Tools

Reorder Buffer and Out-of-Order Execution

Follow these step-by-step instructions to understand and explore Reorder Buffer and Out-of-Order Execution using the interactive simulator.

Step 1: Understanding the Interface

  1. Observe the Initial State

    • Notice that all ROB entries start in the Empty state
    • The Register Alias Table (RAT) shows all registers mapped to their initial values
    • The instruction queue is empty and ready to accept programs
    • Performance metrics show zero values

  2. Familiarize with the Components

    • Instruction Input Panel: Interface to load sample programs or enter custom assembly
    • Reorder Buffer Table: 16-entry ROB showing instruction status and values
    • Register Alias Table: Maps architectural registers to ROB entries or committed values
    • Execution Timeline: Visual representation of instruction flow through pipeline stages
    • Commit Log: Historical record of committed instructions
    • Performance Metrics: Real-time statistics on ROB utilization and efficiency

Step 2: Basic Out-of-Order Execution

  1. Load the Basic Example

    • Click "Basic Example" to load a simple instruction sequence
    • Observe the assembly code with arithmetic operations
    • Notice the mix of ADD, SUB, and MUL instructions

  2. Execute Single Steps

    • Click "Step" to advance one cycle at a time
    • Watch how instructions are issued to ROB entries
    • Observe the state transitions: Issue → Execute → Complete → Commit

  3. Analyze Issue Phase

    • Notice that multiple instructions can be issued in the same cycle
    • Watch how ROB entries are allocated sequentially (tail pointer advancement)
    • Observe register renaming in the RAT when destinations are mapped to ROB entries

  4. Study Execution Phase

    • See how instructions execute out-of-order based on operand availability
    • Notice that MUL operations take longer than ADD/SUB operations
    • Observe how independent instructions execute in parallel

Step 3: Register Renaming and Dependency Handling

  1. Load the Dependencies Sample

    • Click "Dependencies" to load a program with register dependencies
    • Study the instruction sequence that demonstrates true and false dependencies

  2. Track Register Renaming

    • Watch how multiple writes to the same register get different ROB entries
    • Observe the RAT updates as new instructions are issued
    • Notice how false dependencies (WAW, WAR) are eliminated

  3. Analyze Dependency Resolution

    • See how dependent instructions wait for their operands
    • Watch result forwarding from completed instructions to waiting ones
    • Observe the execution timeline showing parallel execution of independent operations

  4. Study Commit Order

    • Notice that instructions commit strictly in program order
    • Watch how the head pointer advances only when instructions complete
    • Observe completed instructions waiting for older instructions to commit first

Step 4: Performance Analysis

  1. Monitor ROB Utilization

    • Run the simulation and watch ROB utilization percentage
    • Notice how utilization indicates instruction-level parallelism
    • Observe the relationship between program structure and ROB occupancy

  2. Analyze Execution Timeline

    • Study the visual timeline showing instruction stages
    • Identify overlapping execution phases
    • Compare execution order with program order

  3. Review Performance Metrics

    • Monitor instructions issued, completed, and committed rates
    • Calculate average CPI (Cycles Per Instruction)
    • Observe how out-of-order execution improves performance

Step 5: Exception Handling and Precise Interrupts

  1. Load the Exception Test Program

    • Click "Exception Test" to load a program with potential exceptions
    • Notice the DIV instruction that may cause division by zero

  2. Trigger an Exception

    • Run the simulation or use "Trigger Exception" button
    • Observe how exceptions are marked in ROB entries
    • Watch the exception handling when the faulting instruction reaches ROB head

  3. Study Recovery Process

    • See how younger instructions are flushed from the pipeline
    • Notice that older instructions continue to commit normally
    • Observe how precise interrupt state is maintained

  4. Analyze Exception Impact

    • Study performance impact of exception handling
    • Notice pipeline recovery overhead
    • Understand the trade-off between speculation and correctness

Step 6: Advanced Instruction Patterns

  1. Create Custom Programs

    • Use the "Custom Assembly" option to enter your own instruction sequences
    • Try different patterns:
      ADD R1, R2, R3
          SUB R4, R5, R6    # Independent - can execute in parallel
          MUL R7, R1, R8    # Depends on ADD result
          AND R9, R4, R10   # Depends on SUB result

