The System V AMD64 ABI you're reading here is the exact contract that GCC, Clang, and every Linux or macOS shared library rely on to call into each other's compiled code without sharing a source language; getting it wrong is why calling-convention mismatches corrupt the stack in mixed-language FFI code. Reading this ABI is also literally step one of analyzing any stripped x86-64 binary in Ghidra or IDA Pro.