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Operating systems for IT Officer

What the subject covers, the key topics, how it has been asked, common mistakes and a two-week plan.

  • 10 min read
  • 8 topic groups
  • 56 terms in the glossary
  • 8 topics

IBPS publishes no official syllabus; this list is based on previous papers.

What the subject covers

An operating system (OS) shares one machine's CPU, memory, disks and devices among many programs, and gives them a safe, simple interface: system calls, files, processes. The exam side has two halves:

Quick revision

Tap a term to open its definition.

Key topics

Basics

Kinds of OS, the kernel and system calls, and booting.

Kinds of OS. Batch, multiprogramming (several jobs in memory), multitasking/time-sharing (fast switching between jobs), multiprocessing (several CPUs), distributed, and real-time.

Real-time system

Real-time systems are hard (a missed deadline is a failure) or soft.

Kernel: monolithic vs microkernel
KernelServices
Monolithic kernelRuns most services inside the kernel
MicrokernelKeeps the kernel small and runs services in user space

System call

Programs enter the kernel through system calls, switching from user mode to kernel mode.

Booting: firmware (BIOS or UEFI), then the boot loader, then the kernel.

Processes and threads

Process vs thread, the PCB, process states and context switches, fork(), zombies and orphans.

Process

A process is a program in execution, with its own address space and a process control block (PCB).

Thread

Threads in one process share its memory and files but have their own stack and registers.

Process states: new → ready → running → waiting (blocked) → ready … → terminated; running → ready on preemption or when the time slice ends.

Context switch

A context switch saves one PCB and loads another.

fork()

In Unix, fork() creates a child process. A program that calls fork() n times in a row (each child also continuing) ends up with 2ⁿ processes, of which 2ⁿ − 1 are child processes.

Zombie vs orphan
ProcessWhat happened
ZombieHas finished but its parent hasn't collected its exit status
OrphanAn orphan's parent has exited

CPU scheduling

Turnaround, waiting and response time, the scheduling algorithms, and a worked FCFS check.

Turnaround time

Turnaround time = completion time − arrival time.

Waiting time

Waiting time = turnaround − burst time.

Response time

Response time = first run − arrival.

Measures: also know throughput and CPU utilisation.

Scheduling algorithms
AlgorithmHow it behaves
FCFSSimple; a long job holds up the short ones behind it (the convoy effect)
SJFNon-preemptive; the optimal average waiting time among non-preemptive algorithms when all jobs are available and burst times are known (SRTF is optimal overall)
SRTFThe preemptive version of SJF
PriorityLow-priority jobs can starve; ageing (slowly raising their priority) fixes this
Round robinA fixed time quantum. Too large and it behaves like FCFS; too small and context switches waste time
Multilevel queue / multilevel feedback queuenot stated

Worked check (our example). P1 arrives at 0 with a burst of 5, P2 at 1 with 3, P3 at 2 with 1. Under FCFS, the completions are 5, 8 and 9:

Worked check (our example): FCFS
ProcessArrivalBurstCompletionTurnaroundWaiting
P105550
P213874
P321976

Turnaround times average 6.33; waiting times average 3.33.

Synchronisation

The critical-section problem, semaphores, mutexes and monitors, and the classic problems.

Critical-section problem

A solution needs mutual exclusion, progress and bounded waiting. Peterson's solution covers two processes.

Semaphore

wait/P decrements and may block; signal/V increments. They are counting or binary.

Mutex

A mutex lock is owned by one thread at a time.

Monitor

Monitors wrap shared data with its access procedures.

Classic problems: producer–consumer (bounded buffer), readers–writers, dining philosophers. Also know race conditions.

Deadlocks

The four Coffman conditions, prevention, avoidance with Banker's algorithm, detection and recovery, starvation.

The four necessary conditions (Coffman)

Mutual exclusion, hold and wait, no preemption, circular wait. Break any one and deadlock can't happen; that is prevention.

Banker's algorithm

Avoidance with Banker's algorithm: Need = Max − Allocation.

Safe state

A state is safe if some order exists in which every process can finish with the Available resources plus what earlier processes release.

Detection and recovery. Use a wait-for graph or resource-allocation graph. With one instance of each resource, a cycle means deadlock; with several instances, a cycle is necessary but not enough. Recover by killing processes or preempting resources.

Starvation

Starvation is not deadlock. In starvation the system progresses but one process never gets served.

Memory management

Fragmentation, paging and the TLB, segmentation, virtual memory, page replacement.
Fragmentation: external vs internal
ComparedExternal fragmentationInternal fragmentation
CauseContiguous allocation (first, best or worst fit) causes itFixed-size blocks cause it
FixCompaction fixes itnot stated

Paging

A logical address is split into page number + offset; the page table maps pages to frames.

TLB

A TLB caches page-table entries.

Effective access time (single-level table)Formula
EAT ≈ hit ratio × (TLB + memory) + miss ratio × (TLB + 2 × memory)

For a single-level table.

