Assembly - The 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 following result:

hello, world