The rest of this post is an excerpt from an email I sent 6 years ago.
The following comments on runtime systems are partially based on a long c.l.l thread with posts by Lucid, Symbolics, and Franz alumni.
Franz uses a 3-layer approach: CL, a low-level Lisp, and C.
Lucid started with Lisp that generated assembler but reluctantly added some C.
Symbolics Lisp Machines used bootstrap code in a Pascal-level language with prefix syntax. A Symbolics alum said that in retrospect they should have used C.
Most Lisp implementations have subprimitives - low-level functions that can circumvent the type system, often with a prefix such as % or :.
Assembly language integration dates to Lisp 1.5 and there are several common approaches.
1. turn the optimizer off - this is easy to use and implement.
2. optimize the assembler block - Naughty Dog GOAL did this.
3. annotate the assembler block with pragmas that indicate side effects. This can be error prone and difficult to use. https://www.pvk.ca/Blog/2014/08/16/how-to-define-new-intrins... is an example of this approach.
Edited to add the SBCL intrinsic link.
I haven't used any lisp dialects for decades, so I have naive questions: is there really sufficient support from compilers+linkers to write a bootloader in lisp? Do I have to do a lot of bootstrapping in assembly to bring up lisp interpreter before I can execute the lisp code or does the ahead-of-time-build result in executable machine code? Can I do inline assembly (not required but a really key benefit IMO)? Are there numerous examples where someone's already written one in lisp?