Subtraction:

0 - 0 = 0, Transfer to next bit = 0
0 - 1 = 1, Transfer to next bit = 1
1 - 0 = 1, Transfer to next bit = 0
1 - 1 = 0, Transfer to next bit = 0

  Addition:

  0 + 0 = 0, Transfer to next bit = 0
  0 + 1 = 1, Transfer to next bit = 0
  1 + 0 = 1, Transfer to next bit = 0
  1 + 1 = 0, Transfer to next bit = 1

  1 + 1 + 1 = 1, Transfer to next bit = 1