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.

COPYING

@@ -1,6 +1,6 @@
 The MIT License
 
-Copyright (c) 2008-2010 Seth J. Morabito <sethm@loomcom.com>
+Copyright (c) 2008-2012 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

README.md

@@ -1,12 +1,11 @@
 SYMON - A 6502 System Simulator
 ===============================
 
-**NOTE: THIS IS ALPHA QUALITY SOFTWARE UNDER ACTIVE DEVELOPMENT. IT IS
+**NOTE: THIS IS BETA QUALITY SOFTWARE UNDER ACTIVE DEVELOPMENT.  Feedback is
-NOT YET FULLY FUNCTIONAL. IT MAY BE USEFUL, BUT IT IS NOT YET INTENDED
+welcome!**
-TO BE USED BY ANYONE BUT DEVELOPERS. Feedback is welcome!**
 
-**Version:** 0.3
+**Version:** 0.5
-**Last Updated:** 14 October, 2012  
+**Last Updated:** 21 October, 2012  
 
 Copyright (c) 2008-2012 Seth J. Morabito <web@loomcom.com>
 
@@ -70,23 +69,36 @@ for testing.
 
 - 'hello.prg' will continuously print "Hello, 6502 World!" to the console.
 
-### 3.3 Running
+The sample directory also contains a ROM image of Lee Davison's
+Ehanced 6502 BASIC. For instructions on loading the rom, please see
+the README file in that directory.
 
-After loading a program, clicking "Run" will start the simulator
+### 3.3 ROM files
+
+Any 12KB file named 'rom.bin' that exists in the same directory where
+Symon is launched will be loaded at address $d000. If the file is
+larger than 12KB, loading will fail. This functionality will be
+improved in a future release!
+
+### 3.4 Running
+
+After loading a program or ROM image, clicking "Run" will start the simulator
 running at address $0300.
 
 
 ## 4.0 To Do
 
-- Accurate timing (all simulated instructions currently take
+- Better debugging tools from the UI, including memory inspection,
-  only one step to execute)
+  disassembly, breakpoints, and execution tracing.
+
+- Better ROM loading (and re-loading)
+
+- More accurate timing.
   
 - Interrupt handling!
 
 - UI needs a ton more polish.
 
-- Add a simple menu interface for common tasks.
-
 - More extensive testing.
 
 - Clean up JavaDoc.

samples/README

@@ -1,9 +1,39 @@
-A Sample Program
+Sample Programs
 ----------------
 
+1. hello.prg
+
   When loaded at address $0300, this program will print "Hello, 6502 World!" in
   infinite loop.
 
-Assembled with the Ophis assembler:
+2. echo.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:
 
    https://hkn.eecs.berkeley.edu/~mcmartin/ophis/
+
+
+3. ehbasic.rom
+
+   This is Lee Davison's Enhanced 6502 BASIC.
+
+   To use this ROM image, just copy the file 'ehbasic.rom' into the directory
+   where you run Symon. Rename the file to 'rom.bin'.  When you start Symon,
+   the ROM file will be automatically loaded at address $d000.
+
+   Click the "Run" button and EhBASIC should automatically start running.
+
+   Type 'C' to do a cold start.
+
+   Then, type $C000 when prompted for the memory size.
+
+   NOTE: EhBASIC only wants upper-case input. This confused me at first!
+
+   More information can be found in the 'ehbasic' directory, and by visiting
+   the EhBASIC web page:
+
+   http://mycorner.no-ip.org/6502/ehbasic/index.html

samples/echo.asm

@@ -27,9 +27,9 @@ getkey: lda iostatus   ; Read the ACIA status
 ;; Write the current char in the accumulator to the console
 
 write:  pha            ; Save accumulator
-        lda iostatus   ; Read the ACIA status
+writel: lda iostatus   ; Read the ACIA status
         and #$10       ; Is the tx register empty?
-        beq write      ; No, wait for it to empty
+        beq writel     ; No, wait for it to empty
         pla            ; Otherwise, load saved accumulator
         sta iobase     ; and write to output.
 

samples/ehbasic/.gitignore

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

samples/ehbasic/Makefile

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

samples/ehbasic/README.txt

@@ -0,0 +1,85 @@
+This directoy contains a very slightly modified version of EhBASIC 2.22
+to support the Symon simulator.
+
+Usage
+-----
+
+The pre-assemled ROM image 'ehbasic.rom' in the "samples" directory is all you
+need unless you want to rebuild this source code.
+
+Just copy the image 'ehbasic.rom' to the directory where you run Symon, and
+rename the file to 'rom.bin'. It will be loaded at memory location $D000 when
+the simulator starts up. Click "Run" and you'll be presented with BASIC.
+
+At the first prompt, type 'C' for a Cold Start
+
+When prompted for free memory, type: $C000
+
+Note that EhBASIC only accepts upper case input, so you'll need to use caps
+lock (the cruise control for cool) to really make the most of it.
+
+
+Building
+--------
+
+
+To build it, you'll need the CC65 tool chain from http://www.cc65.org/
+and a suitable version of 'make'. Just typing:
+
+   % make
+   
+in this directory should re-build everything. You'll get a listing,
+some object files, and the ROM image itself.
+
+
+Changes from the EhBASIC 2.22
+-----------------------------
+
+  - Minor syntax changes to allow assembly with the cc65 tool chain.
+
+  - Memory map modified for Symon.
+
+  - At reset, configure the 6551 ACIA for 8-N-1, 2400 baud.
+ 
+  - Monitor routines 'ACIAin' and 'ACIAout' modified for the 6551 ACIA.
+    Specifically, reading and writing will check the 6551 status register for
+    the status of rx and tx registers before receive or transmit.
+
+EhBASIC is copyright Lee Davison <leeedavison@googlemail.com>
+
+My changes are so slight that I hesitate to even say this, but for "CYA"
+reasons:
+
+Changes are copyright Seth Morabito <web@loomcom.com> and are distributed under
+the same license as EhBASIC. I claim no commercial interest whatsoever. Any
+commercial use must be negotiated directly with Lee Davison.
+
+
+Original EhBASIC 2.22 README
+----------------------------
+
+ Enhanced BASIC is a BASIC interpreter for the 6502 family microprocessors. It
+ is constructed to be quick and powerful and easily ported between 6502 systems.
+ It requires few resources to run and includes instructions to facilitate easy
+ low level handling of hardware devices. It also retains most of the powerful
+ high level instructions from similar BASICs.
+
+ EhBASIC is free but not copyright free. For non commercial use there is only one
+ restriction, any derivative work should include, in any binary image distributed,
+ the string "Derived from EhBASIC" and in any distribution that includes human
+ readable files a file that includes the above string in a human readable form
+ e.g. not as a comment in an HTML file.
+
+ For commercial use please contact me,  Lee Davison, at leeedavison@googlemail.com
+ for conditions.
+
+ For more information on EhBASIC, other versions of EhBASIC and other projects
+ please visit my site at ..
+
+	 http://mycorner.no-ip.org/index.html
+
+
+ P.S. c't magazin, henceforth refered to as "those thieving german bastards", are
+ prohibited from using this or any version of EhBASIC for any of their projects
+ or products. The excuse "we don't charge people for it" doesn't wash, it adds
+ value to your product so you owe me.

