Ie Storage Devices

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Ie Storage Devices

  1. 1. Storage Devices
  2. 2. Primary & Secondary Storage <ul><li>Primary Storage </li></ul><ul><ul><li>Volatile storage, such as RAM, is primary storage </li></ul></ul><ul><ul><li>All contents lost when machine switched off </li></ul></ul><ul><ul><li>Good practice to make regular back-ups </li></ul></ul><ul><li>Secondary Storage </li></ul><ul><ul><li>Non-volatile, more permanent storage for software and data files </li></ul></ul><ul><ul><li>For example: </li></ul></ul><ul><ul><ul><li>Magnetic Tape, Magnetic Disks, CD-ROM, microfilm </li></ul></ul></ul>
  3. 3. File Processing <ul><li>Method usually depends upon whether every record needs to be processed. </li></ul><ul><li>Sequential Processing </li></ul><ul><ul><li>Every record has to be read. Typically used in a payroll situation. </li></ul></ul><ul><li>Random Processing </li></ul><ul><ul><li>Each record has its own address allowing direct access </li></ul></ul><ul><ul><ul><li>Address can be calculated from a unique key </li></ul></ul></ul><ul><ul><ul><li>Or retrieved from a separate index </li></ul></ul></ul><ul><ul><li>Typical in Database Management Systems </li></ul></ul>
  4. 4. Floppy Disks & Zip Disks <ul><ul><li>3.5” Floppy Disk </li></ul></ul><ul><ul><ul><li>Thin plastic disk (mylar) covered in metal oxide in a hard plastic case. </li></ul></ul></ul><ul><ul><ul><li>HD (high-density) disk can store 1.44 Mb </li></ul></ul></ul><ul><ul><li>Zip Disk </li></ul></ul><ul><ul><ul><li>Slightly larger than floppy but similar construction </li></ul></ul></ul><ul><ul><ul><li>Hold 100Mb or 250Mb of data </li></ul></ul></ul><ul><ul><li>Disk Storage </li></ul></ul><ul><ul><ul><li>Diskette has two surfaces, each with (typically) 80 concentric circles called tracks . </li></ul></ul></ul><ul><ul><ul><li>Tracks are divided into sectors when disk is formatted </li></ul></ul></ul>
  5. 5. Hard Disks <ul><li>PC </li></ul><ul><ul><li>One or more platters in a sealed casing. </li></ul></ul><ul><ul><li>Typically 5Gb to 30Gb as of Spring 2001. </li></ul></ul><ul><li>Minis & mainframes </li></ul><ul><ul><li>May be fixed or removable </li></ul></ul><ul><ul><li>Fixed are faster, more reliable, greater storage capacity </li></ul></ul><ul><ul><li>Similar storage in concentric track, but on multiple platters, giving rise to concept of cylinders. </li></ul></ul><ul><ul><li>Cylinder is the set of tracks that multiple read/write heads access in one position. </li></ul></ul>
  6. 6. Hard Disk layout
  7. 7. Magnetic Tape <ul><li>Magnetic Tape is obviously a serial medium. </li></ul><ul><ul><li>Have to read through tape to find a required record </li></ul></ul><ul><ul><li>Updates require creation of a new tape </li></ul></ul><ul><ul><ul><li>Data cannot be changed in situ </li></ul></ul></ul><ul><li>Cheap and convenient for backup </li></ul><ul><ul><li>Cartridge tape for PCs can store up to 20Gb </li></ul></ul>
  8. 8. Magnetic Tape <ul><li>Data record in frames across the tape </li></ul><ul><li>One frame represents one byte </li></ul><ul><li>Frames form tracks along the tape, typically 9 tracks – 8 data tracks and one parity track </li></ul>1 0 1 1 1 1 0 1 1 0 0 0 1 1 0 1 1 0 0 0 1 0 0 1
  9. 9. Other Devices <ul><li>CD-ROM </li></ul><ul><ul><li>Stores up to 700Mb. </li></ul></ul><ul><ul><li>Various data types </li></ul></ul><ul><ul><li>Read-only. Read by reflecting a laser beam off the surface and detecting the presence or absence of tiny pits which will have been burned into the surface to represent binary digits </li></ul></ul><ul><li>CD-R </li></ul><ul><ul><li>Similar to CD-ROM, but usually gold in colour </li></ul></ul><ul><ul><li>Intended use for firms to store their own data for rapid retrieval </li></ul></ul><ul><ul><li>Favoured by ‘pirates’ to produce illicit copies of software </li></ul></ul><ul><li>CD-RW </li></ul>
  10. 10. Other Devices <ul><li>Magneto-Optical Disks </li></ul><ul><ul><li>Integrate optical and laser technology for read and write storage. </li></ul></ul><ul><ul><li>A 5.5” disk can store up to 2.6 Gb. </li></ul></ul><ul><ul><li>Still under development, but may replace magnetic disks when price and speed permit. </li></ul></ul><ul><li>Microfiche </li></ul><ul><ul><li>COM (computer output on microfilm) devices create the4” by 6” hard-copy films that are microfiche </li></ul></ul><ul><ul><li>Each sheet may contain upto 270 frames each containing a page of information which can be viewed using a special viewer </li></ul></ul>

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