1. 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. Comp 1001: IT & Architecture - Joe Carthy 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 (2 10 bytes)1 Kilobyte (Kb) 1024 Kb (2 20 bytes)1 Megabyte (Mb) 1024 Mb (2 30 bytes)1 Gigabyte (Gb) 1024 Gb (2 40 bytes)1 Terabyte (Tb) 1024 Tb (2 50 bytes)1 Petabyte (Pb) Review: Bits and Bytes

3. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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. Comp 1001: IT & Architecture - Joe Carthy 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 data your backup copy. Traditionally tapes have been used as the standard backup medium

11. Comp 1001: IT & Architecture - Joe Carthy 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