samples/ehbasic/min_mon.asm

@@ -0,0 +1,152 @@
+
+; minimal monitor for EhBASIC and 6502 simulator V1.05
+
+; To run EhBASIC on the simulator load and assemble [F7] this file, start the simulator
+; running [F6] then start the code with the RESET [CTRL][SHIFT]R. Just selecting RUN
+; will do nothing, you'll still have to do a reset to run the code.
+
+	.feature labels_without_colons
+	.include "basic.asm"
+
+; put the IRQ and MNI code in RAM so that it can be changed
+
+IRQ_vec	= VEC_SV+2		; IRQ code vector
+NMI_vec	= IRQ_vec+$0A	; NMI code vector
+
+; setup for the 6502 simulator environment
+
+IO_AREA = $C000
+ACIAdata	= IO_AREA		; simulated ACIA r/w port
+ACIAstatus  = IO_AREA+1
+ACIAcommand = IO_AREA+2
+ACIAcontrol = IO_AREA+3
+
+; now the code. all this does is set up the vectors and interrupt code
+; and wait for the user to select [C]old or [W]arm start. nothing else
+; fits in less than 128 bytes
+
+.segment "MONITOR"
+	.org	$FF00			; pretend this is in a 1/8K ROM
+
+; reset vector points here
+
+RES_vec
+	CLD				; clear decimal mode
+	LDX	#$FF			; empty stack
+	TXS				; set the stack
+
+; Initialize the ACIA
+ACIA_init
+	LDA	#$09
+	STA	ACIAcommand
+	LDA	#$1A			; Set output for 8-N-1 2400
+	STA	ACIAcontrol
+
+; set up vectors and interrupt code, copy them to page 2
+
+	LDY	#END_CODE-LAB_vec	; set index/count
+LAB_stlp
+	LDA	LAB_vec-1,Y		; get byte from interrupt code
+	STA	VEC_IN-1,Y		; save to RAM
+	DEY				; decrement index/count
+	BNE	LAB_stlp		; loop if more to do
+
+; now do the signon message, Y = $00 here
+
+LAB_signon
+	LDA	LAB_mess,Y		; get byte from sign on message
+	BEQ	LAB_nokey		; exit loop if done
+
+	JSR	V_OUTP		; output character
+	INY				; increment index
+	BNE	LAB_signon		; loop, branch always
+
+LAB_nokey
+	JSR	V_INPT		; call scan input device
+	BCC	LAB_nokey		; loop if no key
+
+	AND	#$DF			; mask xx0x xxxx, ensure upper case
+	CMP	#'W'			; compare with [W]arm start
+	BEQ	LAB_dowarm		; branch if [W]arm start
+
+	CMP	#'C'			; compare with [C]old start
+	BNE	RES_vec		; loop if not [C]old start
+
+	JMP	LAB_COLD		; do EhBASIC cold start
+
+LAB_dowarm
+	JMP	LAB_WARM		; do EhBASIC warm start
+
+; byte out to simulated ACIA
+ACIAout
+	PHA				; save accumulator
+ACIAout_w
+	LDA	ACIAstatus		; Read 6551 status
+	AND	#$10			; Is tx buffer full?
+	BEQ	ACIAout_w		; if not, loop back
+	PLA				; Otherwise, restore accumulator
+	STA	ACIAdata		; write byte to 6551
+	RTS
+
+; byte in from simulated ACIA
+
+ACIAin
+	LDA	ACIAstatus		; Read 6551 status
+	AND	#$08			;
+	BEQ	LAB_nobyw		; If rx buffer empty, no byte
+
+	LDA	ACIAdata		; Read byte from 6551
+	SEC				; Flag byte received
+	RTS
+
+LAB_nobyw
+	CLC				; flag no byte received
+no_load				; empty load vector for EhBASIC
+no_save				; empty save vector for EhBASIC
+	RTS
+
+; vector tables
+
+LAB_vec
+	.word	ACIAin		; byte in from simulated ACIA
+	.word	ACIAout		; byte out to simulated ACIA
+	.word	no_load		; null load vector for EhBASIC
+	.word	no_save		; null save vector for EhBASIC
+
+; EhBASIC IRQ support
+
+IRQ_CODE
+	PHA				; save A
+	LDA	IrqBase		; get the IRQ flag byte
+	LSR				; shift the set b7 to b6, and on down ...
+	ORA	IrqBase		; OR the original back in
+	STA	IrqBase		; save the new IRQ flag byte
+	PLA				; restore A
+	RTI
+
+; EhBASIC NMI support
+
+NMI_CODE
+	PHA				; save A
+	LDA	NmiBase		; get the NMI flag byte
+	LSR				; shift the set b7 to b6, and on down ...
+	ORA	NmiBase		; OR the original back in
+	STA	NmiBase		; save the new NMI flag byte
+	PLA				; restore A
+	RTI
+
+END_CODE
+
+LAB_mess
+	.byte	$0D,$0A,"6502 EhBASIC [C]old/[W]arm ?",$00
+					; sign on string
+
+; system vectors
+
+.segment "VECTORS"
+	.org	$FFFA
+
+	.word	0			; NMI vector
+	.word	RES_vec		; RESET vector
+	.word	0			; IRQ vector
+

samples/ehbasic/symon.config

@@ -0,0 +1,18 @@
+MEMORY {
+RAM1: start = $0000, size = $C000;
+ROM1: start = $D000, size = $2F00, fill = yes;
+MONITOR: start = $FF00, size = $FA, fill = yes;
+ROMV: start = $FFFA, size = $6, file = %O, fill = yes;
+}
+
+SEGMENTS {
+CODE:     load = ROM1, type = ro;
+DATA:     load = ROM1, type = ro;
+MONITOR:  load = MONITOR, type = ro;
+VECTORS:  load = ROMV, type = ro;
+}
+
+SYMBOLS {
+__STACKSIZE__ = $0300;
+}
+

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

@@ -42,9 +42,10 @@ public class Bus {
         MemoryRange memRange = device.getMemoryRange();
         for (Device d : devices) {
             if (d.getMemoryRange().overlaps(memRange)) {
-                throw new MemoryRangeException("The device being added " +
+                throw new MemoryRangeException("The device being added at " +
+                                               String.format("$%04X", memRange.startAddress()) +
                                                " overlaps with an existing " +
-                                               "device.");
+                                               "device, '" + d + "'");
             }
         }
 
@@ -114,7 +115,7 @@ public class Bus {
                 return d.read(devAddr);
             }
         }
-        throw new MemoryAccessException("Read failed!  No device at address.");
+        throw new MemoryAccessException("Bus read failed. No device at address " + String.format("$%04X", address));
     }
 
     public void write(int address, int value) throws MemoryAccessException {
@@ -127,7 +128,7 @@ public class Bus {
                 return;
             }
         }
-        throw new MemoryAccessException("Write failed!  No device at address.");
+        throw new MemoryAccessException("Bus write failed. No device at address " + String.format("$%04X", address));
     }
 
     public SortedSet<Device> getDevices() {

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

@@ -29,6 +29,10 @@ public class Cpu implements InstructionTable {
     public static final int NMI_VECTOR_L = 0xfffe;
     public static final int NMI_VECTOR_H = 0xffff;
 
