Seth Morabito
795ccfde5d
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.
1478 lines
44 KiB
Java
1478 lines
44 KiB
Java
package com.loomcom.symon;
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import com.loomcom.symon.exceptions.MemoryAccessException;
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/**
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* Main 6502 CPU Simulation.
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*/
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public class Cpu implements InstructionTable {
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public static final int DEFAULT_BASE_ADDRESS = 0x200;
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/* Process status register mnemonics */
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public static final int P_CARRY = 0x01;
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public static final int P_ZERO = 0x02;
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public static final int P_IRQ_DISABLE = 0x04;
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public static final int P_DECIMAL = 0x08;
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public static final int P_BREAK = 0x10;
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// Bit 5 is always '1'
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public static final int P_OVERFLOW = 0x40;
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public static final int P_NEGATIVE = 0x80;
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// NMI vector
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public static final int IRQ_VECTOR_L = 0xfffa;
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public static final int IRQ_VECTOR_H = 0xfffb;
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// Reset vector
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public static final int RST_VECTOR_L = 0xfffc;
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public static final int RST_VECTOR_H = 0xfffd;
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// IRQ vector
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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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/* User Registers */
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private int a; // Accumulator
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private int x; // X index register
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private int y; // Y index register
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/* Internal Registers */
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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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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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/* Internal scratch space */
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private int lo = 0, hi = 0; // Used in address calculation
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private int tmp; // Temporary storage
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/* Unimplemented instruction flag */
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private boolean opTrap = false;
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private int addr; // The address the most recent instruction
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// was fetched from
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/* Status Flag Register bits */
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private boolean carryFlag;
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private boolean negativeFlag;
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private boolean zeroFlag;
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private boolean irqDisableFlag;
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private boolean decimalModeFlag;
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private boolean breakFlag;
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private boolean overflowFlag;
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/* The number of steps taken since the last reset. */
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private long stepCounter = 0L;
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/**
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* Construct a new CPU.
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*/
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public Cpu() {
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}
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/**
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* Set the bus reference for this CPU.
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*/
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public void setBus(Bus bus) {
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this.bus = bus;
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}
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/**
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* Return the Bus that this CPU is associated with.
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*/
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public Bus getBus() {
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return bus;
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}
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/**
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* Reset the CPU to known initial values.
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*/
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public void reset() throws MemoryAccessException {
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// Registers
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sp = 0xff;
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// Set the PC to the address stored in the reset vector
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pc = address(bus.read(RST_VECTOR_L), bus.read(RST_VECTOR_H));
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// Clear instruction register.
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ir = 0;
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// Clear status register bits.
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carryFlag = false;
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irqDisableFlag = false;
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decimalModeFlag = false;
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breakFlag = false;
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overflowFlag = false;
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zeroFlag = false;
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// Clear illegal opcode trap.
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opTrap = false;
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// Reset step counter
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stepCounter = 0L;
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// Reset registers.
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a = 0;
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x = 0;
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y = 0;
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}
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public void step(int num) throws MemoryAccessException {
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for (int i = 0; i < num; i++) {
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step();
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}
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}
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/**
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* Performs an individual machine cycle.
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*/
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public void step() throws MemoryAccessException {
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// Store the address from which the IR was read, for debugging
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addr = pc;
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// Fetch memory location for this instruction.
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ir = bus.read(pc);
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irAddressMode = (ir >> 2) & 0x07;
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irOpMode = ir & 0x03;
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// Increment PC
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incrementPC();
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// Clear the illegal opcode trap.
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clearOpTrap();
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// Decode the instruction and operands
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instSize = Cpu.instructionSizes[ir];
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for (int i = 0; i < instSize - 1; i++) {
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args[i] = bus.read(pc);
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// Increment PC after reading
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incrementPC();
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}
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// Increment step counter
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stepCounter++;
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// Get the data from the effective address (if any)
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effectiveAddress = 0;
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switch (irOpMode) {
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case 0:
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case 2:
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switch (irAddressMode) {
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case 0: // #Immediate
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break;
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case 1: // Zero Page
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effectiveAddress = args[0];
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break;
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case 2: // Accumulator - ignored
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break;
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case 3: // Absolute
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effectiveAddress = address(args[0], args[1]);
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break;
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case 5: // Zero Page,X / Zero Page,Y
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if (ir == 0x96 || ir == 0xb6) {
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effectiveAddress = zpyAddress(args[0]);
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} else {
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effectiveAddress = zpxAddress(args[0]);
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}
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break;
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case 7: // Absolute,X / Absolute,Y
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if (ir == 0xbe) {
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effectiveAddress = yAddress(args[0], args[1]);
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} else {
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effectiveAddress = xAddress(args[0], args[1]);
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}
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break;
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}
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break;
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case 1:
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switch (irAddressMode) {
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case 0: // (Zero Page,X)
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tmp = args[0] + getXRegister();
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effectiveAddress = address(bus.read(tmp), bus.read(tmp + 1));
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break;
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case 1: // Zero Page
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effectiveAddress = args[0];
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break;
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case 2: // #Immediate
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effectiveAddress = -1;
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break;
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case 3: // Absolute
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effectiveAddress = address(args[0], args[1]);
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break;
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case 4: // (Zero Page),Y
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tmp = address(bus.read(args[0]),
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bus.read((args[0] + 1) & 0xff));
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effectiveAddress = (tmp + getYRegister()) & 0xffff;
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break;
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case 5: // Zero Page,X
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effectiveAddress = zpxAddress(args[0]);
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break;
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case 6: // Absolute, Y
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effectiveAddress = yAddress(args[0], args[1]);
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break;
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case 7: // Absolute, X
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effectiveAddress = xAddress(args[0], args[1]);
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break;
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}
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break;
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}
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// Execute
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switch (ir) {
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/** Single Byte Instructions; Implied and Relative **/
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case 0x00: // BRK - Force Interrupt - Implied
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if (!getIrqDisableFlag()) {
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// Set the break flag before pushing.
