Core CS Resource• Study Guide

Operating Systems

Processes, Threads, CPU Scheduling, Deadlocks, Memory Management & Linux Internals

Operating System Interview Questions — Complete Notes#

Table of Contents#

[[#1. OS Fundamentals]] [[#2. Process Management]] [[#3. CPU Scheduling]] [[#4. Threads]] [[#5. Process Synchronization]] [[#6. Deadlocks]] [[#7. Memory Management]] [[#8. Virtual Memory]] [[#9. File Systems]] [[#10. I/O Management]] [[#11. System Calls]] [[#12. Kernel]] [[#13. Multiprocessing & Distributed Systems]] [[#14. Real-Time Systems]] [[#15. Classic OS Problems]] [[#16. Linux-Specific Questions]] [[#17. Quick Revision]]

1. OS Fundamentals#

Q1. What is an Operating System?#

An Operating System (OS) is system software that acts as an intermediary between users/applications and computer hardware. It manages hardware resources and provides services for programs.

Responsibilities

Process Management
Memory Management
File Management
Device Management
Security & Protection
CPU Scheduling

Examples

Windows
Linux
macOS
Android
iOS

Interview One-Liner:

"An OS is a resource manager and an abstraction layer between hardware and applications."

Q2. Why do we need an Operating System?#

Without an OS:

Applications would directly access hardware.
Resource conflicts would occur.
Memory management would be difficult.
Security would be nearly impossible.

Benefits

Ease of use
Resource sharing
Security
Reliability
Multitasking

Q3. What are the different types of Operating Systems?#

1. Batch Operating System

Jobs are collected and executed in batches.

Advantages

High throughput

Disadvantages

No interaction during execution

2. Multiprogramming OS

Multiple programs reside in memory simultaneously.

Goal

Keep CPU busy.

3. Multitasking OS

Multiple tasks appear to execute simultaneously through context switching.

Example:

Browsing
Music
Coding simultaneously

4. Multiprocessing OS

Uses multiple processors.

Examples:

Modern CPUs

5. Distributed OS

Multiple systems work together as one system.

6. Real-Time OS (RTOS)

Guarantees response within deadlines.

Examples:

Airbag Systems
Medical Equipment

Q4. What is the difference between Multiprogramming, Multitasking and Multiprocessing?#

Multiprogramming	Multitasking	Multiprocessing
Multiple programs in memory	Multiple tasks executed concurrently	Multiple CPUs execute tasks
Single CPU	Usually single CPU	Multiple CPUs
Goal: CPU utilization	Goal: User responsiveness	Goal: Parallelism

Q5. What are the goals of an Operating System?#

Primary Goals

Convenience
Efficiency
Resource Management
Fairness
Security
Reliability

2. Process Management#

Q6. What is a Process?#

A process is a program in execution.

A process consists of:

Code Section
Data Section
Heap
Stack
Registers
Program Counter

Example

Running Chrome Browser = Process

Q7. Program vs Process#

Program	Process
Passive entity	Active entity
Stored on disk	Stored in memory
Static	Dynamic
No state	Has state

Interview Example

Chrome.exe on disk → Program

Chrome currently running → Process

Q8. What are the states of a Process?#

text
New
 ↓
Ready
 ↓
Running
 ↓
Waiting
 ↓
Ready
 ↓
Terminated

New

Process is being created.

Ready

Waiting for CPU allocation.

Running

Currently executing.

Waiting

Waiting for I/O or event.

Terminated

Execution completed.

Q9. What is PCB (Process Control Block)?#

PCB stores information about a process.

Process ID
Process State
Program Counter
CPU Registers
Scheduling Information
Memory Information
Open Files

Importance

Without PCB, context switching is impossible.

Q10. What is Context Switching?#

Process of saving one process state and loading another.

Steps

Save current PCB
Load next PCB
Resume execution

Drawback

Consumes CPU time.

Interview Tip

Context switching is pure overhead.

No useful work is performed.

Q11. What is Process Scheduling?#

Mechanism of selecting a process from the ready queue and assigning CPU.

Objectives

Maximize CPU Utilization
Minimize Waiting Time
Minimize Turnaround Time
Maximize Throughput

Q12. What is a Zombie Process?#

A process that has finished execution but whose PCB still exists.

Reason

Parent has not collected exit status.

Solution

c
wait();

Interview Tip

Zombie = Dead process occupying process table entry.

