Register Usage

Visual Studio 2015

Other Versions

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.