Operating Systems Notes
Subject Overview
Operating Systems covers OS structure and services, process management (states, scheduling, threads), CPU scheduling algorithms (FCFS, SJF, Round Robin), process synchronization and deadlocks (Banker's algorithm), and memory management from paging/
Subject at a glance
4
Units
44
Topics
3
Credits
4
Infographics
Unit-wise Syllabus
4 units — open any unit for detailed topic-wise notes, diagrams and important exam questions
- Definition
- evolution
- components and services of an OS
- structure and architecture
- batch systems
- multiprogramming
- time-sharing
- parallel
- distributed and real-time systems
- OS services and system calls
- system programs
- OS generations
Batch
Jobs grouped and run without interaction
Multiprogramming
Several programs in memory share the CPU
Time-sharing
Fast switching gives each user a share
Real-time
Guaranteed response within a deadline
Distributed
Many machines work as one system
Study guide
The operating system is the software that manages the computer's hardware and runs your programs. You learn its definition, evolution, components and services, its structure and architecture, and the types of system: batch, multiprogramming, time-sharing, parallel, distributed and real-time. You also study OS services, system calls and system programs. A good way to remember the types is to link each to a real example, such as time-sharing and a modern desktop.
- Process definition
- states and state transitions
- process scheduling
- PCB
- threads and multithreading
- scheduling objectives and algorithms
- preemptive/non-preemptive CPU scheduling (FCFS, SJF, Round Robin)
- performance evaluation of scheduling algorithms
- 1. New: Process is being created
- 2. Ready: Waiting for the CPU
- 3. Running: Executing on the CPU
- 4. Waiting: Waiting for I/O or an event
- 5. Terminated: Finished execution
Study guide
A process is a program in execution. This unit covers process states and transitions, the process control block (PCB), threads and multithreading, and CPU scheduling: objectives, preemptive and non-preemptive algorithms such as FCFS, SJF and Round Robin, and how to evaluate them. Practise Gantt chart problems and calculate waiting and turnaround times; they are the numerical core of this unit.
- Inter-process communication
- race conditions
- critical section problem
- mutual exclusion
- semaphores
- monitors
- deadlock system model and characterization
- prevention
- avoidance
- Banker's algorithm
- deadlock detection and recovery
- Mutual exclusion
- A resource can be used by only one process at a time
- Hold and wait
- A process holds one resource while waiting for another
- No preemption
- Resources cannot be taken away forcibly
- Circular wait
- A closed chain of processes each waiting for the next
Study guide
When processes share resources they must coordinate. You study inter-process communication, race conditions, the critical-section problem, mutual exclusion, semaphores and monitors, then deadlocks: the system model, characterisation, prevention, avoidance with the Banker's algorithm, detection and recovery. Learn the four conditions for deadlock and solve a Banker's algorithm problem step by step.
- Logical and physical address mapping
- swapping
- memory allocation
- MFT/MVT
- internal/external fragmentation and compaction
- paging
- segmentation
- demand paging
- page replacement algorithms
- frame allocation
- thrashing
- I/O hardware principles
- disk structure and scheduling algorithms
Division
Fixed-size pages and frames
Variable-size logical segments
Fragmentation
Internal
External
Programmer view
Invisible to the programmer
Matches program structure (code, data, stack)
Address
Page number + offset
Segment number + offset
Study guide
Memory and storage management decide how efficiently a computer uses what it has. You study logical and physical addresses, swapping, contiguous allocation (MFT and MVT), internal and external fragmentation and compaction, paging, segmentation, demand paging, page replacement algorithms, frame allocation and thrashing, followed by I/O hardware and disk structure and scheduling. Practise page replacement tables for FIFO and compare their page faults.
Why Operating Systems Matters
Operating Systems explains what happens behind every click and every program you run. It is a core subject for interviews, competitive exams and any career in software, systems, cloud or cybersecurity, and its ideas such as scheduling, memory and concurrency appear everywhere in modern computing.
How to Prepare for Operating Systems
- Practise scheduling problems (FCFS, SJF, Round Robin) with Gantt charts until they are routine.
- Memorise the four necessary conditions for deadlock and the steps of the Banker's algorithm.
- Do page replacement problems by hand and count the page faults carefully.
- Write definitions of key terms (process, thread, semaphore, paging) in one or two crisp sentences.
Operating Systems: Frequently Asked Questions
What is the difference between a process and a thread?
A process is a program in execution with its own memory space, while a thread is a lightweight unit of execution inside a process that shares the process's resources.
What is a deadlock?
A deadlock occurs when a set of processes are each waiting for a resource held by another process in the set, so none can proceed.
Which Operating Systems topics are most important for exams?
CPU scheduling problems, deadlock and the Banker's algorithm, paging and page replacement, and process synchronisation are the most frequently tested.