Comment by ggm

Yes. Exactly. But, tbf I could/should have tried harder to say that. I remember the Emacs manuals on vax systems used to say although it was a 24bit addressable space inside Emacs, they didn't guarantee if would be able to address all 24bits, but they'd try. And, from memory there are recursion limits you may need to turn off, and tail call optimisation and stuff is hard.