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Hybrid kernel is a kernel architecture based on a combination of microkernel and monolithic kernel architecture used in computer operating systems. This kernel approach combines the speed and simpler design of monolithic kernel with the modularity and execution safety of microkernel.
A hybrid kernel runs some services in the kernel space to reduce the performance overhead of a traditional microkernel, while still running kernel code as servers in the user space. For instance, a hybrid kernel design may keep the virtual files system and bus controllers inside the kernel and the file system drivers and storage drivers as user mode programs outside the kernel. Such a design keeps the performance and design principles of a monolithic kernel.
The Microsoft NT kernel is a well-known example of a hybrid kernel that powers Windows NT, Windows 2000, Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008 and Windows 7. It is referred to as a monolithic kernel as emulation subsystems run in the user mode server processes. One of the most important thing about it is its structure, which is a collection of modules that communicate via well-known interfaces, with a small microkernel limited to core functions such as first-level interrupt handling, thread scheduling and synchronization primitives. This allows for the possibility of using either direct procedure calls or interprocess communication to communicate between modules, and hence for the potential location of modules in different address spaces.