Hazards

Definition

Hazards in computer architecture refer to potential issues that can disrupt the smooth execution of instructions within a pipeline. These hazards can lead to incorrect results or performance degradation, impacting the efficiency of the datapath components and organization. Understanding these hazards is essential for designing systems that can handle instruction-level parallelism effectively.

5 Must Know Facts For Your Next Test

1. Hazards can be classified into three main types: data hazards, control hazards, and structural hazards, each posing different challenges for pipeline execution.
2. Data hazards can be resolved through techniques like forwarding (bypassing) and stalling (inserting no-operation instructions).
3. Control hazards often involve branch predictions to improve flow in the pipeline and minimize stalls due to conditional jumps.
4. Structural hazards typically arise from limited hardware resources, such as when multiple instructions attempt to use a single memory or ALU unit at once.
5. Mitigating hazards is crucial for optimizing the performance of modern processors, as they enable higher throughput and better resource utilization.

Review Questions

• How do data hazards affect instruction execution in a pipelined architecture?
  • Data hazards occur when an instruction needs data from a prior instruction that has not yet finished executing. This can cause stalls, where subsequent instructions must wait for the necessary data before they can proceed. Techniques like forwarding help resolve these issues by allowing data to be used as soon as it’s available rather than waiting for it to be written back to memory.
• What strategies can be employed to address control hazards in pipelined processors?
  • Control hazards can be managed through branch prediction and delay slots. Branch prediction attempts to guess the outcome of a branch instruction before it's resolved, allowing the pipeline to continue executing subsequent instructions. If the prediction is correct, execution is smooth; if incorrect, previously executed instructions may need to be flushed, causing delays.
• Evaluate the impact of structural hazards on processor performance and how they can be mitigated in design.
  • Structural hazards can significantly degrade processor performance by causing conflicts when multiple instructions try to access shared resources simultaneously. To mitigate this, designers can increase resource availability by adding more functional units or designing the pipeline with greater resource separation. Effective scheduling algorithms can also help by rearranging instructions to minimize resource contention during execution.

Related terms

Data Hazard: A type of hazard that occurs when an instruction depends on the result of a previous instruction that has not yet completed.

Control Hazard: A hazard that arises from the pipelining of instructions, where the next instruction to execute is dependent on the outcome of a branch instruction.

Structural Hazard: A situation where two or more instructions require the same resource at the same time, leading to potential conflicts in accessing hardware components.