OS selects a page frame to replace and saves its current contents to disk. 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.