Added stack functions.

src/main/java/com/loomcom/lm6502/Cpu.java

@@ -621,8 +621,11 @@ public class Cpu implements InstructionTable {
 			break;
 		}
 	}
-		
+	
-	public String statusString() {
+	/**
+	 * Returns a string representing the CPU state. 
+	 */
+	public String toString() {
 		String opcode = CpuUtils.opcode(ir, operands[0], operands[1]);
 		StringBuffer sb = new StringBuffer(String.format("$%04X", addr) + "  ");
 		sb.append(String.format("%-12s", opcode));
@@ -633,10 +636,31 @@ public class Cpu implements InstructionTable {
 		return sb.toString();
 	}
 	
+	/**
+	 * Push an item onto the stack, and decrement the stack counter.
+	 * Silently fails to push onto the stack if SP is
+	 * TODO: Unit tests.  
+	 */
+	protected void push(int data) {
+		bus.write(sp, data);
+		if (sp > 0x100) { sp--; }
+	}
+	
+
+	/**
+	 * Pop a byte off the user stack, and increment the stack counter. 
+	 * TODO: Unit tests.
+	 */
+	protected int pop() {
+		int data = bus.read(sp);
+		if (sp < 0x1ff) { sp++; }
+		return data;
+	}
+
 	/*
 	 * Increment the PC, rolling over if necessary.
 	 */
-	private void incProgramCounter() {
+	protected void incProgramCounter() {
 		if (pc == 0xffff) {
 			pc = 0;
 		} else {

src/main/java/com/loomcom/lm6502/Simulator.java

@@ -61,22 +61,29 @@ public class Simulator {
 		bus.write(0x0302, 0xea); // NOP
 		bus.write(0x0303, 0xea); // NOP
 		bus.write(0x0304, 0xea); // NOP
-		bus.write(0x0305, 0xea); // NOP
+		bus.write(0x0305, 0xa0); // LDY #$1A
-		bus.write(0x0306, 0xa9); // LDA #$1A
+		bus.write(0x0306, 0x1a);
-		bus.write(0x0307, 0x1a);
+		bus.write(0x0307, 0xea); // NOP
 		bus.write(0x0308, 0xea); // NOP
-		bus.write(0x0309, 0xea); // NOP
+		bus.write(0x0309, 0xa2); // LDX #$03
-		bus.write(0x030a, 0xa9); // LDA #$03
+		bus.write(0x030a, 0x03);
-		bus.write(0x030b, 0x03);
+		
-		bus.write(0x030c, 0x4c); // JMP #$0300
+		bus.write(0x030b, 0xa9); // LDA #$00
-		bus.write(0x030d, 0x00); 
+		bus.write(0x030c, 0x00);
-		bus.write(0x030e, 0x03);
+		bus.write(0x030d, 0xa2); // LDX #$00
+		bus.write(0x030e, 0x00);
+		bus.write(0x030f, 0xa0); // LDY #$00
+		bus.write(0x0310, 0x00);
+		
+		bus.write(0x0311, 0x4c); // JMP #$0300
+		bus.write(0x0312, 0x00); 
+		bus.write(0x0313, 0x03);
 
 		cpu.reset();
 
-		for (int i = 0; i < 60; i++) {
+		for (int i = 0; i <= 23; i++) {
 			cpu.step();
-			System.out.println(cpu.statusString());
+			System.out.println(cpu.toString());
 		}
 
 	}