Using __asm

I have a problem. My ASM code works fine when included as part of main, but when I make a function that takes a reference it doesn't work. What I mean is:

Code:

---

// The following function generates no errors,
// but when called does not effect the variable passed.
// If the __asm lines are put in the main program, however
// and used with integer num, it works.
// How do I use assembly lines to access variables passed
// to functions? =|
void zapit (int &num) {
    __asm mov ecx, [num]
    __asm xor  ecx, ecx
    __asm mov [num], ecx
}

---

¬¬ ;

i think your problem is here:
void zapit(int &num) ....

you feed the reference into the function when its called to let it know where the variable is. you need to have a pointer to accept the reference. in other words:
void zapit(int *num) ....
i know almost nothing about inline asm

You could just do -

Code:

---

void zapit (int& num) {
       
                __asm mov eax,num               
                __asm mov [eax],0

   
}

---

> __asm xor ecx, ecx

I don't know any inline assembly, but isn't this just xoring the number with itself? (just making it 0)

>I don't know any inline assembly, but isn't this just xoring the number with itself? (just making it 0)

Yes, it's a quicker instruction than mov so is often used when setting a register to zero , but in the above function it's not the register that needs setting to zero so I don't think it would be of any use.

That's interesting - thanks Sorensen...

And I'm with ygf - why would you have the function prototype the way you do, and not the way he does?

IOW, why this:
void foo(int& bar)

instead of this:
void foo(int *bar)

It's just a reference instead of a pointer, there's not really any difference in this example (except how the variable is passed into the function). Both would do exactly the same thing, using the reference cleans up the syntax a little. Although it may not be immeadiately clear when you are passing a parameter into a function by reference instead of value using C++ references.

I think you are changing the address instead of the value. Try something similar to this:

Code:

---

// The following function generates no errors,
// but when called does not effect the variable passed.
// If the __asm lines are put in the main program, however
// and used with integer num, it works.
// How do I use assembly lines to access variables passed
// to functions? =|
void zapit (int &num) {
    __asm mov di, num
    __asm mov ecx, [di]
    __asm xor  ecx, ecx
    __asm mov [di], ecx
}

---

I'm not sure if I have the correct register names. I'm using an old compiler. Here is what I used:

Code:

---

void zapit (int &num) {
    __asm mov di, num
    __asm mov ax, [di]
    __asm xor ax, ax
    __asm mov [di], ax
}

---

Surprising that people don't know what references are o_O;

Anyway... um, I don't really care what it does, I'm trying to get communication with the variable I passed. This asm code works, but not in functions... and I want to know how to get it to work hehe...

None of the suggestions have worked so far... I think ASM doesn't understand references or something, but it creates an error when I try to use pointers

What compiler are you using?

Quote:

---

Originally posted by vVv
>Surprising that people don't know what references are o_O;

Score: -1, Troll.

#include <iostream.h>
int buf;
void blah( int& ref )
{
buf = ref;
__asm__( "
movl buf,%eax;
xorl %eax,%eax;
movl %eax,buf;
");
ref = buf;
}

main()
{
int var =20;
blah( var );
cout << var << endl;
}

---

That's not really answering the question. You're using C/C++, to assign the value to the reference.


>I think ASM doesn't understand references or something

ASM understands references as they are just the same as pointers underneath the syntax. However, the mnemonics used by C/C++ compilers for inline assembly varies from compiler to compiler. This definitely works on MSVC for all values of num -

Code:

---

void zapit (int& num) {
       
                __asm mov eax,[num]               
                __asm mov DWORD PTR[eax],0
 
}

---

...or using a similar method to your original -

Code:

---

void zapit (int& num) {
       
            __asm mov eax,[num]       
        __asm xor ecx,ecx
        __asm mov [eax],ecx
 
}

---