<P> In modern operating systems, memory which is accessed less frequently can be temporarily stored on disk or other media to make that space available for use by other programs . This is called swapping, as an area of memory can be used by multiple programs, and what that memory area contains can be swapped or exchanged on demand . </P> <P> "Virtual memory" provides the programmer or the user with the perception that there is a much larger amount of RAM in the computer than is really there . </P> <P> Multitasking refers to the running of multiple independent computer programs on the same computer; giving the appearance that it is performing the tasks at the same time . Since most computers can do at most one or two things at one time, this is generally done via time - sharing, which means that each program uses a share of the computer's time to execute . </P> <P> An operating system kernel contains a scheduling program which determines how much time each process spends executing, and in which order execution control should be passed to programs . Control is passed to a process by the kernel, which allows the program access to the CPU and memory . Later, control is returned to the kernel through some mechanism, so that another program may be allowed to use the CPU . This so - called passing of control between the kernel and applications is called a context switch . </P>

Sometimes called the systerm board is the main circuit board of the system unit