Virtual Memory and Caching

Why this module exists

Virtual memory is not just page tables. It is a set of caches and copying rules that make the system fast and safe: TLBs, copy-on-write, and page sharing.

1) TLBs

The Translation Lookaside Buffer (TLB) caches recent page table translations. It reduces translation cost but introduces new invariants:

• TLB entries must be invalidated on page table changes.
• Replacement policy matters under heavy locality shifts.

2) Copy-on-write (COW)

Fork would be too expensive if it copied the entire address space. COW avoids that:

• parent and child share pages marked read-only
• on first write, the page is copied

COW correctness depends on precise reference counting and fault handling.

What you will build

• A TLB simulator with LRU replacement
• A copy-on-write reference count simulator

Key takeaways

• TLBs are a cache, so eviction policy matters.
• COW trades memory for deferred work on writes.
• Small bookkeeping bugs cause silent data corruption.