Problems with PMode

[Vladaz]

Hello. I have PMode problem again. I started new brand OS project and something don't work. I attached my source code of my bootloader. When i emulate it with bochs, it writes me an error:
Bochs is exiting with the following message:
[CPU ] jump_protected: cs == 0

Can you help me? Can you tell me what i need to correct in my code?

[Vladaz]

boot.asm

[Curufir]

First you switch to Pmode

Code: Select all

   mov eax, cr0
   or eax, 1
   mov cr0, eax
      
   jmp code_selector:flush_pipe

Then you immediately try to load CS with the NULL selector.

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   jmp 0:0x8000

You have something totally confused here, I'm not surprised Bochs doesn't like it.

[Vladaz]

Now i written my simple kernel that just shoud write a letter on the screen, but it does not do that. It just reboots.
There is my boot and kernel:
http://avnet.sytes.net/~vladas/boot.asm
http://avnet.sytes.net/~vladas/kernel.asm
can someone help me, please? ???

[oswizard]

In boot.asm:

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SectorLoader:      
    xor bx, bx
    mov es, bx
    mov bx, 0x8000

and

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flush_pipe: 
    jmp code_selector:0x80000

In the first, you are loading the kernel to 0000:8000 (0x00008000) and in pmode you are jumping to 0x000800000. Big difference!

Overall it looks pretty good. Everything else seems fine.

Good luck

[Vladaz]

It still reboots, even when I changed it :-[

[oswizard]

In boot.asm:

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gdtr:
   dw gdt_end-gdt
   dd gdt

For the GDT, the limit must be gdt_end-gdt-1. It doesn't make too much sense, except that this allows a GDT with length 0xFFFF, really meaning 0x10000, but small enough to fit within a 16-bit word. I don't know if this would be enough to cause a reboot, though.

Good Luck

[oswizard]

From the NASM manual:

For example, the following code will generate the longword 0x00000104:


org 0x100
dd label
label:

Here the assembler will decide that the label "gdtr" is equal to ORG + offset, or 0x7C??. Because you set DS to 0 the instruction "lgdt [gdtr]" will try to load from DS base + offset, or 0x00?? (which is definately not where it should be).

Umm... if "gdtr" is equal to ORG+offset, wouldn't "lgdt [gdtr]" load from DS:ORG+offset, or 0000:7C00+offset? The bootloader I wrote has "org 0" with a CS and DS of 0x07C0, so a bootloader with CS and DS equal to 0x0000, the ORG would have to be 0x7C00.

[Brendan]

Hi,

Umm... if "gdtr" is equal to ORG+offset, wouldn't "lgdt [gdtr]" load from DS:ORG+offset, or 0000:7C00+offset? The bootloader I wrote has "org 0" with a CS and DS of 0x07C0, so a bootloader with CS and DS equal to 0x0000, the ORG would have to be 0x7C00.

Ooops - you're right. I made the mistake that I thought Vladaz was making, and messed everything up. There was no problem with the boot code, and my examples are wrong too :'(.

I should be more careful - thanks for noticing my mistake. I'll edit my last post so no-one tries to use it..


Cheers,

Brendan