“An Operating system also provides services to application programs. When an application program needs to read from, or write to the disk, or draw letters on the screen, it does not have to do all of the complicated I/O instructions necessary to accomplish the task. The OS has a number of small routines that it keeps in RAM at all times for such purposes. All an application needs to do to use one of these routines is to load up some information in the registers, and then jump to the address of the proper OS routine.”