Int 21 Fn 19  - DOS 1+ - Get Current Default Drive                         [D]

   AH = 19h

Return: AL = drive (00h = A:, 01h = B:, etc)

Note:  Novell NetWare uses the fact that DOS 2.x COMMAND.COM issues this call
     from a particular location every time it starts a command to
     determine when to issue an automatic EOJ

See Also: AH=0Eh,AH=47h,AH=BBh