This document discusses data hazards and solutions in pipelining. There are three types of hazards in pipelining: structural hazards, data hazards, and control hazards. Data hazards include read after write (RAW), write after read (WAR), write after write (WAW), and read after read (RAR) hazards. Solutions to data hazards include stalling the pipeline, forwarding, and choosing instruction types and orders to minimize stalls. Forwarding can reduce or eliminate stalls by forwarding operands from instructions earlier in the pipeline to dependent instructions.

1. Topic:
Data Hazards & Solutions
APPENDIX A - PIPELINING 1

2. Introduction
1 What is Pipelining?
2 The Major Hurdle of Pipelining-
Structural Hazards
Data Hazards
Control Hazards
3.solution of hurdles in pipelining.
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3. What Is Pipelining
Laundry Example
A, B, C and D
each have one load of clothes
to wash, dry, and fold
Washer takes 30 minutes
Dryer takes 40 minutes
“Folder” takes 20 minutes
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A B C D

4. What Is Pipelining
Sequential laundry takes 6 hours for 4 loads
If they learned pipelining, how long would laundry take?
4
A
B
C
D
30 40 20 30 40 20 30 40 20 30 40 20
6 PM 7 8 9 10 11 Midnight
T
a
s
k
O
r
d
e
r
Time

5. What Is Pipelining
Start work ASAP
Pipelined laundry takes 3.5
hours for 4 loads
5
A
B
C
D
6 PM 7 8 9 10 11 Midnight
T
a
s
k
O
r
d
e
r
Time
30 40 40 40 40 20

6. Pipelining Lessons
Pipelining doesn’t help latency of
single task, it helps throughput of
entire workload
Multiple tasks operating
simultaneously
Time to “fill” pipeline and time to
“drain” it reduces speedup
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7. Pipeline Hurdles
Structural hazards
Data hazards
Control hazards
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8. Pipeline Hurdles
Definition
Structural hazard
◦two different instructions use same h/w in same cycle
Data hazard
◦two different instructions use same storage
◦must appear as if the instructions execute in correct order
Control hazard
◦one instruction affects which instruction is next
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9. Data Hazards
There are four types of Data Hazards
RAW (Read after Write)
WAR (Write after Read)
WAW (Write after Write)
RAR (Read after Read)
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10. Data Hazards
Read After Write (RAW)
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I: add r1,r2,r3
J: sub r4,r1,r3
I instruction
complete first
than j
instruction read

11. Data Hazards
Write After Read (WAR)
Write After Write (WAW)
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I: sub r4,r1,r3
J: add r1,r2,r3
I: sub r1,r4,r3
J: add r1,r2,r3

12. Data Hazards
12
Read After Read (RAR)
I: sub r4,r1,r3
J: add r2,r1,r5
•Only RAW effects in pipelining
•WAR,WAW,RAR do not effects in
pipelining

13. Data Hazards
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This is another
representation
of the stall.
LW R1, 0(R2) IF ID EX MEM WB
SUB R4, R1, R5 IF ID EX MEM WB
AND R6, R1, R7 IF ID EX MEM WB
OR R8, R1, R9 IF ID EX MEM WB

14. Example
ADD R3,R4,R5
SUB R1,R2,R3
DIV R1,R2,R3
ST R2,R4
MUL R4,R3,R2
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15. Solution
1. F D E M WB
2. F - - - D E M WB
3. F D E M WB
4. F D E M WB
5. F - - - D E M WB
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16. Example
1.ADD R3,R4,R5 > 2 clock cycle
2.SUB R1,R2,R3 > 4 clock cycle
3.DIV R1,R3,R2 > 5 clock cycle
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17. Solution
1. F D E E M WB
2. F - - - - D E E E E M WB
3. - - - - F D - - - E E E E E M WB
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18. Hazards solution
Forwarding:
To minimize or remove the stalls
Type of instruction:
Arithmetic> (add,sub,div,mul)
Movement> (load,store,move)
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19. Forwording
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1.ADD R3,R4,R5 > 2 clock cycle
2.SUB R1,R2,R3 > 4 clock cycle
3.DIV R1,R3,R2 > 5 clock cycle
4.LD R2,R1 > 3 clock cycle

20. Solution
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1. F D E E M WB
2. F - - D E E E E M WB
3. F - - - - D E E E E E M WB
4. F - - - - - D E E E M WB

21. THANK YOU
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