Arithmetic: R-type
	000000	01000	01001	01010	00000	100000
	b31-26	b25-21	b20-16	b15-11	b10-6	b5-0
	opcode	$rs	$rt	$rd	shamt	function
	add, sub
		add $rd, $rs, $rt    # R[d] <- R[s] + R[t]
		sub $rd, $rs, $rt    # R[d] <- R[s] - R[t]
		R-type: 3 registers
		add or subtract 2C values
		note that there is nothing to indicate whether the registers used
			actually contain values of the proper type
			unlike high-level languages, there is no type;
			the bitstrings are simply used as 2C
		opcode: 0
		function		add: 32
				sub: 34
	addu, subu
		Unsigned add or subtract
		Ignore overflow
		opcode: 0
		function		add: 33
				sub: 35