Clarifications re: writable XLEN

1 files changed, 28 insertions, 0 deletions

diff --git a/src/priv-csrs.tex b/src/priv-csrs.tex
index 003f5df..3dc2613 100644
--- a/src/priv-csrs.tex
+++ b/src/priv-csrs.tex
@@ -437,3 +437,31 @@ values to an \warl\ field.  Implementations must always
 deterministically return the same legal value after a given illegal
 value is written.
 
+\section{CSR Width Modulation}
+
+If the width of a CSR is changed (for example, by changing M-XLEN or U-XLEN, as
+described in Section~\ref{xlen-control}), the values of the {\em writable}
+fields and bits of the new-width CSR are, unless specified otherwise,
+determined from the previous-width CSR as though by this algorithm:
+
+\begin{enumerate}
+
+\item The value of the previous-width CSR is copied to a temporary register of
+the same width.
+
+\item For the read-only bits of the previous-width CSR, the bits at the same
+positions in the temporary register are set to zeros.
+
+\item The width of the temporary register is changed to the new width. If the
+new width $W$ is narrower than the previous width, the least-significant $W$
+bits of the temporary register are retained and the more-significant bits are
+discarded. If the new width is wider than the previous width, the temporary
+register is zero-extended to the wider width.
+
+\item Each writable field of the new-width CSR takes the value of the bits at
+the same positions in the temporary register.
+
+\end{enumerate}
+
+Changing the width of a CSR is not a read or write of the CSR and thus
+does not trigger any side effects.