x86 assembly occurrence of a character

Introduction

The small assembly exercise below is for (Intel and AMD 32-bit) x86 architectures and uses the NASM syntax , an assembler, available for free and

that can be used on different platforms such as Windows or Linux.

Note that external functions used comes from the standard C library.

Issue

Imagine an array of characters (which does not necessarily end with 0). It has its size and we would like to test the presence of a given character

in this array. The aim will be to write a function that takes as input an array of characters, size and character. If this character is present in

the array, it returns a nonzero value, otherwise it returns zero.

Here are what give this function in C:

/the function
int is_in_array (char *array , int size, char c); 
//implementation:
char tab[] = {'n', 'e', 'u', 'e'};
is_in_array (tab, sizeof(tab), 'u'); //Return a value other than 0
is_in_array (tab, sizeof(tab), 'a'); //Return value 0


Simply insert your code in: 
extern printf

section .data
	array  db 'dadedidadedavivoufufifamasibifisaz'
	yes db 'oui', 10, 0
	no db 'non', 10, 0

section .text
	global main

is_in_array:
	;Insert your code there


main:
	push ebp
	mov ebp, esp
	
       ;Test if m is in the array 
	push dword 'm'
	;Lenght of array  (here 34) 
	push dword 34 
	;string address in eax
	push array

        ;Call is_in_array  with adress of array, 
        ;the size, and the value you are searching 
	call is_in_array  
	test eax, eax
	jnz is_there ;If eax != 0 display yes
	push no ;Then display no
	jmp screendisplay
        ;screendisplay of the string with printf
   is_there
	push yes
   screendisplay:
	call printf

	mov eax, 0
	leave
	ret

Certainly this will not be enough .....

Solution

is_in_array:
	;Retrieves the address of the array  (first parameter) in edi
	mov edi, [esp + 4] 
	;Retrieves the size of the array (second parameter) in  ecx
	mov ecx, [esp + 8]
	;Retrieves the character to find (third parameter) in eax
	mov eax, [esp + 12]

	;Search the character
	repne scasb
	;If flag ZERO (ZF) has a value of 1 it means character is found
	;In the other case it is not found
	;Simply add the value of ZF in eax
	mov eax, 0
	;If ZF = 1 then al = 1 (al being the 8 least significant bits of eax)
	setz al 

	ret

Explanation

ZF = 0
ecx = length
eax = character
edi = array 
//The Loop that defines "repne scasb"
While ecx != 0 ET ZF = 0 Do
    if al == [edi] Then
        ZF = 1
    FinSi
    ecx = ecx - 1
    edi = edi + 1
EndWhile

eax = 0
//The condition that defines "setz"
Si ZF = 1 Alors
    eax = 1
EndIf