Lecture 5 - Instruction Level Parallelism (OOO)

Question 1

Dynamic Scheduling [Definition, advantages and disadvantages]

Answer 1

• Dynamic scheduling rearranges the order of instructions to reduce stalls while maintaining data flow.
• (Advantage) It handles cases where dependencies are unknown at compile time and tolerates cache delays (through hiding latency).
• (Disadvantage) It increases hardware complexity and complicates exception handling.
• Out-of-order execution and completion create the possibility for WAR (Write After Read) and WAW (Write After Write) hazards

Question 2

Register Renaming (Definition, Explain Mechanism)

Answer 2

• Register Renaming is a technique used to allow instructions to use different physical registers, even if its the same logical register.
• Register renaming is introduced in hardware to minimize WAR and WAW hazards, and also allow out-of-order execution.
• It is provided by reservation stations (RS)

Question 3

Tomasulo’s Algorithm
(Definition, Mechanism - 3 Steps IEW)

Answer 3

Tomasulo’s Algorithm is a technique that uses dynamic scheduling of instructions that allow out-of-oder executions.

Tomasulo’s Algorithm involves three main steps:

1. Issue:
   - Get the next instruction from the FIFO queue.
   - If a reservation station is available, issue the instruction to the RS with operand values (if available).
   - If operand values are not available, stall the instruction.
2. Execute:
   - When an operand becomes available, store it in all RSs waiting for it.
   - When all operands are ready in an RS, execute the instruction.
3. Write Result:
   - Write the result of the operation onto the CDB.