Most operating systems (OSs) in use are monolithic. In these, all source code is compiled into one logically undivided block of object code, though the source and/or object code may be segmented into different address spaces, unlike in a Single Address Space OS (SASOS). On this page, OSs are arranged in three groups and levels: 1) Top group: types or classes of OSs. 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.
Hybrid kernels are a quasi-classification of operating system (OS) kernel architecture, based on combining traits of monolithic kernel and microkernel designs. The goal is to use a kernel structure implemented as a monolithic kernel, but structured in some ways as a microkernel. As in monolithic kernels, all (or nearly all) services run in kernel space, the reverse of a microkernel, where services run in user space. As in monolithic kernels, no performance overhead occurs due to microkernel message passing and context switching between kernel and user mode, but no benefits occur of running services in user space. This classification is controversial. The term has been dismissed by some as a marketing phrase. In OSs, the usually accepted classifications are monolithic kernels and microkernels, with nanokernels and picokernels being more extreme versions of microkernels.