What is an inverted page table and when is it used?

Inverted page table has one entry per physical frame (not per virtual page), containing process ID and virtual page number. Size is proportional to physical memory, not virtual space - important for 64-bit systems where virtual space is huge. Lookup requires searching/hashing by (PID, virtual page). Hash table implementation provides O(1) average lookup. Used in IA-64, PowerPC. TLB is crucial as table search is expensive. Hybrid approaches combine with traditional page tables.