The Foreign Function & Memory API (JEP 454) introduced a new way of interacting with libraries written in other languages. It can be used as replacement for JNI. This talk examines the inner workings of the Foreign Function Interface (FFI) API. After having it implemented for PowerPC, I'd like to discuss how the JVM handles native function calls and Java callbacks, focusing on key concepts like stack layouts, calling conventions, and cross-platform challenges. The session is intended for developers curious about the technical foundations of free and open Java technologies and how JVM enhancements like the FFI API are realized. It will shed light on the low-level mechanisms that enable seamless integration of Java with native code while maintaining the performance and safety Java developers expect.