Does every function end up in kernel mode?

Can we say that while programming, showing something on output, adding values etc., we always interact with system? I mean whether every function in app ends up(finally) in kernel开发者_StackOverflow中文版. I don't know if this approach varies from OS to OS so I mean Windows.

I appreciate Your response, and I am sorry for my English.

No, adding two values together will pretty sure not use any system code.

You always interact with the system in that the CPU (or some other processor like a GPU) has to execute your code.

Not every instruction executed by the CPU will involve a kernel-mode operation, though.

No, for example in Windows all messaging and COM objects don't end in Kernel-mode but they may use some kernel-mode resources like HANDLEs.