Assembly - STOS Instruction

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