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setBreakFlag();
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// Push program counter + 2 onto the stack
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stackPush((pc + 2 >> 8) & 0xff); // PC high byte
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stackPush(pc + 2 & 0xff); // PC low byte
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stackPush(getProcessorStatus());
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// Set the Interrupt Disabled flag. RTI will clear it.
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setIrqDisableFlag();
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// Load interrupt vector address into PC
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pc = address(bus.read(IRQ_VECTOR_L), bus.read(IRQ_VECTOR_H));
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}
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break;
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case 0x08: // PHP - Push Processor Status - Implied
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stackPush(getProcessorStatus());
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break;
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case 0x10: // BPL - Branch if Positive - Relative
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if (!getNegativeFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0x18: // CLC - Clear Carry Flag - Implied
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clearCarryFlag();
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break;
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case 0x20: // JSR - Jump to Subroutine - Implied
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stackPush((pc - 1 >> 8) & 0xff); // PC high byte
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stackPush(pc - 1 & 0xff); // PC low byte
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pc = address(args[0], args[1]);
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break;
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case 0x28: // PLP - Pull Processor Status - Implied
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setProcessorStatus(stackPop());
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break;
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case 0x30: // BMI - Branch if Minus - Relative
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if (getNegativeFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0x38: // SEC - Set Carry Flag - Implied
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setCarryFlag();
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break;
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case 0x40: // RTI - Return from Interrupt - Implied
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setProcessorStatus(stackPop());
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lo = stackPop();
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hi = stackPop();
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setProgramCounter(address(lo, hi));
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break;
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case 0x48: // PHA - Push Accumulator - Implied
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stackPush(a);
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break;
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case 0x50: // BVC - Branch if Overflow Clear - Relative
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if (!getOverflowFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0x58: // CLI - Clear Interrupt Disable - Implied
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clearIrqDisableFlag();
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break;
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case 0x60: // RTS - Return from Subroutine - Implied
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lo = stackPop();
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hi = stackPop();
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setProgramCounter((address(lo, hi) + 1) & 0xffff);
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break;
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case 0x68: // PLA - Pull Accumulator - Implied
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a = stackPop();
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setArithmeticFlags(a);
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break;
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case 0x70: // BVS - Branch if Overflow Set - Relative
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if (getOverflowFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0x78: // SEI - Set Interrupt Disable - Implied
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setIrqDisableFlag();
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break;
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case 0x88: // DEY - Decrement Y Register - Implied
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y = --y & 0xff;
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setArithmeticFlags(y);
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break;
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case 0x8a: // TXA - Transfer X to Accumulator - Implied
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a = x;
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setArithmeticFlags(a);
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break;
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case 0x90: // BCC - Branch if Carry Clear - Relative
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if (!getCarryFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0x98: // TYA - Transfer Y to Accumulator - Implied
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a = y;
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setArithmeticFlags(a);
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break;
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case 0x9a: // TXS - Transfer X to Stack Pointer - Implied
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setStackPointer(x);
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break;
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case 0xa8: // TAY - Transfer Accumulator to Y - Implied
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y = a;
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setArithmeticFlags(y);
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break;
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case 0xaa: // TAX - Transfer Accumulator to X - Implied
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x = a;
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setArithmeticFlags(x);
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break;
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case 0xb0: // BCS - Branch if Carry Set - Relative
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if (getCarryFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0xb8: // CLV - Clear Overflow Flag - Implied
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clearOverflowFlag();
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break;
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case 0xba: // TSX - Transfer Stack Pointer to X - Implied
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x = getStackPointer();
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setArithmeticFlags(x);
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break;
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case 0xc8: // INY - Increment Y Register - Implied
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y = ++y & 0xff;
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setArithmeticFlags(y);
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break;
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case 0xca: // DEX - Decrement X Register - Implied
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x = --x & 0xff;
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setArithmeticFlags(x);
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break;
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case 0xd0: // BNE - Branch if Not Equal to Zero - Relative
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if (!getZeroFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0xd8: // CLD - Clear Decimal Mode - Implied
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clearDecimalModeFlag();
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break;
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case 0xe8: // INX - Increment X Register - Implied
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x = ++x & 0xff;
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setArithmeticFlags(x);
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break;
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case 0xea: // NOP
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// Do nothing.
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break;
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case 0xf0: // BEQ - Branch if Equal to Zero - Relative
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if (getZeroFlag()) {
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pc = relAddress(args[0]);
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}
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break;
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case 0xf8: // SED - Set Decimal Flag - Implied
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setDecimalModeFlag();
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break;
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/** JMP *****************************************************************/
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case 0x4c: // JMP - Absolute
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pc = address(args[0], args[1]);
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break;
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case 0x6c: // JMP - Indirect
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lo = address(args[0], args[1]); // Address of low byte
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hi = lo + 1; // Address of high byte
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pc = address(bus.read(lo), bus.read(hi));
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/* TODO: For accuracy, allow a flag to enable broken behavior
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* of early 6502s:
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*
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* "An original 6502 has does not correctly fetch the target
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* address if the indirect vector falls on a page boundary
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* (e.g. $xxFF where xx is and value from $00 to $FF). In this
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* case fetches the LSB from $xxFF as expected but takes the MSB
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* from $xx00. This is fixed in some later chips like the 65SC02
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* so for compatibility always ensure the indirect vector is not
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* at the end of the page."
