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

-Layered 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 layersProperties
Simplicity of construction.
Simplicity of Debugging
Problems
Precise definition of layers
Example: Memory manager requires device driver of backing store (due to virtual memory)
The device driver requires CPU scheduler (since if the driver waits for IO, another task should be scheduled)
CPU scheduler may require virtual memory for large amount of information of some processes
Less efficiency: due to the number of layers a request should pass .