Bump version to 0.9.1.0

README.md

@@ -3,7 +3,7 @@ SYMON - A 6502 System Simulator
 
 **NOTE: THIS SOFTWARE IS UNDER ACTIVE DEVELOPMENT. Feedback is welcome!**
 
-**Version:** 0.9.0.1
+**Version:** 0.9.1.0
 
 **Last Updated:** 29 December, 2013
 
@@ -154,7 +154,7 @@ Maven will build Symon, run unit tests, and produce a jar file in the
 Symon is meant to be invoked directly from the jar file. To run with
 Java 1.5 or greater, just type:
 
-    $ java -jar symon-0.9.0.jar
+    $ java -jar symon-0.9.1.0.jar
 
 When Symon is running, you should be presented with a simple graphical
 interface.
@@ -196,6 +196,8 @@ running.
 
 ## 5.0 Revision History
 
+  - **0.9.1:** 26 January, 2014 - Support for IRQ and NMI handling.
+
   - **0.9.0:** 29 December, 2013 - First pass at a 6545 CRTC simulation.
 
   - **0.8.5:** 30 March, 2013 - ASCII display for memory window.
@@ -233,8 +235,6 @@ running.
     and disassembly.
 
   - More accurate timing.
-  
-  - Smarter interrupt handling.
 
   - UI needs a ton more polish.
 

pom.xml

@@ -4,7 +4,7 @@
     <groupId>com.loomcom.symon</groupId>
     <artifactId>symon</artifactId>
     <packaging>jar</packaging>
-    <version>0.9.0.1</version>
+    <version>0.9.1.0</version>
     <name>symon</name>
     <url>http://www.loomcom.com/symon</url>
     <properties>

samples/README

@@ -6,18 +6,22 @@ Sample Programs
   When loaded at address $0300, this program will print "Hello, 6502 World!" in
   infinite loop.
 
-2. echo.prg
+2. echo_poll.prg
 
   When loaded at address $0300, this program will echo back to the console
   anything typed.
 
 
-Both hello.prg and echo.prg were assembled with the Ophis assembler:
+Both hello.prg and echo_poll.prg were assembled with the Ophis assembler:
 
    https://hkn.eecs.berkeley.edu/~mcmartin/ophis/
 
+3. echo_irq.rom
 
-3. ehbasic.rom
+  This is another echo program, and behaves identically to echo_poll.prg,
+  except it is interrupt-driven.
+
+4. ehbasic.rom
 
    This is Lee Davison's Enhanced 6502 BASIC.
 

samples/echo_irq/.gitignore

@@ -0,0 +1,4 @@
+*.rom
+*.lst
+*.map
+*.o

samples/echo_irq/Makefile

@@ -0,0 +1,13 @@
+CA=ca65
+LD=ld65
+
+all: echo_irq
+
+echo_irq: echo_irq.o
+	$(LD) -C symon.config -vm -m echo_irq.map -o echo_irq.rom echo_irq.o
+
+echo_irq.o:
+	$(CA) --listing -o echo_irq.o echo_irq.asm
+
+clean:
+	rm -f *.o *.rom *.map *.lst

samples/echo_irq/echo_irq.asm

@@ -0,0 +1,48 @@
+;;
+;; Read input from the keyboard, and echo to console.
+;;
+
+
+IOBASE   = $8800
+IOSTATUS = IOBASE + 1
+IOCMD    = IOBASE + 2
+IOCTRL   = IOBASE + 3
+
+.segment "CODE"
+.org $C000
+
+START:  CLI
+        LDA #$09
+        STA IOCMD      ; Set command status
+        LDA #$1A
+        STA IOCTRL     ; 0 stop bits, 8 bit word, 2400 baud
+
+;;
+;; Infinite idle loop, waiting for interrupt.
+;;
+IDLE:   LDA #$FF
+        TAX
+IDLE1:  DEX
+        BNE IDLE1
+        JMP IDLE
+
+;;
+;; IRQ handler
+;;
+IRQ:    LDA IOBASE      ; Get the character in the ACIA.
+        PHA             ; Save accumulator
+ECHO1:  LDA IOSTATUS    ; Read the ACIA status
+        AND #$10        ; Is the tx register empty?
+        BEQ ECHO1       ; No, wait for it to empty
+        PLA             ; Otherwise, load saved accumulator,
+        STA IOBASE      ; write to output,
+        RTI             ; and return
+
+; system vectors
+
+.segment "VECTORS"
+.org    $FFFA
+
+.word   IRQ         ; NMI vector
+.word   START       ; RESET vector
+.word   IRQ         ; IRQ vector

samples/echo_irq/symon.config

@@ -0,0 +1,16 @@
+MEMORY {
+RAM1: start = $0000, size = $8000;
+ROM1: start = $C000, size = $3FFA, fill = yes;
+ROMV: start = $FFFA, size = $6, file = %O, fill = yes;
+}
+
+SEGMENTS {
+CODE:     load = ROM1, type = ro;
+DATA:     load = ROM1, type = ro;
+VECTORS:  load = ROMV, type = ro;
+}
+
+SYMBOLS {
+__STACKSIZE__ = $0300;
+}
+

src/main/java/com/loomcom/symon/Bus.java

@@ -151,6 +151,30 @@ public class Bus {
         throw new MemoryAccessException("Bus write failed. No device at address " + String.format("$%04X", address));
     }
 
