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I recently came across a program that looked like a win32 program but going through the source code I came across the interrupt calls like

int 3

eventually I realised that it was a native application(the author mentioned it in the comments)

I think I know much about win32 but next to nothing about native windows applications.Are they device drivers or are they ring-0 applications(probably device drivers are ring-0 themselves).

I request to please lead me from where I could learn to write such appications.
And please explain how could we use interrupts, I thought they were obsolete and all we need to know is the api.

int 3 is often used to trigger a debugger...
You put it somewhere that you don't want your program to go, and if it ends up going there when you run it, blam, your debugger traps the execution at that point (from which you might continue it).