OS fetches the page referenced and places it into the freed page frame. OS changes the mem map and restarts the instruction that caused the trap. The Pentium uses 32-bit virtual addresses. If each page table entry occupies 4 bytes, that's 4MB of memory, just to store the page table. Every instruction fetch requires a memory reference.
Many instructions have a memory operand.
Page Table Design Alternatives Single page table stored in an array of fast hardware registers. OS loads registers from memory when a process is started. Context switches require the entire page table to be loaded. If it is large, this will be expensive. Page table kept entirely in main memory.