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* (http://www.obelisk.demon.co.uk/6502/reference.html#JMP)
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*/
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break;
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/** ORA - Logical Inclusive Or ******************************************/
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case 0x09: // #Immediate
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a |= args[0];
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setArithmeticFlags(a);
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break;
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case 0x01: // (Zero Page,X)
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case 0x05: // Zero Page
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case 0x0d: // Absolute
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case 0x11: // (Zero Page),Y
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case 0x15: // Zero Page,X
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case 0x19: // Absolute,Y
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case 0x1d: // Absolute,X
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a |= bus.read(effectiveAddress);
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setArithmeticFlags(a);
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break;
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/** ASL - Arithmetic Shift Left *****************************************/
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case 0x0a: // Accumulator
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a = asl(a);
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setArithmeticFlags(a);
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break;
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case 0x06: // Zero Page
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case 0x0e: // Absolute
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case 0x16: // Zero Page,X
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case 0x1e: // Absolute,X
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tmp = asl(bus.read(effectiveAddress));
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bus.write(effectiveAddress, tmp);
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setArithmeticFlags(tmp);
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break;
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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 = 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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/** AND - Logical AND ***************************************************/
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case 0x29: // #Immediate
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a &= args[0];
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setArithmeticFlags(a);
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break;
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case 0x21: // (Zero Page,X)
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case 0x25: // Zero Page
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case 0x2d: // Absolute
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case 0x31: // (Zero Page),Y
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case 0x35: // Zero Page,X
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case 0x39: // Absolute,Y
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case 0x3d: // Absolute,X
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a &= bus.read(effectiveAddress);
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setArithmeticFlags(a);
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break;
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/** ROL - Rotate Left ***************************************************/
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case 0x2a: // Accumulator
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a = rol(a);
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setArithmeticFlags(a);
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break;
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case 0x26: // Zero Page
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case 0x2e: // Absolute
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case 0x36: // Zero Page,X
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case 0x3e: // Absolute,X
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tmp = rol(bus.read(effectiveAddress));
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bus.write(effectiveAddress, tmp);
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setArithmeticFlags(tmp);
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break;
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/** EOR - Exclusive OR **************************************************/
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case 0x49: // #Immediate
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a ^= args[0];
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setArithmeticFlags(a);
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break;
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case 0x41: // (Zero Page,X)
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case 0x45: // Zero Page
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case 0x4d: // Absolute
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case 0x51: // (Zero Page,Y)
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case 0x55: // Zero Page,X
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case 0x59: // Absolute,Y
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case 0x5d: // Absolute,X
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a ^= bus.read(effectiveAddress);
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setArithmeticFlags(a);
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break;
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/** LSR - Logical Shift Right *******************************************/
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case 0x4a: // Accumulator
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a = lsr(a);
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setArithmeticFlags(a);
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break;
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case 0x46: // Zero Page
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case 0x4e: // Absolute
