[otbn] Fix CSRRW in ISS when destination is x0
This was (wrongly) skipping the instruction entirely when the
destination is x0, rather than just skipping the read.
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/dv/otbnsim/sim/insn.py b/hw/ip/otbn/dv/otbnsim/sim/insn.py
index d25bcd4..6b9ce2f 100644
--- a/hw/ip/otbn/dv/otbnsim/sim/insn.py
+++ b/hw/ip/otbn/dv/otbnsim/sim/insn.py
@@ -291,13 +291,12 @@
self.grs1 = op_vals['grs1']
def execute(self, state: OTBNState) -> None:
- if self.grd == 0:
- return
-
- old_val = state.read_csr(self.csr)
new_val = state.gprs.get_reg(self.grs1).read_unsigned()
- state.gprs.get_reg(self.grd).write_unsigned(old_val)
+ if self.grd != 0:
+ old_val = state.read_csr(self.csr)
+ state.gprs.get_reg(self.grd).write_unsigned(old_val)
+
state.write_csr(self.csr, new_val)
