INT 21 - DOS 1+ - WRITE STRING TO STANDARD OUTPUT
        AH = 09h
        DS:DX -> '$'-terminated string
Return: AL = 24h (the '$' terminating the string, despite official docs which
                state that nothing is returned) (at least DOS 3.3-5.0)
Notes:   Break checked, and INT 23 called if pressed
        standard output is always the screen under DOS 1.x, but may be
          redirected under DOS 2+
        under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
SeeAlso: AH=06h"OUTPUT"