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.
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Seth Morabito committed
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25 files changed
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@@ -29,6 +29,10 @@ public class Cpu implements InstructionTable {
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public static final int NMI_VECTOR_L = 0xfffe;
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public static final int NMI_VECTOR_H = 0xffff;
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// The delay in microseconds between steps.
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// TODO: Make configurable
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private static final int CLOCK_IN_NS = 1000;
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/* The Bus */
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private Bus bus;
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@@ -41,11 +45,13 @@ public class Cpu implements InstructionTable {
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private int pc; // Program Counter register
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private int sp; // Stack Pointer register, offset into page 1
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private int ir; // Instruction register
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private int lastPc; // The program counter before the most recent step.
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private int[] args = new int[3]; // Decoded instruction args
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private int instSize; // # of operands for the instruction
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/* Scratch space for addressing mode and effective address
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* calculations */
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* calculations */
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private int irAddressMode; // Bits 3-5 of IR: [ | | |X|X|X| | ]
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private int irOpMode; // Bits 6-7 of IR: [ | | | | | |X|X]
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private int effectiveAddress;
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@@ -435,8 +441,8 @@ public class Cpu implements InstructionTable {
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/** BIT - Bit Test ******************************************************/
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case 0x24: // Zero Page
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case 0x2c: // Absolute
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tmp = a & bus.read(effectiveAddress);
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setZeroFlag(tmp == 0);
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tmp = bus.read(effectiveAddress);
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setZeroFlag((a & tmp) == 0);
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setNegativeFlag((tmp & 0x80) != 0);
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setOverflowFlag((tmp & 0x40) != 0);
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break;
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@@ -553,7 +559,6 @@ public class Cpu implements InstructionTable {
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case 0x99: // Absolute,Y
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case 0x9d: // Absolute,X
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bus.write(effectiveAddress, a);
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setArithmeticFlags(a);
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break;
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@@ -562,7 +567,6 @@ public class Cpu implements InstructionTable {
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case 0x8c: // Absolute
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case 0x94: // Zero Page,X
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bus.write(effectiveAddress, y);
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setArithmeticFlags(y);
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break;
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@@ -571,7 +575,6 @@ public class Cpu implements InstructionTable {
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case 0x8e: // Absolute
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case 0x96: // Zero Page,Y
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bus.write(effectiveAddress, x);
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setArithmeticFlags(x);
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break;
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@@ -704,6 +707,8 @@ public class Cpu implements InstructionTable {
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setOpTrap();
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break;
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}
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delayLoop(ir);
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}
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/**
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@@ -715,7 +720,7 @@ public class Cpu implements InstructionTable {
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* @param operand The operand
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* @return
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*/
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public int adc(int acc, int operand) {
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private int adc(int acc, int operand) {
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int result = (operand & 0xff) + (acc & 0xff) + getCarryBit();
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int carry6 = (operand & 0x7f) + (acc & 0x7f) + getCarryBit();
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setCarryFlag((result & 0x100) != 0);
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@@ -729,7 +734,7 @@ public class Cpu implements InstructionTable {
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* Add with Carry (BCD).
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*/
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public int adcDecimal(int acc, int operand) {
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private int adcDecimal(int acc, int operand) {
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int l, h, result;
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l = (acc & 0x0f) + (operand & 0x0f) + getCarryBit();
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if ((l & 0xff) > 9) l += 6;
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@@ -748,12 +753,8 @@ public class Cpu implements InstructionTable {
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* Common code for Subtract with Carry. Just calls ADC of the
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* one's complement of the operand. This lets the N, V, C, and Z
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* flags work out nicely without any additional logic.
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*
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* @param acc
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* @param operand
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* @return
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*/
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public int sbc(int acc, int operand) {
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private int sbc(int acc, int operand) {
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int result;
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result = adc(acc, ~operand);
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setArithmeticFlags(result);
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@@ -762,12 +763,8 @@ public class Cpu implements InstructionTable {
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/**
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* Subtract with Carry, BCD mode.
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*
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* @param acc
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* @param operand
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* @return
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*/
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public int sbcDecimal(int acc, int operand) {
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private int sbcDecimal(int acc, int operand) {
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int l, h, result;
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l = (acc & 0x0f) - (operand & 0x0f) - (carryFlag ? 0 : 1);
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if ((l & 0x10) != 0) l -= 6;
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@@ -784,23 +781,19 @@ public class Cpu implements InstructionTable {
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/**
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* Compare two values, and set carry, zero, and negative flags
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* appropriately.
