Explain when sideleg/sedeleg must exist

Closes #366

1 files changed, 6 insertions, 0 deletions

diff --git a/src/machine.tex b/src/machine.tex
index 41298c3..a7a3ba6 100644
--- a/src/machine.tex
+++ b/src/machine.tex
@@ -1093,10 +1093,16 @@ S-mode or U-mode to the S-mode trap handler. If U-mode traps are
 supported, S-mode may in turn set corresponding bits in the {\tt
   sedeleg} and {\tt sideleg} registers to delegate traps that occur in
 U-mode to the U-mode trap handler.
+In systems with S-mode, the {\tt medeleg} and {\tt mideleg} registers must
+exist, whereas the {\tt sedeleg} and {\tt sideleg} registers should only exist
+if the N extension for user-mode interrupts is also implemented.
 
 In systems with two privilege modes (M/U) and support for U-mode
 traps, setting a bit in {\tt medeleg} or {\tt mideleg} will
 delegate the corresponding trap in U-mode to the U-mode trap handler.
+In systems with only M-mode and U-mode, the {\tt medeleg} and {\tt mideleg}
+registers should only be implemented if the N extension for user-mode
+interrupts is implemented.
 
 In systems with only M-mode, or with both M-mode and U-mode but
 without U-mode trap support, the {\tt medeleg} and {\tt mideleg}