17. Extract the Nibble from an 8-bit Register

#include <stdio.h>

unsigned char extractNibble(unsigned char reg, int pos) {
    if(pos)
    {
        return reg >> 4;
    }
    else
    {
        return reg &= 0xF;
    }
}

int main() {
    unsigned char reg;
    int pos;
    scanf("%hhu %d", &reg, &pos);
    printf("%d", extractNibble(reg, pos));
    return 0;
}

If we need the upper nibble, right shift to remove the lower 4 bits. If we need the lower nibble, bitwise-and with a lower nibble mask.