25. Pack Multiple Fields into a 16-bit Control Register

#include <stdio.h>
#include <stdint.h>

union Reg {
    struct {
        uint8_t mode : 3;
        uint8_t speed : 5;
        uint8_t reserved : 2;
        uint8_t status : 6;
    } f;
    uint16_t r;
};

uint16_t pack_register(uint8_t mode, uint8_t speed, uint8_t status) {
    Reg reg;
    reg.f.mode = mode & ((1 << 3) - 1);
    reg.f.speed = speed & ((1 << 5) - 1);
    reg.f.reserved = 0;
    reg.f.status = status & ((1 << 6) - 1);
    return reg.r;
}

int main() {
    uint8_t mode, speed, status;
    scanf("%hhu %hhu %hhu", &mode, &speed, &status);

    uint16_t reg = pack_register(mode, speed, status);
    printf("%u", reg);
    return 0;
}