+    public void assertIrq() {
+        if (cpu != null) {
+            cpu.assertIrq();
+        }
+    }
+
+    public void clearIrq() {
+        if (cpu != null) {
+            cpu.clearIrq();
+        }
+    }
+
+    public void assertNmi() {
+        if (cpu != null) {
+            cpu.assertNmi();
+        }
+    }
+
+    public void clearNmi() {
+        if (cpu != null) {
+            cpu.clearNmi();
+        }
+    }
+
     public SortedSet<Device> getDevices() {
         // Expose a copy of the device list, not the original
         return new TreeSet<Device>(devices);

src/main/java/com/loomcom/symon/Cpu.java

@@ -113,7 +113,9 @@ public class Cpu implements InstructionTable {
      * Reset the CPU to known initial values.
      */
     public void reset() throws MemoryAccessException {
-        // Registers
+        /* TODO: In reality, the stack pointer could be anywhere
+           on the stack after reset. This non-deterministic behavior might be
+           worth while to simulate. */
         state.sp = 0xff;
 
         // Set the PC to the address stored in the reset vector
@@ -131,6 +133,8 @@ public class Cpu implements InstructionTable {
         state.overflowFlag = false;
         state.negativeFlag = false;
 
+        state.irqAsserted = false;
+
         // Clear illegal opcode trap.
         state.opTrap = false;
 
@@ -150,12 +154,20 @@ public class Cpu implements InstructionTable {
     }
 
     /**
-     * Performs an individual machine cycle.
+     * Performs an individual instruction cycle.
      */
     public void step() throws MemoryAccessException {
         // Store the address from which the IR was read, for debugging
         state.lastPc = state.pc;
 
+        // Check for Interrupts before doing anything else.
+        // This will set the PC and jump to the interrupt vector.
+        if (state.nmiAsserted) {
+            handleNmi();
+        } else if (state.irqAsserted && !getIrqDisableFlag()) {
+            handleIrq(state.pc);
+        }
+
         // Fetch memory location for this instruction.
         state.ir = bus.read(state.pc);
         irAddressMode = (state.ir >> 2) & 0x07;
@@ -247,16 +259,7 @@ public class Cpu implements InstructionTable {
             /** Single Byte Instructions; Implied and Relative **/
             case 0x00: // BRK - Force Interrupt - Implied
                 if (!getIrqDisableFlag()) {
-                    // Set the break flag before pushing.
-                    setBreakFlag();
-                    // Push program counter + 1 onto the stack
-                    stackPush((state.pc + 1 >> 8) & 0xff); // PC high byte
-                    stackPush(state.pc + 1 & 0xff);        // PC low byte
-                    stackPush(state.getStatusFlag());
-                    // Set the Interrupt Disabled flag.  RTI will clear it.
-                    setIrqDisableFlag();
-                    // Load interrupt vector address into PC
-                    state.pc = address(bus.read(IRQ_VECTOR_L), bus.read(IRQ_VECTOR_H));
+                    handleIrq(state.pc + 1);
                 }
                 break;
             case 0x08: // PHP - Push Processor Status - Implied
@@ -729,6 +732,35 @@ public class Cpu implements InstructionTable {
         delayLoop(state.ir);
     }
 
+    private void handleIrq(int returnPc) throws MemoryAccessException {
+        handleInterrupt(returnPc, IRQ_VECTOR_L, IRQ_VECTOR_H);
+        clearIrq();
+    }
+
+    private void handleNmi() throws MemoryAccessException {
+        handleInterrupt(state.pc, NMI_VECTOR_L, NMI_VECTOR_H);
+        clearNmi();
+    }
+
+    /**
+     * Handle the common behavior of BRK, /IRQ, and /NMI
+     *
+     * @throws MemoryAccessException
+     */
+    private void handleInterrupt(int returnPc, int vectorLow, int vectorHigh) throws MemoryAccessException {
+        // Set the break flag before pushing.
+        setBreakFlag();
+        // Push program counter + 1 onto the stack
+        stackPush((returnPc >> 8) & 0xff); // PC high byte
+        stackPush(returnPc & 0xff);        // PC low byte
+        stackPush(state.getStatusFlag());
+        // Set the Interrupt Disabled flag.  RTI will clear it.
+        setIrqDisableFlag();
+
+        // Load interrupt vector address into PC
+        state.pc = address(bus.read(vectorLow), bus.read(vectorHigh));
+    }
+
     /**
      * Add with Carry, used by all addressing mode implementations of ADC.
      * As a side effect, this will set the overflow and carry flags as
@@ -1161,6 +1193,34 @@ public class Cpu implements InstructionTable {
         return state.getStatusFlag();
     }
 
+    /**
+     * Simulate transition from logic-high to logic-low on the INT line.
+     */
+    public void assertIrq() {
+       state.irqAsserted = true;
+    }
+
+    /**
+     * Simulate transition from logic-low to logic-high of the INT line.
+     */
+    public void clearIrq() {
+        state.irqAsserted = false;
+    }
+
+    /**
+     * Simulate transition from logic-high to logic-low on the NMI line.
+     */
+    public void assertNmi() {
+        state.nmiAsserted = true;
+    }
+
+    /**
+     * Simulate transition from logic-low to logic-high of the NMI line.
+     */
+    public void clearNmi() {
+        state.nmiAsserted = false;
+    }
+
     /**
      * Push an item onto the stack, and decrement the stack counter.
      * Will wrap-around if already at the bottom of the stack (This
@@ -1305,6 +1365,8 @@ public class Cpu implements InstructionTable {
         public int[] args = new int[2];
         public int instSize;
         public boolean opTrap;
+        public boolean irqAsserted;
+        public boolean nmiAsserted;
 
