Memory Ordering

Question 1

What is a LSQ?

Answer 1

Load-Store Queue - used to solve dependency issues in memory accesses

Question 2

What is the structure of an LSQ?

Answer 2

It is similar to a ROB and replaces RSes for Loads and Stores. Entries are placed in program order. Each entry has:

1. Load vs Store ?
2. Address
3. Value
4. Completed?

Question 3

What is Store-to-Load Forwarding?

Answer 3

If we try to load something that has already been stored in the queue, we just pull that value, we don’t go to memory

Question 4

What do we do if we have a load, but there are pending stores in the queue?

Answer 4

Get the address from memory anyways! But, when the store(s) return, it must check all the loads after it in the queue

Question 5

How do we remove items from the LSQ?

Answer 5

On commit!

• For load instructions, save value to register file and remove it from the queue
• For store instructions, save value to data cache and remove it from the queue