Bit field

A bit field is a data structure used in computer programming. It consists of a number of adjacent computer memory locations which have been allocated to hold a sequence of bits, stored so that any single bit or group of bits within the set can be addressed. A bit field is most commonly used to represent integral types of known, fixed bit-width.

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 microprocessors and other logic devices, collections of bit fields called "flags" are commonly used to control or to indicate the intermediate state or outcome of particular operations. Microprocessors typically have a status register that is composed of such flags, used to indicate various post-operation conditions, for example an arithmetic overflow. The flags can be read and used to decide subsequent operations, such as in processing 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.

Implementation

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. In this case, the programmer can declare a structure for a bit field which labels and determines the width of several subfields. Adjacently declared bit fields of the same type can then be packed by the compiler into a reduced number of words, compared with the memory used if each 'field' were to be declared separately.

For languages lacking native bitfields, or where the programmer wants strict control over the resulting bit representation, it is possible to manually manipulate bits within a larger word type. In this case, the programmer can set, test, and change the bits in the field using combinations of masking and bitwise operations.

Examples

C programming language

Declaring a bit field in C

#include <stdio.h>

// opaque and show
#define YES 1
#define NO  0

// line styles
#define SOLID  1
#define DOTTED 2
#define DASHED 3

// primary colors
#define BLUE  4  /* 100 */
#define GREEN 2  /* 010 */
#define RED   1  /* 001 */

// mixed colors
#define BLACK   0                    /* 000 */
#define YELLOW  (RED | GREEN)        /* 011 */
#define MAGENTA (RED | BLUE)         /* 101 */
#define CYAN    (GREEN | BLUE)       /* 110 */
#define WHITE   (RED | GREEN | BLUE) /* 111 */

const char * colors[8] = {"Black", "Red", "Green", "Yellow", "Blue", "Magenta", "Cyan", "White"};

// bit field box properties
struct box_props
{
     unsigned int opaque       : 1;
     unsigned int fill_color   : 3;
     unsigned int              : 4; // fill to 8 bits
     unsigned int show_border  : 1;
     unsigned int border_color : 3;
     unsigned int border_style : 2;
     unsigned int              : 0; // fill to nearest byte (16 bits)
     unsigned char width       : 4, // Split a byte into 2 fields of 4 bits
                   height      : 4;
};

Example of emulating bit fields with a primitive and bit operators in C:

/* Each prepocessor directive defines a single bit */
#define KEY_UP       (1 << 0)  /* 000001 */
#define KEY_RIGHT    (1 << 1)  /* 000010 */
#define KEY_DOWN     (1 << 2)  /* 000100 */
#define KEY_LEFT     (1 << 3)  /* 001000 */
#define KEY_BUTTON1  (1 << 4)  /* 010000 */
#define KEY_BUTTON2  (1 << 5)  /* 100000 */

int gameControllerStatus = 0;

/* Sets the gameControllerStatus using OR */
void keyPressed(int key) {
     gameControllerStatus |= key;
}

/* Turns the key in gameControllerStatus off using AND and ~ (binary NOT)*/
void keyReleased(int key) {
    gameControllerStatus &= ~key;
}

/* Tests whether a bit is set using AND */
int isPressed(int key) {
    return gameControllerStatus & key;
}

Processor status register

A simple example of a bitfield status register is included in the design of the eight-bit 6502 processor. One eight-bit field held seven pieces of information:

• Bit 7. Negative flag
• Bit 6. Overflow flag
• Bit 5. Unused
• Bit 4. Break flag
• Bit 3. Decimal flag
• Bit 2. Interrupt-disable flag
• Bit 1. Carry flag
• Bit 0. Zero flag

Unix process exit code

Another example is the Unix exit status code, which can be used as a flag word to pass status information to another process. For example, a program which monitors the status of eight burglar alarm switches could set the bits in the exit code, passing along to another process information about which of the switches are closed or open.

Extracting bits from flag words

A subset of flags in a flag field may be extracted by ANDing with a mask, and in many languages, the shift operator (<<) can be also be used to determine the value of one or more bits.

Changing bits in flag words

Writing, reading or toggling bits in flags can be done only using the OR, AND and NOT operations - operations which can be performed quickly in the processor. To set a bit, OR the status byte with a mask byte. Any bits set in the mask byte or the status byte will be set in the result.

To toggle a bit, XOR the status byte and the mask byte. This will set a bit if it is cleared or clear a bit if it is set.

See also

• Bitboard, used in chess and similar games.
• Bit array
• Word (computer architecture)
• Flag word
• Mask (computing)
• Program status word
• Status register
• FLAGS register (computing)
• Control register

External links

• Explanation from a book
• Description from another wiki
• Use case in a C++ guide
• C++ libbit bit library (alternative URL)