Some viruses only attack a system when the program that conceals them is run. Other, more sophisticated, viruses stay in a computer’s memory ready to attack a program that is run on the computer. These are resident or memory-resident viruses.
The concept of a resident virus is simple. A computer can run several programs at the same time. This is called multi-tasking. At any moment there may be various programs such as a word processor, a web browser and a media player running simultaneously.
A resident virus is written to take advantage of multi-tasking. However, unlike other programs, resident viruses hide in a system’s Random Access Memory (RAM), waiting to be activated when something happens, such as when a user clicks a mouse to start a program. In this way they can easily infect other programs.
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