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

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

#define GENERIC_PACK(val, pos, len) (((uint16_t)(val) & ((1U << (len)) - 1)) << (pos))

uint16_t pack_register(uint8_t mode, uint8_t speed, uint8_t status) {
    uint16_t reg = 0;
    reg |= GENERIC_PACK(mode,   0, 3); // Field at bit 0, 3 bits wide
    reg |= GENERIC_PACK(speed,  3, 5); // Field at bit 3, 5 bits wide
    reg |= GENERIC_PACK(status, 10, 6); // Field at bit 10, 6 bits wide
    return reg;
}

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;
}