Operand forwarding
Operand forwarding (or data forwarding) is an optimization in pipelined CPUs to limit performance deficits which occur due to pipeline stalls.[1][2] 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.
| 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 |
| 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 |
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.
gollark: I should probably find a scale or something and start tracking that.
gollark: Like basically every other business where people are physically present which is considered nonessential, I guess.
gollark: Online classes for physical skills based around other people are probably a *bit* useful if done right, but not very.
gollark: I'm not entirely sure what you're saying there, but yes, I'm *intellectually* aware exercise is good and all, I just dislike actually doing any.
gollark: I know *intellectually* that exercise is important and very good and stuff, personally, but seemingly that's not enough to make me actually do anything.
See also
References
- "CMSC 411 Lecture 19, Pipelining Data Forwarding". University of Maryland Baltimore County Computer Science and Electrical Engineering Department. Retrieved 2020-01-22.
- "High performance computing, Notes of class 11". hpc.serc.iisc.ernet.in. September 2000. Archived from the original on 2013-12-27. Retrieved 2014-02-08.
External links
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