Information & Communication Technology 1996-2003

GCSE Revision


Computer Hardware - Storage Media & Devices

Storage devices are pieces of equipment that are used to store the data used by a computer, such as a floppy disk drive. Storage media are the actual materials on which the data is stored, such as a floppy disk.

Note spelling: disk (USA), disc (UK), you can use either but you must be consistent.


Tape - data is stored as magnetic fluctuations along the length of a specially coated plastic tape, similar to the common audio tape. The coded data is processed (or translated) into useful information by the computer. Very slow, as need to wind tape (perhaps a long way) to find any specific position. Can hold thousands of megabytes (MB) of data depending on the length of the tape. Useful for daily backups of all work and programs on a large network, can be set to run automatically during the night and then will only be needed for disaster recovery where a reliable restoration is more important than speed. Can be damaged by stray magnetic fields from electrical equipment.

Floppy Disk - data is stored magnetically and uses a material similar to tape. It needs to be formatted to provide a layout of tracks and segments to organise where the data is stored. One spin of the disk provides access to all the data hence much faster access than tape (about 36 KB per second) but it only starts spinning when requested. Each disk can hold only 1.44 megabytes of data. Good for transferring work and small programs between computers and for backing up your work projects, but is easily damaged if carried around unprotected and heavily used.  Easily damaged by magnetic fields such as that produced by speakers and mobile phones. Disks cost less than £0.20 and a disk drive costs around £2, if available at all these days. Now totally superseded by the USB Memory Stick as a means of mobile storage.

Storage
capacity
  Floppy disk = BookBookBook
    1.44MB   360 A4 pages
(approx)

Hard Disk - made of stronger material (aluminium) and fixed permanently together with its drive mechanism inside the computer. Data is stored magnetically. Spinning at the required speed all the time it is very much faster to access than a floppy disk or CD-ROM (faster than 1 MB per second). Can hold thousands of megabytes (gigabytes) of data. Used for storing all programs and work files for very fast access by the computer. Can be damaged by stray magnetic fields but reasonable safe when kept inside the computer. Cost around £40.00 (40GB). Other common sizes are 80 and 120GB.

     Internal memory (ROM/RAM) Hard disk CD-ROM Floppy disk
Access
speed
  >>> >> >
    Instant 1000KB/s   60KB/s   36KB/s

Removable-disk ('Zip') hard drives - two sizes most common are 100MB and 250MB. Zip disks, being about the same physical size as the floppy disk once became a popular replacement as a mobile means of storage but now totally superseded by the USB Memory Stick.

Compact Disk (CD-ROM) - uses laser (light) technology to store the data. Comes with the data already on it for reading only (ROM = Read Only Memory) and hence cannot be written to for everyday storage. Like a floppy disk it has to spin up to the correct speed each time it is accessed. Much faster to access than a floppy but currently slower than a hard disk (single-speed 15 KB per second so a 40X speed CD-ROM drive is 40 times faster). Can hold up to 650 MB (megabytes) of data. Useful for the distribution of today’s large programs and information libraries, which you can then copy (all or in part) onto your hard disk. One of the safest forms of storage, provided that you donít attack it with a sharp or heavy object and cannot be damaged by stray magnetic fields. CD-ROM drives cost under £20 and are now the minimum expected standard fitted to any computer.

Widely used by the music industry as they give better quality sound and do not stretch or wear out like audio cassette tape.

Record once CD-R and rewriteable CD-RW drives are now available for under £30. Cost of a blank CD disk is under £0.20. They have largely replaced both the standard floppy disk drive and the CD-ROM drive as the expected standard specification for most of today's computers.

Storage
capacity
  CD-ROM x 1 = 451 x Floppy disk
    650MB   1.44MB

Digital Versatile Disk (DVD-ROM) - also known as a Digital Video Disk is a very large capacity CD-ROM with similar access speed. It can store up to 4.7 GB (gigabytes) of data which is enough to store a feature-length film of 2-6 hours in length depending on the picture quality desired. A DVD drive can also read a normal CD disk and the so called 'combo' drives can also write to CD-RW disks. At a cost of under £50 they are now becoming the minimum standard, especially on laptops.

Rewriteable DVD-RW drives are currently around £50-100 and usually available as an optional extra. Disks are very cheap at around £0.20 for DVD-R and £0.40 for DVD-RW.

DVD recorders are now replacing the standard video cassette recorder for use in home recording.

