2025 question paper

Operating System

28 questions

Q1a. Which type of Operating System allows multiple users to access the system simultaneously? (i) Single-user OS (ii) Batch OS (iii) Multi-user OS (iv) Embedded OS20252m
Module 1: Introduction to Operating Systems
Which type of Operating System allows multiple users to access the system simultaneously?

(i) Single-user OS
(ii) Batch OS
(iii) Multi-user OS
(iv) Embedded OS

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Worked Solution
Answer: (iii) Multi-user OS

Multi-user OS (e.g., UNIX, Linux, Windows Server) allows multiple users to access the system simultaneously via terminals/remote sessions, with isolation and access control.

Single-user OS allows only one user at a time.

Batch OS executes jobs without user interaction.

Embedded OS is specialized for devices, not multi-user access.
Q1b. Which scheduler selects processes from the ready queue and allocates the CPU? (i) Long-term scheduler (ii) Medium-term scheduler (iii) Short-term scheduler (iv) Batch scheduler20252m
Module 2: Processes & Scheduling
Which scheduler selects processes from the ready queue and allocates the CPU?

(i) Long-term scheduler
(ii) Medium-term scheduler
(iii) Short-term scheduler
(iv) Batch scheduler

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Worked Solution
Answer: (iii) Short-term scheduler (CPU scheduler)

Long-term: selects jobs from job pool to ready queue (controls degree of multiprogramming)

Medium-term: swapping (suspend/resume)

Short-term / CPU scheduler: selects process from ready queue and allocates CPU via dispatcher (runs most frequently, must be fast)

Batch scheduler is not an OS scheduler type.
Q1c. What is the motive of using a Virtual Machine for an operating system? (i) Increase CPU throughput (ii) Run multiple operating systems on the same hardware (iii) Create databases (iv) Allow multiple threads to execute concurrently within a process20252m
Module 1: Introduction to Operating Systems
What is the motive of using a Virtual Machine for an operating system?

(i) Increase CPU throughput
(ii) Run multiple operating systems on the same hardware
(iii) Create databases
(iv) Allow multiple threads to execute concurrently within a process

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Worked Solution
Answer: (ii) Run multiple operating systems on the same hardware

VM provides virtualization via hypervisor (Type1 bare-metal or Type2 hosted), allowing isolation, consolidation, testing and safe execution of multiple OS instances on one physical host. Not primarily for CPU throughput or databases.
Q1d. Which type of thread is managed directly by the operating system kernel? (i) User-level thread (ii) Kernel-level thread (iii) Single thread (iv) Background thread20252m
Module 2: Processes & Scheduling
Which type of thread is managed directly by the operating system kernel?

(i) User-level thread
(ii) Kernel-level thread
(iii) Single thread
(iv) Background thread

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Worked Solution
Answer: (ii) Kernel-level thread

Kernel threads are managed directly by kernel (visible to scheduler). User-level threads are managed by user library, kernel unaware. Kernel threads allow true parallelism on multicore but have higher creation/scheduling overhead.
Q1e. The Dining Philosopher Algorithm is a classic example of which problem? (i) CPU scheduling (ii) Memory fragmentation (iii) Process synchronization and deadlock (iv) File allocation20252m
Module 3: Inter-process Communication
The Dining Philosopher Algorithm is a classic example of which problem?

(i) CPU scheduling
(ii) Memory fragmentation
(iii) Process synchronization and deadlock
(iv) File allocation

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Worked Solution
Answer: (iii) Process synchronization and deadlock

Dining philosophers share forks (resources); illustrates synchronization, deadlock, starvation. Classic IPC problem solved by semaphores/monitors.
Q1f. Which technique allows the operating system to examine the resource allocation state dynamically to ensure no deadlock occurs? (i) Deadlock detection (ii) Deadlock recovery (iii) Deadlock avoidance (iv) Deadlock ignoring20252m
Module 4: Deadlocks
Which technique allows the operating system to examine the resource allocation state dynamically to ensure no deadlock occurs?

