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Units of information

Official AQA 7517 section 4.5.3.

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AqaA LevelComputer ScienceFundamentals of data representation

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Bits and bytes1 objectives

Bits and bytes: Know that: • the bit is the fundamental unit of information • a byte is a group of 8 bits. A bit is either 0 or 1. Know that the 2n different values can be represented with n bits. For example, 3 bits can be configured in 23 = 8 different ways. 000, 001, 010, 011, 100, 101, 110, 111

Units1 objectives

Units: Know that quantities of bytes can be described using binary prefixes representing powers of 2 or using decimal prefixes representing powers of 10, eg one kibibyte is written as 1KiB = 210 B and one kilobyte is written as 1 kB = 103 B. Know the names, symbols and corresponding powers of 2 for the binary prefixes: • kibi, Ki - 2 10 • mebi, Mi - 2 20 • gibi, Gi - 2 30 • tebi, Ti - 2 40 Know the names, symbols and corresponding powers of 10 for the decimal prefixes: • kilo, k - 10 3 • mega, M - 10 6 • giga, G - 10 9 • tera, T - 10 12 Historically the terms kilobyte, megabyte, etc have often been used when kibibyte, mebibyte, etc are meant.

BitByteBinary prefixDecimal prefix

Use the power-of-two rule: For a question asking how many different values n bits can represent, use 2^n.
Check both the prefix spelling and symbol: Before calculating or selecting an answer, identify whether the unit uses a decimal prefix such as k, M, G or T, or a binary prefix such as Ki, Mi, Gi or Ti.

Confusing bits with bytes: A bit is a single unit with a value of 0 or 1. A byte is a group of 8 bits.
Confusing kB with KiB: Remember that kB uses kilo and equals 10^3 B, or 1000 B. KiB uses kibi and equals 2^10 B, or 1024 B. The symbols must be read exactly.

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