编译的程序如何与操作系统交互？

When a program is compiled it is converted to machine code which can be "understood" by the machine. How does this machine code interact with the operating system in order to do things like getting input from the keyboard ? To me, it seems that the machine code should run at a lower level than the operating system and therefore, I can't understand how the OS can act as an intermediary between the compiled application and the hardware. When a program is compiled it is converted to m