| /* |
| * Copyright (c) 2016 Seth J. Morabito <web@loomcom.com> |
| * |
| * Permission is hereby granted, free of charge, to any person obtaining |
| * a copy of this software and associated documentation files (the |
| * "Software"), to deal in the Software without restriction, including |
| * without limitation the rights to use, copy, modify, merge, publish, |
| * distribute, sublicense, and/or sell copies of the Software, and to |
| * permit persons to whom the Software is furnished to do so, subject to |
| * the following conditions: |
| * |
| * The above copyright notice and this permission notice shall be |
| * included in all copies or substantial portions of the Software. |
| * |
| * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, |
| * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF |
| * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND |
| * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE |
| * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION |
| * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION |
| * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. |
| */ |
| |
| package com.loomcom.symon; |
| |
| import java.util.logging.Level; |
| import java.util.logging.Logger; |
| |
| import com.loomcom.symon.exceptions.MemoryAccessException; |
| import com.loomcom.symon.util.Utils; |
| |
| /** |
| * This class provides a simulation of the MOS 6502 CPU's state machine. |
| * A simple interface allows this 6502 to read and write to a simulated bus, |
| * and exposes some of the internal state for inspection and debugging. |
| */ |
| public class Cpu implements InstructionTable { |
| |
| private final static Logger logger = Logger.getLogger(Cpu.class.getName()); |
| |
| /* Process status register mnemonics */ |
| public static final int P_CARRY = 0x01; |
| public static final int P_ZERO = 0x02; |
| public static final int P_IRQ_DISABLE = 0x04; |
| public static final int P_DECIMAL = 0x08; |
| public static final int P_BREAK = 0x10; |
| // Bit 5 is always '1' |
| public static final int P_OVERFLOW = 0x40; |
| public static final int P_NEGATIVE = 0x80; |
| |
| // NMI vector |
| public static final int NMI_VECTOR_L = 0xfffa; |
| public static final int NMI_VECTOR_H = 0xfffb; |
| // Reset vector |
| public static final int RST_VECTOR_L = 0xfffc; |
| public static final int RST_VECTOR_H = 0xfffd; |
| // IRQ vector |
| public static final int IRQ_VECTOR_L = 0xfffe; |
| public static final int IRQ_VECTOR_H = 0xffff; |
| |
| /* Simulated behavior */ |
| private CpuBehavior behavior; |
| |
| /* The Bus */ |
| private Bus bus; |
| |
| /* The CPU state */ |
| private final CpuState state = new CpuState(); |
| |
| /* start time of op execution, needed for speed simulation */ |
| private long opBeginTime; |
| |
| /** |
| * Construct a new CPU. |
| */ |
| public Cpu() { |
| this(CpuBehavior.NMOS_WITH_INDIRECT_JMP_BUG); |
| } |
| |
| public Cpu(CpuBehavior behavior) { |
| this.behavior = behavior; |
| } |
| |
| /** |
| * Set the bus reference for this CPU. |
| */ |
| public void setBus(Bus bus) { |
| this.bus = bus; |
| } |
| |
| /** |
| * Return the Bus that this CPU is associated with. |
| */ |
| public Bus getBus() { |
| return bus; |
| } |
| |
| public void setBehavior(CpuBehavior behavior) { |
| this.behavior = behavior; |
| } |
| |
| public CpuBehavior getBehavior() { |
| return behavior; |
| } |
| |
| /** |
| * Reset the CPU to known initial values. |
| */ |
