> Using SBCL as a macro-assembler is extremely cool, and then allowing CL code to call into the VM is where it really blows my mind.

Absolutely. I think TFA is hooking into the actual code emitter used by SBCL's evaluator-compiler, since it's not actually primitive, but implemented in Lisp and loaded into the image itself.

My guess at context: from the earliest days of Lisp, I think there was an expectation that Lisp systems would expose as much of their internals as possible... including their internal JIT, which is being plugged into here. I think the name for it was the "LISP Assembly Program" (LAP).

Try pair reading it with a good thinking LLM like GPT 5.5 or Claude Opus. I found it help me a lot.

I have started learning SBCL internal from the beginning of this year with the help of GPT, and I really want to contribute to SBCL compiler someday in future.