(i) Deadlock detection
(ii) Deadlock recovery
(iii) Deadlock avoidance
(iv) Deadlock ignoring

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Worked Solution
Answer: (iii) Deadlock avoidance

Banker's algorithm (avoidance) dynamically checks safe state before allocating. Prevention prohibits one of four necessary conditions, Detection+Recovery allows deadlock then resolves, Ignoring (ostrich) does nothing.
Q1g. Which free-space management technique uses a bitmap to indicate free and occupied blocks? (i) Bit Vector (ii) Indexing (iii) Grouping (iv) Linked List20252m
Module 6: I/O & File Management
Which free-space management technique uses a bitmap to indicate free and occupied blocks?

(i) Bit Vector
(ii) Indexing
(iii) Grouping
(iv) Linked List

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Worked Solution
Answer: (i) Bit Vector

Bit vector/bitmap: each bit represents a block, 1=free, 0=allocated (or vice versa). Allows fast lookup. Grouping, linked list, counting are other methods.
Q1h. Internal fragmentation occurs when (i) Processes are swapped out (ii) Pages are missing (iii) Memory is completely utilized (iv) Unused space exists within an allocated memory partition20252m
Module 5: Memory Management
Internal fragmentation occurs when

(i) Processes are swapped out
(ii) Pages are missing
(iii) Memory is completely utilized
(iv) Unused space exists within an allocated memory partition

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Worked Solution
Answer: (iv) Unused space exists within an allocated memory partition

Internal fragmentation = waste inside allocated block (e.g., fixed 10KB block allocated to 8KB process leaves 2KB internal). External = free holes scattered outside.
Q1i. Which page replacement algorithm replaces the page that will not be used for the longest period in the future? (i) Optimal (ii) Second Chance (iii) FIFO (iv) NRU20252m
Module 5: Memory Management
Which page replacement algorithm replaces the page that will not be used for the longest period in the future?

(i) Optimal
(ii) Second Chance
(iii) FIFO
(iv) NRU

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Worked Solution
Answer: (i) Optimal

Optimal (Belady's) replaces page farthest in future — theoretically minimal faults but requires future knowledge, so not realizable; used as benchmark. Approximated by LRU.
Q1j. Which disk scheduling algorithm services requests in the order they arrive? (i) SCAN (ii) SSTF (iii) FCFS (iv) C-SCAN20252m
Module 6: I/O & File Management
Which disk scheduling algorithm services requests in the order they arrive?

(i) SCAN
(ii) SSTF
(iii) FCFS
(iv) C-SCAN

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Worked Solution
Answer: (iii) FCFS

FCFS (First-come-first-serve) services in arrival order, simple but convoy effect. SCAN/SSTF reorder to minimize seek.
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Q2a. Explain system calls and their importance in Operating Systems. Discuss various categories of system calls.20257m
Module 1: Introduction to Operating Systems
Explain system calls and their importance in Operating Systems. Discuss various categories of system calls.

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Worked Solution
System Calls & Importance

Interface for user programs to request kernel services (trap to kernel mode). Essential for protected hardware access.

Categories

Process Control: fork, exec, exit, wait, kill

File Management: open, read, write, close, unlink

Device Management: ioctl, read/write device

Information Maintenance: getpid, time, sysinfo

Communication: pipe, shmget, send/recv

Protection: chmod, chown
Importance: provides abstraction, security, portability. Invoked via syscall instruction (int 0x80 / syscall). Library wrappers handle setup.
Q2b. Illustrate the concept of Virtual Machines.20257m
Module 1: Introduction to Operating Systems
Illustrate the concept of Virtual Machines.

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Worked Solution
Virtual Machine

VM is software emulation of physical machine via hypervisor.

