Int 21 Fn FF  UP - Rational Systems Dos/4gw - Api                          [E]

   AH = FFh
   DH = function (00h-17h) (also see separate entries below)
   DL = subfunction or argument

Return: CF clear if valid function number
       AX = status???
   CF set if invalid function

See Also: INT 15/AX=BFDCh