CoreX OS - A Custom x86 Operating System

A custom 32-bit operating system built from scratch in C and x86 Assembly, demonstrating low-level systems programming and OS development concepts.

🚀 Features

Core System

• Custom Bootloader - Written in x86 Assembly, loads kernel into memory
• 32-bit Protected Mode - Full transition from Real Mode to Protected Mode
• GDT (Global Descriptor Table) - Proper memory segmentation setup
• IDT (Interrupt Descriptor Table) - 32 exception handlers + 16 IRQ handlers
• PIC (Programmable Interrupt Controller) - IRQ remapping to avoid conflicts

Memory Management

• Physical Memory Manager (PMM) - Bitmap-based page frame allocator
• Paging Support - 4KB page tables with identity mapping
• Dynamic Memory Allocation - Page-level memory allocation and deallocation

I/O & Drivers

• VGA Text Mode Driver - 80x25 color text output with scrolling
• PS/2 Keyboard Driver - Scancode to ASCII conversion with shift/caps support
• PIT Timer - Programmable Interval Timer for time-based operations

User Interface

• Interactive Shell - Command-line interface with multiple built-in commands
  • help - Display available commands
  • clear - Clear the screen
  • version - Show OS version information
  • meminfo - Display memory statistics
  • echo - Echo text to screen

File System

• In-Memory File System - Simple file creation, reading, and deletion
• Directory Support - Basic directory structure

🏗️ Architecture

CoreX OS
├── Bootloader (Assembly)
│   ├── Stage 1: Boot sector (512 bytes)
│   └── Stage 2: Kernel loader
├── Kernel (C + Assembly)
│   ├── Core
│   │   ├── Entry point (kernel_stub.asm)
│   │   ├── Main kernel (kernel.c)
│   │   └── Interrupt handlers (isr.asm)
│   ├── Memory Management
│   │   ├── PMM (pmm.c)
│   │   └── Paging (paging.c)
│   ├── Drivers
│   │   ├── VGA (kernel.c)
│   │   ├── Keyboard (keyboard.c)
│   │   └── Timer (timer.c)
│   └── System
│       ├── IDT (idt.c)
│       ├── PIC (pic.c)
│       ├── Shell (shell.c)
│       └── File System (fs.c)

🛠️ Building

Prerequisites

• NASM - Netwide Assembler
• GCC - GNU Compiler Collection (with -m32 support)
• LD - GNU Linker
• QEMU - For testing (qemu-system-i386)

Build Commands

# Build bootloader
nasm -f bin bootloader/boot.asm -o bootloader/boot.bin

# Build kernel
nasm -f elf32 kernel/kernel_stub.asm -o kernel/kernel_stub.o
nasm -f elf32 kernel/isr.asm -o kernel/isr.o
gcc -m32 -ffreestanding -c kernel/*.c -Iinclude
ld -m i386pe -T kernel/linker.ld -o kernel/kernel.bin <objects>

# Create OS image
cat bootloader/boot.bin kernel/kernel.bin > os-image.bin

# Run in QEMU
qemu-system-i386 -drive format=raw,file=os-image.bin

Or use the provided Makefile:

make all
make run

🎯 Technical Highlights

Low-Level Programming

• Direct hardware manipulation via port I/O
• Inline assembly for critical operations
• Manual stack and register management
• Interrupt handling in Assembly

Systems Concepts

• Bootloader development and disk I/O
• CPU mode transitions (Real → Protected)
• Memory segmentation and paging
• Interrupt-driven I/O
• Device driver development

Code Quality

• Modular architecture with clear separation of concerns
• Comprehensive comments and documentation
• Proper error handling
• Clean code structure following OS development best practices

📚 Learning Outcomes

This project demonstrates proficiency in:

• Low-level programming (C, x86 Assembly)
• Operating system concepts (memory management, process scheduling, I/O)
• Hardware interaction (BIOS, VGA, keyboard, timer)
• Systems architecture (x86 architecture, interrupts, paging)
• Debugging (QEMU, GDB, serial debugging)

🔧 Current Status

Working:

• ✅ Bootloader successfully loads kernel
• ✅ Protected mode transition
• ✅ IDT with exception and IRQ handlers
• ✅ VGA text output with scrolling
• ✅ Physical memory manager
• ✅ Shell command system
• ✅ File system operations

In Development:

• 🔨 Interactive keyboard input (hardware interrupt handling)
• 🔨 Multi-tasking scheduler
• 🔨 Virtual file system
• 🔨 Network stack

📸 Screenshots

CoreX OS booting and initializing subsystems

Interactive shell with command support

🎓 Educational Value

This project was built as a learning exercise to understand:

• How operating systems work at the lowest level
• The boot process from power-on to running code
• Memory management and virtual memory
• Hardware abstraction and device drivers
• System call interfaces

📖 Resources Used

• OSDev Wiki
• Intel x86 Architecture Manuals
• "Operating Systems: Three Easy Pieces" by Remzi H. Arpaci-Dusseau
• Various OS development tutorials and documentation

📝 License

This project is open source and available under the MIT License.

👤 Author

Your Name

• GitHub: Shashank0701-byte
• LinkedIn: Shashank_Chakraborty

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Built with ❤️ and lots of debugging