Add commentary that we favor zero-extension unless SW demands otherwise

1 files changed, 10 insertions, 0 deletions

diff --git a/src/supervisor.tex b/src/supervisor.tex
index ca5502e..bd432a1 100644
--- a/src/supervisor.tex
+++ b/src/supervisor.tex
@@ -1302,6 +1302,16 @@ a page-fault exception will occur.  The 27-bit VPN is translated into a
 44-bit PPN via a three-level page table, while the 12-bit page offset
 is untranslated.
 
+\begin{commentary}
+When mapping between narrower and wider addresses, RISC-V usually
+zero-extends a narrower address to a wider size.  The mapping
+between 64-bit virtual addresses and the 39-bit usable address
+space of Sv39 is not based on zero-extension but instead follows an
+entrenched convention that allows an OS to use one or a few of the
+most-significant bits of a full-size (64-bit) virtual address to
+quickly distinguish user and supervisor address regions.
+\end{commentary}
+
 \begin{figure*}[h!]
 {\footnotesize
 \begin{center}