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case 0x56: // Zero Page,X
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case 0x5e: // Absolute,X
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tmp = lsr(bus.read(effectiveAddress));
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bus.write(effectiveAddress, tmp);
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setArithmeticFlags(tmp);
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break;
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/** ADC - Add with Carry ************************************************/
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case 0x69: // #Immediate
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if (decimalModeFlag) {
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a = adcDecimal(a, args[0]);
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} else {
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a = adc(a, args[0]);
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}
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break;
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case 0x61: // (Zero Page,X)
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case 0x65: // Zero Page
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case 0x6d: // Absolute
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case 0x71: // (Zero Page),Y
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case 0x75: // Zero Page,X
|
|
case 0x79: // Absolute,Y
|
|
case 0x7d: // Absolute,X
|
|
if (decimalModeFlag) {
|
|
a = adcDecimal(a, bus.read(effectiveAddress));
|
|
} else {
|
|
a = adc(a, bus.read(effectiveAddress));
|
|
}
|
|
break;
|
|
|
|
|
|
/** ROR - Rotate Right **************************************************/
|
|
case 0x6a: // Accumulator
|
|
a = ror(a);
|
|
setArithmeticFlags(a);
|
|
break;
|
|
case 0x66: // Zero Page
|
|
case 0x6e: // Absolute
|
|
case 0x76: // Zero Page,X
|
|
case 0x7e: // Absolute,X
|
|
tmp = ror(bus.read(effectiveAddress));
|
|
bus.write(effectiveAddress, tmp);
|
|
setArithmeticFlags(tmp);
|
|
break;
|
|
|
|
|
|
/** STA - Store Accumulator *********************************************/
|
|
case 0x81: // (Zero Page,X)
|
|
case 0x85: // Zero Page
|
|
case 0x8d: // Absolute
|
|
case 0x91: // (Zero Page),Y
|
|
case 0x95: // Zero Page,X
|
|
case 0x99: // Absolute,Y
|
|
case 0x9d: // Absolute,X
|
|
bus.write(effectiveAddress, a);
|
|
break;
|
|
|
|
|
|
/** STY - Store Y Register **********************************************/
|
|
case 0x84: // Zero Page
|
|
case 0x8c: // Absolute
|
|
case 0x94: // Zero Page,X
|
|
bus.write(effectiveAddress, y);
|
|
break;
|
|
|
|
|
|
/** STX - Store X Register **********************************************/
|
|
case 0x86: // Zero Page
|
|
case 0x8e: // Absolute
|
|
case 0x96: // Zero Page,Y
|
|
bus.write(effectiveAddress, x);
|
|
break;
|
|
|
|
|
|
/** LDY - Load Y Register ***********************************************/
|
|
case 0xa0: // #Immediate
|
|
y = args[0];
|
|
setArithmeticFlags(y);
|
|
break;
|
|
case 0xa4: // Zero Page
|
|
case 0xac: // Absolute
|
|
case 0xb4: // Zero Page,X
|
|
case 0xbc: // Absolute,X
|
|
y = bus.read(effectiveAddress);
|
|
setArithmeticFlags(y);
|
|
break;
|
|
|
|
|
|
/** LDX - Load X Register ***********************************************/
|
|
case 0xa2: // #Immediate
|
|
x = args[0];
|
|
setArithmeticFlags(x);
|
|
break;
|
|
case 0xa6: // Zero Page
|
|
case 0xae: // Absolute
|
|
case 0xb6: // Zero Page,Y
|
|
case 0xbe: // Absolute,Y
|
|
x = bus.read(effectiveAddress);
|
|
setArithmeticFlags(x);
|
|
break;
|
|
|
|
|
|
/** LDA - Load Accumulator **********************************************/
|
|
case 0xa9: // #Immediate
|
|
a = args[0];
|
|
setArithmeticFlags(a);
|
|
break;
|
|
case 0xa1: // (Zero Page,X)
|
|
case 0xa5: // Zero Page
|
|
case 0xad: // Absolute
|
|
case 0xb1: // (Zero Page),Y
|
|
case 0xb5: // Zero Page,X
|
|
case 0xb9: // Absolute,Y
|
|
case 0xbd: // Absolute,X
|
|
a = bus.read(effectiveAddress);
|
|
setArithmeticFlags(a);
|
|
break;
|
|
|
|
|
|
/** CPY - Compare Y Register ********************************************/
|
|
case 0xc0: // #Immediate
|
|
cmp(y, args[0]);
|
|
break;
|
|
case 0xc4: // Zero Page
|
|
case 0xcc: // Absolute
|
|
cmp(y, bus.read(effectiveAddress));
|
|
break;
|
|
|
|
|
|
/** CMP - Compare Accumulator *******************************************/
|
|
case 0xc9: // #Immediate
|
|
cmp(a, args[0]);
|
|
break;
|
|
case 0xc1: // (Zero Page,X)
|
|
case 0xc5: // Zero Page
|
|
case 0xcd: // Absolute
|
|
case 0xd1: // (Zero Page),Y
|
|
case 0xd5: // Zero Page,X
|
|
case 0xd9: // Absolute,Y
|
|
case 0xdd: // Absolute,X
|
|
cmp(a, bus.read(effectiveAddress));
|
|
break;
|
|
|
|
|
|
/** DEC - Decrement Memory **********************************************/
|
|
case 0xc6: // Zero Page
|
|
case 0xce: // Absolute
|
|
case 0xd6: // Zero Page,X
|
|
case 0xde: // Absolute,X
|
|
tmp = (bus.read(effectiveAddress) - 1) & 0xff;
|
|
bus.write(effectiveAddress, tmp);
|
|
setArithmeticFlags(tmp);
|
|
break;
|
|
|
|
|
|
/** CPX - Compare X Register ********************************************/
|
|
case 0xe0: // #Immediate
|
|
cmp(x, args[0]);
|
|
break;
|
|
case 0xe4: // Zero Page
|
|
case 0xec: // Absolute
|
|
cmp(x, bus.read(effectiveAddress));
|
|
break;
|
|
|
|
|
|
/** SBC - Subtract with Carry (Borrow) **********************************/
|
|
case 0xe9: // #Immediate
|
|
if (decimalModeFlag) {
|
|
a = sbcDecimal(a, args[0]);
|
|
} else {
|
|
a = sbc(a, args[0]);
|
|
}
|
|
break;
|
|
case 0xe1: // (Zero Page,X)
|
|
case 0xe5: // Zero Page
|
|
case 0xed: // Absolute
|
|
case 0xf1: // (Zero Page),Y
|
|
case 0xf5: // Zero Page,X
|
|
case 0xf9: // Absolute,Y
|
|
case 0xfd: // Absolute,X
|
|
if (decimalModeFlag) {
|
|
a = sbcDecimal(a, bus.read(effectiveAddress));
|
|
} else {
|
|
a = sbc(a, bus.read(effectiveAddress));
|
|
}
|
|
break;
|
|
|
|
|
|
/** INC - Increment Memory **********************************************/
|
|
case 0xe6: // Zero Page
|
|
case 0xee: // Absolute
|
|
case 0xf6: // Zero Page,X
|
|
case 0xfe: // Absolute,X
|
|
tmp = (bus.read(effectiveAddress) + 1) & 0xff;
|
|
bus.write(effectiveAddress, tmp);
|
|
setArithmeticFlags(tmp);
|
|
break;
|
|
|
|
/** Unimplemented Instructions ****************************************/
|
|
default:
|
|
setOpTrap();
|
|
break;
|
|
}
|
|
|
|
delayLoop(ir);
|
|
}
|
|
|
|
/**
|
|
* Add with Carry, used by all addressing mode implementations of ADC.
|
|
* As a side effect, this will set the overflow and carry flags as
|
|
* needed.
