Assembly - The 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 following result:

Hello, world!