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Data as Bit Patterns

Memory in a computer consists of a very large number of electronic 'circuits' which can be in one of two possible states .......'ON' (represented by 1) or 'OFF' represented by 0.

8 such circuits which are in the states :
ON, ON, OFF, OFF, ON, OFF, OFF, ON

...could be represented by the digits :
1 1 0 0 1 0 0 1

Each 1 or 0 is called a BIT (short for BInary digiT).

8 bits used together is known as a BYTE.

 

 

Representation of characters.

Most computers use one byte (8 bits) to represent one character. This allows a unique code for each character and one spare bit. This spare bit might be used as a parity bit (see below) or as an indicator that an extended character set (eg which uses some graphic characters) is being used.

Most PCs use ASCII (American Standard Code for Information Interchange) codes. Each character has a unique 7-bit code...

Eg A is 01000001

Some mainframes use a code called EBCDIC (Extended Binary Coded Decimal Interchange Code).

 

 

Representation of Graphics.

In a bitmap system, a graphic is composed of a large number of pixels.

Each pixel has a colour and this colour is stored as a bit pattern. Eg Each pixel may be stored as one byte giving a possible 256 different colours.

Memory for storing graphics usually comes on a separate graphics card specific to the type of monitor.

 

 

Representation of Sound.

Sounds may also be stored as bit patterns. Sound may be input using a microphone, CD, electronic keyboard with MIDI (Musical Instrument Digital Interface). 

Sound waves are analogue and need to be converted to digital signals before processing.

 

 

Notes.

  1. Parity. A computer system which uses Even Parity uses data which always has an even number of '1' bits in each byte. This is used for error checking - particularly when data is moved from one place to another.The computer always checks the parity of data when sending and receiving.

    Eg
    The ASCII code for 'P' is 1010000...add a parity bit of 0.

    The ASCII code for 'Q' is 1010001..add a parity bit of 1.

  2. We can see that the same bit pattern may represent a character, a pixel on a graphic, a sound etc... The computer will know the context of the data and will be able to process it accordingly.