         /* Status Flag Register bits */
         public boolean carryFlag;
@@ -1341,6 +1403,7 @@ public class Cpu implements InstructionTable {
             this.args[1] = s.args[1];
             this.instSize = s.instSize;
             this.opTrap = s.opTrap;
+            this.irqAsserted = s.irqAsserted;
             this.carryFlag = s.carryFlag;
             this.negativeFlag = s.negativeFlag;
             this.zeroFlag = s.zeroFlag;

src/main/java/com/loomcom/symon/devices/Acia.java

@@ -23,10 +23,8 @@
 
 package com.loomcom.symon.devices;
 
-import com.loomcom.symon.exceptions.*;
-
-import java.util.logging.Level;
-import java.util.logging.Logger;
+import com.loomcom.symon.exceptions.MemoryAccessException;
+import com.loomcom.symon.exceptions.MemoryRangeException;
 
 
 /**
@@ -58,6 +56,8 @@ public class Acia extends Device {
     private int commandRegister;
     private int controlRegister;
 
+    private boolean receiveIrqEnabled = false;
+    private boolean transmitIrqEnabled = false;
     private boolean overrun = false;
 
     private long lastTxWrite   = 0;
@@ -105,7 +105,7 @@ public class Acia extends Device {
                 reset();
                 break;
             case 2:
-                commandRegister = data;
+                setCommandRegister(data);
                 break;
             case 3:
                 setControlRegister(data);
@@ -115,13 +115,23 @@ public class Acia extends Device {
         }
     }
 
+
+    private void setCommandRegister(int data) {
+        commandRegister = data;
+
+        // Bit 1 controls receiver IRQ behavior
+        receiveIrqEnabled = (commandRegister & 0x02) == 0;
+        // Bits 2 & 3 controls transmit IRQ behavior
+        transmitIrqEnabled = (commandRegister & 0x08) == 0 && (commandRegister & 0x04) != 0;
+    }
+
     /**
      * Set the control register and associated state.
      *
      * @param data
      */
-    public void setControlRegister(int data) {
-        this.controlRegister = data;
+    private void setControlRegister(int data) {
+        controlRegister = data;
 
         // If the value of the data is 0, this is a request to reset,
         // otherwise it's a control update.
@@ -245,18 +255,28 @@ public class Acia extends Device {
 
     public synchronized void rxWrite(int data) {
         rxFull = true;
+
+        if (receiveIrqEnabled) {
+            getBus().assertIrq();
+        }
+
         rxChar = data;
     }
 
     public synchronized int txRead() {
         txEmpty = true;
+
+        if (transmitIrqEnabled) {
+            getBus().assertIrq();
+        }
+
         return txChar;
     }
 
     public synchronized void txWrite(int data) {
         lastTxWrite = System.nanoTime();
-        txEmpty = false;
         txChar = data;
+        txEmpty = false;
     }
 
     /**
@@ -278,6 +298,8 @@ public class Acia extends Device {
         txEmpty = true;
         rxChar = 0;
         rxFull = false;
+        receiveIrqEnabled = false;
+        transmitIrqEnabled = false;
     }
 
 }

src/test/java/com/loomcom/symon/AciaTest.java

@@ -1,13 +1,83 @@
 package com.loomcom.symon;
 
-import org.junit.*;
-
 import com.loomcom.symon.devices.Acia;
-import com.loomcom.symon.exceptions.FifoUnderrunException;
+import org.junit.Test;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.*;
 
 public class AciaTest {
+
+    @Test
+    public void shouldTriggerInterruptOnRxFullIfRxIrqEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = new Acia(0x000);
+        acia.setBus(mockBus);
+
+        // Disable TX IRQ, Enable RX IRQ
+        acia.write(2, 0x00);
+
+        acia.rxWrite('a');
+
+        verify(mockBus, atLeastOnce()).assertIrq();
+    }
+
+    @Test
+    public void shouldNotTriggerInterruptOnRxFullIfRxIrqNotEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = new Acia(0x000);
+        acia.setBus(mockBus);
+
+        // Disable TX IRQ, Disable RX IRQ
+        acia.write(2, 0x02);
+
+        acia.rxWrite('a');
+
+        verify(mockBus, never()).assertIrq();
+    }
+
+    @Test
+    public void shouldTriggerInterruptOnTxEmptyIfTxIrqEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = new Acia(0x000);
+        acia.setBus(mockBus);
+
+        // Enable TX IRQ, Disable RX IRQ
+        acia.write(2, 0x06);
+
+        // Write data
+        acia.write(0, 'a');
+
+        verify(mockBus, never()).assertIrq();
+
+        // Transmission should cause IRQ
+        acia.txRead();
+
+        verify(mockBus, atLeastOnce()).assertIrq();
+    }
+
+    @Test
+    public void shouldNotTriggerInterruptOnTxEmptyIfTxIrqNotEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = new Acia(0x000);
+        acia.setBus(mockBus);
+
+        // Disable TX IRQ, Disable RX IRQ
+        acia.write(2, 0x02);
+
+        // Write data
+        acia.write(0, 'a');
+
+        // Transmission should cause IRQ
+        acia.txRead();
+
+        verify(mockBus, never()).assertIrq();
+    }
+
     @Test
     public void newAciaShouldHaveTxEmptyStatus() throws Exception {
         Acia acia = new Acia(0x000);

src/test/java/com/loomcom/symon/BusTest.java

@@ -10,113 +10,145 @@ import com.loomcom.symon.exceptions.*;
  */
 public class BusTest extends TestCase {
 
