Page 1 of 1
Error entering protected mode
Posted: Fri Oct 17, 2008 2:11 pm
by posman
Hi.
I've just modified my boot loader but now it doesn't enter protected mode. Debugging (with bochs), when the line "move cr0, eax" is executed, the next code that is executed is at address 0x0000
Maybe LGDT has an invalidad value? Does DS has to have a specified value? At the moment when I load GDT, DS = 0x8000
These are images of debug
Before loading GDT
After
This is the part of code for the gdt:
Code: Select all
cli ; Inhabilitar interrupciones
lgdt [gdt_desc] ; leer GDT
mov eax, cr0
or eax, 1
mov cr0, eax
jmp 0x08:limpiarRegistros
...
use32
...
gdt:
gdt_null:
dd 0
dd 0
gdt_code: ; segmento de codigo
dw 0x0FFFF
dw 0
db 0
db 10011010b
db 11001111b
db 0
gdt_data: ; segmento de datos
dw 0x0FFFF
dw 0
db 0
db 10010010b
db 11001111b
db 0
gdt_end: ; para calcular el tamano de la GDT
gdt_desc:
dw gdt_end - gdt - 1 ; limite
dd gdt ; direccion de la GDT
Re: Error entering protected mode
Posted: Fri Oct 17, 2008 2:58 pm
by cr2
try adding this inside the function that enters protected mode(no "JMP 0x08:limpiarRegistros")
Code: Select all
mov ax, 0x10
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
jmp 0x08:setCodeSeg
setCodeSeg:
P.S. Translate your code comments into english next time.
Re: Error entering protected mode
Posted: Fri Oct 17, 2008 3:16 pm
by kmtdk
well
if you send the compilede data, i could take a greater look at it ..
KMT dk
Re: Error entering protected mode
Posted: Fri Oct 17, 2008 10:22 pm
by egos
Your descriptors contain zero base, but you are using displacements relatively the address 0x80000.
Try this:
Code: Select all
gdt:
dq 0
desc 0x80000, (0x100000000-0x80000)/0x1000-1, DF_CODE or DF_DUALACTION or DF_USE32 or DF_PAGED
desc 0x80000, (0x100000000-0x80000)/0x1000-1, DF_DATA or DF_DUALACTION or DF_USE32 or DF_PAGED
Or this:
Code: Select all
...
jmp pword 8:startup32
use32
org 0x80000+$
startup32:
...
Re: Error entering protected mode
Posted: Sat Oct 18, 2008 12:44 am
by egos
Code: Select all
mov ax, 0x08
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
jmp 0x08:setCodeSeg
setCodeSeg:
, cs must be initialized firstly!
This right:
Code: Select all
jmp 0x08:setCodeSeg
...
setCodeSeg:
mov eax, 0x10
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
Re: Error entering protected mode
Posted: Sat Oct 18, 2008 3:41 am
by pcmattman
cr2, cs must be initialized firstly!
The segment registers are preserved over a far jump, so it should work either way. The value of 0x8 for DS, ES, FS and GS is incorrect, though.
EDIT: Also, that far jump is only to load the new CS (because you can't do mov cs,ax) - protected mode is technically already active at this point.
Re: Error entering protected mode
Posted: Sat Oct 18, 2008 4:35 am
by egos
The selector 8 is correct for data segment registers if the code segment (that it selects) is readable. Loading of data segment registers depends on CS.CPL, therefore to initialize cs firstly is more right.
Re: Error entering protected mode
Posted: Sat Oct 18, 2008 5:15 am
by Combuster
...and CS.CPL is zero in real mode so it doesn't matter anyway
Re: Error entering protected mode
Posted: Sat Oct 18, 2008 8:56 am
by cr2
Sorry for the 0x08(I promise I will not post on OSDEV.ORG when I should be sleeping).
Re: Error entering protected mode
Posted: Sat Oct 18, 2008 9:01 am
by cr2
Can we see limpiarRegistros?