.model flat, stdcall        ; Simplified with Windows conventions, please.
    .radix 16                   ; Hex by default, please.
    option casemap :none        ; Case sensitive identifiers, please.

    ; Declarations of Windows API functions:
                extern  ExitProcess@4       : proc
                extern  MessageBoxW@16      : proc

    ; Declaration of this program's entry point symbol:
                public  startup

    .const
    ; Data:
titleString     dw  0054, 0068, 0065, 0020, 006D, 0061, 0063, 0068, 0069, 006E, 0065
                dw  0020, 0063, 006F, 0064, 0065, 0020, 0073, 0061, 0079, 0073, 003A
                dw  0000

helloString     dw  0048, 0065, 006C, 006C, 006F, 002C, 0020, 0077, 006F, 0072, 006C
                dw  0064, 0021, 0009, 0000

    .code
    ; Call forwarders, to be able to use the API function addresses directly.
exitProcessJump:
                db  68                          ; push ...
                dd  offset ExitProcess@4                ; address of ExitProcess
                db  0C3                         ; ret
messageBoxJump:
                db  68                          ; push ...
                dd  offset MessageBoxW@16               ; address of MessageBox
                db  0C3                         ; ret

    ; Main program:
startup:
                ; Present a message box:
                db  68, 40, 00, 01, 00          ; push flags argument 00010040
                db  68                          ; push ...
                dd  offset titleString                  ; offset titleString
                db  68                          ; push ...
                dd  offset helloString                  ; offset helloString
                db  6A, 00                      ; push parent window handle, 0 = none.
                db  0E8                         ; call ...
                dd  (offset messageBoxJump) - ($ + 4)   ; MessageBox

                ; Exit process:
                db  6A, 00                      ; push exit code argument, 0 = success.
                db  0E8                         ; call ...
                dd  (offset exitProcessJump) - ($ + 4)  ; ExitProcess

    ; This end directive has no effect, it's just required to mark the end of the source code.
    end