Kernel Mode vs. User Mode

To protect user applications from accessing and/or modifying critical operating system data, Windows uses two processor access modes (even if the processor on which Windows is running supports more than two): user mode and kernel mode.

User application code runs in user mode, whereas operating system code (such as system services and device drivers) runs in kernel mode. Kernel mode refers to a mode of execution in a processor that grants access to all system memory and all CPU instructions. By providing the operating system software with a higher privilege level than the application software has, the processor provides a necessary foundation for operating system designers to ensure that a misbehaving application can't disrupt the stability of the system as a whole.

Although each Windows process has its own private memory space, the kernel-mode operating system and device driver code share a single virtual address space. Each page in virtual memory is tagged as to what access mode the processor must be in to read and/or write the page.

Windows doesn't provide any protection to private read/write system memory being used by components running in kernel mode. In other words, once in kernel mode, operating system and device driver code has complete access to system space memory and can bypass Windows security to access objects.

User applications switch from user mode to kernel mode when they make a system service call.

A transition from user mode to kernel mode (and back) does not affect thread scheduling per se—a mode transition is not a context switch.

Thus, it's normal for a user thread to spend part of its time executing in user mode and part in kernel mode. In fact, because the bulk of the graphics and windowing system also runs in kernel mode, graphics-intensive applications spend more of their time in kernel mode than in user mode.