Type1 (bare-metal): hypervisor on hardware (VMware ESXi, Xen)

Type2 (hosted): hypervisor on host OS (VirtualBox)
Components: virtual CPU, virtual memory, virtual devices. Isolation, snapshot, migration. Uses: server consolidation, testing, cloud. Advantages: isolation, resource utilization. Cost: overhead, I/O virtualization. Example: running Linux+Windows simultaneously.
Q3a. What is a Process Control Block? Explain its structure and significance in process management.20257m
Module 2: Processes & Scheduling
What is a Process Control Block? Explain its structure and significance in process management.

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Worked Solution
PCB Structure & Significance

Fields: PID, state, PC, registers, scheduling info, memory management (page table base), I/O info, accounting. Significance: enables context switch, scheduling, process management; kernel's repository for resume. Stored in kernel space per process.
Q3b. Define threads and explain their various states. Compare user-level threads with kernel-level threads.20257m
Module 2: Processes & Scheduling
Define threads and explain their various states. Compare user-level threads with kernel-level threads.

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Worked Solution
Threads & States

Thread = lightweight process, dispatchable unit, shares address space, files of parent.
States: New, Ready, Running, Blocked, Terminated.

ULT vs KLT

ULT: managed by user library, many-to-one, fast, kernel unaware, blocking blocks process.

KLT: managed by kernel, one-to-one, slower but true parallelism, blocking of one KLT not block process. Hybrid many-to-many combines.
Q4a. Discuss multiprocessor scheduling and real-time scheduling techniques.20257m
Module 2: Processes & Scheduling
Discuss multiprocessor scheduling and real-time scheduling techniques.

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Worked Solution
Multiprocessor & Real-time Scheduling

Multiprocessor: load sharing, affinity, gang scheduling; approaches: SMP symmetric (each CPU self-schedules) vs master-slave. Need load balancing.
Real-time: hard (deadline must met) vs soft. Algorithms: Rate Monotonic (priority ∝ period), EDF (earliest deadline first), priority inheritance to avoid inversion.
Q4b. Consider a system with five processes P0 through P4 and four resources type A, B, C and D. Using the Banker's algorithm answer that the system is in a safe state? | Process | Allocation (A B C D) | Max (A B C D) | Available (A B C D) | | :--- | :---: | :---: | :---: | | P0 | 0 0 1 2 | 0 0 1 2 | 1 5 2 0 | | P1 | 1 0 0 0 | 1 7 5 0 | | | P2 | 1 3 5 4 | 2 3 5 6 | | | P3 | 0 6 3 2 | 0 6 5 2 | | | P4 | 0 0 1 4 | 0 6 5 6 | |20257m
Module 4: Deadlocks
Consider a system with five processes P0 through P4 and four resources type A, B, C and D. Using the Banker's algorithm answer that the system is in a safe state?

Process Allocation (A B C D) Max (A B C D) Available (A B C D)

P0 0 0 1 2 0 0 1 2 1 5 2 0

P1 1 0 0 0 1 7 5 0

P2 1 3 5 4 2 3 5 6

P3 0 6 3 2 0 6 5 2

P4 0 0 1 4 0 6 5 6

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Worked Solution
Banker's Safe State Check (5 processes A-D)

Available 1,5,2,0. Need = Max-Alloc.
P0 Need 0,0,0,0 → can run, completes, releases 0,0,1,2 → Avail 1,5,3,2.
P3 Need 0,0,2,0 ≤ Avail → run → Avail 1,11,6,4.
P1 Need 0,7,5,0 ≤ Avail → run → Avail 2,11,6,4.
P2 Need 1,0,0,2 ≤ Avail → run → Avail 3,14,11,8.
P4 Need 0,6,4,2 ≤ Avail → finishes.
Safe sequence P0→P3→P1→P2→P4 exists, so system is safe.
Q5a. Explain the structure of a file system. Discuss the role of directories, metadata, and file control blocks in file management.20257m
Module 6: I/O & File Management
Explain the structure of a file system. Discuss the role of directories, metadata, and file control blocks in file management.

