Need to stall for one cycle.

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Presentation transcript:

Need to stall for one cycle. Load-Use Data Hazard Need to stall for one cycle. Why??

Load-Use Hazard Detection Check when the 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

How to Stall the Pipeline Force control values in ID/EX register to 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

Stall/Bubble in the Pipeline Stall inserted here

Stall/Bubble in the Pipeline Or, more accurately…

Datapath with Hazard Detection