The x64 architecture provides for 16 general-purpose registers (hereafter referred to as integer registers) as well as 16 XMM 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 | 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 needed by caller |
XMM6:XMM15 | Nonvolatile | Must be preserved as needed by callee. |
See Also