Thread: Calling assembly from c

Hello, I have a problem with calling assembly from c.

typedef unsigned char digit; // apn is a pointer to array of digits.

Functions:

add(apn number1,apn number2,apn result)

subtract(apn number1,apn number2,apn result)

multiply(apn number1,apn number2,apn result)

I must call these functions from C and then these funct. must use assembly to add,sub,mul operations. That means i need assembly code for this functions. Numbers are in a char array. Example;

number1="123"//number1 and number2 are digit array.

number2="999"

then when i call add(number1,number2,result) from c

result will be "1122"

May you help me to write assembly code for these functions?(Please remember that numbers are char array) If you give me some links to learn from or explain something about this topic, you will be helped me to write

Originally Posted by Kuluzuva

May you help me to write assembly code for these functions?

You should post this in an ASM forum.

Also, if you've never written ASM before it's time to crack the books and start learning... but be warned, it's a truly nebulous subject.

Actually, I've written Asm before. I'm searching information about this topic from morning but I got nothing... I cant find sites, information, anything. If you write some links to look for me, I'll be happier...

So then what exactly do you have a question about?

Also, if you're working on an x86: Intel x86 Function-call Conventions - Assembly View.

If you are having trouble implementing an assembly in to C, you may wish to state your platform first as the syntax differs. For example you can process the char array and extract numerics out of them (i.e. '3' - '0' = 3) and pass them as a parameter in your assembly, i.e. in GNU syntax

Code:

       asm ( 
             assembly template
           : output operands                  /* optional */
           : input operands                   /* optional */
           : list of clobbered registers      /* optional */
           );

See: GCC-Inline-Assembly-HOWTO
(or MSVC: Inline Assembler)

My compiler is gcc and assembler is nasm. I made my makefile. I need to learn taking arguments from function to assembly code. For example, these functions take 3 arguments and how i will use them in assembly? I wrote something that is:

PHP Code:


 .MODEL SMALL

 .CODE

global _add_num

_add_num PROC

       push    ebp ;Pushing arguments from function
       mov    ebp,esp
       pushad
       mov eax,0
       mov edx,0
       mov esi,[ebp+8]
       mov esp,[ebp+4]
       

       mov edi,esi
       sub ecx,ecx
       not ecx
       sub al,al
       cld
       repne scasb
       neg ecx
       sub ecx,3
       mov ebx,ecx ;ebx has lenght of first argument

       mov edi,esp
       sub ecx,ecx
       not ecx
       sub al,al
       cld
       repne scasb
       neg ecx
       sub ecx,3
       mov edi,[ebp+4]
       mov ebp,ecx ;ebp has lenght of first argument

       sub eax,eax
       sub edx,edx
       mov ecx,2

       L1:
       mov al,[esi+ebp]
       mov dl,[esp+ebx] 
       sub dl,48 ;sub ascii code
       add al,dl 
       mov [edi+ebx],al       ;result is in eax
       dec ebp
       dec ebx
       loop L1

       pop ebp
 ret

_add_num ENDP

 END 


This code is correct for add function:
add(char* number1,char* number2,char* result)

And I'm getting memory for result before calling function.
There is any wrong about this code?