System Structure

• Simple Structure

-->View the OS as a series of levels

-->Each level performs a related subset of functions

-->Each level relies on the next lower level to perform more primitive functions

-->This decomposes a problem into a number of more manageable subproblems

• Layred Approach

The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers.
A process is a program in execution. A process needs certain resources: CPU time, memory (address space), files, and I/O devices, to accomplish its task.