IRQ and NMI handling at CPU level.

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

@@ -160,6 +160,14 @@ public class Cpu implements InstructionTable {
         // 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();
+        }
+
         // Fetch memory location for this instruction.
         state.ir = bus.read(state.pc);
         irAddressMode = (state.ir >> 2) & 0x07;
@@ -251,16 +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.
+                    handleIrq();
-                    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));
                 }
                 break;
             case 0x08: // PHP - Push Processor Status - Implied
@@ -733,6 +732,37 @@ public class Cpu implements InstructionTable {
         delayLoop(state.ir);
     }
 
+    private void handleIrq() throws MemoryAccessException {
+        handleInterrupt(IRQ_VECTOR_L, IRQ_VECTOR_H);
+
+        clearIrq();
+    }
+
+    private void handleNmi() throws MemoryAccessException {
+        handleInterrupt(NMI_VECTOR_L, NMI_VECTOR_H);
+
+        clearNmi();
+    }
+
+    /**
+     * Handle the common behavior of BRK, /IRQ, and /NMI
+     *
+     * @throws MemoryAccessException
+     */
+    private void handleInterrupt(int vectorLow, int vectorHigh) throws MemoryAccessException {
+        // 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(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

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

@@ -20,7 +20,7 @@ public class CpuAccumulatorModeTest extends TestCase {
 
         // Load the reset vector.
         bus.write(0xfffc, Bus.DEFAULT_LOAD_ADDRESS & 0x00ff);
-    bus.write(0xfffd, (Bus.DEFAULT_LOAD_ADDRESS & 0xff00)>>>8);
+        bus.write(0xfffd, (Bus.DEFAULT_LOAD_ADDRESS & 0xff00) >>> 8);
 
         cpu.reset();
 

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

@@ -345,6 +345,153 @@ 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 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));