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Modes
Two things that people like to confuse:
- Kernel Mode
Code executing in kernel mode runs in a special mode of the CPU. This mode allows access to critical commands which control CPU or peripheral hardware. Examples are: "hlt" to stop the CPU or "in/out" to access devices on Intel systems.
- Super-user mode
Programs running in this mode have an extended set of rights to perform certain actions. But they CANNOT access those protected commands which are only available in kernel mode. If they want to do something like halting the system they need to switch to kernel mode and request a kernel mode service routing to do it. The Super-user rights only give them PERMISSION to request these kernel mode operations.