ITPEC FE Morning October 2023 Question 17

Page fault — the page is valid in the virtual address space but not in physical memory, so the hardware interrupts and the OS loads it.

In a paging system the page table marks which virtual pages are present in main memory. Accessing a page that is mapped but absent raises a page fault interrupt; the OS reads the page in from auxiliary storage, replacing another page if memory is full, updates the page table and restarts the instruction. It is a normal event of virtual memory, not an error.

Answer (b)

Why not others:

• •(a) Fatal error — an unrecoverable failure that stops a program or the system; a page fault is handled and execution continues
• •(c) Scheduling error — relates to allocating the CPU among processes, not to memory access
• •(d) Segmentation fault — an access to an address the process is not allowed to use; here the page is mapped, so the access is legal

Key rule: A page fault means "valid but not loaded" and is resolved by paging in; a segmentation fault means "not allowed" and usually terminates the process.

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