Two's-Complement Arithmetic

In order to add signed numbers, simply perform the standard addition. If the numbers are correctly signed, the answer will come out with the correct sign.
In order to subtract signed numbers, take the two's-complement of the number being subtracted and add the two numbers.
When adding and subtracting be sure the answer does not exceed the range of the signed numbers.
(+127 to –128 for 8-bit numbers; +32,767 to –32,768 for 16-bit numbers)