[hazeii]

Overlays (at least in RT-11) are just chunks of code that would load at a specific address in memory. So if your program didn't fit into memory, a good chunk of it could run in an overlay area.

So for example your code could have an always-resident portion say from 0 to 100000 octal (the lowest half of memory) and an overlay area from 100000 to 140000. We had a program that has an input, calculation and runtime phase so it suited the overlay model; programs that needed all their code in memory at the same time by comparison would not be suited to overlays (especially on 8" floppies, where loading an overlay was accompanied by plenty of mechanical noise).