Register Usage
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The x64 architecture provides for 16 general-purpose registers (hereafter referred to as integer registers) as well as 16 XMM/YMM registers available for floating-point use. Volatile registers are scratch registers presumed by the caller to be destroyed across a call. Nonvolatile registers are required to retain their values across a function call and must be saved by the callee 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 needed by caller; used in syscall/sysret instructions |
| R12:R15 | Nonvolatile | Must be preserved by callee |
| RDI | Nonvolatile | Must be preserved by callee |
| RSI | Nonvolatile | Must be preserved by callee |
| RBX | Nonvolatile | Must be preserved by callee |
| RBP | Nonvolatile | May be used as a frame pointer; must be preserved by callee |
| RSP | Nonvolatile | Stack pointer |
| XMM0, YMM0 | Volatile | First FP argument; first vector-type argument when __vectorcall is used |
| XMM1, YMM1 | Volatile | Second FP argument; second vector-type argument when __vectorcall is used |
| XMM2, YMM2 | Volatile | Third FP argument; third vector-type argument when __vectorcall is used |
| XMM3, YMM3 | Volatile | Fourth FP argument; fourth vector-type argument when __vectorcall is used |
| XMM4, YMM4 | Volatile | Must be preserved as needed by caller; fifth vector-type argument when __vectorcall is used |
| XMM5, YMM5 | Volatile | Must be preserved as needed by caller; sixth vector-type argument when __vectorcall is used |
| XMM6:XMM15, YMM6:YMM15 | Nonvolatile (XMM), Volatile (upper half of YMM) | Must be preserved as needed by callee. YMM registers must be preserved as needed by caller. |