Assembly (ASM) source to invoke a message box on Windows x64

messagebox-x64.asm

BITS 64

; Assembly with NASM for 64-bit
; nasm -f win64 -o messagebox.o messagebox.asm

; Linking for 64-bit with MinGW
; x86_64-w64-mingw32-ld -o messagebox.exe messagebox.o -lkernel32 -luser32

extern MessageBoxA
extern ExitProcess
section .data
    title db 'whatever', 0
    content db 'hack the planet 31337', 0

section .text
global main

main:
    ; Prepare arguments for MessageBoxA
    push 0               ; MB_OK
    mov rax, rsp         ; Pointer to MB_OK
    push rax             ; Push pointer to MB_OK for the MessageBoxA call
    mov r9, rax          ; UType = MB_OK (4th argument)
    pop rax              ; Clean the stack (remove MB_OK pointer)
    lea r8, [rel title]  ; lpCaption (3rd argument)
    lea rdx, [rel content] ; lpText (2nd argument)
    xor rcx, rcx         ; hWnd = NULL (1st argument)

    ; Call MessageBoxA
    call MessageBoxA

    ; Clean the stack
    add rsp, 8           ; Clean up after push

    ; ExitProcess call
    xor ecx, ecx         ; uExitCode = 0
    call ExitProcess