this gives the appearance of many processes running at once. User programs are usually stored on a hard disk and need to be loaded into memory before being executed. This presents the need for memory management, as the memory of the computer would need to be searched for a free area in which to load a users program. When the user was finished running the program, the memory consumed by it would need to be freed up and made available for another user when required (CIT). Process scheduling and management is also necessary, so that all programs can be executed and run without conflict. Some programs might need to be executed more frequently than others, for example, printing. Conversely, some programs may need to be temporarily halted, then restarted again, so this introduces the need for inter-program communication. In modern operating systems, we speak more of a process (a portion of a program in some stage of execution (CIT, 3)) than a program. This is because only a portion of the program is loaded at any one time. The rest of the program sits waiting on the disk until it is needed, thereby saving memory space. UNIX users speak of the operating system as having three main parts: the kernel, the shell and the file system. While DOS users tend not to use the term kernel and only sometimes use the term shell, the terms remain relevant. The kernel, also known as the "Real Time Executive", is the low-level core of the OS and is loaded into memory right after the loading of the BIOS whenever the system is started. The kernel handles the transfer of data among the various parts of the system, such as from hard disk to RAM to CPU. It also assigns memory to the various system-level processes that occur whenever the computer does anything. The kernel is also responsible for scheduling the CPU's operations and for letting the shell access the CPU (PC Mag, 1). The shell is the visible user interface to the OS and is a program that ...