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An operating-system kernel design retaining essential protected mechanisms while moving services into user-space services. Address spaces, software threads and IPC form a useful small foundation: the operating-system kernel protects private state and controls communication, while a file system service interprets requests about files. This separation of mechanism and policy permits different service implementations to use the same protected interface. ^[ref]

The useful question is which authority a service needs. A device driver may require access to a device while having no permission to overwrite an unrelated application's computer memory. A microkernel makes such protection domains possible; their actual boundaries depend on the chosen memory protection and security capabilities.

For the microkernel file-read example, distinguish system call entry/exit, security capability lookup, payload transfer, process scheduling and context switches from the service's actual file lookup and read work. Entering a privileged processor privilege level and switching to a different address space are different events. A change of address space can affect the translation lookaside buffer; running different code and touching different computer memory can cause cache misses. Neither event implies that every translation lookaside buffer entry or cache line must be discarded.

A larger copied payload requires more transfer work; sharing memory pages can avoid copying that bulk data but introduces mapping and synchronization work. A service that requests another service adds another IPC exchange. If the cached file data were absent, communication with a device driver and the input-output wait would also matter. Blocking behind other requests affects observed response time even when the individual IPC operations are cheap.

These are cost sources, not timings or a universal ranking against a monolithic kernel. Implementation and CPU details matter: the construction analysis separates protected entry costs, address space switching, translation lookaside buffer effects and memory cache effects. ^[ref]