Q13. What is an Orphan Process?#

A child process whose parent terminates before it.

What Happens?

OS assigns it to:

text
init process

(or systemd in modern Linux)

Q14. Zombie vs Orphan Process#

Zombie	Orphan
Child terminated	Parent terminated
PCB remains	Process still running
Wastes process table entry	Adopted by init

3. Threads#

Q15. What is a Thread?#

A thread is the smallest unit of CPU execution.

Multiple threads can exist inside a process.

Shared Resources

Code
Heap
Data

Private Resources

Stack
Registers
Program Counter

Q16. Process vs Thread#

Process	Thread
Heavyweight	Lightweight
Own memory	Shared memory
Expensive switching	Cheap switching
IPC required	Direct communication

Interview Tip

Threads are preferred for performance.

Processes are preferred for isolation.

Q17. What is Multithreading?#

Executing multiple threads concurrently inside a process.

Advantages

Better CPU utilization
Responsiveness
Resource sharing
Faster execution

Examples

Browser Tabs
VS Code
IDEs

Q18. User Thread vs Kernel Thread#

User Thread	Kernel Thread
Managed by user library	Managed by OS
Faster	Slower
OS unaware	OS aware
Blocking affects all threads	Blocking affects one thread

4. CPU Scheduling#

Q19. What is CPU Scheduling?#

Process of selecting which ready process gets CPU next.

Scheduling Criteria

CPU Utilization
Throughput
Waiting Time
Response Time
Turnaround Time

Q20. What are the Scheduling Algorithms?#

FCFS (First Come First Serve)

Processes execute in arrival order.

Advantage

Simple

Disadvantage

Convoy Effect

SJF (Shortest Job First)

Smallest burst time executes first.

Advantage

Minimum average waiting time

Disadvantage

Starvation

Priority Scheduling

Higher priority process executes first.

Problem

Starvation

Solution

Aging

Round Robin

Each process receives a fixed time quantum.

Advantage

Fair scheduling

Used In

Time-sharing systems

Multilevel Queue Scheduling

Processes divided into different queues.

Examples:

System Processes
Interactive Processes
Batch Processes

Multilevel Feedback Queue

Processes can move between queues.

Used in modern operating systems.

[!tip]

Most Frequently Asked OS Questions So Far:

✓ What is OS?
✓ Types of OS
✓ Process vs Program
✓ Process States
✓ PCB
✓ Context Switching
✓ Zombie Process
✓ Orphan Process
✓ Thread vs Process
✓ Multithreading
✓ CPU Scheduling
✓ FCFS
✓ SJF
✓ Round Robin
✓ Priority Scheduling

5. Process Synchronization#

Q21. What is Process Synchronization?#

Process synchronization ensures that multiple processes or threads can safely access shared resources without causing data inconsistency.

Why Needed?

When multiple processes access shared data simultaneously:

Race conditions may occur
Data corruption may occur
Incorrect results may be produced

Goal

Maintain data consistency.

Q22. What is the Critical Section Problem?#

A critical section is a part of a program where shared resources are accessed.

Example

c
count++;

If two threads execute simultaneously, incorrect results may occur.

Requirements of a Good Solution

Mutual Exclusion
Progress
Bounded Waiting

Q23. What is a Race Condition?#

A race condition occurs when the final result depends on the order of execution of concurrent processes.

Example

Initial Value:

text
count = 0

Thread A:

c
count++;

Thread B:

c
count++;

Expected:

text
count = 2

Possible Actual:

text
count = 1

Solution

Mutex
Semaphore
Monitor

Q24. What is Mutual Exclusion?#

Mutual exclusion ensures that only one process enters the critical section at a time.

Goal

Prevent simultaneous access to shared resources.

Q25. What is Peterson's Solution?#

A software-based solution to the critical section problem for two processes.

Uses

Turn Variable
Flag Array

Properties

✔ Mutual Exclusion

✔ Progress

✔ Bounded Waiting

Limitation

Works only for two processes.

Q26. What is a Semaphore?#

A semaphore is a synchronization mechanism used to control access to shared resources.

Operations

c
wait()
signal()

Working

wait() decreases semaphore value
signal() increases semaphore value

Q27. Types of Semaphore#

Binary Semaphore

Values:

text
0 or 1

Used like a lock.

Counting Semaphore

Values:

text
0 to N

Used when multiple instances of a resource exist.

