1. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
Data 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.

2. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
Computers 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.

3. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
The 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

4. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
Random 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.

5. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
The 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.

6. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
PROM 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.

7. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

8. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

9. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

10. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

11. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

12. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

13. GCSE ICT 3rd
Edition
GCSE ICT 3rd
Data storage
4
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.

14. GCSE ICT 3rd
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GCSE ICT 3rd
Data storage
4
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.

15. GCSE ICT 3rd
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GCSE ICT 3rd
Data storage
4
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.

16. GCSE ICT 3rd
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GCSE ICT 3rd
Data storage
4
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.