Getting PA of a structure/inside a class

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Insightsoft
Posts: 63
Joined: Wed Jul 22, 2009 6:44 am

Getting PA of a structure/inside a class

Post by Insightsoft » Tue Oct 26, 2010 6:11 pm

I have implemented the following code with a ASM file inside my project...
but I would like to use your way... it is more easy to keep track...


The Case: I'm building my own OS with C++, based in "classes"/not linear C... however, when I need to some low-level instructions, I have included "in project" assembly files...

for example:

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class definition _GDT
{
    protected struct record for IDTR
    member functions
    Install();
}


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_GDT::Install()
{
    call my low-level asm code to perform the lidt...
}


How do I use this beauty code?

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_asm lidt [_idtr]



The problem is that _idtr, in my case, is inside class capsule...

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_asm lidt [this->_idtr]

does not work

Is there any way to take the physical address of that structure that reside inside the class?
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Andyhhp
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Re: Getting PA of a structure/inside a class

Post by Andyhhp » Wed Oct 27, 2010 6:03 pm

Which build environment are you using? MSVC based or GCC based?

There is no standard way to do this, but there are ways to get it working in both.

~Andrew
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Insightsoft
Posts: 63
Joined: Wed Jul 22, 2009 6:44 am

Re: Getting PA of a structure/inside a class

Post by Insightsoft » Wed Oct 27, 2010 11:26 pm

Visual C++ under Visual Studio

I solved the problem using this:

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      _asm   push eax
      _asm   mov   eax, dword ptr [this]              ;the pa of the class
      _asm   lgdt [eax]                                         ;because the struct is the first declaration inside the class
      _asm   pop eax


if the struct is not the first declaration we will need to add eax the correct displacement

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      _asm   push eax
      _asm   mov   eax, dword ptr [this]              ;the pa of the class
      _asm   add eax,  ????                              ;where is the location of the struct inside the class
      _asm   lgdt [eax]                                       
      _asm   pop eax



Its working....
But is there any other way?
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Andyhhp
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Re: Getting PA of a structure/inside a class

Post by Andyhhp » Wed Oct 27, 2010 11:53 pm

Unfortunatly that is not safe. It assumes that no extra fields are inserted at the top of the structure, which is not the case if you are doing debugging, or if you are using virtual base classes.

Anyway, the proper way to reference fields in data structure is as so:

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_asm {
  lea eax,<object_name> ; Load Effective Address
  mov ecx, [eax]<object_name>.<field_name>
}

This will move the field value into ecx.

For reference, see http://msdn.microsoft.com/en-us/library/fabdxz08.aspx

Anyway, for your specific problem, you have to ensure that your GDT is a packed structure.

then your code can be as simple as:

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lea eax,<GDT object name>
lgdt [eax]


It is safe to assume that eax is fine - all other registers are not safe to assume anything about. (Typically, ecx is the 'this' pointer)

~Andrew
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Insightsoft
Posts: 63
Joined: Wed Jul 22, 2009 6:44 am

Re: Getting PA of a structure/inside a class

Post by Insightsoft » Thu Oct 28, 2010 2:25 am

Thanks...

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      __asm
      {
         mov      eax, [this]_IDT._idtr
         lidt   [eax]
      }



Thanks anyway...
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