I need help with these questions for my homework.

(1) Why are the first level caches usually split (instructions and data are in different caches) while the L2 is usually unified (instructions and data are both in the same cache)?

(2) Why is miss rate not a good metric for evaluating cache performance? What is the appropriate metric? Give its definition. What is the reason for using a combination of first and second- level caches rather than using the same chip area for a larger first-level cache?

(3) The original motivation for using virtual memory was compatibility. What does that mean in this context? What are two other motivations for using virtual memory?