Bootloader

Legacy BIOS bootloader

The purpose of this project is to understand what really happens when a PC boots, from BIOS startup, all the way to jumping into my own kernel. The long-term goal is to grow this into a minimal OS.

What it does (current)

• Stage 1 (boot sector, 512 bytes)
  • Loaded by BIOS at 0x7C00
  • Uses BIOS Interrupt disk services (INT 13h) to load Stage 2
• Stage 2 (real mode (16 bit) -> protected mode (32 bit) -> long mode (64 bit))
  • Sets up a GDT
  • Enables A20 Fast Gate
  • Switches to 32-bit protected mode
  • Builds identity-mapped page tables (illustration can be found at identity_paging.md)
  • Enables paging + long mode
  • Jumps to the kernel entry point
• Kernel (64-bit)
  • Currently prints to VGA and halts

Project layout

• boot/ — stage1 + stage2 boot code
• kernel/ — 64-bit kernel + entry stub
• drivers/ — simple hardware drivers (VGA for now)
• common/ — shared headers (e.g. types.h)
• run_stages.sh — builds a raw disk image and runs QEMU

Build & run

Requirements:

• nasm
• clang (cross targets: i386-elf, x86_64-elf)
• ld.lld
• llvm-objcopy
• qemu-system-x86_64

Run:

./run_stages.sh