|
|
*
|
|
* @param acc The current value of the accumulator
|
|
* @param operand The operand
|
|
* @return
|
|
*/
|
|
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);
|
|
setOverflowFlag(carryFlag ^ ((carry6 & 0x80) != 0));
|
|
result &= 0xff;
|
|
setArithmeticFlags(result);
|
|
return result;
|
|
}
|
|
|
|
/**
|
|
* Add with Carry (BCD).
|
|
*/
|
|
|
|
private int adcDecimal(int acc, int operand) {
|
|
int l, h, result;
|
|
l = (acc & 0x0f) + (operand & 0x0f) + getCarryBit();
|
|
if ((l & 0xff) > 9) l += 6;
|
|
h = (acc >> 4) + (operand >> 4) + (l > 15 ? 1 : 0);
|
|
if ((h & 0xff) > 9) h += 6;
|
|
result = (l & 0x0f) | (h << 4);
|
|
result &= 0xff;
|
|
setCarryFlag(h > 15);
|
|
setZeroFlag(result == 0);
|
|
setNegativeFlag(false); // BCD is never negative
|
|
setOverflowFlag(false); // BCD never sets overflow flag
|
|
return result;
|
|
}
|
|
|
|
/**
|
|
* 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.
|
|
*/
|
|
private int sbc(int acc, int operand) {
|
|
int result;
|
|
result = adc(acc, ~operand);
|
|
setArithmeticFlags(result);
|
|
return result;
|
|
}
|
|
|
|
/**
|
|
* Subtract with Carry, BCD mode.
|
|
*/
|
|
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;
|
|
h = (acc >> 4) - (operand >> 4) - ((l & 0x10) != 0 ? 1 : 0);
|
|
if ((h & 0x10) != 0) h -= 6;
|
|
result = (l & 0x0f) | (h << 4);
|
|
setCarryFlag((h & 0xff) < 15);
|
|
setZeroFlag(result == 0);
|
|
setNegativeFlag(false); // BCD is never negative
|
|
setOverflowFlag(false); // BCD never sets overflow flag
|
|
return (result & 0xff);
|
|
}
|
|
|
|
/**
|
|
* Compare two values, and set carry, zero, and negative flags
|
|
* appropriately.
|
|
*/
|
|
private void cmp(int reg, int operand) {
|
|
int tmp = (reg - operand) & 0xff;
|
|
setCarryFlag(reg >= operand);
|
|
setZeroFlag(tmp == 0);
|
|
setNegativeFlag((tmp & 0x80) != 0); // Negative bit set
|
|
}
|
|
|
|
/**
|
|
* Set the Negative and Zero flags based on the current value of the
|
|
* register operand.
|
|
*/
|
|
private void setArithmeticFlags(int reg) {
|
|
zeroFlag = (reg == 0);
|
|
negativeFlag = (reg & 0x80) != 0;
|
|
}
|
|
|
|
/**
|
|
* Shifts the given value left by one bit, and sets the carry
|
|
* flag to the high bit of the initial value.
|
|
*
|
|
* @param m The value to shift left.
|
|
* @return the left shifted value (m * 2).
|
|
*/
|
|
private int asl(int m) {
|
|
setCarryFlag((m & 0x80) != 0);
|
|
return (m << 1) & 0xff;
|
|
}
|
|
|
|
/**
|
|
* Shifts the given value right by one bit, filling with zeros,
|
|
* and sets the carry flag to the low bit of the initial value.
|
|
*/
|
|
private int lsr(int m) {
|
|
setCarryFlag((m & 0x01) != 0);
|
|
return (m >>> 1) & 0xff;
|
|
}
|
|
|
|
/**
|
|
* Rotates the given value left by one bit, setting bit 0 to the value
|
|
* of the carry flag, and setting the carry flag to the original value
|
|
* of bit 7.
|
|
*/
|
|
private int rol(int m) {
|
|
int result = ((m << 1) | getCarryBit()) & 0xff;
|
|
setCarryFlag((m & 0x80) != 0);
|
|
return result;
|
|
}
|
|
|
|
/**
|
|
* Rotates the given value right by one bit, setting bit 7 to the value
|
|
* of the carry flag, and setting the carry flag to the original value
|
|
* of bit 1.
|
|
*/
|
|
private int ror(int m) {
|
|
int result = ((m >>> 1) | (getCarryBit() << 7)) & 0xff;
|
|
setCarryFlag((m & 0x01) != 0);
|
|
return result;
|
|
}
|
|
|
|
/**
|
|
* @return the negative flag
|
|
*/
|
|
public boolean getNegativeFlag() {
|
|
return negativeFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the negative flag is set, 0 if it is clear.
|
|
*/
|
|
public int getNegativeBit() {
|
|
return (negativeFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param register the register value to test for negativity
|
|
*/
|
|
public void setNegativeFlag(int register) {
|
|
negativeFlag = (register < 0);
|
|
}
|
|
|
|
/**
|
|
* @param negativeFlag the negative flag to set
|
|
*/
|
|
public void setNegativeFlag(boolean negativeFlag) {
|
|
this.negativeFlag = negativeFlag;
|
|
}
|
|
|
|
public void setNegativeFlag() {
|
|
this.negativeFlag = true;
|
|
}
|
|
|
|
public void clearNegativeFlag() {
|
|
this.negativeFlag = false;
|
|
}
|
|
|
|
/**
|
|
* @return the carry flag
|
|
*/
|
|
public boolean getCarryFlag() {
|
|
return carryFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the carry flag is set, 0 if it is clear.
