Release 0.9.9.0

Infrastructure for several machine types, Bus reworkings, 6850 ACIA and more

README.md

@@ -3,15 +3,16 @@ SYMON - A 6502 System Simulator
 
 **NOTE: THIS SOFTWARE IS UNDER ACTIVE DEVELOPMENT. Feedback is welcome!**
 
-**Version:** 0.9.0.1
+**Version:** 0.9.9.0
 
-**Last Updated:** 29 December, 2013
+**Last Updated:** 26 July, 2014
 
-Copyright (c) 2013 Seth J. Morabito <web@loomcom.com>
+Copyright (c) 2014 Seth J. Morabito <web@loomcom.com>
+
+Portions Copyright (c) 2014 Maik Marten <maikmarten@googlemail.com>
 
 See the file COPYING for license.
 
-
 ![Symon Simulator in Action] (https://github.com/sethm/symon/raw/master/screenshots/full.jpg)
 
 ## 1.0 About
@@ -22,7 +23,8 @@ in Java. Its core goals are accuracy, ease of development, clear
 documentation, and extensive test suites for validating correctness.
 
 Symon simulates a complete system with a 1 MHz NMOS 6502, 32KB of RAM,
-16KB of ROM, a 6551 ACIA, a 6522 VIA, and a 6545 CRTC.
+16KB of ROM, a MOS 6551 or Motorola 6850 ACIA, a MOS 6522 VIA, and an
+experimental 6545 CRTC.
 
 Symon has extensive unit tests to verify correctness, and fully passes
 Klaus Dormann's 6502 Functional Test Suite as of version 0.8.2
@@ -31,21 +33,33 @@ for more information about this functional test suite).
 
 ## 2.0 Requirements
 
-  - Java 1.5 or higher
+  - Java 1.7 or higher
   - Maven 2.0.x or higher (for building from source)
   - JUnit 4 or higher (for testing)
 
 ## 3.0 Features
 
-### 3.1 Memory Map
+Symon can simiulate multiple 6502 based architectures. At present, two
+machines are implemented: Symon (the default), and MULTICOMP.
+
+### 3.1 Memory Maps
+
+#### 3.1.1 Symon Memory Map
 
   - `$0000`--`$7FFF`: 32KB RAM
   - `$8000`--`$800F`: 6522 VIA
-  - `$8800`--`$8803`: 6551 ACIA (Serial Console)
-  - `$9000`--`$9001`: 6545 CRTC
+  - `$8800`--`$8803`: MOS 6551 ACIA (Serial Console)
+  - `$9000`--`$9001`: MOS 6545 CRTC
   - `$C000`--`$FFFF`: 16KB ROM
 
-The CRT Controller uses memory address `$7000` as the start of Video memory.
+The CRT Controller uses memory address `$7000` as the start of Video
+memory.
+
+#### 3.1.2 MULTICOMP Memory Map
+
+  - `$0000`--`$DFFF`: 56KB RAM
+  - `$E000`--`$FFFF`: 8KB ROM
+  - `$FFD0`--`$FFD1`: Motorola 6850 ACIA
 
 ### 3.2 Serial Console and CPU Status
 
@@ -53,10 +67,9 @@ The CRT Controller uses memory address `$7000` as the start of Video memory.
 
 The main window of the simulator acts as the primary Input/Output
 system through a virtual serial terminal. The terminal is attached to
-a simulated MOS 6551 ACIA, including a programmable baud rate
-generator that tries to approximate the correct "feel" of the
-programmed baud rate. (The sample Enhanced BASIC ROM image is
-programmed for 9600 baud)
+a simulated ACIA, including a programmable baud rate generator that
+tries to approximate the correct "feel" of the programmed baud rate.
+(The sample Enhanced BASIC ROM image is programmed for 9600 baud)
 
 It also provides CPU status. Contents of the accumulator, index
 registers, processor status flags, disassembly of the instruction
@@ -66,13 +79,16 @@ register, and stack pointer are all displayed.
 
 The console supports font sizes from 10 to 20 points.
 
-### 3.3 16 KB ROM Loading
+### 3.3 ROM Loading
 
 ![ROM Loading] (https://github.com/sethm/symon/raw/master/screenshots/load_rom.png)
 
-Symon can load any 16 KB (16384 bytes) ROM image from the "File"
-menu. The selected ROM will be placed in memory from locations `$C000`
-to `$FFFF`
+Symon can load any appropriately sized ROM image. The Symon
+architecture expects as 16KB (16384 byte) ROM image, while the
+MULTICOMP architecture expects an 8KB (8192 byte) ROM image. Images
+are loaded via the "Load ROM..." action in the "File" menu. The
+selected ROM file will be loaded into memory at the correct ROM base
+address.
 
 ### 3.4 Memory Window
 
@@ -154,24 +170,29 @@ 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.
 
 ### 4.2 ROM images
 
-The simulator requires a 16KB ROM image loaded at address `$C000` to `$FFFF` to
-work properly. Without a ROM in memory, the simulator will not be able to
-reset, since the reset vector for the 6502 is located in this address space.
+The simulator requires a ROM image loaded into memory to work
+properly. Without a ROM in memory, the simulator will not be able to
+reset, since the reset vector for the 6502 is located in the ROM
+address space.
 
-By default, any 16KB file named `rom.bin` that exists in the same directory
-where Symon is launched will be loaded as a ROM image. ROM images can also
-be swapped out at run-time with the "Load ROM Image..." in the File menu.
+By default, any file named `rom.bin` that exists in the same directory
+where Symon is launched will be loaded as a ROM image. ROM images can
+also be swapped out at run-time with the "Load ROM Image..." in the
+File menu.
 
-The "samples" directory contains a ROM image named 'ehbasic.rom', containing
-Lee Davison's Enhanced 6502 BASIC. This serves as a good starting point for
-exploration.
+The "samples" directory contains a ROM image for the Symon
+architecture named 'ehbasic.rom', containing Lee Davison's Enhanced
+6502 BASIC. This serves as a good starting point for exploration.
+
+*Note*: Presently, EhBASIC only works with the Symon machine
+ architecture, not with MULTICOMP.
 
 ### 4.3 Loading A Program
 
@@ -184,7 +205,7 @@ reset.
 
 There are two very simple sample program in the "samples" directory,
 for testing.
-  
+
 - 'echo.prg' will echo back anything typed at the console.
 
 - 'hello.prg' will continuously print "Hello, 6502 World!" to the console.
@@ -196,6 +217,11 @@ running.
 
 ## 5.0 Revision History
 
+  - **0.9.9:** 26 July, 2014 - MULTICOMP and multi-machine support
+    contributed by Maik Marten <maikmarten@googlemail.com>
+
+  - **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.
@@ -232,10 +258,6 @@ running.
   - Better debugging tools from the UI, including breakpoints
     and disassembly.
 
-  - More accurate timing.
-  
-  - Smarter interrupt handling.
-
   - UI needs a ton more polish.
 
   - More extensive testing.
@@ -247,6 +269,8 @@ running.
   - Allow displaying ACIA status and dumping ACIA buffers, for
     debugging.
 
+  - Symbolic debugging.
+
 ## 7.0 Acknowledgements
 
 This project would not have been possible without the following resources:
@@ -254,7 +278,7 @@ This project would not have been possible without the following resources:
   - [Graham Edgecombe's JTerminal project] (https://github.com/grahamedgecombe/jterminal),
     which forms the core of Symon's console.
 
