comp 1001: it & architecture - joe carthy 1 information representation: summary all information...

Comp 1001: IT & Architecture - Joe Carthy

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 2’s 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

• 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)

• Review: Bits and Bytes


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: 1.4MbZip 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 technologyWORM: Write Once/Read Many timesAlso 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 ( <100 Mb)– Cartridge: 100Mb to <10 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


11

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

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