Podcast
Questions and Answers
What is the primary function of the Forwarding Unit?
What is the primary function of the Forwarding Unit?
- To handle ALU operations
- To bypass load and store operations (correct)
- To extend the signed immediate
- To register write-back operations
In which stage of the pipeline is the register written?
In which stage of the pipeline is the register written?
- MEM/WB (correct)
- ID/EX
- IF/ID
- EX/MEM
What is the purpose of the_MEM/WB.RegisterRd signal?
What is the purpose of the_MEM/WB.RegisterRd signal?
- To indicate a store operation
- To indicate a load operation
- To stall the pipeline
- To specify the write register (correct)
How many registers are written by a single MIPS instruction?
How many registers are written by a single MIPS instruction?
In which cycle is the load value available?
In which cycle is the load value available?
What is the function of the ALUSrc signal?
What is the function of the ALUSrc signal?
What is the purpose of the RegDst signal?
What is the purpose of the RegDst signal?
In which stage of the pipeline is the instruction address generated?
In which stage of the pipeline is the instruction address generated?
What is the purpose of the Extend Unit?
What is the purpose of the Extend Unit?
In which cycle is the store value needed?
In which cycle is the store value needed?
What do the arrows in the pipeline diagram represent?
What do the arrows in the pipeline diagram represent?
What do the tails of the arrows represent in the pipeline diagram?
What do the tails of the arrows represent in the pipeline diagram?
What do the heads of the arrows represent in the pipeline diagram?
What do the heads of the arrows represent in the pipeline diagram?
What type of hazard is represented by an arrow pointing backwards in time?
What type of hazard is represented by an arrow pointing backwards in time?
Which instructions have a data hazard in the given pipeline diagram?
Which instructions have a data hazard in the given pipeline diagram?
What is the purpose of the fancier pipeline diagram?
What is the purpose of the fancier pipeline diagram?
What is the main challenge in the given pipeline diagram?
What is the main challenge in the given pipeline diagram?
What is the key question to ask when dealing with data hazards in the pipeline?
What is the key question to ask when dealing with data hazards in the pipeline?
What is the main function of the forwarding unit in the outlined datapath?
What is the main function of the forwarding unit in the outlined datapath?
What determines the selection of ALU sources in the outlined datapath?
What determines the selection of ALU sources in the outlined datapath?
What is the purpose of the multiplexers in the datapath?
What is the purpose of the multiplexers in the datapath?
Under what condition will the operands come from the register file?
Under what condition will the operands come from the register file?
What happens when there is a hazard?
What happens when there is a hazard?
What is the purpose of the forwarding hardware?
What is the purpose of the forwarding hardware?
What is necessary to detect an EX/MEM data hazard?
What is necessary to detect an EX/MEM data hazard?
What is the main benefit of using forwarding hardware?
What is the main benefit of using forwarding hardware?
What is the problem with the following sequence of instructions: add $1, $2, $3; add $1, $1, $4;?
What is the problem with the following sequence of instructions: add $1, $2, $3; add $1, $1, $4;?
What is the purpose of the ForwardA and ForwardB signals?
What is the purpose of the ForwardA and ForwardB signals?
What is the condition for forwarding the ALU source operand?
What is the condition for forwarding the ALU source operand?
What is the function of the Hazard Detection Unit?
What is the function of the Hazard Detection Unit?
What is the purpose of the IF/ID, ID/EX, EX/MEM, and MEM/WB buffers?
What is the purpose of the IF/ID, ID/EX, EX/MEM, and MEM/WB buffers?
What is the function of the EX/MEM.RegisterRd register?
What is the function of the EX/MEM.RegisterRd register?
What is the purpose of the ForwardA signal?
What is the purpose of the ForwardA signal?
What is the condition for detecting a MEM/WB hazard?
What is the condition for detecting a MEM/WB hazard?
What is the value of the PC at the end of cycle 3?
What is the value of the PC at the end of cycle 3?
In which register is the result of the instruction stored?
In which register is the result of the instruction stored?
What is the value of the second operand of the instruction?
What is the value of the second operand of the instruction?
What is the address of the memory location being accessed?
What is the address of the memory location being accessed?
In which stage is the instruction decoded?
In which stage is the instruction decoded?
What is the value of Rs?
What is the value of Rs?
What is the operation being performed in cycle 3?
What is the operation being performed in cycle 3?
What is the value of Rt?
What is the value of Rt?
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Study Notes
Load/Store Bypassing: Extending the Datapath
- A harder case of load/store instructions:
lw $1, 0($2)andsw $1, 0($4) - The load value is available in the MEM stage, and the store value is needed in the EX stage
Datapath
- The datapath needs to be extended to handle load/store instructions
- A forwarding unit is needed to select the correct ALU inputs for the EX stage
Dependency Arrows
- Arrows indicate the flow of data between instructions
- The tails of the arrows show when a register is written, and the heads show when it is read
- Any arrow that points backwards in time represents a data hazard in the basic pipelined datapath
Pipeline Diagram
- A fancier pipeline diagram showing the clock cycles and dependencies between instructions
- Data hazards exist between instructions 1 & 2 and 1 & 3
Eliminating Hazards
- The hazards need to be eliminated to ensure the correct value of register $2 is used
- The AND and OR instructions will use the correct value for register $2
- When is the data actually produced and consumed?
Forwarding Hardware
- A forwarding unit selects the correct ALU inputs for the EX stage
- If there is no hazard, the ALU's operands will come from the register file
- If there is a hazard, the operands will come from either the EX/MEM or MEM/WB pipeline registers instead
Simplified Datapath with Forwarding Muxes
- A simplified datapath with forwarding multiplexers (muxes) for the ALU inputs
- The forwarding muxes are controlled by the ForwardA and ForwardB signals
Detecting EX/MEM Data Hazards
- The hardware needs to determine if a hazard exists
- Register $1 is written by both of the previous instructions, but only the most recent result should be forwarded
MEM/WB Hazard Equations
- Equations for detecting and handling MEM/WB hazards for the first and second ALU sources
- The equations check if the MEM/WB register is being written and if its contents match the ID/EX register Rs or Rt
Simplified Datapath with Forwarding
- A simplified datapath with forwarding multiplexers and hazard detection logic
- The forwarding unit ensures the correct value of register $2 is used in the AND and OR instructions
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