A Network Operating System (NOS) is an operating system installed on a server computer that manages and controls network resources. It allows multiple computers connected in a network to communicate and share resources such as files, printers, users, applications, and security.

Functions of Network Operating System

A NOS provides the following services:

• File sharing between computers
• Printer sharing across the network
• User and group management
• Security and access control
• Managing network resources and applications

Types of Network Operating Systems

1. Client-Server Network Operating System

In a client-server network, a central server provides services, and all other computers (clients) depend on it for resources and data.

Working

• The server controls all network operations
• Clients request services from the server
• Server responds with required data or services

Examples

• Windows Server 2003
• Windows Server 2008
• Linux Server
• Novell NetWare

Real-Life Example

In a school computer lab:

• The teacher’s computer acts as the server
• Student computers are clients
• Students access files and printers through the teacher’s computer

Another example:
In a company:

• Server stores all employee data
• Employees (clients) access files from the server

2. Peer-to-Peer Network Operating System

In a peer-to-peer network, there is no dedicated server. Every computer can act as both a client and a server.

Working

• Each computer shares its own resources directly
• No central control system
• All computers are equal in the network

Examples

• Windows 95
• Windows NT Workstation
• Windows 10 (in basic home sharing setup)

Real-Life Example

In a small group of friends:

• Each person shares files directly from their laptop
• One computer can send files to another without a central server

Another example:
In a small office:

• All computers are connected
• Each system shares printers and files directly

Advantages of NOS

• Centralized management (in client-server systems)
• Better security control
• Easy sharing of resources
• Remote access is possible

Disadvantages of NOS

• Server systems are expensive
• High dependency on the server
• Maintenance and updates are required regularly
• If the server fails, the whole network is affected

A Real-Time Operating System (RTOS) is an operating system designed to process data and respond to inputs within a strict time limit. Its main objective is to provide a fast and predictable response to external events.

Objectives of RTOS

Primary Objective

• To provide a quick response and meet strict scheduling deadlines.

Secondary Objective

• To ensure efficient resource utilization and user convenience.

Features of RTOS

• Responds immediately to external inputs (like sensors or devices)
• Used in systems where timing is critical
• Guarantees response within a fixed time limit
• Commonly used in embedded and control systems

Examples of RTOS Applications

• Missile control systems
• Air traffic control systems
• Robotics
• Medical equipment
• Industrial automation systems

Types of Real-Time Operating Systems

1. Soft Real-Time Operating System

Definition

A Soft Real-Time Operating System is a system where missing a deadline is not critical, but performance may degrade.

Characteristics

• Flexible timing constraints
• System continues working even if deadlines are missed
• Priority is given to important tasks, but no strict guarantee of completion time

Examples

• Video streaming (YouTube)
• Online gaming
• Multimedia applications
• Virtual reality systems

Example (Easy Understanding)

If a video stream is slightly delayed, it may buffer or reduce quality, but the system still works.

2. Hard Real-Time Operating System

Definition

A Hard Real-Time Operating System strictly guarantees that tasks are completed within the required time limit. If the deadline is missed, the system may fail.

Characteristics

• Strict time constraints
• No delay is acceptable
• Failure can cause serious damage or danger
• Used in safety-critical systems

Examples

• Airbag control systems in cars
• Automatic parachute systems
• Missile launching systems
• Medical life-support systems

Example (Easy Understanding)

If an airbag system does not activate at the exact time of a crash, it may result in failure and serious injury.

Advantages of RTOS

• High efficiency and performance
• Best for embedded systems
• Accurate and error-controlled operations
• Better memory management
• Focuses on important real-time tasks

Disadvantages of RTOS

• Can handle only limited tasks at a time
• Expensive to implement
• Complex design and algorithms
• Requires high system resources

A System Call is a programmatic interface through which a user program requests services from the Operating System kernel.

It acts as a bridge between user programs and the kernel.

Basic Structure

• User Program → System Call → Kernel → Hardware Resources → Response back to User Program

Dual Mode Operation (Important Concept)

An operating system works in two modes:

1. User Mode

• Programs run in this mode
• Cannot directly access hardware
• Limited permissions (for security)

2. Kernel Mode

• Operating system runs in this mode
• Has full access to hardware resources (CPU, RAM, I/O devices)
• Executes system calls

Mode Switching

When a user program needs any hardware resource:

• It makes a system call
• System switches from user mode → kernel mode
• After execution, it returns to user mode

Why System Calls are Needed

• For security (user cannot directly access hardware)
• For controlled access to system resources
• To manage processes, files, devices, and communication

Types of System Calls

1. Process Management System Calls

These system calls are used to control processes (programs in execution).

Examples:

• Create process
• Terminate process
• Fork (create child process)
• Execute process
• Wait and signal
• Load and end process

Example (Simple)

When you open an application like a browser, the OS uses process management system calls to create and run it.

2. File Management System Calls

These are used to handle files.

Examples:

• Create file
• Delete file
• Open / Close file
• Read / Write file
• Reposition file pointer
• Get / set file attributes

Example (Simple)

When you save a document in MS Word, the OS uses file system calls to write data into storage.

3. Device Management System Calls

These manage input/output devices.

Examples:

• Request and release device
• Read / Write device
• Get or set device attributes
• I/O control operations

Example (Simple)

When you print a document, the OS uses device system calls to communicate with the printer.

4. Information Management System Calls

These provide system-related information.

Examples:

• Get or set system time and date
• Get system data
• Get process, file, or device attributes
• Get system information like memory usage

5. Communication System Calls

These are used for inter-process communication (IPC).

Models:

(A) Message Passing Model

• Processes communicate by sending and receiving messages
• Examples:
  • Send message
  • Receive message
  • Create / delete communication links

(B) Shared Memory Model

• Processes communicate using shared memory
• One process writes, another reads

Examples:

• Pipes
• Message queues
• Shared memory

Advantages of System Calls

• Safe access to hardware resources
• Better security
• Efficient resource management
• Supports multitasking and process control

What is a File?

A file is a collection of related data stored on a secondary storage device such as a hard disk, SSD, or pen drive.

A file can be defined in different ways:

1. General Definition

A file is a collection of related data defined by the user and stored on secondary storage.

Example: A student record file containing names, marks, and addresses of students.

2. Structural Definition

A file is a collection of records, where each record consists of multiple fields, and each field stores a specific piece of information.

Example:
A student record:

• Record → Student details
• Fields → Roll No, Name, Marks, Address

3. Technical Definition

A file is a collection of bits, bytes, characters, or words that are stored and processed by a computer system.

Example of File Structure

Explanation of Example

• Each row represents a record
• Each column represents a field
• Entire table represents a file