Disk Storage Devices

Disk drives of various kinds are used to store and transfer data files. When data is enterd into a computer it must be saved as a file to preserve it when the computer is switched off: Also software applications need to install files on the main disk drive in the computer in order to run.

The most important type of drivein the computer is the hard disk which is used to store the software and data on which the computer run.

Most computers will also have removable disk drives to install software and backup data files.

Disk performance is principally a question of capacity: how much data can be stored on the disk?
Other factors that influence disk performance are the seek time and the transfer rate.

Hard Disk

The hard disk is usually a device fixed inside the computer. It stores large volumes of data which can be accessed and retrived quickly. The seek time of a hard disk is much lower and the transfer rate much higher than any other type of disk drive.

• Hard disk consist of a number of rigid magnetic disks in a protectives casing. Data is recorded magnetically onto concentric circular tracks each one divides into a number of sectors. Data is accessed by an arm moving over the surface of the disk.

• Hard disk can hold very large amounts of data and are usually very fast in comparison with other storage devices. They very considerably in terms of piece depending on their speed and capacity. Hard disk on sale at the time of writing very in capacity from 80 GB to 750 GB.

• When  you install software the setup istructions indicate the amount of free disk space required for the software to run effectively. Applications typically take up 50-200 MB each though multimedia applications and games can require far more.

• Most hard disk are fixed inside the computer. Removable hard disk are available but tend to be much more expensive.

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Measuring data and storage

A computer process digital data. When you use an input device the signal from the device are converted into a steam of bits the smallest componnents of computer data. Each bit can be in one of two states 1 or 0 with the 1 standing for On and the 0 stand for Off.


Memery and the disks are the components used to store data for processing. The unit used to messure data are bits , bytes , megabytes and gigabytes.

• Bits are combined with sets of eight to form charactors such as letters from the alpherbet. For a example 01000001 is used torepresent the letters A in binary code. This group of eight bits makes a byte.

• A killobyte (KB) is 1024 bytes. The size of a data file is often measured in killobyte.

• A megabyte (MB) is often used to measure the storage capacity of a disk or the amount of main memary. The totall size of a collection of files within a folder may be messured in megabyte. A megabyte is 1048576 bytes. 1 MB is about as much data as 500 pages of double spaced text.

• A gigabyte (GB) is equivalent to 1024 MB. The memary capacity of a many larger storage devices such as hard drive is messured in gigabytes. 

• A terabyte (TB) is equivalent to 1024 GB.

Analogue and Digital data

Computer system work with Digital data. In a digital system every unit of information is reduce to a combinations of bits. Each bit can have one of two valuves: ON or Off.


Many data transmission media use analogue transmitions. An analogue transmition uses variatoin in a signal wave to represent data. When someone speak to you, you hear a sound wave and interpret the data as word depending on the variations in tone and pitch.


Therefor many computer device include componnents to convert data between analogue and digital formats. For a example  a high point of wave might represent On the low point Off. Example of componnents that convert between analogue and digital include the sound adapter and a computer modem.

Sound input and output

Sound card and speaker
Many computers especially home computers are equipped with speakers. Speakers take the signal genarated by a sound card and convert it into sound.
If your computer has speakers you can listen to music CD's or hear sound effects genarated by software. This capability is often used by educational softwares and computer games.

Speech Synthesiser

A speech synthasiser receied a output of a computer and turn it into speech played throught the speaker.
A microphone can be attached to a sound card provide audio input with suitable software a microphone can also be used to issue commands and create files.

Visual output devices

output device display information from the computer to the user. the main output devices display data on a screen print data on paper, and play data as sound.
With modern software the screen display is the most important interface the user has with a computer. All commands are issued using a mouse or keyboard to select elements of a graphical user interface such as menues and toolbar.

Displaying data on screen requires two components.

• The graphic card is often connected to an expansion slot (PCI or AGP) on the motherboard. Many graphic cards are optimised to display 3D graphics effects. They often incorporated sophisticated. microprocessor and have thrir memory so that the screen display can be kept updated as smoothly as possible.
• The visual display unit displays an image on a screeen recording to the data supplied to it by the graphic card.

These componnents need to be compatible in order to work together. Almost all modern display system use the SVGA standerd. SVGA support high colour depth resolution and refresh rate. Older display standerd include VGA and XGA which are not compatible of such high performance.