Pseudoinstructions: logical
	Instruction			Real instructions				Semantics
	not $rd, $rs			addi    $at, $0, -1				# R[1] = -1
				xor     $rd, $rs, $at				# R[d] = R[s] ^ R[1]
	Better way:
	not $rd, $rs			nor $rd, $rs, $0				# R[d] = ~R[s]
	Why does this work?
			a	b	a | b	~(a|b)	~(0|b)	~b
			0	0	0	1	1	1
			0	1	1	0	0	0
			1	0	1	0
			1	1	1	0