Debugging would be easier in user mode, surely. You could attach with gdb or whatever. Last I checked, IPC overhead was not a barrier to performance in modern

Debugging would be easier in user mode, surely. You could attach with gdb or whatever. Last I checked, IPC overhead was not a barrier to performance in modern microkernels. QNX and XNU (used by OS X) both use microkernel-type IPC and are fast. XNU is even able to survive with an old Mach kernel. If a kernel driver ends up in a faulty state (accesses invalid memory, does the wrong interrupt thing) then surely the whole kernel will panic? NT is frequently referred to as "hybrid", but it's a frequently misunderstood design. First, while it uses IPC internally, it's not really comparable to microkernels since there's no context switch; every participant in the IPC communication is in the kernel. This is done purely as a way to isolate internals behind a rigorous, message-oriented API. Secondly, almost all of NT's services run in the kernel. A fairly small number of services run in userland: There's the user-mode driver system (UMDF, available since Vista), and graphics drivers also have the option of running partly in usermode.