Bit field

Declaring a bit field in C and C++:[6]

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

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

// primary colors
#define BLUE  0b100
#define GREEN 0b010
#define RED   0b001

// 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 BoxProps
{
  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 char              : 0; // fill to nearest byte (16 bits)
  unsigned char width        : 4, // Split a byte into 2 fields of 4 bits
                height       : 4;
};

The layout of bit fields in a C struct is implementation-defined. For behavior that remains predictable across compilers, it may be preferable to emulate bit fields with a primitive and bit operators:

/* Each of these preprocessor directives defines a single bit,
   corresponding to one button on the controller.  
   Button order matches that of the Nintendo Entertainment System. */
#define KEY_RIGHT  0b00000001
#define KEY_LEFT   0b00000010
#define KEY_DOWN   0b00000100
#define KEY_UP     0b00001000
#define KEY_START  0b00010000
#define KEY_SELECT 0b00100000
#define KEY_B      0b01000000
#define KEY_A      0b10000000

unsigned char gameControllerStatus = 0;

/* Sets the gameControllerStatus using OR */
void KeyPressed(unsigned char key) 
{ 
  gameControllerStatus |= key; 
}

/* Clears the gameControllerStatus using AND and ~ (binary NOT)*/
void KeyReleased(unsigned char key) 
{ 
  gameControllerStatus &= ~key; 
}

/* Tests whether a bit is set using AND */
unsigned char IsPressed(unsigned char key) 
{ 
  return gameControllerStatus & key; 
}

The status register of a processor is a bit field consisting of several flag bits. Each flag bit describes information about the processor's current state.[8] As an example, the status register of the 6502 processor is shown below:

These bits are set by the processor following the result of an operation. Certain bits (such as the Carry, Interrupt-disable, and Decimal flags) may be explicitly controlled using set and clear instructions. Additionally, branching instructions are also defined to alter execution based on the current state of a flag.

For an instance, after an ADC (Add with Carry) instruction, the BVS (Branch on oVerflow Set) instruction may be used to jump based on whether the overflow flag was set by the processor following the result of the addition instruction.

A subset of flags in a flag field may be extracted by ANDing with a mask. A large number of languages support the shift operator (<<) where 1 << n aligns a single bit to the nth position. Most also support the use of the AND operator (&) to isolate the value of one or more bits.

If the status-byte from a device is 0x67 and the 5th flag bit indicates data-ready. The mask-byte is ${\displaystyle 2^{5}=0x20}$. ANDing the status-byte 0x67 (0110 0111 in binary) with the mask-byte 0x20(0010 0000 in binary) evaluates to 0x20. This means the flag bit is set i.e., the device has data ready. If the flag-bit had not been set, this would have evaluated to 0 i.e., there is no data available from the device.

To check the nth bit from a variable v, perform the operation:

bool nth_is_set = (v & (1 << n)) != 0;
bool nth_is_set = (v >> n) & 1;

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.