Contents | Tests | Glossary | | | Office 2007

Ex 1.1.3 - Memory and Storage
 
KNOWLEDGE
 
TYPES OF MEMORY
 
Computer memory is where program code and data are stored.  Computers have two types of memory:
RAM (random access memory)
RAM is the short term memory available for temporarily storing applications and data whilst you are creating your word processed documents or spreadsheet workbooks.  If you close down an application or document it will be removed from the RAM memory.  The RAM memory will then be free to allow other programs to run.  If your computer power supply is lost whilst you are working all the information in the RAM will be also lost.  A typical computer will have 256MB or 512MB of RAM available to it.  The more RAM you have generally the quicker your computer will work.  Information is processed in RAM more quickly than in ROM.
 
ROM (read only memory)
ROM is the computers long term memory where information is stored permanently.  The basic code required to run the computer is recorded here when the computer is built.  The information in ROM never changes and is not lost even when the computer is turned off.  Although information processed in ROM is slower than RAM it is still processed much quicker than to magnetic disc (hard or floppy).
 
MEASUREMENTS OF DATA STORAGE
 
Computers are digital devices and the "binary" measurement system is the primary system used to describe them.  Binary means two.

Denary, the decimal system that we commonly use means ten.  The denary system uses ten symbols: 0, 1, 2, 3, 4, 5, 6, 7, 8 & 9.  These ten symbols can define values as small as two (2) or as big as ten million (10,000,000).

Given that binary (the system used in computing) means two then ONLY two symbols are required: 0 & 1.  The digit 0 can be considered to be the OFF state and the digit 1 can be considered to be the ON state.  If you consider the fact that magnetic floppy discs are used to store data and you were to inspect the disc with a microscope the surface of the disc is either magnetised, i.e. ON = 1, or it is not magnetised, i.e. OFF = 0.  There is no other state the disc can be in.  Other devices that are part of the computer system can be looked at in a similar way.

The five measurements of data storage we will consider are:

Measurement Description
Bit Bit is short for binary digit.  It is the smallest element of computer storage.  A single zero, 0, or a single 1 is a bit.  If you analysed the surface of a floppy disc physically you would find that one minute part of the disc is not magnetised.  We would represent that minute part of the disc with a zero, 0, using the binary digit system.  If the state of that minute part of the disc were changed and it became magnetised then we would say it could be represented by the symbol 1.
Byte Eight bits make up one byte.

8 bits = 1 byte

Computer memory is measured in bytes as a byte holds the equivalent of a single character - e.g. the character "Z" or the symbol "&".  The character "A" can be represented by the eight binary digits 01000001 (remember 8 bits = 1 byte).  In physical terms on the surface of the disc there would be eight minute parts of the disc, all next to each other, two of which are magnetised and six of which are not.
Kilobyte Because a byte is a very small unit of measurement, it only represents one character, we often talk in terms of kilobytes.  In distance measurement we have the metre and them the kilometre, where a kilometre is 1000 metres.  The same is almost true of the kilobyte.  It is actually:

1 kilobyte = 1024 bytes

We also shorten kilobyte to KB.  So 650 kilobytes would normally be written as 650KB.
Megabyte Kilobytes are OK to describe the file size of a word processed document or a single small graphic image.  Once you have a folder that has a large number files in it it is better to describe the data in terms of megabytes.

1 megabyte = 1,048,576 bytes
(1 megabyte is approx. 1 million bytes)

1 megabyte = 1,024 kilobytes

1MB = 1,024KB

The data that can be saved on a single floppy disc is equivalent to 1.44MB, i.e. just under one and a half megabytes.  In real terms that means 1,440,000 individual characters.
Gigabyte As file sizes increase and images and now music and video are stored in digital format we need measurements that are even larger than megabytes.

1 gigabyte = 1,073,741,824 bytes
(1 gigabyte is approx. 1 billion bytes)

1 gigabyte = 1,024 megabytes

1GB = 1,024MB

Hard discs can save 40GB of data and DVD discs can store 4.5GB of data.
Terabyte As file sizes increase and images and now music and video are stored in digital format we need measurements that are even larger than gigabytes.

1 terabyte = 1,099,511,627,776 bytes
(1 terabyte is approx. 1 trillion bytes)

1 terabyte = 1,024 gigabytes

1TB = 1,024GB

Hard discs can save 40GB of data and DVD discs can store 4.5GB of data.
 
