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