| public void reset() throws MemoryAccessException { |
| /* TODO: In reality, the stack pointer could be anywhere |
| on the stack after reset. This non-deterministic behavior might be |
| worth while to simulate. */ |
| state.sp = 0xff; |
| |
| // Set the PC to the address stored in the reset vector |
| state.pc = Utils.address(bus.read(RST_VECTOR_L), bus.read(RST_VECTOR_H)); |
| |
| // Clear instruction register. |
| state.ir = 0; |
| |
| // Clear status register bits. |
| state.carryFlag = false; |
| state.zeroFlag = false; |
| state.irqDisableFlag = false; |
| state.decimalModeFlag = false; |
| state.breakFlag = false; |
| state.overflowFlag = false; |
| state.negativeFlag = false; |
| |
| state.irqAsserted = false; |
| |
| // Clear illegal opcode trap. |
| state.opTrap = false; |
| |
| // Reset step counter |
| state.stepCounter = 0L; |
| |
| // Reset registers. |
| state.a = 0; |
| state.x = 0; |
| state.y = 0; |
| |
| peekAhead(); |
| } |
| |
| public void step(int num) throws MemoryAccessException { |
| for (int i = 0; i < num; i++) { |
| step(); |
| } |
| } |
| |
| /** |
| * Performs an individual instruction cycle. |
| */ |
| public void step() throws MemoryAccessException { |
| opBeginTime = System.nanoTime(); |
| |
| // Store the address from which the IR was read, for debugging |
| state.lastPc = state.pc; |
| |
| // Check for Interrupts before doing anything else. |
| // This will set the PC and jump to the interrupt vector. |
| if (state.nmiAsserted) { |
| handleNmi(); |
| } else if (state.irqAsserted && !getIrqDisableFlag()) { |
| handleIrq(state.pc); |
| } |
| |
| // Fetch memory location for this instruction. |
| state.ir = bus.read(state.pc); |
| int irAddressMode = (state.ir >> 2) & 0x07; // Bits 3-5 of IR: [ | | |X|X|X| | ] |
| int irOpMode = state.ir & 0x03; // Bits 6-7 of IR: [ | | | | | |X|X] |
| |
| incrementPC(); |
| |
| clearOpTrap(); |
| |
| // Decode the instruction and operands |
| state.instSize = Cpu.instructionSizes[state.ir]; |
| for (int i = 0; i < state.instSize - 1; i++) { |
| state.args[i] = bus.read(state.pc); |
| // Increment PC after reading |
| incrementPC(); |
| } |
| |
| state.stepCounter++; |
| |
| // Get the data from the effective address (if any) |
| int effectiveAddress = 0; |
| int tmp; // Temporary storage |
| |
| switch (irOpMode) { |
| case 0: |
| case 2: |
| switch (irAddressMode) { |
| case 0: // #Immediate |
| break; |
| case 1: // Zero Page |
| effectiveAddress = state.args[0]; |
| break; |
| case 2: // Accumulator - ignored |
| break; |
| case 3: // Absolute |
| effectiveAddress = Utils.address(state.args[0], state.args[1]); |
| break; |
| case 5: // Zero Page,X / Zero Page,Y |
| if (state.ir == 0x96 || state.ir == 0xb6) { |
| effectiveAddress = zpyAddress(state.args[0]); |
| } else { |
| effectiveAddress = zpxAddress(state.args[0]); |
| } |
| break; |
| case 7: // Absolute,X / Absolute,Y |
| if (state.ir == 0xbe) { |
| effectiveAddress = yAddress(state.args[0], state.args[1]); |
| } else { |
| effectiveAddress = xAddress(state.args[0], state.args[1]); |
| } |
| break; |
| } |
| break; |
| case 1: |
| switch (irAddressMode) { |
| case 0: // (Zero Page,X) |
| tmp = (state.args[0] + state.x) & 0xff; |
| effectiveAddress = Utils.address(bus.read(tmp), bus.read(tmp + 1)); |
| break; |
| case 1: // Zero Page |
| effectiveAddress = state.args[0]; |
| break; |
| case 2: // #Immediate |
| effectiveAddress = -1; |
| break; |
| case 3: // Absolute |
| effectiveAddress = Utils.address(state.args[0], state.args[1]); |
