1. Backing storage devices like magnetic tapes, floppy disk drives, and hard disk drives are used to store programs and data when not in active use or when the computer is turned off. Main memory temporarily stores programs and data being used. 2. Computer memory is measured in bytes, with one byte made up of eight bits that can store one character. Larger units of measurement include kilobytes, megabytes, and gigabytes. 3. Hard disk drives can store gigabytes of data and allow faster access than floppy disks, which typically store 1.44 megabytes. Floppy disks and hard disks allow direct access to data while magnetic tapes only allow sequential access.

1. 1
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Storing data
Data storage devices can be divided into 2 main categories:
Backing storage is used to store programs and data when
they are not being used or when a computer is switched off.
When programs and data are needed they are copied into main
memory but also still remain on backing storage.
Magnetic tape drives, floppy disk drives and hard disk drives are
all examples of backing storage devices.
The Main Memory contains two types of memory chip called
ROM and RAM which hold program instructions and data.
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BITS and BYTES
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.
1 0 0 0 0 0 1 0
The eight bit binary code in this
byte represents the letter A
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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 that we
use to measure computer memory.
Measurement Size (bytes) Symbol
kilobyte 1,024 Kb
megabyte 1,048,576 Mb
gigabyte 1,073,741,824 Gb
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Random Access Memory (RAM)
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 which is wiped clean when the
computer is turned off is called volatile
memory.
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Read Only Memory (ROM)
The main use of ROM memory chips in a computer
is to store the program that runs when the
computer is turned on which loads the operating
system (e.g. Windows 2000) 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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PROM and EPROM
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.

2. 2
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Hard disk drives
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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Floppy disks
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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CD-ROM
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 CDs
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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Digital versatile disk (DVD)
•DVD is the latest way of storing data
•DVD discs are expected to replace ordinary compact
discs and video tapes in the future
•A DVD disc can store up to 17 gigabytes of data. This is
enough storage space for at least four full-length feature
films!

3. 3
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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.
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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Direct and serial access
Floppy disks, hard disks and CDs 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
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.