dp instruction

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FlashBurn

Joined: 06 Jan 2005
Posts: 87

FlashBurn 08 Jan 2005, 11:35

There is something false with this instruction. It saves the values in the wrong order! It seems that it first saves the dword and then the word, but it has to be the other way around!

08 Jan 2005, 11:35

MCD

Joined: 21 Aug 2004
Posts: 602
Location: Germany

MCD 11 Jan 2005, 10:23

Take a look at Fasm documentation. both the dd and dp instructions in Fasm allow defining data values with a colon, but the data will be stored in a different order as with other assemblers. This is also true for short strings used as numerical constants, like

Code:

mov eax, 'ABCD'

becomes

Code:

mov eax, 44434241h

11 Jan 2005, 10:23

FlashBurn

Joined: 06 Jan 2005
Posts: 87

FlashBurn 11 Jan 2005, 13:38

The problem is that I have to use the dp instruction when I want to declare a variable which I then want to load with lidt and lgdt. But I have to use mov instructions to give the variable the right values, but this is overhead I donÂ´t want. Also what other values use the dp instruction? I think this are the only and then there comes first the word and then the dword and not the orher way around! It would be a shame if I have to change the assembler because of this!

11 Jan 2005, 13:38

beppe85

Joined: 23 Oct 2004
Posts: 181

beppe85 11 Jan 2005, 14:16

To quote "Programmer's Manual":

Quote:

The dp directive and its synonym df accept the values consisting of two numerical expressions separated with colon, the first value will become the high word and the second value will become the low double word of the far pointer value.

There's nothing wrong with this directive, it follows the little-endiam rule.

You can define a macro:

Code:

macro descriptor adword, aword
{
  dd adword
  dw aword
}

11 Jan 2005, 14:16

FlashBurn

Joined: 06 Jan 2005
Posts: 87

FlashBurn 11 Jan 2005, 14:26

OK, I solved the problem. IÂ´ve written this:

Code:

GDTR dw limit
         dd base

And this code doesnÂ´t work with fasm. But If I write it like this:

Code:

GDTR:
        dw limit
        dd base

It will work.

@beppe85

Your macro is also wrong for me, because I need the word before the doubleword!

11 Jan 2005, 14:26

beppe85

Joined: 23 Oct 2004
Posts: 181

beppe85 11 Jan 2005, 14:31

Yup, I missed that. Just invert the dd/dw.

11 Jan 2005, 14:31

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 12 Jan 2005, 00:10

Code:

label GDTR pword ;maybe it's other size operator, i am not sure
 dw limit 
 dd base

12 Jan 2005, 00:10

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