| break; |
| case 4: // (Zero Page),Y |
| tmp = Utils.address(bus.read(state.args[0]), bus.read((state.args[0] + 1) & 0xff)); |
| effectiveAddress = (tmp + state.y) & 0xffff; |
| break; |
| case 5: // Zero Page,X |
| effectiveAddress = zpxAddress(state.args[0]); |
| break; |
| case 6: // Absolute, Y |
| effectiveAddress = yAddress(state.args[0], state.args[1]); |
| break; |
| case 7: // Absolute, X |
| effectiveAddress = xAddress(state.args[0], state.args[1]); |
| break; |
| } |
| break; |
| } |
| |
| // Execute |
| switch (state.ir) { |
| |
| /** Single Byte Instructions; Implied and Relative **/ |
| case 0x00: // BRK - Force Interrupt - Implied |
| if (!getIrqDisableFlag()) { |
| handleIrq(state.pc + 1); |
| } |
| break; |
| case 0x08: // PHP - Push Processor Status - Implied |
| // Break flag is always set in the stack value. |
| stackPush(state.getStatusFlag() | 0x10); |
| break; |
| case 0x10: // BPL - Branch if Positive - Relative |
| if (!getNegativeFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0x18: // CLC - Clear Carry Flag - Implied |
| clearCarryFlag(); |
| break; |
| case 0x20: // JSR - Jump to Subroutine - Implied |
| stackPush((state.pc - 1 >> 8) & 0xff); // PC high byte |
| stackPush(state.pc - 1 & 0xff); // PC low byte |
| state.pc = Utils.address(state.args[0], state.args[1]); |
| break; |
| case 0x28: // PLP - Pull Processor Status - Implied |
| setProcessorStatus(stackPop()); |
| break; |
| case 0x30: // BMI - Branch if Minus - Relative |
| if (getNegativeFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0x38: // SEC - Set Carry Flag - Implied |
| setCarryFlag(); |
| break; |
| case 0x40: // RTI - Return from Interrupt - Implied |
| setProcessorStatus(stackPop()); |
| int lo = stackPop(); |
| int hi = stackPop(); |
| setProgramCounter(Utils.address(lo, hi)); |
| break; |
| case 0x48: // PHA - Push Accumulator - Implied |
| stackPush(state.a); |
| break; |
| case 0x50: // BVC - Branch if Overflow Clear - Relative |
| if (!getOverflowFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0x58: // CLI - Clear Interrupt Disable - Implied |
| clearIrqDisableFlag(); |
| break; |
| case 0x60: // RTS - Return from Subroutine - Implied |
| lo = stackPop(); |
| hi = stackPop(); |
| setProgramCounter((Utils.address(lo, hi) + 1) & 0xffff); |
| break; |
| case 0x68: // PLA - Pull Accumulator - Implied |
| state.a = stackPop(); |
| setArithmeticFlags(state.a); |
| break; |
| case 0x70: // BVS - Branch if Overflow Set - Relative |
| if (getOverflowFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0x78: // SEI - Set Interrupt Disable - Implied |
| setIrqDisableFlag(); |
| break; |
| case 0x88: // DEY - Decrement Y Register - Implied |
| state.y = --state.y & 0xff; |
| setArithmeticFlags(state.y); |
| break; |
| case 0x8a: // TXA - Transfer X to Accumulator - Implied |
| state.a = state.x; |
| setArithmeticFlags(state.a); |
| break; |
| case 0x90: // BCC - Branch if Carry Clear - Relative |
| if (!getCarryFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0x98: // TYA - Transfer Y to Accumulator - Implied |
| state.a = state.y; |
| setArithmeticFlags(state.a); |
| break; |
| case 0x9a: // TXS - Transfer X to Stack Pointer - Implied |
| setStackPointer(state.x); |
| break; |
| case 0xa8: // TAY - Transfer Accumulator to Y - Implied |
| state.y = state.a; |
| setArithmeticFlags(state.y); |
| break; |
| case 0xaa: // TAX - Transfer Accumulator to X - Implied |
