INT 21 - DOS 1+ - NULL FUNCTION FOR CP/M COMPATIBILITY
AH = 20h
Return: AL = 00h
Note:   corresponds to the CP/M BDOS function "get/set default user
(sublibrary) number", which is meaningless under MSDOS
SeeAlso: AH=1Dh,AH=1Eh,AX=4459h

See also: 2118

2120 - DOS 1+ - NULL FUNCTION FOR CP/M COMPATIBILITY