disk Total time Rev. by Luciano Gualà (2008) 5 - 16 RAID  Redundant Array of Independent Disks  At least 7 different versions in common use (Not a hierarchy)  Set of physical disks viewed as single logical drive by the operating system  Data distributed (striped) across physical drives  Can use redundant capacity to store parity information and provide fault tolerance Rev. by Luciano Gualà (2008) 5 - 17 Magnetic Tape  Only sequential access  Slower than magnetic and optical disks  Very very cheap  Backup and archive Rev. by Luciano Gualà (2008) 5 - 18 Optical Storage: CD-ROM  Originally for audio  650 Mbytes giving over 70 minutes audio  Polycarbonate coated with highly reflective coat, usually aluminum  Data stored as pits  Read by reflecting laser  Audio is single speed     Constant linear velocity 1.2 m/s Track (spiral) is 5.27km long Gives 4391 seconds = 73.2 minutes Rev. by Luciano Gualà (2008) 5 - 19 Random Access on CD-ROM  Difficult, due to constant density and single track  Move head to rough position  Set correct speed  Read address  Adjust to required location Rev. by Luciano Gualà (2008) 5 - 20 CD-ROM for & against  Large capacity  Easy to mass produce  Removable  Robust  Expensive for small runs  Slower than magnetic disk  Read only Rev. by Luciano Gualà (2008) 5 - 21 Other Optical Storage  CD-R (for Recordable)  Writable, but ... Write Once Read Many (WORM)  Now affordable  Compatible with CD-ROM drives  CD-RW (for ReWritable)     Erasable, hence writable many times (~1000) Different technology (phase change vs pit) Getting cheaper Mostly, but not always, CD-ROM drive compatible Rev. by Luciano Gualà (2008) 5 - 22 DVD - Digital Video/Versatile Disk  Optical (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 compatible  Also writable (DVD-R, DVD-RW), but not yet fully standardized Rev. by Luciano Gualà (2008) 5 - 23