Top 10 Interview Questions for Embedded Hardware Engineers - Memory Systems

R:

Superpipelining, Superscalar, Dynamic Pipeline

2. Define the aspects of locality in memory access.

Temporal locality R: Tendency to reference locations recently referenced

Spatial locality R: Tendency to reference locations "near" those recently referenced

3. Define the sources for these 2 locality types defined above.

Temporal locality R: Instruction fetch (loops), Locals (loops, repeated invocations), Data Structures

Spatial locality R: Instruction sequence, Indexing Arrays, Locals in a stack frame, Contiguous allocation

4. Describe the idea of cache memory:

R:

Processor accesses "Memory hierarchy" -> Accesses first check small, fast "cache" -> If "miss" in cache, retrieve data from main memory -> Cache exploits locality to provide high performance.

5. Define some benefits and limitations of cache memory:

R:

Performance depends on application structure, amount of locality present.

Effectively reduce the average access time.

Don't work well if there is no locality (pathological access patterns).

A probabilistic way to improve performance.

6. Define the following cache fundamentals terms:

cache hit R: an access where the data is found in the cache.

cache miss R: an access which isn't

hit time R: time to access the cache

miss penalty R: time to move data from further level to closer, then to cpu

hit ratio R: percentage of time the data is found in the cache

miss ratio R: (1 - hit ratio)

7. How do you compute the size of an associative cache memory?

R:

Cache size = Number of sets * block size * associativity

8. Explain the following policy cache.

write-through R: all writes go to both cache and main memory

write-back R: writes go only to cache. Modified cache lines are written back to memory when the line is replaced.

write-allocate R: on a store miss, bring written line into the cache

write-around R: on a store miss, ignore cache

9. Explain the following terms regarding cache memory miss.

Compulsory (or cold-start) misses R: first access to the data.

Capacity misses R: we missed only because the cache isn't big enough.

Conflict misses R: we missed because the data maps to the same line as other data that forced it out of the cache.

10. Define the term "Virtual memory

R:

Virtual memory is the name of the technique that allows us to view main memory as a cache of a larger memory space (on disk). Is just a mapping function from virtual memory addresses to physical memory locations, which allows cacheing of virtual pages in physical memory.