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Questions and Answers

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?

  • MEM/WB (correct)
  • ID/EX
  • IF/ID
  • EX/MEM

    • 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?

    <p>1 (B)</p> Signup and view all the answers

    In which cycle is the load value available?

    <p>MEM (C)</p> Signup and view all the answers

    What is the function of the ALUSrc signal?

    <p>To select the ALU operation (A)</p> Signup and view all the answers

    What is the purpose of the RegDst signal?

    <p>To specify the write register (A)</p> Signup and view all the answers

    In which stage of the pipeline is the instruction address generated?

    <p>IF/ID (C)</p> Signup and view all the answers

    What is the purpose of the Extend Unit?

    <p>To sign-extend the immediate (A)</p> Signup and view all the answers

    In which cycle is the store value needed?

    <p>MEM (D)</p> Signup and view all the answers

    What do the arrows in the pipeline diagram represent?

    <p>The flow of data between instructions (A)</p> Signup and view all the answers

    What do the tails of the arrows represent in the pipeline diagram?

    <p>When register $2 is written (D)</p> Signup and view all the answers

    What do the heads of the arrows represent in the pipeline diagram?

    <p>When register $2 is read (A)</p> Signup and view all the answers

    What type of hazard is represented by an arrow pointing backwards in time?

    <p>Data hazard (C)</p> Signup and view all the answers

    Which instructions have a data hazard in the given pipeline diagram?

    <p>Instructions 1 &amp; 2, and 1 &amp; 3 (C)</p> Signup and view all the answers

    What is the purpose of the fancier pipeline diagram?

    <p>To eliminate data hazards in the pipeline (B)</p> Signup and view all the answers

    What is the main challenge in the given pipeline diagram?

    <p>Eliminating data hazards between instructions (A)</p> Signup and view all the answers

    What is the key question to ask when dealing with data hazards in the pipeline?

    <p>When is the data actually produced and consumed? (C)</p> Signup and view all the answers

    What is the main function of the forwarding unit in the outlined datapath?

    <p>To select the correct ALU inputs for the EX stage. (C)</p> Signup and view all the answers

    What determines the selection of ALU sources in the outlined datapath?

    <p>ForwardA and ForwardB control signals (D)</p> Signup and view all the answers

    What is the purpose of the multiplexers in the datapath?

    <p>To select the correct ALU inputs for the EX stage. (D)</p> Signup and view all the answers

    Under what condition will the operands come from the register file?

    <p>When there is no hazard (B)</p> Signup and view all the answers

    What happens when there is a hazard?

    <p>The operands will come from the EX/MEM or MEM/WB pipeline registers. (C)</p> Signup and view all the answers

    What is the purpose of the forwarding hardware?

    <p>To enhance performance by reducing stalls (D)</p> Signup and view all the answers

    What is necessary to detect an EX/MEM data hazard?

    <p>Checking the pipeline registers for dependencies (C)</p> Signup and view all the answers

    What is the main benefit of using forwarding hardware?

    <p>Improved performance by reducing stalls (A)</p> Signup and view all the answers

    What is the problem with the following sequence of instructions: add $1, $2, $3; add $1, $1, $4;?

    <p>It is trying to write to the same register multiple times. (C)</p> Signup and view all the answers

    What is the purpose of the ForwardA and ForwardB signals?

    <p>To detect and handle MEM/WB hazards. (C)</p> Signup and view all the answers

    What is the condition for forwarding the ALU source operand?

    <p>When MEM/WB.RegisterRd = ID/EX.RegisterRs (C)</p> Signup and view all the answers

    What is the function of the Hazard Detection Unit?

    <p>To detect and handle MEM/WB hazards. (C)</p> Signup and view all the answers

    What is the purpose of the IF/ID, ID/EX, EX/MEM, and MEM/WB buffers?

    <p>To pipeline the execution of instructions. (C)</p> Signup and view all the answers

    What is the function of the EX/MEM.RegisterRd register?

    <p>To store the destination register of the instruction. (C)</p> Signup and view all the answers

    What is the purpose of the ForwardA signal?

    <p>To forward the value of the first ALU source operand. (B)</p> Signup and view all the answers

    What is the condition for detecting a MEM/WB hazard?

    <p>When MEM/WB.RegWrite = 1 and MEM/WB.RegisterRd = ID/EX.RegisterRs. (A)</p> Signup and view all the answers

    What is the value of the PC at the end of cycle 3?

    <p>103 (D)</p> Signup and view all the answers

    In which register is the result of the instruction stored?

    <p>$12 (B)</p> Signup and view all the answers

    What is the value of the second operand of the instruction?

    <p>3 (D)</p> Signup and view all the answers

    What is the address of the memory location being accessed?

    <p>103 (C)</p> Signup and view all the answers

    In which stage is the instruction decoded?

    <p>ID (A)</p> Signup and view all the answers

    What is the value of Rs?

    <p>2 (D)</p> Signup and view all the answers

    What is the operation being performed in cycle 3?

    <p>SUB (C)</p> Signup and view all the answers

    What is the value of Rt?

    <p>5 (D)</p> Signup and view all the answers

    Study Notes

    Load/Store Bypassing: Extending the Datapath

    • A harder case of load/store instructions: lw $1, 0($2) and sw $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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    Related Documents

    L12-Forwarding 6.pdf

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