| state.x = state.a; |
| setArithmeticFlags(state.x); |
| break; |
| case 0xb0: // BCS - Branch if Carry Set - Relative |
| if (getCarryFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0xb8: // CLV - Clear Overflow Flag - Implied |
| clearOverflowFlag(); |
| break; |
| case 0xba: // TSX - Transfer Stack Pointer to X - Implied |
| state.x = getStackPointer(); |
| setArithmeticFlags(state.x); |
| break; |
| case 0xc8: // INY - Increment Y Register - Implied |
| state.y = ++state.y & 0xff; |
| setArithmeticFlags(state.y); |
| break; |
| case 0xca: // DEX - Decrement X Register - Implied |
| state.x = --state.x & 0xff; |
| setArithmeticFlags(state.x); |
| break; |
| case 0xd0: // BNE - Branch if Not Equal to Zero - Relative |
| if (!getZeroFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0xd8: // CLD - Clear Decimal Mode - Implied |
| clearDecimalModeFlag(); |
| break; |
| case 0xe8: // INX - Increment X Register - Implied |
| state.x = ++state.x & 0xff; |
| setArithmeticFlags(state.x); |
| break; |
| case 0xea: // NOP |
| // Do nothing. |
| break; |
| case 0xf0: // BEQ - Branch if Equal to Zero - Relative |
| if (getZeroFlag()) { |
| state.pc = relAddress(state.args[0]); |
| } |
| break; |
| case 0xf8: // SED - Set Decimal Flag - Implied |
| setDecimalModeFlag(); |
| break; |
| |
| /** JMP *****************************************************************/ |
| case 0x4c: // JMP - Absolute |
| state.pc = Utils.address(state.args[0], state.args[1]); |
| break; |
| case 0x6c: // JMP - Indirect |
| lo = Utils.address(state.args[0], state.args[1]); // Address of low byte |
| |
| if (state.args[0] == 0xff && (behavior == CpuBehavior.NMOS_WITH_INDIRECT_JMP_BUG || behavior == CpuBehavior.NMOS_WITH_ROR_BUG)) { |
| hi = Utils.address(0x00, state.args[1]); |
| } else { |
| hi = lo + 1; |
| } |
| |
| state.pc = Utils.address(bus.read(lo), bus.read(hi)); |
| /* TODO: For accuracy, allow a flag to enable broken behavior of early 6502s: |
| * |
| * "An original 6502 has does not correctly fetch the target |
| * address if the indirect vector falls on a page boundary |
| * (e.g. $xxFF where xx is and value from $00 to $FF). In this |
| * case fetches the LSB from $xxFF as expected but takes the MSB |
| * from $xx00. This is fixed in some later chips like the 65SC02 |
| * so for compatibility always ensure the indirect vector is not |
| * at the end of the page." |
| * (http://www.obelisk.demon.co.uk/6502/reference.html#JMP) |
| */ |
| break; |
| |
| /** ORA - Logical Inclusive Or ******************************************/ |
| case 0x09: // #Immediate |
| state.a |= state.args[0]; |
| setArithmeticFlags(state.a); |
| break; |
| case 0x01: // (Zero Page,X) |
| case 0x05: // Zero Page |
| case 0x0d: // Absolute |
| case 0x11: // (Zero Page),Y |
| case 0x15: // Zero Page,X |
| case 0x19: // Absolute,Y |
| case 0x1d: // Absolute,X |
| state.a |= bus.read(effectiveAddress); |
| setArithmeticFlags(state.a); |
| break; |
| |
| /** ASL - Arithmetic Shift Left *****************************************/ |
| case 0x0a: // Accumulator |
| state.a = asl(state.a); |
| setArithmeticFlags(state.a); |
| break; |
| case 0x06: // Zero Page |
| case 0x0e: // Absolute |
| case 0x16: // Zero Page,X |
| case 0x1e: // Absolute,X |
| tmp = asl(bus.read(effectiveAddress)); |
| bus.write(effectiveAddress, tmp); |
| setArithmeticFlags(tmp); |
| break; |
| |
| /** BIT - Bit Test ******************************************************/ |
| case 0x24: // Zero Page |
