Assembly - The STOS Instruction

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The STOS instruction copies the data item from AL (for bytes - STOSB), AX (for words - STOSW) or EAX (for doublewords - STOSD) to the destination string, pointed to by ES:DI in memory.

The following example demonstrates use of the LODS and STOS instruction to convert an upper case string to its lower case value:

section	.text
    global _start         ;must be declared for using gcc
_start:	;tell linker entry point
        mov    ecx, len
        mov    esi, s1
        mov    edi, s2
loop_here:
	lodsb
	or      al, 20h
	stosb
	loop    loop_here	
	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 ;source
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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