Arithmetic, gulp. Addition & Subtraction


Addition and subtraction using unsigned or two's complement



X = X + Y
except when we try to do (unsigned)

  1111 1111 +
  0000 0001
  _________ 

1 0000 0000

We have an overflow. 

In 2's comp it gets worse.

0111 1111 +    127
0000 0001        1
_________      ___

1000 0000     -128!