Paging, page tables, and page faults: When does a page fault occur?

• A. The page is already loaded in RAM.
• B. The CPU is in privileged mode.
• C. The TLB translation is present.
• D. The page is not in RAM, prompting loading from disk.

Answer: (D)

In virtual memory, a page fault happens when a process accesses a virtual page that is not currently loaded into physical memory. The hardware traps to the operating system, which then loads the needed page from disk into RAM, updates the page table (and the TLB), and resumes execution. That makes the statement about the page not being in RAM, prompting loading from disk, the correct description.

If the page is already in RAM, there’s no page fault because the translation can be completed directly. The CPU mode (privileged vs user) and whether a translation is currently in the TLB are not what define a page fault; a page fault is specifically about the page’s presence in physical memory.