Bit field

A bit field assigns a selected group of bits within a stored value to a particular component or meaning. For a contiguous field, specify its width $n$, starting position $s$ counted from the least significant bit, and interpretation. In a pixel format, separate bit fields can hold red, green and blue component codes. The Linux frame-buffer interface describes such fields with explicit offset and length members.^[ref]

For a nonnegative integer word $W$, the unsigned value of the field isIn a language with the usual unsigned bitwise operations, this corresponds to shifting right by $s$ and applying bitwise AND with $2^n-1$.

For an original eight-bit example, let $W=181$, written 0xB5 or binary 10110101. Select three bits beginning at position 4, so the field consists of bits 6–4. Extraction givesThe field's bits are 011. To replace them by value 6, clear the field with mask 0x70, then insert 6 << 4. Restricted to eight-bit words, the calculation isThe other five bits are unchanged. The range $0\le v<2^n$ specifies what fits in an unsigned field.

A language-level bit-field declaration is a related abstraction, but its physical placement follows that language and implementation. In the C programming language, unsigned int code : 3; declares a three-bit field; C11 leaves allocation order within a storage unit implementation-defined and the storage unit's alignment unspecified.^[ref] Such a declaration alone therefore does not specify a portable external byte sequence. An explicit layout must also state the relevant endianness and storage rules.