Linux Evolution, GNU, GPL and System Architecture

Linux evolution

Linux grew from a Unix-like kernel into a complete operating-system family by combining the kernel with GNU tools, libraries.

GNU project

The GNU project develops a free Unix-compatible software environment

GPL

The GNU General Public License permits use, study, modification and redistribution while requiring redistributed derivative source code to remain available.

distribution

A Linux distribution packages the kernel, installer, system libraries, utilities, package manager.

kernel and user space

Kernel space executes privileged code that manages memory, devices, processes and filesystems

system call

A system call is the protected entry point through which a user-space process asks the kernel to perform work such as opening a file.

process and service

A process is a running program instance; a service is a managed background capability often supervised by a service manager.

Linux server

A Linux system configured to provide network or local services under controlled identities, resources and operational policies.

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)

System Calls

What are System Calls?

System calls provide the interface between user programs and the operating system kernel.

When a program needs OS services (file I/O, process creation, memory allocation), it makes a system call.

Categories of System Calls

Category	Examples
Process Control	fork(), exec(), exit(), wait(), getpid()
File Management	open(), read(), write(), close(), unlink(), stat()
Device Management	ioctl(), read(), write() (for devices)
Information Maintenance	getpid(), alarm(), sleep(), time()
Communication	pipe(), socket(), send(), recv(), shmget()

System Call Flow

User Program
    |
    | calls  printf("hello") → internally calls write()
    |
    ↓
std C library (write() wrapper)
    |
    | System Call Instruction (int 0x80 / syscall)
    ↓
KERNEL (system call handler)
    |
    | sys_write() → device driver → I/O
    ↓
Returns to user program

Dual Mode Operation

What is Dual Mode?

The CPU operates in two modes to protect the OS from user programs:

Mode	Also Called	Privilege
User Mode	Unprivileged mode	Limited instructions; cannot directly access hardware
Kernel Mode	Supervisor/System mode	All instructions; full hardware access

Mode Switching

User Program executes → User Mode
     |
     | System Call or Interrupt
     ↓
Kernel Mode (OS handles request)
     |
     | Return from system call
     ↓
User Mode (program resumes)

Mode bit:

0 = Kernel Mode
1 = User Mode

Privileged vs Non-Privileged Instructions

Instruction Type	User Mode	Kernel Mode
I/O operations	Not allowed	Allowed
Memory management	Not allowed	Allowed
Interrupt control	Not allowed	Allowed
Normal arithmetic	Allowed	Allowed
Function calls	Allowed	Allowed

Process Management

What is a Process?

A process is a program in execution — an active entity.

Program vs Process:

Feature	Program	Process
State	Passive (on disk)	Active (in memory)
Resources	None (static file)	CPU, memory, I/O
Number	One copy on disk	Multiple processes can run from same program
Example	`notepad.exe` file	Running instance of Notepad

Process in Memory

+------------------+   High address
|      Stack       |   ← Function calls, local variables (grows down)
|        ↓         |
|                  |
|        ↑         |
|       Heap       |   ← Dynamic memory (malloc, new)
+------------------+
|    BSS Segment   |   ← Uninitialized global/static variables
+------------------+
|    Data Segment  |   ← Initialized global/static variables
+------------------+
|    Text Segment  |   ← Program code (instructions)
+------------------+   Low address

Process Creation

In Unix/Linux:

pid_t child_pid = fork();  // Creates child process (copy of parent)
if (child_pid == 0) {
    // This is the child process
    exec("new_program");   // Replace with different program
} else {
    wait(NULL);            // Parent waits for child to finish
}

Process hierarchy — Process Tree:

init (PID 1)
├── systemd
│   ├── sshd
│   │   └── bash (user session)
│   │       └── vim
│   └── cron
│       └── backup_script.sh
└── kthreadd (kernel threads)

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