# Data Hazards and Stalls

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Data Hazards and Stalls. Data hazards and forwarding Data Hazards and Stalls. An Example. Assuming that register R2 = 10 initially. And the sub instruction will result in R2 = -20. Pipelined dependencies in a five instruction sequence using simplified datapaths to show the dependencies. - PowerPoint PPT Presentation

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Data Hazards and Stalls
Data hazards and forwarding Data Hazards and Stalls

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An Example
Assuming that register R2 = 10 initially. And the sub instruction will result in R2 = -20.

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Sheet1

sub\$2, \$1, \$3# Register \$2 written by sub

and\$12, \$2, \$5# 1st operand (\$2) depends on sub

or\$13, \$6, \$2# 2nd operand (\$2) depend on sub

add\$14, \$2, \$2# 1st (\$2) & 2nd (\$2) depend on sub

sw\$15, 100(\$2)# Index (\$2) depends on sub

Sheet2

Sheet3

Pipelined dependencies in a five instruction sequence using simplified datapaths to show the dependencies.
Note: only add and sw get the right value R2 = - 20

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An Example (contd)
Hazard condition (between sub and and at the indicated stages):

EX/MEM.RegisterRd = ID/EX.RegisterRs = \$2

General
1a.EX/MEM.RegisterRd = ID/EX.RegisterRs1b.EX/MEM.RegisterRd = ID/EX.RegisterRt2a.MEM/WB.RegisterRd = ID/EX.RegisterRs2b.MEM/WB.RegisterRd = ID/EX.RegisterRt
Question: can you identify which of the 4 conditionsHere captures the hazards condition between sub and or ?

\course\cpeg323-05F\Topic6c-323.ppt

Sheet1

sub\$2, \$1, \$3# Register \$2 written by sub

and\$12, \$2, \$5# 1st operand (\$2) depends on sub

or\$13, \$6, \$2# 2nd operand (\$2) depend on sub

add\$14, \$2, \$2# 1st (\$2) & 2nd (\$2) depend on sub

sw\$15, 100(\$2)# Index (\$2) depends on sub

Sheet2

Sheet3

Forwarding/Bypassing
ALUinputs can also from pipeline registers

Extra multiplexors

Hazard detection units

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The dependencies between the pipeline registers move forward in time, so it is possible to supply the inputs to the ALU needed by the and instruction and or instruction by forwarding the results found in the pipeline registers rather than stall.

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the ALU and pipeline registerbefore adding forwarding.
a. No forwarding

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Forwarding control and the multiplexors are added
b. With forwarding

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The control values for the forwarding multiplexors.

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Forwarding Conditions
1. EX hazard:

if(EX/MEM.RegWriteand (EX/MEM.RegisterRd =/= 0)and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) ForwardA = 10

if (EX/MEM.RegWriteand (EX/MEM.RegisterRd =/= 0)and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) ForwardB = 10

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2. MEM hazard:

if(MEM/WB.RegWriteand (MEM/WB.RegisterRd =/= 0)and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01

if(MEM/WB.RegWriteand (MEM/WB.RegisterRd =/= 0)and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01
Cond

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Note:here both EX and MEM stages may have the exact hazard conditions. What should we do ?
Add \$1, \$1, \$2Add \$1, \$1, \$3Add \$1, \$1, \$4.
Priority in Forwarding

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Note:When both EX and MEM stages may have the exact hazard conditions.
If (MEM/WB.RegWriteand (MEM/WB.RegisterRd =/= 0)and (EX/MEM.RegisterRd =/= ID/EX.RegisterRs)and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) Forward A = 01

if (MEM/WB.RegWriteand (MEM/WB.RegisterRd =/= 0)and (EX/MEM.RegisterRd =/= ID/EX.RegisterRt)and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01
Priority in Forwarding

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The datapath modified to resolve hazards via forwarding.

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The Snapshots of Our Example Through Forwarding

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Clock cycles 3 of the instruction sequence.

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Clock cycles 4 of the instruction sequence.
add \$9, \$4, \$2 or \$4, \$4, \$2 and \$4, \$2, \$5 sub \$2 before

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Clock cycles 5 of the instruction sequence.

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Clock cycles 6 of the instruction sequence.
after after add \$9, \$4, \$2 or \$4 and \$4...

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Hazards and Stalls

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A pipelined sequence of instructions.

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The way stalls are really inserted into the pipeline.

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Forwarding with Load and Stores
The problem

The solution

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A Hazard Detection Unit
In the ID stage, the following should be checked by the hazard detection unit, so it can insert the stall between a load and its use.if (ID/EX.MemRead and ((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt = IF/ID.RegisterRt)) stall the pipeline

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Pipelined control overview, showing the two multiplexors for forwarding, the hazard detection unit, and the forwarding unit.

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Snapshots of the running example through the pipeline

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Clock cycles 2 of the instruction sequence in the example.

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Clock cycles 3 of the instruction sequence in the example.

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Clock cycles 4 the instruction sequence in the example.

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Clock cycles 5 of the instruction sequence in the example.

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Clock cycles 6 of the instruction sequence in the example.

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Clock cycles 7 of the instruction sequence in the example. (note the forwarding of r4)

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Branch Hazards and Handling
Branch hazardsBranch hazard handling methodsStaticDynamic

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