memory arbiter
The main concept of memory arbitar is sharing of the single memory with multiple user fairly.
So for that agent who want to access memory should raise request to arbiter, and arbiter will grant its request according the algorithem it has implemented in.
Algorithems like first come first serve are not fair enough for arbiter.
Look for arbiter, here it is specific case about memory.
Bus arbiter also work alike memory arbiter.
Hope this might help you to start