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.
-                    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();
                 }
                 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