Pipelined Datapath and Control

1. Pipelined Datapath and Control Lecture for CPSC 5155 Edward Bosworth, Ph.D. Computer Science Department Columbus State University

2. MIPS Pipelined Datapath §4.6 Pipelined Datapath and Control MEM Right-to-left flow leads to hazards WB Chapter 4 — The Processor — 2

3. Pipeline registers • Need registers between stages • To hold information produced in previous cycle Chapter 4 — The Processor — 3

4. The Pipeline Registers • IF/ID This provides an execution context for the ID (Instruction Decode and Register Fetch) stage of execution. • ID/EX This provides an execution context for the EX (Execute) phase of instruction execution. In particular, the discrete control signals generated by the control unit as a result of instruction decoding are stored here. • EX/MEM This provides an execution context for the MEM (Memory Access or R-Type Instruction Completion) phase of instruction execution. In addition , this register stores copies of the control signals required to complete both the MEM and WB phase of execution for this instruction. • MEM/WB This provides an execution context for the WB (Write Back) phase of instruction execution.

5. Pipeline Operation • Cycle-by-cycle flow of instructions through the pipelined datapath • “Single-clock-cycle” pipeline diagram • Shows pipeline usage in a single cycle • Highlight resources used • c.f. “multi-clock-cycle” diagram • Graph of operation over time • We’ll look at “single-clock-cycle” diagrams for load & store Chapter 4 — The Processor — 5

6. IF for Load, Store, … Chapter 4 — The Processor — 6

7. ID for Load, Store, … Chapter 4 — The Processor — 7

8. EX for Load Chapter 4 — The Processor — 8

9. MEM for Load Chapter 4 — The Processor — 9

10. WB for Load Wrongregisternumber Chapter 4 — The Processor — 10

11. Corrected Datapath for Load Chapter 4 — The Processor — 11

12. EX for Store Chapter 4 — The Processor — 12

13. MEM for Store Chapter 4 — The Processor — 13

14. WB for Store Chapter 4 — The Processor — 14

15. Multi-Cycle Pipeline Diagram • Form showing resource usage Chapter 4 — The Processor — 15

16. Multi-Cycle Pipeline Diagram • Traditional form Chapter 4 — The Processor — 16

17. Single-Cycle Pipeline Diagram • State of pipeline in a given cycle Chapter 4 — The Processor — 17

18. Pipelined Control (Simplified) Chapter 4 — The Processor — 18

19. Pipelined Control • Control signals derived from instruction • As in single-cycle implementation Chapter 4 — The Processor — 19

20. The Control Signals by Phase • Instruction Fetch There are no control signals specific to this stage. • Instruction Decode There are no instruction–specific control signals in this step. • Execute • There are three control signals associated with this step. • RegDst This selects which field, IR[20:16] or IR[15:11] will be used as theregister destination number for the Write Register in WB. The five bit value selected is written into EX/MEM and copied to MEM/WB. • ALUOp This is the two–bit selector of the ALU operation. • ALUSrc This discrete control signal selects the B input to the ALU.

21. Control Signals by Phase • Memory Access • There are three control signals associated with this step. • BranchThis indicates that a branch instruction is in this stage. • MemRead The ALU output is used as a memory address that is read.This is set by the LW instruction. • MemWrite The ALU output is used as a memory address, to whichthe contents of the specified register are written.This is set by the SW instruction. • Write Back • There are two control signals associated with this step. • MemToReg This selects either the ALU output or memory output tobe written back to the register file • RegWrite This causes the selected value to be written to the specified register.

22. Pipelined Control Chapter 4 — The Processor — 22

23. Size of the Pipeline Registers

24. Data Hazards in ALU Instructions • Consider this sequence: sub $2, $1,$3and $12,$2,$5or $13,$6,$2add $14,$2,$2sw $15,100($2) • We can resolve hazards with forwarding • How do we detect when to forward? §4.7 Data Hazards: Forwarding vs. Stalling Chapter 4 — The Processor — 24

25. Dependencies & Forwarding Chapter 4 — The Processor — 25

26. Detecting the Need to Forward • Pass register numbers along pipeline • e.g., ID/EX.RegisterRs = register number for Rs sitting in ID/EX pipeline register • ALU operand register numbers in EX stage are given by • ID/EX.RegisterRs, ID/EX.RegisterRt • Data hazards when 1a. EX/MEM.RegisterRd = ID/EX.RegisterRs 1b. EX/MEM.RegisterRd = ID/EX.RegisterRt 2a. MEM/WB.RegisterRd = ID/EX.RegisterRs 2b. MEM/WB.RegisterRd = ID/EX.RegisterRt Fwd fromEX/MEMpipeline reg Fwd fromMEM/WBpipeline reg Chapter 4 — The Processor — 26

27. Detecting the Need to Forward • But only if forwarding instruction will write to a register! • EX/MEM.RegWrite, MEM/WB.RegWrite • And only if Rd for that instruction is not $zero • EX/MEM.RegisterRd ≠ 0,MEM/WB.RegisterRd ≠ 0 Chapter 4 — The Processor — 27

28. Forwarding Paths Chapter 4 — The Processor — 28

29. Forwarding Conditions • EX hazard • if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs))ForwardA = 10 #Two bit control signal to MUX • if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt))ForwardB = 10 • MEM hazard • if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRs))ForwardA = 01 • if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRt))ForwardB = 01 Chapter 4 — The Processor — 29

30. Double Data Hazard • Consider the sequence: add $1,$1,$2add $1,$1,$3add $1,$1,$4 • Both hazards occur • Want to use the most recent • Revise MEM hazard condition • Only fwd if EX hazard condition isn’t true Chapter 4 — The Processor — 30

31. Revised Forwarding Condition • MEM hazard • if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0)and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01 • if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0)and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01 Chapter 4 — The Processor — 31

32. Datapath with Forwarding Chapter 4 — The Processor — 32

33. Load-Use Data Hazard Need to stall for one cycle Chapter 4 — The Processor — 33

34. Load-Use Hazard Detection • Check when using instruction is decoded in ID stage • ALU operand register numbers in ID stage are given by • IF/ID.RegisterRs, IF/ID.RegisterRt • Load-use hazard when • ID/EX.MemRead and ((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt = IF/ID.RegisterRt)) • If detected, stall and insert bubble Chapter 4 — The Processor — 34

35. How to Stall the Pipeline • Force control values in ID/EX registerto 0 • EX, MEM and WB do nop (no-operation) • Prevent update of PC and IF/ID register • Using instruction is decoded again • Following instruction is fetched again • 1-cycle stall allows MEM to read data for lw • Can subsequently forward to EX stage Chapter 4 — The Processor — 35

36. Stall/Bubble in the Pipeline Stall inserted here Chapter 4 — The Processor — 36

37. Stalls and Performance • Stalls reduce performance • But are required to get correct results • Compiler can arrange code to avoid hazards and stalls • Requires knowledge of the pipeline structure The BIG Picture Chapter 4 — The Processor — 37