Hello world Assembly x86-16 (MS-DOS, Compact Memory Model)

; Hello world program
; Language: x86 (16-bit) Assembly (Compact Memory Model)
; Library: MS-DOS interrupts

segment code
..start:
    mov ax, data
    mov ds, ax
    mov ax, stack
    mov ss, ax
    mov sp, stacktop
    mov dx, message
    mov ah, 0x9
    int 0x21
    mov ax, 0x4C00
    int 0x21

segment data
    message db 'Hello world!', 0xD, 0xA, '$'

segment stack class=stack
    resb 0x400
stacktop: