Added commentary on purpose of static rounding modes.

src/f.tex

@@ -164,7 +164,12 @@ \section{Floating-Point Control and Status Register}
 
 \begin{commentary}
 The C99 language standard effectively mandates the provision of a
-dynamic rounding mode register.
+dynamic rounding mode register.  In typical implementations, writes to
+the dynamic rounding mode CSR state will serialize the pipeline.
+
+Static rounding modes are used to implement specialized arithmetic
+operations that often have to switch frequently between different
+rounding modes.
 \end{commentary}
 \newpage