ITPEC FE Subject A October 2024 Question 13

Look for the page in virtual memory on the hard disk — a major page fault means the page is not in physical memory at all, so the OS must read it from secondary storage.

A page fault occurs when a program touches a virtual page with no valid mapping to a physical frame:

• •Minor (soft) fault: the page is already in physical memory, for example shared with another process or still in the page cache, and the OS only updates the page table; no disk I/O
• •Major (hard) fault: the page is not in physical memory; the OS finds a free frame (evicting a page if needed), reads the page from the swap area or file on disk, updates the page table and restarts the instruction

Answer (c)

Why not others:

• •(a) — page faults are handled by the OS transparently; the user is never asked for input
• •(b) — the CPU cache is managed by hardware and holds copies of data already in main memory, so a page missing from memory cannot be found there
• •(d) — finding the page in physical memory is how a minor fault is resolved; in a major fault the page is not in physical memory

Key rule: Minor fault = page already in RAM, fix the mapping; major fault = page on disk, costly I/O. A flood of major faults is thrashing.

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