-  public BusTest(String testName) {
-    super(testName);
-  }
-
-  public static Test suite() {
-    return new TestSuite(BusTest.class);
-  }
-
-  public void testCreatingWithStartAndEndAddresses() {
-    Bus b = null;
-
-    b = new Bus(0x00, 0xff);
-    assertEquals(0x00, b.startAddress());
-    assertEquals(0xff, b.endAddress());
-
-    b = new Bus(0x20, 0xea);
-    assertEquals(0x20, b.startAddress());
-    assertEquals(0xea, b.endAddress());
-  }
-
-  public void testCreatingWithSize() {
-    Bus b = null;
-
-    b = new Bus(256);
-    assertEquals(0x00, b.startAddress());
-    assertEquals(0xff, b.endAddress());
-
-    b = new Bus(4096);
-    assertEquals(0x000, b.startAddress());
-    assertEquals(0xfff, b.endAddress());
-
-    b = new Bus(65536);
-    assertEquals(0x0000, b.startAddress());
-    assertEquals(0xffff, b.endAddress());
-  }
-
-  public void testAddDevice() throws MemoryRangeException {
-    Device memory = new Memory(0x0000, 0x00ff, true);
-    Device rom    = new Memory(0x0100, 0x02ff, false);
-
-    Bus b = new Bus(0x0000, 0xffff);
-
-    assertEquals(0, b.getDevices().size());
-    b.addDevice(memory);
-    assertEquals(1, b.getDevices().size());
-    b.addDevice(rom);
-    assertEquals(2, b.getDevices().size());
-  }
-
-  public void testOverlappingDevicesShouldFail() throws MemoryRangeException {
-    Device memory = new Memory(0x0000, 0x0100, true);
-    Device rom    = new Memory(0x00ff, 0x0200, false);
-
-    Bus b = new Bus(0x0000, 0xffff);
-
-    b.addDevice(memory);
-
-    try {
-      b.addDevice(rom);
-      fail("Should have thrown a MemoryRangeException.");
-    } catch (MemoryRangeException ex) {
-      // expected
+    public BusTest(String testName) {
+        super(testName);
     }
-  }
 
-  public void testIsCompleteWithFirstDeviceNotStartingAtStartAddress() throws MemoryRangeException {
-    Device memory = new Memory(0x00ff, 0xff00, true);
+    public static Test suite() {
+        return new TestSuite(BusTest.class);
+    }
 
-    Bus b = new Bus(0x0000, 0xffff);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(memory);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-  }
+    public void testCreatingWithStartAndEndAddresses() {
+        Bus b = null;
 
-  public void testIsCompleteWithOneDevice() throws MemoryRangeException {
-    Device memory = new Memory(0x0000, 0xffff, true);
-    Bus b = new Bus(0x0000, 0xffff);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(memory);
-    assertTrue("Address space should have been complete!", b.isComplete());
-  }
+        b = new Bus(0x00, 0xff);
+        assertEquals(0x00, b.startAddress());
+        assertEquals(0xff, b.endAddress());
 
-  public void testIsCompleteWithTwoDevices() throws MemoryRangeException {
-    Device memory = new Memory(0x0000, 0x7fff, true);
-    Device rom    = new Memory(0x8000, 0xffff, false);
+        b = new Bus(0x20, 0xea);
+        assertEquals(0x20, b.startAddress());
+        assertEquals(0xea, b.endAddress());
+    }
 
-    Bus b = new Bus(0x0000, 0xffff);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(memory);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(rom);
-    assertTrue("Address space should have been complete!", b.isComplete());
-  }
+    public void testCreatingWithSize() {
+        Bus b = null;
 
-  public void testIsCompleteWithThreeDevices() throws MemoryRangeException {
-    Device memory = new Memory(0x0000, 0x7fff, true);
-    Device rom1   = new Memory(0x8000, 0xBfff, false);
-    Device rom2   = new Memory(0xC000, 0xffff, false);
+        b = new Bus(256);
+        assertEquals(0x00, b.startAddress());
+        assertEquals(0xff, b.endAddress());
 
