How does the CPU do subtraction?

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The whole reason we use 2’s-complement is that addition is the same whether the numbers are positive or negative – there are no special cases to consider, like there are with 1’s-complement or signed-magnitude representations.

So to find A-B, we can just negate B and add; that is, we find A + (-B), and because we’re using 2’s-complement, we don’t worry if (-B) is positive or negative, because the addition-algorithm works the same either way.

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