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Appendix C

This appendix discusses the implementation of pipeline hazards detection and forwarding techniques in the MIPS R4000 architecture, including structural hazards, data hazards, and control hazards. It also explores the limitations of pipelining and the techniques used to mitigate hazards.

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Appendix C

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  1. Appendix C Pipeline implementation Pipeline hazards, detection and forwarding Multiple-cycle operations MIPS R4000 • CDA5155 Fall 2014, Peir / University of Florida

  2. Limits of Pipelining • Increasing the number of pipeline stages in a given logic block by a factor of n generally allows increasing clock speed & throughput by a factor of almost n. • Usually less than n because of overheads such as latches and balance of delay in each stage. • But, pipelining has a natural limit: • At least 1 layer of logic gates per pipeline stage! • Practical minimum is usally several gates (2-10). • Commercial designs are approaching this point!!

  3. Simple RISC Datapath

  4. Basic RISC Pipelining • Basic idea: • Each instruction spends 1 clock cycle in each of the 5 execution stages. • During 1 clock cycle, the pipeline can be processing (different stages of) 5 different instructions.

  5. Adding Pipeline Registers

  6. Pipeline Hazards • Hazards are circumstances which may lead to stalls (delays, “bubbles”) in the pipeline if not addressed. • Three major types: • Structural hazards: • Lack of HW resources to keep all instructions moving. • Data hazards • Data results of earlier instrs. not yet avail. when needed. • Control hazards • Control decisions resulting from earlier instrs. (branches) not yet made; don’t know which new instrs. to execute.

  7. Structural Hazard Example • Suppose you had a combined instruction+data memory with only 1 read port

  8. Hazards Produce “Bubbles”

  9. Another View

  10. Example Data Hazard

  11. Forwarding for Data Hazards

  12. Another Forwarding Example

  13. Three Types of Data Hazards • Let i be an earlier instruction, j a later one. • RAW (read after write) • j tries to read a value before i writes it • WAW (write after write) • i and j write to same place, but in the wrong order. • Only occurs if >1 pipeline stage can write. • WAR (write after read) • j writes a new value to a location before i has read the old one. • Only occurs if writes can happen before reads in pipeline.

  14. An Unavoidable Stall - Load

  15. Stalling for Load Dependent

  16. Data Hazard Prevention • A clever compiler can often reschedule instructions (code motion) to avoid a stall. • A simple example: • Original code: lw r2, 0(r4) add r1, r2, r3  Note: Stall happens here! lw r5, 4(r4) • Transformed code: lw r2, 0(r4) lw r5, 4(r4) add r1, r2, r3  No stall needed!

  17. MIPS Instruction Format

  18. 5-Stage Pipeline

  19. Operations of Pipe Stages

  20. Data Hazard Detection

  21. Hazard Detection Logic for Load • NOTE, The right part of the equ. should be IF/ID.IR (Fig. C.25) • Example: Detecting whether an instruction that has just been fetched needs to be stalled because of dependence from a preceding load.

  22. Forwarding Situations in MIPS • Same as Figure C.26

  23. Forwarding to The ALU • Provide multiple path to the input of the ALU

  24. ID/EX • EX/MEM • Control • IF/ID • Add • MEM/WB • Branch • Add • 4 • Shift • left 2 • Register File • Read Addr 1 • Instruction • Memory • Data • Memory • Read • Data 1 • Read Addr 2 • Read • Address • PC • Read • Data • Address • Write Addr • ALU • Read • Data 2 • Write Data • Write Data • ALU • cntrl • 16 • 32 • Sign • Extend • EX/MEM.RegisterRd • ID/EX.RegisterRt • Forward • Unit • MEM/WB.RegisterRd • ID/EX.RegisterRs Datapath with Forwarding Hardware • PCSrc

  25. ID/EX.MemRead • 0 • ID/EX.RegisterRt Adding the Hazard Hardware • PCSrc • Hazard • Unit • ID/EX • EX/MEM • 0 • IF/ID • 1 • Control • Add • MEM/WB • Branch • Add • 4 • Shift • left 2 • Read Addr 1 • Instruction • Memory • Data • Memory • Register • File • Read • Data 1 • Read Addr 2 • Read • Address • PC • Read • Data • Address • Write Addr • ALU • Read • Data 2 • Write Data • Write Data • ALU • cntrl • 16 • 32 • Sign • Extend • Forward • Unit

  26. Branch Hazard • Suppose the new PC value is not computed until the MEM stage. • Then we must stall 3 clocks after every branch!

  27. Early Branch Resolution • Branch resolution at ID stage

  28. Predict-Not-Taken • (Branch resolves in ID) • Same as Fig. C.12

  29. Delayed Branches • Machine code sequence: • Branch instruction • Delay slot instruction(s) • Post-branch instructions • Branch is taken (if taken) at this point • Same as Fig. C.13

  30. Filling the Branch-Delay Slot • For (b), (c) must no side-effect! •  Note, dynamic branch prediction will be covered in Chap. 3

  31. Multi-Cycle Execution • Figure C.33 The MIPS pipeline with three additional unpipelined, floating-point, functional units.

  32. Latency & Initiation Interval • Latency: • Extra delay cycles before result is available. • Initiation interval: • Minimum number of cycles before a new input can be given to that functional unit.

  33. Pipelined Multiple-FP Operations • Figure C.35 A pipeline that supports multiple outstanding FP operations.

  34. Pipelining FP Instructions • Notice instructions may complete out-of-order: • MULTD IF ID M1 M2 M3 M4 M5 M6 M7 ME WB • ADDD IF ID A1 A2 A3 A4 ME WB • LD IF ID EXME WB • SD IF ID EXME WB • Raises the possibility of WAW hazards, and structural hazards in MEM & WB stages. • Structural hazards may occur especially often with non-pipelined DIV unit. • Out-of-order completion impacts exception handling.

  35. Issues in Multi-Cycle Operations • Stall for RAW is longer and more frequent (Fig. C.37) • WAW is possible; WAR is not (why?) • Structural Hazard possible for non-pipelined unit • Multiple WBs are likely (Fig. C.38) • Handling hazards • At Issue (ID) stage: • Check structural hazards: functional unit, WB port • Check RAW hazards: Issue with forwarding • Check WAW hazards: Not issue to make sure write in order • Detect and stall instruction before MEM and WB stages • More uniform handling given in Chapter 3.

  36. Maintaining Precise Exception • Settle for imprecise exception • Buffer and complete in order • Require large buffers and comparators • History file, future file approaches • Software trap handling when exception occurs • Hybrid scheme: Issue when certain no exception for early instruction • All instructions before can be completed • No instructions after can be completed

  37. Real MIPS R4000 Pipeline • IF,IS - Instruction cache fetch, First & Second halves. • RF - Inst. decode, Register Fetch, hazard check… • EX - Execution (EA calc, ALU op, target calc…) • DF,DS - Data cache access, First & Second halves. • TC - Tag Check, did cache access hit? • Note, use data before resolving hit/miss. • WB - Write-Back for loads & register-register ops. •  Read through C.43 – C.51

  38. 2-Cycle Load Delay

  39. Branch Delay

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