Storage Devices

• There are 2 main types of memory used in computers:

  RAM and ROM.
• RAM stands for Random Access Memory
• ROM stands for Read Only Memory
• RAM usually stores the operating system, running programs and data currently being used.
• ROM usually stores the computing bootup instructions (Boostrap)
• When the computer is turned off the contents of RAM are lost as this type of memory is volatile.
• When the computer is turned off the contents of ROM are retained as it is a type of non-volatile memory.
• When applications or programs are loaded, they are copied into RAM from secondary storage (hard disk).
• The CPU can only access information that has been loaded into RAM.
• Virtual Memory


  It is a part of the hard drive that is used as an extension to RAM. When RAM is full it will push some of the data that has not been recently accessed into virtual memory. When the data is needed again it will need to be re-loaded into RAM before it can be accessed.
• Secondary Storage

  Refers to non-volatile storage:

  • magnetic storage such as the hard disk
  • optical storage that uses laser light such as CD-ROM
  • solid state devices using flash memory
• Tracks, Sectors and Platters

  Concentric tracks are created on a magnetic disk. Disk spins at high speeds such as 3600-7200rpm. Spinning platters are each read by drive heads. Data is read or written as a sector moves under the head.
• Hard disks have very high capacity, fast read and write speeds and have relatively cheap storage per TB.
• Solid state drives (SSDs)

  Electrically programmable non-volatile flash memory. Its advantages are that its:

  • more durable as SSD cards have no moving parts so can survive drops, extreme heat and cold, and extreme pressure
  • typically less capacity than a hard disk
  • faster than hard disk drives
  • consume less power
  • more portable

  However, a disadvantage is that its more expensive than hard disk drives.
• USB flash drives

  They can store up to 128Gb of data with prices starting at under £5. They can use password protection and are useful for transferring data between computers.
• CDs, DVDs and BluRay

  Why are the capacities of these discs different when physical size is the same?:

  Each of these have different laser wavelengths which burn smaller pits and the spiral track can therefore be more tightly wound, creating a longer track.
• Optical Disks are available as:

  • read only
  • recordable and
  • re-writable formats
• Software mailing:

  • often best suited to optical disks
  • cheap to manufacture and distribute
  • robust during carriage
  • lightweight
• CD formats

  CDs come in 3 different formats:

  • CD-ROM
  • CD-R
  • CD-RW

  CD-ROM is "pressed" at the time of manufacture

  CD-R can be written to once

  CD-RW can be written, read many times, erased and written again
• Optical Disk Uses

  CD-ROM is widely used for software distribution

  CD-R may be used for copying a game, software, audio or video files or documents. These can then easily be stored offline

  CD-RW useful for short or medium term backup, or transferring files from one computer to another

  DVD and BluRay are high capacity discs which can store feature length films
• Hard Disk

  Capacity: 512GB-6TB

  Access Speed: Fast
• CD-ROM

  Capacity: 700MB

  Access Speed: Medium
• DVD

  Capacity: 4.7-8.5GB

  Access Speed: Medium
• BluRay Disk

  Capacity: 25-50GB

  Access Speed: Medium
• Solid State Disk

  Capacity: 4GB-2TB

  Access Speed: Very Fast