DOS Fn 1bH: Get Drive Info (current disk)

                                                         Compatibility: 2.0+ 
 Expects: AH    1bH
 Returns: DS:BX address of a FAT ID byte (might indicate disk type)
          DX    total clusters (allocation units) on the disk
          AL    sectors per cluster
          CX    bytes per sector
    Info: Returns information on the size and type of the default disk.

          Disk size in bytes = (DX * AL * CX)

          Find free space with DOS Fn 36h Disk Free or 32h Disk Info

 Warning: Notice that this changes the value of DS segment register.

Versions: DOS 1.x held the FAT in memory and returned DS:BX => FAT
          DOS 2.0+ may retain only a portion of the FAT in memory.

See Also: Fns 1cH  36H  32H
          File Allocation Table
          DOS Functions