Storage
capacity
  DVD x 1 = 7 x CD-ROM
    4.7GB   650MB

Optical Disk - looks and is used just like a floppy disk but contains a rewriteable compact disk inside. With 100 megabytes capacity it is most suitable for the large picture files created on modern computers and for storing photographs in some older digital cameras. Now superseded by the memory stick in computers and memory cards in cameras as the latter require no moving parts to wear out.

Magnetic Strip - a short strip of magnetic tape for storing a small amount of data. The simplest type has a personal identity number (PIN) permanently stored on it, eg. credit card, cash-point card - used to withdraw cash from the Automatic Teller Machines (ATMs) on the walls of banks. Others can be written to as well, perhaps to store a running total, eg. phone card, debit card, library card.

Smart Card - contains a processing microchip to provide it with intelligence as well as memory - it is really a tiny computer, eg. the SIMcard used in a mobile phone identifies you through a PIN number, identifies and connects to your service provider and provides a menu of options, as well as storing phone numbers and phone settings.

Bar Code - a series of printed parallel black lines of varying thickness on a white background used to represent (and therefore store) a decimal number code. Almost everything you buy has a bar code either on it or on its packaging. A computer is used to print the bar codes which can later be scanned back into a computer very quickly. They can then be processed or translated into useful information for display, eg at a supermarket checkout or for creating a library database.

Data Logger - a remote input/output device, which stores data received from sensors that can then be input into a computer at another time or place. It can be left alone to collect data over a long period of time and in hostile places, such as on a satellite, at the bottom of the ocean or at the South Pole.

Microchip - there are many chips inside a computer, some perform all the processing tasks (eg. the Intel Pentium CPU), and some are used for the storage of data as internal memory. They have the fastest access of all storage media. Because of this, all files read from the hard disk into memory are also copied into a separate memory "cache" and the next time you attempt to load them from the hard disk, the computer will look in the cache first. There are two types of internal memory:

ROM (Read Only Memory) chips are used to store the permanent operating system data (the BIOS). In some types of computer, such as a pocket book, they may be used to store major programs such as a word processor. They are however more difficult to replace when a new version is released. They are also used to store the permanent programming inside a domestic device such as a television or washing machine.

RAM (Random Access Memory) chips are used for temporary storage. They can be written to as well as read. A program and your work files have to be transferred from your hard disk into RAM when you want to work with them. You must then save your new work back onto the hard disk, as all RAM memory is lost when the computer is switched off. Computers today are normally sold with 16, 32, 64, 128 and 256 megabytes of RAM plus at least a 512KB cache RAM 

USB Memory Stick or Pen Drive - The ultimate (and latest) replacement for the floppy disk as a means of mobile storage. High memory capacity on microchip therefore no moving parts and almost instant access. It plugs directly into the USB port on any computer. Costs: 128MB for £10-20, 256MB for £30 on up to around 4GB.

Memory Card - similar to the memory stick but designed to be used in devices such as portable music players, personal digital assistants, digital cameras, camcorders, mobile phones or PCís. There are two types - SD (Secure Digital) and CF (Compact Flash). A 512MB card costs around £25-30


Bits and Bytes

A byte is the unit of memory representing one unit of data.

It is a code made up of 8 bits (binary digits).
For example 01100001 represents the lower case letter 'a'.

(Imagine 8 people standing in a row on the top of a hill at night, each holding a torch. 0 means 'off' and 1 means 'on'. As nothing else is possible for each torch only these two numbers are needed to represent the signal. This, in brief, is what we mean by 'digital' or more exactly 'binary digital'.)

A byte is the amount of memory needed to store one keyboard character.

1 kilobyte (KB) = 1000 bytes (10^3)
1 megabyte (MB) = a million bytes (1000 kilobytes) (10^6)
1 gigabyte (GB) = a billion bytes (1000 megabytes) (10^9
1 terabyte (TB) = 1000 gigabytes (10^12)
1 petabyte (PB) = 1000 terabytes (10^15)

  • About 4 kilobytes (4KB) is needed to store one full A4 page of text.
  • A ten volume encyclopaedia or 20 musical recordings might occupy 600 megabytes (600MB) on a CD-ROM.
  • A full-length feature film might occupy 4 gigabytes (4GB) on a DVD.
  • You would need something in the order of petabytes (billions of megabytes) to store all the data for a complete human being in an electronic state, either on a microchip to produce a hologram person such as "Rimmer" in "Red Dwarf" or for transporting as a beam of energy to another planet as in "Startrek".

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