3. Building a Datapath

3.8. Branch Instructions

• ex) beq $t0, $t1, immediate
  1. read operand
  2. compare → sub를 통해
  3. zero인지 확인
  4. jump
  5. PC+4+(imm*4) 계산
     1. sign extend
     2. shift left
     3. add to PC+4

3.9. Composing the Elements

• First-cut data path does an instruction in one clock cycle
• Use Multiplexers where alternate data sources are used for different instructions

3.10. R-Type/Load/Store Datapath

3.11. Full Datapath

4. A Simple Implementation Scheme

4.1. ALU Control

• ALU used for

  • Load/Store : F = add
  • Branch : F = subtract
  • R-type : F depends on funct field

  → opcode로 구분!

  ALU control	Function
  0000	AND
  0001	OR
  0010	add
  0110	subtract
  0111	set-on-less-than
  1101	NOR

• Assume 2-bit ALUOp derived from opcode

  • Combinational logic derives ALU control
  1. opcode → ALUOp
  2. ALUOp + funct → ALU control

  opcode	ALUOp	Operation	funct	ALU function	ALU control
  lw	00	load word	XXXXXX	add	0010
  sw	00	store word	XXXXXX	add	0010
  beq	01	branch equal	XXXXXX	subtract	0110
  R-type	10	add	100000	add	0010
  		subtract	100010	subtract	0110
  		AND	100100	AND	0000
  		OR	100101	OR	0001
  		set-on-less-than	101010	set-on-less-than	0111

• Truth table for ALU control

  ALUOp		funct field						ALU control
  ALUOp1	ALUOp0	F5	F4	F3	F2	F1	F0
  0	0	x	x	x	x	x	x	0010
  x	1	x	x	x	x	x	x	0110
  1	x	x	x	0	0	0	0	0010
  1	x	x	x	0	0	0	0	0010
  1	x	x	x	0	0	1	0	0110
  1	x	x	x	0	1	0	0	0000
  1	x	x	x	0	1	0	1	0001
  1	x	x	x	1	0	1	0	0111

4.2. The Main Control Unit

• Control signals derived from instruction