Question

I’m taking a Computer Systems class as a pre-req for my Masters and came across something I found fascinating and hard to see practical use of and that is “faking subtraction” and the fact that there doesn’t need to be a subtraction instruction.

Something like:

x – y

Can be written as:

x + (~y + 1)

Now, that’s all well and good but it seems like that is overly complicated for a simple subtraction, especially when you could just easily put “x – y”. Are there situations where it would be necessary to do this, or is it just something that CAN be done but isn’t.

Answer 1

The two’s complement implementation is done in hardware, so you do not need to implement them like that for builtin datatypes.

If you are making an n-bit integer arithmetic library, then you need to emulate the integer addition, subtraction, multiplication and division etc operations, in which case such a technique might be implemented to add the n-bit length numbers, but using the carry flag to do so is a better implementation in my opinion.