What is an Operating System?; Functions of an Operating System; UNIX System Architecture

What is an Operating System?

Why is an OS Needed?

Basic functions of a computer:

Data Storage — save data to storage devices

Data Processing — compute and transform data 3. Data Movement — transfer data between components 4. Control — coordinate all operations

Problem: Any user cannot communicate/interact directly with computer hardware to do different tasks. There is a need for some interface between user and hardware.

Solution: The Operating System

Definition of Operating System

An Operating System (OS) is system software that:

Manages computer hardware and software resources
Provides a common interface between users and hardware
Controls and coordinates use of hardware among various application programs for various users
Acts as a resource manager — allocates CPU, memory, I/O to processes

OS as an Intermediary

Applications (User Programs)
           ↕
    OPERATING SYSTEM
           ↕
      Hardware (CPU, Memory, I/O)

Popular Operating Systems

OS	Family	Use Case
Windows 11	Microsoft	Desktop, Gaming
Ubuntu / Debian	Linux	Development, Server
macOS	Apple/Unix	Creative, Development
Android	Linux-based	Mobile
iOS	Apple	Mobile
Unix	AT&T Bell Labs	Server, Academic
Linux	Open-source Unix-like	Server, Embedded

Functions of an Operating System

Function	Description
Process Management	Create, schedule, terminate processes; handle synchronization
Memory Management	Allocate/deallocate memory; virtual memory management
File Management	Create/delete files and directories; access control
I/O Management	Control I/O devices; provide device driver interface
Security & Protection	Protect resources from unauthorized access
Networking	TCP/IP stack; network communication
Error Detection & Handling	Detect hardware errors; take corrective action
User Interface	CLI (command line) or GUI (graphical) interface

UNIX System Architecture

Overview

UNIX is one of the most influential operating systems ever designed (Bell Labs, 1969).

Major Subsystems of UNIX:

File Subsystem — manages files and directories
Process Control Subsystem — manages processes

UNIX Architecture Diagram

+-------------------------------------------------------+
|                  USER SPACE                           |
|   User Programs / Applications                        |
|   (Shell, Compiler, text editor, database client)    |
+------------------+-------------------------------------|
|       System Call Interface (API boundary)            |
+------------------+------------------------------------+
|                  KERNEL SPACE                         |
|   File Subsystem    |   Process Control Subsystem     |
|   (file operations) |   (process scheduling, IPC)    |
|   +---------------+ |   +-------------------------+   |
|   | File tables   | |   | CPU Scheduler           |   |
|   | Inodes        | |   | Memory Management       |   |
|   | Buffer cache  | |   | IPC (pipes, signals)    |   |
|   +---------------+ |   +-------------------------+   |
+----------------------+----------------------------------+
|         Hardware Control (Device Drivers)              |
+-------------------------------------------------------+
|               HARDWARE                                |
|   CPU | Memory | Disk | Network | Terminal            |
+-------------------------------------------------------+

UNIX Key Concepts

Concept	Description
Everything is a file	Devices, sockets, pipes are represented as files
Shell	Command-line interface to the kernel (bash, sh, zsh, ksh)
PID	Every process has a unique Process ID
UID/GID	User ID and Group ID for security
File permissions	rwx for owner, group, others
Kernel	Core of the OS managing all resources
Daemon	Background process (like a service in Windows)