-  - [Andrew Jacobs' 6502 Pages] (http://www.obelisk.demon.co.uk/6502/), for 
+  - [Andrew Jacobs' 6502 Pages] (http://www.obelisk.demon.co.uk/6502/), for
     wonderfully detailed information about the 6502
 
   - [Neil Parker's "The 6502/65C02/65C816 Instruction Set Decoded"] (http://www.llx.com/~nparker/a2/opcodes.html),

pom.xml

@@ -4,7 +4,7 @@
     <groupId>com.loomcom.symon</groupId>
     <artifactId>symon</artifactId>
     <packaging>jar</packaging>
-    <version>0.9.0.1</version>
+    <version>0.9.9.0</version>
     <name>symon</name>
     <url>http://www.loomcom.com/symon</url>
     <properties>
@@ -88,7 +88,7 @@
                     <appendAssemblyId>false</appendAssemblyId>
                     <archive>
                         <manifest>
-                            <mainClass>com.loomcom.symon.Simulator</mainClass>
+                            <mainClass>com.loomcom.symon.Main</mainClass>
                         </manifest>
                     </archive>
                 </configuration>

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

@@ -26,6 +26,11 @@ package com.loomcom.symon;
 import com.loomcom.symon.devices.Device;
 import com.loomcom.symon.exceptions.MemoryAccessException;
 import com.loomcom.symon.exceptions.MemoryRangeException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.HashMap;
+import java.util.Map;
 
 import java.util.SortedSet;
 import java.util.TreeSet;
@@ -45,15 +50,19 @@ public class Bus {
     // The CPU
     private Cpu cpu;
 
-    // Ordered list of IO devices.
-    private SortedSet<Device> devices;
+    // Ordered sets of IO devices, associated with their priority
+    private Map<Integer, SortedSet<Device>> deviceMap;
+    
+    // an array for quick lookup of adresses, brute-force style
+    private Device[] deviceAddressArray;
+    
 
     public Bus(int size) {
         this(0, size - 1);
     }
 
     public Bus(int startAddress, int endAddress) {
-        this.devices = new TreeSet<Device>();
+        this.deviceMap = new HashMap<Integer, SortedSet<Device>>();
         this.startAddress = startAddress;
         this.endAddress = endAddress;
     }
@@ -65,7 +74,49 @@ public class Bus {
     public int endAddress() {
         return endAddress;
     }
+    
+    private void buildDeviceAddressArray() {
+        int size = (this.endAddress - this.startAddress) + 1;
+        deviceAddressArray = new Device[size];
+   
+        // getDevices() provides an OrderedSet with devices ordered by priorities
+        for(Device device : getDevices()) {
+            MemoryRange range = device.getMemoryRange();
+            for(int address = range.startAddress; address <= range.endAddress; ++address) {
+                deviceAddressArray[address - this.startAddress] = device;
+            }
+        }
+        
+    }
 
+    /**
+     * Add a device to the bus.
+     *
+     * @param device
+     * @param priority
+     * @throws MemoryRangeException
+     */
+    public void addDevice(Device device, int priority) throws MemoryRangeException {
+        
+        MemoryRange range = device.getMemoryRange();
+        if(range.startAddress() < this.startAddress || range.startAddress() > this.endAddress) {
+            throw new MemoryRangeException("start address of device " + device.getName() + " does not fall within the address range of the bus");
+        }
+        if(range.endAddress() < this.startAddress || range.endAddress() > this.endAddress) {
+            throw new MemoryRangeException("end address of device " + device.getName() + " does not fall within the address range of the bus");
+        }
+  
+        
+        SortedSet<Device> deviceSet = deviceMap.get(priority);
+        if(deviceSet == null) {
+            deviceSet = new TreeSet<Device>();
+            deviceMap.put(priority, deviceSet);
+        }
+        
+        deviceSet.add(device);
+        buildDeviceAddressArray();
+    }
+    
     /**
      * Add a device to the bus. Throws a MemoryRangeException if the device overlaps with any others.
      *
@@ -73,21 +124,9 @@ public class Bus {
      * @throws MemoryRangeException
      */
     public void addDevice(Device device) throws MemoryRangeException {
-        // Make sure there's no memory overlap.
-        MemoryRange memRange = device.getMemoryRange();
-        for (Device d : devices) {
-            if (d.getMemoryRange().overlaps(memRange)) {
-                throw new MemoryRangeException("The device being added at " +
-                                               String.format("$%04X", memRange.startAddress()) +
-                                               " overlaps with an existing " +
-                                               "device, '" + d + "'");
-            }
-        }
-
-        // Add the device
-        device.setBus(this);
-        devices.add(device);
+        addDevice(device, 0);
     }
+    
 
     /**
      * Remove a device from the bus.
@@ -95,9 +134,10 @@ public class Bus {
      * @param device
      */
     public void removeDevice(Device device) {
-        if (devices.contains(device)) {
-            devices.remove(device);
+        for(SortedSet<Device> deviceSet : deviceMap.values()) {
+            deviceSet.remove(device);
         }
+        buildDeviceAddressArray();
     }
 
     public void addCpu(Cpu cpu) {
@@ -111,49 +151,81 @@ public class Bus {
      * device.
      */
     public boolean isComplete() {
-        // Empty maps cannot be complete.
-        if (devices.isEmpty()) {
-            return false;
+        if(deviceAddressArray == null) {
+            buildDeviceAddressArray();
         }
-
-        // Loop over devices and add their size
-        int filledMemory = 0;
-        for (Device d : devices) {
-            filledMemory += d.getSize();
+        
+        for(int address = startAddress; address <= endAddress; ++address) {
+            if(deviceAddressArray[address - startAddress] == null) {
+                return false;
+            }
         }
-
-        // Returns if the total size of the devices fill the bus' memory space
-        return filledMemory == endAddress - startAddress + 1;
+        
+        return true;
     }
 
     public int read(int address) throws MemoryAccessException {
-        for (Device d : devices) {
+        Device d = deviceAddressArray[address - this.startAddress];
+        if(d != null) {
             MemoryRange range = d.getMemoryRange();
-            if (range.includes(address)) {
-                // Compute offset into this device's address space.
-                int devAddr = address - range.startAddress();
-                return d.read(devAddr);
-            }
+            int devAddr = address - range.startAddress();
+            return d.read(devAddr);
         }
+        
         throw new MemoryAccessException("Bus read failed. No device at address " + String.format("$%04X", address));
     }
 
     public void write(int address, int value) throws MemoryAccessException {
-        for (Device d : devices) {
+        Device d = deviceAddressArray[address - this.startAddress];
+        if(d != null) {
             MemoryRange range = d.getMemoryRange();
-            if (range.includes(address)) {
-                // Compute offset into this device's address space.
-                int devAddr = address - range.startAddress();
-                d.write(devAddr, value);
-                return;
-            }
+            int devAddr = address - range.startAddress();
+            d.write(devAddr, value);
+            return;
         }
+        
         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);
+        // create an ordered set of devices, ordered by device priorities
+        SortedSet<Device> devices = new TreeSet<Device>();
+        
+        List<Integer> priorities = new ArrayList<Integer>(deviceMap.keySet());
+        Collections.sort(priorities);
+        
+        for(int priority : priorities) {
+            SortedSet<Device> deviceSet = deviceMap.get(priority);
+            for(Device device : deviceSet) {
+                devices.add(device);
+            }
+        }
+        
+        return devices;
     }
 
     public Cpu getCpu() {

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

@@ -25,6 +25,8 @@ package com.loomcom.symon;
 
 import com.loomcom.symon.exceptions.MemoryAccessException;
 import com.loomcom.symon.util.HexUtil;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 
 /**
  * This class provides a simulation of the MOS 6502 CPU's state machine.
@@ -75,6 +77,9 @@ public class Cpu implements InstructionTable {
     /* Internal scratch space */
     private int lo = 0, hi = 0;  // Used in address calculation
     private int tmp; // Temporary storage
+    
+    /* start time of op execution, needed for speed simulation */
+    private long opBeginTime;
 
     /**
      * Construct a new CPU.
@@ -113,7 +118,9 @@ 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 +138,8 @@ public class Cpu implements InstructionTable {
         state.overflowFlag = false;
         state.negativeFlag = false;
 
+        state.irqAsserted = false;
+
         // Clear illegal opcode trap.
         state.opTrap = false;
 
@@ -150,12 +159,21 @@ public class Cpu implements InstructionTable {
     }
 
     /**
-     * Performs an individual machine cycle.
+     * Performs an individual instruction cycle.
      */
     public void step() throws MemoryAccessException {
+        opBeginTime = System.nanoTime();
         // 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 +265,7 @@ 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 +738,35 @@ 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 +1199,34 @@ 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
@@ -1264,11 +1330,10 @@ public class Cpu implements InstructionTable {
         if (clockSteps == 0) {
             clockSteps = 1;
         }
-        long startTime = System.nanoTime();
-        long stopTime = startTime + (CLOCK_IN_NS * clockSteps);
-        // Busy loop
-        while (System.nanoTime() < stopTime) {
-            ;
+        long opScheduledEnd = opBeginTime + clockSteps;
+        long now = System.nanoTime();
+        while(now < opScheduledEnd) {
+            now = System.nanoTime();
         }
     }
 
@@ -1305,6 +1370,8 @@ 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 +1408,7 @@ 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/Main.java

