Sibling segments, those at the same depth level, share the same memory, called overlay region or destination region.
Often linkers provide support for overlays.
The following example shows the control statements that instruct the OS/360 Linkage Editor to link an overlay program, indented to show structure (segment names are arbitrary):
These statements define a tree consisting of the permanently resident segment, called the root, and two overlays A and B which will be loaded following the end of MOD2.
Overlay A itself consists of two overlay segments, AA, and AB.
At execution time overlays A and B will both utilize the same memory locations; AA and AB will both utilize the same locations following the end of MOD3.
All the segments between the root and a given overlay segment are called a path.
As of 2015, most business applications are intended to run on platforms with virtual memory.
A developer on such a platform can design a program as if the memory constraint does not exist unless the program's working set exceeds the available physical memory.
Most importantly, the architect can focus on the problem being solved without the added design difficulty of forcing the processing into steps constrained by the overlay size.
Still, overlays remain useful in embedded systems.
For example, the Space Shuttle Primary Avionics System Software (PASS) uses programmed overlays.
In the home computer era overlays were popular because the operating system and many of the computer systems it ran on lacked virtual memory and had very little RAM by current standards: the original IBM PC had between 16K and 64K, depending on configuration.
Overlays were a popular technique in Commodore BASIC to load graphics screens.
In order to detect when an overlay was already loaded, a flag variable could be used.
"Several DOS linkers in the 1980s supported [overlays] in a form nearly identical to that used 25 years earlier on mainframe computers."
containing memory overlays had a de facto standard extension, .OVL.
The GFA BASIC compiler was able to produce .OVL files.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Overlay (programming).