64 Bit Kernel With Grub

os.hacker64 · Post by **os.hacker64** » Tue Feb 12, 2008 5:01 pm

My understanding is that GRUB will load an ELF64 kernel into long mode is this correct?

ucosty · Post by **ucosty** » Tue Feb 12, 2008 5:24 pm

Not as far as I'm aware.

os.hacker64 · Post by **os.hacker64** » Tue Feb 12, 2008 5:28 pm

How do the 64 bit Linuxes do it?

ucosty · Post by **ucosty** » Tue Feb 12, 2008 5:31 pm

Some sort of stub, no doubt.

xyzzy · Post by **xyzzy** » Wed Feb 13, 2008 1:17 am

Grub will enter it in 32-bit protected mode. It's up to you to write some 32-bit ASM code in that will get you into long mode.

bluecode · Post by **bluecode** » Wed Feb 13, 2008 2:58 am

Grub woun't load a elf64 file either. At least not unpatched.

AJ · Post by AJ » Wed Feb 13, 2008 3:01 am

I tackle this by loading a 32 bit elf second stage boot loader as the 'kernel' in GRUB. The actual kernel is loaded as a module, which my 32 bit elf loader relocates.

Cheers,
Adam

ucosty · Post by **ucosty** » Wed Feb 13, 2008 3:36 am

I should also mention that Linux kernel images are treated as a special case by GRUB as they're not multiboot compliant.

binutils · Post by **binutils** » Wed Feb 13, 2008 6:57 am

grub-32 loads linux_kernel which is compiled by pure-64 gcc, i.e. 64bit kernel.
But, grub-32 itself doesn't compiled by pure-64 gcc, ATM.
Better off to try lilo.

xyzzy · Post by **xyzzy** » Wed Feb 13, 2008 7:21 am

binutils wrote:grub-32 loads linux_kernel which is compiled by pure-64 gcc, i.e. 64bit kernel.

GRUB puts any Linux kernel it loads on x86, regardless of whether it's 32-bit or 64-bit, in 16-bit real-mode. Therefore, Linux still has some 16-bit real mode startup code on both i386 and x86_64, with some extra code for x86_64 to get it into long mode.

binutils · Post by **binutils** » Wed Feb 13, 2008 7:39 am

AlexExtreme wrote:
binutils wrote:grub-32 loads linux_kernel which is compiled by pure-64 gcc, i.e. 64bit kernel.
GRUB puts any Linux kernel it loads on x86, regardless of whether it's 32-bit or 64-bit, in 16-bit real-mode. Therefore, Linux still has some 16-bit real mode startup code on both i386 and x86_64, with some extra code for x86_64 to get it into long mode.

weird, gnu grub is in real-mode? i thought it is protected-mode.
lilo is ofcourse real-mode, it depend on bin86.

bluecode · Post by **bluecode** » Wed Feb 13, 2008 7:41 am

binutils wrote:weird, gnu grub is in real-mode? i thought it is protected-mode.

grub does not just support multiboot.

xyzzy · Post by **xyzzy** » Wed Feb 13, 2008 7:44 am

Linux has always been started in real-mode on i386 and x86_64. AFAIK Grub does run in 32-bit pmode, but it definitely drops back to 16-bit real mode when loading a Linux kernel