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