Operating Systems
Complete Notes

What is an Operating System?

An Operating System (OS) is system software that manages hardware resources and provides services to applications. It acts as an intermediary between users and hardware.

Main Functions of OS

• Resource Management: CPU, Memory, Disk, I/O devices
• Process Management: Create, schedule, terminate processes
• Memory Management: Allocate and manage RAM
• File Management: Organize and manage files
• Security: Protect system from unauthorized access
• User Interface: Command-line or graphical interface

Types of Operating Systems

OS Architecture

User Applications
        ↓
System Call Interface
        ↓
Kernel (Core of OS)
  - Process Management
  - Memory Management
  - File Management
  - I/O Management
        ↓
Hardware (CPU, Memory, Disk, I/O)

Process Basics

A process is an instance of a program in execution. Each process has its own memory space and resources.

Process vs Program

Process States

New → Ready → Running → Waiting → Terminated
  ↑_________________________↓
     (Context Switch)

States:
- New: Process created
- Ready: Waiting for CPU
- Running: Currently executing
- Waiting: Waiting for I/O or event
- Terminated: Execution complete

Process Control Block (PCB)

Data structure that stores information about a process:

• PID: Process Identifier
• Program Counter: Next instruction address
• Registers: Saved register values
• Memory: Base and limit registers
• I/O Status: Open files, devices
• Priority: Scheduling priority

Context Switching

Context switching is saving one process's state and loading another's. Overhead but necessary for multitasking.

Scheduling Goals

• Maximize CPU Utilization - Keep CPU busy
• Maximize Throughput - Complete more processes
• Minimize Turnaround Time - Reduce total execution time
• Minimize Waiting Time - Reduce time in ready queue
• Minimize Response Time - Quick first response

Scheduling Algorithms

FCFS Example

Processes: P1(24), P2(3), P3(3)  (Burst time in ms)

Timeline:
P1: 0 -------- 24
P2:           24 -- 27
P3:              27 -- 30

Waiting Times:
P1: 0, P2: 24, P3: 27
Average: 17ms

Critical Section Problem

When multiple processes access shared data, race conditions occur. Critical section is code accessing shared data that must execute atomically.

Solution Requirements

• Mutual Exclusion: Only one process in critical section
• Progress: Processes can't be blocked indefinitely
• Bounded Waiting: Limited waits before entering

Synchronization Tools

Semaphore Example

Semaphore S = 1;  // Binary semaphore

Process P1:          Process P2:
wait(S);             wait(S);
// Critical Section  // Critical Section
signal(S);           signal(S);

Only ONE process enters at a time!

Deadlock Definition

A deadlock is a situation where a process waits indefinitely for a resource held by another waiting process. Results in system hang.

Conditions for Deadlock (All 4 must exist)

• Mutual Exclusion: Resources can't be shared
• Hold and Wait: Processes hold resources while waiting
• No Preemption: Resources can't be forcibly taken
• Circular Wait: Circular chain of processes

Deadlock Handling Strategies

Deadlock Detection

Wait-for graph: Node = process, edge = wait relationship. Cycle = deadlock.

Memory Hierarchy

Memory Allocation Strategies

• Contiguous: Entire process in continuous memory
• Paging: Fixed-size pages, can be non-contiguous
• Segmentation: Variable-size segments

Paging

Divide memory into fixed-size pages (typically 4KB). Process doesn't need contiguous space.

Logical Address: Page Number | Offset
Physical Address: Frame Number | Offset

Example: 4KB pages
Logical address 8000 → Page 1 (8000/4096) + Offset (8000%4096)

Virtual Memory Concept

Virtual memory allows processes to use more memory than physically available. Unused pages stored on disk.

Demand Paging

Load pages only when needed, not all at once.

• Page Hit: Page in memory, fast access
• Page Fault: Page on disk, slow access, must load

Page Replacement Algorithms

Working Set

Set of pages actively used by process. OS tries to keep working set in memory to minimize faults.

File System Basics

File system organizes data into files and directories. Manages storage space and retrieval.

File Attributes

• Name: Human-readable identifier
• Type: Extension (.txt, .exe, etc.)
• Size: Number of bytes
• Permissions: Read, write, execute
• Timestamps: Creation, modification, access time
• Owner: User who created file

File Organization

Directory Structure

• Single-level: All files in one directory
• Two-level: Users have personal directories
• Tree-structured: Hierarchical directories

I/O Organization

User Program (I/O Request)
         ↓
I/O Manager / Device Driver
         ↓
I/O Controller / Hardware
         ↓
Physical Device (Disk, Printer, etc.)

I/O Techniques

Disk Scheduling Algorithms

Security Goals

• Confidentiality: Keep data private
• Integrity: Prevent unauthorized changes
• Availability: System accessible when needed

User Authentication

• Password: Something you know
• Biometric: Something you are
• Physical Token: Something you have

Access Control

• DAC (Discretionary): Owner controls permissions
• MAC (Mandatory): System enforces security policy
• RBAC (Role-based): Permissions based on role

File Permissions (Unix)

rwxrwxrwx
├─ Owner (User)
├─ Group
└─ Others

r = read (4)
w = write (2)
x = execute (1)

Example: 755 = rwxr-xr-x
Owner: read+write+execute (7)
Group: read+execute (5)
Others: read+execute (5)