# Data Units

Candidates should be able to:

• define the terms bit, nibble, byte, kilobyte, megabyte, gigabyte, terabyte
• understand that data needs to be converted into a binary format to be processed by a computer.

## Data units in computer systems

• Bit
• This is a single unit of memory and can only store 2 possible binary values, either 0 or 1.
• Nibble
• A nibble is a unit of memory made up of 4 bits. This means it can store 16 possible binary values, 0000 to 1111. Numbers encoded using the binary coded decimal (BCD) system use 1 nibble to encode each digit of the number (rather than converting the whole number into binary). For example, to encode the denary number 75 using the BCD system would mean the 7 would be encoded as 0111 and the 5 as 0101, using 2 nibbles of memory.
• Byte
• A byte is a unit of memory made up of 8 bits. This means it can store 256 possible binary values, 00000000 through to 11111111. The denary (decimal) number 75 could therefore be encoded as binary 1001011, using 1 byte of memory.
• Kilobyte
• 1024 bytes are called a kilobyte (kB). When talking about computer storage rather than computer memory a kilobyte is often referred to as 1000 (103) bytes. 1kB of memory could store roughly one full A4 page of text.
• Megabyte
• 1024 kilobytes are called a megabyte (MB). When talking about computer storage rather than computer memory a megabyte is often referred to as 1000 kilobytes (106 bytes). A typical MP3 music file is around 4MB.
• Gigabyte
• 1024 megabytes are called a gigabyte (GB). When talking about computer storage rather than computer memory a gigabyte is often referred to as 1000 megabytes (109 bytes). A typical DVD can store around 4.7GB of data.
• Terabyte
• 1024 gigabytes are called a terabyte (TB). When talking about computer storage rather than computer memory a terabyte is often referred to as 1000 gigabytes (1012 bytes). The first 1TB hard drive was produced in 2007.

Why does data need to be converted into a binary format to be processed by a computer?
Computers can only process and store binary numbers. This means that numbers and text, data from sensors and all analogue sounds, images and video etc. has to be converted into binary (digitised) before it can be processed by a computer.