Very few Emacs users—no matter how advanced—ever need to worry about the specifics of the Elisp bytecode, or even, for that matter, that it exists. Still, as guys like Chris Wellons have shown, it can sometimes be useful to have a basic understanding of the bytecodes.

R Bernstein has put together a comprehensive, book-length documentation on Elisp bytecodes. After a short introduction, the documentation considers the bytecode environment including the compiler, interpreter, and bytecode optimization. Then there’s a long section on the individual bytecode instructions.

Finally, there are sections on the changes in bytecodes between Emacs versions, a table of opcodes, and a reference section. There’s also a GitHub repository of the document source.

As I said, you probably will never need this but if you do, you’ll be very glad to have Bernstein’s documentation. It’s another example of the vibrant Emacs community.