Int 21 Fn 1D  - Dos 1+ - Null Function For Cp/m Compatibility              [D]

   AH = 1Dh

Return: AL = 00h

Note:  corresponds to the CP/M BDOS function "get bit map of read-only
     drives", which is meaningless under MS-DOS

See Also: AH=18h,AH=1Eh,AH=20h,AX=4459h