Int 21 Fn 18  - Dos 1+ - Null Function For Cp/m Compatibility              [D]

   AH = 18h

Return: AL = 00h

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

See Also: AH=1Dh,AH=1Eh,AH=20h,AX=4459h,INT 60/DI=0513h