index.md (746B)
1 +++ 2 title = 'Addition & subtraction with signed integers' 3 +++ 4 # Addition & subtraction with signed integers 5 Just like with base-10 numbers, but you carry if you have more than the binary version of 1. 6 7 To add (2’s complement): add bitwise, ignoring the leftmost carry-out. 8 9 To subtract (2’s complement): for X-Y, form 2’s complement of Y and then add. 10 11   12 13 An overflow happens when both terms have the same sign and the result has a different sign. 14 15 In 1’s complement, the carry-out can’t be ignored — if it’s 0, the result is correct; if it’s 1, a 1 has to be added to the result. 16 17 You can stick the same value into more bits by repeating the leftmost digit.