diff --git a/cpu/decoder.cpp b/cpu/decoder.cpp
index b47702b..7c6699f 100644
--- a/cpu/decoder.cpp
+++ b/cpu/decoder.cpp
@@ -300,7 +300,22 @@ void Cpu::executeInstruction()
     {
       switch(op)
         {
-        case 0x00: break; // NOP
+        case 0xD3: // Undefined, treat as NOP
+        case 0xE3:
+        case 0xE4:
+        case 0xF4:
+        case 0xDB:
+        case 0xEB:
+        case 0xEC:
+        case 0xFC:
+        case 0xDD:
+        case 0xED:
+        case 0xFD:
+          break;
+
+        case 0x00: // defined NOP
+          break;
+
         case 0x0A: // Load A, [BC]
           state.A = bus->read8(state.BC);
           mcycles = 2;