Assembly - MOVS Instruction

The MOVS instruction is used to copy a data item (byte, word or doubleword) from the source string to the destination string. The source string is pointed by DS:SI and the destination string is pointed by ES:DI.

The following example explains the concept −

section .text
   global _start        ;must be declared for using gcc
 
_start:                 ;tell linker entry point
   mov ecx, len
   mov esi, s1
   mov edi, s2
   cld
   rep movsb
 
   mov edx,20         ;message length
   mov ecx,s2         ;message to write
   mov ebx,1         ;file descriptor (stdout)
   mov eax,4         ;system call number (sys_write)
   int 0x80         ;call kernel
 
   mov eax,1         ;system call number (sys_exit)
   int 0x80         ;call kernel
 
section .data
s1 db 'Hello, world!',0 ;string 1
len equ $-s1

section  .bss
s2 resb 20              ;destination

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

Hello, world!