Certain programs, however, can be marked as being memory resident, which means that the operating system is not permitted to swap them out to a storage device; they will always remain in memory.

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 access immediately.

Another term for memory resident is RAM resident. In DOS systems, memory-resident programs are called pop-up utilities or TSRs (terminate and stay resident).