<P> At power - on or reset, the system begins in supervisor mode . Once an operating system kernel has been loaded and started, the boundary between user mode and supervisor mode (also known as kernel mode) can be established . </P> <P> Supervisor mode is used by the kernel for low level tasks that need unrestricted access to hardware, such as controlling how memory is accessed, and communicating with devices such as disk drives and video display devices . User mode, in contrast, is used for almost everything else . Application programs, such as word processors and database managers, operate within user mode, and can only access machine resources by turning control over to the kernel, a process which causes a switch to supervisor mode . Typically, the transfer of control to the kernel is achieved by executing a software interrupt instruction, such as the Motorola 68000 TRAP instruction . The software interrupt causes the microprocessor to switch from user mode to supervisor mode and begin executing code that allows the kernel to take control . </P> <P> In user mode, programs usually have access to a restricted set of microprocessor instructions, and generally cannot execute any instructions that could potentially cause disruption to the system's operation . In supervisor mode, instruction execution restrictions are typically removed, allowing the kernel unrestricted access to all machine resources . </P> <P> The term "user mode resource" generally refers to one or more CPU registers, which contain information that the running program isn't allowed to alter . Attempts to alter these resources generally causes a switch to supervisor mode, where the operating system can deal with the illegal operation the program was attempting, for example, by forceably terminating ("killing") the program). </P>

Where is the operating system installed for use