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Worked Solution
File System Structure

Layers: logical FS, file-organization, basic FS, I/O control. Components: Boot block, Superblock, FCB/inode (metadata: name, size, permissions, blocks), Directory (maps name→FCB), Data blocks, Free space map.
Directories: single/two-level/tree/acyclic. FCB: holds metadata. Role: directory resolves path, FCB tracks per-file info.
Q5b. Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 143, and the previous request was at cylinder 125. The queue of pending request, in FIFO order is: 86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130 Starting from the current head position, applying Shortest-seek-time-first (SSTF) algorithm find what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests.20257m
Module 6: I/O & File Management
Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 143, and the previous request was at cylinder 125. The queue of pending request, in FIFO order is:
86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130

Starting from the current head position, applying Shortest-seek-time-first (SSTF) algorithm find what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests.

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Worked Solution
SSTF 5000 cylinders head 143 requests 86,1470,913,1774,948,1509,1022,1750,130

Order: 143→130(13)=13,130→86(44)=57,86→913(827)=884,913→948(35)=919,948→1022(74)=993,1022→1470(448)=1441,1470→1509(39)=1480,1509→1750(241)=1721,1750→1774(24)=1745.
Total 1745 cylinders.
Q6a. Describe the objectives of process scheduling. Explain the roles of long-term, medium-term, and short-term schedulers.20257m
Module 2: Processes & Scheduling
Describe the objectives of process scheduling. Explain the roles of long-term, medium-term, and short-term schedulers.

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Worked Solution
Scheduling Objectives

Max utilization, throughput, minimize turnaround/waiting/response, fairness.
Roles as earlier: LT admit, MT suspend, ST dispatch.
Q6b. Consider the main memory with the capacity of 4 page frames. Assume that the pages of a process are referenced in the order as given below: 1, 3, 4, 4, 3, 2, 1, 7, 5, 6, 4, 2, 1, 2 Which of FIFO or LRU would be better in this case as a page replacement policy? Why?20257m
Module 5: Memory Management
Consider the main memory with the capacity of 4 page frames. Assume that the pages of a process are referenced in the order as given below:
1, 3, 4, 4, 3, 2, 1, 7, 5, 6, 4, 2, 1, 2

Which of FIFO or LRU would be better in this case as a page replacement policy? Why?

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Worked Solution
4 frames ref 1,3,4,4,3,2,1,7,5,6,4,2,1,2

Sim FIFO faults ~10, LRU faults ~8 → LRU better due to locality keeping recent pages (3,4). FIFO evicts early 1 quickly leading to extra fault when 1 reappears. LRU approximates working set.
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Q7a. Explain the Producer-Consumer problem and discuss how semaphores can be used to solve it.20257m
Module 3: Inter-process Communication
Explain the Producer-Consumer problem and discuss how semaphores can be used to solve it.

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Worked Solution
Producer-Consumer

As Q23: bounded buffer problem, solution with semaphores empty/full/mutex as above.
Q7b. Consider the following set of processes with the length of the CPU burst given in milliseconds: | Process | Burst Time | Priority | | :---: | :---: | :---: | | A | 10 | 3 | | B | 1 | 1 | | C | 2 | 3 | | D | 1 | 4 | | E | 5 | 2 | The processes are assumed to have arrived in order A, B, C, D, E all at time 0. Draw the Gantt charts that illustrate the execution of SJF scheduling algorithm. Show which algorithm results in the minimum average waiting time.20257m
Module 2: Processes & Scheduling
Consider the following set of processes with the length of the CPU burst given in milliseconds:

Process Burst Time Priority

A 10 3

B 1 1

C 2 3

D 1 4

E 5 2

The processes are assumed to have arrived in order A, B, C, D, E all at time 0. Draw the Gantt charts that illustrate the execution of SJF scheduling algorithm. Show which algorithm results in the minimum average waiting time.

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Worked Solution
SJF as Q72: sorted B1,D1,C2,E5,A10 → Gantt B0-1,D1-2,C2-4,E4-9,A9-19, Avg WT 3.2, proving SJF optimal for avg WT vs FCFS 9.6.
Q8a. Discuss deadlock recovery techniques. Explain process termination and resource preemption methods used for recovery from deadlock.20257m
Module 4: Deadlocks
Discuss deadlock recovery techniques. Explain process termination and resource preemption methods used for recovery from deadlock.

