In this guide, we will discuss STOS Instruction in Assembly Programming Language. 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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