Example:

text
Printer Pool
Connection Pool

Q28. What is a Mutex?#

Mutex stands for:

text
Mutual Exclusion

A lock that allows only one thread to access a resource.

Operations

c
lock()
unlock()

Important

The thread that acquires the mutex must release it.

Q29. Semaphore vs Mutex#

Semaphore	Mutex
Signaling mechanism	Locking mechanism
Integer variable	Object
No ownership	Ownership exists
Binary/Counting	Binary only
Used for synchronization	Used for mutual exclusion

Interview Tip

Mutex = Lock

Semaphore = Counter

Q30. What is a Monitor?#

A monitor is a high-level synchronization construct.

Contains

Shared Data
Procedures
Synchronization Logic

Advantage

Simpler and safer than semaphores.

Q31. What is a Spinlock?#

A lock where a thread repeatedly checks until the lock becomes available.

c
while(lock == BUSY);

Advantages

No context switching

Disadvantages

CPU wastage

Used When

Waiting time is very short.

6. Deadlocks#

Q32. What is a Deadlock?#

A deadlock occurs when two or more processes wait indefinitely for resources held by each other.

Example

Process P1 holds Resource A and waits for B.

Process P2 holds Resource B and waits for A.

Both wait forever.

Q33. Necessary Conditions for Deadlock#

All four conditions must occur simultaneously.

1. Mutual Exclusion

Resource cannot be shared.

2. Hold and Wait

Process holds one resource and waits for another.

3. No Preemption

Resources cannot be forcibly taken away.

4. Circular Wait

Processes form a cycle of waiting.

Memory Trick

text
MHNC

M → Mutual Exclusion
H → Hold and Wait
N → No Preemption
C → Circular Wait

Q34. How Can Deadlocks Be Handled?#

Deadlock Prevention

Break one of the four conditions.

Deadlock Avoidance

Allocate resources carefully.

Example:

text
Banker's Algorithm

Deadlock Detection

Allow deadlocks and detect them later.

Deadlock Recovery

Recover after detection.

Methods:

Kill Process
Resource Preemption

Q35. What is Banker's Algorithm?#

A deadlock avoidance algorithm.

Principle

Grant resources only if the system remains in a safe state.

Invented By

Edsger Dijkstra

Q36. What is a Safe State?#

A state where all processes can complete execution in some order.

Safe State

text
All processes can finish.

Unsafe State

text
Deadlock may occur.

Q37. Deadlock vs Starvation#

Deadlock	Starvation
Processes wait forever	One process waits forever
Circular dependency	Scheduling issue
No progress	Some processes continue
Resource problem	Priority problem

Q38. What is Starvation?#

A process waits indefinitely because higher-priority processes keep getting resources.

Example

Priority Scheduling

Low-priority process never executes.

Q39. What is Aging?#

A technique used to prevent starvation.

Idea

Increase process priority gradually over time.

Benefit

Eventually every process gets CPU time.

7. Classic Synchronization Problems#

Q40. What is the Producer Consumer Problem?#

A classic synchronization problem.

Producer

Produces data.

Consumer

Consumes data.

Problems

Buffer Overflow

Producer produces too fast.

Buffer Underflow

Consumer consumes too fast.

Solution

Using:

Semaphore
Mutex

Real World Examples

Kafka Consumers
RabbitMQ Queues
Job Queues
Producer-Consumer Systems

[!tip]

Most Asked Interview Questions From This Section:

✓ Critical Section Problem

✓ Race Condition

✓ Peterson's Solution

✓ Semaphore

✓ Binary vs Counting Semaphore

✓ Mutex

✓ Mutex vs Semaphore

✓ Monitor

✓ Spinlock

✓ Deadlock

✓ Four Deadlock Conditions

✓ Banker's Algorithm

✓ Safe State

✓ Starvation

✓ Aging

✓ Producer Consumer Problem

7. Classic Synchronization Problems#

Q41. What is the Readers-Writers Problem?#

A synchronization problem where:

Readers

Only read data.
Do not modify data.

Writers

Modify data.

Rules

Multiple readers can read simultaneously.
Only one writer can write at a time.
No reader should read while a writer is writing.

Goal

Maximize concurrency while maintaining consistency.

Real-World Example

Database systems:

text
SELECT → Reader
INSERT/UPDATE/DELETE → Writer

Q42. What is the Dining Philosophers Problem?#

A classic synchronization and deadlock problem.

