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A computer uses two types of storage. A main store consisting of ROM and RAM, and backing stores which can be internal, eg hard disk, or external, eg a CD or USB flash drive.


Main store (or computer memory) is divided into Read Only Memory (ROM) and Random Access Memory (RAM).


ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off. For example, ROM stores the instructions for the computer to start up when it is turned on again.


RAM is a fast temporary type of memory in which programs, applications and data are stored. Here are some examples of what's stored in RAM:

·the operating system


·the graphical user interface (GUI)

If a computer loses power, all data stored in its RAM is lost.

Storage capacity and file size

Storage capacities and file sizes are measured from lowest to highest in:







An operating system abbreviates these measurements, eg 1 megabyte becomes 1MB (megabyte).

Relationship between measurements

The table below outlines the relationship between bits (smallest) and terabytes (largest). Read each row of the table from left to right:

Relationship between measurement


Equal to

8 bits

1 byte

1024 bytes

1 kilobyte

1024 kilobytes

1 megabyte

1024 megabytes

1 gigabyte

1024 gigabytes

1 terabyte

The size of a file and a storage device's capacity will always be written in its simplest form. For example, an operating system would report a 1 terabyte hard disk's size as 1TB not as 1024GB, although both are correct.

Example file sizes

The table below lists files commonly found on a computer and their typical file size (compressed):


File size







A file’s size can be influenced by a number of factors but ultimately the more information a file stores, the larger it will be.

The hard disk

The main internal backing store is a computer's hard disk.

A hard disk stores:

·the operating system

·softwareapplications or programs

·the majority of your data files

Hard disks spin at very high speeds (around 7,200 RPM - revolutions per minute) within a sealed unit inside the computer. Hard disks store large amounts of data - 200GB to 1TB is common in desktop computers. The data stored on a hard disk is retained until deleted, but it needs to be loaded into main store RAM before it can be used.

Floppy and Zip disks

Floppy disks hold very small amounts of data, just 1.44MB and a computer needs a floppy drive to read them. They are used to:

·transfer small files of data from one machine to another

·backup important small files that are stored on your hard disk

·store restricted files that you don't want other users of your computer seeing

Zip disk

Zip disks are like large floppy disks but can store 250MB or more of data. To read them a computer needs a zip drive. Their use is similar to that of floppy disks.

Newer storage mediums such as DVD-RWs and memory sticks have replaced floppy/zip disks.

External backing stores: optical discs

There are several different types of optical disc, although they all look pretty much the same.

CD (Compact Disc)

Optical discs that use the same technology as music CDs. They store up to 700MB of data. CDs can be used for multimediaapplications such as encyclopaedias and can store pictures, sounds and video clips or anything else that will fit.

There are several formats on the market, such as:

·CD-ROMs - read only, the data is written to them before they are sold.

·CD-R - meaning CD-Recordable, the user can write data to the CD once or fill it over time using multi-session (writing to the same disc on separate occasions).

·CD-RW - meaning CD-ReWritable, the CD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten.

DVD (Digital Versatile Disc)

DVDs are the same physical size as CDs but hold much more data - a single sided disc can hold up to 4.7GB. DVDs are commonly used for storing video so you will often see them measured in minutes, eg 4.7GB = 120 minutes.

There are several formats on the market, such as:

·DVD-ROMs - read only, the data is written to them before they are sold.

·DVD-R - meaning DVD-Recordable, the user can write data to the DVD once or fill it over time using multi-session.

·DVD-RW - meaning DVD-ReWritable, the DVD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten.

CD/DVD drives

To read from and write data to CDs and DVDs you will need a suitable drive. Today you can buy CD/DVD drives that are able to:

·read all CD and DVD formats

·write to CD-R, CD-RW, DVD-R and DVD-RW

Data is written to and read from the discs using a laser.

External backing stores

Magnetic tape

Principally used for backup of important files from the hard disk and for the long term storage and archiving of data.


·They are small, robust, portable and provide low cost storage per GB.


·They are very slow to write to and read from.

