Operating Systems for UPSC ISS: Scheduling, Deadlock, Paging

Operating Systems for UPSC ISS is the second heaviest topic in the entire Computer section, behind only internet and security.

Quick answer. Operating systems have produced 28 questions in the last ten papers, an average of 2.8 a year, or 7 marks. That is a seventh of the whole Computer section sitting in one article. The questions cluster around six things: what the OS does, the types of OS, process states, CPU scheduling, deadlock, and memory management.

Statistics Paper I, Section IV. Syllabus item: operating systems. Three words in the syllabus, 28 questions in ten years.

Table of Contents

What the operating system does

The OS is the interface between hardware and user. Without it, no application can run. Its functions are asked directly, so learn them as a list.

Function	What it manages
Process management	Creating, scheduling and terminating processes
Memory management	Allocating and freeing memory, paging, swapping
File management	Naming, saving, deleting, organising files and permissions
Device management	Input and output devices through drivers
Security management	Access control and protection from unauthorised use

The file manager handles naming, saving and deleting files. All three, asked as a set.

The goals of an operating system are efficient use of the CPU, the memory, and the input and output devices. All three, asked as a set.

A trap worth naming. An OS is crucial because it controls the hardware and manages files and directories. Running antivirus software and allowing a Google search are not OS functions. Those belong to utility software and to an application. Both have appeared as wrong options.

Types of operating system

Type	Defining feature
Batch processing	Similar jobs grouped and run together. No user interaction while the batch runs
Time sharing	Many users share one machine, the CPU switches so fast each feels alone
Multitasking	Several programs run at once on one machine
Multiprocessing	More than one physical CPU, sharing common memory and a common bus
Real time, RTOS	Responds to an event within a predetermined time. Used in ATMs, flight control, medical devices
Embedded	Built into a specific device and does only that device’s job
Distributed	Many machines, each with its own memory, connected by a network. Does not share common memory or a common bus

Distributed against multiprocessor. This is the question. A multiprocessor system has several CPUs sharing one memory and one bus. A distributed system has separate machines with no shared memory and no shared bus, which is why they must communicate by passing messages. The question “which system does not share common memory and common bus” has one answer, and it is distributed.

Linux is not a real time operating system. It is a general purpose, time sharing OS. MTOS, LynxOS and RTX are real time systems. Linux appearing in a list of RTOS names is the trap.

UNIX and Linux

Point	Fact
Core component of UNIX	The kernel, which talks directly to the hardware
The shell	The outer layer that interprets user commands. Not the core
UNIX is written in	C
Linux is	Open source, freely modifiable
Linux is classified as	Multi user, multitasking and multiprocessor
Windows, iOS, MS DOS	Proprietary, not open source

If the goal is to reduce the cost of developing and maintaining software, the answer is an open source operating system, which in practice means Linux.

The core component of UNIX has been asked twice, in 2018 and 2023, and again in 2026. The answer is the kernel every time.

Process states

A process is a program in execution. It moves through a fixed set of states.

State	Meaning
New	Being created
Ready	Waiting for the CPU to be assigned
Running	Instructions are being executed
Waiting or Blocked	Waiting for input, output or some event
Terminated	Finished

The sequence is New, Ready, Running, Waiting, Terminated. A process can move from Running back to Ready when its time slice expires, and from Waiting back to Ready when the event it needed occurs.

Deadlock is not a process state. It is a situation involving two or more processes, not a state a single process sits in. That was asked directly in 2026 and it is the cleanest possible test of whether the list above was memorised or understood.

CPU scheduling

Scheduling decides the order in which ready processes get the CPU.

Algorithm	How it chooses	Pre-emptive?
First Come First Serve, FCFS	Whoever asked first	No
Shortest Job First, SJF	Shortest execution time first	The basic form is not
Priority scheduling	Highest priority rank first	Either form exists
Round Robin, RR	Each process gets a fixed time slice, then goes to the back of the queue	Yes

Round Robin is the one allocated a specific time period or time slice. That is the question, asked in exactly those words, and the time slice is also called the time quantum.

Deadlock

A deadlock is a situation where processes are each holding a resource and waiting for one another, so none can proceed. Four conditions must hold at the same time for it to be possible.

