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Progress Update on Jumps, Branches, and Forwarding Unit Development

In Week 9, significant strides were made in completing jumps and branches for our project. We focused on implementing three types of jumps, ensuring the correct values were registered. The complexity of branches required the creation of a dedicated Branch Unit for zero checks and address updates. Comprehensive testing revealed timing issues and control signal adjustments. Our forwarding unit was completed with added logic for accurate value prioritization. We are nearing project completion, with only advanced hazard management and testing remaining to address.

eleanor-rowe
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Progress Update on Jumps, Branches, and Forwarding Unit Development

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  1. Group 1 – Week 9 Almost done…

  2. Jumps and Branches • Worked on completeing both the jumps and branches • With jumps had to be concerned with three different types: • Regular jumps fairly straight forward. • Jump and Links had to make sure that the right values were written to the right register • Jump Register had to add another control signal • Branches were more complicated • Created a Branch Unit which did the check for zero and updated the branch address. • Through testing discovered the several different problems with pushing it to the ID stage • Still trying to determine if all the timing issues have been worked out

  3. Jump and Link Unit

  4. General Testing • Ran through a large array of tests trying to find small bugs • Clock signal needed to be inverted • Had the wrong control signal input into data hazard • Control signal had to be reversed so that the processor wouldn't crash on restart • Determine how to get stalling to work properly • Now space is becoming an issue

  5. Forwarding & Hazards • Finished forwarding unit according to book • Noticed the book doesn’t forward from WB back to MEM (Load then Store), added that logic too • Prioritized so that we always forward furthest back in the pipeline • MEM wins over WB, its value is more accurate • Bug found: don’t forget to take FPR / GPR into account • Used hazard equation in book, pretty simple

  6. Tests to make sure both Rs & Rd are FPR or both GPR top right • Tests for zeros at bottom right • Register address comparisons at center, outputs at top • Additional Equations: • If Wb.GPR Write && MEM.Write && (Wb.Rd != 0) && (Wb.Rd == Ex.Rd)

  7. Hazards & Forwarding for Branches • Pushing up branch comparison to ID creates problems • Our forwarding unit only forwards back to the EX input muxes • Had to create new forwarding unit from MEM & WB back to ID • Data hazard when branch Rs in ID stage == Rd for any instruction in EX • Added to stall unit

  8. This unit is simpler than the other forwarding unit. • - branches only use Rs, not Rt • - we never use FPR’s here, no need to test both signals • only one mux to control, not two • Equations: • If (ID.Rs == Mem.Rd) && (Mem.Rd !=0) Mem.GPRWrite && ID.Branch • If (ID.Rs == WB.Rd) && (WB.Rd !=0) && (Wb.Rd != Mem.Rd) && WB.GPRWrite && ID.Branch

  9. Next Steps • Finish! • Hardware • Everything is done except Trap & RFE units • Have control signals for this already • Testing • Only complicated hazards, stalls, and forwarding situations to go

  10. Not catching exceptions = bad

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