Scenario

Five philosophers sit around a table.
Five forks are placed between them.
A philosopher needs both left and right forks to eat.

Problem

All philosophers pick one fork simultaneously.

Result:

text
Deadlock

Solutions

Resource ordering
Semaphore
Arbitrator (Waiter)

Interview Tip

Used to explain:

Deadlocks
Resource Allocation
Starvation

8. Memory Management#

Q43. What is Memory Management?#

Memory management is the process of managing RAM efficiently.

Responsibilities

Allocation
Deallocation
Protection
Address Translation

Goal

Maximize memory utilization.

Q44. What is Logical Address vs Physical Address?#

Logical Address

Generated by CPU.

Physical Address

Actual address in RAM.

Mapping

Performed by:

text
MMU (Memory Management Unit)

Q45. What is Swapping?#

Moving processes between RAM and secondary storage.

Why?

When RAM becomes insufficient.

Steps

Process moved to disk.
Space freed in RAM.
Process loaded back when needed.

Drawback

Disk is slower than RAM.

Q46. What is Contiguous Memory Allocation?#

Each process gets one continuous block of memory.

Advantages

Simple implementation.

Disadvantages

Leads to fragmentation.

Q47. What is Fragmentation?#

Unused memory that cannot be utilized efficiently.

Types

Internal Fragmentation

Unused memory inside allocated block.

Example:

text
Need = 18 KB
Allocated = 20 KB

Waste = 2 KB

External Fragmentation

Free memory exists but scattered.

Example:

text
10 KB Free
20 KB Free
15 KB Free

Need = 40 KB

Cannot allocate.

Q48. Internal vs External Fragmentation#

Internal	External
Waste inside block	Waste between blocks
Fixed allocation	Variable allocation
Paging suffers	Segmentation suffers

Q49. What is Compaction?#

Technique to eliminate external fragmentation.

Idea

Move processes together.

Result

One large free block.

Drawback

Expensive operation.

9. Paging#

Q50. What is Paging?#

Paging is a memory management technique where:

Logical Memory

Divided into:

text
Pages

Physical Memory

Divided into:

text
Frames

Mapping

Page Table maps pages to frames.

Q51. Why is Paging Used?#

Benefits

✔ Eliminates external fragmentation

✔ Better memory utilization

✔ Supports virtual memory

Q52. What is a Page Table?#

A data structure used to map:

text
Logical Address → Physical Address

Maintained By

Operating System

Q53. What is Effective Access Time (EAT)?#

Actual time required to access memory considering page faults.

Formula

text
EAT =
(1-p) × Memory Access Time
+
p × Page Fault Service Time

Where:

text
p = Page Fault Probability

10. Segmentation#

Q54. What is Segmentation?#

Memory divided according to logical units.

Examples:

Code
Stack
Heap
Data

Segment Table

Stores information about segments.

Q55. Paging vs Segmentation#

Paging	Segmentation
Fixed-size blocks	Variable-size blocks
Pages and Frames	Segments
Eliminates external fragmentation	May suffer external fragmentation
User invisible	User visible

Interview Tip

Paging = Physical View

Segmentation = Logical View

11. Virtual Memory#

Q56. What is Virtual Memory?#

A technique that allows execution of processes larger than physical memory.

Idea

Only required pages remain in RAM.

Remaining pages stay on disk.

Advantages

Large address space
Better multiprogramming
Efficient memory usage

Q57. What is Demand Paging?#

Pages are loaded only when required.

Benefits

Faster startup
Reduced memory usage

Used In

Almost all modern operating systems.

Q58. What is a Page Fault?#

Occurs when a required page is not present in RAM.

Steps

Trap to OS.
Locate page on disk.
Load page into frame.
Update page table.
Resume execution.

Interview Tip

Page fault is normal.

Too many page faults indicate a problem.

Q59. What is Thrashing?#

System spends more time swapping pages than executing processes.

Symptoms

High CPU wait
Low throughput
Excessive page faults

Causes

Insufficient RAM
High degree of multiprogramming

Solution

Increase RAM
Reduce multiprogramming

Q60. What is Locality of Reference?#

Programs tend to access the same memory locations repeatedly.

Types

Temporal Locality

Recently used data likely to be used again.

Example:

c
for(i=0;i<100;i++)

Spatial Locality

Nearby memory locations likely to be accessed.