-    Bus b = new Bus(0x0000, 0xffff);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(memory);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(rom1);
-    assertFalse("Address space was unexpectedly complete!", b.isComplete());
-    b.addDevice(rom2);
-    assertTrue("Address space should have been complete!", b.isComplete());
-  }
+        b = new Bus(4096);
+        assertEquals(0x000, b.startAddress());
+        assertEquals(0xfff, b.endAddress());
+
+        b = new Bus(65536);
+        assertEquals(0x0000, b.startAddress());
+        assertEquals(0xffff, b.endAddress());
+    }
+
+    public void testAddDevice() throws MemoryRangeException {
+        Device memory = new Memory(0x0000, 0x00ff, true);
+        Device rom = new Memory(0x0100, 0x02ff, false);
+
+        Bus b = new Bus(0x0000, 0xffff);
+
+        assertEquals(0, b.getDevices().size());
+        b.addDevice(memory);
+        assertEquals(1, b.getDevices().size());
+        b.addDevice(rom);
+        assertEquals(2, b.getDevices().size());
+    }
+
+    public void testOverlappingDevicesShouldFail() throws MemoryRangeException {
+        Device memory = new Memory(0x0000, 0x0100, true);
+        Device rom = new Memory(0x00ff, 0x0200, false);
+
+        Bus b = new Bus(0x0000, 0xffff);
+
+        b.addDevice(memory);
+
+        try {
+            b.addDevice(rom);
+            fail("Should have thrown a MemoryRangeException.");
+        } catch (MemoryRangeException ex) {
+            // expected
+        }
+    }
+
+    public void testIsCompleteWithFirstDeviceNotStartingAtStartAddress() throws MemoryRangeException {
+        Device memory = new Memory(0x00ff, 0xff00, true);
+
+        Bus b = new Bus(0x0000, 0xffff);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(memory);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+    }
+
+    public void testIsCompleteWithOneDevice() throws MemoryRangeException {
+        Device memory = new Memory(0x0000, 0xffff, true);
+        Bus b = new Bus(0x0000, 0xffff);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(memory);
+        assertTrue("Address space should have been complete!", b.isComplete());
+    }
+
+    public void testIsCompleteWithTwoDevices() throws MemoryRangeException {
+        Device memory = new Memory(0x0000, 0x7fff, true);
+        Device rom = new Memory(0x8000, 0xffff, false);
+
+        Bus b = new Bus(0x0000, 0xffff);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(memory);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(rom);
+        assertTrue("Address space should have been complete!", b.isComplete());
+    }
+
+    public void testIsCompleteWithThreeDevices() throws MemoryRangeException {
+        Device memory = new Memory(0x0000, 0x7fff, true);
+        Device rom1 = new Memory(0x8000, 0xBfff, false);
+        Device rom2 = new Memory(0xC000, 0xffff, false);
+
+        Bus b = new Bus(0x0000, 0xffff);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(memory);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(rom1);
+        assertFalse("Address space was unexpectedly complete!", b.isComplete());
+        b.addDevice(rom2);
+        assertTrue("Address space should have been complete!", b.isComplete());
+    }
+
+    public void testSetAndClearIrq() throws Exception {
+        Bus b = new Bus(0x0000, 0xffff);
+        Cpu c = new Cpu();
+        b.addCpu(c);
+
+        assertFalse(c.getCpuState().irqAsserted);
+
+        b.assertIrq();
+
+        assertTrue(c.getCpuState().irqAsserted);
+
+        b.clearIrq();
+
+        assertFalse(c.getCpuState().irqAsserted);
+    }
+
+    public void testSetAndClearNmi() throws Exception {
+        Bus b = new Bus(0x0000, 0xffff);
+        Cpu c = new Cpu();
+        b.addCpu(c);
+
+        assertFalse(c.getCpuState().nmiAsserted);
+
+        b.assertNmi();
+
+        assertTrue(c.getCpuState().nmiAsserted);
+
+        b.clearNmi();
+
+        assertFalse(c.getCpuState().nmiAsserted);
+    }
 
 }

src/test/java/com/loomcom/symon/CpuAccumulatorModeTest.java

@@ -7,303 +7,303 @@ import junit.framework.*;
 
 public class CpuAccumulatorModeTest extends TestCase {
 
-  protected Cpu cpu;
-  protected Bus bus;
-  protected Memory mem;
+    protected Cpu cpu;
+    protected Bus bus;
+    protected Memory mem;
 
-  public void setUp() throws MemoryRangeException, MemoryAccessException {
-    this.cpu = new Cpu();
-    this.bus = new Bus(0x0000, 0xffff);
-    this.mem = new Memory(0x0000, 0x10000);
-    bus.addCpu(cpu);
-    bus.addDevice(mem);
+    public void setUp() throws MemoryRangeException, MemoryAccessException {
+        this.cpu = new Cpu();
+        this.bus = new Bus(0x0000, 0xffff);
+        this.mem = new Memory(0x0000, 0x10000);
+        bus.addCpu(cpu);
+        bus.addDevice(mem);
 
-    // Load the reset vector.
-    bus.write(0xfffc, Bus.DEFAULT_LOAD_ADDRESS & 0x00ff);
-    bus.write(0xfffd, (Bus.DEFAULT_LOAD_ADDRESS & 0xff00)>>>8);
+        // Load the reset vector.
+        bus.write(0xfffc, Bus.DEFAULT_LOAD_ADDRESS & 0x00ff);
+        bus.write(0xfffd, (Bus.DEFAULT_LOAD_ADDRESS & 0xff00) >>> 8);
 
-    cpu.reset();
+        cpu.reset();
 
-    // Assert initial state
-    assertEquals(0, cpu.getAccumulator());
-      assertEquals(0, cpu.getXRegister());
-      assertEquals(0, cpu.getYRegister());
-    assertEquals(0x200, cpu.getProgramCounter());
-    assertEquals(0xff, cpu.getStackPointer());
-    assertEquals(0x20, cpu.getProcessorStatus());
-  }
+        // Assert initial state
+        assertEquals(0, cpu.getAccumulator());
+        assertEquals(0, cpu.getXRegister());
+        assertEquals(0, cpu.getYRegister());
+        assertEquals(0x200, cpu.getProgramCounter());
+        assertEquals(0xff, cpu.getStackPointer());
+        assertEquals(0x20, cpu.getProcessorStatus());
+    }
 
