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Avoiding Stalls By Scheduling Instructions

Consider the common high-level language statement 

  a := b + c

We get

LW R1,B  IF ID EX MEM WB                
LW R2,C    IF ID EX MEM WB              
ADD R3,R1,R2      IF ID stall EX MEM WB          
SW, A,R3        IF stall ID EX MEM WB        


The stall seems inevitable. However, consider the sequence 
  a := b + c
  d := e - f


 
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