Representation of data in computer systems

Units

• Bit = 1
• Nibble = 4 bits
• Byte = 8 bits
• Kilobyte = 1024 bytes
• Megabyte = 1024 kilobytes
• Gigabyte = 1024 megabytes
• Terabyte - 1024 gigabytes
• A bit is a binary digit, and is the basic unit which takes the value of either 1 or 0.
• By turning electricity on (1) and off (0) along a calbe messages can be sent between computers.
• Byte is the basic practical data volume unit - it is a unit of information that computers can send at any one time.
• Bytes are split into nibbles to make it easier to read. Each nibble can then be converted into a hexadecimal letter. Hexadecimal numbers normally have a # sign before them.

When a key is pressed on a keyboard...

• ...a code is generated that the computer can convert into a symbol for display or printing.
• it is important that systems agree on the codes being used for the data to be correctly presented. In 1960 American Standards Association agreed on a set of codes to represent the main character set used in telegraph systems, ASCII. They agreed to deal with basic textual messages.
• A character set is the characters available to a computer. ASCII has 127 codes (95 printable and 32 non-printable), plus 'null' code (00000000), used as a control character in certain applications. In binary, 127 = 1111111, so this system uses 7 bits to code the character set of a computer.
• ASCII fails to deal with a range of other characters such as non-English ones and mathematical symbols. ASCII was extended to use 8 bits rather than 7, which meant that another128 character could be included, with one character taking up only one byte to store.
• The Unicode system for encoding characters uses up to 32 bits (4 bytes), providing more than 4 billion possibilities. In this, the original 128 ASCII characters still have the same values, so ASCII is technically a subset of the Unicode system for coding characters. A lot of the 32 bit system is unused, and the 16 bit version is sufficient for most needs.

What is a pixel?

• The smallest element of an image. The dots that make the image on screen.
• When enlarged, the image becomes less defines and the pixels can be clearly seen. If a pixel is to be able to represen more than one colour then more bits have to be used to make a range of colours available. With one bit, we can only have two colours - black and white.
• Colour depth