CPU 13 AArch64 Calling Convention

Inside the book

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• CPU Programming Series AArch64 Calling Convention (AAPCS64) C/C++ Interoperability in Practice Prepared by Ayman Alheraki simplifycpp.org January 2026 Contents Contents 2 Preface 14 Purpose of This Booklet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 What You Will Be Able to Do After This Booklet . . . . . . . . . . . . . . . . 15 Minimal “Hello ABI” Example . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Where AAPCS64 Fits in the CPU Programming Series . . . . . . . . . . . . . . . . 16 Why This Booklet Exists (Even If You “Know Assembly”) . . . . . . . . . . . 16 A Quick Map to Neighbor Booklets . . . . . . . . . . . . . . . . . . . . . . . 17 Prerequisites and Assumed Knowledge . . . . . . . . . . . . . . . . . . . . . . . . . 17 Required C/C++ Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Tooling Assumptions (Minimal) . . . . . . . . . . . . . . . . . . . . . . . . . 18 Scope, Limits, and ABI Discipline Philosophy . . . . . . . . . . . . . . . . . . . . . 18 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Out of Scope (By Design) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 ABI Discipline Philosophy: “Correct First, Fast Second” . . . . . . . . . . . . 19 A “Wrong but Looks Fine” Example (Register Preservation Bug) . . . . . . . . 19 A Minimal Non-Leaf Template (Calls Another Function) . . . . . . . . . . . . 20 2 3 1 AAPCS64 Overview and Design Goals 22 1.1 What a Calling Convention Really Defines . . . . . . . . . . . . . . . . . . . . 22 1.1.1 The Four Things Eve