1 3 Computing System Fundamentals 3.2 Computer Architecture

3.2.2 Bits and Bytes

3 Bit The smallest unit of storage in memory or storage is the bit (b), which can store a zero or a one. Bits represent the fact that the transistors that make up computer memory are effectively switches that can only be ON (1) or OFF (0).

4 Byte Bits are grouped together in eights to form a byte (B) e.g Sometimes a group of bytes representing one item of data or one instruction is referred to as a word (typically 2, 4 or 8 bytes, depending on the computer, hence 32- or 64-bit processors).

5 Byte The same prefixes as for other SI units are used: ‣ kB - a kilobyte holds 1024 bytes (about 1000), ‣ MB - a megabyte holds 1024 kilobytes (about a million bytes), ‣ GB - a gigabyte holds 1024 megabytes (about a thousand million bytes), ‣ TB - a terabyte holds 1024 gigabytes (about a million million bytes), etc.

6 Byte Note that the increases are not in 1000s but in 1024s - computer storage is measured in units of 2 10 (1024), not in units of 10 3 (1000). Hence (in computing only, not elsewhere): ‣ kilo- represents 2 10, ‣ mega- represents 2 20, etc. A hard disk of 500 GB can store the equivalent of characters = ~62.5 million million words = ~ 200 thousand million pages.

7 Binary See section 3.5 Get used to this series of numbers 2 1 = 2, 2 2 = 4, 2 3 = 8, 2 4 = 16, 2 5 = 32, 2 6 = 64, 2 7 = 128, 2 8 = 256, 2 9 = 512, 2 10 = 1024.