MxCsr
The register state also includes MxCsr. The calling convention divides this register into a volatile portion and a nonvolatile portion. The volatile portion consists of the 6 status flags, MXCSR[0:5], while the remainder of the register, MXCSR[6:15], is considered nonvolatile.
The nonvolatile portion is set to the following standard values at the start of program execution:
MXCSR[6] : Denormals are zeros - 0
MXCSR[7:12] : Exception masks all 1's (all exceptions masked)
MXCSR[13:14] : Rounding control - 0 (round to nearest)
MXCSR[15] : Flush to zero for masked underflow - 0 (off)
A callee that modifies any of the nonvolatile fields within MXCSR must restore them before returning to its caller. Furthermore, a caller that has modified any of these fields must restore them to their standard values before invoking a callee unless by agreement the callee expects the modified values.
There are two exceptions to the rules regarding the non-volatility of the control flags:
In functions where the documented purpose of the given function is to modify the nonvolatile MxCsr flags.
When it is provably correct that the violation of these rules results in a programs that behaves/means the same as a program where these rules are not violated, for example, through whole-program analysis.
No assumptions can be made about the state of the volatile portion of MXCSR across a function boundary, unless specifically described in a function's documentation.