Can you explain what happens inside the operating system when system calls are made?

When a system call is invoked, there is a transition from user mode to kernel mode of the operating system. This is necessary because system calls provide access to protected resources and OS functions that cannot be executed directly from user space.

Main steps:

1. The program calls a function that wraps the system call (e.g., read, write).
2. An interrupt or a special instruction occurs (e.g., syscall on x86_64), which switches the processor to kernel mode.
3. The OS checks the call parameters, access rights, and performs the necessary operation (e.g., reading a file).
4. The result is returned to user space.
5. The processor switches back to user mode, and the program continues to run.

Thus, system calls provide safe and controlled access to OS resources.