stval is optionally written with bad instruction, as with mtval

1 files changed, 14 insertions, 0 deletions

diff --git a/src/supervisor.tex b/src/supervisor.tex
index 04104f4..0f03772 100644
--- a/src/supervisor.tex
+++ b/src/supervisor.tex
@@ -597,6 +597,20 @@ variable-length instructions, {\tt stval} will point to the portion
 of the instruction that caused the fault while {\tt sepc} will point
 to the beginning of the instruction.
 
+The {\tt stval} register can optionally also be used to return the
+faulting instruction bits on an illegal instruction exception ({\tt
+  sepc} points to the faulting instruction in memory).
+
+If this feature is not provided, then {\tt stval} is set to zero on
+an illegal instruction fault.
+
+If the feature is provided, after an illegal instruction trap, {\tt
+  stval} will contain the entire faulting instruction provided the
+instruction is no longer than XLEN bits.  If the instruction is less
+than XLEN bits long, the upper bits of {\tt stval} are cleared to
+zero. If the instruction is more than XLEN bits long, {\tt stval}
+will contain the first XLEN bits of the instruction.
+
 \subsection{Supervisor Address Translation and Protection ({\tt satp}) Register}
 \label{sec:satp}