binary    hex
0000      0
0001      1
0010      2
0011      3
...
1001      9
1010      a
1011      b
1100      c
1101      d
1110      e
1111      f

0xfeed0dad

1111 1110 1110 1101 0000 1101 1010 1101

1110100010

0011 1010 0010
3    a    2

6+7=3 with a carry

in binary:
0+0=0
0+1=1
1+0=1
1+1=0 with a carry


result(x[i],y[i]) = x[i] xor y[i]
carry(x[i],y[i]) = x[i] and y[i]
result(result(x[i],y[i]),carry(x[i-1],y[i-1]))
carry(result(x[i],y[i]),carry(x[i-1],y[i-1]))