• Mike (3/3) Dec 02 2007 From COM to assembler ... I've got a lot of questions today ... need ass...
• Jarrett Billingsley (12/19) Dec 02 2007 Right. EAX gets you the value in EAX, [EAX] gets you the value in memor...
• Mike (13/42) Dec 02 2007 Thanks to you both. I did that:
• novice2 (22/24) Dec 02 2007 lea register, variable

Mike <vertex gmx.at> writes:

From COM to assembler ... I've got a lot of questions today ... need assembler
but my knowledge is somehow limited. That means I've got a lot of nice access
violations :)

Anyway: what's the difference between EAX and [EAX]? I suspect it's value vs.
pointer dereferencing, but I'm not sure. And: how do you get the address of a
variable in assembler anyway? I tried "&var", but that doesn't compile
(obvisously), "[var]" crashes.

-Mike

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"Mike" <vertex gmx.at> wrote in message news:fiut5k$8c8$1 digitalmars.com...
 From COM to assembler ... I've got a lot of questions today ... need 
 assembler but my knowledge is somehow limited. That means I've got a lot 
 of nice access violations :)

 Anyway: what's the difference between EAX and [EAX]? I suspect it's value 
 vs. pointer dereferencing, but I'm not sure.

Right.  EAX gets you the value in EAX, [EAX] gets you the value in memory at 
the address held in EAX.

And: how do you get the address of a variable in assembler anyway? I tried 
"&var", but that doesn't compile (obvisously), "[var]" crashes.

Use lea with just the name of the var:

int x = 5;

asm
{
    lea EAX, x;
    mov [EAX], 3; // save 3 to the memory location in EAX
}

writefln(x);

This prints 3. 

Mike <vertex gmx.at> writes:

Thanks to you both. I did that:

long var;
long *pvar = &var;

asm
{
mov EAX, pvar
push EAX
}

for now to find out if at least the concept works :)

So ... the code pushes the pointer to var on the stack and later calls a
function which fills var with the (correct) value. Works. Interestingly "mov
EAX, [pvar]" does the same thing, although it shouldn't ... it should - as I
understand it - push the content of var (which is 0) to the stack, the function
should then try to write to address 0 and fail with an access violation. Or
shouldn't it? Does [] only dereference registers, not values?

Jarrett: go work on MiniD, you're just answering my questions all day long,
you've got better things to do :)

-Mike

Jarrett Billingsley Wrote:

 "Mike" <vertex gmx.at> wrote in message news:fiut5k$8c8$1 digitalmars.com...
 From COM to assembler ... I've got a lot of questions today ... need 
 assembler but my knowledge is somehow limited. That means I've got a lot 
 of nice access violations :)

 Anyway: what's the difference between EAX and [EAX]? I suspect it's value 
 vs. pointer dereferencing, but I'm not sure.

 
 Right.  EAX gets you the value in EAX, [EAX] gets you the value in memory at 
 the address held in EAX.
 
And: how do you get the address of a variable in assembler anyway? I tried 
"&var", but that doesn't compile (obvisously), "[var]" crashes.

 
 Use lea with just the name of the var:
 
 int x = 5;
 
 asm
 {
     lea EAX, x;
     mov [EAX], 3; // save 3 to the memory location in EAX
 }
 
 writefln(x);
 
 This prints 3. 
 
 

novice2 <sorry noem.ail> writes:

 Anyway: what's the difference between EAX and [EAX]?

EAX="opearnd is EAX register itself"
[EAX]="operand is memory pointed by address in EAX"

And: how do you get the address of a variable in assembler anyway?

lea register, variable

small eample:

#void main()