|
|
*/
|
|
public int getCarryBit() {
|
|
return (carryFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param carryFlag the carry flag to set
|
|
*/
|
|
public void setCarryFlag(boolean carryFlag) {
|
|
this.carryFlag = carryFlag;
|
|
}
|
|
|
|
/**
|
|
* Sets the Carry Flag
|
|
*/
|
|
public void setCarryFlag() {
|
|
this.carryFlag = true;
|
|
}
|
|
|
|
/**
|
|
* Clears the Carry Flag
|
|
*/
|
|
public void clearCarryFlag() {
|
|
this.carryFlag = false;
|
|
}
|
|
|
|
/**
|
|
* @return the zero flag
|
|
*/
|
|
public boolean getZeroFlag() {
|
|
return zeroFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the zero flag is set, 0 if it is clear.
|
|
*/
|
|
public int getZeroBit() {
|
|
return (zeroFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param zeroFlag the zero flag to set
|
|
*/
|
|
public void setZeroFlag(boolean zeroFlag) {
|
|
this.zeroFlag = zeroFlag;
|
|
}
|
|
|
|
/**
|
|
* Sets the Zero Flag
|
|
*/
|
|
public void setZeroFlag() {
|
|
this.zeroFlag = true;
|
|
}
|
|
|
|
/**
|
|
* Clears the Zero Flag
|
|
*/
|
|
public void clearZeroFlag() {
|
|
this.zeroFlag = false;
|
|
}
|
|
|
|
/**
|
|
* @return the irq disable flag
|
|
*/
|
|
public boolean getIrqDisableFlag() {
|
|
return irqDisableFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the interrupt disable flag is set, 0 if it is clear.
|
|
*/
|
|
public int getIrqDisableBit() {
|
|
return (irqDisableFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param irqDisableFlag the irq disable flag to set
|
|
*/
|
|
public void setIrqDisableFlag(boolean irqDisableFlag) {
|
|
this.irqDisableFlag = irqDisableFlag;
|
|
}
|
|
|
|
public void setIrqDisableFlag() {
|
|
this.irqDisableFlag = true;
|
|
}
|
|
|
|
public void clearIrqDisableFlag() {
|
|
this.irqDisableFlag = false;
|
|
}
|
|
|
|
|
|
/**
|
|
* @return the decimal mode flag
|
|
*/
|
|
public boolean getDecimalModeFlag() {
|
|
return decimalModeFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the decimal mode flag is set, 0 if it is clear.
|
|
*/
|
|
public int getDecimalModeBit() {
|
|
return (decimalModeFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param decimalModeFlag the decimal mode flag to set
|
|
*/
|
|
public void setDecimalModeFlag(boolean decimalModeFlag) {
|
|
this.decimalModeFlag = decimalModeFlag;
|
|
}
|
|
|
|
/**
|
|
* Sets the Decimal Mode Flag to true.
|
|
*/
|
|
public void setDecimalModeFlag() {
|
|
this.decimalModeFlag = true;
|
|
}
|
|
|
|
/**
|
|
* Clears the Decimal Mode Flag.
|
|
*/
|
|
public void clearDecimalModeFlag() {
|
|
this.decimalModeFlag = false;
|
|
}
|
|
|
|
/**
|
|
* @return the break flag
|
|
*/
|
|
public boolean getBreakFlag() {
|
|
return breakFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the break flag is set, 0 if it is clear.
|
|
*/
|
|
public int getBreakBit() {
|
|
return (carryFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param breakFlag the break flag to set
|
|
*/
|
|
public void setBreakFlag(boolean breakFlag) {
|
|
this.breakFlag = breakFlag;
|
|
}
|
|
|
|
/**
|
|
* Sets the Break Flag
|
|
*/
|
|
public void setBreakFlag() {
|
|
this.breakFlag = true;
|
|
}
|
|
|
|
/**
|
|
* Clears the Break Flag
|
|
*/
|
|
public void clearBreakFlag() {
|
|
this.breakFlag = false;
|
|
}
|
|
|
|
/**
|
|
* @return the overflow flag
|
|
*/
|
|
public boolean getOverflowFlag() {
|
|
return overflowFlag;
|
|
}
|
|
|
|
/**
|
|
* @return 1 if the overflow flag is set, 0 if it is clear.
|
|
*/
|
|
public int getOverflowBit() {
|
|
return (overflowFlag ? 1 : 0);
|
|
}
|
|
|
|
/**
|
|
* @param overflowFlag the overflow flag to set
|
|
*/
|
|
public void setOverflowFlag(boolean overflowFlag) {
|
|
this.overflowFlag = overflowFlag;
|
|
}
|
|
|
|
/**
|
|
* Sets the Overflow Flag
|
|
*/
|
|
public void setOverflowFlag() {
|
|
this.overflowFlag = true;
|
|
}
|
|
|
|
/**
|
|
* Clears the Overflow Flag
|
|
*/
|
|
public void clearOverflowFlag() {
|
|
this.overflowFlag = false;
|
|
}
|
|
|
|
/**
|
|
* Set the illegal instruction trap.
|
|
*/
|
|
public void setOpTrap() {
|
|
this.opTrap = true;
|
|
}
|
|
|
|
/**
|
|
* Clear the illegal instruction trap.
|
|
*/
|
|
public void clearOpTrap() {
|
|
this.opTrap = false;
|
|
}
|
|
|
|
/**
|
|
* Get the status of the illegal instruction trap.
