Boot code doesn't work on real computer

[JW]

Hello,

I've just started developing my own OS. So far I've written a boot code and a small kernel. The OS works on both Bochs and VMWare. But when I try to run it on a real computer, it seems to crash. I figured out on what line it was crashing (you can see it in the attachment). It's when I move the kernel from 0x7E00 to 0x100000 (1MB). I have no idea why it's crashing. Please help.

If the attachment doesn't work, use this link: http://jwfiles.europe.webmatrixhosting.net/os/boot.asm

Thanks in advance.

ps. I added my attachment, but there already was a boot.asm on the forum which isn't mine. So use the link.

[Adek336]

your memory moving code does not set the direction flag. Depending on its value when you get to "rep movsb", the memory copied will be in range ESI to ESI+ECX or ESI-ECX to ESI. Try inserting one of these instructions: cld std. Not sure which will suffice.

Anyways, I'm impressed with your code, it's just perfectly commented! Did you write it all yourself?

Cheers,
Adrian

[Pype.Clicker]

try replacing 'jmp boot' with 'jmp 0000:boot'. Your real hardware may be jumping 0x7c0:0000 instead of 0x0000:0x7c00 ...

[bubach]

isn?t 0x7c0:0000 and 0x0000:0x7c00 the same physical address?
Should it really make a diffrent?

/ Christoffer

[JW]

Thanks for your replies.

I will try to set cs to zero and the direction flag.

Anyways, I'm impressed with your code, it's just perfectly commented! Did you write it all yourself?

Yes I wrote it myself. Thanks ;D (but I think it could be commented better, especially the A20 part)

[JW]

It's working....

I had to set the ecx register instead of the cx register in the code where I move the kernel from 0x7E00 to 0x100000 ( maybe I had to set cs to zero too ).

[Pype.Clicker]

isn?t 0x7c0:0000 and 0x0000:0x7c00 the same physical address?
Should it really make a diffrent?

/ Christoffer

the difference *does* matters.
You have assembled stuff so that the code may have

Code: Select all

jmp 0x7D40

, which is a near jump (absolute address within the same segment). If CS == 0, you'll be jumping at 0x0000:7D40 which is in your bootsector. Fine. Everyone's happy ... But if CS == 7C0, you'll be jumping at 0x07C0:7D40, which should be somewhere at 0x00F940 or something ... out of your bootsector, without a doubt.

Things will even become more ugly if you have something like

Code: Select all

mov ax,cs
mov ds,ax

In which case your data will not be where you expect them to be ... which is harmless in most cases (as the ds:<reg> will still be okay), but which will have sensibly bad effect when trying to give the absolute address of the GDT to the processor, for instance ...