54. Pipeline Hazards

• Hazard is a dependency that breaks pipelining

  • Data hazard program needs a value that has not been computed yet

  • Control hazard program does not know which instruction is next

• Data hazard example:

  
       add  $t1, $t2, $t3   # IF ID                     EX ME WB-$t1 is set here
       addi $t4, $t1, 1     #    IF ID-$t1 is read here EX ME WB
  
  

• Solution 1: processor inserts delays

  
       add  $t1, $t2, $t3   # IF ID EX ME WB-$t1 is set here
       addi $t4, $t1, 1     #    IF XX XX XX ID-$t1 is read here EX ME WB
  
  

• Solution 2: processor reorders instructions to avoid data hazards