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)

| 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); */
/* a–f in regs, g on stack */

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

2. ARM AAPCS (AArch32/AArch64)

AArch64:

| Item | Rule | |------|------| | Integer args | x0–x7 | | Float args | v0–v7 | | Return | x0/x1 or v0 | | Stack align | 16-byte | | Callee-saved | x19–x28, fp (x29), lr (x30) |

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

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

3. RISC-V psABI (RV64)

| Item | Rule | |------|------| | Integer args | a0–a7 | | Callee-saved | s0–s11, sp | | Return | a0, a1 | | Stack align | 16-byte |

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

| Symptom | Cause | Fix | |---------|-------|-----| | Crash on call | Stack misaligned | Align rsp mod 16 | | Wrong float arg | XMM vs GPR mismatch | Match prototype | | Corrupt callee-saved | Asm clobber missing | List in clobber or save | | FFI garbage | Windows vs SysV mix | Match toolchain ABI | | Variadic UB | Wrong type to va_arg | Cast to promoted type |

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

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: mohitmishra786
• Source: mohitmishra786/low-level-dev-skills
• License: MIT
• Homepage: https://www.lowleveldevskills.com

Install and usage instructions live in the source repository linked above.