Windows Driver Kit: Kernel-Mode Driver Architecture
Register Usage for x64 64-Bit
The x64 architecture provides for 16 general-purpose registers (also called integer registers) as well as 16 XMM registers available for floating-point use. Volatile registers are scratch registers that the user can assume are destroyed across a call. Nonvolatile registers are required to retain their values across a function call and must be saved by the called function if used. The following table describes how each register is used across function calls.
| Register | Status | Use |
|---|
| RAX | Volatile | Return value register. |
| RCX | Volatile | First integer argument. |
| RDX | Volatile | Second integer argument. |
| R8 | Volatile | Third integer argument. |
| R9 | Volatile | Fourth integer argument. |
| R10:R11 | Volatile | Must be preserved as required by caller; used in syscall/sysret instructions. |
| R12:R15 | Nonvolatile | Must be preserved by called function. |
| RDI | Nonvolatile | Must be preserved by called function. |
| RSI | Nonvolatile | Must be preserved by called function. |
| RBX | Nonvolatile | Must be preserved by called function. |
| RBP | Nonvolatile | Can be used as a frame pointer. Must be preserved by called function. |
| RSP | Nonvolatile | Stack Pointer. |
| XMM0 | Volatile | First FP argument. |
| XMM1 | Volatile | Second FP argument. |
| XMM2 | Volatile | Third FP argument. |
| XMM3 | Volatile | Fourth FP argument. |
| XMM4:XMM5 | Volatile | Must be preserved as required by caller. |
| XMM6:XMM15 | Nonvolatile | Must be preserved as required by called function. |