Condition	Meaning
Mutual exclusion	At least one resource can be held by only one process at a time
Hold and wait	A process holds one resource while waiting for another
No pre-emption	A resource cannot be forcibly taken away
Circular wait	A closed chain of processes, each waiting on the next

All four are necessary. Break any one of them and deadlock cannot occur. A 2022 question listed the first three and asked under which conditions deadlock may arise. All three listed are genuine necessary conditions, so all three belong in the answer.

Interprocess communication

When two processes need to exchange data, there are two mechanisms.

Method	How it works	Used in
Shared memory	Both processes read and write a common region of memory	Systems that share memory
Message passing	One process sends a message, the other receives it	Distributed systems

Cache memory is not an interprocess communication method. It is a hardware speed device with no role in letting processes talk to each other, and it has sat in the options as a distractor.

In a distributed system the answer is always message passing, because by definition there is no shared memory to use.

Memory management

Technique	What it divides	Block size
Paging	Logical memory into pages, physical memory into frames	Fixed
Segmentation	Memory into segments following the program’s logical structure	Variable
Swapping	Moves whole processes between memory and disk	Whole process
Demand paging	Brings a page into memory only when it is needed	Fixed

Paging with swapping is also known as demand paging.

A page fault occurs when a program accesses a page that is not currently in memory. The OS then fetches it from disk. A page fault is normal behaviour, not an error.

The virtual address is mapped to the physical address by paging, using a page table.

A correction worth making. Most notes say paging divides memory into blocks called pages. More precisely, logical memory is divided into pages and physical memory into frames, and the page table maps one to the other. If a question separates the two words, this is why.

Worked Example 1: Following one process through its states

A program is launched, waits for the user to type something, then finishes.

Stage	State	Why
1. Launched	New	Being created by the OS
2. Queued	Ready	Waiting its turn for the CPU
3. Executing	Running	The CPU is assigned to it
4. Time slice expires	Ready	Pushed back into the queue, not blocked
5. Asks for keyboard input	Waiting	Blocked until the user types
6. User types	Ready	The event occurred, back to the queue
7. Finishes	Terminated	Done

Steps 4 and 5 are the ones that separate candidates. A process pushed out by Round Robin goes to Ready. A process waiting for input goes to Waiting. They are not the same and the paper has asked for the sequence.

Worked Example 2: Round Robin in action

Three processes, each needing 10 milliseconds of CPU, with a time slice of 4 milliseconds. What is the order of execution?

Round	P1	P2	P3
1	4 ms, 6 left	4 ms, 6 left	4 ms, 6 left
2	4 ms, 2 left	4 ms, 2 left	4 ms, 2 left
3	2 ms, done	2 ms, done	2 ms, done

Every process gets the CPU before any process gets it twice. That fairness is the entire purpose of Round Robin, and it is the reason it is pre-emptive while FCFS is not.

Worked Example 3: Breaking a deadlock

Four cars arrive at a four way junction at once, each blocking the one to its left.

Condition	Present?	Break it by
Mutual exclusion	Yes, one car per space	Cannot be removed here
Hold and wait	Yes, each holds its space and waits	Requiring all space before entering
No pre-emption	Yes, nobody can be pushed out	Allowing reversing
Circular wait	Yes, a closed loop of four	Making one car reverse

Break one condition and the deadlock ends. In real systems circular wait is the easiest to attack, usually by forcing resources to be requested in a fixed order.

How This Gets Asked

The system with no shared memory [2019]

Which one of the following systems does not share common memory and common bus?

Distributed system. A multiprocessor system shares both. This single distinction then explains the 2024 question about interprocess communication in distributed systems, where the answer is message passing, precisely because there is no shared memory available.

Two questions, five years apart, resting on one definition.

The state that is not a state [2026]

Which one of the following is not a process state? (a) Ready (b) Executing (c) Deadlock (d) Terminated

Deadlock. Ready, executing and terminated are all states a single process occupies. Deadlock is a condition involving several processes and their resources.

This is what a well set two mark question looks like. Nothing obscure, no calculation, and it separates candidates who listed the states from candidates who read about them.

