foil on Section 2.6 ( and see Chapter 26)
Microkernel structured operating system
+ tailored configuration
+ small - easier to engineer, debug and maintain
+ services run above the kernel
easier to engineer, tune, update, maintain
+ bounded kernel-execution time
+ policy - mechanism separation
kernel provides minimal mechanism
flexible expression of policy at user level
+ can emulate existing OS while providing new facilities
e.g. UNIX emulation + multiprocessor support
- a service runs more slowly at user level than in the kernel
invoked by inter-process communication not system call
e.g.file service, communications