Assembly - The MOVS Instruction

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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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