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) 2008-2013 Seth J. Morabito <sethm@loomcom.com>
+ *                         Maik Merten <maikmerten@googlemail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+
+package com.loomcom.symon;
+
+import com.loomcom.symon.machines.MulticompMachine;
+import com.loomcom.symon.machines.SymonMachine;
+import java.util.Locale;
+import javax.swing.JOptionPane;
+import javax.swing.SwingUtilities;
+import javax.swing.UIManager;
+
+public class Main {
+    
+     /**
+     * Main entry point to the simulator. Creates a simulator and shows the main
+     * window.
+     *
+     * @param args
+     */
+    public static void main(String args[]) throws Exception {
+        
+        Class machineClass = SymonMachine.class;
+        for(int i = 0; i < args.length; ++i) {
+            String arg = args[i].toLowerCase(Locale.ENGLISH);
+            if(arg.equals("-machine") && (i+1) < args.length) {
+                String machine = args[i+1].trim().toLowerCase(Locale.ENGLISH);
+                if(machine.equals("symon")) {
+                    machineClass = SymonMachine.class;
+                } else if(machine.equals("multicomp")) {
+                    machineClass = MulticompMachine.class;
+                }
+            }
+        }
+        
+        while(true) {        
+            if(machineClass == null) {
+                Object[] possibilities = {"Symon", "Multicomp"};
+                String s = (String)JOptionPane.showInputDialog(
+                                null,
+                                "Please choose the machine type to be emulated:",
+                                "Machine selection",
+                                JOptionPane.PLAIN_MESSAGE,
+                                null,
+                                possibilities,
+                                "Symon");
+                
+                
+                if(s != null && s.equals("Multicomp")) {
+                    machineClass = MulticompMachine.class;
+                } else {
+                    machineClass = SymonMachine.class;
+                }
+            }
+        
+            final Simulator simulator = new Simulator(machineClass);
+        
+            SwingUtilities.invokeLater(new Runnable() {
+                public void run() {
+                    try {
+                        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
+                        // Create the main UI window
+                        simulator.createAndShowUi();
+                    } catch (Exception e) {
+                        e.printStackTrace();
+                    }
+                }
+            });
+        
+        
+            Simulator.MAIN_CMD cmd = simulator.waitForCommand();
+            if(cmd.equals(Simulator.MAIN_CMD.SELECTMACHINE)) {
+                machineClass = null;
+            } else {
+                break;
+            }
+        }
+       
+    }
+    
+}

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

@@ -23,14 +23,12 @@
 
 package com.loomcom.symon;
 
-import com.loomcom.symon.devices.Acia;
-import com.loomcom.symon.devices.Crtc;
 import com.loomcom.symon.devices.Memory;
-import com.loomcom.symon.devices.Via;
 import com.loomcom.symon.exceptions.FifoUnderrunException;
 import com.loomcom.symon.exceptions.MemoryAccessException;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 import com.loomcom.symon.exceptions.SymonException;
+import com.loomcom.symon.machines.Machine;
 import com.loomcom.symon.ui.*;
 import com.loomcom.symon.ui.Console;
 
@@ -52,29 +50,6 @@ import java.util.logging.Logger;
  */
 public class Simulator {
 
-    // Constants used by the simulated system. These define the memory map.
-    private static final int BUS_BOTTOM = 0x0000;
-    private static final int BUS_TOP    = 0xffff;
-
-    // 32K of RAM from $0000 - $7FFF
-    private static final int MEMORY_BASE = 0x0000;
-    private static final int MEMORY_SIZE = 0x8000;
-
-    // VIA at $8000-$800F
-
-    private static final int VIA_BASE = 0x8000;
-
-    // ACIA at $8800-$8803
-    private static final int ACIA_BASE = 0x8800;
-
-    // CRTC at $9000-$9001
-    private static final int CRTC_BASE = 0x9000;
-    private static final int VIDEO_RAM_BASE = 0x7000;
-
-    // 16KB ROM at $C000-$FFFF
-    private static final int ROM_BASE = 0xC000;
-    private static final int ROM_SIZE = 0x4000;
-
     // UI constants
     private static final int  DEFAULT_FONT_SIZE    = 12;
     private static final Font DEFAULT_FONT         = new Font(Font.MONOSPACED, Font.PLAIN, DEFAULT_FONT_SIZE);
@@ -84,23 +59,17 @@ public class Simulator {
     // to refresh the status view on every simulated clock cycle. Instead, we will only refresh the status view
     // after this number of steps when running normally.
     //
-    // Since we're simulating a 1MHz 6502 here, we have a 1 us delay between steps. Setting this to 10000
-    // should give us a status update every 10 ms.
+    // Since we're simulating a 1MHz 6502 here, we have a 1 us delay between steps. Setting this to 20000
+    // should give us a status update about every 100 ms.
     //
     // TODO: Work around the event dispatch thread with custom painting code instead of relying on Swing.
     //
-    private static final int MAX_STEPS_BETWEEN_UPDATES = 10000;
+    private static final int MAX_STEPS_BETWEEN_UPDATES = 20000;
 
     private final static Logger logger = Logger.getLogger(Simulator.class.getName());
 
-    // The simulated peripherals
-    private final Bus    bus;
-    private final Cpu    cpu;
-    private final Acia   acia;
-    private final Via    via;
-    private final Crtc   crtc;
-    private final Memory ram;
-    private       Memory rom;
+    // The simulated machine
+    private Machine machine;
 
     // Number of CPU steps between CRT repaints.
     // TODO: Dynamically refresh the value at runtime based on performance figures to reach ~ 30fps.
@@ -142,43 +111,25 @@ public class Simulator {
     private JButton runStopButton;
     private JButton stepButton;
     private JButton resetButton;
-    private JComboBox stepCountBox;
+    private JComboBox<String> stepCountBox;
 
     private JFileChooser      fileChooser;
     private PreferencesDialog preferences;
 
+    private final Object commandMonitorObject = new Object();
+    private MAIN_CMD command = MAIN_CMD.NONE;
+    public static enum MAIN_CMD  {
+        NONE,
+        SELECTMACHINE
+    }
+    
     /**
      * The list of step counts that will appear in the "Step" drop-down.
      */
     private static final String[] STEPS = {"1", "5", "10", "20", "50", "100"};
 
-    public Simulator() throws MemoryRangeException, IOException {
-        this.bus = new Bus(BUS_BOTTOM, BUS_TOP);
-        this.cpu = new Cpu();
-        this.ram = new Memory(MEMORY_BASE, MEMORY_BASE + MEMORY_SIZE - 1, false);
-        this.via = new Via(VIA_BASE);
-        this.acia = new Acia(ACIA_BASE);
-        this.crtc = new Crtc(CRTC_BASE, ram);
-
-        bus.addCpu(cpu);
-
-        bus.addDevice(ram);
-        bus.addDevice(via);
-        bus.addDevice(acia);
-        bus.addDevice(crtc);
-
-        // TODO: Make this configurable, of course.
-        File romImage = new File("rom.bin");
-        if (romImage.canRead()) {
-            logger.info("Loading ROM image from file " + romImage);
-            this.rom = Memory.makeROM(ROM_BASE, ROM_BASE + ROM_SIZE - 1, romImage);
-        } else {
-            logger.info("Default ROM file " + romImage +
-                        " not found, loading empty R/W memory image.");
-            this.rom = Memory.makeRAM(ROM_BASE, ROM_BASE + ROM_SIZE - 1);
-        }
-
-        bus.addDevice(rom);
+    public Simulator(Class machineClass) throws Exception {
+        this.machine = (Machine) machineClass.getConstructors()[0].newInstance();
     }
 
     /**
@@ -212,7 +163,7 @@ public class Simulator {
         stepButton = new JButton("Step");
         resetButton = new JButton("Reset");
 
-        stepCountBox = new JComboBox(STEPS);
+        stepCountBox = new JComboBox<String>(STEPS);
         stepCountBox.addActionListener(new ActionListener() {
             public void actionPerformed(ActionEvent actionEvent) {
                 try {
@@ -266,10 +217,12 @@ public class Simulator {
         traceLog = new TraceLog();
 
         // Prepare the memory window
-        memoryWindow = new MemoryWindow(bus);
+        memoryWindow = new MemoryWindow(machine.getBus());
 
