assembly - ASM x86_64 Hello world program -

i new asm, , i'm trying create basic hello world program, using function :

section .text      global main  print_msg:       push rbp      mov rbp, rsp       mov rax, 1      mov rdi, 1      mov rsi, buffer       ;to change      mov rdx, buffersize   ;to change      syscall       mov rsp, rbp      pop rbp       ret  main:       mov rdi, buffer      mov rsi, buffersize      call print_msg       mov rax, 60      mov rdi, 0      syscall  section .rodata  buffer:     db 'hello, world !', 0x0a buffersize: equ $-buffer 

this code work, because directly copied buffer in rsi , buffersize in rdx, in "print_msg" function, want copy received arguements in these 2 registers, saw :

mov rsi, [rsp + 8] mov rdx, [rsp + 12] 

but doesn't work here.

x86-64 uses registers pass arguments, code illustrates:

 mov rdi, buffer  mov rsi, buffersize  call print_msg 

if loaded rdi , rsi arguments, why expect them on stack in called function? call doesn't registers, , puts return address on stack. thus, 2 arguments still happily in rdi , rsi. mov them correct place:

 mov rdx, rsi  mov rsi, rdi  mov rax, 1  mov rdi, 1  syscall