+    // The delay in microseconds between steps.
+    // TODO: Make configurable
+    private static final int CLOCK_IN_NS = 1000;
+
     /* The Bus */
     private Bus bus;
 
@@ -41,6 +45,8 @@ public class Cpu implements InstructionTable {
     private int pc;  // Program Counter register
     private int sp;  // Stack Pointer register, offset into page 1
     private int ir;  // Instruction register
+
+    private int lastPc; // The program counter before the most recent step.
     private int[] args = new int[3];  // Decoded instruction args
     private int instSize;   // # of operands for the instruction
 
@@ -435,8 +441,8 @@ public class Cpu implements InstructionTable {
             /** BIT - Bit Test ******************************************************/
             case 0x24: // Zero Page
             case 0x2c: // Absolute
-                tmp = a & bus.read(effectiveAddress);
+                tmp = bus.read(effectiveAddress);
-                setZeroFlag(tmp == 0);
+                setZeroFlag((a & tmp) == 0);
                 setNegativeFlag((tmp & 0x80) != 0);
                 setOverflowFlag((tmp & 0x40) != 0);
                 break;
@@ -553,7 +559,6 @@ public class Cpu implements InstructionTable {
             case 0x99: // Absolute,Y
             case 0x9d: // Absolute,X
                 bus.write(effectiveAddress, a);
-                setArithmeticFlags(a);
                 break;
 
 
@@ -562,7 +567,6 @@ public class Cpu implements InstructionTable {
             case 0x8c: // Absolute
             case 0x94: // Zero Page,X
                 bus.write(effectiveAddress, y);
-                setArithmeticFlags(y);
                 break;
 
 
@@ -571,7 +575,6 @@ public class Cpu implements InstructionTable {
             case 0x8e: // Absolute
             case 0x96: // Zero Page,Y
                 bus.write(effectiveAddress, x);
-                setArithmeticFlags(x);
                 break;
 
 
@@ -704,6 +707,8 @@ public class Cpu implements InstructionTable {
                 setOpTrap();
                 break;
         }
+
+        delayLoop(ir);
     }
 
     /**
@@ -715,7 +720,7 @@ public class Cpu implements InstructionTable {
      * @param operand The operand
      * @return
      */
-    public int adc(int acc, int operand) {
+    private int adc(int acc, int operand) {
         int result = (operand & 0xff) + (acc & 0xff) + getCarryBit();
         int carry6 = (operand & 0x7f) + (acc & 0x7f) + getCarryBit();
         setCarryFlag((result & 0x100) != 0);
@@ -729,7 +734,7 @@ public class Cpu implements InstructionTable {
      * Add with Carry (BCD).
      */
 
-    public int adcDecimal(int acc, int operand) {
+    private int adcDecimal(int acc, int operand) {
         int l, h, result;
         l = (acc & 0x0f) + (operand & 0x0f) + getCarryBit();
         if ((l & 0xff) > 9) l += 6;
@@ -748,12 +753,8 @@ public class Cpu implements InstructionTable {
      * Common code for Subtract with Carry.  Just calls ADC of the
      * one's complement of the operand.  This lets the N, V, C, and Z
      * flags work out nicely without any additional logic.
-     *
-     * @param acc
-     * @param operand
-     * @return
      */
-    public int sbc(int acc, int operand) {
+    private int sbc(int acc, int operand) {
         int result;
         result = adc(acc, ~operand);
         setArithmeticFlags(result);
@@ -762,12 +763,8 @@ public class Cpu implements InstructionTable {
 
     /**
      * Subtract with Carry, BCD mode.
-     *
-     * @param acc
-     * @param operand
-     * @return
      */
-    public int sbcDecimal(int acc, int operand) {
+    private int sbcDecimal(int acc, int operand) {
         int l, h, result;
         l = (acc & 0x0f) - (operand & 0x0f) - (carryFlag ? 0 : 1);
         if ((l & 0x10) != 0) l -= 6;
@@ -784,23 +781,19 @@ public class Cpu implements InstructionTable {
     /**
      * Compare two values, and set carry, zero, and negative flags
      * appropriately.
-     *
-     * @param reg
-     * @param operand
      */
-    public void cmp(int reg, int operand) {
+    private void cmp(int reg, int operand) {
+        int tmp = (reg - operand) & 0xff;
         setCarryFlag(reg >= operand);
-        setZeroFlag(reg == operand);
+        setZeroFlag(tmp == 0);
-        setNegativeFlag((reg - operand) > 0);
+        setNegativeFlag((tmp & 0x80) != 0); // Negative bit set
     }
 
     /**
      * Set the Negative and Zero flags based on the current value of the
      * register operand.
-     *
-     * @param reg The register.
      */
-    public void setArithmeticFlags(int reg) {
+    private void setArithmeticFlags(int reg) {
         zeroFlag = (reg == 0);
         negativeFlag = (reg & 0x80) != 0;
     }
@@ -1134,6 +1127,10 @@ public class Cpu implements InstructionTable {
         this.y = val;
     }
 
+    public int getLastProgramCounter() {
+        return lastPc;
+    }
+
     public int getProgramCounter() {
         return pc;
     }
@@ -1285,15 +1282,15 @@ public class Cpu implements InstructionTable {
      */
     public String toString() {
         String opcode = opcode(ir, args[0], args[1]);
-        StringBuffer sb = new StringBuffer(String.format("$%04X", addr) +
+        StringBuffer sb = new StringBuffer(getInstructionByteStatus());
-                                           "   ");
+        sb.append("  ");
         sb.append(String.format("%-14s", opcode));
-        sb.append("A=" + getAccumulatorStatus() + "  ");
+        sb.append("A:" + String.format("%02x", a) + " ");
-        sb.append("X=" + getXRegisterStatus() + "  ");
+        sb.append("X:" + String.format("%02x", x) + " ");
-        sb.append("Y=" + getYRegisterStatus() + "  ");
+        sb.append("Y:" + String.format("%02x", y) + " ");
-        sb.append("PC=" + getProgramCounterStatus() + "  ");
+        sb.append("F:" + String.format("%02x", getProcessorStatus()) + " ");
-        sb.append("SP=" + getStackPointerStatus() + "  ");
+        sb.append("S:" + String.format("1%02x", sp) + " ");
-        sb.append("P=" + getProcessorStatusString());
+        sb.append(getProcessorStatusString());
         return sb.toString();
     }
 
@@ -1397,6 +1394,19 @@ public class Cpu implements InstructionTable {
         bus.write(RST_VECTOR_L, address & 0x00ff);
     }
 
