May a compiler ever generate code to unload parts of the code segment during execution?

Question

Apart from Dll concept that provides ability of loading/unloading methods or functions at run-time, I'm wondering if a compiler may ever say something like, ok as this particular part of the code takes considerable amount of space in code segment and is never gonna be used again after this point during program execution, it'd be good to generate some code to unload that part of code segment after reaching that particular point during program execution so that overall space took by code segment gets smaller. Is it something just fictional or may that happen?

Answer 1

Sure. There's a technique called overlaying that loads different code into the same bit of address space at different times. Sometimes it was done manually, other times compilers helped. Sometimes the loading is done in software, sometimes in hardware (with address multiplexing, so that e.g. during boot time one bit of address space reads from a ROM chip, but after boot it switches to address RAM or a different ROM instead).

Overlaying was much more common when computers had less memory, e.g. in the early days of DOS where you had 640K at best and often not even that. These days it still has applications for embedded systems where memory and/or address space are at a premium.