The application's view of an operating system is formed by the semantics of the services it provides and their call interface syntax.
These services are provided through a set of subroutines called "system calls". The internal details of the Operating System are normally invisible to the application programmer - the system calls make up the bulk of the view.
System calls define the application/Operating-System boundary and therefore some special things have to happen at system call time (which we will discuss later).
Because of this, you will likely never see a system call made from inside the O/S, nor an internal O/S routine called directly from outside the O/S.
