INT 21 UP - Rational Systems DOS/4GW - API(Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running. Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes.
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
SeeAlso: INT 15/AX=BFDCh