Arithmetic: overflow
	So far, we have always assumed that we have enough bits to represent a value.
	However, a computer must work with a finite number of bits.
	What happens if the result doesn't fit in the available bits?
		This is called overflow.
		Overflow occurs when the result of an operation is greater than the max possible value
			or less than the min value.
	In UB:
		carry	1	1	1	1
				1	0	1	1		11
				0	1	1	1		7
			result	0	0	1	0		2
	What should the result be?
	Overflow occurs in UB when there is a carry out of the msb.  (Value is too large for N bits.)
	What about 2C?
	In our example of 2C addition, there was a carry out of the msb, but the result was correct!
		carry	1	1	1	1
				1	0	1	1		-5
				0	1	1	1		7
			result	0	0	1	0		2
	In fact, whenever we add a positive and a negative number, overflow won't occur,
		because the result will be less than the magnitude of the larger value.
	When does overflow occur in 2C?
		carry	1	0	1	1
				1	0	1	1		-5
				1	0	0	1		-7
			result	0	1	0	0		4
		When the signs of the values are the same, and the sign of the result is different.
		Another way of detecting this is by the fact that the carry in to the msb is
			different from the carry out.