CPU bug fixes and Simulator enhancements.

Bug Fixes:

- Fixed several bugs in the CPU that caused processor status flags to
  be set incorrectly.  Instructions affected were: STA, STX, STY, CMP,
  CPX, CPY, BIT.

- Made some internal-use-only methods on the CPU class private.

- Fixed incorrect disassembly of (Indirect,X) and (Indirect),Y
  instructions. Although this didn't affect behavior, it certainly
  caused me some confusion in debugging.

- Added missing "BCS" instruction to instruction table.

Enhancements:

- Now includes a full version of Lee Davison's Enhanced 6502 BASIC
  bundled as source code and a ROM image. Get that REAL COMPUTER
  EXPERIENCE!(tm)

- If a file named "rom.bin" exists in the same directory where the
  simulator is executed, it will be loaded at addresses $d000-$ffff.

- Gave the CPU an idle loop to make simulated timing a little more
  realistic (but this is still an area needing major improvement)

- Changed the CPU's toString() method to give better debugging output.

- Added a small typeahead buffer to the Console.

- Better exception messaging.

Misc:

- Bumped version to 0.5, updated README.

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

@@ -412,7 +412,8 @@ public class CpuAbsoluteYModeTest extends TestCase {
         bus.loadProgram(0x99, 0x10, 0xab); // STA $ab10,Y
         cpu.step();
         assertEquals(0x00, bus.read(0xab40));
-        assertTrue(cpu.getZeroFlag());
+        // STA should have NO effect on flags
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -434,7 +435,7 @@ public class CpuAbsoluteYModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0xab40));
         assertFalse(cpu.getZeroFlag());
-        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDX - Load X Register - $be */
@@ -523,7 +524,7 @@ public class CpuAbsoluteYModeTest extends TestCase {
         cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-        assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag()); // $80 - $ff = $81
     }
 
     /* SBC - Subtract with Carry - $f9 */

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

@@ -412,7 +412,7 @@ public class CpuIndirectXModeTest extends TestCase {
         bus.loadProgram(0x9d, 0x10, 0xab); // STA $ab10,X
         cpu.step();
         assertEquals(0x00, bus.read(0xab40));
-        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -434,7 +434,7 @@ public class CpuIndirectXModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0xab40));
         assertFalse(cpu.getZeroFlag());
-        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDA - Load Accumulator - $bd */
@@ -494,7 +494,7 @@ public class CpuIndirectXModeTest extends TestCase {
         cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-        assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag()); // m - y < 0
     }
 
     /* SBC - Subtract with Carry - $fd */

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

@@ -709,7 +709,8 @@ public class CpuZeroPageXModeTest extends TestCase {
         bus.loadProgram(0x94, 0x10); // STY $10,X
         cpu.step();
         assertEquals(0x00, bus.read(0x40));
-        assertTrue(cpu.getZeroFlag());
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -730,7 +731,7 @@ public class CpuZeroPageXModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0x40));
         assertFalse(cpu.getZeroFlag());
-        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* STA - Store Accumulator - $95 */
@@ -742,7 +743,8 @@ public class CpuZeroPageXModeTest extends TestCase {
         bus.loadProgram(0x95, 0x10); // STA $10,X
         cpu.step();
         assertEquals(0x00, bus.read(0x40));
-        assertTrue(cpu.getZeroFlag());
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -763,7 +765,7 @@ public class CpuZeroPageXModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0x40));
         assertFalse(cpu.getZeroFlag());
-        assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDY - Load Y Register - $b4 */
@@ -852,7 +854,7 @@ public class CpuZeroPageXModeTest extends TestCase {
         cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-        assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag()); // m - y < 0
     }
 
     /* DEC - Decrement Memory Location - $d6 */

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

@@ -7,96 +7,97 @@ import junit.framework.TestCase;
 
 public class CpuZeroPageYModeTest extends TestCase {
 
-  protected Cpu cpu;
-  protected Bus bus;
-  protected Memory mem;
+    protected Cpu    cpu;
+    protected Bus    bus;
+    protected Memory mem;
 
-  protected void setUp() throws Exception {
-    this.cpu = new Cpu();
-    this.bus = new Bus(0x0000, 0xffff);
-    this.mem = new Memory(0x0000, 0x10000);
-    bus.addCpu(cpu);
-    bus.addDevice(mem);
+    protected void setUp() throws Exception {
+        this.cpu = new Cpu();
+        this.bus = new Bus(0x0000, 0xffff);
+        this.mem = new Memory(0x0000, 0x10000);
+        bus.addCpu(cpu);
+        bus.addDevice(mem);
 
