This section introduces branching, an essential concept in pipelined processors, and its impact on the pipeline’s performance.

● What are Branch Instructions?: Branch instructions are used to change the flow of control in a program (e.g., if statements, loops, function calls).
● Branching and Pipelining: Branch instructions create challenges in pipelined architectures because the next instruction depends on the outcome of the branch decision.
● The Challenge of Control Flow: Without knowledge of the branch outcome, the processor cannot fetch the correct instruction, causing delays and inefficiencies.

Control hazards, also known as branch hazards, occur when the pipeline must wait to determine the correct instruction to fetch after a branch instruction.

● What is a Control Hazard?: A control hazard arises when the pipeline needs to know the result of a branch to decide the next instruction.
● Branch Decision Delay: In a pipelined processor, control hazards cause delays because the branch decision must be made before the correct instruction can be fetched and executed.
● Impact on Performance: The longer the delay due to branching, the more performance is impacted, especially in processors with deep pipelines.

To mitigate control hazards and keep the pipeline flowing smoothly, branch prediction techniques are used.

● Static Branch Prediction: A simple form of branch prediction that assumes branches will always go in one direction (e.g., always taken or always not taken).
○ Example: Predicting that an if statement will always evaluate as true.
● Dynamic Branch Prediction: More sophisticated prediction that uses runtime information and history to make a prediction about the branch direction.
○ Branch History Table (BHT): A table that records the history of branch outcomes (taken or not taken).
○ Two-Level Adaptive Prediction: Uses multiple levels of history to improve prediction accuracy, including past branch behavior.
● Branch Target Buffer (BTB): A cache used to store the target addresses of branches, allowing the pipeline to fetch the correct instruction while waiting for the branch outcome.
● Return Address Stack (RAS): A specialized stack that stores the return addresses for function calls, helping to predict the target address of function returns.

Branch prediction is not always accurate, and mispredictions can cause significant performance penalties.

● Branch Misprediction: Occurs when the processor incorrectly predicts the outcome of a branch, resulting in the wrong instruction being fetched.
● Pipeline Flush: When a branch misprediction occurs, the instructions already in the pipeline must be discarded (flushed), and the correct instruction must be fetched.
● Penalty of Misprediction: The cost of a misprediction is the time it takes to flush the pipeline and fetch the correct instruction, which can severely affect performance, especially in deep pipelines.

A technique used to mitigate branch penalties by filling the delay between the branch decision and the fetching of the correct instruction.

● What is a Delay Slot?: A delay slot is a slot in the pipeline after a branch instruction, where a useful instruction can be executed while the branch decision is being made.
● Filling Delay Slots: Instructions that can be executed without depending on the branch outcome (e.g., independent operations) are scheduled to fill the delay slot, minimizing the performance loss.
● Limitations of Delay Slots: Not all instructions can be scheduled in the delay slot, and modern processors have largely moved away from this technique in favor of more advanced branch prediction mechanisms.

While pipelining provides a significant performance boost, it also has inherent limits and challenges that must be addressed.

● Structural Hazards: Occur when there aren’t enough resources (e.g., ALUs, memory ports) to handle all the instructions in the pipeline simultaneously. For example, if the processor cannot access memory while the pipeline is processing instructions, this causes a stall.
● Data Hazards: When an instruction depends on the result of a previous instruction that has not yet completed, data hazards can occur. These hazards are particularly challenging in pipelined systems.
○ RAW (Read-After-Write) Hazards: A later instruction needs data from an earlier instruction that hasn’t yet completed.
○ WAR (Write-After-Read) Hazards: A later instruction writes to a register before an earlier instruction reads it.
○ WAW (Write-After-Write) Hazards: Two instructions write to the same register, and the order of the writes must be carefully managed.
● Pipeline Depth and Power Consumption: The deeper the pipeline, the greater the power consumption and the higher the complexity of managing pipeline hazards. As pipelines get deeper, managing these risks becomes more difficult, and the benefits of pipelining are offset by higher power usage and complexity.
● Pipeline Stall and Complexity: In order to handle hazards, processors may introduce pipeline stalls (delays), which reduce the overall throughput of the system. The more complex the pipeline, the more difficult it is to manage and avoid stalls.