Information Representation: Summary

1
Information Representation Summary

• All Information is stored and transmitted in
  digital form in a computer system
• Text is typically represented by ASCII or Unicode
  binary codes
• Integers are typically represented as pure binary
  or 2s complement binary numbers
• Real numbers are represented in scientific
  notation form in binary
• Audio is converted to digital (binary) by ADC
  and from binary to analog by DAC
• Images are represented by pixels which are
  represented by binary numbers
• Video can be regarded as a combination of Image
  and Audio representations

2
Storage Devices

• Review Bits and Bytes

• One binary digit i.e. 1 or 0 is called a bit
• A group of 8 bits is one byte
• Byte is the unit of storage measurement

Number of Bytes Unit
1024 bytes (210 bytes) 1 Kilobyte (Kb)
1024 Kb (220 bytes) 1 Megabyte (Mb)
1024 Mb (230 bytes) 1 Gigabyte (Gb)
1024 Gb (240 bytes) 1 Terabyte (Tb)
1024 Tb (250 bytes) 1 Petabyte (Pb)
3
Storage Devices

• Storage devices are may be classified as
• Short-term (volatile) or
• Long-term (permanent)
• Short-term
• RAM  (Main Memory)
• Loses contents on power off
•  Capacity    n Megabytes
•   (2004 n 256 .. 1024 for PCs)
• 100,000 times faster than disk !
• e.g. 27.7 hours is 100,000 times longer than 1
  second
• Much more expensive than other storage devices

4
Storage Devices

• Long Term (Secondary/Auxiliary)
• Information stored for years
• No power required to store information
•  
•        Disk Magnetic medium
• Random access storage medium 
•               Hard Disk
• Internal
• External (Removable)
• Capacity n Gigabytes
• (n 30 to 100 Gb for PCs) 
• Floppy Disk 1.4Mb
• Zip Disk (100/250/750 Mb)

5
Storage Devices

• RAID Technology Redundant Array of Inexpensive
  Disks
• Use collection of independent disks to do same
  job as a larger disk.
• Increases availability of data
• Improves access time
• Uses either disk mirroring or striping
• May be hot-swappable
•  
• Capacity x00 Gb to Tb range

6
Storage Devices Optical Technology

• CD-ROM/CD-R
• (ROM Read-Only Memory CD-R Recordable)
• Capacity 650 Mb
•   CD-R is an example of WORM technology
• WORM Write Once/Read Many times
• Also have rewriteable CDs - more expensive
• CDs are much slower to access data than magnetic
  disks
•  Uses laser to read/write digital data on surface
  of disk

7
Storage Devices Optical Technology

• DVD Digital Versatile Disk
• Capacity 4.7 Gb
• (or n times 4.7 Gb where n .5, 2, 4)
•  
• At moment - use mainly DVD-ROM
•  Also DVD-RAM (rewriteable DVD)

8
Storage Devices Tape Technology

• Tapes Magnetic and Optical  
•        Slower to access than Disk
•        Cheaper than disk
•        Sequential storage medium
•        Very good for offline storage
• Capacity
• Old Reel  ( lt100 Mb)
• Cartridge 100Mb to lt10 Gb
• DAT 4Gb upwards      
•    Optical tape x Terabytes capacity
• fast access relative to other tapes

9
Storage Devices Flash Memory

• Flash memory is a form of non-volatile RAM
  (NV-RAM)
• USB disks e.g. Disgo are small devices that
  store from 16 MB to 2 GB in this form of memory
• Plug into any USB port
• Most modern PCs and Operating Systems allow you
  use them without installing additional software
  (drivers)
• Much more convenient and reliable than Floppy
  disks
• Also much large capacity than floppy disks

10
Storage Devices Importance of Backups

• Most important storage principle
• Always have a Backup !
• It is almost impossible to overstate the
  importance of this principle
• Hard disks do and will wear out or breakdown
  (head crash)
• They may also be stolen or lost (with your laptop
  or desktop)
• Files may be deleted by accident/on purpose
• CDs and DVDs are very useful for storing a copy
  of your hard disk datayour backup copy.
• Traditionally tapes have been used as the
  standard backup medium

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Storage Devices Review

• Short and Long-term storage devices
• RAM Short-term x00 to low Gigabyte range
• 100,000 times faster than disk
• Disk Long-term, Magnetic medium
• Widely used
• X Gb (X 30 .. 200 for PCs)
• CD/DVD Long-term Optical medium Very robust
• Widely used Slower than disk
• CD 650 Mb
• DVD 4.7 Gb upwards
• Tape Long-term Sequential access Magnetic
  Slow Cheap
• DAT 4 Gb upwards
• Optical terabyte range
• NV-RAM devices Long-term
• USB 16 Mb to 2GB
• Always have a Backup