Units of information revision notes

Bits and bytes

The bit is the fundamental unit of information. A bit has one of two possible values: 0 or 1. A byte is a group of 8 bits.

Because each bit has two possible states, the number of different values that can be represented by n bits is 2^n. For example, 3 bits can be configured in 2^3 = 8 different ways: 000, 001, 010, 011, 100, 101, 110 and 111. The calculation counts every possible choice for each bit.

A useful exam method is to identify the number of bits first, then apply 2^n. For example, 5 bits can represent 2^5 = 32 different values. This is a count of possible configurations; it does not by itself identify what the configurations mean.

Binary prefixes

Binary prefixes describe quantities of bytes using powers of 2. The required binary prefixes are:

| Prefix name | Symbol | Value | |---|---:|---:| | kibi | Ki | 2^10 B | | mebi | Mi | 2^20 B | | gibi | Gi | 2^30 B | | tebi | Ti | 2^40 B |

For example, 1 KiB = 2^10 B.

Decimal prefixes

Decimal prefixes describe quantities of bytes using powers of 10. The required decimal prefixes are:

| Prefix name | Symbol | Value | |---|---:|---:| | kilo | k | 10^3 B | | mega | M | 10^6 B | | giga | G | 10^9 B | | tera | T | 10^12 B |

For example, 1 kB = 10^3 B.

Key distinction and common errors

Do not treat KiB and kB as interchangeable. KiB uses the binary prefix kibi and means 2^10 B, whereas kB uses the decimal prefix kilo and means 10^3 B. The same distinction applies to MiB and MB, GiB and GB, and TiB and TB.

Historically, terms such as kilobyte and megabyte have often been used when kibibyte and mebibyte were meant. In an exam, use the symbol and prefix given, and state whether the calculation uses a power of 2 or a power of 10.