  2. Experiment with Loop Patterns

    • Create simple loops with independent iterations
    • Observe how ROB handles repetitive patterns
    • Study the impact of loop unrolling on ROB utilization

  3. Test Memory Operations

    • Include LOAD and STORE instructions in your programs
    • Observe memory operation handling in ROB
    • Study interaction between memory operations and register dependencies

Step 7: Comparative Analysis

  1. Run Simulation at Different Speeds

    • Adjust the simulation speed slider
    • Use slow speeds for detailed observation
    • Use fast speeds for overall behavior analysis

  2. Compare Different Instruction Mixes

    • Test compute-intensive programs (many ALU operations)
    • Test memory-intensive programs (frequent LOAD/STORE)
    • Test mixed workloads with various instruction types

  3. Analyze ROB Size Impact

    • Observe behavior with different ROB occupancy levels
    • Notice when ROB becomes full and causes stalls
    • Understand the relationship between ROB size and performance

Step 8: Interactive Experimentation

  1. Manual Instruction Addition

    • Use the operation dropdown to add individual instructions
    • Build custom dependency chains
    • Experiment with different instruction sequences

  2. State Inspection

    • Click on ROB entries to examine detailed state
    • Study register values and ROB mappings
    • Trace instruction flow through the timeline

  3. Real-time Monitoring

    • Watch performance metrics update in real-time
    • Monitor ROB utilization patterns
    • Observe commit log for instruction retirement order

Step 9: Performance Optimization Study

  1. Identify Bottlenecks

    • Look for patterns where ROB utilization is low
    • Identify dependency chains that limit parallelism
    • Study resource conflicts and their impact

  2. Optimization Experiments

    • Reorder independent instructions to improve parallelism
    • Add independent operations to fill execution slots
    • Study the impact of instruction scheduling on performance

  3. Trade-off Analysis

    • Compare performance gains vs. hardware complexity
    • Understand when out-of-order execution provides benefits
    • Analyze scenarios where in-order might be sufficient

Step 10: Advanced Scenarios

  1. Complex Dependency Patterns

    • Create programs with complex dependency graphs
    • Test scenarios with multiple dependency chains
    • Study how ROB handles intricate instruction relationships

  2. Resource Contention

    • Create scenarios where multiple instructions compete for resources
    • Observe how the simulator handles resource conflicts
    • Study the impact of limited functional units

  3. Exception Recovery

    • Design programs that test various exception scenarios
    • Study recovery mechanisms and their performance impact
    • Understand the relationship between speculation depth and recovery cost

Expected Learning Outcomes

After completing this procedure, you should understand:

  • How Reorder Buffer enables out-of-order execution while maintaining program correctness
  • The role of register renaming in eliminating false dependencies
  • The mechanism of precise interrupt handling in speculative processors
  • Performance benefits and trade-offs of out-of-order execution
  • Real-world applications and implementation considerations

Analysis Questions

During your experiments, consider these questions:

  1. How does ROB size affect instruction-level parallelism?
  2. What types of programs benefit most from out-of-order execution?
  3. How does exception handling impact overall performance?
  4. What are the trade-offs between ROB complexity and performance gains?
  5. How do different instruction mixes affect ROB utilization?

Troubleshooting Tips

  • If simulation appears stuck, check for dependency deadlocks
  • Use step-by-step execution to understand complex behaviors
  • Reset simulation if you encounter unexpected states
  • Adjust simulation speed for better observation of fast transitions
  • Use the help modal for instruction format reference

Documentation

  1. Record Observations

    • Document interesting patterns you observe
    • Note performance improvements in different scenarios
    • Record any unexpected behaviors and their explanations

  2. Create Test Cases

    • Design specific scenarios to test ROB features
    • Document instruction sequences that demonstrate key concepts
    • Share findings with classmates for discussion

  3. Performance Analysis

    • Calculate performance improvements from out-of-order execution
    • Compare CPI values between different instruction patterns
    • Analyze ROB utilization efficiency across various workloads