Solutions for Structural Hazards

Structural hazards happen in a pipelined processor when two instructions need the same hardware at the same time. Because the hardware cannot be shared in that moment, the pipeline may slow down or stop. To avoid this problem, computer architects use several simple techniques.

1. Add More Hardware (Resource Duplication)

The easiest way to remove a structural hazard is to add another copy of the hardware unit that is causing the conflict.

Example:

• Use separate instruction memory and data memory, so instruction fetch and data access can happen together.
• Add extra ALUs or functional units.

Benefit: No waiting; faster performance.

2. Instruction Scheduling

Instructions can be reordered so that they do not use the same hardware in the same cycle.

Types:

• Compiler Scheduling: The compiler arranges instructions to avoid conflicts.
• Dynamic Scheduling: Hardware reorders instructions during execution.

Benefit: No extra hardware needed.

3. Use Separate Units for Different Tasks

Sometimes a single unit (like ALU or memory) is used by different pipeline stages. The solution is to split the hardware into different units.

Example:

• Separate integer unit and floating-point unit
• Separate address calculation unit and execution unit

This reduces competition for the same resource.

4. Pipeline the Functional Units

• Multi-cycle units (like multipliers) can be made pipelined internally.
• This allows a new operation to start every cycle, even if the previous one is not yet finished.

5. Insert Stalls (Hardware Interlocks)

If no other solution works, the hardware can pause the pipeline for one or more cycles.

Effect:

• Slows down the pipeline
• But ensures correct execution

Used only when necessary.