Set associative cache

• The example assumes one cache line per index
  • Called a direct-mapped cache
  • A different access to a line evicts the resident cache line
  • This is either a capacity or a conflict miss
• Conflict misses can be reduced by providing multiple lines per index
• Access to an index returns a set of cache lines
  • For an n-way set associative cache there are n lines per set
• Carry out multiple tag comparisons in parallel to see if any one in the set hits

2-way set associative

Set associative cache

• When you need to evict a line in a particular set you run a replacement policy
  • LRU is a good choice: keeps the most recently used lines (favors temporal locality)
  • Thus you reduce the number of conflict misses
• Two extremes of set size: direct-mapped (1-way) and fully associative (all lines are in a single set)
  • Example: 32 KB cache, 2-way set associative, line size of 64 bytes: number of indices or number of sets=32*1024/(2*64)=256 and hence index is 8 bits wide
  • Example: Same size and line size, but fully associative: number of sets is 1, within the set there are 32*1024/64 or 512 lines; you need 512 tag comparisons for each access