Question 1

What is a page fault and what happens when one occurs?

Accepted Answer

An exception triggered when a process accesses a virtual memory page not currently loaded in physical RAM — the OS loads it from disk. Page fault handling: OS pauses the process, finds the page on disk (swap), loads it into a free frame, updates the page table, and resumes the process. Frequent page faults cause thrashing.

Question 2

What is copy-on-write (COW) in operating systems?

Accepted Answer

Parent and child processes share the same memory pages; a copy is made only when one of them modifies a page — making fork() very fast. After fork(), parent and child share pages marked read-only. When either writes, the OS copies just that page. COW avoids copying the entire address space upfront — critical for fork()+exec() efficiency.

Question 3

What is a zombie process?

Accepted Answer

A process that has finished executing but its entry remains in the process table until the parent reads its exit status via wait(). When a child finishes, it becomes a zombie until the parent calls wait() to collect its exit status. If the parent never calls wait(), zombies accumulate. If the parent dies, init (PID 1) adopts and reaps them.

Question 4

What is the difference between paging and segmentation in memory management?

Accepted Answer

Paging divides memory into fixed-size pages; segmentation divides memory into variable-size logical segments (code, stack, heap). Paging: fixed-size (e.g., 4KB) — eliminates external fragmentation but has internal fragmentation. Segmentation: variable-size logical units — matches programmer's view but has external fragmentation. Modern OSes use both together.

Question 5

What is an LRU (Least Recently Used) page replacement algorithm?

Accepted Answer

Evicts the page that has not been accessed for the longest time — approximates the optimal algorithm. LRU assumes recently used pages will be used again soon. Implemented with a doubly linked list + hashmap for O(1) operations. Commonly used in OS page replacement and CPU cache management.

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