Bit field

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A bit field is used in computer programming to store multiple, logical, neighboring bits, where each of the sets of bits, and single bits can be addressed. A bit field is most commonly used to represent integral types of known, fixed bit-width. A well-known usage of bit-fields is to represent a set of bits, and/or series of bits, known as flags.[citation needed] For example, the first bit in a bit field can be used to determine the state of a particular attribute associated with the bit field.

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

"A bit field is set up with a structure declaration that labels each field and determines its width."[1] In C and C++ bit fields can be created using unsigned int, signed int, or _Bool (in C99).

You can set, test, and change the bits in the field using a mask, bitwise operators, and the proper membership operator of a struct (. or ->). ORing a value will turn the bits on if they are not already on, and leave them unchanged if they are, e.g. bf.flag |= MASK; To turn a bit off, you can AND its reciprocal, e.g. bf->flag &= ~MASK; And finally you can toggle a bit (turn it on if it is off and off if it is on) with the XOR operator, e.g. (*bf).flag ^= MASK; To test a bit you can use an AND expression, e.g. (flag_set & MASK) ? true : false;

Having the value of a particular bit can be simply done by left shifting (≪) 0, n amount of times (or, x ≪ n - √x amount of times, where x is a power of 2), where n is the index of the bit you want (the right most bit being the start), e.g. if you want the value of the 4th bit in a binary number, you can do: 0 << 4; which will yield 8, or, 1 << 3; or, 2 << 2; etc. The benefits of this become apparent when iterating through a series of bits one at a time in a for loop, or when needing the powers of large numbers to check high bits.

If a language doesn't support bit fields, but supports bit manipulation, you can do something very similar. Since a bit field is just a group of neighboring bits, and so is any other primitive data type, you can substitute the bit field for a primitive, or array of primitives. For example, with a 32 bit integer represented as 32 contiguous bits, you could use it the same way as a bit field with one difference; with a bitfield you can represent a particular bit or set of bits using its named member, and a flag whose value is between 0, and 2 to the nth power, where n is the length of the bits.

Examples

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              : 2; // fill to 16 bits
};

[1]

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

/* Each preprocessor 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 ~ */
void keyReleased(int key) {
    gameControllerStatus &= ~key;
}
 
/* Tests whether a bit is set using AND */
int isPressed(int key) {
    return gameControllerStatus & key;
}

See also

References

  1. ↑ 1.0 1.1 Prata, Stephen (2007). C primer plus (5th ed.). Indianapolis, Ind: Sams. ISBN 0-672-32696-5. 

External links

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