Metasm is an open source ruby framework developed by Yoann Guillot. It can: work with binary files, assemble, disassemble, debug running processes, manipulate C source code, play the ruby interpreter, and plenty other things. Our talk follows a simple guideline based on a real life case: the development of a code tracer. Starting from a trivial tracing algorithm, we will show that the use of Metasm allows to efficiently build a multi-platforms tool, then we will extend its capacity by taking advantages of the native Windows API. At the end we will use the tool to debug the firmware of a network card, running on the NIC and not on the main CPU.