Toward real microkernels

(IPC), and basic scheduling. All servers---even device drivers ---run in user mode and are treated exactly like any other application by the kernel. Since each server has its own address space, all these objects are protected from one another. When the microkernel idea was introduced in the COMMUNICATIONS OF THE ACM September 1996/Vol. 39, No. 9 71 late 1980s, the software technology advantages seemed obvious: . Different application program interfaces (APIs), different file systems, and perhaps even different basic operating system strategies can coexist in one system. They are implemented as competing or cooperating servers. . The system becomes more flexible and extensible. It can be more easily and effectively adapted to new hardware or new applications. Only selected servers need to be modified or added to the system. In particular, the impact of such modifications can be restricted to a subset of the system, so all other processes ar