Abi And Calling Conventions

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)

/* void foo(int a, int b, int c, int d, int e, int f, int g); */
/* a–f in regs, g on stack */

Windows x64 differs — see skills/compilers/msvc-cl.

2. ARM AAPCS (AArch32/AArch64)

AArch64:

Thumb interworking: LSB of function pointer set for Thumb code.

See skills/low-level-programming/assembly-arm.

3. RISC-V psABI (RV64)

See skills/low-level-programming/assembly-riscv.

4. Stack frame layout (conceptual)

high addresses
├── return address
├── saved frame pointer
├── local variables
├── spill slots / alignment padding
└── outgoing args (if any)
low addresses (rsp)

5. Variadic functions

System V: al holds number of vector args used; register_save_area on stack for va_start. Prefer typed wrappers over raw va_arg in portable FFI.

6. Verify in compiler output

gcc -O2 -S -o - foo.c   # study .cfi_* and mov args
objdump -d -M intel ./a.out

7. Agent usage

/abi-and-calling-conventions Which registers hold args 5–8 in AArch64 for my C function?

Common Problems

Related Skills

• skills/low-level-programming/assembly-x86 — x86-64 asm
• skills/low-level-programming/assembly-arm — AAPCS examples
• skills/low-level-programming/assembly-riscv — RISC-V asm
• skills/rust/rust-ffi — extern "C" guarantees
• skills/binaries/elf-inspection — symbol and relocation views