My_Bootloader

A two-stage x86 bootloader written in x86 assembly and C, targeting a 1.44MB floppy image. Built as a learning project to understand the PC boot process from the ground up.

What it does

Stage 1 fits inside the 512-byte MBR boot sector. It reads drive geometry dynamically from the BIOS, loads stage 2 from disk using BIOS INT 13h with a 3-attempt retry loop, and jumps to it.

Stage 2 start in 16-bit real mode with full BIOS access and does the work to switch to 32 bit:

• Enables the A20 line via the 8042 keyboard controller
• Sets up a flat-model GDT (null, 32-bit code, 32-bit data, 16-bit code, 16-bit data descriptors)
• Switches the CPU from 16-bit real mode to 32-bit protected mode
• Far jumps into 32-bit code, reloads segment registers, zeroes BSS, and calls into C

Finally, stage 2 traverses the FAT12 filesystem to find and load kernel.bin, booting into the kernel itself

Stage 2 also implements real mode ↔ protected mode switching to allow BIOS calls from 32-bit C code — disk reads and drive parameter queries go through this transition.

Build

Requires an i686-elf cross-compiler (binutils + GCC). Build it following the OSDev GCC Cross-Compiler guide

make

Output is a floppy.img that can be run directly in QEMU:

qemu-system-i386 -fda build/main_floppy.img

Or in Bochs using the provided bochs_config.

Structure

src/
  bootloader/
    stage1/   - MBR boot sector (NASM)
    stage2/   - FAT12 loader, A20, GDT, protected mode switch (NASM + C)
  kernel/     - basic kernel used to prove that the bootloader works

What I learned

• Bootloaders and why they are split into 2 stages
• FAT12 filesystem layout and cluster chain traversal
• BIOS disk I/O
• x86 segmentation, GDT descriptor format, and why the fields are laid out the way they are
• How to build a freestanding C environment with a custom linker script and cross-compiler

References and credits

Built following nanobyte_dev's "Building an OS" YouTube series. Several implementation details and code excerpts adapted from OSDev Wiki tutorials. The intel manual for 64 and IA-32 Architectures was also very useful.

Big thanks to nanobyte and the os dev community!

Next

This project is complete as a standalone bootloader. I initially wanted to build an entire hobby OS from the ground up, Bootloader included, but this project has shown me has intricate a bootloader can get. Thus, the next project will be a separate kernel booted via Limine, focused on memory management, paging and scheduling.