ReOrder Buffer

Question 1

What are the problems with Tomasulo’s algorithm?

Answer 1

Main problem: we are saving state out-of-order, so when we have a problem, it is hard to recover.

1. Exception handling
2. Branch mispredictions
3. Phantom exception - exception thrown from wrong branch path

Question 2

How do we fix the problems in Tomasulo’s algorithm?

Answer 2

We need to save values to registers according to program order. Use a reorder buffer (ROB)

Question 3

What is a reorder buffer?

Answer 3

Buffer that remembers program order and keeps results of instructions until it is safe to write

Question 4

How is a ROB structured?

Answer 4

Table with three values: Register, Value, Done (the value has been written to the buffer, not regs). Has two pointers (issue, commit)

Question 5

What is a unified reservation station? What is its drawback?

Answer 5

A reservation station that feeds into different ALUs. This makes the dispatch logic more complicated.