-  /*
-   * The following opcodes are tested for correctness in this file:
-   *
-	 * ASL - $0a
-	 * ROL - $2a
-	 * LSR - $4a
-	 * ROR - $6a
-	 */
+   /*
+    * The following opcodes are tested for correctness in this file:
+    *
+    * ASL - $0a
+    * ROL - $2a
+    * LSR - $4a
+    * ROR - $6a
+	*/
 
 	/* ASL - Arithmetic Shift Left - $0a */
 
-	public void test_ASL() throws MemoryAccessException {
-		bus.loadProgram(0xa9, 0x00,  // LDA #$00
-										0x0a,        // ASL A
+    public void test_ASL() throws MemoryAccessException {
+        bus.loadProgram(0xa9, 0x00,  // LDA #$00
+                        0x0a,        // ASL A
 
-										0xa9, 0x01,  // LDA #$01
-                    0x0a,        // ASL A
+                        0xa9, 0x01,  // LDA #$01
+                        0x0a,        // ASL A
 
-										0xa9, 0x02,  // LDA #$02
-                    0x0a,        // ASL A
+                        0xa9, 0x02,  // LDA #$02
+                        0x0a,        // ASL A
 
-										0xa9, 0x44,  // LDA #$44
-                    0x0a,        // ASL A
+                        0xa9, 0x44,  // LDA #$44
+                        0x0a,        // ASL A
 
-										0xa9, 0x80,  // LDA #$80
-                    0x0a);       // ASL A
+                        0xa9, 0x80,  // LDA #$80
+                        0x0a);       // ASL A
 
-    cpu.step(2);
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x02, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x02, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x04, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x04, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x88, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x88, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-		assertTrue(cpu.getCarryFlag());
-	}
+        cpu.step(2);
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getCarryFlag());
+    }
 
 	/* ROL - Rotate Left - $2a */
 
-	public void test_ROL() throws MemoryAccessException {
-    bus.loadProgram(0xa9, 0x00,  // LDA #$00
-										0x2a,        // ROL A   (m=%00000000, c=0)
-                    0xa9, 0x01,  // LDA #$01
-										0x2a,        // ROL A   (m=%00000010, c=0)
-                    0x38,        // SEC     (m=%00000010, c=1)
-                    0x2a,        // ROL A   (m=%00000101, c=0)
-                    0x2a,        // ROL A   (m=%00001010, c=0)
-                    0x2a,        // ROL A   (m=%00010100, c=0)
-                    0x2a,        // ROL A   (m=%00101000, c=0)
-                    0x2a,        // ROL A   (m=%01010000, c=0)
-                    0x2a,        // ROL A   (m=%10100000, c=0)
-                    0x2a,        // ROL A   (m=%01000000, c=1)
-                    0x2a);       // ROL A   (m=%10000001, c=0)
+    public void test_ROL() throws MemoryAccessException {
+        bus.loadProgram(0xa9, 0x00,  // LDA #$00
+                        0x2a,        // ROL A   (m=%00000000, c=0)
+                        0xa9, 0x01,  // LDA #$01
+                        0x2a,        // ROL A   (m=%00000010, c=0)
+                        0x38,        // SEC     (m=%00000010, c=1)
+                        0x2a,        // ROL A   (m=%00000101, c=0)
+                        0x2a,        // ROL A   (m=%00001010, c=0)
+                        0x2a,        // ROL A   (m=%00010100, c=0)
+                        0x2a,        // ROL A   (m=%00101000, c=0)
+                        0x2a,        // ROL A   (m=%01010000, c=0)
+                        0x2a,        // ROL A   (m=%10100000, c=0)
+                        0x2a,        // ROL A   (m=%01000000, c=1)
+                        0x2a);       // ROL A   (m=%10000001, c=0)
 
-    cpu.step(2);
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x02, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x02, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x05, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x05, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x0a, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x0a, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x14, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x14, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x28, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x28, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x50, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x50, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0xa0, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0xa0, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x40, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertTrue(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x40, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x81, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
-	}
+        cpu.step();
+        assertEquals(0x81, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
+    }
 
 	/* LSR - Logical Shift Right - $4a */
 
-	public void test_LSR() throws MemoryAccessException {
-    bus.loadProgram(0xa9, 0x00,  // LDA #$00
-										0x4a,        // LSR A
+    public void test_LSR() throws MemoryAccessException {
+        bus.loadProgram(0xa9, 0x00,  // LDA #$00
+                        0x4a,        // LSR A
 
-										0xa9, 0x01,  // LDA #$01
-                    0x4a,        // LSR A
+                        0xa9, 0x01,  // LDA #$01
+                        0x4a,        // LSR A
 
-										0xa9, 0x02,  // LDA #$02
-                    0x4a,        // LSR A
+                        0xa9, 0x02,  // LDA #$02
+                        0x4a,        // LSR A
 
-										0xa9, 0x44,  // LDA #$44
-                    0x4a,        // LSR A
+                        0xa9, 0x44,  // LDA #$44
+                        0x4a,        // LSR A
 
-										0xa9, 0x80,  // LDA #$80
-                    0x4a,        // LSR A
+                        0xa9, 0x80,  // LDA #$80
+                        0x4a,        // LSR A
 
-                    0x38,        // SEC
-										0xa9, 0x02,  // LDA #$02
-                    0x4a);       // LSR $05
+                        0x38,        // SEC
+                        0xa9, 0x02,  // LDA #$02
+                        0x4a);       // LSR $05
 
