System Structure

• Simple Structure


• 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 layers

Properties


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