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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.

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