I know, there are lots of topics similiar to this. I have tried out a lot of the solutions I found on google, and this forum. And nothing really helped. So I try my luck here.
My problem is the following:
I am just starting to develop a bootloader. I use this tutorial as a learning source:
https://github.com/cfenollosa/os-tutorialI am at the point where I try to get into protected mode. The thing is, that the code I wrote, (mostly the same as the tutorials code) is working in qemu, but not if I write and boot it from a USB stick on my Notebook. I have already tried different settings in the GDT, for 64-Bit mode and so on. In the moment where I try to boot it from a USB-Stick it just restarts, but it doesn't in qemu, so I don't really have a chance to debug it in a development environment. Here is my code:
main.asm
Code:
[bits 16]
[org 0x7c00]
start:
call switch_to_pm
jmp $
%include "./include/gdt.asm"
%include "./include/32bit-print.asm"
%include "./include/32bit-switch.asm"
[bits 32]
BEGIN_PM:
mov ebx, MSG_PROT_MODE
call print_pm
cli
hlt
MSG_PROT_MODE db 'Started prot mode', 0
BOOTSTRING_START db 'Started bootloader', 0
times 510 - ($ - $$) db 0
dw 0xaa55
gdt.asm
Code:
[bits 16]
gdt:
dd 0x00
dd 0x00
gdt_code:
dw 0xffff
dw 0x0000
db 0x00
db 10011010b
db 11001111b
db 0x00
gdt_data:
dw 0xffff
dw 0x0000
db 0x00
db 10010010b
db 11001111b
db 0x00
gdt_end:
gdt_descriptor:
dw gdt_end - gdt - 1
dd gdt
CODE_SEG equ gdt_code - gdt
DATA_SEG equ gdt_data - gdt
32bit-print.asm
Code:
[bits 32]
VIDEO_MEMORY equ 0xb8000
WHITE_ON_BLACK equ 0x0f
print_pm:
pusha
mov edx, VIDEO_MEMORY
.loop:
mov al, [ebx]
mov ah, WHITE_ON_BLACK
cmp al, 0
je .done
mov [edx], ax
add ebx, 1
add edx, 2
jmp .loop
.done:
popa
ret
32bit-print.asm
Code:
[bits 16]
switch_to_pm:
cli
lgdt [gdt_descriptor]
mov eax, cr0
or eax, 0x1
mov cr0, eax
jmp CODE_SEG:init_pm
[bits 32]
init_pm:
mov ax, DATA_SEG
mov ds, ax
mov ss, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ebp, 0x90000
mov esp, ebp
call BEGIN_PM
I do assemble it on a Linux Machine with the following commands (and running in qemu):
Code:
nasm -f bin -o bootloader.bin bootloader.asm
qemu-system-x86-64 bootloader.bin
Writing to the USB-Stick:
Code:
sudo dd if=bootloader.bin of=/dev/sdb bs=512 count=1
I should also mention, that I've got to work everything in before of the tutorial, including reading sectors from the disk/USB-Stick. So it shouldn't be a problem with Floppy/HDD-Emulation.
I am pretty thankful for any help. I would love to go on, but doesnt want to advance in things before I do understand everything before.
Greetings
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