INT 21 - DOS 2+ - GET FREE DISK SPACE
        AH = 36h
        DL = drive number (00h = default, 01h = A:, etc)
Return: AX = FFFFh if invalid drive
        else
            AX = sectors per cluster
            BX = number of free clusters
            CX = bytes per sector
            DX = total clusters on drive
Notes:  free space on drive in bytes is AX * BX * CX
        total space on drive in bytes is AX * CX * DX
        "lost clusters" are considered to be in use
        according to Dave Williams' MSDOS reference, the value in DX is
          incorrect for non-default drives after ASSIGN is run
SeeAlso: AH=1Ch