This article delves into the internal memory management techniques employed by GNU Emacs, focusing on how it represents Lisp values. Emacs uses a 64-bit slot, `Lisp_Object`, to hold various data types like integers and strings. By utilizing the least significant bits of a pointer, Emacs implements a form of tagged pointers. This approach optimizes memory usage by reclaiming space, a critical consideration given the constraints of early computing.

Tagged unions, like C++'s `std::variant`, offer type safety but can lead to memory inefficiency because the size of the structure is determined by the largest element. In contrast, tagged pointers keep data on the heap, using the lower bits of the pointer as a type tag. This method provides several possible types without allocating inline bytes. However, it's limited to only 8 fundamental types.

For more complex scenarios, fat pointers are used, wherein additional space is allocated to store tag information. This results in doubled memory size compared to a 64-bit tagged pointer. Emacs, however, sticks to the 64-bit word design. The engineering choice reflects the resource-conscious era when a typical workstation had just 256 KB of RAM. Emacs bridges statically typed C to dynamically typed Lisp.

Ultimately, Emacs's design prioritizes memory efficiency and leverages techniques like tagged pointers and unions to manage diverse data types within the confines of a single machine word. Understanding these internal mechanisms is crucial for appreciating the performance characteristics of Emacs and its ability to handle complex data structures efficiently, even with the memory constraints of the past.