Question about pure space

[Gerd Möllmann]

I'm trying to understand pure space better for the scratch/igc branch.

>From my flimsy recollection, there is a pure[] array in which Lisp
objects can be stored. That array still seems to exist, and it is quite
large on macOS, ca. 6Mb.

The odd thing is that I can't find where it is used. The function
pure_alloc always uses purebeg, which is initialized to NULL, and then
gets malloc'd.

What the heck?

Is pure[] used at all?
If yes, how?
(If not, why is it there?)