[EntrySign](https://bughunters.google.com/blog/5424842357473280/zen-and-the-art-of-microcode-hacking) opened the door to custom microcode on AMD Zen CPUs earlier this year. Using a weakness in the signature verification we can load custom microcode updates and modify behavior of stock AMD Zen 1-5 CPUs. While AMD has released patches to address this weakness on some CPUs, we can still use unpatched systems for our analysis. In this talk we cover what we found out about microcode, what we saw in the microcode ROM, the tooling we build, how we worked to find out more and how you can write & test your own microcode on your own AMD Zen systems. We have our tools up on [GitHub](https://github.com/AngryUEFI) for everyone to play around with and hopefully help us understand microcode more than we currently do.