-    // Load the reset vector.
-    bus.write(0xfffc, Cpu.DEFAULT_BASE_ADDRESS & 0x00ff);
-    bus.write(0xfffd, (Cpu.DEFAULT_BASE_ADDRESS & 0xff00)>>>8);
+        // Load the reset vector.
+        bus.write(0xfffc, Cpu.DEFAULT_BASE_ADDRESS & 0x00ff);
+        bus.write(0xfffd, (Cpu.DEFAULT_BASE_ADDRESS & 0xff00) >>> 8);
 
-    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());
-  }
+        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());
+    }
 
-  /*
-   * The following opcodes are tested for correctness in this file:
-   *
-   * STX - $96
-   * LDX - $b6
-   *
-   */
+    /*
+    * The following opcodes are tested for correctness in this file:
+    *
+    * STX - $96
+    * LDX - $b6
+    *
+    */
 
-  /* STX - Store X Register - $96 */
+    /* STX - Store X Register - $96 */
 
-  public void test_STX() throws MemoryAccessException {
-    cpu.setYRegister(0x30);
-    cpu.setXRegister(0x00);
-    bus.loadProgram(0x96, 0x10);  // STX $10,Y
-    cpu.step();
-    assertEquals(0x00, bus.read(0x40));
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+    public void test_STX() throws MemoryAccessException {
+        cpu.setYRegister(0x30);
+        cpu.setXRegister(0x00);
+        bus.loadProgram(0x96, 0x10);  // STX $10,Y
+        cpu.step();
+        assertEquals(0x00, bus.read(0x40));
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
 
-    cpu.reset();
-    cpu.setYRegister(0x30);
-    cpu.setXRegister(0x0f);
-    bus.loadProgram(0x96, 0x10);  // STX $10,Y
-    cpu.step();
-    assertEquals(0x0f, bus.read(0x40));
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        cpu.reset();
+        cpu.setYRegister(0x30);
+        cpu.setXRegister(0x0f);
+        bus.loadProgram(0x96, 0x10);  // STX $10,Y
+        cpu.step();
+        assertEquals(0x0f, bus.read(0x40));
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
 
-    cpu.reset();
-    cpu.setYRegister(0x30);
-    cpu.setXRegister(0x80);
-    bus.loadProgram(0x96, 0x10);  // STX $10,Y
-    cpu.step();
-    assertEquals(0x80, bus.read(0x40));
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-  }
+        cpu.reset();
+        cpu.setYRegister(0x30);
+        cpu.setXRegister(0x80);
+        bus.loadProgram(0x96, 0x10);  // STX $10,Y
+        cpu.step();
+        assertEquals(0x80, bus.read(0x40));
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
+    }
 
-  /* LDX - Load X Register - $b6 */
+    /* LDX - Load X Register - $b6 */
 
-  public void test_LDX() throws MemoryAccessException {
-    bus.write(0x40, 0x00);
-    bus.write(0x41, 0x0f);
-    bus.write(0x42, 0x80);
+    public void test_LDX() throws MemoryAccessException {
+        bus.write(0x40, 0x00);
+        bus.write(0x41, 0x0f);
+        bus.write(0x42, 0x80);
 
-    bus.loadProgram(0xb6, 0x10,
-                    0xb6, 0x11,
-                    0xb6, 0x12);
+        bus.loadProgram(0xb6, 0x10,
+                        0xb6, 0x11,
+                        0xb6, 0x12);
 
-    cpu.setYRegister(0x30);
+        cpu.setYRegister(0x30);
 
-    cpu.step();
-    assertEquals(0x00, cpu.getXRegister());
-    assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        cpu.step();
+        assertEquals(0x00, cpu.getXRegister());
+        assertTrue(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
 
-    cpu.step();
-    assertEquals(0x0f, cpu.getXRegister());
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        cpu.step();
+        assertEquals(0x0f, cpu.getXRegister());
+        assertFalse(cpu.getZeroFlag());
+        assertFalse(cpu.getNegativeFlag());
 
-    cpu.step();
-    assertEquals(0x80, cpu.getXRegister());
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-  }
+        cpu.step();
+        assertEquals(0x80, cpu.getXRegister());
+        assertFalse(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+    }
 
 }