Operand forwarding

Operand forwarding is an optimization in pipelined CPUs to limit performance deficits which occur due to Pipeline stalls. 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.

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			Read Operands SUB	Execute SUB	Write result

With operand forwarding

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.

External links

• Data Hazards
• Introduction to Pipelining