Operand forwarding

Operand forwarding (or data forwarding, register bypass) is an optimization in pipelined CPUs to limit performance deficits which occur due to pipeline stalls caused by data hazards. A data hazard can lead to a pipeline stall when the current operation has to wait for the results of an earlier operation which has not yet finished.

It is very common that an instruction requires a value computed by the immediately preceding instruction. It may take a few clock cycles to write a result to the register file and then read it back for the subsequent instruction. To improve performance, the register file write/read is bypassed. The result of an instruction is forwarded directly to the execute stage of a subsequent instruction.

Example

ADD A B C #A=B+C SUB D C A #D=C-A

If these two assembly pseudocode instructions run in a pipeline, after fetching and decoding the second instruction, the pipeline stalls, waiting until the result of the addition is written and read.

+ Without operand forwarding 1 2 3 4 5 6 7 8 Fetch ADD Decode ADD Read Operands ADD Execute ADD Write result Fetch SUB Decode SUB stall stall Read Operands SUB Execute SUB Write result

+ With operand forwarding 1 2 3 4 5 6 7 Fetch ADD Decode ADD Read Operands ADD Execute ADD Write result Fetch SUB Decode SUB stall Read Operands SUB: use result from previous operation Execute SUB Write result

In some cases all stalls from such read-after-write data hazards can be completely eliminated by operand forwarding:Gurpur M. Prabhu. "Computer Architecture Tutorial". Sections "Forwarding". and "Data Hazard Classification".Dr. Orion Lawlor. "Pipelining, Pipeline Stalls, and Operand Forwarding".Larry Snyder. "Pipeline Review".

+ With operand forwarding (enhanced) 1 2 3 4 5 6 Fetch ADD Decode ADD Read Operands ADD Execute ADD Write result Fetch SUB Decode SUB Read Operands SUB: use result from previous operation Execute SUB Write result

Technical realization The CPU control unit must implement logic to detect dependencies where operand forwarding makes sense. A multiplexer can then be used to select the proper register or flip-flop to read the operand from.

See also Feed forward (control)

References

External links Introduction to Pipelining