<P> A program may also trigger an interrupt to the operating system . If a program wishes to access hardware, for example, it may interrupt the operating system's kernel, which causes control to be passed back to the kernel . The kernel then processes the request . If a program wishes additional resources (or wishes to shed resources) such as memory, it triggers an interrupt to get the kernel's attention . </P> <P> Modern microprocessors (CPU or MPU) support multiple modes of operation . CPUs with this capability offer at least two modes: user mode and supervisor mode . In general terms, supervisor mode operation allows unrestricted access to all machine resources, including all MPU instructions . User mode operation sets limits on instruction use and typically disallows direct access to machine resources . CPUs might have other modes similar to user mode as well, such as the virtual modes in order to emulate older processor types, such as 16 - bit processors on a 32 - bit one, or 32 - bit processors on a 64 - bit one . </P> <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>

How many types of operating system of computer