RAW Hazard Detection

Again, a RAW (Read After Write) data hazard refers to a situation where we refer to a result that has not yet been calculated, for example: 
   i1.  add   $2, $1, $3     # $2 = $1 + $3
   i2.  add   $4, $2, $3     # $4 = $2 + $3
RAW data hazards could happen at both EX and MEM stages.

EX Hazard
This kind of hazards requires forwarding for operations that use results during the EX stage. It occurs when

  • The current instruction being decoded is in the ID stage and the source register is in the IF/ID register and

  • The prior instruction is in the EX stage and the destination register is in the ID/EX register.

Therefore, a pair of hazard conditions are as follows: 
   1a.  IF/ID.Rs = ID/EX.Rw
   1b.  IF/ID.Rt = ID/EX.Rw
where “IF/ID.Rs” refers to the Rs register whose value is found in the pipeline register IF/ID.

Hazard of the above instruction sequence can be detected by Condition 1a.