int main()
{
    u_int8_t a = 0;
    int i = 0;

    for (i = 0; i < 8; i++) {
        fprintf(stderr, "%d..%d\n", i, a >> i & 1);
    }

    a &= ~(0b00000111 << 4);
    a |= (3 << 4); // writing 3

    for (i = 0; i < 8; i++) {
        fprintf(stderr, "%d..%d\n", i, a >> i & 1);
    }
    return(0);
}