diff --git a/OSBindings/Mac/Clock SignalTests/6522Tests.swift b/OSBindings/Mac/Clock SignalTests/6522Tests.swift
index 66617a990..41100aacc 100644
--- a/OSBindings/Mac/Clock SignalTests/6522Tests.swift	
+++ b/OSBindings/Mac/Clock SignalTests/6522Tests.swift	
@@ -16,6 +16,8 @@ class MOS6522Tests: XCTestCase {
 		action(bridge)
 	}
 
+	// MARK: Timer tests
+
 	func testTimerCount() {
 		with6522 {
 			// set timer 1 to a value of $000a
@@ -90,4 +92,22 @@ class MOS6522Tests: XCTestCase {
 			XCTAssert($0.valueForRegister(5) == 0x00, "High order byte should be 0x00; was \($0.valueForRegister(5))")
 		}
 	}
+
+
+	// MARK: Data direction tests
+	func testDataDirection() {
+		with6522 {
+			// set low four bits of register B as output, the top four as input
+			$0.setValue(0x0f, forRegister: 2)
+
+			// ask to output 0x8c
+			$0.setValue(0x8c, forRegister: 0)
+
+			// set current input as 0xda
+			$0.portBInput = 0xda
+
+			// test that the result of reading register B is therefore 0x8a
+			XCTAssert($0.valueForRegister(0) == 0x8a, "Data direction register should mix input and output; got \($0.valueForRegister(0))")
+		}
+	}
 }
diff --git a/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.h b/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.h
index d2bed3fe8..b5110ae86 100644
--- a/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.h	
+++ b/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.h	
@@ -11,6 +11,8 @@
 @interface MOS6522Bridge : NSObject
 
 @property (nonatomic, readonly) BOOL irqLine;
+@property (nonatomic) uint8_t portBInput;
+@property (nonatomic) uint8_t portAInput;
 
 - (void)setValue:(uint8_t)value forRegister:(NSUInteger)registerNumber;
 - (uint8_t)valueForRegister:(NSUInteger)registerNumber;
diff --git a/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.mm b/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.mm
index e47ccc844..8dd444a05 100644
--- a/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.mm	
+++ b/OSBindings/Mac/Clock SignalTests/MOS6522Bridge.mm	
@@ -15,11 +15,18 @@ class VanillaVIA: public MOS::MOS6522<VanillaVIA> {
 	public:
 		MOS6522Bridge *bridge;
 		bool irq_line;
+		uint8_t port_a_value;
+		uint8_t port_b_value;
 
 		void set_interrupt_status(bool new_status)
 		{
 			irq_line = new_status;
 		}
+
+		uint8_t get_port_input(int port)
+		{
+			return port ? port_b_value : port_a_value;
+		}
 };
 
 @implementation MOS6522Bridge
@@ -57,4 +64,24 @@ class VanillaVIA: public MOS::MOS6522<VanillaVIA> {
 	return _via.irq_line;
 }
 
+- (void)setPortAInput:(uint8_t)portAInput
+{
+	_via.port_a_value = portAInput;
+}
+
+- (uint8_t)portAInput
+{
+	return _via.port_a_value;
+}
+
+- (void)setPortBInput:(uint8_t)portBInput
+{
+	_via.port_b_value = portBInput;
+}
+
+- (uint8_t)portBInput
+{
+	return _via.port_b_value;
+}
+
 @end