DATA STORAGE MEDIA
 
Data storage media include:
Hard discs
Hard discs are the primary form of data storage media in that they sit permanently inside the computer and are used to store both data files (such as your word processed letter or spreadsheet file) and the operating system and application software files.  Hard discs can also sit outside the main computer and are referred to as external hard discs.  Such hard discs are used not to store operating system and application software but are rather used to backup the internal hard disc and/or store data files in the same way that floppy discs or CD's are used.  External hard discs that can store 40GB are common and higher capacities such as 100MB, 200MB and above are also available (as of Autumn 2005).  Hard discs are typically the fastest of the data storage media and it should be noted that they work fastest if the connection to the computer is via a Firewire or USB 2.0 port rather than the older USB 1.0 ports.  External hard discs typically cost £100+.
 
Floppy discs
The 3 1/2" floppy disc that you carry around with you is a form of magnetic disc as is the "hard disc" which can be found inside your computer.  Floppy discs are capable of storing up to 1.44MB whereas hard discs that can store 40GB are common.  Floppy discs are a relatively cheap form of data storage media and given the fact they are portable everyone uses them.  They can store up to 500 pages of text documents.  Data can also be deleted and replaced by new data.  The disadvantage with floppy discs these days is that whilst they are still useful for storing word processed documents the file size of graphics and video are such that they fill up floppies very quickly.  Floppy discs that are carried around from office to office can easily get damaged or lost.  They are also prone to picking up and passing on viruses from computer to computer if care is not taken to scan them regularly with anti virus software.  Floppy discs typically cost 20p.
 
CD-ROM's
CD-ROM is short for compact disc read only memory - this is often referred to as simple CD or CD-R.  This form of disc can only have date "read from it" once it has been created.  You can not save further data to it.  The CD that you buy with your application software on, Microsoft Office for example, is a CD-R disc.  The CD-R is a form of optical disc.  The CD drive that you place the CD-R in has a laser beam in it which reads the data that has been etched on the surface of the disc.  CD-R's typically store as much as 700MB, or the equivalent of 500 floppy discs.  CD-R's are used not only to store application software they are also useful for storing large numbers of graphics, music and video.  CD-R's that contain text, graphics, music and video are referred to as multimedia CD's.  If you wish to save data to a CD disc you will need to buy a drive that has this ability - it will be referred to as as CD writer.  With a CD writer you will generally be able to write to CD-R discs as well as CD-RW discs.  CD-RW discs allow you to save data to the CD and then delete it later and save further data, like you can with magnetic discs.  CD-RW discs are more expensive than CD-R discs.  CD-R discs typically cost 25p whilst CD-R discs cost 50p.
 
DVD's
DVD's, or digital versatile disc to give it its full name, are similar to CD's in that they are a form of optical disc.  They work in a similar fashion to CD's and are available in both read only format and read/write format, provided you have a special DVD recorder.  The difference between CD's and DVD's is the amount of data that can be stored on a DVD is about 4.5GB (4,500MB) compared to the 700MB of data on a CD.  The amount of data that can be stored on a DVD is equivalent to the contents of a typical movie - this includes the whole film, the sound and the music.  Recordable DVD's are also available and are typically £1.20 per disc.
 
Tape cartridges
Magnetic tape is similar in nature to music cassette tapes.  In order to retrieve a particular file on a magnetic tape the tape has to be wound forward to the point where the data is.  This could take as little as a few seconds or as long as a few minutes.  Because of this nobody uses magnetic tape for accessing data on a daily basis in the way that they do with data on magnetic or optical discs.  Hence, magnetic tape is normally used for storing backup data rather than data that is in daily use - they are slow but they are cheap.  At the end of the working day most large companies make copies of the files created or edited that day by saving them to tape.  A series of tapes may be used one for each day of the week.  At the start of the next week new data could be backed up on top of old data thus deleting it.  Big institutions like high street banks keep their backup tapes for months so that any fraudulent activity discovered can be traced back.  Small capacity data cartridges can cost as little as £10 but high capacity ones used for full system backup can cost thousands of pounds.
 
USB stick
 
If you have a USB stick you can place it into a free USB slot at the back of your machine and save your files to Removable Disk (E).  Like the floppy disk a USB stick is a portable storage device.  Typical sizes include 256MB and 512MB - data capacity will always be increasing.  As an external drive the speed of the USB is always going to be slower than a hard disc, however data transfer rates are much higher than for a floppy disc.
 
FURTHER INFORMATION
 
To read further information about the topic(s) discussed in this exercise try visiting the following web site(s):

www.orcalogic.co.uk - Memory Supplier

www.hp.co.uk - Storage/Storage Devices
 
 


Basic Concepts of IT (Copyright STW)