Overlays were common on MS-DOS in the 80s. A typical scheme was to have a configuration file / script for the linker. You would list one or more blocks, each

Overlays were common on MS-DOS in the 80s. A typical scheme was to have a configuration file / script for the linker. You would list one or more blocks, each of which had a list of mutually exclusive .obj files to be swapped in and out as needed. It could be challenging to track dependencies between objects to make sure you never called between modules which were supposed to be mutually exclusive, which was why you sometimes had multiple swap buffers. Later linkers, or proprietary schemes like in Turbo Pascal version 5, would swap things per subroutine, rather than per compilation unit (.obj file), which was much easier to use. (I did once see some people make a C function that was literally over 50 printed pages long, and the overlay manager didn’t help them much with OOM, among other problems that thing had)