Bitfields

Syntax

unsigned int <name> : <n(bit limit)>;

Examples

unsigned int FLAG : 1;

This means FLAG can have either 0 or 1 — just a single bit.

unsigned int COUNTER : 2;

This means COUNTER can have up to 2 bits, i.e., values from 0–3.

Bitfields help you save memory when you know a value only needs a limited range — super useful for hardware registers and protocols.

Why unsigned?

Because bitfields simulate hardware bits, which are always positive numbers.

Example: Controller Register

Turn on/off and control volume (up to 10):

struct ControllerRegister {
    unsigned int ON_OFF : 1;  // 1 bit
    unsigned int VOLUME : 4;  // 4 bits
};

• ON_OFF is 1 bit → either 0 or 1
• VOLUME is 4 bits → 2^4 = 16 values, but we just need up to 10, so we can modularize it

void toggle(struct ControllerRegister *ctlreg) {
    ctlreg->ON_OFF ^= ctlreg->ON_OFF;
}

void vol_change(struct ControllerRegister *ctlreg) {
    ctlreg->VOLUME = (ctlreg->VOLUME + 1) % 10;
}

This is a simple way to toggle and change volume.

• Total memory used for ControllerRegister = 5 bits only!

What happens if we ignore the bit limit?

void vol_change(struct ControllerRegister *ctlreg) {
    ctlreg->VOLUME = ctlreg->VOLUME + 1;
}

• If VOLUME exceeds 4 bits (max 15), it wraps automatically.
• Only the rightmost bits are stored; any extra left bits are ignored. But is crucial to do our wrapping implementation