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I want to build a real mode OS. I'm using Ubuntu and NASM. I'm using the real-mode example from the tutorials. I add the multiboot numbers at the beginning. I have a problem. Apparently I can't begin my real mode (NASM) program with a multiboot header - or maybe I can't multiboot a 16 bit program. Someone told me Grub switches into protected mode for multiboot compliant kernels. The code uses BIOS interrupts, so protected mode will be a problem.

QEMU (or grub) complains: error invalid arch-dependent ELF magic. I googled it. No help. If I remove the multiboot numbers at the beginning. It complains: no multiboot header.

I tried using NASM directive 'bits 32' for the first bytes which are the multiboot header (such as 0x1BADB002 ) and then use 'bits 16' for the rest of the code. That makes no difference.

File grub.cfg must have the following menu entry:
menuentry "myos" {
multiboot /boot/myos.kernel
}

File grub-mkrescue creates file myos.iso based on that menuentry

qemu-system-i386 -cdrom myos.iso says: invalid arch-dependent ELF magic. The only place magic is involved is in the multiboot header

GRUB can only boot multiboot-compliant kernels, which are 32-bit or (someone correct me if I'm wrong) 64-bit kernels. There's no point making your kernel multiboot-compliant if it's 16-bit.

omarrx024 wrote:GRUB can only boot multiboot-compliant kernels, which are 32-bit or (someone correct me if I'm wrong) 64-bit kernels. There's no point making your kernel multiboot-compliant if it's 16-bit.

It doesn't work if I remove the word 'multiboot' from:

menuentry "myos" {
multiboot /boot/myos.kernel
}

I have grub installed on the computer. There's no way to boot except using grub.

Grub can chain load another bootloader. So you put your 16-bit bootloader in the bootsector of the partition containing your OS and then chain load it from Grub.

http://www.gnu.org/software/grub/manual ... m-directly

I notice that the meaty skeleton tutorial is set up to boot using QEMU and no physical boot device. That method relies on GRUB, which I cant use for 16-bit kernel. Also I notice I can boot my simple 16-bit kernel from a flashdrive with only a MSDOS MBR in sector 1, with my kernel beginning in sector 2. I don't need an image of a DOS bootable floppy or a harddrive image. So apparently the only extra software needed is a MSDOS MBR. I believe I should be able to use that in an ISO file along with my simple 16-bit kernel to boot using the linux -cdrom (software) device that is used by meaty skeleton via QEMU. I concatenated the kernel onto the MBR and made an ISO from that. But QEMU says no bootable device.

There must be a simple method using the following 5 components:

DOS MBR in sector #1 (or el torito equivalent)
16-bit kernel following the MBR
Xorriso ISO creating program
-cdrom (software) device used in meaty skeleton
QEMU emulator

I tried some ideas using the above, but always QEMU seems to be looking for a bootable device (hardwae or bootable image?).

Thanks. Bill S.

NOTE Before Reading: I have not actually tried this, it is all theory to me.

GRUB does allow you to boot a 16-bit kernel, in fact that is how memtest86+ is loaded: Now, I have disassembled memtest86+.bin - it has no multiboot header and the first few lines are just segment fixes: So, you can boot 16-bit, but without multiboot. And it seems Grub will load your binary to 0x7C00? which memtest86+ relocates to 0x90000.

But my recommendation would be to either use 32-bit multiboot complaint kernel OR roll your own boot sectors / loaders.

One more choice you do have - if you require multiboot - is to make a 32-bit multiboot complaint kernel and drop back down to 16-bit after grub loads you in.



Best Regards,

B!

If all you want from GRUB is to load your file as a boot sector at 0x7C00 and transfer control to it in real mode, use the "chainloader" command.

OK. I got it. I made isolinux.cfg file that makes an ISO for my kernel then I use QEMU to boot the ISO - I just need to streamline it with a script. Thanks everybody for your input it is very instructive information. Bill S.

iansjack wrote:Grub can chain load another bootloader. So you put your 16-bit bootloader in the bootsector of the partition containing your OS and then chain load it from Grub.

http://www.gnu.org/software/grub/manual ... m-directly

I did a 16-bit bootloader and chainloading with grub. It works great. Thanks.

Bill S.