Emacs Internal #02: Data First — Deconstructing Lisp_Object in C
Summary
The article presents a system-design view of GNU Emacs, focusing on Lisp_Object as a 64-bit tagged union and how tag bits are embedded in pointers. It explains the tagged pointer layout, fixnum representation, and the macros used to extract data, then maps McCarthy's Lisp axioms to the Emacs C implementation and discusses the broader data-first approach. The piece sets up a continuation exploring the tagged union pattern across languages in future posts.