·Serial access means all the data on the tape must be read before you can access the data you need.

External hard disks

An external hard disk can store very large amounts of data, eg 1TB, and can be plugged into your computer via a USB or FireWire port to provide extra storage. They're essentially the same as the hard disk in your computer but due to the casing are usable externally.


·High capacity, eg 1TB or more so they can be used to backup data or move large amounts of data between machines.


·They're often quite large and therefore inconvenient to carry around.

·They have moving parts so are more likely to break, especially if dropped.

Memory sticks

A memory stick is 'pen top' sized USB device and can be used in a similar way to a floppy disk but it is inserted into the USB port - it is then seen by the computer as a removable drive. They typically come in sizes from 512MB to 32GB upwards depending on the price paid.


·Memory sticks can hold large quantities of data.

·They are extremely portable, so the user can take them wherever they go.

·They're durable, because they have no moving parts.


·Portable storage devices in general are more likely to be lost, stolen or damaged.

Backing up and archiving data

Data needs to be backed up for many reasons:

·a user may delete an important file

·hard disks can fail

·a virus can wipe out data

·a fire may destroy the building where the data was being stored (businesses will often store their backups off-site)

If the data wasn’t backed up then the consequences could be disastrous depending on what data was lost. If a business lost details of all the payments it had yet to receive the business could go bankrupt as they wouldn’t know what was owed to them or by who.

Businesses typically use magnetic tape to backup important data. Your average home user is unlikely to backup their data but those who do will most likely use an external hard disk.

Frequency of backups

Some data is more valuable than other data and some data is changed more frequently than other data. These are the kind of issues that must be taken into account when deciding how often to backup data.

The value of the data should determine how frequently it is backed up.

If the data doesn’t change often then it doesn’t need to be backed up as often, maybe just after each change. If the data changes frequently then it should be backed up frequently (maybe every evening).


Some data may not be being used very often but it may still be useful or needed in the future. In this case data can be archived. Archived data is copied to a suitable storage medium (perhaps DVDs or magnetic tape) then it is stored safely and securely. The original data is then deleted from the computer system. This is done to free up storage space for new data.

A virus is a simple program designed to cause harm to someone else's computer. A virus spreads by duplicating and attaching itself to other files. The extent of the damage depends on the virus. There are tens of thousands of viruses.

Some virus damage is insignificant but inconvenient. Some virus damage is disastrous, putting the computer system out of action by erasing files or corrupting data.

How do you get a virus?

Viruses are written by malicious programmers who wish to cause problems for other computer users.

The primary source of infection these days are emailattachments followed by illegal software and infected files from the Internet. If you have up to date anti-virus software installed this will immediately warn you of any infection. If not, there is usually no evidence of the virus and the user is not usually aware of it until something goes wrong.

How can reduce the risk of getting a virus?

Viruses can be prevented by taking sensible precautions, including:

·Keeping your operating system up to date.

·Using up to date anti-virussoftware.

·Not opening an emailattachment unless you are expecting it and know the source (many email servers scan emails with anti-virus software on the user's behalf).

·Not allowing other users to use their own memory stick on your system.

·Only downloading files from reputable web sites.

·Avoiding software from unreliable sources.

Anti-virus software can inspect computer files and email attachments for viruses and remove or quarantine any which are found.

It is good practice to backup your data regularly. If a virus does damage your data, you can restore the damaged files from backup.


Worms:These viruses spread via computer networks. It is self replicating computer program.

TROJAN VIRUSES: A Trojan virus is one that opens your computer up to malicious users allowing them to read your files.

VIRUSES:“A virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes.”

Difference Between a Virus, Worm and Trojan Horse:

Virus cannot replicate themselves but worm and trojan can do that.

A virus cannot be spread without a human action such as running an infected file or program but worm and trojan have the capabilities to spread themselves automatically from computer to computer through network connetion.

A virus do not consume system memory but worm consumes too much system memory and network bandwidth because of their copying nature.

Trojans are used by malicious users to access your computer information but viruses and worms can’t do so, they simply infect your computer.

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