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# 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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