Improve RV64 32x32->64 text; move to commentary

1 files changed, 9 insertions, 4 deletions

diff --git a/src/m.tex b/src/m.tex
index 66c11f8..9f0ee4c 100644
--- a/src/m.tex
+++ b/src/m.tex
@@ -60,10 +60,15 @@ with the less-significant words of the multiplicand (which are unsigned).
 
 MULW is an RV64 instruction that multiplies the lower 32 bits of the source
 registers, placing the sign-extension of the lower 32 bits of the result
-into the destination register.  In RV64, MUL can be used to obtain the upper
-32 bits of the 64-bit product, but signed arguments must be proper 32-bit
-signed values, whereas unsigned arguments must have their upper 32 bits
-clear.
+into the destination register.
+
+\begin{commentary}
+In RV64, MUL can be used to obtain the upper 32 bits of the 64-bit product,
+but signed arguments must be proper 32-bit signed values, whereas unsigned
+arguments must have their upper 32 bits clear.  If the
+arguments are not known to be sign- or zero-extended, an alternative is to
+shift both arguments left by 32 bits, then use MULH[[S]U].
+\end{commentary}
 
 \section{Division Operations}