What is a TLB and why is it important?

TLB (Translation Lookaside Buffer) is a hardware cache of recent virtual-to-physical address translations. Without TLB, every memory access requires page table lookup (one or more memory accesses). TLB hit rate is typically >99% due to locality. TLB miss triggers page table walk. TLB is flushed on context switch (or uses ASIDs to avoid). Size is small (64-1024 entries) but critical for performance - TLB miss can cost 10-100 cycles.