NASM 2.05 based x86 Instruction Reference[ch327]
A.5.302 STMXCSR: Store Streaming SIMD Extension Control/Status STMXCSR m32 ; 0F AE /3 [KATMAI,SSE] STMXCSR stores the contents of the MXCSR control/status register to the specified memory location. MXCSR is used to enable masked/unmasked exception handling, to set rounding modes, to set flush-to-zero mode, and to view exception status flags. The reserved bits in the MXCSR register are stored as 0s. For details of the MXCSR register, see the Intel processor docs. See also LDMXCSR (section A.5.133).