Control hazards arise in pipelined processors when instructions are not executed in the order they appear in the program. This can lead to incorrect results or unexpected behavior. Here are the different types of control hazards:
1. Branch Hazards
- Definition: Occur when a conditional branch instruction (e.g., if, else, goto) changes the program's flow, making it difficult to predict the next instruction to fetch.
- Example: If a conditional branch instruction is encountered in the pipeline, the processor may fetch the wrong instruction, leading to a delay.
- Solution: Branch prediction techniques, such as static branch prediction (predicting the branch based on historical data) or dynamic branch prediction (using the processor's history of recent branches to predict the next branch), are used to avoid this hazard.
2. Jump Hazards
- Definition: Similar to branch hazards, jump hazards occur when a jump instruction (e.g., jmp, call) changes the program's flow, leading to incorrect instruction fetching.
- Example: A jump instruction may cause the processor to fetch an instruction from a different location in memory, leading to a delay.
- Solution: Techniques similar to branch prediction are used to handle jump hazards.
3. Procedure Call Hazards
- Definition: Occur when a procedure call instruction changes the program's flow and requires saving the current state of the program.
- Example: Saving the return address and other registers to the stack before jumping to the procedure takes time and can cause a delay.
- Solution: Techniques like delayed branching or pipelined procedure calls are used to reduce the impact of this hazard.
4. Data Hazards
- Definition: Data hazards occur when an instruction needs data that has not yet been produced by a previous instruction in the pipeline. This can happen when instructions depend on each other's results.
- Example: If an instruction needs the result of a previous instruction that is still in the pipeline, the processor may have to stall until the result is available.
- Solution: Data forwarding or bypassing techniques are used to pass the result of one instruction to the next instruction without waiting for it to be written back to memory.
By understanding and addressing these control hazards, pipelined processors can achieve higher performance and efficiency.
