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OPERATING SYSTEM
Chapter 1 : Operating System Services and Components
Definitions โข Types โข Diagrams โข Examples โข Advantages & Disadvantages
1. Operating System โ Concept, Functions
1.1 Definition
Definition : An Operating System (OS) is a system software that acts as an intermediary between the computer hardware and the user. It manages all the hardware resources of a computer and provides a platform for application programs to run. It controls the execution of programs, manages memory, files, input/output devices, and ensures security and efficient resource utilization.
Key Points (5 Lines)
- OS is the first program loaded when a computer starts (bootstrap/boot program).
- It acts as a resource manager โ managing CPU, memory, storage, and I/O devices.
- OS provides a user interface (GUI or CLI) to interact with the computer.
- It ensures security and protection by controlling access to resources.
- OS coordinates between hardware and software, enabling multitasking.
1.2 Functions of OS
Function Description
Process Management Creates, schedules, and terminates processes; handles context switching.
Memory Management Allocates and deallocates RAM to processes; prevents memory conflicts.
File Management Creates, reads, writes, deletes files; maintains directory structure.
I/O Management Controls communication with input/output devices via drivers.
Security & Protection Controls user access, prevents unauthorized use of resources.
Error Detection Detects and handles hardware and software errors.
Resource Allocation Allocates CPU time, memory, and devices to processes fairly.
Networking Manages network connections, protocols, and data transfer.
1.2 Types of Operating System
Type: Batch OS
Definition: Jobs are collected in batches and processed one after another without user interaction. CPU processes jobs in sequence. No direct interaction with the user during execution.
Characteristics:
- Jobs collected, stored, and processed sequentially without user interaction.
- A Job Control Language (JCL) was used to describe the jobs.
- Monitor program manages job transitions automatically.
- CPU is never idle โ moves to next job automatically.
Advantages vs Disadvantages
n ADVANTAGES n DISADVANTAGES
Efficient use of CPU No user interaction
Good for large repetitive jobs Difficult to debug
Less setup time per job High turnaround time
Diagram:
+------------------+ | Job Pool (Batch) | | Job1, Job2, Job3 | +--------+---------+ | v +--------+---------+ | CPU Execution | +--------+---------+ | v | Output Results | +------------------+
Example: IBM OS/360, early mainframe systems, payroll processing.
Type: Multiprogrammed OS
Definition: Multiple programs are kept in main memory at the same time. When one process waits for I/O, the CPU switches to another process. Increases CPU utilization significantly.
Characteristics:
Fast response time Data communication overhead
Multiple users simultaneously Reliability issues
Diagram:
Time Slice (Quantum = 10ms):
User1 | User2 | User3 | User1 | User ... [10ms] [10ms] [10ms] [10ms] [10ms]
CPU switches between users so fast, each feels they have dedicated access.
Example: UNIX, Linux, Windows โ multiple users logged in at same time.
Type: Real Time OS (RTOS)
Definition: Processes data as it comes in, typically without buffering delays. Must respond within a guaranteed time constraint (deadline). Two types: Hard RTOS and Soft RTOS.
Characteristics:
- Each process gets equal CPU time slice (quantum).
- Round-robin scheduling is typically used.
- Context switch happens at each quantum expiry.
- User gets interactive response within milliseconds.
Advantages vs Disadvantages
n ADVANTAGES n DISADVANTAGES
Deterministic response Expensive
High reliability Limited multitasking
Precise timing control Complex design
Diagram:
Hard RTOS: MUST meet deadline (no exceptions) Soft RTOS: Tries to meet deadline (some misses OK)
Sensor Input --> [RTOS] --> Output in microseconds | Guaranteed Deadline
Example: Air traffic control, medical devices (pacemaker), anti-lock brakes (ABS), missile systems.
Type: Distributed OS
Definition: Manages a group of independent computers and presents them to the user as a single coherent system. Resources are shared across the network. Each node has its own OS.
Characteristics:
- Multiple autonomous computers connected via network.
- Transparent to the user โ appears as one machine.
- Load balancing distributes computation across nodes.
- Failure of one node does not crash the entire system.
Advantages vs Disadvantages
n ADVANTAGES n DISADVANTAGES
Resource sharing Network dependency
Fault tolerance Complex security
Scalability Synchronization challenges
Diagram:
Node A <----Network----> Node B | | Local OS Local OS | | Resources Resources
Users see ONE unified system.
Example: Google's distributed systems, Amazon AWS, Amoeba OS.
Type: Mobile OS
Definition: Designed specifically for mobile devices. Manages touch interface, battery, GPS, camera, mobile networks, and small screen display. Lightweight and power-efficient.
Characteristics:
- Designed for touch-based, battery-powered devices.
