CHAPTER 4 - GCSE - ICT
Data Storage!
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The programs and data needed by computers are stored using data storage devices. Data
storage devices are divided into two main categories; main memory and backing storage.!
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Main memory!
When programs and data are ready to be used they are copied from backing storage into
a collection of microchips inside the computer called the main memory. The two most
common types of microchip, which together form the main memory of a computer, are
called ROM and RAM (explained later).!
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Computers store and process data using binary numbers; Binary numbers are used by
computers because they are very simple to make electronic circuits for.!
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This is because a binary number is represented by a pattern of Os and 1s (1 for ‘on’ and 0
for ‘off’). A single unit in binary is called a bit, which stands for binary digit.!
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Computer memory is measured in bytes with one byte being made up of eight bits. One
byte can store one character, such as A!
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ln the example below one complete byte is shown. This byte contains the binary code!
that is used to represent the letter A.!
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0
1
0
0
0
0
0
1
Figure 4.7 The eight bit binary code for the letter A
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ln the example below 3 bytes have been used to store the word CAT in the computer's
memory. One byte is needed for each letter. The bytes shown contain the binary code that
represents each of the letters C, A and T.!
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0
1
0
0
0
0
1
1
0
1
0
0
0
0
0
1
This byte represents the “C” This byte represents the “A”
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Figure 4.2 The three bytes representing the word CAT!
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0
1
0
1
0
1
0
0
This byte represents the “T”
CHAPTER 4 - GCSE - ICT
The size of a computer's memory is normally measured in kilobytes (Kb),!
megabytes (Mb) or gigabytes (Gb). The table below shows some of the main!
units of size used to measure computer memory.!
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Measurement
Size (Bytes)
Symbol
kilobyte
1,024
Kb
megabyte
1,048,576
Mb
gigabyte
1,073,741,824
Gb
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ROM !
ROM stands for Read-Only Memory. The programs and data stored on ROM are!
permanent and cannot be changed. When the computer is switched off, the!
contents of ROM are not lost. This sort of memory, which ¡sn't Wiped clean when!
the computer is turned off, is called non-volatile memory.!
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The main use of ROM memory chips in a computer is to store a program that runs!
when the computer is first turned on. This program loads the operating system!
(e.g. Windows XP) from backing storage.!
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RAM!
RAM stands for Random Access Memory. RAM is the computer's 'working memory'
where it stores the programs and data that are being used at any given time. The
contents of RAM can be changed because it only stores programs and
data temporarily. When the computer is turned off the contents of
RAM are lost. This sort of memory, which is wiped clean
When the computer is turned off, is called volatile
memory.!
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CHAPTER 4 - GCSE - ICT
PROM and EPROM!
Sometimes, When new computer systems or software are being developed, special types
of read-only memory that can be programmed by the software developer are required.
PROM and EPROM are both special types of programmable read only memory. PROM
stands for Programmable Read-Only Memory.!
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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 changed Whenever necessary. It is useful for, say, developing
programs Which Will eventually be held in ROM chips inside devices like washing
machines or video recorders.
CHAPTER 4 - GCSE - ICT
Backing storage!
Backing storage is used to store programs and data When they are not being used or
when a computer is switched off. Magnetic tape drives, floppy disk drives and hard disk
drives are all examples of backing storage devices. Hard disks, floppy disks, magnetic
tapes, CD-ROMs and DVDs are all examples of backing storage media. Backing storage
devices are used to transfer data to and from backing storage media, Which is what the
data is actually stored on. So a floppy disk is an example of backing storage media and a
floppy disk drive is an example of a backing storage device.!
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Hard disks!
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. !
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Data stored on hard disks can be accessed much more quickly than data stored on floppy
disks. Hard disks can store much more data than floppy disks. A typical hard disk inside a
personal computer can hold many gigabytes of data.!
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CHAPTER 4 - GCSE - ICT
Floppy disks!
A standard floppy disk is 31/2 inches wide and can store up to 1.44 Mb of data. Floppy
disks are only really useful for transferring small data files between!
computers, such as short word-processed letters. !
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An outer hard plastic cover and inner woven cloth liners protect a round plastic disk coated
with a magnetic material. When the disk is put into a disk drive a metal shutter slides back
revealing the surface. A small plastic write-protect tab can be clicked back to reveal a hole
in the outer cover. When this tab is clicked back the disk becomes write-protected. This
means no data can be added to or removed from the disk.!
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Floppy disks can be damaged quite easily if they are not treated carefully. This can!
result in the loss or corruption of data. Following these straightforward rules will!
help to protect data stored on floppy disks:!
• never slide back the metal cover and touch the surface of the disk;!
• keep disks away from magnetic fields -- for example don't put them on top of computer
monitors which generate magnetic fields;!
• avoid storing disks in very hot or cold places;!
