Implement LRU Eviction Policy for Cache Replacement

Replacement Policies

When a cache set is full and a new tag arrives, a replacement policy picks the victim line. LRU (Least Recently Used) evicts the line untouched longest. It approximates temporal locality well though real hardware often uses pseudo-LRU trees for speed.

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Implementing LRU

• Counter LRU: stamp each way with access time
• Stack LRU: linked list per set (true order, more logic)
• Clock: single bit per line, good enough for many L2 caches

For example, accessing ways A, B, A in a 2-way set leaves B as LRU; the next conflicting miss evicts B.

For this exercise, you will add LRU tracking to your set-associative simulator. This task asks you to log victim way indices and verify LRU order on a scripted access sequence before benchmarking realistic traces.

Working Through the Exercise

Keep the relevant datasheet, ISA manual, or architecture textbook chapter open while you implement. When your output disagrees with the reference trace on the same program, the bug is usually a mis-decoded opcode, a stale register read, or a flag bit left unchanged after arithmetic.

• Trace first: walk one failing input by hand before changing random lines
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

For this exercise, you will use those habits while implementing the requirement in the starter code. Microarchitectural product names change across CPU generations, but the control ideas (fetch, bypass, cache lines, vector lanes) stay stable enough to debug from first principles.