-    cpu.step(2);
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertTrue(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x01, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x01, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x22, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x22, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x40, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x40, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    // Setting Carry should not affect the result.
-    cpu.step(3);
-    assertEquals(0x01, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
-	}
+        // Setting Carry should not affect the result.
+        cpu.step(3);
+        assertEquals(0x01, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
+    }
 
 	/* ROR - Rotate Right - $6a */
 
-	public void test_ROR() throws MemoryAccessException {
-    bus.loadProgram(0xa9, 0x00,  // LDA #$00
-										0x6a,        // ROR A   (m=%00000000, c=0)
-										0xa9, 0x10,  // LDA #$10
-                    0x6a,        // ROR A   (m=%00001000, c=0)
-                    0x6a,        // ROR A   (m=%00000100, c=0)
-                    0x6a,        // ROR A   (m=%00000010, c=0)
-                    0x6a,        // ROR A   (m=%00000001, c=0)
-                    0x6a,        // ROR A   (m=%00000000, c=1)
-                    0x6a,        // ROR A   (m=%10000000, c=0)
-                    0x6a,        // ROR A   (m=%01000000, c=0)
-                    0x6a,        // ROR A   (m=%00100000, c=0)
-                    0x6a);       // ROR A   (m=%00010000, c=0)
+    public void test_ROR() throws MemoryAccessException {
+        bus.loadProgram(0xa9, 0x00,  // LDA #$00
+                        0x6a,        // ROR A   (m=%00000000, c=0)
+                        0xa9, 0x10,  // LDA #$10
+                        0x6a,        // ROR A   (m=%00001000, c=0)
+                        0x6a,        // ROR A   (m=%00000100, c=0)
+                        0x6a,        // ROR A   (m=%00000010, c=0)
+                        0x6a,        // ROR A   (m=%00000001, c=0)
+                        0x6a,        // ROR A   (m=%00000000, c=1)
+                        0x6a,        // ROR A   (m=%10000000, c=0)
+                        0x6a,        // ROR A   (m=%01000000, c=0)
+                        0x6a,        // ROR A   (m=%00100000, c=0)
+                        0x6a);       // ROR A   (m=%00010000, c=0)
 
-    cpu.step(2);
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step(2);
-    assertEquals(0x08, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step(2);
+        assertEquals(0x08, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x04, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x04, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x02, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x02, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x01, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x01, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x00, cpu.getAccumulator());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertTrue(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x00, cpu.getAccumulator());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x80, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x80, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x40, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x40, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x20, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
+        cpu.step();
+        assertEquals(0x20, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
 
-    cpu.step();
-    assertEquals(0x10, cpu.getAccumulator());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getCarryFlag());
-	}
+        cpu.step();
+        assertEquals(0x10, cpu.getAccumulator());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+        assertFalse(cpu.getCarryFlag());
+    }
 
 }

src/test/java/com/loomcom/symon/CpuTest.java

@@ -345,6 +345,197 @@ public class CpuTest extends TestCase {
         assertFalse(cpu.getNegativeFlag());
     }
 
