Overlay structured program:
A program consisting of overlays is referred as overlay structured program in which an overlay is a part of program which has similar load origin as some other part(s) of the program. Such type of a program contains
a. A permanently resident portion, called the root
b. A set of overlays
The overlay structure of a program is planned by identifying mutually exclusive modules- i.e. modules that do not call each other. The major idea is that such modules do not require residing simultaneously in memory. Therefore they are located in different overlays with similar load origin.