Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Operand forwarding
open-in-new
<h2 id="example">Example</h2> ADD A B C #A=B+C SUB D C A #D=C-A <p>If these two <a href="/facts/Assembly_language/7XuV0cla">assembly</a> 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. </p> Without operand forwarding<table><tbody><tr><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th></tr><tr><td>Fetch ADD</td><td>Decode ADD</td><td>Read Operands ADD</td><td>Execute ADD</td><td>Write result</td><td></td><td></td><td></td></tr><tr><td></td><td>Fetch SUB</td><td>Decode SUB</td><td><i>stall</i></td><td><i>stall</i></td><td>Read Operands SUB</td><td>Execute SUB</td><td>Write result</td></tr></tbody></table> With operand forwarding<table><tbody><tr><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th></tr><tr><td>Fetch ADD</td><td>Decode ADD</td><td>Read Operands ADD</td><td>Execute ADD</td><td>Write result</td><td></td><td></td></tr><tr><td></td><td>Fetch SUB</td><td>Decode SUB</td><td><i>stall</i></td><td>Read Operands SUB: use result from previous operation</td><td>Execute SUB</td><td>Write result</td></tr></tbody></table> <p>In some cases all stalls from such read-after-write data hazards can be completely eliminated by operand forwarding:<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> With operand forwarding (enhanced)<table><tbody><tr><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th></tr><tr><td>Fetch ADD</td><td>Decode ADD</td><td>Read Operands ADD</td><td>Execute ADD</td><td>Write result</td><td></td></tr><tr><td></td><td>Fetch SUB</td><td>Decode SUB</td><td>Read Operands SUB: use result from previous operation</td><td>Execute SUB</td><td>Write result</td></tr></tbody></table> <h2 id="technical-realization">Technical realization</h2> <p>The CPU <a href="/facts/Control_unit/8QRX3lwf">control unit</a> must implement logic to detect dependencies where operand forwarding makes sense. A <a href="/facts/Multiplexer/lVro0Sfz">multiplexer</a> can then be used to select the proper <a href="/facts/Processor_register/32RvbUdz">register</a> or <a href="/facts/Flip-flop_(electronics)/vA99xjM7">flip-flop</a> to read the operand from. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Feed_forward_(control)/N2lyuRNm">Feed forward (control)</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://www.cs.uaf.edu/2010/fall/cs441/lecture/09_16_pipelining.html">Introduction to Pipelining</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"CMSC 411 Lecture 19, Pipelining Data Forwarding". University of Maryland Baltimore County Computer Science and Electrical Engineering Department. Retrieved 2020-01-22. <a href="http://www.csee.umbc.edu/~squire/cs411_l19.html" target="_blank">http://www.csee.umbc.edu/~squire/cs411_l19.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"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. <a href="https://web.archive.org/web/20131227033204/http://hpc.serc.iisc.ernet.in/~govind/hpc/L10-Pipeline.txt" target="_blank">https://web.archive.org/web/20131227033204/http://hpc.serc.iisc.ernet.in/~govind/hpc/L10-Pipeline.txt</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Gurpur M. Prabhu. "Computer Architecture Tutorial". Sections "Forwarding". and "Data Hazard Classification". <a href="https://web.cs.iastate.edu/~prabhu/Tutorial/PIPELINE/forward.html" target="_blank">https://web.cs.iastate.edu/~prabhu/Tutorial/PIPELINE/forward.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Dr. Orion Lawlor. "Pipelining, Pipeline Stalls, and Operand Forwarding". <a href="https://www.cs.uaf.edu/2011/fall/cs441/lecture/09_20_pipelining.html" target="_blank">https://www.cs.uaf.edu/2011/fall/cs441/lecture/09_20_pipelining.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Larry Snyder. "Pipeline Review". <a href="https://courses.cs.washington.edu/courses/cse378/09au/lectures/cse378au09-15.pdf" target="_blank">https://courses.cs.washington.edu/courses/cse378/09au/lectures/cse378au09-15.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> </ol>