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A user-mode kernel
This is the standard layer diagram of an operating system. At the bottom is hardware with an operating system controlling it. The OS is broken into generic code, which is architecture-independent, and the archictecture layer and drivers, which implement the hardware-specific functions needed by the generic kernel. Above that is the system call layer and user-level processes like 'ls', 'ps', and 'netscape', which use it.
The user-mode kernel is implemented at this layer, but it is structured exactly like the host kernel. It has a generic kernel layer, which may be exactly the same code as in the host kernel, with the architecture-specific functions implemented by a user-mode architecture layer and drivers, which are implemented in terms of system calls.