         // Composite Video and 6545 CRTC
-        videoWindow = new VideoWindow(crtc, 2, 2);
+        if(machine.getCrtc() != null) {
+            videoWindow = new VideoWindow(machine.getCrtc(), 2, 2);
+        }
 
         mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
@@ -282,7 +235,20 @@ public class Simulator {
         mainWindow.setVisible(true);
 
         console.requestFocus();
+        handleReset();
     }
+    
+    public MAIN_CMD waitForCommand() {
+        synchronized(commandMonitorObject) {
+            try {
+                commandMonitorObject.wait();
+            } catch (InterruptedException ex) {
+                ex.printStackTrace();
+            }
+        }
+        return command;
+    }
+    
 
     private void handleStart() {
         // Shift focus to the console.
@@ -312,7 +278,7 @@ public class Simulator {
         try {
             logger.log(Level.INFO, "Reset requested. Resetting CPU.");
             // Reset and clear memory
-            cpu.reset();
+            machine.getCpu().reset();
             // Clear the console.
             console.reset();
             // Reset the trace log.
@@ -321,7 +287,8 @@ public class Simulator {
             SwingUtilities.invokeLater(new Runnable() {
                 public void run() {
                     // Now update the state
-                    statusPane.updateState(cpu);
+                    statusPane.updateState(machine.getCpu());
+                    memoryWindow.updateState();
                 }
             });
         } catch (MemoryAccessException ex) {
@@ -342,7 +309,8 @@ public class Simulator {
                     if (traceLog.isVisible()) {
                         traceLog.refresh();
                     }
-                    statusPane.updateState(cpu);
+                    statusPane.updateState(machine.getCpu());
+                    memoryWindow.updateState();
                 }
             });
         } catch (SymonException ex) {
@@ -355,15 +323,15 @@ public class Simulator {
      * Perform a single step of the simulated system.
      */
     private void step() throws MemoryAccessException {
-        cpu.step();
+        machine.getCpu().step();
 
-        traceLog.append(cpu.getCpuState());
+        traceLog.append(machine.getCpu().getCpuState());
 
         // Read from the ACIA and immediately update the console if there's
         // output ready.
-        if (acia.hasTxChar()) {
+        if (machine.getAcia().hasTxChar()) {
             // This is thread-safe
-            console.print(Character.toString((char) acia.txRead()));
+            console.print(Character.toString((char) machine.getAcia().txRead()));
             console.repaint();
         }
 
@@ -371,14 +339,13 @@ public class Simulator {
         // TODO: Interrupt handling.
         try {
             if (console.hasInput()) {
-                acia.rxWrite((int) console.readInputChar());
+                machine.getAcia().rxWrite((int) console.readInputChar());
             }
         } catch (FifoUnderrunException ex) {
             logger.severe("Console type-ahead buffer underrun!");
         }
 
-        if (stepsSinceLastCrtcRefresh++ > stepsBetweenCrtcRefreshes) {
-            videoWindow.refreshDisplay();
+        if (videoWindow != null && stepsSinceLastCrtcRefresh++ > stepsBetweenCrtcRefreshes) {
             stepsSinceLastCrtcRefresh = 0;
         }
 
@@ -389,7 +356,8 @@ public class Simulator {
             SwingUtilities.invokeLater(new Runnable() {
                 public void run() {
                     // Now update the state
-                    statusPane.updateState(cpu);
+                    statusPane.updateState(machine.getCpu());
+                    memoryWindow.updateState();
                 }
             });
             stepsSinceLastUpdate = 0;
@@ -403,7 +371,7 @@ public class Simulator {
     private void loadProgram(byte[] program, int startAddress) throws MemoryAccessException {
         int addr = startAddress, i;
         for (i = 0; i < program.length; i++) {
-            bus.write(addr++, program[i] & 0xff);
+            machine.getBus().write(addr++, program[i] & 0xff);
         }
 
         logger.log(Level.INFO, "Loaded " + i + " bytes at address 0x" +
@@ -411,42 +379,21 @@ public class Simulator {
 
         // After loading, be sure to reset and
         // Reset (but don't clear memory, naturally)
-        cpu.reset();
+        machine.getCpu().reset();
 
         // Reset the stack program counter
-        cpu.setProgramCounter(preferences.getProgramStartAddress());
+        machine.getCpu().setProgramCounter(preferences.getProgramStartAddress());
 
         // Immediately update the UI.
         SwingUtilities.invokeLater(new Runnable() {
             public void run() {
                 // Now update the state
-                statusPane.updateState(cpu);
+                statusPane.updateState(machine.getCpu());
+                memoryWindow.updateState();
             }
         });
     }
 
-    /**
-     * Main entry point to the simulator. Creates a simulator and shows the main
-     * window.
-     *
-     * @param args
-     */
-    public static void main(String args[]) {
-        SwingUtilities.invokeLater(new Runnable() {
-            public void run() {
-                try {
-                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
-                    // Create the main UI window
-                    Simulator simulator = new Simulator();
-                    simulator.createAndShowUi();
-                    // Reset the simulator.
-                    simulator.handleReset();
-                } catch (Exception e) {
-                    e.printStackTrace();
-                }
-            }
-        });
-    }
 
     /**
      * The main run thread.
@@ -488,7 +435,8 @@ public class Simulator {
 
             SwingUtilities.invokeLater(new Runnable() {
                 public void run() {
-                    statusPane.updateState(cpu);
+                    statusPane.updateState(machine.getCpu());
+                    memoryWindow.updateState();
                     runStopButton.setText("Run");
                     stepButton.setEnabled(true);
                     stepCountBox.setEnabled(true);
@@ -510,7 +458,7 @@ public class Simulator {
          * @return True if the run loop should proceed to the next step.
          */
         private boolean shouldContinue() {
-            return isRunning && !(preferences.getHaltOnBreak() && cpu.getInstruction() == 0x00);
+            return isRunning && !(preferences.getHaltOnBreak() && machine.getCpu().getInstruction() == 0x00);
         }
     }
 
@@ -530,7 +478,7 @@ public class Simulator {
                     if (f.canRead()) {
                         long fileSize = f.length();
 
-                        if (fileSize > MEMORY_SIZE) {
+                        if (fileSize > machine.getMemorySize()) {
                             throw new IOException("Program will not fit in available memory.");
                         } else {
                             byte[] program = new byte[(int) fileSize];
@@ -577,22 +525,19 @@ public class Simulator {
                     if (romFile.canRead()) {
                         long fileSize = romFile.length();
 
-                        if (fileSize != ROM_SIZE) {
-                            throw new IOException("ROM file must be exactly " + String.valueOf(ROM_SIZE) + " bytes.");
+                        if (fileSize != machine.getRomSize()) {
+                            throw new IOException("ROM file must be exactly " + String.valueOf(machine.getRomSize()) + " bytes.");
                         } else {
-                            if (rom != null) {
-                                // Unload the existing ROM image.
-                                bus.removeDevice(rom);
-                            }
+                            
                             // Load the new ROM image
-                            rom = Memory.makeROM(ROM_BASE, ROM_BASE + ROM_SIZE - 1, romFile);
-                            bus.addDevice(rom);
+                            Memory rom = Memory.makeROM(machine.getRomBase(), machine.getRomBase() + machine.getRomSize() - 1, romFile);
+                            machine.setRom(rom);
 
                             // Now, reset
-                            cpu.reset();
+                            machine.getCpu().reset();
 
                             logger.log(Level.INFO, "ROM File `" + romFile.getName() + "' loaded at " +
-                                                   String.format("0x%04X", ROM_BASE));
+                                                   String.format("0x%04X", machine.getRomBase()));
                         }
                     }
                 }
@@ -617,6 +562,34 @@ public class Simulator {
             preferences.getDialog().setVisible(true);
         }
     }
+    
+    class SelectMachineAction extends AbstractAction {
+        Simulator simulator;
+        
+        public SelectMachineAction() {
+            super("Switch emulated machine...", null);
+            putValue(SHORT_DESCRIPTION, "Select the type of the machine to be emulated");
+            putValue(MNEMONIC_KEY, KeyEvent.VK_M);
+        }
+
+        public void actionPerformed(ActionEvent actionEvent) {
+            if(runLoop != null) {
+                runLoop.requestStop();
+            }
+
+            memoryWindow.dispose();
+            traceLog.dispose();
+            if(videoWindow != null) {
+                videoWindow.dispose();
+            }
+            mainWindow.dispose();
+
+            command = MAIN_CMD.SELECTMACHINE;
+            synchronized(commandMonitorObject) {
+                commandMonitorObject.notifyAll();
+            }
+        }
+    }
 
     class QuitAction extends AbstractAction {
         public QuitAction() {
@@ -743,11 +716,13 @@ public class Simulator {
             loadProgramItem = new JMenuItem(new LoadProgramAction());
             loadRomItem = new JMenuItem(new LoadRomAction());
             JMenuItem prefsItem = new JMenuItem(new ShowPrefsAction());
+            JMenuItem selectMachineItem = new JMenuItem(new SelectMachineAction());
             JMenuItem quitItem = new JMenuItem(new QuitAction());
 
             fileMenu.add(loadProgramItem);
             fileMenu.add(loadRomItem);
             fileMenu.add(prefsItem);
+            fileMenu.add(selectMachineItem);
             fileMenu.add(quitItem);
 
             add(fileMenu);
@@ -792,14 +767,16 @@ public class Simulator {
             });
             viewMenu.add(showMemoryTable);
 
