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GGB86 - Gabriel's Good Bootloader for x86

Simple bootloader for x86 machines and the FAT 32 filesystem. Loads kernels from the boot disk, from reserved FAT 32 sectors to the 0x1000 address, switches to 32 bit protected mode and transfers control to the kernel.

A dumb kernel is included for testing.

Quickstart

You'll need the Netwide Assembler (NASM) and the GNU C toolchain (GCC, LD and Make) to build. Also, your system needs a cat implementation in the $PATH.

Use make to create everything: the bootloader and the dumb kernel, with both linked in the os_image32.img file. The bios_x86 file is just the MBR boot sector (i.e., the bootloader itself) and FAT boilerplate and the kernel32 is just the compiled kernel.

The script boot.sh launches the image in a QEMU-KVM virtual machine.

How it works

The FAT filesystem itself has as header a jmp instruction so bootloaders keep working (I don't know if they would be very useful otherwise...). So the first sector of the disk image has the FAT stuff and the bootloader. In the FAT metadata, 32 sectors are reserved, being the first three:

  • First: FAT metadata and the bootloader;
  • Second: FSInfo struct of FAT 32;
  • Third: Reserved for the root directory.

The remaining 29 are reserved for the kernel, which could also be a second stage bootloader. The entry of the kernel should of course be at the start of the fourth sector, as the bootloader loads those 8 sectors to the address 0x1000 and simply jumps there.

TODO

  • Change from NASM to GNU AS.