Comment by drfuchs

TeX was developed on DECsystem-10/20 machines (36-bit words, your choice of byte size). The various operating systems (Tops20, Sail/Waits, ITS) were superior to Unix in a few ways, one of which was that when a process was suspended (think control-Z, or even control-C) you could issue the built-in shell SAVE command that would save the entire state of the suspended process into a new executable, data segment as well as code. So, on these systems, you could run TeX, have it load a bunch of macros and such, and then SAVE the result as a real, pre-configured executable for the world to run. Easy-peasy.

This was true for the original TeX78 written in Sail, as well as the ultimate TeX82 written in Knuth's WEB macro language on top of Pascal (that nowadays typically gets transpiled to C). Other programs did similar stuff; the feature was in the OS way before TeX started.

(Gory detail: Actually, TeX went a little further, to free up all possible address space for the final executable: The version that could initialize various hash tables and hyphenation trie tables could dump (nee serialize) a binary file of the resulting data structures; then a slimmed-down version that didn't have that code would read the binary info back in to recreate the initial data structure state, and that's what you'd SAVE the production executable from.)

For the unix-y versions of TeX, there was effort made to mimic this sort of thing in user-land with "undump", but admirable as it was, it was a hack, I'm told. These days, everything is so fast, it's not clear that this feature would be worth it anyway.

Source: me; I was there.