-            final JCheckBoxMenuItem showVideoWindow = new JCheckBoxMenuItem(new ToggleVideoWindowAction());
-            videoWindow.addWindowListener(new WindowAdapter() {
-                @Override
-                public void windowClosing(WindowEvent e) {
-                    showVideoWindow.setSelected(false);
-                }
-            });
-            viewMenu.add(showVideoWindow);
+            if(videoWindow != null) {
+                final JCheckBoxMenuItem showVideoWindow = new JCheckBoxMenuItem(new ToggleVideoWindowAction());
+                videoWindow.addWindowListener(new WindowAdapter() {
+                    @Override
+                    public void windowClosing(WindowEvent e) {
+                        showVideoWindow.setSelected(false);
+                    }
+                });
+                viewMenu.add(showVideoWindow);
+            }
 
             add(viewMenu);
         }

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

@@ -23,169 +23,47 @@
 
 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.MemoryRangeException;
 
 
 /**
- * This is a simulation of the MOS 6551 ACIA, with limited
- * functionality.  Interrupts are not supported.
- * <p/>
- * Unlike a 16550 UART, the 6551 ACIA has only one-byte transmit and
- * receive buffers. It is the programmer's responsibility to check the
- * status (full or empty) for transmit and receive buffers before
- * writing / reading.
+ * Abstract base class for ACIAS such as the 6551 and 6580
  */
-public class Acia extends Device {
 
-    public static final int ACIA_SIZE = 4;
-
-    static final int DATA_REG = 0;
-    static final int STAT_REG = 1;
-    static final int CMND_REG = 2;
-    static final int CTRL_REG = 3;
+public abstract class Acia extends Device {
 
+    private String name;
+    
     /**
      * Register addresses
      */
-    private int baseAddress;
+    int baseAddress;
 
-    /**
-     * Registers. These are ignored in the current implementation.
-     */
-    private int commandRegister;
-    private int controlRegister;
-
-    private boolean overrun = false;
-
-    private long lastTxWrite   = 0;
-    private long lastRxRead    = 0;
-    private int  baudRate      = 0;
-    private long baudRateDelay = 0;
-
-    /**
+    boolean receiveIrqEnabled = false;
+    boolean transmitIrqEnabled = false;
+    boolean overrun = false;
+    
+	long lastTxWrite   = 0;
+    long lastRxRead    = 0;
+    int  baudRate      = 0;
+    long baudRateDelay = 0;
+	
+	/**
      * Read/Write buffers
      */
-    private int rxChar = 0;
-    private int txChar = 0;
+    int rxChar = 0;
+    int txChar = 0;
 
-    private boolean rxFull  = false;
-    private boolean txEmpty = true;
-
-    public Acia(int address) throws MemoryRangeException {
-        super(address, address + ACIA_SIZE - 1, "ACIA");
+    boolean rxFull  = false;
+    boolean txEmpty = true;
+	
+	
+    public Acia(int address, int size, String name) throws MemoryRangeException {
+        super(address, address + size - 1, name);
+        this.name = name;
         this.baseAddress = address;
     }
 
-    @Override
-    public int read(int address) throws MemoryAccessException {
-        switch (address) {
-            case DATA_REG:
-                return rxRead();
-            case STAT_REG:
-                return statusReg();
-            case CMND_REG:
-                return commandRegister;
-            case CTRL_REG:
-                return controlRegister;
-            default:
-                throw new MemoryAccessException("No register.");
-        }
-    }
-
-    @Override
-    public void write(int address, int data) throws MemoryAccessException {
-        switch (address) {
-            case 0:
-                txWrite(data);
-                break;
-            case 1:
-                reset();
-                break;
-            case 2:
-                commandRegister = data;
-                break;
-            case 3:
-                setControlRegister(data);
-                break;
-            default:
-                throw new MemoryAccessException("No register.");
-        }
-    }
-
-    /**
-     * Set the control register and associated state.
-     *
-     * @param data
-     */
-    public void setControlRegister(int data) {
-        this.controlRegister = data;
-
-        // If the value of the data is 0, this is a request to reset,
-        // otherwise it's a control update.
-
-        if (data == 0) {
-            reset();
-        } else {
-            // Mask the lower three bits to get the baud rate.
-            int baudSelector = data & 0x0f;
-            switch (baudSelector) {
-                case 0:
-                    baudRate = 0;
-                    break;
-                case 1:
-                    baudRate = 50;
-                    break;
-                case 2:
-                    baudRate = 75;
-                    break;
-                case 3:
-                    baudRate = 110; // Real rate is actually 109.92
-                    break;
-                case 4:
-                    baudRate = 135; // Real rate is actually 134.58
-                    break;
-                case 5:
-                    baudRate = 150;
-                    break;
-                case 6:
-                    baudRate = 300;
-                    break;
-                case 7:
-                    baudRate = 600;
-                    break;
-                case 8:
-                    baudRate = 1200;
-                    break;
-                case 9:
-                    baudRate = 1800;
-                    break;
-                case 10:
-                    baudRate = 2400;
-                    break;
-                case 11:
-                    baudRate = 3600;
-                    break;
-                case 12:
-                    baudRate = 4800;
-                    break;
-                case 13:
-                    baudRate = 7200;
-                    break;
-                case 14:
-                    baudRate = 9600;
-                    break;
-                case 15:
-                    baudRate = 19200;
-                    break;
-            }
-
-            // Recalculate the baud rate delay.
-            baudRateDelay = calculateBaudRateDelay();
-        }
-    }
 
     /*
      * Calculate the delay in nanoseconds between successive read/write operations, based on the
@@ -215,48 +93,54 @@ public class Acia extends Device {
      */
     public void setBaudRate(int rate) {
         this.baudRate = rate;
+		this.baudRateDelay = calculateBaudRateDelay();
     }
 
     /**
      * @return The contents of the status register.
      */
-    public int statusReg() {
-        // TODO: Overrun, Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
-        int stat = 0;
-        if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
-            stat |= 0x08;
-        }
-        if (txEmpty && System.nanoTime() >= (lastTxWrite + baudRateDelay)) {
-            stat |= 0x10;
-        }
-        return stat;
-    }
+    public abstract int statusReg();
 
     @Override
     public String toString() {
-        return "ACIA@" + String.format("%04X", baseAddress);
+        return name + "@" + String.format("%04X", baseAddress);
     }
 
     public synchronized int rxRead() {
         lastRxRead = System.nanoTime();
+        overrun = false;
         rxFull = false;
         return rxChar;
     }
 
     public synchronized void rxWrite(int data) {
+        if(rxFull) {
+            overrun = true;
+        }
+        
         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;
     }
 
     /**
@@ -273,11 +157,4 @@ public class Acia extends Device {
         return rxFull;
     }
 
-    private synchronized void reset() {
-        txChar = 0;
-        txEmpty = true;
-        rxChar = 0;
-        rxFull = false;
-    }
-
 }

