MXCSR

MXCSR stands for the Multimedia Extensions Control and Status Register. It is a 32-bit register used by the x86 SSE (Streaming SIMD Extensions) floating-point unit to control and report the behavior of floating-point operations performed by SSE instructions. The register complements the legacy x87 floating-point unit by providing explicit control over SIMD floating-point handling.

The MXCSR consists of control fields and status flags. The control fields include six exception-masks that determine whether common floating-point exceptions (such as invalid operation, denormal operand, division by zero, overflow, underflow, and inexact result) generate traps or are masked. It also contains a denormals-are-zero bit and a flush-to-zero bit that affect how denormal numbers are treated, and a rounding-control field that selects the operating rounding mode (such as round-to-nearest, toward zero, toward positive infinity, or toward negative infinity). Together, these fields let software tailor precision, performance, and robustness for numeric workloads.

The status portion of MXCSR records the occurrence of floating-point exceptions, enabling software to poll or react to exception events after they happen. The two parts of the register are designed so that exceptions can be masked (not signaled) while still updating the status bits for later inspection.

Accessing and manipulating MXCSR is done with specific instructions such as LDMXCSR and STMXCSR, or with the FXSAVE/FXRSTOR and XSAVE family to save and restore processor state across context switches. The MXCSR is distinct from the x87 control word and is specific to SSE-enabled floating-point operations.

In practice, MXCSR provides fine-grained, software-controllable handling of floating-point behavior for modern vectorized workloads, enabling predictable numerical results and performance optimizations.