Set-associative cache
Beginner
A cache where each block can go in a few spots, not just one. That way, two blocks that want the same place can both stay.
Novice
A cache divided into sets of a few lines (ways). An address picks one set; the line can go in any way of that set. Direct-mapped is 1 way per set; fully associative is one set holding every line.
Expert
An N-way cache reads N tags in parallel and compares them, so more ways cost energy and hit time. Sets = capacity ÷ (line × ways); index bits = log2(sets). 8–16 ways is common in L1 and L2.
Explained in Caches and coherence (Architectures).
See also: Cache, Cache line (block), 3Cs (compulsory, capacity, conflict misses).