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Chap 01 02


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Chap 01 02

  1. 1. 1- Mass Storage (Secondary Storage) <ul><li>On-line versus off-line </li></ul><ul><li>Typically much larger than main memory </li></ul><ul><li>Less volatile than main memory </li></ul><ul><li>Access much slower than main memory </li></ul>
  2. 2. 1- Mass Storage Systems <ul><li>Magnetic Systems </li></ul><ul><ul><li>Disk </li></ul></ul><ul><ul><li>Tape </li></ul></ul><ul><li>Optical Systems </li></ul><ul><ul><li>CD </li></ul></ul><ul><ul><li>DVD </li></ul></ul><ul><li>Flash Drives </li></ul>
  3. 3. 1- Figure 1.9 A magnetic disk storage system magnetic coating
  4. 4. 1- <ul><li>each track is split into sectors/arcs , each sector can be accessed as an independent string of bits </li></ul><ul><li>a sector could contain upto a few KB </li></ul><ul><li>tracks and sectors are not a permanent part of a disk's physical structure. They are marked magnetically through formatting the disk. </li></ul><ul><li>hard disks v floppy disks </li></ul><ul><li>capacity varies - number of disks, density of sectors and tracks </li></ul><ul><li>for speed, heads might float off the disk and not touch it. one particle of dust could jam between head and disk - head crash. </li></ul>
  5. 5. 1- Disk performance <ul><li>Seek time – moving heads from one track to another </li></ul><ul><li>Rotation delay/latency time – half the time a disk takes to rotate (the average time it takes for the desired data to rotate round to a head). Eg several thousand revs per minute </li></ul><ul><li>Access time – seek time + rotation delay </li></ul><ul><li>Transfer rate – rate data transferred to/from disk eg MB a second </li></ul>
  6. 6. 1- Disk Fragmentation <ul><li>Data can become fragmented over time. </li></ul><ul><li>De-fragmentation: the process of re ordering data to make storage more efficient. </li></ul>
  7. 7. 1- Figure 1.10 Magnetic tape storage
  8. 8. Optical Systems <ul><li>CD has reflective material covered with a clear protective coating (600-700MB) </li></ul><ul><li>Data recorded by creating variations in the reflective surfaces </li></ul><ul><li>Data retrieved by laser beam that monitors irregularities on the reflective surface as it spins </li></ul><ul><li>A single track spirals from the middle outwards </li></ul><ul><li>Track divided into sectors (2KB) </li></ul><ul><li>Data stored at a uniform linear density over entire track –more data on outer part of disk than inner </li></ul><ul><li>DVDs –have multiple semi transparent layers which are distinct surfaces when viewed by a precisely focused laser. More storage (several GB). </li></ul>
  9. 9. 1- Figure 1.11 CD storage
  10. 10. Flash Drives <ul><li>No heads or spinning or moving lasers, just electronic circuitry –fast! Not sensitive to physical shock </li></ul><ul><li>Bits are stored by sending electronic signals directly to the storage medium where they cause electrons to be trapped in tiny chambers of silicon dioxide </li></ul><ul><li>Repeated erasing damages the silicon dioxide chambers. So not used for main memory, used for digital cameras, PDAs </li></ul>1-
  11. 11. 1- Files <ul><li>File: A unit of data stored in mass storage system </li></ul><ul><ul><li>Fields and keyfields </li></ul></ul><ul><li>Physical record (eg all the data on a sector) versus Logical record (eg a staff member details) </li></ul><ul><ul><li>One physical record could hold many logical </li></ul></ul><ul><ul><li>One logical record could spread over many physical </li></ul></ul><ul><li>Buffer: A memory area used for the temporary storage of data (usually as a step in transferring the data) </li></ul>
  12. 12. 1- Figure 1.12 Logical records versus physical records on a disk
  13. 13. 1- Representing Information : Representing Text <ul><li>Each character (letter, punctuation, etc.) is assigned a unique bit pattern. </li></ul><ul><ul><li>ASCII : Uses patterns of 7-bits to represent most symbols used in written English text </li></ul></ul><ul><ul><li>Unicode : Uses patterns of 16-bits to represent the major symbols used in languages world side </li></ul></ul><ul><ul><li>ISO standard : Uses patterns of 32-bits to represent most symbols used in languages world wide </li></ul></ul>
  14. 14. 1- Figure 1.13 The message “Hello.” in ASCII
  15. 15. 1- Representing Numeric Values <ul><li>Using ASCII, to store 25 need 16 bits, largest number is 99 </li></ul><ul><li>Binary notation : Uses bits to represent a number in base two; 16 bits could represent 0 to 65535 </li></ul><ul><li>Limitations of computer representations of numeric values </li></ul><ul><ul><li>Overflow – occurs when a value is too big to be represented </li></ul></ul><ul><ul><li>Truncation – occurs when a value cannot be represented accurately </li></ul></ul>
  16. 16. 1- Representing Images <ul><li>Bit map techniques </li></ul><ul><ul><li>Pixel: short for “picture element” </li></ul></ul><ul><ul><li>Black & white picture: 1 for black, 0 for white or 8 bits to record shades of greyness </li></ul></ul><ul><ul><li>Colour picture, two approaches: </li></ul></ul><ul><ul><ul><li>RGB – each pixel is 3 colours, record intensity of each colour, need 3 bytes </li></ul></ul></ul><ul><ul><ul><li>Luminance (brightness- the sum of RGB components), red chrominance (difference between luminance and amount of red light in the pixel) and blue chrominance </li></ul></ul></ul><ul><ul><li>can’t rescale </li></ul></ul><ul><li>Vector techniques – scalable egTrueType, PostScript </li></ul>
  17. 17. 1- Representing Sound <ul><li>Sampling techniques </li></ul><ul><ul><li>Eg 8000 samples/sec for long distance phone </li></ul></ul><ul><ul><li>Used for high quality recordings eg 44,100 samples/sec </li></ul></ul><ul><ul><li>Need 16 bits a sample, so each second of music is more than a million bits </li></ul></ul><ul><ul><li>Records actual audio </li></ul></ul><ul><li>MIDI </li></ul><ul><ul><li>Used in music synthesizers, video game sound, </li></ul></ul><ul><ul><li>Records “musical score” – what instrument is to play which note and for how long . So a clarinet playing D for 2 seconds can be encoded in 3 bytes rather than over 2 million </li></ul></ul>
  18. 18. 1- Figure 1.14 The sound wave represented by the sequence 0, 1.5, 2.0, 1.5, 2.0, 3.0, 4.0, 3.0, 0