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Posted: **Tue Jun 03, 2008 8:09 am**

Hi, I was reading the articles at http://www.jamesmolloy.co.uk/tutorial_h ... 20IDT.html.

And I saw a piece of code like this:

Code: Select all

[GLOBAL gdt_flush]    ; Allows the C code to call gdt_flush().

gdt_flush:
   mov eax, [esp+4]  ; Get the pointer to the GDT, passed as a parameter.
   lgdt [eax]        ; Load the new GDT pointer

   mov ax, 0x10      ; 0x10 is the offset in the GDT to our data segment
   mov ds, ax        ; Load all data segment selectors
   mov es, ax
   mov fs, ax
   mov gs, ax
   mov ss, ax
   jmp 0x08:.flush   ; 0x08 is the offset to our code segment: Far jump!
.flush:
   ret

I can not understand two lines:
1) mov eax, [esp+4]. I usually use ebp register to access a parameter in a function. But there use esp register to do this job. I can not imagine what's the current stack like at this time.

2)jmp 0x08:.flush. I do not know where the instruction will jmp to.

Can any one give me some tips?Thanks.

Posted: **Tue Jun 03, 2008 8:21 am**

Hi,

1) If you read the second article, "2. Genesis", you'd know that in _cdecl when a function is called it will find its return address at the stack pointer, and its parameters immediately above the stack pointer on the stack.

so mov eax, [esp+4] moves the second item from the stack into EAX, which is of course the function's first parameter.

Note that you can only use EBP once you've set up ebp - i.e. you need to make yourself a stack frame. This isn't needed in this function, as it's a leaf function.

2) it jumps to the label ".flush", which is declared directly below the jmp statement...

This was all explained in the tutorial text, by the way.

Cheers,

James

OSDev.org

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