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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 (see docs/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