The Unabridged Pentium 4 IA32 Processor Genealogy

Fixed the MXCSR Serialization Problem

The earlier versions of the Pentium® 4 processor did not serialize program execution when the contents of the x87 FP unit's FCW register was modified (to select a different rounding algorithm or a different precision). A simple prediction scheme was implemented to capture common renaming cases involving the FCW. Intel® documentation does not define the prediction scheme.

However, the same was not true of the MXCSR register. Loading a new value into this register caused program execution to be serialized.

The 90nm version of the processor detects common-case modifications to the MXCSR register (e.g., changing the rounding algorithm or precision) and ensures that they will not cause program execution to be ...