diff --git a/InstructionSets/68k/Decoder.cpp b/InstructionSets/68k/Decoder.cpp
index 9db1367c8..a0a8c72f8 100644
--- a/InstructionSets/68k/Decoder.cpp
+++ b/InstructionSets/68k/Decoder.cpp
@@ -92,8 +92,26 @@ constexpr Operation Predecoder<model>::operation(OpT op) {
 }
 
 template <Model model>
-template <uint8_t operation, bool validate> Preinstruction Predecoder<model>::validated(Preinstruction original) {
-	return original;
+template <uint8_t op, bool validate> Preinstruction Predecoder<model>::validated(Preinstruction original) {
+	if constexpr (!validate) {
+		return original;
+	}
+
+	switch(op) {
+		default: return original;
+
+		// NBCD: don't permit address registers
+		case OpT(Operation::NBCD):
+			switch(original.mode<0>()) {
+				default: return original;
+
+				case AddressingMode::AddressRegisterDirect:
+				case AddressingMode::ProgramCounterIndirectWithDisplacement:
+				case AddressingMode::ProgramCounterIndirectWithIndex8bitDisplacement:
+				case AddressingMode::ImmediateData:
+					return Preinstruction();
+			}
+	}
 }
 
 /// Decodes the fields within an instruction and constructs a `Preinstruction`, given that the operation has already been
diff --git a/OSBindings/Mac/Clock SignalTests/m68kDecoderTests.mm b/OSBindings/Mac/Clock SignalTests/m68kDecoderTests.mm
index 9df10b9c4..85a287482 100644
--- a/OSBindings/Mac/Clock SignalTests/m68kDecoderTests.mm	
+++ b/OSBindings/Mac/Clock SignalTests/m68kDecoderTests.mm	
@@ -86,7 +86,7 @@ template <int index> NSString *operand(Preinstruction instruction) {
 			case Operation::ABCD:		instruction = @"ABCD";	break;
 			case Operation::SBCD:		instruction = @"SBCD";	break;
 			case Operation::NBCD:		instruction = @"NBCD";	break;
-			case Operation::ADDb:		instruction = @"ADD.b";	break;
+//			case Operation::ADDb:		instruction = @"ADD.b";	break;
 
 			// For now, skip any unmapped operations.
 			default: continue;