src/main/java/com/loomcom/symon/devices/Acia6551.java

@@ -0,0 +1,208 @@
+/*
+ * Copyright (c) 2008-2013 Seth J. Morabito <sethm@loomcom.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package com.loomcom.symon.devices;
+
+import com.loomcom.symon.exceptions.MemoryAccessException;
+import com.loomcom.symon.exceptions.MemoryRangeException;
+
+
+/**
+ * This is a simulation of the MOS 6551 ACIA, with limited
+ * functionality.  Interrupts are not supported.
+ * <p/>
+ * Unlike a 16550 UART, the 6551 ACIA has only one-byte transmit and
+ * receive buffers. It is the programmer's responsibility to check the
+ * status (full or empty) for transmit and receive buffers before
+ * writing / reading.
+ */
+public class Acia6551 extends Acia {
+
+    public static final int ACIA_SIZE = 4;
+
+    static final int DATA_REG = 0;
+    static final int STAT_REG = 1;
+    static final int CMND_REG = 2;
+    static final int CTRL_REG = 3;
+
+
+    /**
+     * Registers. These are ignored in the current implementation.
+     */
+    private int commandRegister;
+    private int controlRegister;
+
+
+    public Acia6551(int address) throws MemoryRangeException {
+        super(address, ACIA_SIZE, "ACIA");
+    }
+
+    @Override
+    public int read(int address) throws MemoryAccessException {
+        switch (address) {
+            case DATA_REG:
+                return rxRead();
+            case STAT_REG:
+                return statusReg();
+            case CMND_REG:
+                return commandRegister;
+            case CTRL_REG:
+                return controlRegister;
+            default:
+                throw new MemoryAccessException("No register.");
+        }
+    }
+
+    @Override
+    public void write(int address, int data) throws MemoryAccessException {
+        switch (address) {
+            case 0:
+                txWrite(data);
+                break;
+            case 1:
+                reset();
+                break;
+            case 2:
+                setCommandRegister(data);
+                break;
+            case 3:
+                setControlRegister(data);
+                break;
+            default:
+                throw new MemoryAccessException("No register.");
+        }
+    }
+
+
+    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
+     */
+    private void setControlRegister(int data) {
+        controlRegister = data;
+        int rate = 0;
+
+        // If the value of the data is 0, this is a request to reset,
+        // otherwise it's a control update.
+
+        if (data == 0) {
+            reset();
+        } else {
+            // Mask the lower three bits to get the baud rate.
+            int baudSelector = data & 0x0f;
+            switch (baudSelector) {
+                case 0:
+                    rate = 0;
+                    break;
+                case 1:
+                    rate = 50;
+                    break;
+                case 2:
+                    rate = 75;
+                    break;
+                case 3:
+                    rate = 110; // Real rate is actually 109.92
+                    break;
+                case 4:
+                    rate = 135; // Real rate is actually 134.58
+                    break;
+                case 5:
+                    rate = 150;
+                    break;
+                case 6:
+                    rate = 300;
+                    break;
+                case 7:
+                    rate = 600;
+                    break;
+                case 8:
+                    rate = 1200;
+                    break;
+                case 9:
+                    rate = 1800;
+                    break;
+                case 10:
+                    rate = 2400;
+                    break;
+                case 11:
+                    rate = 3600;
+                    break;
+                case 12:
+                    rate = 4800;
+                    break;
+                case 13:
+                    rate = 7200;
+                    break;
+                case 14:
+                    rate = 9600;
+                    break;
+                case 15:
+                    rate = 19200;
+                    break;
+            }
+
+            setBaudRate(rate);
+        }
+    }
+
+
+    /**
+     * @return The contents of the status register.
+     */
+    @Override
+    public int statusReg() {
+        // TODO: Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
+        int stat = 0;
+        if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
+            stat |= 0x08;
+        }
+        if (txEmpty && System.nanoTime() >= (lastTxWrite + baudRateDelay)) {
+            stat |= 0x10;
+        }
+        if (overrun) {
+            stat |= 0x04;
+        }
+        return stat;
+    }
+
+
+    private synchronized void reset() {
+        txChar = 0;
+        txEmpty = true;
+        rxChar = 0;
+        rxFull = false;
+        receiveIrqEnabled = false;
+        transmitIrqEnabled = false;
+    }
+
+}

src/main/java/com/loomcom/symon/devices/Acia6850.java

@@ -0,0 +1,133 @@
+/*
+ * Copyright (c) 2008-2013 Seth J. Morabito <sethm@loomcom.com>
+ *                         Maik Merten <maikmerten@googlemail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package com.loomcom.symon.devices;
+
+import com.loomcom.symon.exceptions.MemoryAccessException;
+import com.loomcom.symon.exceptions.MemoryRangeException;
+
+
+/**
+ * This is a simulation of the Motorola 6850 ACIA, with limited
+ * functionality.  Interrupts are not supported.
+ * <p/>
+ * Unlike a 16550 UART, the 6850 ACIA has only one-byte transmit and
+ * receive buffers. It is the programmer's responsibility to check the
+ * status (full or empty) for transmit and receive buffers before
+ * writing / reading.
+ */
+public class Acia6850 extends Acia {
+
+    public static final int ACIA_SIZE = 2;
+
+
+    static final int STAT_REG = 0;	// read-only
+    static final int CTRL_REG = 0;	// write-only
+	
+    static final int RX_REG = 1;	// read-only
+    static final int TX_REG = 1;	// write-only
+
+    
+    /**
+     * Registers. These are ignored in the current implementation.
+     */
+    private int commandRegister;
+
+
+    public Acia6850(int address) throws MemoryRangeException {
+        super(address, ACIA_SIZE, "ACIA6850");
+        setBaudRate(2400);
+    }
+
+    @Override
+    public int read(int address) throws MemoryAccessException {
+        switch (address) {
+            case RX_REG:
+                return rxRead();
+            case STAT_REG:
+                return statusReg();
+
+            default:
+                throw new MemoryAccessException("No register.");
+        }
+    }
+
+    @Override
+    public void write(int address, int data) throws MemoryAccessException {
+        switch (address) {
+            case TX_REG:
+                txWrite(data);
+                break;
+            case CTRL_REG:
+                setCommandRegister(data);
+                break;
+            default:
+                throw new MemoryAccessException("No register.");
+        }
+    }
+    
+    private void setCommandRegister(int data) {
+        commandRegister = data;
+		
+        // Bits 0 & 1 control the master reset
+        if((commandRegister & 0x01) != 0 && (commandRegister & 0x02) != 0) {
+            reset();
+        }
+
+        // Bit 7 controls receiver IRQ behavior
+        receiveIrqEnabled = (commandRegister & 0x80) != 0;
+        // Bits 5 & 6 controls transmit IRQ behavior
+        transmitIrqEnabled = (commandRegister & 0x20) != 0 && (commandRegister & 0x40) == 0;
+    }
+
+
+
+    /**
+     * @return The contents of the status register.
+     */
+    @Override
+    public int statusReg() {
+        // TODO: Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
+        int stat = 0;
+        if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
+            stat |= 0x01;
+        }
+        if (txEmpty && System.nanoTime() >= (lastTxWrite + baudRateDelay)) {
+            stat |= 0x02;
+        }
+        if (overrun) {
+            stat |= 0x20;
+        }
+		
+        return stat;
+    }
+
+
+    private synchronized void reset() {
+        overrun = false;
+        rxFull = false;
+        txEmpty = true;
+    }
+
+}

src/main/java/com/loomcom/symon/machines/Machine.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2008-2014 Seth J. Morabito <sethm@loomcom.com>
+ *                         Maik Merten <maikmerten@googlemail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package com.loomcom.symon.machines;
+
+import com.loomcom.symon.Bus;
+import com.loomcom.symon.Cpu;
+import com.loomcom.symon.devices.Acia;
+import com.loomcom.symon.devices.Crtc;
+import com.loomcom.symon.devices.Memory;
+import com.loomcom.symon.devices.Via;
+import com.loomcom.symon.exceptions.MemoryRangeException;
+
+
+public interface Machine {
+    
+    public Bus getBus();
+    
+    public Cpu getCpu();
+    
+    public Memory getRam();
+    
+    public Acia getAcia();
+    
+    public Via getVia();
+    
+    public Crtc getCrtc();
+    
+    public Memory getRom();
+    
+    public void setRom(Memory rom) throws MemoryRangeException;
+    
+    public int getRomBase();
+    
+    public int getRomSize();
+    
+    public int getMemorySize();
+    
+}

