Permanently in memory. Normally, a computer does not have enough memory to hold all the programs you use. When you want to run a program, therefore, the operating system is obliged to free some memory by copying data or programs from main memory to a disk. This process is known as swapping.
The programs and data used most frequently are the ones that should be memory resident. This includes central portions of the operating system and special programs, such as calendars and calculators, that you want to be able to accessimmediately.