+    public void testIrq() throws Exception {
+        // Ensure the IRQ disable flag is cleared
+        cpu.clearIrqDisableFlag();
+
+        // Set the IRQ vector
+        bus.write(0xffff, 0x12);
+        bus.write(0xfffe, 0x34);
+
+        // Create an IRQ handler at 0x1234
+        cpu.setProgramCounter(0x1234);
+        bus.loadProgram(0xa9, 0x33,  // LDA #$33
+                        0x69, 0x01); // ADC #$01
+
+        cpu.setProgramCounter(0x0200);
+        // Create a little program at 0x0200
+        bus.loadProgram(0x18,        // CLC
+                        0xa9, 0x01,  // LDA #$00
+                        0x69, 0x01); // ADC #$01
+
+        cpu.step();
+        assertEquals(0x0201, cpu.getProgramCounter()); // First instruction executed.
+        assertEquals(0x00, cpu.getAccumulator());
+
+        cpu.step();
+        assertEquals(0x0203, cpu.getProgramCounter());
+        assertEquals(0x01, cpu.getAccumulator());
+
+        cpu.assertIrq();
+
+        cpu.step();
+        assertTrue(cpu.getIrqDisableFlag()); // Should have been set by the IRQ
+        assertEquals(0x1236, cpu.getProgramCounter());
+        assertEquals(0x33, cpu.getAccumulator());
+
+        // Be sure that the IRQ line is no longer held low
+        assertFalse(cpu.getCpuState().irqAsserted);
+    }
+
+    public void testIrqHonorsIrqDisabledFlag() throws Exception {
+        // Ensure the IRQ disable flag is set
+        cpu.setIrqDisableFlag();
+
+        // Set the IRQ vector
+        bus.write(0xffff, 0x12);
+        bus.write(0xfffe, 0x34);
+
+        // Create an IRQ handler at 0x1234
+        cpu.setProgramCounter(0x1234);
+        bus.loadProgram(0xa9, 0x33,  // LDA #$33
+                        0x69, 0x01); // ADC #$01
+
+        cpu.setProgramCounter(0x0200);
+        // Create a little program at 0x0200
+        bus.loadProgram(0x18,        // CLC
+                        0xa9, 0x01,  // LDA #$00
+                        0x69, 0x01); // ADC #$01
+
+        cpu.step();
+        assertEquals(0x0201, cpu.getProgramCounter()); // First instruction executed.
+        assertEquals(0x00, cpu.getAccumulator());
+
+        cpu.step();
+        assertEquals(0x0203, cpu.getProgramCounter());
+        assertEquals(0x01, cpu.getAccumulator());
+
+        cpu.assertIrq(); // Should be ignored, because the disable flag is set.
+
+        cpu.step();
+        assertTrue(cpu.getIrqDisableFlag()); // Should have been left alone.
+        assertEquals(0x0205, cpu.getProgramCounter());
+        assertEquals(0x02, cpu.getAccumulator());
+    }
+
+    public void testIrqPushesCorrectReturnAddressOntoStack() throws Exception {
+        // Ensure the IRQs are enabled
+        cpu.clearIrqDisableFlag();
+
+        // Set the IRQ vector
+        bus.write(0xffff, 0x10);
+        bus.write(0xfffe, 0x00);
+
+        // Create an IRQ handler at 0x1000 that just RTIs.
+        cpu.setProgramCounter(0x1000);
+        bus.loadProgram(0xea, 0xea, 0x40); // NOP, NOP, RTI
+
+        cpu.setProgramCounter(0x0200);
+
+        // Create a little program at 0x0200 with three instruction sizes.
+        bus.loadProgram(0x18,               // CLC
+                        0xa9, 0x01,         // LDA #$01
+                        0x6d, 0x06, 0x02,   // ADC $0207
+                        0x00,               // BRK
+                        0x03);              // $03 (data @ $0206)
+
+        cpu.step(); // CLC
+        assertEquals(0x0201, cpu.getProgramCounter()); // First instruction executed.
+        assertEquals(0x00,   cpu.getAccumulator());
+
+        cpu.assertIrq();
+        cpu.step(); // NOP
+        assertEquals(0x1001, cpu.getProgramCounter());
+        cpu.step(); // NOP
+        assertEquals(0x1002, cpu.getProgramCounter());
+        cpu.step(); // RTI: PC -> 0x0201
+        assertEquals(0x0201, cpu.getProgramCounter());
+        cpu.step(); // LDA $#01
+        assertEquals(0x0203, cpu.getProgramCounter());
+
+        cpu.assertIrq();
+        cpu.step(); // NOP
+        assertEquals(0x1001, cpu.getProgramCounter());
+        cpu.step(); // NOP
+        assertEquals(0x1002, cpu.getProgramCounter());
+        cpu.step(); // RTI: PC -> 0x0203
+        assertEquals(0x0203, cpu.getProgramCounter());
+    }
+
+    public void testNmi() throws Exception {
+        // Set the NMI vector to 0x1000
+        bus.write(0xfffb, 0x10);
+        bus.write(0xfffa, 0x00);
+
+        // Create an NMI handler at 0x1000
+        cpu.setProgramCounter(0x1000);
+        bus.loadProgram(0xa9, 0x33,  // LDA #$33
+                        0x69, 0x01); // ADC #$01
+
+        // Create a little program at 0x0200
+        cpu.setProgramCounter(0x0200);
+        bus.loadProgram(0x18,        // CLC
+                        0xa9, 0x01,  // LDA #$00
+                        0x69, 0x01); // ADC #$01
+
+        cpu.step();
+        assertEquals(0x0201, cpu.getProgramCounter()); // First instruction executed.
+        assertEquals(0x00, cpu.getAccumulator());
+
+        cpu.step();
+        assertEquals(0x0203, cpu.getProgramCounter());
+        assertEquals(0x01, cpu.getAccumulator());
+
+        cpu.assertNmi();
+
+        cpu.step();
+        assertTrue(cpu.getIrqDisableFlag()); // Should have been set by the NMI
+        assertEquals(0x1002, cpu.getProgramCounter());
+        assertEquals(0x33, cpu.getAccumulator());
+
+        // Be sure that the NMI line is no longer held low
+        assertFalse(cpu.getCpuState().nmiAsserted);
+    }
+
+    public void testNmiIgnoresIrqDisableFlag() throws Exception {
+        // Set the IRQ disable flag, which should be ignored by the NMI
+        cpu.setIrqDisableFlag();
+
+        // Set the NMI vector to 0x1000
+        bus.write(0xfffb, 0x10);
+        bus.write(0xfffa, 0x00);
+
+        // Create an NMI handler at 0x1000
+        cpu.setProgramCounter(0x1000);
+        bus.loadProgram(0xa9, 0x33,  // LDA #$33
+                        0x69, 0x01); // ADC #$01
+
+        // Create a little program at 0x0200
+        cpu.setProgramCounter(0x0200);
+        bus.loadProgram(0x18,        // CLC
+                        0xa9, 0x01,  // LDA #$00
+                        0x69, 0x01); // ADC #$01
+
+        cpu.step();
+        assertEquals(0x0201, cpu.getProgramCounter()); // First instruction executed.
+        assertEquals(0x00, cpu.getAccumulator());
+
+        cpu.step();
+        assertEquals(0x0203, cpu.getProgramCounter());
+        assertEquals(0x01, cpu.getAccumulator());
+
+        cpu.assertNmi();
+
+        cpu.step();
+        assertTrue(cpu.getIrqDisableFlag()); // Should have been set by the NMI
+        assertEquals(0x1002, cpu.getProgramCounter());
+        assertEquals(0x33, cpu.getAccumulator());
+
+        // Be sure that the NMI line is no longer held low
+        assertFalse(cpu.getCpuState().nmiAsserted);
+    }
+
+
     public void testAddress() {
         assertEquals(0xf1ea, cpu.address(0xea, 0xf1));
         assertEquals(0x00ea, cpu.address(0xea, 0x00));