Screen Version

School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

Motivation

• Early operating systems had very little structure.
• A strictly layered approach was promoted by [Dijkstra 1968].
• Later OS (more or less) followed that approach (e.g., Unix).

Problems with layered approach

• Widening range of services and applications
  ==> OS bigger, more complex, slower, more error prone.
• Need to support same OS on different hardware.
• Like to support various OS environments.
• Distribution
  ==> impossible to provide all services from same (local) kernel.

Idea: break up the OS

Monolithic vs. client-server OS structure

Kernel:

 $.$

Contains code which must run in supervisor mode;

 $.$

Isolates hardware dependence from higher levels;

 $.$

Is small and fast

==>

extensible system;

 $.$

Kernel provides mechanisms.

User-level servers:

• Are hardware independent/portable,
• Provide ``OS environment''/''OS personality'' (maybe several),
• May be invoked:
  • from application (via message-passing IPC)
  • from kernel (upcalls);
• Servers implement policies[BH70].