Tools

Dynamic Scheduling with Tomasulo Algorithm

Procedure

Follow these step-by-step instructions to understand and explore the Tomasulo Algorithm for dynamic scheduling using the interactive simulator.

Step 1: Understanding the Interface

  1. Observe the Initial State

    • Notice that all reservation stations start in the Free state
    • The register alias table shows all registers mapped to their architectural values
    • The instruction queue is empty and ready to accept instructions
    • All functional units are idle

  2. Familiarize with the Components

    • Instruction Input Panel: Interface to add different types of instructions
    • Reservation Stations: Tables showing Add/Sub, Multiply/Divide, and Load/Store stations
    • Register Alias Table: Maps architectural registers to reservation station tags
    • Functional Units Status: Shows current execution status of each unit
    • Common Data Bus Monitor: Displays result broadcasts
    • Instruction Timeline: Visual representation of instruction flow through pipeline stages

Step 2: Basic Instruction Issue

  1. Issue a Simple ADD Instruction

    • In the instruction input panel, select ADD as the operation
    • Set destination register to R1
    • Set source registers to R2 and R3
    • Click "Issue Instruction"

  2. Observe the Results

    • A reservation station in the Add/Sub group becomes Waiting
    • Register R1 in the alias table now points to the reservation station tag
    • If R2 and R3 have values, they are copied; otherwise, tags are recorded
    • The instruction appears in the instruction timeline

  3. Issue Additional Instructions

    • Add a few more instructions with different register dependencies
    • Observe how reservation stations fill up
    • Notice how register renaming eliminates false dependencies

Step 3: Instruction Execution and Dependencies

  1. Create a True Dependency Chain

    • Issue: ADD R1, R2, R3
    • Issue: SUB R4, R1, R5 (depends on previous ADD)
    • Issue: MUL R6, R4, R7 (depends on previous SUB)

  2. Monitor Execution Flow

    • Click "Step Execution" or "Run Simulation"
    • Watch how the first ADD executes immediately (if operands available)
    • Observe SUB waiting for R1 value from the ADD instruction
    • Notice MUL waiting for R4 value from the SUB instruction

  3. Analyze Data Flow

    • Follow values as they move through the Common Data Bus
    • See how results broadcast to all waiting instructions
    • Watch reservation stations transition: Waiting → Ready → Executing → Complete

Step 4: Out-of-Order Execution

  1. Demonstrate Independent Instructions

    • Issue a sequence like:
      ADD R1, R2, R3      // Takes 2 cycles
          MUL R4, R5, R6      // Takes 10 cycles  
          SUB R7, R8, R9      // Takes 2 cycles, independent
          ADD R10, R11, R12   // Takes 2 cycles, independent

  2. Observe Out-of-Order Completion

    • Start execution and watch the timeline
    • Notice that SUB and the second ADD complete before MUL
    • Instructions complete out of program order but maintain correctness
    • Resource utilization increases with parallel execution

Step 5: Register Renaming Effects

  1. Create False Dependencies

    • Issue instructions that demonstrate WAR and WAW hazards:
      ADD R1, R2, R3      // Write R1
          SUB R4, R1, R5      // Read R1 (RAW - true dependency)
          MUL R1, R6, R7      // Write R1 (WAW - false dependency)
          ADD R8, R4, R1      // Read R1 (should get MUL result)

  2. Analyze Renaming Behavior

    • Watch how different writes to R1 get different reservation station tags
    • Observe that the SUB reads the first R1 value
    • Notice that the final ADD waits for the MUL result
    • Verify that register renaming eliminates the false WAW hazard

Step 6: Load/Store Operations

  1. Memory Operations

    • Issue load instructions: LOAD R1, 100(R2)
    • Issue store instructions: STORE R3, 200(R4)
    • Mix with computational instructions

  2. Address Calculation

    • Watch how effective addresses are calculated
    • Observe memory unit utilization
    • Note variable latencies for cache hits/misses

Step 7: Performance Analysis

  1. Measure Instruction Throughput

    • Run a sequence of mixed instructions
    • Record issue rate, completion rate, and average cycles per instruction
    • Compare with in-order execution

  2. Resource Utilization Study

    • Monitor reservation station occupancy
    • Track functional unit utilization
    • Identify bottlenecks in the system

  3. Hazard Impact Analysis

    • Compare execution time with and without dependencies
    • Measure the effect of different instruction mixes
    • Analyze the impact of resource constraints

Step 8: Advanced Scenarios

  1. Resource Contention

    • Issue more instructions than available reservation stations
    • Observe structural hazards and stalling behavior
    • Study the impact of limited functional units

  2. Complex Dependency Patterns

    • Create complex dependency chains and trees
    • Mix different instruction types with varying latencies
    • Analyze instruction scheduling decisions

  3. Performance Tuning

    • Experiment with different numbers of reservation stations
    • Vary functional unit counts and latencies
    • Study the trade-offs in hardware complexity vs. performance

Step 9: Comparative Analysis

  1. In-Order vs. Out-of-Order

    • Use the comparison mode to see both execution styles
    • Measure performance differences
    • Understand when out-of-order provides benefits

  2. Different Instruction Mixes

    • Test with compute-intensive workloads
    • Try memory-intensive patterns
    • Analyze mixed workload performance

Step 10: Experiment Documentation

  1. Record Observations

    • Document state transitions you observe
    • Note performance improvements achieved
    • Record any unexpected behaviors

  2. Answer Analysis Questions

    • How does register renaming eliminate false dependencies?
    • What factors limit instruction-level parallelism?
    • How do resource constraints affect performance?

  3. Design Experiments

    • Create scenarios to test specific aspects
    • Vary parameters to understand trade-offs
    • Compare with theoretical predictions