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