This category is on operating systems (OSs) which main trait is that they have a microkernel architecture. A microkernel is a minimal OS kernel providing only basic OS services (system calls), while other services (usually done by kernels) are done by user-space programs called servers. Usually, microkernels provide services such as address space (memory) management, thread management, and inter-process communication, but not networking, or display. Later extensions of microkernel designs led to new architectures such as nanokernels, exokernels, and hardware abstraction layers (HAL). Monolithic kernels (DOS, Linux, most Unix, Windows, etc.) go in their own category. 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.