OSDev.org

Posted: **Wed May 25, 2011 8:49 am**

I had kind of left the whole OS devel side of programming for a while because its a little confusing when you are just starting out

but I'm back with a cleared head

Okay, I have a simple (one assembly source file) kernel image for GRUB to boot, but it's telling me it's not a valid executable format! I'm compiling with NASM with an output of flat binary. I have a Multiboot header (just the required values). Here is my one assembly file

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[BITS 32]
%include "grub.inc"

start:

	cli				; Block interrupts (they can awaken the CPU from halt!)
	hlt				; Halt the CPU

ALIGN 4
mboot:
	dd MULTIBOOT_HEADER_MAGIC
	dd MULTIBOOT_HEADER_FLAGS
	dd MULTIBOOT_CHECKSUM

Of course, that doesn't do anything, but everything is stripped down just to get it booted.
Here is "grub.inc"

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MULTIBOOT_PAGE_ALIGN	equ 1<<0
MULTIBOOT_HEADER_MAGIC	equ 0x1BADB002
MULTIBOOT_HEADER_FLAGS	equ MULTIBOOT_PAGE_ALIGN
MULTIBOOT_CHECKSUM	equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)

I compile with:

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nasm -f bin -o kernel.bin ./kernel_entry.asm

and then I mount my virtual hard disk using a loop back, and copy that image to "/boot/"

I use qemu to boot the hdd with this call:

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qemu -hda ./SimpleKernel.hdd -m 128 "$@"

The "$@" is because that is actually in a shell script. That way I can pass extra parameters to qemu if needed.

once in grub, I type "kernel /boot/kernel.bin" and I get this message:

Error 13: Invalid or unsupported executable format

I am using grub v0.97 (yes I know, it's old, but I had it laying around, and it was simple to install

)

Any help would be greatly appreciated!

Thanks,
Caleb

Posted: **Wed May 25, 2011 9:13 am**

It would be much easier for you if you would assemble it as an ELF (not flat binary). But, if you really want to use flat binary, what you can do (iirc) is include the aout kludge in the multiboot header. This will probably require a linker script to get the values for.

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extern kernelCodeAddr
extern kernelBssAddr
extern kernelEndAddr

;...

mboot:
        dd MULTIBOOT_HEADER_MAGIC
        dd MULTIBOOT_HEADER_FLAGS
        dd MULTIBOOT_CHECKSUM
        dd mboot
        dd kernelCodeAddr
        dd kernelBssAddr
        dd kernelEndAddr
        dd start

Posted: **Wed May 25, 2011 9:42 am**

I just tried compiling with ELF format, and I get the same error :/

I just changed the "-f bin" in the nasm call to "-f elf" and I got the same result. If I can use ELF format, I would assume that would be simpler. I just didn't realize I could use it.

I had originally had an example from here: http://www.osdever.net/tutorials/view/w ... e-c-kernel, but that didn't even run using grub! :O

I stripped it down to what you saw in my original post (removing the C code, and therefore the linker script and the aout kludge) to just make it a straight up assembly binary to see if that even worked.

Let's start over, since it appears my tutorial wasn't even valid. how would you (anyone really) go about compiling a simple kernel for grub?

EDIT

I was getting frustrated and starting googling (again lol) and found this tutorial: http://www.osdever.net/bkerndev/Docs/basickernel.htm his linker script was slightly different, but his multiboot header is the same. It worked! Of course, my k_printc function needs some work (it's printing characters diagonally

) but I'm very happy since it's printing something at all!

I now need to figure out what's wrong with the original linker script... lol

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