Start Your Journey With Operating Systems Beginner-Friendly Concepts, Diagrams & Practice

What is an Operating System?

An OS is a software layer that manages hardware resources (CPU, memory, storage, I/O devices) and provides a convenient interface for users and applications to interact with the system.

Why is an OS Important?

• Resource Management – Efficiently allocates CPU time, memory, and I/O
• Abstraction – Hides complex hardware details from users
• Multitasking – Runs multiple programs simultaneously
• Security – Protects data and resources from unauthorized access
• User Interface – Provides CLI or GUI for interaction

1. Process Management

Handles creation, scheduling, and termination of processes. Manages synchronization and communication between processes.

2. Memory Management

Allocates and deallocates memory to processes. Implements virtual memory and handles paging/segmentation.

3. File System Management

Organizes files into directories, manages file permissions, and handles read/write operations on storage devices.

4. Device Management

Manages I/O devices through drivers, handles interrupts, and buffers data transfers between devices and memory.

5. Security & Protection

Controls access to resources through authentication, authorization, and encryption mechanisms.

Processes vs Threads

• Process – Heavyweight, isolated memory space, independent execution
• Thread – Lightweight, shares memory within a process, multiple threads per process

Process States

A process goes through several states during its lifecycle:

Context Switching

When the CPU switches from one process to another, it saves the current process state (registers, program counter) and loads the saved state of the next process. This is called context switching.

1. First-Come, First-Served (FCFS)

The simplest algorithm. Processes are executed in the order they arrive. Non-preemptive — once a process gets the CPU, it runs to completion.

2. Shortest Job First (SJF)

Executes the process with the smallest burst time first. Can be preemptive (SRTF) or non-preemptive. Minimizes average waiting time.

3. Round Robin (RR)

Each process gets a fixed time quantum (e.g., 4ms). Processes are scheduled in a circular queue. If a process doesn't finish within its quantum, it's preempted and moved to the end of the queue.

4. Priority Scheduling

Each process has a priority. The CPU is allocated to the process with the highest priority. Can be preemptive or non-preemptive. May cause starvation for low-priority processes.

Paging

Physical memory is divided into fixed-size blocks called frames. Process memory is divided into blocks of the same size called pages. A page table maps logical pages to physical frames, eliminating external fragmentation.

Segmentation

Memory is divided into variable-sized segments (code, data, stack, heap). Each segment has a base address and limit. Supports logical division of a program but can cause external fragmentation.

Virtual Memory

Allows execution of processes that are not entirely in memory. Uses paging with demand paging — pages are loaded only when needed. This enables running larger programs than physical RAM allows.

Four Necessary Conditions for Deadlock

1. Mutual Exclusion – Resources cannot be shared; only one process at a time
2. Hold and Wait – A process holds at least one resource while waiting for others
3. No Preemption – Resources cannot be forcibly taken from a process
4. Circular Wait – A circular chain of processes exists where each holds a resource needed by the next

Deadlock Prevention

Prevent at least one of the four conditions:

• Mutual Exclusion – Use spooling or shareable resources (not always possible)
• Hold and Wait – Require processes to request all resources at once
• No Preemption – Allow preemption of resources
• Circular Wait – Impose a total ordering of resource types

Deadlock Avoidance — Banker's Algorithm

The OS simulates resource allocation and only grants a request if the resulting state is safe (all processes can eventually complete). This is known as the Banker's Algorithm.

Files and Directories

• File – A named collection of related data stored on disk
• Directory – A container that holds files and other directories (folders)
• Path – The location of a file in the directory tree (absolute or relative)
• Metadata – File attributes like size, permissions, timestamps, owner

File Allocation Methods

• Contiguous Allocation – Each file occupies a contiguous block of disk space. Fast access but causes external fragmentation.
• Linked Allocation – Each file is a linked list of disk blocks. No fragmentation but slow random access.
• Indexed Allocation – An index block contains pointers to all data blocks. Supports direct access with minimal overhead.

I/O Hardware

• Device Controller – Electronic component that manages a specific device
• I/O Port – Communication channel between CPU and device
• Interrupts – Signals sent by devices to get CPU attention
• DMA (Direct Memory Access) – Allows devices to transfer data directly to/from memory without CPU involvement

I/O Software Layers

• User-level I/O – Library calls (e.g., fread(), write())
• Kernel I/O Subsystem – Buffering, caching, spooling, scheduling
• Device Drivers – Low-level code that communicates with device controllers

Disk Scheduling Algorithms

When multiple I/O requests are pending, the OS uses scheduling to minimize seek time:

• FCFS – Serves requests in arrival order
• SSTF (Shortest Seek Time First) – Serves the closest request first
• SCAN (Elevator) – Disk arm moves in one direction, servicing requests, then reverses
• C-SCAN – Like SCAN but only services requests in one direction

Authentication

Verifying the identity of a user or process. Common methods:

• Passwords – Knowledge-based (what you know)
• Biometrics – Fingerprint, face, iris (what you are)
• Two-Factor Auth (2FA) – Combines password with a code (what you have)
• Kerberos – Network authentication protocol using tickets

Access Control

Determines who can access what resources and how:

• Discretionary Access Control (DAC) – Owner decides permissions (Linux file permissions)
• Mandatory Access Control (MAC) – System-wide policy enforced by the OS (SELinux)
• Role-Based Access Control (RBAC) – Permissions assigned to roles, users are assigned roles

User Permissions

• Read (r) – View file contents or list directory
• Write (w) – Modify file or directory contents
• Execute (x) – Run as program or traverse directory