How to get pointer values in ASM

[modshah]

Hi

I tried to pass a pointer value to an assembly procedure. But I don't know how to get the value pointed by the pointer.

Code:

C
--
extern void Sum(int,int *);

int main()
{
int a = 10,b = 15;
Sum(a,&b);
printf("%d",b); //I expect the asm proc to make b = b + a using pointers
return 0;
}

ASM
----

_Sum:
   pop ebp
   mov ebp,esp
   mov ax,[ebp+4]  ; Getting my first arg (i.e.) 'a'
   mov bx,[ebp+8]  ; I don't what am I getting - is it the pointer address ?
                   ; or is it the value of *b

   ; And what should I do to get the 15+10 stored in b without returning but by                            ;   using the pointer

   push ebp
   ret


--modshah

[modshah]

Sorry I wrongly typed the asm code. It was a

<push ebp>

and

<pop ebp>

Thanks
modshah

[gaffi]

mov bx,[ebp+8]  ; bx = address of b

mov ax,[bx]     ; ax = value of b

just use the register with the address like a variable's name and you'll get its contents

greetings,
Daniel Raffler

[modshah]

Thanks Daniel. But could you please tell me how to do the following C code in assembly.

void sum(int a,int *b)
{
*b = *b + a;
//That is I want to get the value of b - ie [bx] as you told me
// But is it the same as saying
//
// mov bx, [bp+4]
// add ax, [bx]
//
// mov [bx],ax - Will this work ?? for *b = *b + a
}

Also lets say I want to pass a string or char array. How to handle that in assembly ?

modshah

[Chris Giese]

Any compiler with an option to produce assembly-language output
will _show_ you how to do something in assembly. For GCC and
Borland C, this option is "-S"

C:\TMP>type foo.c
void sum(int a, int *b) { *b = *b + a; }

C:\TMP>gcc -S -O2 -Wall -W foo.c

C:\TMP>type foo.s
        .file   "foo.c"
gcc2_compiled.:
___gnu_compiled_c:
.text
        .p2align 2
.globl _sum
_sum:
        pushl %ebp
        movl %esp,%ebp
        movl 12(%ebp),%edx
        movl (%edx),%eax
        addl 8(%ebp),%eax
        movl %eax,(%edx)
        movl %ebp,%esp
        popl %ebp
        ret

C:\TMP>gcc -S -O2 -Wall -W -fomit-frame-pointer foo.c

C:\TMP>type foo.s
        .file   "foo.c"
gcc2_compiled.:
___gnu_compiled_c:
.text
        .p2align 2
.globl _sum
_sum:
        movl 8(%esp),%edx
        movl (%edx),%eax
        addl 4(%esp),%eax
        movl %eax,(%edx)
        ret

C:\TMP>bcc32 -S -O2 -w foo.c
Borland C++ 5.5.1 for Win32 Copyright (c) 1993, 2000 Borland
foo.c:

C:\TMP>type foo.asm
{snip debug info and TASM32 stuff}
_sum    proc    near
        push      ebp
        mov       ebp,esp
        mov       eax,dword ptr [ebp+8]
        mov       edx,dword ptr [ebp+12]
        add       dword ptr [edx],eax
        pop       ebp
        ret

[modshah]

Thanks chris I figured a way out.

//
// *b = *b + a
// using pointers
//

// C Code
// CallSum.c
#include<stdio.h>

int main()
{
int a=10,b=15;
Sum(a,&b);
printf("%d\n",b);
}

// ASM Code
// Sum.asm

global _Sum

SEGMENT .text
[BITS 32]        
_Sum:
       push ebp
       mov ebp,esp
       mov eax,[ebp+8]  
       mov ebx,[ebp+12]  ;The BYTE PTR didn't work
       add eax,[ebx]     ;some one told me PTR is not supported in NASM
       mov [ebx],eax     ;Is that true ?
       pop ebp
       ret