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Hello,

I looked over loads of forum posts and I couldn't find a solution. I would really appreciate some help as I've been trying to run this for 3 full days. Thanks in advance.

I'm trying to follow the Bare Bones tutorial to right my first kernel. What I'm trying to do it to write a kernel that loops forever printing "Hello, World!".
I followed all the steps in the tutorial except for a few changes since it's written for x86 and I'm trying to create a x86_64 kernel. The changes I've done were the following:

change linker.ld lines from:
push eax
push ebx

to:
push rax
push rbx

After this change I can compile and link with:

nasm -f elf64 -o loader.o -s loader.s

x86_64-pc-linux-gcc -masm=intel -o kernel.o -c kernel.c -Wall -Wextra -nostdlib -nostartfiles -nodefaultlibs
Which gives me the following error:
kernel.c: In function 'kmain':
kernel.c:5: warning: unused variable 'boot_loader_name'
kernel.c:3: warning: unused parameter 'magic'

x86_64-pc-linux-ld -T linker.ld -o kernel.bin loader.o kernel.o

I then create the floppy image (tried with stage1 and stage2 from i386 and x86_64) , run qemu and when I type 'kernel 200+5' I get:
Erro 13: Invalid or unsupported executable format

I also trying with target elf64-little and tried using grub2's grub-mkrecovery. The last didn't give me the error but I'm still not sure if it was working.

Extra question: Just let me ask something that it's on my mind. As far as I know stage1 and stage2 are running in real mode. Does it still support the push rax/rbx from loader.s? Aren't them only allowed in long mode?

Here is the output of some commands and my code:

$file kernel.bin
kernel.bin: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped

$file kernel.o
kernel.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

$file loader.o
loader.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

$file floppy.img
floppy.img: x86 boot sector; GRand Unified Bootloader, stage1 version 0x3

$objdump -h kernel.bin
kernel.bin: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 000000bb 0000000000100000 0000000000100000 00100000 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .rodata 00000002 0000000000101000 0000000000101000 00101000 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .eh_frame 00000058 0000000000101008 0000000000101008 00101008 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .bss 00004000 0000000000101060 0000000000101060 00101060 2**2
ALLOC
4 .comment 00000031 0000000000000000 0000000000000000 00101060 2**0
CONTENTS, READONLY

$objdump -h kernel.o
kernel.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 0000009b 0000000000000000 0000000000000000 00000040 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000000 0000000000000000 0000000000000000 000000dc 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0000000000000000 0000000000000000 000000dc 2**2
ALLOC
3 .rodata 00000002 0000000000000000 0000000000000000 000000dc 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 00000012 0000000000000000 0000000000000000 000000de 2**0
CONTENTS, READONLY
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000f0 2**0
CONTENTS, READONLY
6 .eh_frame 00000050 0000000000000000 0000000000000000 000000f0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

$objdump -h loader.o
loader.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 0000001f 0000000000000000 0000000000000000 00000240 2**4
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .bss 00004000 0000000000000000 0000000000000000 00000260 2**2
ALLOC
2 .comment 0000001f 0000000000000000 0000000000000000 00000260 2**0
CONTENTS, READONLY

$mbcheck loader.o
loader.o: The Multiboot header is found at the offset 576.
loader.o: Page alignment is turned on.
loader.o: Memory information is turned on.
loader.o: Address fields is turned off.
loader.o: All checks passed.

$mbcheck (for kernel.o and kernel.bin)
kernel.bin: No Multiboot header.

$mbcheck floppy.img
floppy.img: The Multiboot header is found at the offset 5367.
floppy.img: Bad checksum (0xffdff805).

kernel.c
#include "vga.h"

void kmain(void *mbd, unsigned int magic){

char *boot_loader_name = (char *) mbd + 64;

char str[] = "Hello, World!";

while(1==1) { vga_print(str, GREEN_BLACK); }

}

TheBase wrote:except for a few changes since it's written for x86 and I'm trying to create a x86_64 kernel

And that is your biggest mistake. The barebones expects to be started in 32 bit mode. You can't just plug in 64 bit code and have it work. In fact, the reason that grub complains about your file is because its no longer 32 bit.

Read the article on x86-64, it tells you the steps you need to do to enter long mode. Otherwise, you'd better be off finding a bootloader that can leave you directly in long mode.