Editing Two's complement (section)

===Subtraction===
Computers usually use the [[method of complements]] to implement subtraction. Using complements for subtraction is closely related to using complements for representing negative numbers, since the combination allows all signs of operands and results; direct subtraction works with two's-complement numbers as well. Like addition, the advantage of using two's complement is the elimination of examining the signs of the operands to determine whether addition or subtraction is needed. For example, subtracting −5 from 15 is really adding 5 to 15, but this is hidden by the two's-complement representation:
<pre style="width:25em">
  11110 000   (borrow)
   0000 1111  (15)
 − 1111 1011  (−5)
 ===========
   0001 0100  (20)
</pre>
Overflow is detected the same way as for addition, by examining the two leftmost (most significant) bits of the borrows; overflow has occurred if they are different.

Another example is a subtraction operation where the result is negative: 15&nbsp;−&nbsp;35 = −20:
<pre style="width:25em">
  11100 000   (borrow)
   0000 1111  (15)
 − 0010 0011  (35)
 ===========
   1110 1100  (−20)
</pre>
As for addition, overflow in subtraction may be avoided (or detected after the operation) by first sign-extending both inputs by an extra bit.