Assembly - SCAS Instruction

The SCAS instruction is used for searching a particular character or set of characters in a string. The data item to be searched should be in AL (for SCASB), AX (for SCASW) or EAX (for SCASD) registers. The string to be searched should be in memory and pointed by the ES:DI (or EDI) register.

Look at the following program to understand the concept −

section .text
   global _start        ;must be declared for using gcc
 
_start:                 ;tell linker entry point

   mov ecx,len
   mov edi,my_string
   mov al , 'e'
   cld
   repne scasb
   je found ; when found
   ; If not not then the following code
 
   mov eax,4
   mov ebx,1
   mov ecx,msg_notfound
   mov edx,len_notfound
   int 80h
   jmp exit
 
found:
   mov eax,4
   mov ebx,1
   mov ecx,msg_found
   mov edx,len_found
   int 80h
 
exit:
   mov eax,1
   mov ebx,0
   int 80h
 
section .data
my_string db 'hello world', 0
len equ $-my_string  

msg_found db 'found!', 0xa
len_found equ $-msg_found

msg_notfound db 'not found!'
len_notfound equ $-msg_notfound   

When the above code is compiled and executed, it produces the following result −

found!