Fundamentals of Superscalar and Out-of-Order Execution Quiz

Explore essential concepts of superscalar processors and out-of-order execution with this quiz, designed to reinforce understanding of parallel instruction execution, pipeline stages, hazards, and related architectural features. Enhance your grasp of how modern processors boost performance through advanced execution strategies.

1. Superscalar Processor Capabilities

   Which main feature distinguishes a superscalar processor from a scalar processor?

   1. It can execute only one instruction at a time.
   2. It can issue multiple instructions per clock cycle.
   3. It always runs at a higher clock speed.
   4. It has no instruction pipeline.

   Explanation: A superscalar processor is characterized by its ability to fetch, issue, and execute more than one instruction per clock cycle, increasing parallelism. Higher clock speed does not specifically define a superscalar architecture. Option C describes a scalar processor, not a superscalar. Lack of an instruction pipeline, as mentioned in Option D, is not accurate for modern processors.

2. Superscalar Pipeline Structure

   In a typical superscalar pipeline, what allows multiple instructions to be executed in parallel?

   1. Duplicated functional units
   2. Single arithmetic logic unit
   3. Absence of register files
   4. Slower instruction fetch phase

   Explanation: Superscalar processors feature multiple execution units, such as multiple ALUs or FPUs, enabling parallel execution of instructions. A single arithmetic logic unit restricts execution to one instruction at a time. A slower instruction fetch phase would reduce performance, not enhance parallelism. All modern CPUs require register files; their absence would prevent operation.

3. Out-of-Order Execution Purpose

   Why do modern processors use out-of-order execution?

   1. To ensure strict program order is always maintained
   2. To eliminate dependency checking between instructions
   3. To reduce the need for branch prediction
   4. To maximize utilization of processor resources when instructions are stalled

   Explanation: Out-of-order execution allows the processor to execute instructions that are not dependent on stalled or waiting instructions, thus increasing efficiency. Maintaining strict program order limits performance improvements, making Option B incorrect. Branch prediction is still required to anticipate the path of execution, so Option C does not align with the main purpose. Dependency checking is crucial for correct execution, making Option D incorrect.

4. Instruction Issue Logic

   What is the main role of the instruction issue unit in a superscalar processor?

   1. To dispatch multiple instructions to available execution units
   2. To control only the fetch stage speed
   3. To replace memory management functions
   4. To permanently store program data

   Explanation: The instruction issue unit analyzes the instructions in the queue and sends several of them to suitable execution units if possible. This boosts throughput by matching instructions with unoccupied functional units. Permanent data storage is not its function. The unit does more than simply changing fetch stage speed, and it does not replace memory management tasks.

5. Hazard Resolution in Out-of-Order Engines

   Which type of hazard is primarily addressed by register renaming in out-of-order execution?

   1. Instruction cache miss
   2. Power consumption
   3. Write-after-read (WAR) hazard
   4. Branch misprediction

   Explanation: Register renaming helps eliminate false dependencies like WAR hazards by giving temporary unique names to registers, allowing instructions to execute independently. Instruction cache misses and branch mispredictions are handled through other architectural techniques. Power consumption is managed at a broader system level and is not directly solved by register renaming.

6. Dependency Types

   Which dependency must always be respected for correct program execution, even with out-of-order execution?

   1. Read-after-write (RAW) dependency
   2. Write-after-write (WAW) dependency
   3. Fetch-after-fetch (FAF) dependency
   4. Write-after-read (WAR) dependency

   Explanation: RAW dependency ensures that an instruction reads the correct value from a previous write, crucial for proper program results. WAW and WAR are handled with techniques like register renaming. 'Fetch-after-fetch' is not recognized as a dependency type in CPU architecture.

7. Basic Out-of-Order Execution Buffer

   Which structure in a processor temporarily holds instructions waiting for their operands before forwarding them to functional units?

   1. Stack pointer
   2. Memory controller
   3. Reservation station
   4. Instruction fetch buffer

   Explanation: Reservation stations hold instructions until operands are available, enabling dynamic scheduling in out-of-order processors. The stack pointer is a register managing stack operations, not instruction issuing. Instruction fetch buffer holds instructions before decoding, not prior to execution. Memory controllers manage access to main memory, not instruction scheduling.

8. Instruction Completion

   What is the function of the reorder buffer (ROB) in an out-of-order execution engine?

   1. It manages branch predictions exclusively.
   2. It ensures instructions retire in original program order.
   3. It decodes all fetched instructions.
   4. It doubles the clock speed.

   Explanation: The reorder buffer (ROB) is critical for tracking instructions and ensuring that, despite being executed out-of-order, they commit their results to the register file or memory in program order. Decoding is performed before instructions reach the ROB. Branch prediction is managed by separate components. The ROB itself has no function related to increasing clock speed.

9. Superscalar Performance Limitations

   Which factor can limit the performance gains of superscalar architectures?

   1. Larger instruction set size
   2. Higher ambient temperature
   3. Longer source code comments
   4. Instruction dependencies that prevent parallel execution

   Explanation: Dependencies, such as data or control dependencies, restrict how many instructions can actually be executed in parallel in a superscalar design. Higher temperature can affect cooling but does not inherently limit parallelism. The size of the instruction set does not directly limit how many instructions can run concurrently. Source code comments have no impact on processor performance.

10. In-Order vs Out-of-Order Example

    Given the instructions: 1) Add R1, R2, R3; 2) Multiply R4, R1, R5; 3) Load R6, [R7], which sequence would an out-of-order processor possibly execute first if data dependencies allow?

    1. Instruction 3 before Instruction 2
    2. Instruction 1 before Instruction 3
    3. Instruction 2 before Instruction 1
    4. Instruction 2 before Instruction 3

    Explanation: If Instruction 3 (Load) has no dependencies on the previous instructions, an out-of-order processor may execute it before Instruction 2 (Multiply), which relies on the result from Instruction 1. Executing Instruction 2 before Instruction 1 is impossible due to a data dependency. Instructions 1 and 3 can be executed in any order if parallelizable, but option A represents a feasible scenario for out-of-order execution. Option D is incorrect unless the Multiply happens to have no dependencies, which the scenario says is not the case.