🛠️ Creating a Custom Bootloader

A bootloader is the tiny program that runs right after your PC powers on — it’s what wakes up your hardware and starts loading your operating system. It’s stored in the first sector of the boot device (usually the first 512 bytes), known as the Master Boot Record (MBR). Since the bootloader runs in a super-limited environment (16-bit real mode, tiny memory, no OS features), writing one means you have to be precise and efficient with assembly code. Your bootloader’s job is to load the next stage (like a kernel or second-stage loader) into memory and transfer control to it.

Writing a custom bootloader gives you full control over the startup process — you can create a custom splash screen, load your OS your way, or even chainload other bootloaders. It’s a foundational step if you’re building your own OS from scratch. Bootloader code must end with the special signature 0x55AA in the last two bytes to tell the BIOS it’s valid and bootable. Because you only have 512 bytes, every instruction counts. Tools like NASM (Netwide Assembler) and emulators like QEMU help you develop and test bootloaders without messing with your actual hardware.

📚 Example Bootloader Code

đź’ˇ Pro Tip: Always test your bootloader code in an emulator like QEMU before trying it on real hardware.