|
|
*/
|
|
public boolean getOpTrap() {
|
|
return this.opTrap;
|
|
}
|
|
|
|
public int getAccumulator() {
|
|
return a;
|
|
}
|
|
|
|
public void setAccumulator(int val) {
|
|
this.a = val;
|
|
}
|
|
|
|
public int getXRegister() {
|
|
return x;
|
|
}
|
|
|
|
public void setXRegister(int val) {
|
|
this.x = val;
|
|
}
|
|
|
|
public int getYRegister() {
|
|
return y;
|
|
}
|
|
|
|
public void setYRegister(int val) {
|
|
this.y = val;
|
|
}
|
|
|
|
public int getLastProgramCounter() {
|
|
return lastPc;
|
|
}
|
|
|
|
public int getProgramCounter() {
|
|
return pc;
|
|
}
|
|
|
|
public void setProgramCounter(int addr) {
|
|
this.pc = addr;
|
|
}
|
|
|
|
public int getStackPointer() {
|
|
return sp;
|
|
}
|
|
|
|
public void setStackPointer(int offset) {
|
|
this.sp = offset;
|
|
}
|
|
|
|
public int getInstructionRegister() {
|
|
return this.ir;
|
|
}
|
|
|
|
public void setInstructionRegister(int op) {
|
|
this.ir = op;
|
|
}
|
|
|
|
public long getStepCounter() {
|
|
return stepCounter;
|
|
}
|
|
|
|
public void setStepCounter(long stepCount) {
|
|
this.stepCounter = stepCount;
|
|
}
|
|
|
|
/**
|
|
* @value The value of the Process Status Register bits to be set.
|
|
*/
|
|
public void setProcessorStatus(int value) {
|
|
if ((value & P_CARRY) != 0)
|
|
setCarryFlag();
|
|
else
|
|
clearCarryFlag();
|
|
|
|
if ((value & P_ZERO) != 0)
|
|
setZeroFlag();
|
|
else
|
|
clearZeroFlag();
|
|
|
|
if ((value & P_IRQ_DISABLE) != 0)
|
|
setIrqDisableFlag();
|
|
else
|
|
clearIrqDisableFlag();
|
|
|
|
if ((value & P_DECIMAL) != 0)
|
|
setDecimalModeFlag();
|
|
else
|
|
clearDecimalModeFlag();
|
|
|
|
if ((value & P_BREAK) != 0)
|
|
setBreakFlag();
|
|
else
|
|
clearBreakFlag();
|
|
|
|
if ((value & P_OVERFLOW) != 0)
|
|
setOverflowFlag();
|
|
else
|
|
clearOverflowFlag();
|
|
|
|
if ((value & P_NEGATIVE) != 0)
|
|
setNegativeFlag();
|
|
else
|
|
clearNegativeFlag();
|
|
}
|
|
|
|
/**
|
|
* @returns The value of the Process Status Register, as a byte.
|
|
*/
|
|
public int getProcessorStatus() {
|
|
int status = 0x20;
|
|
if (getCarryFlag()) {
|
|
status |= P_CARRY;
|
|
}
|
|
if (getZeroFlag()) {
|
|
status |= P_ZERO;
|
|
}
|
|
if (getIrqDisableFlag()) {
|
|
status |= P_IRQ_DISABLE;
|
|
}
|
|
if (getDecimalModeFlag()) {
|
|
status |= P_DECIMAL;
|
|
}
|
|
if (getBreakFlag()) {
|
|
status |= P_BREAK;
|
|
}
|
|
if (getOverflowFlag()) {
|
|
status |= P_OVERFLOW;
|
|
}
|
|
if (getNegativeFlag()) {
|
|
status |= P_NEGATIVE;
|
|
}
|
|
return status;
|
|
}
|
|
|
|
/**
|
|
* @return A string representing the current status register state.
|
|
*/
|
|
public String getProcessorStatusString() {
|
|
StringBuffer sb = new StringBuffer("[");
|
|
sb.append(getNegativeFlag() ? 'N' : '.'); // Bit 7
|
|
sb.append(getOverflowFlag() ? 'V' : '.'); // Bit 6
|
|
sb.append("-"); // Bit 5 (always 1)
|
|
sb.append(getBreakFlag() ? 'B' : '.'); // Bit 4
|
|
sb.append(getDecimalModeFlag() ? 'D' : '.'); // Bit 3
|
|
sb.append(getIrqDisableFlag() ? 'I' : '.'); // Bit 2
|
|
sb.append(getZeroFlag() ? 'Z' : '.'); // Bit 1
|
|
sb.append(getCarryFlag() ? 'C' : '.'); // Bit 0
|
|
sb.append("]");
|
|
return sb.toString();
|
|
}
|
|
|
|
public String getOpcodeStatus() {
|
|
return opcode(ir, args[0], args[1]);
|
|
}
|
|
|
|
public String getAddressStatus() {
|
|
return String.format("$%04X", addr);
|
|
}
|
|
|
|
public String getAccumulatorStatus() {
|
|
return String.format("$%02X", a);
|
|
}
|
|
|
|
public String getXRegisterStatus() {
|
|
return String.format("$%02X", x);
|
|
}
|
|
|
|
public String getYRegisterStatus() {
|
|
return String.format("$%02X", y);
|
|
}
|
|
|
|
public String getProgramCounterStatus() {
|
|
return String.format("$%04X", pc);
|
|
}
|
|
|
|
public String getStackPointerStatus() {
|
|
return String.format("$%02X", sp);
|
|
}
|
|
|
|
/**
|
|
* Returns a string representing the CPU state.
