Building your own Operating System (OS) is one of the most challenging and rewarding projects in computer science. It teaches you how computers really work beneath the applications and frameworks we use every day.
In this article, we'll explore the complete roadmap for creating an operating system from scratch using the C programming language.
What Is an Operating System?
An Operating System is software that manages computer hardware and provides services to applications.
Popular operating systems include:
- Linux
- Windows
- macOS
- FreeBSD
An OS is responsible for:
- Process management
- Memory management
- File systems
- Device drivers
- User interfaces
- Hardware abstraction
Without an operating system, applications cannot easily communicate with hardware.
Prerequisites
Before building an operating system, you should understand:
Programming Languages
C
C is the primary language used for kernel development because it provides:
- Direct memory access
- Low-level control
- High performance
- Minimal runtime dependencies
Assembly
Assembly is required for:
- Bootloader development
- CPU initialization
- Interrupt handling
- Context switching
Computer Science Fundamentals
You should know:
Data Structures
- Arrays
- Linked Lists
- Stacks
- Queues
- Trees
- B-Trees
- Hash Tables
- Bitmaps
Algorithms
- Searching
- Sorting
- Scheduling algorithms
- Memory allocation algorithms
Understanding Computer Architecture
Operating systems interact directly with hardware.
Important topics:
CPU
Learn:
- Registers
- Instructions
- Privilege levels
- Interrupts
- Exceptions
Memory
Understand:
- RAM
- Virtual Memory
- Paging
- Segmentation
- Memory Mapping
Storage
Learn about:
- Hard Drives
- SSDs
- File Systems
Development Environment
Install:
Compiler
GCC Cross Compiler
Example:
x86_64-elf-gcc
Using a cross-compiler prevents your host operating system from interfering with kernel compilation.
Assembler
NASM
Used for writing assembly code.
Emulator
QEMU
Allows testing your operating system safely.
Example:
qemu-system-x86_64
Debugger
GDB
Used for debugging kernel code.
Operating System Architecture
A basic operating system consists of:
+----------------+
| Applications |
+----------------+
| System Calls |
+----------------+
| Kernel |
+----------------+
| Drivers |
+----------------+
| Hardware |
+----------------+
Step 1: Create a Bootloader
When a computer starts:
- BIOS/UEFI executes.
- Bootloader loads.
- Bootloader loads kernel into memory.
- Kernel starts running.
Simple bootloader example:
[BITS 16]
[ORG 0x7C00]
mov ah, 0x0E
mov al, 'H'
int 0x10
jmp $
times 510-($-$$) db 0
dw 0xAA55
This displays the letter H on the screen.
Step 2: Create the Kernel Entry Point
Kernel entry:
void kernel_main()
{
while (1)
{
}
}
This is the first C function executed by the kernel.
Step 3: Display Text on Screen
Using VGA text mode:
#define VGA_MEMORY ((unsigned short*)0xB8000)
void print_char(char c)
{
VGA_MEMORY[0] = (0x0F << 8) | c;
}
Usage:
print_char('A');
You should now see a character displayed on the screen.
Step 4: Build a Terminal
A terminal allows user interaction.
Features:
- Print text
- Read keyboard input
- Execute commands
Example:
myOS>
Commands:
help
clear
version
Step 5: Keyboard Driver
The keyboard generates interrupts.
Kernel responsibilities:
- Read scan codes
- Convert to characters
- Send input to terminal
Example:
User presses A
↓
Keyboard Interrupt
↓
Kernel Receives Code
↓
Display A
Step 6: Interrupt Handling
Interrupts allow hardware to communicate with the CPU.
Examples:
- Keyboard interrupt
- Timer interrupt
- Disk interrupt
Interrupt Descriptor Table (IDT):
struct idt_entry
{
unsigned short offset_low;
unsigned short selector;
unsigned char zero;
unsigned char flags;
unsigned short offset_high;
};
Step 7: Memory Management
Memory management is one of the most important kernel subsystems.
