Most of the code consists of small functions such as

    let load_prog_platter a = !platters.(0) <- a; a

for which comments would just get in the way. I don't know if you know OCaml, but that's roughly the same as

    plat load_prog_platter(plat a) {
        (*platters)[0] = a;
        return a;
    }

I don't think commenting that should be necessary.

He did comment one part, though: "(* we deliberately use Array.get to check bounds; use unsafe_ for speed *)".

(The only big function, exec, is an Opcode -> Action table.)