Back in the day when one built things with PDP-11s as one tended to do, there was very little memory and no virtual memory.
One had to organize one's code into "overlays" - regions of code that could replace each other in RAM.
If you neglected to swap in an overlay before calling a routine in it? Guess what happend.
In production projects managing memory was half the challenge in writing code.
No one has been doing any real coding for years.