Responsibilities:
- Allocate memory
- Free memory
- Protect memory
- Virtual memory mapping
Simple bitmap allocator:
unsigned char bitmap[1024];
Each bit represents a memory page.
Step 8: Physical Memory Manager
Tracks available RAM pages.
Functions:
void* alloc_page();
void free_page(void* ptr);
Step 9: Virtual Memory
Modern operating systems use paging.
Benefits:
- Process isolation
- Security
- Larger address spaces
Virtual address:
0x400000
May map to:
0x1A3000
Physical memory.
Step 10: Process Management
A process is a running program.
Process structure:
struct process
{
int pid;
void* stack;
void* page_directory;
};
Kernel responsibilities:
- Create process
- Destroy process
- Schedule process
Step 11: Task Scheduler
Scheduler decides which process runs next.
Common algorithms:
Round Robin
P1 → P2 → P3 → P1
Priority Scheduling
High Priority First
Step 12: Context Switching
Switching between processes requires saving CPU state.
Save:
- Registers
- Stack Pointer
- Instruction Pointer
Restore next process state.
This creates multitasking.
Step 13: System Calls
Applications communicate with the kernel using system calls.
Example:
write();
read();
open();
close();
Flow:
Application
↓
System Call
↓
Kernel
↓
Hardware
Step 14: File System
A file system stores data permanently.
Examples:
- FAT12
- FAT16
- FAT32
- EXT2
- EXT4
Beginner recommendation:
FAT12
because it is simple to implement.
Basic operations:
open()
read()
write()
delete()
Step 15: Disk Driver
The kernel must communicate with storage devices.
Responsibilities:
- Read sectors
- Write sectors
- Cache data
Example:
read_sector();
write_sector();
Step 16: User Mode and Kernel Mode
Modern CPUs support privilege levels.
Kernel Mode:
Ring 0
User Mode:
Ring 3
Benefits:
- Security
- Stability
- Isolation
Step 17: ELF Executable Loader
ELF is a common executable format.
The loader:
- Reads executable.
- Maps memory.
- Creates process.
- Starts execution.
Step 18: Device Drivers
Drivers communicate with hardware.
Examples:
- Keyboard
- Mouse
- Audio
- Network
- Graphics
Step 19: Networking
Networking requires implementing:
- Ethernet
- ARP
- IP
- ICMP
- UDP
- TCP
Example stack:
Application
↓
TCP
↓
IP
↓
Ethernet
Step 20: Graphical User Interface
After the kernel is stable:
Build:
- Framebuffer driver
- Window manager
- Fonts
- Mouse support
Simple GUI architecture:
Desktop
├── Window
├── Window
└── Window
Recommended Project Roadmap
Follow this order:
- Bootloader
- Kernel Entry
- VGA Text Output
- Keyboard Driver
- Terminal
- IDT
- Timer
- Physical Memory Manager
- Paging
- Heap Allocator
- Processes
- Scheduler
- System Calls
- FAT12 File System
- ELF Loader
- User Programs
- Networking
- GUI
Useful Resources
Recommended books:
- Operating Systems: Three Easy Pieces
- Modern Operating Systems
- Operating System Concepts
- Computer Systems: A Programmer's Perspective
- Intel Software Developer Manual
Recommended websites:
- OSDev Wiki
- LittleOSBook
- BrokenThorn OS Development Series
Final Thoughts
Building an operating system is a long-term project that can take months or even years. Start small. Focus first on booting the machine, printing text, handling the keyboard, and managing memory. Once those fundamentals are working, gradually add multitasking, file systems, networking, and eventually a graphical interface.
The most important advice is simple: build one subsystem at a time. Every modern operating system, from Linux to Windows, started as a small kernel capable of doing only a few basic tasks. With patience and consistency, you can build your own operating system from scratch in C and gain a deep understanding of how computers truly work.
