Re: Problem in 2's compliment

Posted by WBahn on
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Problem-in-2-s-compliment-tp4036032p4036039.html

I don't understand what you mean by "any value of zero" or "like till not operation".

If you set x=0 and pass that through the Not gate, you will get all 1's (which is the two's complement representation of -1). If you then increment this, you will get all 0's (since the leading one is lost). But this is what you should get, since -(0) = 0.

But you don't need to implement arithmetic negation this way. Your ALU can selectively invert any combination of the inputs and outputs, right?

Look at the table of ALU control signals and the operations that result. One of those operations is -x and another is -y. Look at the effect of putting the control signals in those states and walk through what happens to your signal from input to output.