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| author | Felix Morgner <felix.morgner@gmail.com> | 2026-02-18 10:50:14 +0100 |
|---|---|---|
| committer | Felix Morgner <felix.morgner@gmail.com> | 2026-02-18 10:50:14 +0100 |
| commit | 63fd09807c528915ac5ba6bc9eb11520cbf073e0 (patch) | |
| tree | a2f795eeab115e51c167954c6397c289cb17ea26 | |
| parent | fc7e9b526e9e8b2cbf06772d4356700f6560c107 (diff) | |
| download | snake.s-63fd09807c528915ac5ba6bc9eb11520cbf073e0.tar.xz snake.s-63fd09807c528915ac5ba6bc9eb11520cbf073e0.zip | |
docs: document expected ABI
| -rw-r--r-- | docs/notes/0000_x86-64-system-v-abi.md | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/docs/notes/0000_x86-64-system-v-abi.md b/docs/notes/0000_x86-64-system-v-abi.md new file mode 100644 index 0000000..e7d2efe --- /dev/null +++ b/docs/notes/0000_x86-64-system-v-abi.md @@ -0,0 +1,63 @@ +# DevNote 0000: x86-64 System V ABI (Linux) + +This reference documents the System V AMD64 ABI used by Linux. + +## 1. General Purpose Registers (GPR) + +| Register | Role | Preservation | Notes | +| :------- | :----------------- | :----------- | :--------------------------------------------------- | +| `rax` | Return Value (1st) | Caller | Also used for syscall numbers. | +| `rbx` | Base / General | **Callee** | | +| `rcx` | Argument 4 | Caller | Used by `loop` instructions. | +| `rdx` | Argument 3 | Caller | Return value (2nd) for 128-bit types. | +| `rsi` | Argument 2 | Caller | | +| `rdi` | Argument 1 | Caller | | +| `rbp` | Frame Pointer | **Callee** | Optional if using stack-pointer relative addressing. | +| `rsp` | Stack Pointer | **Callee** | Must be preserved (balanced). | +| `r8` | Argument 5 | Caller | | +| `r9` | Argument 6 | Caller | | +| `r10` | Temporary | Caller | Used as Arg 4 in **syscalls**. | +| `r11` | Temporary | Caller | Clobbered by `syscall` instruction. | +| `r12` | General | **Callee** | | +| `r13` | General | **Callee** | | +| `r14` | General | **Callee** | | +| `r15` | General | **Callee** | | + +## 2. Function Calling Convention + +### Parameters +1. **Integers & Pointers**: Passed in registers `rdi`, `rsi`, `rdx`, `rcx`, `r8`, `r9`. +2. **Floating Point**: Passed in `xmm0` through `xmm7`. +3. **Stack Arguments**: Additional arguments are pushed onto the stack in **reverse order** (right-to-left). + +### Return Values +* **Integers/Pointers**: `rax`. +* **Floating Point**: `xmm0`. + +### Stack Alignment +The stack pointer (`rsp`) must be **16-byte aligned** before making a `call`. +* When a function is called, the return address is pushed, misaligning the stack by 8 bytes. +* Prologue usually pushes `rbp` (another 8 bytes), re-aligning it to 16 bytes. + +### The Red Zone +The 128 bytes below the stack pointer (`rsp - 128` to `rsp`) is reserved for the current function. Leaf functions (functions that call no other functions) can use this space without adjusting `rsp`. + +## 3. Kernel System Calls + +The convention for making syscalls differs slightly from function calls. + +* **Instruction**: `syscall` +* **Syscall Number**: `rax` +* **Return Value**: `rax` (Values between -4095 and -1 indicate error). +* **Clobbered**: `rcx`, `r11` (and `rax` for return). + +**Syscall Arguments:** + +| Arg | Register | Note | +| :-- | :-------- | :--------------------------------------- | +| 1 | `rdi` | Same as function call | +| 2 | `rsi` | Same as function call | +| 3 | `rdx` | Same as function call | +| 4 | **`r10`** | **Different** (Function calls use `rcx`) | +| 5 | `r8` | Same as function call | +| 6 | `r9` | Same as function call |
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