Information and Communication Technology FundamentalsCredits Hours: 2+1

Instructor: Ayesha Bint Saleem

Storing Data• Penning it down on a piece of paper• Typing it using typewriter and getting a

hard copy• Storing it electronically on computer

6A-3

Describing Storage Devices• Store data when computer is off

• Re-use data at a later time

• Two processes• Writing data

• Recording data over some surface

• Reading data• And transferring it to computer memory

6A-4

Describing Storage Devices• Storage terms

• Media is the physical material storing data• Storage devices manage the media

• Writing data to storage device• Reading data from storage device• Diskette: storage medium, Diskette Drive: storage

device

• Magnetic Storage• Magnetic devices use a magnet

• Optical Storage• Optical devices use lasers

• Solid-state devices have physical switches

6A-5

Magnetic Storage Devices• Most common form of storage• Hard drives, floppy drives, tape• All magnetic drives work the same

• Similar Techniques for reading and writing

• Surface of Storage media covered with Magnetically sensitive material• Reacts to magnetic field• Example; Iron Oxide

6A-6

Magnetic Storage Devices• Diskette

• Single thin disk of plastic• Stores data on both sides• Each side has its own read/write head

• Hard Disk• Contain multiple disks

• Platters

• Made of rigid material• Aluminum

6A-7

Magnetic Storage Devices

Floppy DiskFloppy Disk

Hard DiskHard Disk

TapeTape

6A-8

Magnetic Storage Devices• One magnet makes another magnet

• Stroke an iron bar with magnet in one direction

• Use electrical current to polarize iron

• Electromagnet

• Polarity and strength dependant on direction and strength of

current

• Transistor stores a bit as ON or OFF• Orientation of Magnetic field used to represent

data• Magnet can represent ON or OFF without continual

source of electricity

6A-9

Magnetic Storage Devices• Data storage

• Media is covered with iron oxide particles• Read/write head is an electro-magnet• Magnet writes charges on the media

• Positive charge is a 1• Negative charge is a 0• Record 1s and 0s by alternating the direction of

current

6A-10

Magnetic Storage Devices• Data retrieval

• Read/Write head passes over surface while no current flows

• Storage medium charges the Read/Write head• Small current flow with direction dependant on

field polarity

• Direction of flow is sensed and data is sent into memory

6A-11

Data Retrieval

6A-12

Magnetic Storage Devices

• Data organization

• Disks must be formatted before use

• Magnetically map disk surface

• Assign addresses to different surface areas

• Computer can go directly to required location:

Random Access Nature

• Reformatting destroys all data present on

disk

6A-13

Magnetic Storage Devices• Data organization

• Format draws tracks on the disk• Concentric rings• Numbered from outermost to innermost, starting

from 0

• Tracks is divided into sectors• Smallest unit Magnetic disk drive can work with• Entire sector read/written• In most Hard Disks a sector can store upto 512

bytes• All sectors on disk numbered in one long sequence• Each sector uniquely addressable and accessible

6A-14

Magnetic Storage Devices• Data organization

• Outermost track longer than innermost track• Same number of sectors/track

• All sectors store same amount of data• Don’t occupy same space• Waste of disk material; Longer tracks can store

more data

• Solution: More sectors to longer tracks• Move towards disk center; number of sectors/track ↓

• More efficient method• Sectors/track given as average

6A-15

Tracks and Sectors

6A-16

Magnetic Storage Devices

• How OS finds data on disk

• Each track and sector is labeled

• Some are reserved

• Location of all data kept in special log

• Labeling called logical formatting

• Different OS format disk in different ways

• Different manner of data management on disk

• Different File system

6A-17

Magnetic Storage Devices• FAT file system

• Standardized file allocation table to keep track of file locations on disk

• Four areas created on disk

• Boot Sector

• Programs that run on startup, checks required files

• Transfers control to OS that continues the process

• Describes disk characteristics: bytes/sector, sectors/track

• File Allocation Table• Log maintaining the location of each file + status of sectors

6A-18

Magnetic Storage Devices• Four areas created on disk

• File Allocation Table

• Writing to Disk: find open area using FAT store file

log file ID + location

• Reading: find location using FAT

• Two copies of FAT, always kept updated

• Root Folder

• Master folder containing all other folders

• Holds all the info about all other folders on disk

• Data Area

• Area where files and Programs are actually written

6A-19

Magnetic Storage Devices• Finding data on disk

• Listing of where files are stored• File Allocation Table (FAT)

• FAT 16: MS-DOS, early Windows

• FAT32• Extended edition of original FAT• Windows 95, 2000, XP

• New Technology File System (NTFS)• Windows NT, supported by other Windows versions as

well• Longer file names allowed

• NTFS 5• High Performance File System (HPFS)

• IBM OS/2

6A-20

Magnetic Storage Devices• Finding data on disk

• Sector grouped into clusters• Seen by OS as a single unit• Smallest place OS will allocate to a single file• Cluster sizes vary depending on size and type

of disk• 4 sectors (diskettes) ; 64 sectors (Hard disks)• Cluster usage tracked in FAT

6A-21

Magnetic Storage Devices• Diskettes

• Also known as floppy disks• Read with a disk drive• Mylar disk• Spin at 300 RPM

• Amount of time required for one revolution 0.2 sec• Time to move from innermost to outermost track 0.17• Both happen simultaneously• Data access time = max(0.2, 0.17) = 0.2• 0.5 sec if disk is moved from a dead stop

