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WAR

``Write after read'': The register must only be written to after previous instructions read it:

DIVF R1, R2, R3  IF ID EX EX EX EX EX MEM WB        
STORE R6, A    IF ID stall stall stall EX MEM WB        
SUBF R6, R4, R5      IF ID EX MEM WB            
                 $^\uparrow$?R6 read here?    
               $^\uparrow$R6 written here      
l|@ @*13p0.9em DIVF R1, R2, R3 & IF & ID & EX & EX & EX & EX & EX & MEM& WB
STORE R6, A & & IF & ID & stall& stall& stall& EX & MEM& WB
SUBF R6, R4, R5 & & & IF & ID & EX & MEM& WB
& & & & & & & & $^\uparrow$?R6 read here?

6in
DIVF R1, R2, R3  IF ID EX EX EX EX EX MEM WB        
STORE R6, A    IF ID stall stall stall EX MEM WB        
SUBF R6, R4, R5      IF ID EX MEM WB            
                 $^\uparrow$?R6 read here?    
               $^\uparrow$R6 written here      
l|@ @*13p0.9em DIVF R1, R2, R3 & IF & ID & EX & EX & EX & EX & EX & MEM& WB
STORE R6, A & & IF & ID & stall& stall& stall& EX & MEM& WB
SUBF R6, R4, R5 & & & IF & ID & EX & MEM& WB
& & & & & & & & $^\uparrow$?R6 read here?
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Next: WAW Up: Pipelining With Multicycle Operations Previous: NEW DATA HAZARDS