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Data Hazards

Data Hazards. 0x30 sub $6 $0 $1 0x34 add $7 $6 $1. Step 3, we calculate $0 - $1. Reg. 0x30 sub $6 $0 $1. IM. DM. Reg. 0x34 add $7 $6 $1. Reg. IM. DM. Reg. ! ! ! HAZARD ! ! ! OLD VALUE OF $6. Step 4, we calculate $1 + $6. Reg. 0x30 sub $6 $0 $1. IM. DM. Reg.

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Data Hazards

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  1. Data Hazards • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • ... • .. • .

  2. Step 3, we calculate $0 - $1 Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 add $7 $6 $1 Reg IM DM Reg ! ! ! HAZARD ! ! ! OLD VALUE OF $6

  3. Step 4, we calculate $1 + $6 Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 add $7 $6 $1 Reg IM DM Reg

  4. Step 5, Now we write $6 Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 add $7 $6 $1 Reg IM DM Reg HAZARD OVER NEW VALUE OF $6 WRITTEN TO REGFILE

  5. Step 2, nop Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 nop Reg IM DM Reg Reg IM DM Reg Reg IM DM Reg

  6. Step 3, nop Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 nop Reg IM DM Reg • 0x38 nop Reg IM DM Reg Reg IM DM Reg

  7. Step 4, add $7 $6 $1 Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 nop Reg IM DM Reg • 0x38 nop Reg IM DM Reg • 0x3C add $7 $6 $1 Reg IM DM Reg

  8. Step 5, new $6 used Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 nop Reg IM DM Reg • 0x38 nop Reg IM DM Reg • 0x3C add $7 $6 $1 Reg IM DM Reg

  9. Data Hazards • Read After Write • Solved by inserting “nop” operations (bubbles) • or by hardware “stalling” the pipe

  10. Cost? • + Hardware inexpensive • - Performance (throughput)

  11. Data Hazards sub $6 $0 $1 add $7 $6 $1 add $7 $7 $6 .. .

  12. We calculate $0 - $1 Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 add $7 $6 $1 Reg IM DM Reg ! ! ! HAZARD ! ! ! OLD VALUE OF $6

  13. We “forward” the new result Reg • 0x30 sub $6 $0 $1 IM DM Reg • 0x34 add $7 $6 $1 Reg IM DM Reg HAZARD SOLVED BY “FORWARDING”

  14. Zeroext. HERE IS THE RESULT Branch logic 0 A ALU 4 B + 31 + Sgn/Ze extend • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • > 0x38 add $7 $7 $1 • ... • ..

  15. Zeroext. WE NEED IT HERE Branch logic 0 A ALU 4 B + 31 + Sgn/Ze extend • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • > 0x38 add $7 $7 $1 • ... • ..

  16. sub $6 $0 $1 • add $7 $6 $1 0 A B DATA WORD • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • > 0x38 add $7 $7 $1 • ... • ..

  17. sub $6 $0 $1 • add $7 $6 $1 = 0 A B REG NR • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • > 0x38 add $7 $7 $1 • ... • ..

  18. Zeroext. Branch logic 0 A ALU 4 B + 31 + Sgn/Ze extend • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • 0x38 add $7 $7 $6 • > 0x3C sw $5 4($0)

  19. add $7 $7 $6 • add $7 $6 $1 • sub $6 $0 $1 = 0 A ALU B = • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • 0x38 add $7 $7 $6 • > 0x3C sw $5 4($0)

  20. add $7 $7 $6 • add $7 $6 $1 • sub $6 $0 $1 = = 0 A ALU B = = • 0x30 sub $6 $0 $1 • 0x34 add $7 $6 $1 • 0x38 add $7 $7 $6 • > 0x3C sw $5 4($0)

  21. Data Hazards sub $6 $0 $1 add $6 $6 $1 add $7 $6 $1 .. .

  22. add $7 $6 $1 • add $6 $6 $1 • sub $6 $0 $1 = = 0 A ALU B = = • 0x30 sub $6 $0 $1 • 0x34 add $6 $6 $1 • > 0x38 add $7 $6 $1 • ... • ..

  23. Performance • No “Bubbles” or “Stalls” • increased throughput • But did we change the critical path? • Topological sort • Static timing analysis

  24. Zeroext. = = Branch logic 0 A ALU 4 B + = = 31 + Sgn/Ze extend

  25. Forwarding Logic W Reg # DRD/DWT W >=1 = Reg # = & & DATA DATA

  26. Critical path ALU? Zeroext. = = Branch logic 0 A ALU 4 B + = = 31 + Sgn/Ze extend

  27. Critical path DATA MEMORY? Zeroext. = = Branch logic 0 A ALU 4 B + = = 31 + Sgn/Ze extend

  28. All problems solved? • NO, what will happen if...... • lw $6 $0($1) • add $4 $6 $1 • add $7 $6 $2

  29. OK, “DM” to “DE” Reg • 0x30 lw $6 $0($1) IM DM Reg Reg IM DM Reg • 0x34 add $4 $6 $1 • 0x38 add $7 $6 $2 Reg IM DM Reg

  30. “DM” to “EX” ? Reg • 0x30 lw $6 $0($1) IM DM Reg Reg IM DM Reg • 0x34 add $4 $6 $1 • 0x38 add $7 $6 $2 Reg IM DM Reg

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