Direct mapped means that every memory location maps to just one cache location. In the diagram, there are 4 cache slots.This means that Cache index 0 might hold memory index 0,4,8 and so on.

Set-associative mapping compromise between a fully associative cache and a direct mapped cache, as it increases speed.

In a naive execution (in comparison to a cache-oblivious execution) of such a seed-and-extend algorithm, the seed mapping locations to be compared to the read would be streamed through the cache.