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Posted: **Sat Mar 21, 2009 12:31 pm**

hello,

i have a kernel in protected mode and want to set up a gdt, so i read the gdt tutorial and tried to put the code pieces i was given there together. Until now, it didn't work. Since i only know at&t syntax assembly language, and the examples of the tutorial are in intel, i had to convert them first.
I think the reason for my gdt-load to not work lies in the conversion part of the code, but i don't know where.

These are the assembly pieces i converted:

This

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gdtr DW 0 ; For limit storage
     DD 0 ; For base storage

setGdt:
   MOV   EAX, [esp + 4]
   MOV   [gdtr + 2], EAX
   MOV   AX, [ESP + 8]
   MOV   [gdtr], AX
   LGDT  [gdtr]
   RET

into this

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gdtr:
limit:  .word 0 # For limit storage
base:   .long 0 # For base storage

.globl setGdt
setGdt:
        movl    4(%esp),        %eax
        movl    %eax,           base
        movw    8(%esp),        %ax
        movw    %ax,            gdtr
        lgdt    gdtr
        ret

and this

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reloadSegments:
   ; Reload CS register containing code selector:
   JMP   0x08:reload_CS ; 0x08 points at the new code selector
.reload_CS:
   ; Reload data segment registers:
   MOV   AX, 0x10 ; 0x10 points at the new data selector
   MOV   DS, AX
   MOV   ES, AX
   MOV   FS, AX
   MOV   GS, AX
   MOV   SS, AX
   RET

into this

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.globl reloadSegments
reloadSegments:
        # Reload CS register containing code selector:
        ljmp    $0x08,  $reload_CS # 0x08 points at the new code selector
reload_CS:
        # Reload data segment registers:
        movw    $0x10,          %ax    # 0x10 points at the new data selector
        movw    %ax,            %ds
        movw    %ax,            %es
        movw    %ax,            %fs
        movw    %ax,            %gs
        movw    %ax,            %ss
        ret

Do you have any ideas?

Thanks in advance, mittwoch

Posted: **Sat Mar 21, 2009 5:09 pm**

I don't think you are referencing your variables correctly here:

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        movw    %ax,            gdtr
        lgdt    gdtr

... and here ...

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        movl    %eax,           base

For a start, I know that the lgdt line should be

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lgdt (gdtr)

Posted: **Sat Mar 21, 2009 6:18 pm**

when referencing memory variables, you need to use "(x)". so to move EAX into BASE, you need to do "movl %eax, (base);" or along those lines.

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