hello world assembly winapi 64-bit

extrn MessageBoxA: PROC
extrn ExitProcess: PROC

.data
titles db 'Win64', 0
msg db 'Hello World!', 0

.code
msgbox_ready:
sub rsp, 28h      ; 32 byte for 4 arguments, 8 byte for 'call' it self
mov rcx, 0        ; hWnd = HWND_DESKTOP
lea rdx, msg      ; LPCSTR lpText
lea r8,  titles   ; LPCSTR lpCaption
mov r9d, 0        ; uType = MB_OK
call MessageBoxA
mov ecx, eax
add rsp, 28h
ret               ; Return from function

main proc
  push rbp        ; Push 8 bytes to align stack to 16 byte boundary before calling a function
  call msgbox_ready
;  call ExitProcess ; Using ExitProcess instead of the pop rbp and ret is an option
  pop rbp         ; Restore RBP and rebalance stack before returning
  ret             ; Return to C startup code and exit program
main endp
End