This category is for submicrokernel operating systems: OSs with non-monolithic architectures based on structures smaller than microkernels. Varieties and synonyms: lightweight kernel, nanokernel, picokernel, femtokernel, exokernel, no-kernel, or closely related topics. Most, maybe all object-oriented OS architectures also qualify as submicrokernels. Traditional OS architectures limit application performance, flexibility, functionality by fixing interfaces and implementations of OS abstractions such as interprocess communication and virtual memory. Submicrokernels address these issues in varied ways. Most are runtime extensible. Exokernels address such issues by application-level management of physical resources: they put applications in control, to run 10x or more faster than normal OSs. On this page, OSs are arranged in three groups and levels: 1) Top group: types or classes of OS. 2) Middle group: OSs for which there are more than one instance of an OS of this name/type, an OS family. 3) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.