Segmentation

Variable-sized logical units (code, data, stack); it can be combined with paging.

Page fault

Virtual memory and demand paging: a page fault loads a page from disk when it is needed.

Dirty (modified) bit

The dirty (modified) bit marks pages changed in memory: only dirty pages must be written back when replaced.

Thrashing

Thrashing is when the system spends more time paging than working.

Page replacement: FIFO, Optimal, LRU
AlgorithmWhat to know
FIFOCan suffer Belady's anomaly: more frames, more faults
OptimalReplaces the page used furthest in future; it is the theoretical best
LRUApproximates Optimal and has no Belady's anomaly

Files and disks

File allocation, disk scheduling (FCFS, SSTF, SCAN, C-SCAN, LOOK, C-LOOK) and RAID levels.

File allocation: contiguous, linked (FAT is a variant), indexed (Unix inodes). Know directory structures and access methods (sequential, direct).

Total head movement

Disk scheduling: total head movement is the sum of the distances between successive positions.

Disk scheduling
AlgorithmOrder of service
FCFSRequests in the order they arrive
SSTFThe nearest request first; it can starve far requests
SCAN (the "elevator")Goes to the end of the disk, then reverses
C-SCANReturns to the start and sweeps one way only
LOOK and C-LOOKLike SCAN and C-SCAN, but turn at the last request instead of the disk end
RAID
LevelWhat it does
0Striping, no redundancy
1Mirroring
5Striping with distributed parity
10Mirror + stripe

Linux and Windows in practice

Linux commands, permissions, shell scripting, and Windows file systems and tools.
Linux commands
TaskCommands
Files and directoriesls (-l long, -a include hidden, -t by modification time, -r reverse), cd, pwd, cp, mv, rm, mkdir
Viewing and searchingcat, less, head, tail, grep, find
Processesps, top, kill
Disk usedf, du
System informationuname (-s kernel name, -r kernel release, -a all)

chmod

Permissions: chmod with rwx = 4/2/1, so 755 = rwxr-xr-x. chown changes the owner.

Shell scriptingBash

A loop looks like

#!/bin/bash
for i in 1 2 3; do …; done

$ expands a variable ($HOME), $1 is the first argument and $? is the last exit status.

Shebang

#!/bin/bash (the shebang) names the interpreter.

Windows
TopicWhat to know
File systemsNTFS, FAT32, exFAT, ReFS
Partition stylesMBR (older, up to 4 primary partitions) and GPT (UEFI, many partitions)
BitLockerFull-volume (often called full-disk) encryption
Hyper-VMicrosoft's hypervisor
Recovery toolsTools such as bootrec repair boot records

How it is asked

Official

From the official IBPS documents

These are the facts about the format from the official IBPS documents:

  • Professional Knowledge in the Mains has 50 questions for 100 marks in 40 minutes (2 marks each, −0.50 for a wrong answer).
  • Each question has five options.

Sources: IBPS official notification CRP SPL-XVI (1 Jul 2026); IBPS Information Handout, Online Main Exam CRP SPL-XVI, “notification §D; handout, Professional Knowledge sample question”

Not official

From memory-based papers (unofficial)

IBPS doesn't publish past papers. Based on memory-based papers shared by candidates (unofficial) for two previous IT Officer Mains exams (January 2023 and December 2024; possibly incomplete), OS questions came in two clear styles:

  • Numerical problems. One paper had a Banker's-algorithm question (count the safe sequences from given Allocation, Max and Available matrices), FCFS average waiting and turnaround times, and total head movement for FCFS disk scheduling. These take longer than one-liners, so budget time for them.
  • Practical one-liners. Linux and Windows came up in both papers:
    • Linux: ls options, uname, the meaning of $ and /bin/bash, valid Bash loop syntax;
    • Windows: file-system names, partition styles, BitLocker, Hyper-V and a boot-repair command.
    • Concept questions on starvation and the dirty bit also appeared.

We don't give a number of OS questions, because IBPS publishes no weightage.

Common mistakes

Our suggestion

Two-week study plan

A suggestion. Adjust it to your own pace.

  1. Day 1

    OS types, kernel, system calls, booting

    Practice: One-page summary in your own words

  2. Days 2–3

    Processes, threads, PCB, states, fork

    Practice: fork() output questions

  3. Days 4–5

    CPU scheduling (FCFS, SJF, SRTF, priority, RR, MLFQ)

    Practice: 2 Gantt charts per algorithm, with average waiting and turnaround

  4. Day 6

    Synchronisation, semaphores, classic problems

    Practice: Trace semaphore values step by step

  5. Days 7–8

    Deadlocks, Banker's algorithm, RAG

    Practice: 5 Banker's problems: find a safe sequence and test a request

  6. Days 9–10

    Paging, TLB, segmentation, virtual memory, page replacement

    Practice: Count page faults for FIFO, LRU and Optimal on the same reference string

