Bit

Also called: binary digit, bits

A binary digit: a single 0 or 1, the smallest unit of information a digital circuit stores. n bits can form 2ⁿ different patterns.

A bit (short for binary digit) is one 0 or 1. It's the smallest piece of information there is: the answer to a single yes/no question.

In hardware a bit is a wire or a storage cell that is either low (0) or high (1). Everything else is built from groups of bits:

4 bits make a nibble, which is one hex digit
8 bits make a byte
a number in a register is a fixed group of bits, such as 8, 16, 32 or 64

Each extra bit doubles the number of patterns, because every old pattern can be followed by a 0 or a 1. So n bits give 2ⁿ patterns. As unsigned numbers, those patterns cover 0 to 2ⁿ − 1.

Bits in a number are named by position. Bit 0 is the rightmost, the LSB. The leftmost is the MSB. A bit's weight is 2 raised to its position number.

A bit has no meaning on its own. 1 might mean the number one, "true", "light on", or the minus sign of a twos complement number. The format you choose decides.

Worked example

Example

How many patterns do 5 bits make?

Count the choices bit by bit.

1.
Each bit has 2 choices.
2.
5 bits: 2 × 2 × 2 × 2 × 2 = 2⁵ = 32 patterns.
3.
As unsigned numbers they run from 00000 (0) to 11111 (31).
4.
The count is 32 but the largest value is 31, because 0 uses up one pattern.

Common mistakes

Confusing the number of patterns (2ⁿ) with the largest unsigned value (2ⁿ − 1).
Thinking 8 bits can hold 256. The largest 8-bit unsigned value is 255.
Mixing up bits and bytes. A byte is 8 bits. Network speeds are usually quoted in bits, file sizes in bytes.

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Bit