+    /*
+    * Perform a busy-loop for CLOCK_IN_NS nanoseconds
+    */
+    private void delayLoop(int opcode) {
+        int clockSteps = Cpu.instructionClocks[0xff & opcode];
+        // Just a precaution. This could be better.
+        if (clockSteps == 0) { clockSteps = 1; }
+        long startTime = System.nanoTime();
+        long stopTime = startTime + (CLOCK_IN_NS * clockSteps);
+        // Busy loop
+        while (System.nanoTime() < stopTime) { ; }
+    }
+
     /**
      * Given an opcode and its operands, return a formatted name.
      *
@@ -1430,10 +1440,10 @@ public class Cpu implements InstructionTable {
                 sb.append(String.format(" ($%04X)", address(op1, op2)));
                 break;
             case XIN:
-                sb.append(String.format(" ($%02X),X", op1));
+                sb.append(String.format(" ($%02X,X)", op1));
                 break;
             case INY:
-                sb.append(String.format(" ($%02X,Y)", op1));
+                sb.append(String.format(" ($%02X),Y", op1));
                 break;
             case REL:
             case ZPG:
@@ -1449,4 +1459,19 @@ public class Cpu implements InstructionTable {
 
         return sb.toString();
     }
+    
+    public String getInstructionByteStatus() {
+        switch (Cpu.instructionSizes[ir]) {
+            case 0:
+            case 1:
+                return String.format("%04X  %02X      ", addr, ir);
+            case 2:
+                return String.format("%04X  %02X %02X   ", addr, ir, args[0]);
+            case 3:
+                return String.format("%04X  %02X %02X %02X", addr, ir, args[0], args[1]);
+            default:
+                return null;
+        }
+    }
+
 }

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

@@ -119,7 +119,7 @@ public interface InstructionTable {
     "TYA", "STA", "TXS",  null,  null, "STA",  null,  null,
     "LDY", "LDA", "LDX",  null, "LDY", "LDA", "LDX",  null,
     "TAY", "LDA", "TAX",  null, "LDY", "LDA", "LDX",  null,
-    null,  "LDA",  null,  null, "LDY", "LDA", "LDX",  null,
+    "BCS", "LDA",  null,  null, "LDY", "LDA", "LDX",  null,
     "CLV", "LDA", "TSX",  null, "LDY", "LDA", "LDX",  null,
     "CPY", "CMP",  null,  null, "CPY", "CMP", "DEC",  null,
     "INY", "CMP", "DEX",  null, "CPY", "CMP", "DEC",  null,
@@ -173,13 +173,13 @@ public interface InstructionTable {
     Mode.ABS, Mode.ABS, Mode.ABS, Mode.NUL,   // 0x8c-0x8f
     Mode.REL, Mode.INY, Mode.NUL, Mode.NUL,   // 0x90-0x93
     Mode.ZPX, Mode.ZPX, Mode.ZPY, Mode.NUL,   // 0x94-0x97
-    Mode.IMP, Mode.ABX, Mode.IMP, Mode.NUL,   // 0x98-0x9b
+    Mode.IMP, Mode.ABY, Mode.IMP, Mode.NUL,   // 0x98-0x9b
-    Mode.NUL, Mode.ABY, Mode.NUL, Mode.NUL,   // 0x9c-0x9f
+    Mode.NUL, Mode.ABX, Mode.NUL, Mode.NUL,   // 0x9c-0x9f
     Mode.IMM, Mode.XIN, Mode.IMM, Mode.NUL,   // 0xa0-0xa3
     Mode.ZPG, Mode.ZPG, Mode.ZPG, Mode.NUL,   // 0xa4-0xa7
     Mode.IMP, Mode.IMM, Mode.IMP, Mode.NUL,   // 0xa8-0xab
     Mode.ABS, Mode.ABS, Mode.ABS, Mode.NUL,   // 0xac-0xaf
-    Mode.NUL, Mode.INY, Mode.NUL, Mode.NUL,   // 0xb0-0xb3
+    Mode.REL, Mode.INY, Mode.NUL, Mode.NUL,   // 0xb0-0xb3
     Mode.ZPX, Mode.ZPX, Mode.ZPY, Mode.NUL,   // 0xb4-0xb7
     Mode.IMP, Mode.ABX, Mode.IMP, Mode.NUL,   // 0xb8-0xbb
     Mode.ABX, Mode.ABY, Mode.ABY, Mode.NUL,   // 0xbc-0xbf
@@ -239,7 +239,7 @@ public interface InstructionTable {
     2, 6, 0, 0, 3, 3, 3, 0, 2, 0, 2, 0, 4, 4, 4, 0,
     2, 6, 0, 0, 4, 4, 4, 0, 2, 5, 2, 0, 0, 5, 0, 0,
     2, 6, 2, 0, 3, 3, 3, 0, 2, 2, 2, 0, 4, 4, 4, 0,
-    0, 5, 0, 0, 4, 4, 4, 0, 2, 4, 2, 0, 4, 4, 4, 0,
+    2, 5, 0, 0, 4, 4, 4, 0, 2, 4, 2, 0, 4, 4, 4, 0,
     2, 6, 0, 0, 3, 3, 5, 0, 2, 2, 2, 0, 4, 4, 6, 0,
     2, 5, 0, 0, 0, 4, 6, 0, 2, 4, 0, 0, 0, 4, 7, 0,
     2, 6, 0, 0, 3, 3, 5, 0, 2, 2, 2, 0, 4, 4, 6, 0,

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

@@ -2,6 +2,7 @@ package com.loomcom.symon;
 
 import com.loomcom.symon.devices.Acia;
 import com.loomcom.symon.devices.Memory;
+import com.loomcom.symon.exceptions.FifoUnderrunException;
 import com.loomcom.symon.exceptions.MemoryAccessException;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 import com.loomcom.symon.exceptions.SymonException;
@@ -14,7 +15,6 @@ import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import java.awt.event.KeyEvent;
 import java.io.*;
-import java.lang.reflect.InvocationTargetException;
 import java.util.Observable;
 import java.util.Observer;
 import java.util.logging.Level;
@@ -27,16 +27,16 @@ public class Simulator implements ActionListener, Observer {
     private static final int BUS_BOTTOM = 0x0000;
     private static final int BUS_TOP    = 0xffff;
 
+    // 32K of RAM
     private static final int MEMORY_BASE = 0x0000;
-    private static final int MEMORY_SIZE = 0xc000; // 48 KB
+    private static final int MEMORY_SIZE = 0xC000;
 
-    private static final int ROM_BASE = 0xe000;
+    // IO area at $D000
-    private static final int ROM_SIZE = 0x2000; // 8 KB
+    private static final int ACIA_BASE = 0xC000;
 
-    private static final int ACIA_BASE = 0xc000;
+    // 8KB ROM at E000-FFFF
-
+    private static final int ROM_BASE = 0xD000;
-    // The delay in microseconds between steps.
+    private static final int ROM_SIZE = 0x3000;
-    private static final int DELAY_BETWEEN_STEPS_NS = 1000;
 
