Control unit: ALU
	ALU operation selection: function depends on type of instruction
			R-type: function determined by function field from instruction
			Load/store: add to compute memory address
			Branch: subtract to compare operands
					(Main control)		(Instruction)
			Instruction		ALUOp		Function		ALU control
			lw, sw		00		xxxxxx		010	add
			beq		01		xxxxxx		110	subtract
			add		10		100000		010	add
			sub		10		100010		110	subtract
			and		10		100100		000	and
			or		10		100101		001	or
			slt		10		101010		111	set on less than
	Fig. 5.15 shows complete truth table for generating these ALU control bits