|
|
*/
|
|
public String toString() {
|
|
String opcode = opcode(ir, args[0], args[1]);
|
|
StringBuffer sb = new StringBuffer(getInstructionByteStatus());
|
|
sb.append(" ");
|
|
sb.append(String.format("%-14s", opcode));
|
|
sb.append("A:" + String.format("%02x", a) + " ");
|
|
sb.append("X:" + String.format("%02x", x) + " ");
|
|
sb.append("Y:" + String.format("%02x", y) + " ");
|
|
sb.append("F:" + String.format("%02x", getProcessorStatus()) + " ");
|
|
sb.append("S:" + String.format("1%02x", sp) + " ");
|
|
sb.append(getProcessorStatusString());
|
|
return sb.toString();
|
|
}
|
|
|
|
/**
|
|
* Push an item onto the stack, and decrement the stack counter.
|
|
* Will wrap-around if already at the bottom of the stack (This
|
|
* is the same behavior as the real 6502)
|
|
*/
|
|
void stackPush(int data) throws MemoryAccessException {
|
|
bus.write(0x100 + sp, data);
|
|
|
|
if (sp == 0) {
|
|
sp = 0xff;
|
|
} else {
|
|
--sp;
|
|
}
|
|
|
|
}
|
|
|
|
|
|
/**
|
|
* Pre-increment the stack pointer, and return the top of the stack.
|
|
* Will wrap-around if already at the top of the stack (This
|
|
* is the same behavior as the real 6502)
|
|
*/
|
|
int stackPop() throws MemoryAccessException {
|
|
if (sp == 0xff) {
|
|
sp = 0x00;
|
|
} else {
|
|
++sp;
|
|
}
|
|
|
|
return bus.read(0x100 + sp);
|
|
}
|
|
|
|
/**
|
|
* Peek at the value currently at the top of the stack
|
|
*/
|
|
int stackPeek() throws MemoryAccessException {
|
|
return bus.read(0x100 + sp + 1);
|
|
}
|
|
|
|
/*
|
|
* Increment the PC, rolling over if necessary.
|
|
*/
|
|
void incrementPC() {
|
|
if (pc == 0xffff) {
|
|
pc = 0;
|
|
} else {
|
|
++pc;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Given two bytes, return an address.
|
|
*/
|
|
int address(int lowByte, int hiByte) {
|
|
return ((hiByte << 8) | lowByte) & 0xffff;
|
|
}
|
|
|
|
/**
|
|
* Given a hi byte and a low byte, return the Absolute,X
|
|
* offset address.
|
|
*/
|
|
int xAddress(int lowByte, int hiByte) {
|
|
return (address(lowByte, hiByte) + getXRegister()) & 0xffff;
|
|
}
|
|
|
|
/**
|
|
* Given a hi byte and a low byte, return the Absolute,Y
|
|
* offset address.
|
|
*/
|
|
int yAddress(int lowByte, int hiByte) {
|
|
return (address(lowByte, hiByte) + getYRegister()) & 0xffff;
|
|
}
|
|
|
|
/**
|
|
* Given a single byte, compute the Zero Page,X offset address.
|
|
*/
|
|
int zpxAddress(int zp) {
|
|
return (zp + getXRegister()) & 0xff;
|
|
}
|
|
|
|
/**
|
|
* Given a single byte, compute the offset address.
|
|
*/
|
|
int relAddress(int offset) {
|
|
// Cast the offset to a signed byte to handle negative offsets
|
|
return (pc + (byte) offset) & 0xffff;
|
|
}
|
|
|
|
/**
|
|
* Given a single byte, compute the Zero Page,Y offset address.
|
|
*/
|
|
int zpyAddress(int zp) {
|
|
return (zp + getYRegister()) & 0xff;
|
|
}
|
|
|
|
public void setResetVector(int address) throws MemoryAccessException {
|
|
bus.write(RST_VECTOR_H, (address & 0xff00) >>> 8);
|
|
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.
|
|
*
|
|
* @param opcode The opcode
|
|
* @param op1 The first operand
|
|
* @param op2 The second operand
|
|
* @return
|
|
*/
|
|
String opcode(int opcode, int op1, int op2) {
|
|
String opcodeName = Cpu.opcodeNames[opcode];
|
|
if (opcodeName == null) {
|
|
return "???";
|
|
}
|
|
|
|
StringBuffer sb = new StringBuffer(opcodeName);
|
|
|
|
switch (Cpu.instructionModes[opcode]) {
|
|
case ABS:
|
|
sb.append(String.format(" $%04X", address(op1, op2)));
|
|
break;
|
|
case ABX:
|
|
sb.append(String.format(" $%04X,X", address(op1, op2)));
|
|
break;
|
|
case ABY:
|
|
sb.append(String.format(" $%04X,Y", address(op1, op2)));
|
|
break;
|
|
case IMM:
|
|
sb.append(String.format(" #$%02X", op1));
|
|
break;
|
|
case IND:
|
|
sb.append(String.format(" ($%04X)", address(op1, op2)));
|
|
break;
|
|
case XIN:
|
|
sb.append(String.format(" ($%02X,X)", op1));
|
|
break;
|
|
case INY:
|
|
sb.append(String.format(" ($%02X),Y", op1));
|
|
break;
|
|
case REL:
|
|
case ZPG:
|
|
sb.append(String.format(" $%02X", op1));
|
|
break;
|
|
case ZPX:
|
|
sb.append(String.format(" $%02X,X", op1));
|
|
break;
|
|
case ZPY:
|
|
sb.append(String.format(" $%02X,Y", op1));
|
|
break;
|
|
}
|
|
|
|
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;
|
|
}
|
|
}
|
|
|
|
}
|