; hello21.asm -- the smallest useful INT 21h program: AH=09h then AH=4Ch with a return code
bits 16
org 0x100                   ; .COM program
    mov ah, 09h             ; AH selects the DOS function: write '$'-terminated string
    mov dx, msg             ; DS:DX -> string (DS = CS in a .COM file)
    int 21h                 ; CD 21 -- the DOS entry point
    mov ax, 4C07h           ; AH=4Ch terminate, AL=7 return code (ERRORLEVEL)
    int 21h
msg db 'Hello from INT 21h, AH=09h', 0Dh, 0Ah, '$'