ICT Unit 3

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  • ICT Unit 3
    • Backing storage is used to store programmes and data when a computer is switched off or when they are not needed.
      • When programmes and data are run they remain on backing storage and a copy is transferred to RAM.
    • Magnetic tape drives and hard disks are both examples of magnetic devices.
    • CD and DVD drives are are examples of optical devices.
    • Memory sticks, pen drives and flash memory cards are examples of solid-state storage devices.
    • It's important to remember that the medium is the thing that stores the data and the device is the equipment that reads the data from the medium and inputs it into the computer system.
    • Hard disk drives
      • Most hard disk drives are fixed inside the computer but some are not fixed and can be removed for either portability or data security.
      • Hard disks can store lots of data, now typically 160-500 GB, but the capacity can be as much as 1.5 TB
      • They are stacks of magnetised circular metal plates. Each plate is divided into concentric tracks and sectors and data is stored in the sectors.
    • Optical disk drives
      • Definition: Equipment designed to read/play CD/DVDs.
      • When fitted to computer equipment they are almost always capable of writing data to a compatible disk.
      • Blu-ray drives are capable of reading normal DVD as well as the high capacity Blu-ray disks. HD-DVD format was an alternative to Blu-ray but has now become standard.
      • CDs and DVDs store data as pits (little indentations) on the surface of a reflective disk. The data is read by moving a laser beam across the surface of the disc and reading the change in position of the reflective beam.
      • CDs
        • CD-ROM can store up to about 1GB of data, but are read only and so the data cant be changed.
        • CD-R is a CD which will let you record data once only, but you can read it many times. Sometimes known as WORM (write once read many)
        • CD-RW is like a CD-R but will yet you record data as many times as you want an will re-write the data.
      • DVDs
        • Is one of the latest ways of storing data, capable of storing up to 10GB of data as a dual-layer disk. DVD has now almost completely overtaken CD as a storage medium.
        • Blu-ray disks can store up to 50 GB as a dual-layer disk.
    • Magnetic tape
      • One advantage of using magnetic tape ad a backing storage is that it can store large amounts of data and is especially useful when speed of access is not important, such as for backups and archives.
      • Direct and serial access
        • Magnetic tape uses serial access to data. To locate the required data on the tape the tape reader has to search from the beginning until it finds the required data item.
        • Hard disks, optical disks and pen drives use direct access to data. To locate the required data on the disk the drive can find the required data item without having to read all the data from the beginning of the disk
    • Backups
      • It is important to make backups of data in case: something goes wrong with the computer and data is lost, data is accidentally deleted or viruses either delete or corrupt your data.
        • In the event of any of these happening, the data can be restored from a backup. Backups should be made on storage devices that are separate from the computer.
    • Archives
      • Data that is no longer regularly used but is still required to be kept can be copied from the hard disk to an external storage device. it can still be retrived if necessary but does not clutter up the system any more.


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