I can't understand why the msb of the output == ng?
Say I have a 4 bit ALU, and I added 0111 to 1000, the result would be 1111.
If my calculation was 7+8=15, the msb is 1, and ng is also 1, but 15 is not less than 0.
I understand that in 2's complement, 1111 is actually -1, so it is less than 0, but that would mean that the ng output is only useful if you already expect to treat the answer as 2's complement?
I couldn't find help in chapter 2 or the lecture. What am I missing here?