| case 0x2c: // Absolute |
| tmp = bus.read(effectiveAddress); |
| setZeroFlag((state.a & tmp) == 0); |
| setNegativeFlag((tmp & 0x80) != 0); |
| setOverflowFlag((tmp & 0x40) != 0); |
| break; |
| |
| /** AND - Logical AND ***************************************************/ |
| case 0x29: // #Immediate |
| state.a &= state.args[0]; |
| setArithmeticFlags(state.a); |
| break; |
| case 0x21: // (Zero Page,X) |
| case 0x25: // Zero Page |
| case 0x2d: // Absolute |
| case 0x31: // (Zero Page),Y |
| case 0x35: // Zero Page,X |
| case 0x39: // Absolute,Y |
| case 0x3d: // Absolute,X |
| state.a &= bus.read(effectiveAddress); |
| setArithmeticFlags(state.a); |
| break; |
| |
| /** ROL - Rotate Left ***************************************************/ |
| case 0x2a: // Accumulator |
| state.a = rol(state.a); |
| setArithmeticFlags(state.a); |
| break; |
| case 0x26: // Zero Page |
| case 0x2e: // Absolute |
| case 0x36: // Zero Page,X |
| case 0x3e: // Absolute,X |
| tmp = rol(bus.read(effectiveAddress)); |
| bus.write(effectiveAddress, tmp); |
| setArithmeticFlags(tmp); |
| break; |
| |
| /** EOR - Exclusive OR **************************************************/ |
| case 0x49: // #Immediate |
| state.a ^= state.args[0]; |
| setArithmeticFlags(state.a); |
| break; |
| case 0x41: // (Zero Page,X) |
| case 0x45: // Zero Page |
| case 0x4d: // Absolute |
| case 0x51: // (Zero Page,Y) |
| case 0x55: // Zero Page,X |
| case 0x59: // Absolute,Y |
| case 0x5d: // Absolute,X |
| state.a ^= bus.read(effectiveAddress); |
| setArithmeticFlags(state.a); |
| break; |
| |
| /** LSR - Logical Shift Right *******************************************/ |
| case 0x4a: // Accumulator |
| state.a = lsr(state.a); |
| setArithmeticFlags(state.a); |
| break; |
| case 0x46: // Zero Page |
| case 0x4e: // Absolute |
| case 0x56: // Zero Page,X |
| case 0x5e: // Absolute,X |
| tmp = lsr(bus.read(effectiveAddress)); |
| bus.write(effectiveAddress, tmp); |
| setArithmeticFlags(tmp); |
| break; |
| |
| /** ADC - Add with Carry ************************************************/ |
| case 0x69: // #Immediate |
| if (state.decimalModeFlag) { |
| state.a = adcDecimal(state.a, state.args[0]); |
| } else { |
| state.a = adc(state.a, state.args[0]); |
| } |
| break; |
| case 0x61: // (Zero Page,X) |
| case 0x65: // Zero Page |
| case 0x6d: // Absolute |
| case 0x71: // (Zero Page),Y |
| case 0x75: // Zero Page,X |
| case 0x79: // Absolute,Y |
| case 0x7d: // Absolute,X |
| if (state.decimalModeFlag) { |
| state.a = adcDecimal(state.a, bus.read(effectiveAddress)); |
| } else { |
| state.a = adc(state.a, bus.read(effectiveAddress)); |
| } |
| break; |
| |
| /** ROR - Rotate Right **************************************************/ |
| case 0x6a: // Accumulator |
| state.a = ror(state.a); |
| setArithmeticFlags(state.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, state.a); |
| break; |
| |
| /** STY - Store Y Register **********************************************/ |
| case 0x84: // Zero Page |
| case 0x8c: // Absolute |
| case 0x94: // Zero Page,X |
| bus.write(effectiveAddress, state.y); |
| break; |
| |
| /** STX - Store X Register **********************************************/ |
| case 0x86: // Zero Page |
| case 0x8e: // Absolute |
| case 0x96: // Zero Page,Y |
| bus.write(effectiveAddress, state.x); |
| break; |
| |
| /** LDY - Load Y Register ***********************************************/ |
| case 0xa0: // #Immediate |
| state.y = state.args[0]; |
