From b45eb6247ddd8ef31a045ead21c659e7c5ecef44 Mon Sep 17 00:00:00 2001
From: Christian Speckner <christian.speckner@mayflower.de>
Date: Sat, 10 Aug 2024 09:29:23 +0200
Subject: [PATCH] Trap if add would switch instruction set.

---
 src/emucore/CortexM0.cxx | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/emucore/CortexM0.cxx b/src/emucore/CortexM0.cxx
index eeb6953cf..961ec562a 100644
--- a/src/emucore/CortexM0.cxx
+++ b/src/emucore/CortexM0.cxx
@@ -999,6 +999,9 @@ CortexM0::err_t CortexM0::execute(uInt16 inst, uInt8 op)
       rc = ra + rb;
       if(rd == 15)
       {
+        if ((rc & 1) == 0)
+          return errIntrinsic(ERR_INVALID_OPERATING_MODE, read_register(15) - 4);
+
         rc &= ~1; //write_register may f this as well
         rc += 2;  //The program counter is special
       }