Understand x86-64 System V ABI Calling Conventions at CPU Level

System V AMD64 ABI

On Linux and macOS, compiled C functions follow the System V AMD64 ABI. The first six integer or pointer arguments arrive in registers; additional args sit on the stack. Return values land in RAX (integer) or XMM0 (float).

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Stack and Saved Registers

• Caller-saved: RAX, RCX, RDX, RSI, RDI, R8-R11 (scratch)
• Callee-saved: RBX, RBP, R12-R15 (preserve if used)
• Stack: must stay 16-byte aligned before CALL

For this exercise, you will trace a function call in assembly and label which registers hold arguments and return values. This task asks you to read the ABI the way Ghidra does when it recovers main's parameters from a stripped binary.

Working Through the Exercise

Keep the relevant datasheet, ISA manual, or architecture textbook chapter open while you implement. When your output disagrees with the reference trace on the same program, the bug is usually a mis-decoded opcode, a stale register read, or a flag bit left unchanged after arithmetic.

• Trace first: walk one failing input by hand before changing random lines
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

For this exercise, you will use those habits while implementing the requirement in the starter code. Microarchitectural product names change across CPU generations, but the control ideas (fetch, bypass, cache lines, vector lanes) stay stable enough to debug from first principles.