Storage Devices

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Storage Devices
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Primary Secondary Storage

• Primary Storage
• Volatile storage, such as RAM, is primary storage
• All contents lost when machine switched off
• Good practice to make regular back-ups
• Secondary Storage
• Non-volatile, more permanent storage for software
  and data files
• For example
• Magnetic Tape, Magnetic Disks, CD-ROM, microfilm

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File Processing

• Method usually depends upon whether every record
  needs to be processed.
• Sequential Processing
• Every record has to be read. Typically used in a
  payroll situation.
• Random Processing
• Each record has its own address allowing direct
  access
• Address can be calculated from a unique key
• Or retrieved from a separate index
• Typical in Database Management Systems

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Floppy Disks Zip Disks

• 3.5 Floppy Disk
• Thin plastic disk (mylar) covered in metal oxide
  in a hard plastic case.
• HD (high-density) disk can store 1.44 Mb
• Zip Disk
• Slightly larger than floppy but similar
  construction
• Hold 100Mb or 250Mb of data
• Disk Storage
• Diskette has two surfaces, each with (typically)
  80 concentric circles called tracks.
• Tracks are divided into sectors when disk is
  formatted

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Hard Disks

• PC
• One or more platters in a sealed casing.
• Typically 5Gb to 30Gb as of Spring 2001.
• Minis mainframes
• May be fixed or removable
• Fixed are faster, more reliable, greater storage
  capacity
• Similar storage in concentric track, but on
  multiple platters, giving rise to concept of
  cylinders.
• Cylinder is the set of tracks that multiple
  read/write heads access in one position.

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Hard Disk layout
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Magnetic Tape

• Magnetic Tape is obviously a serial medium.
• Have to read through tape to find a required
  record
• Updates require creation of a new tape
• Data cannot be changed in situ
• Cheap and convenient for backup
• Cartridge tape for PCs can store up to 20Gb

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Magnetic Tape

• Data record in frames across the tape
• One frame represents one byte
• Frames form tracks along the tape, typically 9
  tracks 8 data tracks and one parity track

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Other Devices

• CD-ROM
• Stores up to 700Mb.
• Various data types
• 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
• WORM Write Once, Read Many
• Similar to CD-ROM, but usually gold in colour
• Intended use for firms to store their own data
  for rapid retrieval
• Favoured by pirates to produce illicit copies
  of software

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Other Devices

• Magneto-Optical Disks
• Integrate optical and laser technology for read
  and write storage.
• A 5.5 disk can store up to 2.6 Gb.
• Still under development, but may replace magnetic
  disks when price and speed permit.
• Microfiche
• COM (computer output on microfilm) devices create
  the4 by 6 hard-copy films that are microfiche
• Each sheet may contain upto 270 frames each
  containing a page of information which can be
  viewed using a special viewer