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A bit field is a data structure that maps to one or more adjacent 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 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
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.
In C, native implementation-defined bit fields can be created using int, unsigned int, signed int, _Bool (in C99), _BitInt(N), unsigned _BitInt(N) (in C23) or other implementation-defined types. In C++, they can be created using any integral or enumeration type; most C compilers also allow this. 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 bit fields, or where the programmer wants 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.
// opaque and show enum Visibility : uint8_t {
NO = 0,
YES = 1
};
// line styles enum LineStyle : uint8_t {
SOLID = 1,
DOTTED = 2,
DASHED = 3
};
// Colors enum Color : uint8_t {
BLACK = 0,
RED = 0b001,
GREEN = 0b010,
BLUE = 0b100,
YELLOW = RED | GREEN, // 011
MAGENTA = RED | BLUE, // 101
CYAN = GREEN | BLUE, // 110
WHITE = RED | GREEN | BLUE // 111
};
const char* colors8 = {"Black", "Red", "Green", "Yellow", "Blue", "Magenta", "Cyan", "White"};
// bit field box properties struct BoxProps {
uint32_t opaque : 1;
uint32_t fill_color : 3;
maybe_unused
uint32_t filler_a : 4; // fill to 8 bits
uint32_t show_border : 1;
uint32_t border_color : 3;
uint32_t border_style : 2;
maybe_unused
uint8_t filler_b : 0; // fill to nearest byte (16 bits)
uint8_t width : 4; // Split a byte into 2 fields of 4 bits
uint8_t height : 4;
};
int main() {
BoxProps my_box = {
.opaque = YES,
.fill_color = RED,
.show_border = YES,
.border_color = BLUE,
.border_style = SOLID,
.width = 5,
.height = 10
};
}
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:
/**
// global state
uint8_t gameControllerStatus = 0;
// Sets the gameControllerStatus using OR
void onKeyPressed(uint8_t key) {
// Clears the gameControllerStatus using AND and ~ (binary NOT)
void onKeyReleased(uint8_t key) {
// Tests whether a bit is set using AND
uint8_t isPressed(uint8_t key) {
* 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.
*/
enum Key : uint8_t {
KEY_RIGHT = 0b00000001,
KEY_LEFT = 0b00000010,
KEY_DOWN = 0b00000100,
KEY_UP = 0b00001000,
KEY_START = 0b00010000,
KEY_SELECT = 0b00100000,
KEY_B = 0b01000000,
KEY_A = 0b10000000
};
gameControllerStatus |= key;
}
gameControllerStatus &= ~key;
}
return gameControllerStatus & key;
}
// Byte 0
const OPAQUE: u8 = 0b0000_0001;
const FILL_COLOR_MASK: u8 = 0b0000_1110;
// Byte 1
const SHOW_BORDER: u8 = 0b0000_0001;
const BORDER_COLOR_MASK: u8 = 0b0000_1110;
const BORDER_STYLE_MASK: u8 = 0b0011_0000;
// Byte 2
const WIDTH_MASK: u8 = 0b0000_1111;
const HEIGHT_MASK: u8 = 0b1111_0000;
}
impl BoxProps {
fn opaque(&self) -> bool {
self.raw[0] & Self::OPAQUE != 0
}
fn set_opaque(&mut self, v: bool) {
self.raw[0] = (self.raw[0] & !Self::OPAQUE) | (v as u8);
}
fn fill_color(&self) -> u8 {
(self.raw[0] & Self::FILL_COLOR_MASK) >> 1
}
fn set_fill_color(&mut self, v: u8) {
self.raw[0] =
(self.raw[0] & !Self::FILL_COLOR_MASK) | ((v & 0b111) << 1);
}
fn width(&self) -> u8 {
self.raw[2] & Self::WIDTH_MASK
}
fn height(&self) -> u8 {
(self.raw[2] & Self::HEIGHT_MASK) >> 4
}
}
| + 6502 status register |
| Carry flag |
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.
If the status-byte from a device is 0x67 and the 5th flag bit indicates data-ready. The mask-byte is 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 either of the following: (both are equivalent)
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.
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