> In fact, some of the Lisp Machines had garbage collection implemented in hardware and allocated everything including stack frames and binding environments in the heap.

I wrote a lot of code for CADR, Symbolics and D-Machine Lispms and I’m not aware of any of them having hardware or microcode GC. That would be crazy anyway. Tagging and other support sure, but you could say that the x86 has hardware support through its pager too.

As for stack frames on the heap, think of how important function calling is, and the impact of allocation, cache, etc of such an approach. They all used PDLs like any other machines.

Now InterLisp-D did spaghetti the stack when you made a closure, at least for some time. I know this because I triggered it with a pathological case of making thousands of closures, bringing any machine to its knees. But a spaghetti stack is a different thing from allocating frames from the heap.