     // Since it is very expensive to update the UI with Swing's Event Dispatch Thread, we can't afford
     // to refresh the view on every simulated clock cycle. Instead, we will only refresh the view after this
@@ -84,18 +84,26 @@ public class Simulator implements ActionListener, Observer {
     private JFileChooser fileChooser;
     private PreferencesDialog  preferences;
 
-    public Simulator() throws MemoryRangeException {
+    public Simulator() throws MemoryRangeException, IOException {
         this.acia = new Acia(ACIA_BASE);
         this.bus = new Bus(BUS_BOTTOM, BUS_TOP);
         this.cpu = new Cpu();
         this.ram = new Memory(MEMORY_BASE, MEMORY_SIZE, false);
 
-        // TODO: Load this ROM from a file, of course!
+        // TODO: Make this configurable, of course.
-        this.rom = new Memory(ROM_BASE, ROM_SIZE, false);
+        File romImage = new File("rom.bin");
+        if (romImage.canRead()) {
+            logger.info("Loading ROM image from file " + romImage);
+            this.rom = Memory.makeROM(ROM_BASE, ROM_SIZE, romImage);
+        } else {
+            logger.info("No ROM file 'rom.bin' found. ROM image will be empty R/W memory.");
+            // Just make it a normal RAM image.
+            this.rom = Memory.makeRAM(ROM_BASE, ROM_SIZE);
+        }
 
         bus.addCpu(cpu);
-        bus.addDevice(acia);
         bus.addDevice(ram);
+        bus.addDevice(acia);
         bus.addDevice(rom);
     }
 
@@ -212,7 +220,7 @@ public class Simulator implements ActionListener, Observer {
      */
     public void actionPerformed(ActionEvent actionEvent) {
         if (actionEvent.getSource() == resetButton) {
-            handleReset();
+            coldReset();
         } else if (actionEvent.getSource() == stepButton) {
             handleStep();
         } else if (actionEvent.getSource() == runStopButton) {
@@ -274,7 +282,7 @@ public class Simulator implements ActionListener, Observer {
         }
     }
 
-    private void handleReset() {
+    private void coldReset() {
         if (runLoop != null && runLoop.isRunning()) {
             runLoop.requestStop();
             runLoop.interrupt();
@@ -282,7 +290,7 @@ public class Simulator implements ActionListener, Observer {
         }
 
         try {
-            logger.log(Level.INFO, "Reset requested. Resetting CPU and clearing memory.");
+            logger.log(Level.INFO, "Cold reset requested. Resetting CPU and clearing memory.");
             // Reset and clear memory
             cpu.reset();
             ram.fill(0x00);
@@ -307,6 +315,7 @@ public class Simulator implements ActionListener, Observer {
     private void handleStep() {
         try {
             step();
+
             // The simulator is lazy about updating the UI for performance reasons, so always request an
             // immediate update after stepping manually.
             SwingUtilities.invokeLater(new Runnable() {
@@ -337,8 +346,6 @@ public class Simulator implements ActionListener, Observer {
      */
     private void step() throws MemoryAccessException {
 
-        delayLoop();
-
         cpu.step();
 
         // Read from the ACIA and immediately update the console if there's
@@ -351,9 +358,13 @@ public class Simulator implements ActionListener, Observer {
 
         // If a key has been pressed, fill the ACIA.
         // TODO: Interrupt handling.
+        try {
             if (console.hasInput()) {
                 acia.rxWrite((int)console.readInputChar());
             }
+        } catch (FifoUnderrunException ex) {
+            logger.severe("Console type-ahead buffer underrun!");
+        }
 
         // This is a very expensive update, and we're doing it without
         // a delay, so we don't want to overwhelm the Swing event processing thread
@@ -374,8 +385,6 @@ public class Simulator implements ActionListener, Observer {
      * Load a program into memory at the simulatorDidStart address.
      */
     private void loadProgram(byte[] program, int startAddress) throws MemoryAccessException {
-        cpu.setResetVector(startAddress);
-
         int addr = startAddress, i;
         for (i = 0; i < program.length; i++) {
             bus.write(addr++, program[i] & 0xff);
@@ -388,6 +397,9 @@ public class Simulator implements ActionListener, Observer {
         // Reset (but don't clear memory, naturally)
         cpu.reset();
 
+        // Reset the stack program counter
+        cpu.setProgramCounter(preferences.getProgramStartAddress());
+
         // Immediately update the UI.
         SwingUtilities.invokeLater(new Runnable() {
             public void run() {
@@ -405,8 +417,8 @@ public class Simulator implements ActionListener, Observer {
                     Simulator simulator = new Simulator();
                     simulator.createAndShowUi();
                     // Reset the simulator.
-                    simulator.handleReset();
+                    simulator.coldReset();
-                } catch (MemoryRangeException e) {
+                } catch (Exception e) {
                     e.printStackTrace();
                 }
             }
@@ -433,15 +445,6 @@ public class Simulator implements ActionListener, Observer {
         }
     }
 
-    /*
-     * Perform a busy-loop for DELAY_BETWEEN_STEPS_NS nanoseconds
-     */
-    private void delayLoop() {
-        long startTime = System.nanoTime();
-        long stopTime = startTime + DELAY_BETWEEN_STEPS_NS;
-        // Busy loop
-        while (System.nanoTime() < stopTime) { ; }
-    }
 
     /**
      * The main run thread.

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

@@ -1,9 +1,12 @@
 package com.loomcom.symon.devices;
 
+import java.io.*;
 import java.util.*;
 
 import com.loomcom.symon.exceptions.*;
 
+import javax.swing.*;
+
 public class Memory extends Device {
 
     private boolean readOnly;
@@ -24,6 +27,17 @@ public class Memory extends Device {
         this(address, size, false);
     }
 
+    public static Memory makeROM(int address, int size, File f) throws MemoryRangeException, IOException {
+        Memory memory = new Memory(address, size, true);
+        memory.loadFromFile(f);
+        return memory;
+    }
+
+    public static Memory makeRAM(int address, int size) throws MemoryRangeException {
+        Memory memory = new Memory(address, size, false);
+        return memory;
+    }
+
     public void write(int address, int data) throws MemoryAccessException {
         if (readOnly) {
             throw new MemoryAccessException("Cannot write to read-only memory at address " + address);
@@ -32,6 +46,34 @@ public class Memory extends Device {
         }
     }
 
+    /**
+     * Load the memory from a file.
+     *
+     * @param file The file to read an array of bytes from.
+     * @throws MemoryRangeException if the file and memory size do not match.
+     * @throws IOException if the file read fails.
+     */
+    public void loadFromFile(File file) throws MemoryRangeException, IOException {
+        if (file.canRead()) {
+            long fileSize = file.length();
+
+            if (fileSize > mem.length) {
+                throw new MemoryRangeException("File will not fit in available memory.");
+            } else {
+                int i = 0;
+                FileInputStream fis = new FileInputStream(file);
+                BufferedInputStream bis = new BufferedInputStream(fis);
+                DataInputStream dis = new DataInputStream(bis);
+                while (dis.available() != 0) {
+                    mem[i++] = dis.readUnsignedByte();
+                }
+            }
+        } else {
+            throw new IOException("Cannot open file " + file);
+        }
+
+    }
+
     public int read(int address) throws MemoryAccessException {
         return this.mem[address];
     }
@@ -40,7 +82,6 @@ public class Memory extends Device {
         Arrays.fill(this.mem, val);
     }
 
