There was a video of him talking about it, but I can't recall the exact words or the name of the video. But his operating system, TempleOS, was based on HolyC

There was a video of him talking about it, but I can't recall the exact words or the name of the video. But his operating system, TempleOS, was based on HolyC which was compiled to pure x64, and had support for inline x64 assembly.