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Worked Solution
Deadlock Recovery

Process termination: abort all deadlocked or one by one (victim selection by priority/cost), rollback.

Resource preemption: preempt resources, roll back process to safe state, starvation avoidance. Requires selecting victim, rollback, starvation prevention.
Q8b. Write an algorithm for Readers-Writers Problem solution using Semaphores.20257m
Module 3: Inter-process Communication
Write an algorithm for Readers-Writers Problem solution using Semaphores.

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Worked Solution
Readers-Writers with Semaphores

shared int readcount=0; sem mutex=1, wrt=1;
Reader: wait(mutex); readcount++; if(readcount==1) wait(wrt); signal(mutex); read; wait(mutex); readcount--; if(readcount==0) signal(wrt); signal(mutex);
Writer: wait(wrt); write; signal(wrt);
Readers preference variant, writer may starve; for writer preference add extra sem.
Q9a. Write a short note on: Process State transitions20257m
Module 2: Processes & Scheduling
Write a short note on:

Process State transitions

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Worked Solution
Process State Transitions

As Q13: New→Ready→Running↔Waiting→Terminated, with suspend states. Explained with diagram and events.
Q9b. Write a short note on: Paging and segmentation20257m
Module 5: Memory Management
Write a short note on:

Paging and segmentation

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Worked Solution
Paging vs Segmentation

Paging: fixed size blocks, no external fragmentation, invisible to user, pure virtual memory.
Segmentation: variable size logical units (code, data), supports protection/sharing by segment, suffers external fragmentation, visible. Combined paging+segmentation used in modern systems (Intel).
Q9c. Write a short note on: Contiguous Memory allocation20257m
Module 5: Memory Management
Write a short note on:

Contiguous Memory allocation

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Worked Solution
Contiguous Memory Allocation

Each process occupies one contiguous block. Simple, fast translation (base+limit), but suffers external fragmentation and limits degree of multiprogramming. Fixed/variable partition are variants. Paging removes need for contiguous.
Q9d. Write a short note on: Boot-block and Bad blocks20257m
Module 6: I/O & File Management
Write a short note on:

Boot-block and Bad blocks

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Worked Solution
Boot-block and Bad Blocks

Boot-block: reserved block per disk containing bootstrap loader, executed at boot.

Bad blocks: damaged sectors marked in bad block table, excluded from allocation, handled via sector sparing or skipping.

Process	Allocation (A B C D)	Max (A B C D)	Available (A B C D)
P0	0 0 1 2	0 0 1 2	1 5 2 0
P1	1 0 0 0	1 7 5 0
P2	1 3 5 4	2 3 5 6
P3	0 6 3 2	0 6 5 2
P4	0 0 1 4	0 6 5 6

Process	Burst Time	Priority
A	10	3
B	1	1
C	2	3
D	1	4
E	5	2

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Operating System

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System Calls & Importance

Categories

Virtual Machine

PCB Structure & Significance

Threads & States

ULT vs KLT

Multiprocessor & Real-time Scheduling

Banker's Safe State Check (5 processes A-D)

File System Structure

SSTF 5000 cylinders head 143 requests 86,1470,913,1774,948,1509,1022,1750,130

Scheduling Objectives

4 frames ref 1,3,4,4,3,2,1,7,5,6,4,2,1,2

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Producer-Consumer

SJF as Q72: sorted B1,D1,C2,E5,A10 → Gantt B0-1,D1-2,C2-4,E4-9,A9-19, Avg WT 3.2, proving SJF optimal for avg WT vs FCFS 9.6.

Deadlock Recovery

Readers-Writers with Semaphores

Process State Transitions

Paging vs Segmentation

Contiguous Memory Allocation

Boot-block and Bad Blocks

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