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 asmskills/low-level-programming/assembly-arm— AAPCS examplesskills/low-level-programming/assembly-riscv— RISC-V asmskills/rust/rust-ffi—extern "C"guaranteesskills/binaries/elf-inspection— symbol and relocation views