Unfortunately, not all data hazards can be solved by forwarding. For example, load has a delay that cannot be eliminated by forwarding. In the example below, the
lw instruction does not have data until end of CC4 and the and instruction wants data at beginning of CC4, which is NOT possible.
Hence, this combination must result in a stall by the hazard detection unit.
| The hazard detection unit operates during the ID stage so that it can insert the stall between the load and its use. |
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A RAW hazard after a load instruction occurs when:
- The load instruction will be in the ID/EX register.
- Instruction that needs the load data will be in the IF/ID register.
if ( (ID/EX.MemRead == 1) and (ID/EX.Rw ≠ 0) and
( (ID/EX.Rw == IF/ID.Rs) or (ID/EX.Rw == IF/ID.Rt) ) )
Stall
Once a hazard is detected, freeze the PC and the IF/ID registers.
No new instruction is fetched and instruction after load is stalled by one cycle by inserting a bubble into the ID/EX register.
| The load instruction in ID/EX register is allowed to proceed. It then forwards data to next instruction after delaying it. |
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