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Assembly – MOVS Instruction

Assembly MOVS instruction

In this guide, we will discuss MOVS Instruction in Assembly Programming Language. 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!

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