SetAssociative

Set-associative is a cache organization used in computer memory systems. In a set-associative cache, the cache is divided into multiple sets. Each memory address maps to exactly one set, but within that set the data can be stored in any one of several cache lines. The number of lines per set is called the associativity; common configurations include 2-way, 4-way, and 8-way sets. The total number of cache lines equals the number of sets times the associativity, and each line holds a block of memory and a tag.

Addressing in a set-associative cache typically divides the memory address into a tag, an index (which selects the set), and an offset (which selects data within the block). The index identifies the candidate set for a given address, while the tag is used to verify whether a line in that set contains the requested data. On a hit, data is retrieved from the corresponding line; on a miss, a new block is loaded into the set, potentially replacing an existing line according to a replacement policy such as LRU (least recently used), pseudo-LRU, or random.

Performance-wise, increasing associativity reduces conflict misses compared with direct-mapped caches and generally improves hit rates, but it raises hardware complexity, access time, and energy consumption. Fully associative caches offer maximum flexibility but are often impractical for large caches due to high complexity. Set-associative caches strike a balance and are widely used in modern CPUs, including L1 and L2 data caches. They can support different write policies (e.g., write-back or write-through) and are compatible with various coherence and consistency mechanisms in multi-core systems.