Assembly - The 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: