Chapter 4 Introduction

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Morgan Kaufmann Publishers 31 January 2013Data Hazards in ALU Instructionsn Consider this sequence:sub $2, $1,$3and $12,$2,$5or $13,$6,$2add $14,$2,$2sw $15,100($2)n We can resolve hazards with forwardingn How do we detect when to forward?§4.7 Data Hazards: Forwarding vs. StallingCSE 420 Chapter 4 — The Processor — 67Dependencies & ForwardingBlue lines show the problem;Red lines represent forwardingCSE 420 Chapter 4 — The Processor — 68
Morgan Kaufmann Publishers 31 January 2013Detecting the Need to Forwardn Pass register numbers along pipelinen e.g., ID/EX.RegisterRs = register number for Rssitting in ID/EX pipeline registern ALU operand register numbers in EX stageare given byn ID/EX.RegisterRs, ID/EX.RegisterRtn Data hazards when1a. EX/MEM.RegisterRd = ID/EX.RegisterRs1b. EX/MEM.RegisterRd = ID/EX.RegisterRt2a. MEM/WB.RegisterRd = ID/EX.RegisterRs2b. MEM/WB.RegisterRd = ID/EX.RegisterRtFwd fromEX/MEMpipeline regFwd fromMEM/WBpipeline regCSE 420 Chapter 4 — The Processor — 69Detecting the Need to Forwardn But only if forwarding instruction will writeto a register!n EX/MEM.RegWrite, MEM/WB.RegWriten And only if Rd for that instruction is not$zeron EX/MEM.RegisterRd ≠ 0,MEM/WB.RegisterRd ≠ 0CSE 420 Chapter 4 — The Processor — 70
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Chapter 4 Introduction

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