From 10700d0766501c9a4ce16c7cab53c908c44d5158 Mon Sep 17 00:00:00 2001
From: Andrew Waterman <andrew@sifive.com>
Date: Tue, 20 Feb 2024 18:07:55 -0800
Subject: Update Smrnmi to account for Smdbltrp extension

This is a backwards compatible change to support resumable double
traps within M-mode when both extensions are implemented.  Although
the resumability feature is not terribly interesting, the reuse of
the Smrnmi state for M-mode double traps aids debugging (and fault
diagnosis for the hardware-error exception case).
---
 src/rnmi.adoc | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

(limited to 'src/rnmi.adoc')

diff --git a/src/rnmi.adoc b/src/rnmi.adoc
index 9938917..cbd19da 100644
--- a/src/rnmi.adoc
+++ b/src/rnmi.adoc
@@ -71,9 +71,15 @@ of holding.
 .Resumable NMI cause `mncause`.
 include::images/bytefield/mncause.edn[]
 
-The `mncause` CSR holds the reason for the NMI, with bit MXLEN-1 set to
-1, and the NMI cause encoded in the least-significant bits or zero if
-NMI causes are not supported.
+The `mncause` CSR holds the reason for the NMI.
+If the reason is an interrupt, bit MXLEN-1 is set to 1, and the NMI
+cause is encoded in the least-significant bits.
+If the reason is an interrupt and NMI causes are not supported, bit MXLEN-1 is
+set to 1, and zero is written to the least-significant bits.
+If the reason is an exception within M-mode that results in a double trap as
+specified in the Smdbltrp extension, bit MXLEN-1 is set to 0 and the
+least-significant bits are set to the cause code corresponding to the
+exception that precipitated the double trap.
 
 .Resumable NMI status register `mnstatus`.
 include::images/bytefield/mnstatus.edn[]
-- 
cgit v1.1

