System Structure

Simple Structure
-Simple structure systems do not have well-defined structures
-The Unix only had limited structure: kernel and system programs
-Everything between the system call interface andphysical hardware is the kernel.


Layered Approach
-The operating system is broken up into a number of layers (or levels), each on top of lower layers.
-Each layer is an implementation of an abstract object that is the encapsulation of data and
operations that can manipulate these data.
-The bottom layer (layer 0) is the hardware.
-The main advantage of layered approach is
modularity.
-The lowest layer is process management.
-Each layer only uses the operations provided by lower layers and does not have to know their implementation.
-Each layer hides the existence of certain data structures, operations and hardware from higher-level layers.