Example:

c
arr[0]
arr[1]
arr[2]

Importance

Basis of:

Cache Memory
Paging
Virtual Memory

[!tip]

Most Asked Questions From This Section:

✓ Fragmentation

✓ Internal vs External Fragmentation

✓ Paging

✓ Page Table

✓ Segmentation

✓ Paging vs Segmentation

✓ Virtual Memory

✓ Demand Paging

✓ Page Fault

✓ Thrashing

✓ Locality of Reference

✓ Compaction

✓ Logical vs Physical Address

These questions alone account for nearly 40% of OS interview questions in placements and product-company interviews.

12. Page Replacement Algorithms#

Q61. Why Do We Need Page Replacement?#

When a page fault occurs and no free frame is available, the OS must decide which page to remove.

This decision is made using page replacement algorithms.

Goal

Reduce page faults
Improve performance

Q62. What is FIFO Page Replacement?#

FIFO (First In First Out) removes the page that entered memory first.

Example

text
Frames = 3

1 2 3 4

Remove 1

Advantages

Simple
Easy to implement

Disadvantages

Poor performance
Suffers from Belady's Anomaly

Q63. What is LRU (Least Recently Used)?#

Removes the page that has not been used for the longest time.

Idea

Past behavior predicts future behavior.

Example

text
Pages used recently
stay in memory.

Advantages

Better than FIFO
Widely used

Disadvantages

Requires tracking page usage

Q64. What is Optimal Page Replacement?#

Removes the page that will not be used for the longest time in the future.

Advantage

Produces minimum page faults.

Disadvantage

Future knowledge is impossible.

Use

Benchmarking other algorithms.

Q65. Compare FIFO, LRU and Optimal#

Algorithm	Performance	Practical
FIFO	Worst	Yes
LRU	Better	Yes
Optimal	Best	No

Interview Answer

text
Optimal > LRU > FIFO

Q66. What is Belady's Anomaly?#

Increasing the number of frames increases page faults.

Occurs In

✔ FIFO

Does NOT Occur In

✔ LRU

✔ Optimal

Interview Favorite

One of the most frequently asked paging questions.

Q67. What is a TLB (Translation Lookaside Buffer)?#

A cache that stores recently used page table entries.

Purpose

Reduce address translation time.

Without TLB

text
Page Table Access
+
Memory Access

With TLB

Often only:

text
Memory Access

Interview Tip

TLB is a cache for page tables.

Q68. What is TLB Hit and TLB Miss?#

TLB Hit

Required entry found in TLB.

Fast access.

TLB Miss

Entry not found.

Need page table lookup.

Slower access.

Q69. What is Cache Memory?#

A small, extremely fast memory located near CPU.

Purpose

Store frequently accessed data.

Memory Hierarchy

text
Registers
↓
Cache
↓
RAM
↓
SSD/HDD

Speed

text
Cache > RAM > Disk

Q70. What is Cache Hit and Cache Miss?#

Cache Hit

Data found in cache.

Fast access.

Cache Miss

Data not found.

Need RAM access.

Slower.

13. File Systems#

Q71. What is a File System?#

A method used by OS to organize and store files.

Responsibilities

File Storage
Retrieval
Access Control
Naming

Examples

FAT32
NTFS
ext4
APFS

Q72. What is a File?#

A named collection of related information stored on secondary storage.

Examples:

text
notes.txt
photo.jpg
video.mp4

Q73. What is a Directory?#

A directory is a collection of files and subdirectories.

Purpose

Organize files logically.

Example:

text
/Documents
/Downloads
/Pictures

Q74. Absolute Path vs Relative Path#

Absolute Path

Starts from root.

Example:

text
/home/user/docs/file.txt

Relative Path

Starts from current directory.

Example:

text
docs/file.txt

Q75. What is File Allocation?#

Process of allocating disk blocks to files.

Methods

Contiguous Allocation

Blocks stored consecutively.

Linked Allocation

Blocks linked together.

Indexed Allocation

Index block stores pointers.

Q76. Contiguous vs Linked vs Indexed Allocation#

Method	Access Speed	Fragmentation
Contiguous	Fastest	High
Linked	Slow	Low
Indexed	Moderate	Low

Q77. What is Inode?#

A data structure in Linux storing file metadata.

Contains

Owner
Permissions
Size
Block Addresses

Does NOT Store

text
Filename

Interview Favorite

"What does inode store?"

Answer:

Everything except filename.

