Units

• Bits are the smallest measure of data
• Computers use 1s and 0s to represent the flow of electricity
• 1 is used to show that electricity is flowing, and 0 shows that it isn't flowing
• All the data we want on a computer to process must be converted into binary code
• Each 1 or 0 in a binary code is a bit (binary digit). E.g. 1010 is 4 bits
• A byte is big enough to store one character
• Most files (like songs, picture and documents) are measured in kB or MB
• High definition videos and complex applications are often measured in gigabytes
• Secondary storage capacity is measured in gigabytes or terabytes
• You might see each unit defined to be 1024 (not 1000) times bigger than the previous unit. The reason is that 1024 is a power of 2 which is helpful when dealing with binary data
• Bit (b) = a single binary digit (1 or 0)
• Nibble = 4 bits
• Byte (B) = 8 bits
• Kilobyte (kB) = 1000 / 1024 bytes
• Megabyte (MB) = 1000 / 1024 kilobytes
• Gigabyte (GB) = 1000 / 1024 megabites
• Terabyte (TB) = 1000 / 1024 gigabytes
• Petabyte (PB) = 1000 / 1024 terabytes
• Each bit can take one of two different values (1 or 0). This means that a nibble (4 bits) can take 2^4 = 16 different values, and a byte (8 bits) can take 2^8 = 256 different values
• Converting between units of data is usually pretty straighforward - just watch out when you need to switch between bits and bytes
• Example

  Ashley has downloaded some images to her computer. Each image is 300 kilobytes.

  How many bits are in each image?

  1. First convert to bytes by multiplying by 1000:

  > 300 kB = 300 x 1000 = 300,000 bytes

  2. There are 8 bits in a byte, so multiply by 8:

  > 300,000 bytes = 300,000 x 8

  = 2,400,000 bits
• Example

  Ashley has downloaded some images to her computer. Each image is 300 kilobytes.

  She wants to copy 400 of these images onto her USB flash drive, which has 0.15 GB of free space left. Does she have enough space to store them all?

  1. Work out the total size of all the images:

  > 400 x 300 = 120,000 kB

  2. Now convert this to GB - first, divide by 1000 to get it in MB, then again to get it in GB:

  > 120,000 kB = 120,000 / 1000 = 120 MB

  >120 MB = 120 / 1000 = 0.12 GB , so yes she has enough space