Adder
	In order to construct a CPU, we need to perform arithmetic and logical operations.
	Basic arithmetic operator: addition
	What's involved in adding binary numbers?
		carry	0	1	1	0			decimal
				0	0	1	1		3
				0	1	1	0		6
			result	1	0	0	1		9
	In each column:
		input: add 2 bits, along with a carry bit from the previous result
		output: 1 bit result, 1 bit carry
	Half-adder
		adds 2 bits (x, y), generates sum (s) and carry (c)
		(note that previous carry is being ignored)
		Truth table						Boolean expressions:
			x	y	s	c
			0	0	0	0		s = \xy + x\y = x XOR y
			0	1	1	0		c = xy
			1	0	1	0
			1	1	0	1