14. Disk Scheduling#

Q78. Why is Disk Scheduling Needed?#

Disk requests arrive simultaneously.

OS decides servicing order.

Goal

Reduce seek time.

Q79. What is Seek Time?#

Time required to move disk head to desired track.

Components of Disk Access

text
Seek Time
+
Rotational Latency
+
Transfer Time

Q80. What is FCFS Disk Scheduling?#

Requests served in arrival order.

Advantages

Simple

Disadvantages

Poor performance

Large seek time.

Q81. What is SSTF (Shortest Seek Time First)?#

Chooses request nearest to current head position.

Advantages

Better performance

Disadvantages

Starvation possible.

Q82. What is SCAN Algorithm?#

Disk head moves in one direction servicing requests.

Then reverses.

Also Called

text
Elevator Algorithm

Q83. What is C-SCAN?#

Circular SCAN.

Working

Move in one direction.
Reach end.
Jump back to beginning.
Continue.

Advantage

Uniform waiting time.

Q84. SCAN vs C-SCAN#

SCAN	C-SCAN
Moves both directions	Single direction
Unequal waiting	More uniform waiting
Simpler	Fairer

[!tip]

Most Asked Questions From This Section:

✓ FIFO

✓ LRU

✓ Optimal

✓ Belady's Anomaly

✓ TLB

✓ Cache Memory

✓ File System

✓ Inode

✓ Contiguous Allocation

✓ Linked Allocation

✓ Indexed Allocation

✓ FCFS Disk Scheduling

✓ SSTF

✓ SCAN

✓ C-SCAN

These are extremely common in placement interviews and OS viva exams.

15. I/O Management#

Q85. What is I/O Management?#

I/O Management is responsible for managing communication between CPU and I/O devices.

Examples

Keyboard
Mouse
Printer
Hard Disk
Network Card

Responsibilities

Device Allocation
Buffering
Scheduling
Interrupt Handling

Q86. What is an Interrupt?#

An interrupt is a signal sent to CPU requesting immediate attention.

Purpose

Instead of continuously checking devices, CPU gets notified when required.

Examples

Keyboard Key Press
Mouse Click
Disk Read Complete

Q87. Types of Interrupts#

Hardware Interrupt

Generated by hardware devices.

Examples:

Keyboard
Disk
Network Card

Software Interrupt

Generated by software.

Examples:

System Calls
Exceptions

Q88. What Happens When an Interrupt Occurs?#

CPU pauses current process.
Saves current state.
Executes Interrupt Service Routine (ISR).
Returns to original process.

Interview Tip

Interrupts improve CPU efficiency.

Q89. What is Polling?#

CPU continuously checks device status.

Example:

c
while(device_not_ready);

Advantages

Simple implementation.

Disadvantages

Wastes CPU cycles.

Q90. Polling vs Interrupts#

Polling	Interrupt
CPU checks device repeatedly	Device notifies CPU
Wastes CPU	Efficient
Simpler	Better performance
High CPU usage	Low CPU usage

Interview Favorite

Interrupts are preferred in modern systems.

Q91. What is DMA (Direct Memory Access)?#

DMA allows devices to transfer data directly between device and memory without CPU involvement.

Traditional Transfer

text
Device
 ↓
CPU
 ↓
Memory

DMA Transfer

text
Device
 ↓
Memory

Advantages

Faster I/O
Reduced CPU overhead

Q92. Why is DMA Faster?#

CPU does not handle every byte transfer.

CPU only:

Starts DMA.
Continues other work.
Receives completion interrupt.

16. System Calls#

Q93. What is a System Call?#

A system call is an interface between user programs and the Operating System.

Purpose

Allows user applications to request kernel services.

Examples:

c
read()
write()
fork()
exec()
open()
close()

Q94. Why Can't User Programs Access Hardware Directly?#

For:

Security
Stability
Resource Protection

Only kernel can directly access hardware.

Q95. User Mode vs Kernel Mode#

User Mode	Kernel Mode
Limited privileges	Full privileges
Applications run here	OS runs here
Cannot access hardware	Can access hardware
Safer	Powerful

Interview Tip

System calls switch execution from User Mode to Kernel Mode.

Q96. What Happens During a System Call?#

User process requests service.
Trap generated.
CPU enters Kernel Mode.
Kernel performs operation.
CPU returns to User Mode.

