Here i explain the concept of Data Hazard and Solution for Data Hazard

1. In the name ofAllah who is most beneficial and most merciful

2. Rules
•You can ask question after completion of topics.
•Your questions should be according to topic.
•Any misbehave during presentation would lead you to
some serious actions like asked to leave the class room.

3. Topic Outline
• What is Pipelining?
• Types of Pipelining.
• What is Data Hazard?
• Types of Data Hazard.
• Solutions for Data Hazards.

4. Pipelining
• A technique used in advanced microprocessors where
the microprocessor begins executing a second
instruction before the first has been completed.
• That is, several instructions are in the pipeline
simultaneously, each at a different processing stage.

5. Types of Pipelining
CONTROL
HAZARD
DATA
HAZARD
STRUCTURE
HAZARD
Pipelining

6. • A situation that prevent the starting next instruction in the next cycle
• They reduce speedup of pipeline execution
• Hazards can always be resolved by waiting
HAZARD

7. • Attempt to use item before it is ready
• Instruction depends on result of prior instruction still in the pipeline
DATA HAZARD
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8. Any Questions???

9. Types of Data Hazard
RAW WAR WAW
DATA
HAZARD

10. RAW(Read AfterWrite)
• It is also known asTRUE DEPENDENCY .
• An instruction modifies a register or memory location
• Succeeding instruction reads data in that location
• Hazard if read takes place before write complete

11. Write After Read (RAW)
• It is also known as Anti dependency
• An instruction reads a register or memory location
• Succeeding instruction writes to location
• Hazard if write completes before read takes place

12. Write AfterWrite (RAW)
•Two instructions both write to same location
•Hazard if writes take place in reverse of order intended
sequence

13. Any Questions???

14. Problem with Data Hazard
Problem

15. • All the instructions after the ADD use the result of the ADD instruction (in R1).The ADD
instruction writes the value of R1 in theWB stage .
• SUB instruction reads the value during ID stage (IDsub).This problem is called a data hazard

16. Two instruction with add and Sub using three register
$sO(in addition) and $t2(in subtraction) =Destination Register
$tO , $t1 and $t3 are Read /Write Register
While $sO is read/write register in Subtraction instruction
Data hazard occur at this stage

17. Solution of Data Hazard
Solution

18. Data Hazards – Forwarding
• Key idea: connect new value directly to next stage
• Still read s0, but ignore in favor of new result

19. Forwarding-Data HazardA instruction goes into four
state
Let two instruction
Add $sO, $tO , $t1
Sub $t2, $sO ,$t3
How to avoid data hazard?
$sO
register
If we use this path as shown by
arrow then we left the $sO
register for instruction SUB

20. Solution
• Usually solved by data or register forwarding (bypassing or short-circuiting).
This is based on the fact that the data selected is not really used in ID but in
the next stage: ALU.
• Forwarding works as follows: ALU result from EX/MEM buffer is always fed
back to the ALU input keys. If the forwarding hardware detects that its
source operand has a new value, the logic selects the newer result rather
than the value read from the register file.

21. Any Question???