• 3 ½ floppy disk holds 1.44 MB• 2880 sectors * 512 bytes/sector

6A-22

Magnetic Storage Devices• Hard disks

• Primary storage device in a computer• 2 or more aluminum platters• Each platter has 2 sides• Spin between 5,400 to 15,000 RPM• Data found in 9.5 ms or less• Drive capacity greater than 40 GB• High rotational speed allows more data to be

recorded• Faster movement can use smaller charges to

make current flow through head

6A-23

Illustrated Hard Disk

6A-24

Magnetic Storage Devices• Removable high capacity disks

• Speed of hard disk• Portability of floppy disk• Several variants have emerged• High capacity floppy disk

• Stores up to 750 MB of data

• Hot swappable hard disks• Provide GB of data• Connect via USB

6A-25

Magnetic Storage Devices• Tape drives

• Best used for• Infrequently accessed data• Back-up solutions

• Slow sequential access• Capacity exceeds 200 GB

6A-26

Optical Storage Devices• CD-ROM

• Most software ships on a CD• Read using a laser

• Lands, binary 1, reflect data• Pits scatter data

• Written from the inside out• CD speed is based on the original

• Original CD read 150 Kbps• A 10 X will read 1,500 Kbps

• Speed slower than Hard Drive• Standard CD holds 650 MB

6A-27

Optical Storage Devices• DVD-ROM

• Digital Video Disk• Use both sides of the disk• Use layers• Capacities can reach 18 GB• DVD players can read CDs

6A-28

Recordable Optical Technologies• CD Recordable (CD-R)

• Create a data or audio CD• Data cannot be changed• Can continue adding until full

6A-29

Recordable Optical Technologies• CD Re-Writable (CD-RW)

• Create a reusable CD• Cannot be read in all CD players• Can reuse about 100 times

6A-30

Recordable Optical Technologies• Photo CD

• Developed by Kodak• Provides for photo storage• Photos added to CD until full• Original pictures cannot be changed

6A-31

Recordable Optical Technologies• DVD Recordable

• Several different formats exist• None are standardized• Allows home users to create DVDs• Cannot be read in all players

6A-32

Recordable Optical Technologies• DVD-RAM

• Allow reusing of DVD media• Not standardized• Cannot be read in all players

6A-33

Solid State Devices• Data is stored physically• No magnets or laser• Less reliable and more expensive than

Magnetic/Optical sotrage• Very fast

• No mechanical motion invloved

6A-34

Solid State Devices• Flash memory

• Found in cameras and USB drives• Combination of RAM and ROM• Long term updateable storage

6A-35

Solid State Devices• Smart cards

• Credit cards with a chip• Chip stores data• Eventually may be used for cash• Hotels use for electronic keys

6A-36

Solid State Devices• Solid-state disks

• Large amount of SDRAM• Extremely fast• Volatile storage• Require battery backups• Most have hard disks copying data

6B-37

Drive Performance• Average access time

• Also known as seek time

• Time to find desired data

• Measured in milliseconds

• Depends on two factors

• RPM

• Time to access a track

• Average Access time half of maximum access time

• Hard drive between 6 and 12 ms

• CD between 80 and 800 ms

• Tape drive slowest

• Solid State fastest

6B-38

Drive Performance• Data transfer rate

• How fast data can be read• Measured in Bps or bps• Hard drive ranges from 15 to 160 MBps• CD ROMS depend on X factor

• 24x CD transfers 24 x 150 KBps

• Floppy disks transfer at 45 KBps

6B-39

Data Transfer Rate

6B-40

Optimizing Performance• Disk optimization

• Handled by operating system tool• Routine disk maintenance• Over time PC’s performance may slow down• Optimization should be run monthly

6B-41

Optimizing Performance• Clean up unnecessary files

• Delete temp files• Uninstall unused programs• Delete obsolete data files• Files should be cleaned weekly

6B-42

Optimizing Performance• Scan a disk for errors

• Bad spots on the media• Unaccounted-for data• Find and fix the error

• Move data to a good spot• Mark the spot as bad

• Disks should be scanned monthly

6B-43

Optimizing Performance• Defragment a disk

• Files fragment when resaved• File does not occupy contiguous sectors when

saved

• Fragmented files load slower• Defragment puts the fragments together• Disks should be defragged monthly• Run scanning before defragmenting

6B-44

Defragment

6B-45

Optimizing Performance• File compression

• Shrinks the size of a file• Takes up less space on disk• Resulting file called archive• Reduce a disks performance• Will increase disk capacity• PKZip, WinZip and WinRAR

6B-46

File Compression

763 KB763 KBon diskon disk

CompressedCompressed157 KB157 KB

6B-47

Drive Interface Standards• Interface

• How the device is connected• Drive controllers allow transfer of data• Dictates transfer rate and access time

6B-48

Drive Interface Standards• Enhanced Integrated Drive Electronics

• EIDE• Generic term for drive controllers• Several names

• Fast IDE• Advanced Technology Attachment (ATA)

• Up to 2 devices per controller• Most computers have 2 EIDE controllers

6B-49

Drive Interface Standards• Small Computer System Interface

• SCSI• Higher transfer rates than EIDE• More than 40 devices per SCSI controller

• Computers may have several SCSI controllers

• Many versions exist• Versions are typically incompatible

• Found in servers and workstations

6B-50

Drive Interface Standards• USB and FireWire

• External drives• Transfer rate is limited• Many devices can be connected

End of Chapter