If this is not your first RubyKaigi, you may know that Ruby has been ported to many game consoles. But how about to NES? In this session, I will explain how I built nesruby — a tiny Ruby interpreter that runs on the NES, inspired by mruby/c. I'll cover the strategy of reusing mruby's bytecode format, the VM implementation in C using cc65, and some interesting internals such as the Symbol class implementation. Note: this talk focuses on the Ruby interpreter side. NES-specific topics (graphics hardware quirks, PPU, etc.) are out of scope, so no prior NES knowledge is required.