Overlays implied a specific kind of resource management about keeping unused code on the disk and loading it on demand. Doing that required help from linker so

Overlays implied a specific kind of resource management about keeping unused code on the disk and loading it on demand. Doing that required help from linker so they were mostly implemented by the language like Turbo Pascal/Turbo C's "FBOV" overlays. You could also keep resources in overlays too but they didn't have a resource management mechanism if I recall correctly. You had to implement it yourself.