-
     public String toString() {
         return "Memory: " + getMemoryRange().toString();
     }

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

@@ -7,6 +7,8 @@ import java.awt.event.MouseListener;
 
 import com.grahamedgecombe.jterminal.JTerminal;
 import com.grahamedgecombe.jterminal.vt100.Vt100TerminalModel;
+import com.loomcom.symon.exceptions.FifoUnderrunException;
+import com.loomcom.symon.util.FifoRingBuffer;
 
 import javax.swing.*;
 import javax.swing.border.BevelBorder;
@@ -22,9 +24,12 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
     private static final int DEFAULT_COLUMNS = 80;
     private static final int DEFAULT_ROWS = 24;
     private static final int DEFAULT_BORDER_WIDTH = 10;
+    // If true, swap CR and LF characters
+    private static final boolean SWAP_CR_AND_LF = true;
+    // If true, send CRLF (0x0d 0x0a) whenever CR is typed
+    private static final boolean SEND_CR_LF_FOR_CR = false;
 
-    private boolean hasInput = false;
+    private FifoRingBuffer typeAheadBuffer;
-    private char keyBuffer;
 
     public Console() {
         this(DEFAULT_COLUMNS, DEFAULT_ROWS);
@@ -32,6 +37,9 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
 
     public Console(int columns, int rows) {
         super(new Vt100TerminalModel(columns, rows));
+        // A small type-ahead buffer, as might be found in any real
+        // VT100-style serial terminal.
+        this.typeAheadBuffer = new FifoRingBuffer(128);
         setBorderWidth(DEFAULT_BORDER_WIDTH);
         addKeyListener(this);
         addMouseListener(this);
@@ -47,11 +55,11 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
      *
      */
     public void reset() {
+        typeAheadBuffer.reset();
         getModel().clear();
         getModel().setCursorColumn(0);
         getModel().setCursorRow(0);
         repaint();
-        this.hasInput = false;
     }
 
     /**
@@ -60,7 +68,7 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
      * @return
      */
     public boolean hasInput() {
-        return hasInput;
+        return !typeAheadBuffer.isEmpty();
     }
 
     /**
@@ -69,6 +77,24 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
      * @param keyEvent The key event.
      */
     public void keyTyped(KeyEvent keyEvent) {
+        char keyTyped = keyEvent.getKeyChar();
+
+        if (SWAP_CR_AND_LF) {
+            if (keyTyped == 0x0a) {
+                keyTyped = 0x0d;
+            }
+            else if (keyTyped == 0x0d) {
+                keyTyped = 0x0a;
+            }
+        }
+
+        if (SEND_CR_LF_FOR_CR && keyTyped == 0x0d) {
+            typeAheadBuffer.push(0x0d);
+            typeAheadBuffer.push(0x0a);
+        } else {
+            typeAheadBuffer.push(keyTyped);
+        }
+
         keyEvent.consume();
     }
 
@@ -78,8 +104,6 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
      * @param keyEvent The key event.
      */
     public void keyPressed(KeyEvent keyEvent) {
-        keyBuffer = keyEvent.getKeyChar();
-        hasInput = true;
         keyEvent.consume();
     }
 
@@ -88,9 +112,8 @@ public class Console extends JTerminal implements KeyListener, MouseListener {
      *
      * @return The character typed.
      */
-    public char readInputChar() {
+    public char readInputChar() throws FifoUnderrunException {
-        hasInput = false;
+        return (char)typeAheadBuffer.pop();
-        return this.keyBuffer;
     }
 
     /**

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

@@ -79,27 +79,27 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0x0d, 0x04, 0x35,  // ORA $3504
                         0x0d, 0x08, 0x35,  // ORA $3508
                         0x0d, 0x10, 0x12); // ORA $1210
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getAccumulator());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x11, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x33, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x77, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0xff, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -120,31 +120,31 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0x0e, 0x03, 0x35,  // ASL $3503
                         0x0e, 0x04, 0x12); // ASL $1204
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x7f00));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x02, bus.read(0x7f01));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x04, bus.read(0x3502));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x88, bus.read(0x3503));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1204));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -197,23 +197,23 @@ public class CpuAbsoluteModeTest extends TestCase {
 
         cpu.step(2);
         assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getOverflowFlag());
+        assertTrue(cpu.getOverflowFlag());
 
         cpu.step(2);
         assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getOverflowFlag());
-
-    cpu.step(2);
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
         assertTrue(cpu.getOverflowFlag());
 
         cpu.step(2);
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getOverflowFlag());
+        assertTrue(cpu.getOverflowFlag());
+
+        cpu.step(2);
+        assertFalse(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertTrue(cpu.getOverflowFlag());
 
         cpu.step(2);
         assertFalse(cpu.getZeroFlag());
@@ -244,12 +244,12 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0x2d, 0x04, 0x12,  // AND $1204
                         0xa9, 0xff,        // LDA #$ff
                         0x2d, 0x05, 0x12); // AND $1205
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getAccumulator());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getAccumulator());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -259,12 +259,12 @@ public class CpuAbsoluteModeTest extends TestCase {
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x88, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getAccumulator());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -294,13 +294,13 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0x2e, 0x01, 0x12,  // ROL $1201 (m=%01000000, c=1)
                         0x2e, 0x01, 0x12); // ROL $1201 (m=%10000001, c=0)
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1200));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x02, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -312,43 +312,43 @@ public class CpuAbsoluteModeTest extends TestCase {
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x0a, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x14, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x28, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x50, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0xa0, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x40, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertTrue(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x81, bus.read(0x1201));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -359,16 +359,12 @@ public class CpuAbsoluteModeTest extends TestCase {
 
     public void test_JMP() throws MemoryAccessException {
         bus.loadProgram(0x4c, 0x00, 0x34);
-    extracted();
+        cpu.step();
         assertEquals(0x3400, cpu.getProgramCounter());
         // No change to status flags.
         assertEquals(0x20, cpu.getProcessorStatus());
     }
 
