A virtualisation on operating system level, other computer programmes within a closed container receive a complete virtual runtime environment. In containers, it is impossible to run two operating systems respectively two versions of the same operating system. In containers, there is no chance of loading drivers. However, a container enables a highly-efficient handling of the available resources (in particular with regard to processor loads as well as the demand of main and mass storage). For this reason, this process is often called a “lightweight virtualisation”. In case of a virtualisation of the operating system, only one host-kernel is running, for UML a special user-mode-kernel controlled by the host-kernel.