Pratik Singhal Pratik Singhal - 30 days ago 13
C Question

How to use QEMU properly with multi boot headers

I am learning the basic of OS making. I have made a multi boot header compliant .asm file and a .c file. The code in the .asm file calls the main function of .c file.

The problem is that QEMU is unable to boot from the file produced after the compilation and linking of the .asm and the .c file .

It simply says that it can't find a bootable device.

Although, I am able to boot from a simple .asm file like :-

mov ax, 0x0e
mov al, 'H'
int 10h
times 510 - ($ - $$) db 0
jmp $
dw 0xaa55

Is there something more which I have to do?

Answer Source

QEMU doesn't have native support for multiboot. Instead, you'll need to create a virtual hard drive image and install some sort of multiboot boot loader (such as grub), then put your multiboot image somewhere on the drive (i.e. in a file on a partition).

As far as actually installing grub onto a virtual HDD, there's multiple ways to do it, but here's the process I always use:

  1. Use qemu-img or dd if=/dev/zero to create your HDD image.
  2. Download a Linux installer ISO (I typically use Arch Linux).
  3. Boot qemu with the blank HDD image and ISO using -hda <HDD-image-filename> -cdrom <ISO-file-name> -boot once=d. The last bit ensures qemu will try to boot from CD first.
  4. Use fdisk/parted/etc to format the disk.
  5. Mount your boot partition (the one you want to install grub to) and use grub-install.
  6. Unmount and shut down the emulator.

Then, you'll be able to boot off the HDD image and use grub or whatever loader you choose to boot your multiboot image.

The reason your simple ASM example works is because you're effectively emulating the MBR, the first sector of a typical hard drive, so QEMU's BIOS will boot from it (specifically, it sees that 0xaa55 signature).