Reorder Buffer

Definition

A reorder buffer is a hardware structure used in out-of-order execution processors that enables the correct sequencing of instruction completion while allowing instructions to execute in a non-sequential order. It helps maintain the appearance of a program's original execution order by holding the results of instructions until they can be committed, ensuring that changes to the architectural state occur in the correct order.

5 Must Know Facts For Your Next Test

1. The reorder buffer allows for dynamic scheduling of instructions, which can improve processor performance by reducing idle time and making better use of execution resources.
2. Each entry in the reorder buffer contains information about an instruction's result and its original program order, allowing for proper reordering during commit.
3. Reorder buffers are critical in maintaining consistency and correctness in the presence of exceptions or interrupts, ensuring that only completed instructions affect the visible state.
4. When an instruction is executed out of order, its result is placed into the reorder buffer and is not visible to other parts of the processor until it is committed.
5. The size of the reorder buffer can impact performance; larger buffers can accommodate more outstanding instructions but may also increase complexity and power consumption.

Review Questions

• How does the reorder buffer improve processor efficiency in out-of-order execution?
  • The reorder buffer improves processor efficiency by allowing instructions to execute as soon as their operands are available, rather than waiting for prior instructions to complete. This dynamic scheduling reduces idle cycles and maximizes resource usage, as multiple instructions can be processed concurrently. By temporarily holding results until they can be committed in the original program order, it also ensures correctness while enhancing overall performance.
• Discuss how a reorder buffer interacts with exception handling and its role in maintaining program correctness.
  • In the case of exceptions or interrupts, the reorder buffer plays a vital role by ensuring that only completed instructions are visible to other parts of the processor. If an exception occurs, any results stored in the reorder buffer that have not yet been committed can be discarded, preventing incorrect changes to the architectural state. This mechanism allows processors to recover gracefully from errors while preserving program correctness and consistency.
• Evaluate the trade-offs involved in increasing the size of a reorder buffer and its impact on overall system performance.
  • Increasing the size of a reorder buffer can enhance performance by accommodating more outstanding instructions, thereby reducing stalls and improving throughput. However, larger buffers come with trade-offs, including increased complexity in management and potentially higher power consumption. Moreover, excessively large buffers may lead to diminishing returns as latency increases due to longer commit times. Thus, optimizing reorder buffer size requires balancing performance gains against these costs.

Related terms

Out-of-Order Execution: A technique used in modern processors to allow instructions to be executed as resources are available, rather than strictly in the order they appear in the code.

Commit Stage: The final stage in a processor's pipeline where the results of instructions are written back to the architectural state of the system, typically in the order they were issued.

Speculative Execution: An optimization technique where a processor guesses the paths of future instructions and executes them before it is known whether they are needed, potentially using a reorder buffer to manage results.