Talk about the kernel as the privileged portion of the O.S. Layered Systems . XINU as an example. Virtual Machines . Emulated Operating System environment for application. Java Virtual machine.
Clients obtain service by sending messages to server processes.
Simple approach where simple kernel mechanism allows policies to be implemented in server processes. A trend towards this latter type of organization. More amenable for moving towards a distributed system. Build minimum mechanism into the kernel and leave policy decisions to the servers. Issues of flexibility vs. performance.