msdn inline asm mistake?

[thomas_remkus]

I am trying to use the code from the MSDN and it's telling me that I have an operand error. Can someone review this?

int main (void)
{
   char format[] = "%s %s\r\n";
   char hello[] = "hello";
   char world[] = "world";

   __asm
   {
      mov eax, offset world
      push eax
      mov eax, offset hello
      push eax
      mov eax, offset format
      push eax
      call printf
      pop ebx
      pop ebx
      pop ebx     
   }

   return 0;
}

[Infro_X]

local variables must be "moved" into eax with lea (load effective address"

lea eax,[world]

mov eax,offset world ,,,isn't possible because that is mov a DWORD (immediate) into eax.
why, because world is created on the stack (ss:esp points to the stack, the prodecdure creates the stack frame (push ebp; mov ebp,esp; sub esp,(localvariables size)
lea eax,[ebp-xxx] is the same as , mov eax,ebp; sub eax,xxx (for local variables created using a stack frame)


so simply put,
lea eax,[localvars] is how you load the address of a local variable
mov eax,offset gobalvar is how you load the address of a global varible

lea eax,[1] is the same as mov eax,1
lea supports displacement+index*(2^x from 0 to ??)+base

some tips for using lea intresstingly
to times anything by 3,5,9,17,?? you can use lea
lea eax,[eax+eax*2]
lea eax,[eax+eax*4]
lea eax,[eax+eax*8]
lea eax,[eax+eax*16]
as well as use lea to do some simple mathmatics
lea eax,[eax*2+5]
lea eax,[e??*??+???] :)

[thomas_remkus]

Thanks! That worked perfectly. Of course, if I put the C variables as global I can still use the "lea" operator.

[thomas_remkus]

Now that I'm back to tinkering with this. I want to understand what it does. I think that I have most of it in my head except for the "pop ebx". Why do I need to do that ... and why three times??

To me, the "load effective address" is putting in a 32 bit register the address of my world variable. Then I push that to the stack because that's how printf will read the parameters (backwards). After the call to printf I don't understand the pop commands.

[thomas_remkus]

Now, let me try this ... is it because I pushed three times and now I need to pop three times? Why didn't printf pop the variables? And why not pop to eax?

[Eóin]

Why didn't printf pop the variables?

Because printf use C calling convention and C call functions don't clean up the stack so the calling code has to. The reason why printf is C call is that it can accept a variable number of arguements hence only the calling code knows how many there were and therefore it knows how many it needs to clean up after.

And why not pop to eax?

eax is left holding the return value of printf, pop into it would lose that return.

[asmfan]

just try
add   esp, 12

[thomas_remkus]

I understand your "add esp, 12". it makes sense I think. it's because there are several things are passed on the stack. I don't know exactly what but I think that a return address is one of them or something.

Is the "add esp, 12" faster than three pops? Each pop I think is 1 cycle, so that's 3. The "add" to a register from a constant is 2 cycles?

These just may be stupid questions but I really find your methods interesting. Are they normally used? What happens to the 12 bytes that were no popped? Vaporized? Lost? 4 8 15 16 23 42?

And, what else is on the stack that my application knew to crash because I did not pop three times??????

[Mark Jones]

Hey Thomas, lots of good info in these two threads:
http://www.masmforum.com/simple/index.php?topic=4365.0
http://www.masmforum.com/simple/index.php?topic=4427.0