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5.1.3 Storage Devices and Media


Revision Points:
Candidates are expected to be able to:
  • describe common backing storage media (such as magnetic tape, CD-ROM, floppy disc and hard disc) and their associated devices;
  • identify typical uses of the storage media in the above;
  • describe the comparative advantages and disadvantages of using different backing storage media;
  • define the difference between internal memory and backing storage, stating the relative benefits of each in terms of speed and permanence.

Backing storage:

The difference between internal memory and backing storage:

Backing storage (also known as secondary storage) means data storage that retains its contents when the computer is switched off.  It can be used to hold both programs and data.

When you run a program or load a file they are copied from the backing store into the internal memory.

When you save a file it is copied from the internal memory to the backing store.

  • It is always slower to access data from backing storage than from internal memory.
  • Data stored in backing storage is permanent so it is NOT lost when the computer is turned off.
  • Data stored in internal memory is lost when the computer is turned off.

Categories of storage media:

Backing storage can be divided into two main categories:

  • Magnetic media which stores the binary data on a disk or tape coated with a material that can be magnetised differently, depending on whether a 0 or 1 is stored.
  • Optical media which stores the binary data in a surface that reflects laser light differently, depending on whether a 0 or 1 is stored.

Magnetic media - Hard discs:

Inside a typical hard drive (2 platters and 4 read/write heads)Data is stored by magnetising the surface of flat, circular plates called platters which have a surface that can be magnetised. They constantly rotate at very high speed. A read/write head floats on a cushion of air a fraction of a millimetre above the surface of the disc. The drive is inside a sealed unit because even a speck of dust could cause the heads to crash.

Programs and data are held on the disc in blocks formed by tracks and sectors. These are created when the hard disc is first formatted and this must take place before the disc can be used. Disc are usually supplied pre-formatted.

For a drive to read data from a disc, the read/write head must move in or out to align with the correct track (the time to do this is called the seek time). It must wait then until the correct sector rotates round until it underneath the the read/write head.  

  • Typical uses:
    • The hard disc is usually the main backing storage media for a typical computer or server. It is used to store:
      • The operating system (e.g. Microsoft Windows)
      • Applications software (e.g. word-processor, database, spreadsheet, etc.)
      • Files such as documents, music, video etc.
    • A typical home/school microcomputer would have a disc capacity of over 100 gigabytes.
  • Advantages:
    • Very fast access to data. Data can be read directly from any part of the hard disc (random access). The access speed is about 1000 KB per second.
  • Disadvantages:
    • Non really!  It can however be a real disaster when they eventually fail because few home users have the data on their home computer hard drive backed up.

Magnetic media - Floppy discs:

These can be found on most microcomputers and accept the usual 3.5 inch floppy discs. High density discs for a PC hold 1.44 MB of data (enough to store about 350 pages of A4 text). A floppy disc needs to be formatted before it can be used but most discs are now sold already formatted for PC's.
  • Typical uses:
    • Floppy discs are useful for transferring data between computers and for keeping a back-up of small files.
  • Advantages:
    • They are very cheap to buy and floppy disc drives are very common.
  • Disadvantages:
    • They have very small storage capacity compared to modern alternatives such as USB memory sticks.
    • They are easily physically damaged if unprotected and magnetic fields can damage the data.
    • They are relatively slow to access because floppy discs rotate far more slowly than hard discs, at only six revolutions per second, and only start spinning when requested. The access speed is about 36 KB per second.
    • Not all modern computers have floppy disk drives.

Magnetic media - Magnetic Tape:

Just like the tape in a tape-recorder, the data is written to or read from the tape as it passes the magnetic heads.

  • Typical uses:
    • Magnetic tapes are often used to make a copy of hard discs for back-up reasons. This is automatically done overnight on the KLB network and the tapes are kept in a safe place away from the server. 
  • Advantages:
    • Magnetic tape is relatively cheap and tape cassettes can store very large quantities of data (typically 26 GB).
  • Disadvantages:
    • Accessing data is very slow and you cannot go directly to an item of data on the tape as you can with a disc. It is necessary to start at the beginning of the tape and search for the data as the tape goes past the heads (serial access).

Optical Media - CD-Rom: 

CD-ROM - means Compact Disc - Read Only Memory. This means you can only read from the disc, not write or store data onto it.

They are also known as optical discs because the data is read by a laser beam reflecting or not reflecting from the disc surface.

Like a floppy disc, a CD-ROM only starts spinning when requested and it has to spin up to the correct speed each time it is accessed. It is much faster to access than a floppy but it is currently slower than a hard disc.

  • Typical uses:
    • Most software programs are now sold on CD-Rom.
  • Advantages:
    • CD-ROM's hold large quantities of data (650 MB).
    • They are relatively tough as long as the surface does not get too scratched.
  • Disadvantages:
    • You cannot save files to a CD-Rom (although CD-R and CD-RW discs now exist which can be written to)

Comparing storage devices:
Storage
capacity

1 CD-ROM (650 MB) = 451 Floppy discs (1.44 MB)

1 DVD (4.7 GB) = 7 CD-ROM's (650 MB)

Access
speed
Harddrive CD-ROM Floppy disc
1000 KB/s 100 KB/s 36 KB/s

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