I think the diagram is just trying to emphasize that operating system functions are usually invoked through a different mechanism then a simple library call.

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An operating system is a collection (or suite) of programs that manages and controls the computer. Operating systems have many functions: ...

The kernel plays an essential role, as it manages how your phone, tablet, and PC's hardware and operating system.

Trusted operating systems seal up systems vulnerabilities like never before. But the complexity of the systems can lead even experienced administrators to create more problems than they solve. For ...

Oracle has been subsuming operating system functions like memory and storage management for years; subsuming the whole operating system was just the next logical step.

The question we ask here is “are tasks or threads necessary in an operating system?” The short answer is that if time slicing is needed, they are absolutely necessary. However, time slicing is ...

The course covers nine areas, with the first six being core operating system functions and the remaining three covering more advanced operating system concepts.

This is a lesson that most people could use. I cannot tell you how many people I know who struggle to get their operating system of choice to behave and function how they want.