src/main/java/com/loomcom/symon/machines/MulticompMachine.java

@@ -0,0 +1,151 @@
+/*
+ * Copyright (c) 2008-2014 Seth J. Morabito <sethm@loomcom.com>
+ *                         Maik Merten <maikmerten@googlemail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package com.loomcom.symon.machines;
+
+import com.loomcom.symon.Bus;
+import com.loomcom.symon.Cpu;
+import com.loomcom.symon.devices.Acia;
+import com.loomcom.symon.devices.Acia6850;
+import com.loomcom.symon.devices.Crtc;
+import com.loomcom.symon.devices.Memory;
+import com.loomcom.symon.devices.Via;
+import com.loomcom.symon.exceptions.MemoryRangeException;
+import java.io.File;
+import java.util.logging.Logger;
+
+
+public class MulticompMachine implements Machine {
+    
+    private final static Logger logger = Logger.getLogger(MulticompMachine.class.getName());
+    
+    // Constants used by the simulated system. These define the memory map.
+    private static final int BUS_BOTTOM = 0x0000;
+    private static final int BUS_TOP    = 0xffff;
+
+    // 56K of RAM from $0000 - $DFFF
+    private static final int MEMORY_BASE = 0x0000;
+    private static final int MEMORY_SIZE = 0xE000;
+
+    // ACIA at $FFD0-$FFD1
+    private static final int ACIA_BASE = 0xFFD0;
+
+
+    // 8KB ROM at $E000-$FFFF
+    private static final int ROM_BASE = 0xE000;
+    private static final int ROM_SIZE = 0x2000;
+
+
+        // The simulated peripherals
+    private final Bus    bus;
+    private final Cpu    cpu;
+    private final Acia   acia;
+    private final Memory ram;
+    private       Memory rom;
+
+
+    public MulticompMachine() throws Exception {
+        this.bus = new Bus(BUS_BOTTOM, BUS_TOP);
+        this.cpu = new Cpu();
+        this.ram = new Memory(MEMORY_BASE, MEMORY_BASE + MEMORY_SIZE - 1, false);
+        this.acia = new Acia6850(ACIA_BASE);
+
+        bus.addCpu(cpu);
+        bus.addDevice(ram);
+        bus.addDevice(acia, 1);
+        
+        // TODO: Make this configurable, of course.
+        File romImage = new File("rom.bin");
+        if (romImage.canRead()) {
+            logger.info("Loading ROM image from file " + romImage);
+            this.rom = Memory.makeROM(ROM_BASE, ROM_BASE + ROM_SIZE - 1, romImage);
+        } else {
+            logger.info("Default ROM file " + romImage +
+                        " not found, loading empty R/W memory image.");
+            this.rom = Memory.makeRAM(ROM_BASE, ROM_BASE + ROM_SIZE - 1);
+        }
+
+        bus.addDevice(rom);
+        
+    }
+
+    @Override
+    public Bus getBus() {
+        return bus;
+    }
+
+    @Override
+    public Cpu getCpu() {
+        return cpu;
+    }
+
+    @Override
+    public Memory getRam() {
+        return ram;
+    }
+
+    @Override
+    public Acia getAcia() {
+        return acia;
+    }
+
+    @Override
+    public Via getVia() {
+        return null;
+    }
+
+    @Override
+    public Crtc getCrtc() {
+        return null;
+    }
+
+    @Override
+    public Memory getRom() {
+        return rom;
+    }
+    
+    public void setRom(Memory rom) throws MemoryRangeException {
+        if(this.rom != null) {
+            bus.removeDevice(this.rom);
+        }
+        this.rom = rom;
+        bus.addDevice(this.rom);
+    }
+
+    @Override
+    public int getRomBase() {
+        return ROM_BASE;
+    }
+
+    @Override
+    public int getRomSize() {
+        return ROM_SIZE;
+    }
+
+    @Override
+    public int getMemorySize() {
+        return MEMORY_SIZE;
+    }
+    
+}

src/main/java/com/loomcom/symon/machines/SymonMachine.java

@@ -0,0 +1,167 @@
+/*
+ * Copyright (c) 2008-2014 Seth J. Morabito <sethm@loomcom.com>
+ *                         Maik Merten <maikmerten@googlemail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package com.loomcom.symon.machines;
+
+import com.loomcom.symon.Bus;
+import com.loomcom.symon.Cpu;
+import com.loomcom.symon.devices.Acia;
+import com.loomcom.symon.devices.Acia6551;
+import com.loomcom.symon.devices.Crtc;
+import com.loomcom.symon.devices.Memory;
+import com.loomcom.symon.devices.Via;
+import com.loomcom.symon.exceptions.MemoryRangeException;
+import java.io.File;
+import java.util.logging.Logger;
+
+
+public class SymonMachine implements Machine {
+    
+    private final static Logger logger = Logger.getLogger(SymonMachine.class.getName());
+    
+    // Constants used by the simulated system. These define the memory map.
+    private static final int BUS_BOTTOM = 0x0000;
+    private static final int BUS_TOP    = 0xffff;
+
+    // 32K of RAM from $0000 - $7FFF
+    private static final int MEMORY_BASE = 0x0000;
+    private static final int MEMORY_SIZE = 0x8000;
+
+    // VIA at $8000-$800F
+
+    private static final int VIA_BASE = 0x8000;
+
+    // ACIA at $8800-$8803
+    private static final int ACIA_BASE = 0x8800;
+
+    // CRTC at $9000-$9001
+    private static final int CRTC_BASE = 0x9000;
+    private static final int VIDEO_RAM_BASE = 0x7000;
+
+    // 16KB ROM at $C000-$FFFF
+    private static final int ROM_BASE = 0xC000;
+    private static final int ROM_SIZE = 0x4000;
+
+
+        // The simulated peripherals
+    private final Bus    bus;
+    private final Cpu    cpu;
+    private final Acia   acia;
+    private final Via    via;
+    private final Crtc   crtc;
+    private final Memory ram;
+    private       Memory rom;
+
+
+    public SymonMachine() throws Exception {
+        this.bus = new Bus(BUS_BOTTOM, BUS_TOP);
+        this.cpu = new Cpu();
+        this.ram = new Memory(MEMORY_BASE, MEMORY_BASE + MEMORY_SIZE - 1, false);
+        this.via = new Via(VIA_BASE);
+        this.acia = new Acia6551(ACIA_BASE);
+        this.crtc = new Crtc(CRTC_BASE, ram);
+
+        bus.addCpu(cpu);
+        bus.addDevice(ram);
+        bus.addDevice(via);
+        bus.addDevice(acia);
+        bus.addDevice(crtc);
+        
+        // TODO: Make this configurable, of course.
+        File romImage = new File("rom.bin");
+        if (romImage.canRead()) {
+            logger.info("Loading ROM image from file " + romImage);
+            this.rom = Memory.makeROM(ROM_BASE, ROM_BASE + ROM_SIZE - 1, romImage);
+        } else {
+            logger.info("Default ROM file " + romImage +
+                        " not found, loading empty R/W memory image.");
+            this.rom = Memory.makeRAM(ROM_BASE, ROM_BASE + ROM_SIZE - 1);
+        }
+
+        bus.addDevice(rom);
+        
+    }
+
+    @Override
+    public Bus getBus() {
+        return bus;
+    }
+
+    @Override
+    public Cpu getCpu() {
+        return cpu;
+    }
+
+    @Override
+    public Memory getRam() {
+        return ram;
+    }
+
+    @Override
+    public Acia getAcia() {
+        return acia;
+    }
+
+    @Override
+    public Via getVia() {
+        return via;
+    }
+
+    @Override
+    public Crtc getCrtc() {
+        return crtc;
+    }
+
+    @Override
+    public Memory getRom() {
+        return rom;
+    }
+    
+    public void setRom(Memory rom) throws MemoryRangeException {
+        if(this.rom != null) {
+            bus.removeDevice(this.rom);
+        }
+        this.rom = rom;
+        bus.addDevice(this.rom);
+    }
+
+    @Override
+    public int getRomBase() {
+        return ROM_BASE;
+    }
+
+    @Override
+    public int getRomSize() {
+        return ROM_SIZE;
+    }
+
+    @Override
+    public int getMemorySize() {
+        return MEMORY_SIZE;
+    }
+    
+    
+
+    
+}

src/main/java/com/loomcom/symon/ui/MemoryWindow.java

@@ -190,14 +190,14 @@ public class MemoryWindow extends JFrame implements ActionListener {
             if (currentPage > 0x00) {
                 setPageNumber(currentPage - 1);
                 updateControls();
-                memoryTable.updateUI();
+                updateState();
             }
         } else if (e.getSource() == nextPageButton) {
             int currentPage = getPageNumber();
             if (currentPage < 0xff) {
                 setPageNumber(currentPage + 1);
                 updateControls();
-                memoryTable.updateUI();
+                updateState();
             }
         } else if (e.getSource() == pageNumberTextField) {
             String pageNumberInput = pageNumberTextField.getText();
@@ -205,7 +205,7 @@ public class MemoryWindow extends JFrame implements ActionListener {
                 // Try to parse a hex value out of the pageNumber.
                 int newPageNumber = Integer.parseInt(pageNumberInput, 16);
                 setPageNumber(newPageNumber & 0xff);
-                memoryTable.updateUI();
+                updateState();
             } catch (NumberFormatException ex) {
                 // An invalid number was entered. Log the error, but otherwise
                 // take no action.
@@ -216,6 +216,13 @@ public class MemoryWindow extends JFrame implements ActionListener {
         }
     }
 
+    /**
+     * Refresh the view of memory
+     */
+    public void updateState() {
+        memoryTable.updateUI();
+    }
+
     /**
      * A JTable that will automatically select all text in a cell
      * being edited.

