OSDev.org

Posted: **Tue Sep 08, 2020 7:51 pm**

I modified the page table setup code a bit to make it identity map the kernel. However, it crashes instantly when I try to set the paging bit in cr0. Here's my code:

Code: Select all

paging:
    mov esi,0
    mov ecx,1024
.1:
    cmp esi,_kernel_start
    jl .2
    cmp esi,_kernel_end
    jge .3
    mov edx,esi
    or edx,3
    mov [edi],edx
.2:
    add esi,4096
    add edi,4
    loop .1
.3:
    mov dword [boot_page_directory],(boot_page_table1+3)
    mov dword [boot_page_directory+(768*4)],(boot_page_table1+0x3)
    mov ecx,boot_page_directory
    mov cr3,ecx
    mov ecx,cr0
    or ecx,0x80010000
    mov cr0,ecx 
    jmp $ ;cr2 pointed to this. replaced with hlt, still the same.

and the register contents are:
cr0=0xe0010011
cr2=0x001010d1
cr3=0x105000
cr4=0
Also, how do I find the corresponding code in my assembly source from the cr2 register?

Posted: **Tue Sep 08, 2020 8:21 pm**

Do "_kernel_start" and "_kernel_end" span a range of addresses that include the MOV to CR0 and the subsequent JMP that are used to enable paging?

I believe objdump will show you addresses that you can match to CR2, but in this case it's safe to say the fault is happening at the JMP instruction.

Posted: **Tue Sep 08, 2020 8:44 pm**

_kernel_start and _kernel_end includes the .text section. Also, I replaced the jmp with hlt, and cr2 still points to 0x1010d1.

Posted: **Tue Sep 08, 2020 8:59 pm**

Is the code you posted in the .text section?

I'd expect the HLT instruction is located at the same address as the JMP instruction you replaced, so CR2 won't change.

Posted: **Tue Sep 08, 2020 9:00 pm**

Yes, the code is in the .text section.
edit: problem solved, I used the code from "Setting up paging" instead.

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