Lecture 7 - ILP Speculative (Unfinished)

Question 1

Hardware Based Speculation
(Describe it)

Answer 1

Instructions are executed along predicted paths, but results are committed only if the prediction is correct. Instruction commit allows updating the register file when the instruction is no longer speculative.

Question 2

Reorder Buffer (ROB)

Answer 2

• The ROB holds the result of an instruction between completion and commit.

It has four fields:
- Instruction type: branch/store/register.
- Destination field: register number.
- Value field: output value.
- Ready field: indicates if execution is completed.

• Reservation stations are modified, with the operand source now being the reorder buffer instead of the functional unit.
• Register values and memory values are not written until an instruction commits.
• On a misprediction, speculated entries in the ROB are cleared.
• Exceptions are not recognized until the instruction is ready to commit.
• The ROB allows instructions to complete out-of-order but makes results visible in-order.