Computer Organization and Architecture

صفحه 1:
William Stallings Computer Organization and Architecture Chapter 6 External Memory 

صفحه 2:
Types of External Memory I Magnetic Disk I RAID 0 Magnetic Tape 0 Optical | CD-ROM 1 CD-R ۱ | DvD 

صفحه 3:
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 

صفحه 4:
Disk Data Layout 

صفحه 5:
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 

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

صفحه 7:
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) 

صفحه 8:
CRC — a Gap1| Id lsap4Datacaps Sync IByte An example format | oS / 0 [rack Head ‏ا‎ CRC Track: | Ga p$ecto Gap$ecto Sync Byte 

صفحه 9:
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 

صفحه 10:
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 

صفحه 11:
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) 

صفحه 12:
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) 

صفحه 13:
Head mechanism 0 Contact | Floppy 0 Fixed gap 0 Aerodynamic gap or flying head | Winchester 

صفحه 14:
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 = 

صفحه 15:
Winchester Hard Disk (2) 0 Universal 0 Cheap U Fastest external storage 0 Getting larger all the time I Multiple Gigabyte now usual 

صفحه 16:
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( 

صفحه 17:
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 

صفحه 18:
Magnetic Tape 0 Only sequential access 0 Slower than magnetic and optical disks 0 Very very cheap 0 Backup and archive 

صفحه 19:
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 

صفحه 20:
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 

صفحه 21:
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 

صفحه 22:
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 

صفحه 23:
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