  7. Day 11

    File systems, disk scheduling, RAID

    Practice: Head movement for all six algorithms on one request queue

  8. Day 12

    Linux commands, permissions, shell scripting; Windows file systems and tools

    Practice: Practise in a Linux terminal (a free VM or WSL)

  9. Days 13–14

    Mixed revision

    Practice: Timed set of 25 questions in 20 minutes; review every wrong answer

Show the plan as a table
Two-week study plan
DaysFocusPractice
1OS types, kernel, system calls, bootingOne-page summary in your own words
2–3Processes, threads, PCB, states, forkfork() output questions
4–5CPU scheduling (FCFS, SJF, SRTF, priority, RR, MLFQ)2 Gantt charts per algorithm, with average waiting and turnaround
6Synchronisation, semaphores, classic problemsTrace semaphore values step by step
7–8Deadlocks, Banker's algorithm, RAG5 Banker's problems: find a safe sequence and test a request
9–10Paging, TLB, segmentation, virtual memory, page replacementCount page faults for FIFO, LRU and Optimal on the same reference string
11File systems, disk scheduling, RAIDHead movement for all six algorithms on one request queue
12Linux commands, permissions, shell scripting; Windows file systems and toolsPractise in a Linux terminal (a free VM or WSL)
13–14Mixed revisionTimed set of 25 questions in 20 minutes; review every wrong answer

Glossary A–Z

56 terms

A

  • AgeingSee Scheduling algorithms.

B

C

  • chmodPermissions: chmod with rwx = 4/2/1, so 755 = rwxr-xr-x. chown changes the owner.
  • chownSee chmod.
  • Context switchA context switch saves one PCB and loads another.
  • Convoy effectSee Scheduling algorithms.
  • Critical-section problemA solution needs mutual exclusion, progress and bounded waiting. Peterson's solution covers two processes.

D

E

F

  • FCFSSee Scheduling algorithms.
  • fork()In Unix, fork() creates a child process. A program that calls fork() n times in a row (each child also continuing) ends up with 2ⁿ processes, of which 2ⁿ − 1 are child processes.

G

  • GPTSee Windows.

H

I

L

  • LRUSee Page replacement: FIFO, Optimal, LRU.

M

  • MBRSee Windows.
  • MicrokernelSee Kernel: monolithic vs microkernel.
  • MonitorMonitors wrap shared data with its access procedures.
  • Monolithic kernelSee Kernel: monolithic vs microkernel.
  • MutexA mutex lock is owned by one thread at a time.

N

O

P

  • Page faultVirtual memory and demand paging: a page fault loads a page from disk when it is needed.
  • PagingA logical address is split into page number + offset; the page table maps pages to frames.
  • PCBSee Process.
  • PermissionsSee chmod.
  • ProcessA process is a program in execution, with its own address space and a process control block (PCB).
  • Process control blockSee Process.

R

S

  • Safe stateA state is safe if some order exists in which every process can finish with the Available resources plus what earlier processes release.
  • SCANSee Disk scheduling.
  • SegmentationVariable-sized logical units (code, data, stack); it can be combined with paging.
  • Semaphorewait/P decrements and may block; signal/V increments. They are counting or binary.
  • Shebang#!/bin/bash (the shebang) names the interpreter.
  • SJFSee Scheduling algorithms.
  • SRTFSee Scheduling algorithms.
  • SSTFSee Disk scheduling.
  • StarvationStarvation is not deadlock. In starvation the system progresses but one process never gets served.
  • System callPrograms enter the kernel through system calls, switching from user mode to kernel mode.

T

  • The four necessary conditions (Coffman)Mutual exclusion, hold and wait, no preemption, circular wait. Break any one and deadlock can't happen; that is prevention.
  • ThrashingThrashing is when the system spends more time paging than working.
  • ThreadThreads in one process share its memory and files but have their own stack and registers.
  • TLBA TLB caches page-table entries.
  • Total head movementDisk scheduling: total head movement is the sum of the distances between successive positions.
  • Turnaround timeTurnaround time = completion time − arrival time.

U

V

W

Z

Not official · based on previous papers

OS topics

8 topics. Practice questions: 0 so far, 2-mark questions coming soon.

OS FAQ

Are numerical OS questions asked in IBPS SO IT Officer?

In the memory-based papers we read (shared by candidates, unofficial), yes: CPU scheduling averages, disk head movement and a Banker's-algorithm question. Practise them until they're quick, because they take longer than one-line questions.

How much Linux do I need?

Everyday commands and their common options, file permissions with chmod, and shell basics such as the shebang, variables with $, and loops. Linux commands appear in both memory-based papers.

Are Windows topics part of the OS section?

The memory-based papers include Windows file systems, partition styles (MBR, GPT), BitLocker and Hyper-V, so learn the basics alongside the OS theory.

Which page replacement algorithm shows Belady's anomaly?

FIFO. With FIFO, adding frames can increase page faults for some reference strings; LRU and Optimal never do this.

How many OS questions come in the Mains?

IBPS doesn't say. Professional Knowledge has 50 questions in total for 100 marks; the split across subjects isn't published.

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