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*
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* @param reg
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* @param operand
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*/
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public void cmp(int reg, int operand) {
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private void cmp(int reg, int operand) {
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int tmp = (reg - operand) & 0xff;
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setCarryFlag(reg >= operand);
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setZeroFlag(reg == operand);
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setNegativeFlag((reg - operand) > 0);
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setZeroFlag(tmp == 0);
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setNegativeFlag((tmp & 0x80) != 0); // Negative bit set
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}
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/**
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* Set the Negative and Zero flags based on the current value of the
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* register operand.
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*
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* @param reg The register.
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*/
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public void setArithmeticFlags(int reg) {
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private void setArithmeticFlags(int reg) {
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zeroFlag = (reg == 0);
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negativeFlag = (reg & 0x80) != 0;
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}
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@@ -1134,6 +1127,10 @@ public class Cpu implements InstructionTable {
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this.y = val;
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}
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public int getLastProgramCounter() {
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return lastPc;
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}
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public int getProgramCounter() {
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return pc;
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}
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@@ -1285,15 +1282,15 @@ public class Cpu implements InstructionTable {
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*/
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public String toString() {
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String opcode = opcode(ir, args[0], args[1]);
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StringBuffer sb = new StringBuffer(String.format("$%04X", addr) +
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" ");
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StringBuffer sb = new StringBuffer(getInstructionByteStatus());
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sb.append(" ");
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sb.append(String.format("%-14s", opcode));
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sb.append("A=" + getAccumulatorStatus() + " ");
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sb.append("X=" + getXRegisterStatus() + " ");
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sb.append("Y=" + getYRegisterStatus() + " ");
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sb.append("PC=" + getProgramCounterStatus() + " ");
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sb.append("SP=" + getStackPointerStatus() + " ");
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sb.append("P=" + getProcessorStatusString());
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sb.append("A:" + String.format("%02x", a) + " ");
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sb.append("X:" + String.format("%02x", x) + " ");
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sb.append("Y:" + String.format("%02x", y) + " ");
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sb.append("F:" + String.format("%02x", getProcessorStatus()) + " ");
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sb.append("S:" + String.format("1%02x", sp) + " ");
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sb.append(getProcessorStatusString());
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return sb.toString();
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}
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@@ -1397,6 +1394,19 @@ public class Cpu implements InstructionTable {
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bus.write(RST_VECTOR_L, address & 0x00ff);
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}
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/*
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* Perform a busy-loop for CLOCK_IN_NS nanoseconds
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*/
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private void delayLoop(int opcode) {
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int clockSteps = Cpu.instructionClocks[0xff & opcode];
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// Just a precaution. This could be better.
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if (clockSteps == 0) { clockSteps = 1; }
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long startTime = System.nanoTime();
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long stopTime = startTime + (CLOCK_IN_NS * clockSteps);
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// Busy loop
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while (System.nanoTime() < stopTime) { ; }
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}
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/**
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* Given an opcode and its operands, return a formatted name.
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*
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@@ -1430,10 +1440,10 @@ public class Cpu implements InstructionTable {
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sb.append(String.format(" ($%04X)", address(op1, op2)));
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break;
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case XIN:
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sb.append(String.format(" ($%02X),X", op1));
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sb.append(String.format(" ($%02X,X)", op1));
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break;
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case INY:
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sb.append(String.format(" ($%02X,Y)", op1));
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sb.append(String.format(" ($%02X),Y", op1));
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break;
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case REL:
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case ZPG:
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@@ -1449,4 +1459,19 @@ public class Cpu implements InstructionTable {
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return sb.toString();
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}
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}
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public String getInstructionByteStatus() {
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switch (Cpu.instructionSizes[ir]) {
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case 0:
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case 1:
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return String.format("%04X %02X ", addr, ir);
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case 2:
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return String.format("%04X %02X %02X ", addr, ir, args[0]);
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case 3:
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return String.format("%04X %02X %02X %02X", addr, ir, args[0], args[1]);
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default:
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return null;
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}
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}
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}
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