-  private void extracted() throws MemoryAccessException {
-    cpu.step();
-  }
-
     /* EOR - Exclusive OR - $4d */
 
     public void test_EOR() throws MemoryAccessException {
@@ -387,17 +383,17 @@ public class CpuAbsoluteModeTest extends TestCase {
         assertTrue(cpu.getNegativeFlag());
         assertFalse(cpu.getZeroFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x77, cpu.getAccumulator());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getZeroFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x44, cpu.getAccumulator());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getZeroFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getAccumulator());
         assertFalse(cpu.getNegativeFlag());
         assertTrue(cpu.getZeroFlag());
@@ -421,31 +417,31 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0x38,              // SEC
                         0x4e, 0x05, 0x12); // LSR $1205
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1200));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1201));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertTrue(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x01, bus.read(0x1202));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x22, bus.read(0x1203));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x40, bus.read(0x1204));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -657,61 +653,61 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0x6e, 0x11, 0x12,  // ROR $1201 (m=%00100000, c=0)
                         0x6e, 0x11, 0x12); // ROR $1201 (m=%00010000, c=0)
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1210));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x08, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x04, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x02, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x01, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1211));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertTrue(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x80, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x40, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x20, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
         assertFalse(cpu.getCarryFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x10, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -723,16 +719,17 @@ public class CpuAbsoluteModeTest extends TestCase {
     public void test_STY() throws MemoryAccessException {
         cpu.setYRegister(0x00);
         bus.loadProgram(0x8c, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1210));
-    assertTrue(cpu.getZeroFlag());
+        // STY should have NO effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
 
         cpu.setYRegister(0x0f);
         bus.loadProgram(0x8c, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x0f, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -741,10 +738,10 @@ public class CpuAbsoluteModeTest extends TestCase {
 
         cpu.setYRegister(0x80);
         bus.loadProgram(0x8c, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x80, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* STA - Store Accumulator - $8d */
@@ -752,16 +749,17 @@ public class CpuAbsoluteModeTest extends TestCase {
     public void test_STA() throws MemoryAccessException {
         cpu.setAccumulator(0x00);
         bus.loadProgram(0x8d, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1210));
-    assertTrue(cpu.getZeroFlag());
+        // STA should have NO effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
 
         cpu.setAccumulator(0x0f);
         bus.loadProgram(0x8d, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x0f, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -770,10 +768,10 @@ public class CpuAbsoluteModeTest extends TestCase {
 
         cpu.setAccumulator(0x80);
         bus.loadProgram(0x8d, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x80, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* STX - Store X Register - $8e */
@@ -781,16 +779,17 @@ public class CpuAbsoluteModeTest extends TestCase {
     public void test_STX() throws MemoryAccessException {
         cpu.setXRegister(0x00);
         bus.loadProgram(0x8e, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1210));
-    assertTrue(cpu.getZeroFlag());
+        // STX should have NO effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
 
         cpu.setXRegister(0x0f);
         bus.loadProgram(0x8e, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x0f, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
@@ -799,10 +798,10 @@ public class CpuAbsoluteModeTest extends TestCase {
 
         cpu.setXRegister(0x80);
         bus.loadProgram(0x8e, 0x10, 0x12);
-    extracted();
+        cpu.step();
         assertEquals(0x80, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDA - Load Accumulator - $ad */
@@ -816,17 +815,17 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xad, 0x11, 0x12,  // LDA $1211
                         0xad, 0x12, 0x12); // LDA $1212
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getAccumulator());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x0f, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x80, cpu.getAccumulator());
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -843,17 +842,17 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xae, 0x11, 0x12,  // LDX $1211
                         0xae, 0x12, 0x12); // LDX $1212
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getXRegister());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x0f, cpu.getXRegister());
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x80, cpu.getXRegister());
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -870,17 +869,17 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xbc, 0x11, 0x12,  // LDY $1211
                         0xbc, 0x12, 0x12); // LDY $1212
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, cpu.getYRegister());
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x0f, cpu.getYRegister());
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x80, cpu.getYRegister());
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -899,20 +898,20 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xcd, 0x11, 0x12,  // CMP $1211
                         0xcd, 0x12, 0x12); // CMP $1212
 
-    extracted();
+        cpu.step();
         assertTrue(cpu.getCarryFlag());    // m > y
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag()); // m - y < 0
 
-    extracted();
+        cpu.step();
         assertTrue(cpu.getCarryFlag());     // m = y
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag()); // m - y == 0
 
-    extracted();
+        cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag());  // m - y > 0
     }
 
     /* CPY - Compare Y Register - $cc */
@@ -928,20 +927,20 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xcc, 0x11, 0x12,  // CPY $1211
                         0xcc, 0x12, 0x12); // CPY $1212
 
-    extracted();
+        cpu.step();
         assertTrue(cpu.getCarryFlag());    // m > y
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag()); // m - y < 0
 
-    extracted();
+        cpu.step();
         assertTrue(cpu.getCarryFlag());     // m = y
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag()); // m - y == 0
 
-    extracted();
+        cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag());  // m - y = $81
     }
 
     /* DEC - Decrement Memory Location - $ce */
@@ -957,22 +956,22 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xce, 0x12, 0x12,  // DEC $1212
                         0xce, 0x13, 0x12); // DEC $1213
 
-    extracted();
+        cpu.step();
         assertEquals(0xff, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1211));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x7f, bus.read(0x1212));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0xfe, bus.read(0x1213));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -991,20 +990,20 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xec, 0x11, 0x12,  // CPX $1211
                         0xec, 0x12, 0x12); // CPX $1212
 
-    extracted();
+        cpu.step();
         assertTrue(cpu.getCarryFlag());    // m > y
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag()); // m - y < 0
 
-    extracted();
+        cpu.step();
         assertTrue(cpu.getCarryFlag());    // m = y
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag()); // m - y == 0
 
-    extracted();
+        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 (borrow) - $ed */
@@ -1213,17 +1212,17 @@ public class CpuAbsoluteModeTest extends TestCase {
                         0xee, 0x11, 0x12,  // INC $1211
                         0xee, 0x12, 0x12); // INC $1212
 
-    extracted();
+        cpu.step();
         assertEquals(0x01, bus.read(0x1210));
         assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x80, bus.read(0x1211));
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
 
-    extracted();
+        cpu.step();
         assertEquals(0x00, bus.read(0x1212));
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());

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

@@ -678,7 +678,8 @@ public class CpuAbsoluteXModeTest extends TestCase {
         bus.loadProgram(0x9d, 0x10, 0xab); // STA $ab10,X
         cpu.step();
         assertEquals(0x00, bus.read(0xab40));
-    assertTrue(cpu.getZeroFlag());
+        // STA should have NO affect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -699,7 +700,7 @@ public class CpuAbsoluteXModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0xab40));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDY - Load Y Register - $bc */
@@ -788,7 +789,7 @@ public class CpuAbsoluteXModeTest extends TestCase {
         cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag()); // $80 - $ff = $81
     }
 
     /* DEC - Decrement Memory Location - $de */

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/CpuImmediateModeTest.java

@@ -200,6 +200,7 @@ public class CpuImmediateModeTest extends TestCase {
                         0x69, 0x01); // ADC #$01
         cpu.step(2);
         assertEquals(0x80, cpu.getAccumulator());
+
         assertTrue(cpu.getNegativeFlag());
         assertTrue(cpu.getOverflowFlag());
         assertFalse(cpu.getZeroFlag());
@@ -493,8 +494,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertTrue(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    // $08 - $0a = negative
+        // $0a - $08 = positive
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
         bus.loadProgram(0xa0, 0xfa,  // LDY #$FA
@@ -502,8 +503,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertTrue(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    // $80 - $FA = negative
+        // $fa - 80 = positive
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     public void test_CPY_DoesNotSetCarryFlagIfYLessThanThanMemory() throws MemoryAccessException {
@@ -512,7 +513,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertFalse(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        // 08 - 0a = negative
+        assertTrue(cpu.getNegativeFlag());
 
         cpu.reset();
         bus.loadProgram(0xa0, 0x70,  // LDY #$70
@@ -520,7 +522,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertFalse(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        // $70 - $80 = negative
+        assertTrue(cpu.getNegativeFlag());
     }
 