- Manages telephony, GPS, camera, sensors.
- App sandboxing provides security isolation.
- Over-the-air (OTA) updates are supported.
Advantages vs Disadvantages
n ADVANTAGES n DISADVANTAGES
Volume in drive C is OS Directory of C:\Users\Yash 05/31/2026 os_notes.pdf C:\Users\Yash> _
Graphical User Interface (GUI)
Definition: A visual interface using icons, windows, menus, and buttons. Users interact using a mouse/touch. Easier to use than CLI.
GUI โ Windows Desktop Layout +----------------------------------------------+ | [Desktop] [_ [] X] | | +----------+ +-----------+ | | | My Files | | Recycle | | | | (icon) | | Bin(icon) | | | +----------+ +-----------+ | | | | [Start Menu] [Taskbar] [Clock: 10:30] | +----------------------------------------------+
n ADVANTAGES n DISADVANTAGES
Easy to use for beginners Slower than CLI for experts
Visual and intuitive Uses more memory/CPU
Multitasking windows Less control over system
No need to memorize commands Requires display hardware
1.4 Components of OS
Component Role
Kernel Core of the OS. Directly interacts with hardware. Manages CPU, memory, I/O.
Shell Interface between user and kernel. Takes user commands (CLI) or actions (GUI).
Process Manager Creates, schedules, and manages all running processes.
Memory Manager Allocates/deallocates RAM. Handles virtual memory and paging.
File System Organizes, stores, retrieves files. Manages directories and permissions.
Device Drivers Special programs that communicate between OS and hardware devices.
I/O Manager Coordinates data transfer between CPU, memory, and I/O devices.
Security Module Handles authentication, authorization, and access control.
Networking Manages network protocols, connections, and data transfer.
OS Architecture โ Layer Diagram USER APPLICATIONS | +------+------+ | SHELL | <-- User Interface Layer +------+------+ | +------+------+ | KERNEL | <-- Core OS Layer +------+------+ | +----------+----------+ | | | [Process] [Memory] [I/O Mgr] Manager Manager | HARDWARE (CPU, RAM, Disk, I/O)
1.5 System Calls
Definition: A System Call is a programmatic way for a user application to request a service from the OS kernel. It provides an interface between user programs and the OS. When a process needs to perform a privileged operation (like file I/O, memory allocation, or process creation), it uses a system call.
Category Examples (Linux) Purpose
Process Control fork(), exec(), exit(), wait(), getpid() Create/manage processes
File Management open(), read(), write(), close(), unlink() File operations
Device Management ioctl(), read(), write() Device I/O control
Information getpid(), alarm(), sleep() System info retrieval
Communication pipe(), socket(), send(), recv() IPC and networking
System Call Flow User Program | | System Call (e.g., read())
Stable & reliable Hardware compatibility issues
Highly customizable Less commercial software
Examples / Versions: Ubuntu, Fedora, Kali Linux, CentOS, Debian
Mac OS
Definition: macOS is Apple's OS for Macintosh computers. Unix-based, known for its sleek design, security, and seamless integration with Apple hardware and services.
n ADVANTAGES n DISADVANTAGES
Excellent UI/UX design Expensive hardware required
Very secure Limited customization
Great for creative work Less gaming titles
Smooth Apple ecosystem Closed ecosystem
Examples / Versions: macOS Sonoma, macOS Ventura
General Advantages & Disadvantages of OS
n ADVANTAGES n DISADVANTAGES
Manages hardware resources efficiently OS itself consumes CPU and memory
Provides user-friendly interface (GUI/CLI) OS crash can bring down entire system
Enables multitasking and multiprocessing Licensed OS can be very expensive
Provides security and access control Security vulnerabilities exist in every OS
Error detection and handling Complex to design and maintain
Efficient file management Boot time can be slow
Quick Revision โ Important Definitions
Term One-Line Definition
OS Software that manages hardware resources and provides services to programs.
Kernel Core part of OS that directly manages hardware.
Shell User interface to OS (CLI or GUI).
System Call Request from user program to OS for a service.
Batch OS Jobs processed sequentially without user interaction.
Time Sharing OS Multiple users share CPU time in small quanta.
Real Time OS OS that guarantees response within a strict time deadline.
Distributed OS OS managing multiple networked computers as one system.
CLI Text-based interface where user types commands.
GUI Visual interface with icons, windows, menus.
Multiprogramming Multiple programs in memory; CPU switches when one waits.
Mobile OS Lightweight OS designed for smartphones/tablets.
Exam Tip: Always write Definition + 3 Characteristics + Example for each type of OS. Draw the layer diagram for OS Architecture. For advantages/disadvantages, write minimum 3 points each.
