Microcode runs in most modern CPUs and translates the outer instruction set (e.g. x86) into a simpler form (usually a RISC architecture). It is updatable to fix bugs in the silicon (see Meltdown/Spectre), but these updates are encrypted and signed, so no one knows how microcode works on conventional CPUs. We successfully reverse engineered part of the microde semantics of AMD CPUs and are able to write our own programs. We also recovered the mapping between the physical readout (electron microscope) and the "virtual" addresses used by microcode itself. In this talk we present background on microcode, our findings, our open source framework to write custom microcode and our custom defensive measures implemented in microcode.