     /* CMP Immediate Mode Tests - 0xc9 */
@@ -564,8 +567,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertTrue(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    // $08 - $0a = negative
+        // $0a - $08 = positive
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
         bus.loadProgram(0xa9, 0xfa,  // LDA #$FA
@@ -573,8 +576,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertTrue(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    // $80 - $FA = negative
+        // $fa - $80 = positive
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     public void test_CMP_DoesNotSetCarryFlagIfYGreaterThanMemory() throws MemoryAccessException {
@@ -583,7 +586,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertFalse(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        // 08 - 0a = negative
+        assertTrue(cpu.getNegativeFlag());
 
         cpu.reset();
         bus.loadProgram(0xa9, 0x70,  // LDA #$70
@@ -591,7 +595,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertFalse(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        // 70 - 80 = negative
+        assertTrue(cpu.getNegativeFlag());
     }
 
     /* CPX Immediate Mode Tests - 0xe0 */
@@ -635,8 +640,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertTrue(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    // $08 - $0a = negative
+        // $0a - $08 = positive
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
         bus.loadProgram(0xa2, 0xfa,  // LDX #$FA
@@ -644,8 +649,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertTrue(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    // $80 - $FA = negative
+        // $fa - $80 = positive
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     public void test_CPX_DoesNotSetCarryFlagIfYGreaterThanMemory() throws MemoryAccessException {
@@ -654,7 +659,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertFalse(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        // $08 - $0a = negative
+        assertTrue(cpu.getNegativeFlag());
 
         cpu.reset();
         bus.loadProgram(0xa2, 0x70,  // LDX #$70
@@ -662,7 +668,8 @@ public class CpuImmediateModeTest extends TestCase {
         cpu.step(2);
         assertFalse(cpu.getCarryFlag());
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        // $70 - $80 = negative
+        assertTrue(cpu.getNegativeFlag());
     }
 
     /* SBC Immediate Mode Tests - 0xe9 */

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/CpuZeroPageModeTest.java

@@ -153,6 +153,8 @@ public class CpuZeroPageModeTest extends TestCase {
 
     public void test_BIT() throws MemoryAccessException {
         bus.write(0x0000, 0xc0);
+        bus.write(0x0010, 0x40);
+        bus.write(0x0020, 0x80);
 
         bus.loadProgram(0xa9, 0x01,  // LDA #$01
                         0x24, 0x00,  // BIT $00
@@ -161,10 +163,10 @@ public class CpuZeroPageModeTest extends TestCase {
                         0x24, 0x00,  // BIT $00
 
                         0xa9, 0x40,  // LDA #$40
-                    0x24, 0x00,  // BIT $00
+                        0x24, 0x20,  // BIT $20
 
                         0xa9, 0x80,  // LDA #$80
-                    0x24, 0x00,  // BIT $00
+                        0x24, 0x10,  // BIT $10
 
                         0xa9, 0xc0,  // LDA #$c0
                         0x24, 0x00,  // BIT $00
@@ -174,24 +176,24 @@ public class CpuZeroPageModeTest extends TestCase {
 
         cpu.step(2);
         assertTrue(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertTrue(cpu.getOverflowFlag());
+
+        cpu.step(2);
+        assertTrue(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
+        assertTrue(cpu.getOverflowFlag());
+
+        cpu.step(2);
+        assertTrue(cpu.getZeroFlag());
+        assertTrue(cpu.getNegativeFlag());
         assertFalse(cpu.getOverflowFlag());
 
         cpu.step(2);
         assertTrue(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
-    assertFalse(cpu.getOverflowFlag());
-
-    cpu.step(2);
-    assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag());
         assertTrue(cpu.getOverflowFlag());
 
-    cpu.step(2);
-    assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
-    assertFalse(cpu.getOverflowFlag());
-
         cpu.step(2);
         assertFalse(cpu.getZeroFlag());
         assertTrue(cpu.getNegativeFlag());
@@ -688,7 +690,8 @@ public class CpuZeroPageModeTest extends TestCase {
         bus.loadProgram(0x84, 0x10);
         cpu.step();
         assertEquals(0x00, bus.read(0x10));
-    assertTrue(cpu.getZeroFlag());
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -707,7 +710,7 @@ public class CpuZeroPageModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0x10));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* STA - Store Accumulator - $85 */
@@ -717,7 +720,8 @@ public class CpuZeroPageModeTest extends TestCase {
         bus.loadProgram(0x85, 0x10);
         cpu.step();
         assertEquals(0x00, bus.read(0x10));
-    assertTrue(cpu.getZeroFlag());
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -736,7 +740,7 @@ public class CpuZeroPageModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0x10));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* STX - Store X Register - $86 */
@@ -746,7 +750,8 @@ public class CpuZeroPageModeTest extends TestCase {
         bus.loadProgram(0x86, 0x10);
         cpu.step();
         assertEquals(0x00, bus.read(0x10));
-    assertTrue(cpu.getZeroFlag());
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -765,7 +770,7 @@ public class CpuZeroPageModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0x10));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDY - Load Y Register - $a4 */
@@ -875,7 +880,7 @@ public class CpuZeroPageModeTest extends TestCase {
         cpu.step();
         assertFalse(cpu.getCarryFlag());    // m < y
         assertFalse(cpu.getZeroFlag());
-    assertFalse(cpu.getNegativeFlag()); // m - y > 0
+        assertTrue(cpu.getNegativeFlag()); // m - y < 0
     }
 
     /* CMP - Compare Accumulator - $c5 */
@@ -904,7 +909,7 @@ public class CpuZeroPageModeTest 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 - $c6 */
@@ -967,7 +972,7 @@ public class CpuZeroPageModeTest 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 - $e5 */

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

@@ -48,7 +48,8 @@ public class CpuZeroPageYModeTest extends TestCase {
         bus.loadProgram(0x96, 0x10);  // STX $10,Y
         cpu.step();
         assertEquals(0x00, bus.read(0x40));
-    assertTrue(cpu.getZeroFlag());
+        // Should have no effect on flags.
+        assertFalse(cpu.getZeroFlag());
         assertFalse(cpu.getNegativeFlag());
 
         cpu.reset();
@@ -67,7 +68,7 @@ public class CpuZeroPageYModeTest extends TestCase {
         cpu.step();
         assertEquals(0x80, bus.read(0x40));
         assertFalse(cpu.getZeroFlag());
-    assertTrue(cpu.getNegativeFlag());
+        assertFalse(cpu.getNegativeFlag());
     }
 
     /* LDX - Load X Register - $b6 */