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

@@ -1,23 +1,94 @@
 package com.loomcom.symon;
 
-import org.junit.*;
-
 import com.loomcom.symon.devices.Acia;
-import com.loomcom.symon.exceptions.FifoUnderrunException;
+import com.loomcom.symon.devices.Acia6551;
+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 Acia6551(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 Acia6551(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 Acia6551(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 Acia6551(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);
+        Acia acia = new Acia6551(0x000);
 
         assertEquals(0x10, acia.read(0x0001));
     }
 
     @Test
     public void aciaShouldHaveTxEmptyStatusOffIfTxHasData() throws Exception {
-        Acia acia = new Acia(0x000);
+        Acia acia = new Acia6551(0x000);
 
         acia.txWrite('a');
         assertEquals(0x00, acia.read(0x0001));
@@ -25,7 +96,7 @@ public class AciaTest {
 
     @Test
     public void aciaShouldHaveRxFullStatusOffIfRxHasData() throws Exception {
-        Acia acia = new Acia(0x000);
+        Acia acia = new Acia6551(0x000);
 
         acia.rxWrite('a');
         assertEquals(0x18, acia.read(0x0001));
@@ -34,18 +105,37 @@ public class AciaTest {
     @Test
     public void aciaShouldHaveTxEmptyAndRxFullStatusOffIfRxAndTxHaveData()
             throws Exception {
-        Acia acia = new Acia(0x000);
+        Acia acia = new Acia6551(0x000);
 
         acia.rxWrite('a');
         acia.txWrite('b');
 
         assertEquals(0x08, acia.read(0x0001));
     }
+    
+    @Test
+    public void aciaShouldOverrunAndReadShouldReset()
+            throws Exception {
+        
+        Acia acia = new Acia6551(0x0000);
+        
+        // overrun ACIA
+        acia.rxWrite('a');
+        acia.rxWrite('b');
+        
+        assertEquals(0x04, acia.read(0x0001) & 0x04);
+        
+        // read should reset
+        acia.rxRead();
+        assertEquals(0x00, acia.read(0x0001) & 0x04);
+        
+    }
+
 
     @Test
     public void readingBuffersShouldResetStatus()
             throws Exception {
-        Acia acia = new Acia(0x0000);
+        Acia acia = new Acia6551(0x0000);
 
         acia.rxWrite('a');
         acia.txWrite('b');

src/test/java/com/loomcom/symon/AciaTest6850.java

@@ -0,0 +1,163 @@
+package com.loomcom.symon;
+
+import com.loomcom.symon.devices.Acia;
+import com.loomcom.symon.devices.Acia6850;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.*;
+
+public class AciaTest6850 {
+    
+    private final static int CMD_STAT_REG = 0;
+    private final static int DATA_REG = 1;
+    
+    
+    private Acia newAcia() throws Exception {
+        Acia acia = new Acia6850(0x0000);
+        // by default rate is limited, have it unlimited for testing
+        acia.setBaudRate(0);
+        return acia;
+    }
+    
+
+    @Test
+    public void shouldTriggerInterruptOnRxFullIfRxIrqEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = newAcia();
+        acia.setBus(mockBus);
+
+        // Disable TX IRQ, Enable RX IRQ
+        acia.write(CMD_STAT_REG, 0x80);
+
+        acia.rxWrite('a');
+
+        verify(mockBus, atLeastOnce()).assertIrq();
+    }
+
+    @Test
+    public void shouldNotTriggerInterruptOnRxFullIfRxIrqNotEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = newAcia();
+        acia.setBus(mockBus);
+
+        // Disable TX IRQ, Disable RX IRQ
+        acia.write(CMD_STAT_REG, 0x00);
+
+        acia.rxWrite('a');
+
+        verify(mockBus, never()).assertIrq();
+    }
+
+    @Test
+    public void shouldTriggerInterruptOnTxEmptyIfTxIrqEnabled() throws Exception {
+        Bus mockBus = mock(Bus.class);
+
+        Acia acia = newAcia();
+        acia.setBus(mockBus);
+
+        // Enable TX IRQ, Disable RX IRQ
+        acia.write(CMD_STAT_REG, 0x20);
+
+        // Write data
+        acia.write(1, '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 = newAcia();
+        acia.setBus(mockBus);
+
+        // Disable TX IRQ, Disable RX IRQ
+        acia.write(CMD_STAT_REG, 0x02);
+
+        // Write data
+        acia.write(DATA_REG, 'a');
+
+        // Transmission should cause IRQ
+        acia.txRead();
+
+        verify(mockBus, never()).assertIrq();
+    }
+
+    @Test
+    public void newAciaShouldHaveTxEmptyStatus() throws Exception {
+        Acia acia = newAcia();
+
+        assertEquals(0x02, acia.read(CMD_STAT_REG) & 0x02);
+    }
+
+    @Test
+    public void aciaShouldHaveTxEmptyStatusOffIfTxHasData() throws Exception {
+        Acia acia = newAcia();
+
+        acia.txWrite('a');
+        assertEquals(0x00, acia.read(CMD_STAT_REG) & 0x02);
+    }
+
+    @Test
+    public void aciaShouldHaveRxFullStatusOnIfRxHasData() throws Exception {
+        Acia acia = newAcia();
+
+        acia.rxWrite('a');
+       
+        assertEquals(0x01, acia.read(CMD_STAT_REG) & 0x01);
+    }
+
+    @Test
+    public void aciaShouldHaveTxEmptyAndRxFullStatusOffIfRxAndTxHaveData()
+            throws Exception {
+        Acia acia = newAcia();
+       
+        acia.rxWrite('a');
+        acia.txWrite('b');
+
+        assertEquals(0x01, acia.read(CMD_STAT_REG) & 0x03);
+    }
+    
+    @Test
+    public void aciaShouldOverrunAndReadShouldReset()
+            throws Exception {
+        
+        Acia acia = newAcia();
+        
+        // overrun ACIA
+        acia.rxWrite('a');
+        acia.rxWrite('b');
+        
+        assertEquals(0x20, acia.read(CMD_STAT_REG) & 0x20);
+        
+        // read should reset
+        acia.rxRead();
+        assertEquals(0x00, acia.read(CMD_STAT_REG) & 0x20);
+        
+    }
+
+    @Test
+    public void readingBuffersShouldResetStatus()
+            throws Exception {
+        Acia acia = newAcia();
+
+        assertEquals(0x00, acia.read(CMD_STAT_REG) & 0x01);
+        
+        acia.rxWrite('a');
+        
+        assertEquals(0x01, acia.read(CMD_STAT_REG) & 0x01);
+        
+        acia.rxRead();
+        
+        assertEquals(0x00, acia.read(CMD_STAT_REG) & 0x01);        
+        
+    }
+}

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

@@ -10,113 +10,129 @@ 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 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/CpuAbsoluteModeTest.java

@@ -14,7 +14,7 @@ public class CpuAbsoluteModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -14,7 +14,7 @@ public class CpuAbsoluteXModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -14,7 +14,7 @@ public class CpuAbsoluteYModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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, 0xffff);
+        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/CpuImmediateModeTest.java

@@ -14,7 +14,7 @@ public class CpuImmediateModeTest extends TestCase {
     public void setUp() throws MemoryRangeException, MemoryAccessException {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -17,7 +17,7 @@ public class CpuImpliedModeTest {
     public void setUp() throws MemoryRangeException, MemoryAccessException {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -16,7 +16,7 @@ public class CpuIndexedIndirectModeTest {
     public void runBeforeEveryTest() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -16,7 +16,7 @@ public class CpuIndirectIndexedModeTest {
     public void runBeforeEveryTest() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -14,7 +14,7 @@ public class CpuIndirectModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -13,7 +13,7 @@ public class CpuIndirectXModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -14,7 +14,7 @@ public class CpuRelativeModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -25,7 +25,7 @@ public class CpuTest extends TestCase {
     public void setUp() throws MemoryRangeException, MemoryAccessException {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 
@@ -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));

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

@@ -14,7 +14,7 @@ public class CpuZeroPageModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -14,7 +14,7 @@ public class CpuZeroPageXModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);
 

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

@@ -14,7 +14,7 @@ public class CpuZeroPageYModeTest extends TestCase {
     protected void setUp() throws Exception {
         this.cpu = new Cpu();
         this.bus = new Bus(0x0000, 0xffff);
-        this.mem = new Memory(0x0000, 0x10000);
+        this.mem = new Memory(0x0000, 0xffff);
         bus.addCpu(cpu);
         bus.addDevice(mem);