The MMIX Supplement provides an in-depth look at the MMIX computer, a modern RISC-style architecture designed by Donald Knuth as a pedagogical tool for teaching assembly language and low-level programming concepts. This supplement explains the MMIX instruction set, registers, memory management, and programming techniques, connecting them to the algorithms and data structures discussed in The Art of Computer Programming. It is an essential resource for students, educators, and programmers who want to understand the practical implementation of algorithms on a real-world-inspired architecture.