The kernel, three times [2018, 2023, 2026]

The core component of UNIX is [2018]

Which one of the following is the core component of UNIX? [2023]

Which one of the following is the central part of UNIX operating system that manages and controls the communication between various hardware and software components? [2026]

Kernel, all three times, with the shell sitting in the options on every occasion.

Three papers over nine years, one word. If you are ever unsure whether learning a single term is worth the effort, this is the answer.

Quick Revision Table

Point	Value
OS is	The principal component of system software
OS functions	Process, memory, file, device and security management
Goals of an OS	Efficient use of CPU, memory and I/O devices
File manager handles	Naming, saving, deleting files
Batch OS	Jobs grouped, no user interaction during the run
Time sharing OS	Many users, rapid CPU switching
Real time OS	Responds within a predetermined time
Not a real time OS	Linux
Embedded OS	Built into one device for one job
Multiprocessor system	Several CPUs, shared memory and bus
Distributed system	Separate machines, no shared memory or bus
Core component of UNIX	Kernel
The shell is	The command interpreter, not the core
UNIX is written in	C
Open source OS	Linux
Linux is	Multi user, multitasking, multiprocessor
Process states	New, Ready, Running, Waiting, Terminated
Deadlock is	A situation, not a process state
Deadlock conditions	Mutual exclusion, hold and wait, no pre-emption, circular wait
Round Robin gives	A fixed time slice, also called a time quantum
Round Robin is	Pre-emptive
IPC methods	Shared memory and message passing
IPC in distributed systems	Message passing
Cache memory as IPC	No
Paging divides	Logical memory into pages, physical into frames
Segmentation blocks	Variable size
Paging with swapping	Demand paging
Page fault	Access to a page not currently in memory

Practice Set

Ten questions. No answer key below.

Write your answers in the comments, all ten in a single comment, in the format 1-b 2-a 3-d and so on. The full answer key is posted in this same thread after seven days.

A year in brackets means the question is from that paper.

The file manager is responsible for 1. Naming files 2. Saving files 3. Deleting files Which of the above are correct? [2017]
Which one of the following systems does not share common memory and common bus? [2019]
Which one of the following is not a real time operating system? [2021]
Consider the following conditions: 1. Mutual exclusion 2. Hold and wait 3. No pre-emption Under which of the above conditions may a deadlock situation arise? [2022]
What is the correct sequence of a cycle of a process in execution? [2022]
Consider the following terms: 1. Shared region 2. Cache memory 3. Message passing 4. Shared memory Which of the above are related to interprocess communication? [2022]
Which of the following are the goals of an operating system? 1. Efficient use of CPU 2. Efficient use of memory 3. Efficient use of I/O devices [2022]
Which one of the following methods is used for interprocess communication in distributed systems? [2024]
Consider the following: I. Multi user II. Multitasking III. Multiprocessor Which of the above are correct in respect of the Linux operating system? [2025]
A page fault in an operating system [2026]

Frequently Asked Questions

How many questions come from operating systems in UPSC ISS?

Twenty eight across the ten papers from 2017 to 2026, about 2.8 a year, worth 7 marks. It is the second heaviest topic in the Computer section after internet and security.

What is the difference between a distributed system and a multiprocessor system?

A multiprocessor system has several CPUs sharing one memory and one bus. A distributed system has separate machines with their own memory, connected by a network, sharing neither.

Is deadlock a process state?

No. The process states are new, ready, running, waiting and terminated. Deadlock is a situation involving several processes competing for resources, and it has been asked as the odd one out.

What are the four conditions for deadlock?

Mutual exclusion, hold and wait, no pre-emption and circular wait. All four must hold at the same time, so breaking any one of them prevents deadlock.

Which scheduling algorithm uses a time slice?

Round Robin. Each process gets a fixed time quantum, then moves to the back of the ready queue, which makes it pre-emptive and fair.

Is Linux a real time operating system?

No. Linux is a general purpose, time sharing, multi user operating system. MTOS, LynxOS and RTX are real time systems, and Linux appearing beside them is a standard trap.

What is the core component of UNIX?

The kernel. It manages memory, processes and hardware communication. The shell is the outer command interpreter and is the usual wrong option.

What is a page fault?

An access to a page that is not currently in main memory. The operating system fetches it from disk and continues. It is normal behaviour in demand paging, not a system error.