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Concept
Bit field
Summary
A bit field is a data structure that consists of one or more adjacent bits which have been allocated for specific purposes, so that any single bit or group of bits within the structure can be set or inspected. A bit field is most commonly used to represent integral types of known, fixed bit-width, such as single-bit Booleans.
The meaning of the individual bits within the field is determined by the programmer; for example, the first bit in a bit field (located at the field's base address) is sometimes used to determine the state of a particular attribute associated with the bit field.
Within CPUs and other logic devices, collections of bit fields called flags are commonly used to control or to indicate the outcome of particular operations. Processors have a status register that is composed of flags. For example if the result of an addition cannot be represented in the destination an arithmetic overflow is set. The flags can be used to decide subsequent operations, such as conditional jump instructions. For example, a JE ... (Jump if Equal) instruction in the x86 assembly language will result in a jump if the Z (zero) flag was set by some previous operation.
A bit field is distinguished from a bit array in that the latter is used to store a large set of bits indexed by integers and is often wider than any integral type supported by the language. Bit fields, on the other hand, typically fit within a machine word, and the denotation of bits is independent of their numerical index.
Bit fields can be used to reduce memory consumption when a program requires a number of integer variables which always will have low values. For example, in many systems storing an integer value requires two bytes (16-bits) of memory; sometimes the values to be stored actually need only one or two bits. Having a number of these tiny variables share a bit field allows efficient packaging of data in the memory.
In C and C++, native implementation-defined bit fields can be created using unsigned int, signed int, or (in C99) _Bool.
Official source
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