Being able to start a process, have it run for a bit to, say, read in initialization data, populating dynamic data structures along the way, and then interrupt the process and save the whole state as a new executable, was a feature built into DEC’s Tops10 and Tops20 operating systems / standard runtimes, along with related custom systems like Waits, under which TeX was developed. It took just two lines of code for TeX to implement its side of this feature on those first platforms.

It came as a bit of a shock at the time that all the Unix-y systems had no such native concept, and that fragile, non-portable user-space schemes were required to mimic this functionality.