Q97. What is fork()?#

Creates a child process.

c
pid = fork();

Returns

Parent:

text
Child PID

Child:

text
0

Q98. What is exec()?#

Replaces current process image with a new program.

Example

c
exec("chrome");

Current program disappears.

New program starts.

Q99. fork() vs exec()#

fork()	exec()
Creates process	Replaces process
New PID created	Same PID
Copies address space	Loads new program

Common Linux Pattern

c
fork();
exec();

Used by shell commands.

Q100. What is Copy-On-Write (COW)?#

Optimization used with fork().

Without COW

Entire memory copied.

With COW

Memory shared initially.

Copy created only when modification occurs.

Benefits

Faster fork()
Less memory usage

17. Kernel#

Q101. What is a Kernel?#

Kernel is the core component of the Operating System.

Responsibilities

Process Management
Memory Management
Scheduling
Device Management
File Management

Q102. Types of Kernel#

Monolithic Kernel

All services run in kernel space.

Examples:

Linux
UNIX

Microkernel

Only essential services run in kernel space.

Examples:

QNX
MINIX

Hybrid Kernel

Combination of Monolithic and Microkernel.

Examples:

Windows
macOS

Q103. Monolithic vs Microkernel#

Monolithic	Microkernel
Faster	More secure
Large kernel	Small kernel
Less modular	Highly modular
Failure may crash system	Better fault isolation

Interview Favorite

Linux → Monolithic

MINIX → Microkernel

18. Multiprocessing & Advanced Systems#

Q104. What is SMP (Symmetric Multiprocessing)?#

Multiple processors share:

Same Memory
Same OS

Benefits

Parallelism
Better Throughput
Reliability

Q105. What is Asymmetric Multiprocessing?#

One processor controls the system.

Other processors execute assigned tasks.

Less common today.

Q106. What is a Real-Time Operating System (RTOS)?#

OS designed to meet strict timing deadlines.

Examples

Airbag Systems
Medical Equipment
Aircraft Control Systems

Q107. Hard RTOS vs Soft RTOS#

Hard RTOS	Soft RTOS
Missing deadline unacceptable	Missing deadline acceptable
Airbags	Video Streaming
Mission Critical	User Experience

19. Linux & Miscellaneous#

Q108. What is a Daemon Process?#

A background process running continuously.

Examples:

text
sshd
httpd
crond

Linux Hint

Most daemon names end with:

text
d

Q109. What Happens During Booting?#

Steps

BIOS/UEFI starts.
Bootloader loads.
Kernel loads.
Drivers initialize.
Services start.
Login screen appears.

Memory Trick

text
BIOS → Bootloader → Kernel → Services

Q110. Throughput vs Turnaround Time vs Waiting Time vs Response Time#

Throughput

Processes completed per unit time.

Turnaround Time

text
Completion Time - Arrival Time

Total time spent in system.

Waiting Time

text
Turnaround Time - Burst Time

Time spent waiting in ready queue.

Response Time

text
First Response - Arrival Time

Important for interactive systems.

OS Interview Rapid Fire (Must Memorize)#

1-Day OS Revision Sheet#

text
OS = Resource Manager

Process = Program in Execution

Thread = Smallest Unit of Execution

PCB = Stores Process Information

Context Switch = Save + Load Process State

Zombie = Dead Child, PCB Exists

Orphan = Parent Dead, Child Alive

Mutex = Lock

Semaphore = Counter

Deadlock Conditions:
1. Mutual Exclusion
2. Hold and Wait
3. No Preemption
4. Circular Wait

Paging = Pages + Frames

Segmentation = Logical Division

Virtual Memory = Illusion of Large Memory

Page Fault = Page Not in RAM

Thrashing = Excessive Swapping

FIFO → Belady's Anomaly

LRU → Most Common

TLB = Cache for Page Table

fork() = Create Process

exec() = Replace Process

Kernel = Core of OS

Linux = Monolithic Kernel

Interrupt = Device Notifies CPU

DMA = Device ↔ Memory Direct Transfer

RTOS = Deadline-Oriented OS

[!tip]

If you master Q1–Q110, you can comfortably answer around 90–95% of Operating System questions asked in:

✓ Internship Interviews

✓ Service Companies (TCS, Infosys, Wipro, Accenture)

✓ Product Companies (Amazon, Microsoft, Atlassian, Walmart, Adobe)

✓ College Viva

✓ GATE/University Exams

✓ Backend & Systems Interviews