abi-and-calling-conventions

name	abi-and-calling-conventions
description	ABI and calling conventions skill for cross-language boundaries. Use when explaining System V AMD64, ARM AAPCS, RISC-V psABI, stack frames, variadic calls, or FFI register rules. Activates on queries about calling convention, ABI, System V AMD64, AAPCS, stack frame, variadic function, or FFI registers.

ABI and Calling Conventions

Purpose

Document application binary interface (ABI) rules: register roles, stack alignment, argument passing, return values, and variadic conventions across System V AMD64, ARM AAPCS, and RISC-V — essential for assembly, FFI, and debugging.

When to Use

• Writing assembly thunks or inline asm clobbers
• Debugging corrupted stack in mixed C/asm
• FFI between Rust/C/Zig (skills/rust/rust-ffi, skills/zig/zig-cinterop)
• Reading disassembly from skills/debuggers/gdb

Workflow

1. System V AMD64 (Linux/macOS)

Item	Rule
Integer args	rdi, rsi, rdx, rcx, r8, r9
XMM args	xmm0–xmm7 (float)
Return int/ptr	rax (+ rdx for 128-bit)
Stack alignment	16-byte before call
Red zone	128 bytes below rsp (Linux)
Callee-saved	rbx, rbp, r12–r15

/* void foo(int a, int b, int c, int d, int e, int f, int g); */