• don't keep disks in damp places or allow them to get wet;!
• don't bend disks;!
• don't lend them to other people -they could come back with files deleted or infected by
viruses;!
• always label disks so that you know what's on them.
CHAPTER 4 - GCSE - ICT
Formatting disks!
Before any type of magnetic disk can be used it must be formatted. The process of
formatting involves:!
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• Deleting any data that is already on the disk. Whenever a magnetic disk is formatted any
data that is already on it Will be permanently erased;!
• Dividing the surface of the disk into invisible circles called tracks, which are further
divided into smaller sections called sectors. Modern computers use high-density 31/2 inch
disks that are formatted With 18 sectors and 80 tracks on each side. One sector of a track
can hold 512 bytes of data;!
• 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;!
• Finding any bad sectors on the disk caused by damage to the disk's surface. Data cannot
be stored in these areas.!
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CHAPTER 4 - GCSE - ICT
USB pen drives!
The use of floppy disks has decreased
in recent years in favour of USB pen
drives sometimes known as flash
memory sticks. 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 3Gb. Pen drives - also
known as flash drives - don't need to be
formatted, have no moving parts and
are much more robust than floppy disks.!
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CD-ROM disks!
CD-ROM stands for Compact Disk Read-Only Memory. A CD-ROM disk looks just like an
ordinary music CD. CD-ROM disks can store approximately 650 megabytes of data, which
is-four hundred times more data than an ordinary 31/2 inch floppy disk. CD-ROM disks
come With information already on them and are read onIy. 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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A CD-ROM disk
CHAPTER 4 - GCSE - ICT
When a compact disk is manufactured, a laser burns small depressions called pits into its
surface. A CD-ROM drive reads data from a disk by shining laser light onto its surface as it
spins around. A sensor in the drive detects the amount of light that is reflected back from
the disk's surface. No light is reflected back from the pits and these are given the binary
value 0. Light is reflected back from areas Where there are no pits and these are given the
binary value 1. This is how data is stored on the surface of a CD-ROM as a binary pattern
of 0s and 1s.!
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Because of their large storage capacity CD-ROMS are excellent for storing archive
material. Many newspapers produce regular CD-ROM disks that contain the complete
contents of their daily editions for six months or even a year. There are also many different
multimedia encyclopaedia titles available on CD-ROM. Multimedia means that sound,
pictures and video are all included together on the disk as Well as ordinary text.!
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Writeable CD-ROM disks!
Writeable CD-ROM 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; WQRM
(Write-Once, Read-Many) disks, which can have data Written to them just once, and
Magneto-Optical disks, which can have data written to them any number of times, just like
a hard disk.
CHAPTER 4 - GCSE - ICT
DVD-ROM disks!
The most common use of DVD-ROM disks is to store and play back feature films. DVDs
are rapidly overtook video cassette tapes as the way people watch films at home. DVDROM is also became 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. From the year 2000 to 2010 PCs usually would come
supplied with read/Write DVD drives rather than the more traditional CD-ROM drives.!
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Now days, DVD-ROM are being replaced by BLURAY DISKS and OnLine video streaming
services provided by several companies.!
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Magnetic tape!
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. Cassettes like the ones used to record music were used as backing storage on
early microcomputers. Cartridges are a special type of cassette that can store a large
amount of data. 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.
CHAPTER 4 - GCSE - ICT
Data is stored along the length of a magnetic tape in tracks, with nine tracks being
common. This gives eight data tracks and one parity track. The parity track is used to
check that data has been correctly read from the tape.!
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1!
0!
1!
1!
1!
1!
0!
1!
1!
0!
0!
0!
Data tracks 2
1!
1!
0!
through 8
1!
1!
0!
0!
0!
1!
0!
0!
1!
0!
0!
1
Track 1
Parity Track
Figure 4. 73 Storing data on a magnetic tape!
Direct and serial access!
Floppy disks, hard disks, CD-ROMS and DVDs 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. This is because the address of the correct track and
sector Where the data is located can be found from the index on the disk. Direct access is
also called random access because you can read the data back in any order - not just the
order in which it was first written. !
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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. This is sometimes also
known as sequential access. This makes it quite time-consuming to find and transfer data
to and from magnetic tape compared With hard disks and CD-ROMS.
CHAPTER 4 - GCSE - ICT
File compression!
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File compression is used to make files smaller so that more data can be stored in the
same amount of space. lt also makes it possible to send large files via the Internet more
quickly, because a compressed file is much smaller than the original file. With less data to
transfer, the whole process takes less time. File compression software is needed to
compress a file. A compressed file on backing store must be decompressed before it can
be used. This can be done using decompression software or by setting files up to be selfextracting, Which means that they can automatically decompress themselves. WinZip is
an example of software that can be used to compress and decompress files.!
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