Information Representation Computer Science CIE AS Level

Bit

A digit in the binary number system written using either of the symbols 0 and1

Byte

A group of 8 bits

Nibble

A group of 4 bits

Decimal Prefix

Kilo-10³ Mega-10⁶ Giga-10⁹ Tera-10¹²

Binary Prefix

Kibi-2¹⁰ Mebi-2²⁰ Gibi-2³⁰ Tebi-2⁴⁰

One’s Complement

The binary number obtained by subtracting each digit in a binary number from 1

Two’s Complement

The one’s complement of a binary number plus 1

Overflow

A condition when the result of a calculation is too large to fit into the number of bits defined for storage

BCD

Storage of a binary value representing one denary digit in a nibble

Packed BCD

When two BCD nibbles are stored in one byte

Use of BCD

• Electronic calculators

  • Keeps numbers in decimal format for easier display and accuracy

• Digital clocks and watches

  • Time is naturally decimal (e.g. 12:45), so BCD makes display logic simpler

• Banking and financial systems

  • Avoids rounding errors when doing decimal calculations, especially with money

• Old digital systems / embedded systems

  • Simpler to implement with hardware that displays digits individually
    

Use of Hexadecimals

• MAC addresses

  • A typical MAC address consists of 12 hexadecimal digits, equivalent to 48 digits in binary

• Colour codes

  • A typical hexadecimal colour code consists of 6 hexadecimal digits, equivalent to 24 digits in binary

• URLs

Benefits of BCD

• display decimal numbers clearly and accurately

• where precision matters

• avoids rounding errors

• where simple hardware-based decimal output is needed

Correction bit for BCD

0110 (6)

Vector Graphic

A graphic consisting of drawing objects defined on a drawing list

Drawing Object

component defined by geometric formulae and associated properties

Drawing List

Contains one set of values for each drawing

Property

Defines one aspect of the appearance of the drawing object

Pixel

The smallest identifiable componen of a bitmap image defined by just two properties: its position in the bitmap matrix and its colout

Colour Depth

The number of bits used to represent one pixel

Bit depth

The number of bits used to represent each of the red grene and blue colours

Image resolution

The number of pixels in the bitmap file defined as the product of the width and the height values

Screen resloution

The product of the width and hieght values for the number of pixels that the screen can display

File Header

A set of bytes at the beginning of a bitmap file which identifes the files and contains meta data

How is a word represented by ASCII

Each character has its own unique code. Each character in the word is replaced by its code. The codes are stored in the order in the word