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Data Hazards and Stalls

Data Hazards and Stalls. Data hazards and forwarding Data Hazards and Stalls. An Example. Assuming that register R2 = 10 initially. And the sub instruction will result in R2 = -20. See P&H Fig. 6.28 3 rd Ed or 4.52 4 th Ed.

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Data Hazards and Stalls

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  1. Data Hazards and Stalls • Data hazards and forwarding • Data Hazards and Stalls \course\cpeg323-08F\Topic6c-323.ppt

  2. An Example Assuming that register R2 = 10 initially. And the sub instruction will result in R2 = -20. \course\cpeg323-08F\Topic6c-323.ppt

  3. See P&H Fig. 6.28 3rd Ed or 4.52 4th Ed Pipelined dependencies in a five instruction sequence using simplified datapaths to show the dependencies. Note: only add and sw get the right value R2 = - 20 \course\cpeg323-08F\Topic6c-323.ppt

  4. An Example (cont’d) Hazard condition (between sub and and at the indicated stages): EX/MEM.RegisterRd = ID/EX.RegisterRs = $2 General 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 Question: can you identify which of the 4 conditions here captures the hazards condition between sub and or ? \course\cpeg323-08F\Topic6c-323.ppt

  5. Forwarding/Bypassing • ALU inputs can also from pipeline registers. • Extra multiplexors. • Hazard detection units. \course\cpeg323-08F\Topic6c-323.ppt

  6. The dependencies between the pipeline registers move forward in time, so it is possible to supply the inputs to the ALU needed by the and instruction and or instruction by forwarding the results found in the pipeline registers rather than stall. See P&H Fig. 6.29 3rd Ed or 4.53 4th Ed \course\cpeg323-08F\Topic6c-323.ppt

  7. See P&H Fig. 6.30 3rd Ed or 4.54 4th Ed The ALU and pipeline register before adding forwarding. \course\cpeg323-08F\Topic6c-323.ppt

  8. See P&H Fig. 6.30 3rd Ed or 4.54 4th Ed Forwarding control and the multiplexors are added \course\cpeg323-08F\Topic6c-323.ppt

  9. See P&H Fig. 6.31 3rd Ed or 4.55 4th Ed The control values for the forwarding multiplexors. \course\cpeg323-08F\Topic6c-323.ppt

  10. Forwarding Conditions 1. EX hazard: if(EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) ForwardA = 10 if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) ForwardB = 10 \course\cpeg323-08F\Topic6c-323.ppt

  11. Con’d 2. 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 \course\cpeg323-08F\Topic6c-323.ppt

  12. Note: Here both EX and MEM stages may have the exact hazard conditions. What should we do ? Priority in Forwarding Add $1, $1, $2 Add $1, $1, $3 Add $1, $1, $4 …. \course\cpeg323-08F\Topic6c-323.ppt

  13. Note: When both EX and MEM stages may have the exact hazard conditions. Priority in Forwarding 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 \course\cpeg323-08F\Topic6c-323.ppt

  14. The datapath modified to resolve hazards via forwarding. See P&H Fig. 6.32 3rd Ed or 4.56 4th Ed \course\cpeg323-08F\Topic6c-323.ppt

  15. The Snapshots of Our Example Through Forwarding \course\cpeg323-08F\Topic6c-323.ppt

  16. Some conventions • The bold lines are those active in a clock cycle. • The italicized register numbers in color with a red circle indicate a hazard. • The forwarding unit is highlighted by shading it when it is forwarding data to the ALU. • The … in the place of operands means that their identity is information not needed by the stage. \course\cpeg323-08F\Topic6c-323.ppt

  17. Clock cycles 3 of the instruction sequence. See P&H Fig. 6.14.11 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  18. Clock cycles 4 of the instruction sequence. See P&H Fig. 6.14.11 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  19. Clock cycles 5 of the instruction sequence. See P&H Fig. 6.14.12 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  20. Clock cycles 6 of the instruction sequence. See P&H Fig. 6.14.12 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  21. Hazards and Stalls \course\cpeg323-08F\Topic6c-323.ppt

  22. A pipelined sequence of instructions. See P&H Fig. 6.34 3rd Ed or 4.58 4th Ed \course\cpeg323-08F\Topic6c-323.ppt

  23. The way stalls are really inserted into the pipeline Bubble \course\cpeg323-08F\Topic6c-323.ppt

  24. See P&H Fig. 6.35 3rd Ed or 4.59 4th Ed The way stalls are really inserted into the pipeline (Another View) \course\cpeg323-08F\Topic6c-323.ppt

  25. Forwarding with Load and Stores • The problem • The solution \course\cpeg323-08F\Topic6c-323.ppt

  26. A Hazard Detection Unit In the ID stage, the following should be checked by the hazard detection unit, so it can insert the stall between aloadand its use. if (ID/EX.MemRead and ((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt = IF/ID.RegisterRt)) stall the pipeline \course\cpeg323-08F\Topic6c-323.ppt

  27. Pipelined control overview, showing the two multiplexors for forwarding, the hazard detection unit, and the forwarding unit. \course\cpeg323-08F\Topic6c-323.ppt

  28. Snapshots of the running example through the pipeline \course\cpeg323-08F\Topic6c-323.ppt

  29. Clock cycles 2 of the instruction sequence in the example. See P&H Fig. 6.14.14 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  30. Clock cycles 3 of the instruction sequence in the example. See P&H Fig. 6.14.14 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  31. Clock cycles 4 of the instruction sequence in the example. See P&H Fig. 6.14.15 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  32. Clock cycles 5 of the instruction sequence in the example. See P&H Fig. 6.14.15 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  33. Clock cycles 6 of the instruction sequence in the example. See P&H Fig. 6.14.16 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  34. Clock cycles 7 of the instruction sequence in the example. (note the forwarding of $4) See P&H Fig. 6.14.16 3rd Ed CD (For more practice section) \course\cpeg323-08F\Topic6c-323.ppt

  35. Branch Hazards and Handling • Branch hazards • Branch hazard handling methods • Static • Dynamic \course\cpeg323-08F\Topic6c-323.ppt

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