MessageBoxA_Masm.asm

; MessageBoxA_Masm.asm by un4ckn0wl3z

.code

COMMENT @
int MessageBoxA(
  [in, optional] HWND   hWnd,
  [in, optional] LPCSTR lpText,
  [in, optional] LPCSTR lpCaption,
  [in]           UINT   uType
);
@

externdef MessageBoxA:proc

main proc
	xor rcx, rcx			; HWND   hWnd
	mov rdx, offset msg		; LPCSTR lpText
	mov r8, offset caption		; LPCSTR lpCaption
	mov r9, 4h			; UINT   uType
	sub rsp, 40			; preserve shadow spill
	call MessageBoxA		; call MessageBoxA
	add rsp, 40			; restore shadow spill
	ret				; return to caller
main endp 

.data

caption db "Hello Assembly", 0
msg db "This is a cool app!", 0

end