System Architecture 

Operating System Mode

　　In most multiuser operating systems, applications are separated from the operating system itself-the operating system kernel code runs in a privileged processor mode, with access to system data and to the hardware; application code runs in a nonprivileged processor mode, with a limited set of interfaces available, limited access to system data, and no direct access to hardware. When a user-mode program call a system service, the processor traps the call and then switches the calling thread to kernel mode. When the system service completes, the operating system switch the thread context back to user mode and allows the caller to continue.

First notice the line dividing the user-mnode and kernel-mode parts of the Windows operating system. The boxes above the line represent user-mode processes, and the components below the line are kernel-mode operating system services.

 Key System Components