Chapter04 storage devices

GCSE ICT 3rd EditionGCSE ICT 3rd

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Data storage4Data storage devices are divided into two main categories:

Backing storage is used to store programs and data when they are not being used or when a computer is switched off.

Main Memory which contains two types of memory chip called ROM and RAM.

When programs and data are needed they are copied into main memory but also still remain on backing storage.


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Data storage4Computers store and process data using binary numbers.

A single unit in binary is called a bit which stands for binary digit.

Computer memory is measured in bytes.

One byte is made up of eight bits.

One byte can store one character.


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Data storage4The size of a computer’s memory is measured in kilobytes (Kb), megabytes (Mb) or gigabytes (Gb).

Measurement Size (bytes) Symbol

kilobyte 1,024 Kb

megabyte 1,048,576 Mb

gigabyte 1,073,741,824 Gb


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Data storage4Random Access Memory or RAM is the computer’s ‘working memory’.

RAM temporarily stores programs and data that are being used at a given time.

The contents of RAM can be changed and are lost when the computer is turned off.

Memory that is wiped clean when a computer is turned off is called volatile memory.


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Data storage4The main use of Read Only Memory or ROM in a computer is to store the program that runs when it is turned on and loads the operating system (e.g. Windows XP) from disk.

The contents of ROM can’t be changed and aren’t lost when the computer is switched off.

Memory which isn't wiped clean when the computer is turned off is called non-volatile memory.


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Data storage4PROM and EPROM are both special types of programmable read only memory.

PROM stands for Programmable Read Only Memory. This type of memory can be programmed once but can’t be changed again afterwards.

EPROM stands for Erasable Programmable Read Only Memory. This type of memory can be programmed and then changed whenever necessary.


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A hard disk is a circular metal disk coated with magnetic material and usually sealed in a hard disk drive inside the computer.

Some hard disk drives are not permanently fixed inside the computer but are removable.

Data stored on a hard disk can be accessed much more quickly than data stored on a floppy disk.

Hard disks can store much more data than a floppy disk. A typical hard disk inside a personal computer can hold several gigabytes of data.


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Data storage4

A standard floppy disk can store up to 1.44 Mb of data.

Before any type of magnetic disk can be used it must be formatted.

A floppy disk a circular piece of plastic coated with a magnetic material and protected by a hard plastic cover.

The size of a floppy disk is measured in inches. Modern floppy disks are 3½ inches wide.


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The formatting process involves:

dividing the surface of the disk into invisible circles called tracks and sectors.

setting up a root directory where the list of files that are on the disk will be kept.

data on a magnetic disk is located by finding the address of its location from an index in the root directory.

each address in the index contains a track and sector number for an individual data segment.


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The use of floppy disks has reduced in recent years in favour of USB pen drives.

These devices are the size of a car key and can be plugged into the USB port of a PC and used just like a hard disk drive.

The main advantage of pen drives is their storage capacity which typically ranges from 32Mb up to 3Mb.

Pen drives also don’t need to be formatted, have no moving parts and are much more robust than floppy disks.


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CD-ROM stands for compact disk read only memory. A CD-ROM looks just like an ordinary compact disk.

CD-ROMs can store approximately 650 megabytes of data which is four hundred times more data than an ordinary 3½ inch floppy disk.

CD-ROM disks come with information already on them and are read only. This means that the information on a CD-ROM cannot be erased or changed, and no new information can be saved.


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Writeable compact disks are supplied blank and can have data put onto them using a special read/write CD drive.

There are two main types of writeable compact disk:WORM (Write-Once, Read-Many) disks which

can have data written to them just once.

Magneto-Optical disks which can have data written to them any number of times just like a hard disk.


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DVD-ROM is also becoming popular as a data storage medium.

The main advantage of using DVD-ROM compared with CD-ROM is its massive storage capacity.

A DVD-ROM disk can store up to 17 gigabytes of data – this is over 26 times more than a single CD-ROM.

Writeable DVD-ROM disks are also available and can have data put onto them using a special read/write DVD drive.

New PCs are increasingly being supplied with read/write DVD drives rather than the more traditional CD-ROM drives.


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Magnetic tape comes in two forms; tape reels, and cassettes or cartridges.

Large tape reels are used to make backup copies of programs and data on large mainframe computers.

Cartridges are used to make backup copies of the programs and data on personal computers and networks.

The main advantage of using magnetic tape as backing storage is that it is relatively cheap and can store large amounts of data.


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Floppy disks, USB pen drives, hard disks, CDs AND DVD-ROM all allow direct access to data.

Direct access means that the required data can be found straight away without having to read through all the data on the disk.

Magnetic tape allows only serial access to data. To locate data on a magnetic tape it has to be searched from the beginning until the required data is found.


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File compression software can be used to make files smaller so that more data can be stored in the same amount of space on backing store.

When a compressed file on backing store needs to be used it must be decompressed.

This can be done using decompression software or by setting files up to be self-extracting which means that they can automatically decompress themselves.

Winzip is an example of software that can be used to compress and decompress files.

Chapter04 storage devices

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