Int 21 Fn 20 - DOS 1+ - Null Function For Cp/m Compatibility [D] AH = 20h Return: AL = 00h Note: corresponds to the CP/M BDOS function "get/set default user (sublibrary) number", which is meaningless under MS-DOS See Also: AH=18h,AH=1Dh,AH=1Eh,AX=4459h