CDC Score Boarding Example| lec 56| Advanced computer Architecture| BhanuPriya

Summary

This video explains the scoreboard approach in dynamic scheduling, focusing on how machine instructions are executed in a pipeline system. The process involves different functional units: integer, multiplier, adder, and divider, each responsible for specific operations. Key stages in the scoreboard include issue, read operand, execute, and write-back, with instructions depending on the availability of functional units. Hazards are managed by ensuring that instructions wait for others to complete, preventing conflicts. The video demonstrates the step-by-step execution of instructions like load, multiply, and add, highlighting how dynamic scheduling optimizes instruction flow in a pipeline.

Takeaways

• 😀 Scoreboarding is a technique used in dynamic scheduling for organizing the execution of instructions in a pipeline system.
• 😀 The main stages of execution in scoreboarding are: Issue, Read Operand, Execute, and Write Back.
• 😀 Functional units in the pipeline include Integer Unit (IU), Adder Unit (Add), Multiplier Unit (Mul), and Divider Unit (Div).
• 😀 In the provided example, the integer unit handles load operations, while multiplication and division use the multiplier and divider units respectively.
• 😀 The load operation uses the integer unit and takes one cycle to execute and write back the result to the destination register.
• 😀 The second load instruction cannot execute until the first load completes because the integer unit is busy.
• 😀 Hazards such as data hazards, where instructions depend on previous results, are managed by ensuring operands are available before execution.
• 😀 The multiplication operation waits for the integer unit to become free, as it requires a value from the load operation.
• 😀 The addition operation cannot start until the division operation completes reading operands to avoid overwriting the result.
• 😀 The scoreboard system coordinates functional unit usage, ensuring instructions execute in the proper order while avoiding data hazards.
• 😀 Scoreboarding helps manage pipeline hazards by tracking when operands are ready and when functional units are free, improving execution efficiency.

Q & A

What is the scoreboard technique in dynamic scheduling?

The scoreboard technique is a method used in dynamic scheduling to manage the execution of instructions in a pipeline system. It tracks the status of functional units, ensuring that operands are available before instructions are executed and handling dependencies between instructions.

How does the scoreboard approach differ from Tomasulo's algorithm?

The key difference is that the scoreboard approach includes an additional stage, the 'read operand' stage, between the 'issue' and 'execute' stages. Tomasulo's algorithm, on the other hand, has only three stages: issue, execute, and write back.

What are the four stages of scoreboard execution?

The four stages of scoreboard execution are: 1) Issue, where the instruction enters the pipeline; 2) Read Operand, where operands are fetched; 3) Execute, where the operation is performed; and 4) Write Back, where the result is written to the destination register.

What are the different functional units discussed in the video?

The functional units discussed include the integer unit, multiplier unit, adder unit, and divider unit. Each unit is responsible for specific operations such as loading, multiplying, adding, and dividing.

What role does the integer unit play in the scoreboard system?

The integer unit is used for load operations, where it loads data from memory into registers. It operates in one cycle and interacts with the scoreboard to track when operands are available for further processing.

How are hazards managed in the scoreboard approach?

Hazards are managed by ensuring that instructions do not overwrite register values or access them before they are ready. For example, an addition operation must wait for a division operation to complete its operand fetch to avoid using outdated register values.

Why does the adder unit have to wait for the division unit to read operands in this example?

The adder unit must wait for the division unit to read operands because both operations require the same register (f6). If the adder writes its result before the division completes, the division may use incorrect, overwritten values. The scoreboard ensures the adder waits until the division has read the operand.

What happens when a functional unit becomes free in the scoreboard approach?

When a functional unit becomes free, it can be assigned to a new instruction that requires it. This helps maintain the pipeline flow and ensures that instructions are executed efficiently without unnecessary delays.

How does the scoreboard handle dependencies between instructions?

The scoreboard handles dependencies by tracking when operands become available and by preventing instructions from executing until their required operands are ready. This avoids data hazards and ensures correct execution order.

What happens if an instruction is blocked due to a busy functional unit?

If an instruction is blocked due to a busy functional unit, it must wait until the unit becomes available. The scoreboard monitors the availability of units and schedules instructions accordingly, ensuring that instructions execute only when their required resources are free.