| setArithmeticFlags(state.y); |
| break; |
| case 0xa4: // Zero Page |
| case 0xac: // Absolute |
| case 0xb4: // Zero Page,X |
| case 0xbc: // Absolute,X |
| state.y = bus.read(effectiveAddress); |
| setArithmeticFlags(state.y); |
| break; |
| |
| /** LDX - Load X Register ***********************************************/ |
| case 0xa2: // #Immediate |
| state.x = state.args[0]; |
| setArithmeticFlags(state.x); |
| break; |
| case 0xa6: // Zero Page |
| case 0xae: // Absolute |
| case 0xb6: // Zero Page,Y |
| case 0xbe: // Absolute,Y |
| state.x = bus.read(effectiveAddress); |
| setArithmeticFlags(state.x); |
| break; |
| |
| /** LDA - Load Accumulator **********************************************/ |
| case 0xa9: // #Immediate |
| state.a = state.args[0]; |
| setArithmeticFlags(state.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 |
| state.a = bus.read(effectiveAddress); |
| setArithmeticFlags(state.a); |
| break; |
| |
| /** CPY - Compare Y Register ********************************************/ |
| case 0xc0: // #Immediate |
| cmp(state.y, state.args[0]); |
| break; |
| case 0xc4: // Zero Page |
| case 0xcc: // Absolute |
| cmp(state.y, bus.read(effectiveAddress)); |
| break; |
| |
| /** CMP - Compare Accumulator *******************************************/ |
| case 0xc9: // #Immediate |
| cmp(state.a, state.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(state.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(state.x, state.args[0]); |
| break; |
| case 0xe4: // Zero Page |
| case 0xec: // Absolute |
| cmp(state.x, bus.read(effectiveAddress)); |
| break; |
| |
| /** SBC - Subtract with Carry (Borrow) **********************************/ |
| case 0xe9: // #Immediate |
| if (state.decimalModeFlag) { |
| state.a = sbcDecimal(state.a, state.args[0]); |
| } else { |
| state.a = sbc(state.a, state.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 (state.decimalModeFlag) { |
| state.a = sbcDecimal(state.a, bus.read(effectiveAddress)); |
| } else { |
| state.a = sbc(state.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 ****************************************/ |
| // TODO: Create a flag to enable highly-accurate emulation of unimplemented instructions. |
| default: |
| setOpTrap(); |
| break; |
| } |
| |
| delayLoop(state.ir); |
| |
| // Peek ahead to the next insturction and arguments |
| peekAhead(); |
| } |
| |
| private void peekAhead() throws MemoryAccessException { |
| state.nextIr = bus.read(state.pc); |
| int nextInstSize = Cpu.instructionSizes[state.nextIr]; |
| for (int i = 1; i < nextInstSize; i++) { |
| int nextRead = (state.pc + i) % bus.endAddress(); |
| state.nextArgs[i - 1] = bus.read(nextRead); |
| } |
| } |
| |
| private void handleIrq(int returnPc) throws MemoryAccessException { |
| handleInterrupt(returnPc, IRQ_VECTOR_L, IRQ_VECTOR_H); |
| clearIrq(); |
| } |
| |
| private void handleNmi() throws MemoryAccessException { |
| handleInterrupt(state.pc, NMI_VECTOR_L, NMI_VECTOR_H); |
| clearNmi(); |
| } |
| |
| /** |
| * Handle the common behavior of BRK, /IRQ, and /NMI |
| * |
| * @throws MemoryAccessException |
| */ |
| private void handleInterrupt(int returnPc, int vectorLow, int vectorHigh) throws MemoryAccessException { |
| // Set the break flag before pushing. |
| setBreakFlag(); |
| // Push program counter + 1 onto the stack |
| stackPush((returnPc >> 8) & 0xff); // PC high byte |
| stackPush(returnPc & 0xff); // PC low byte |
| stackPush(state.getStatusFlag()); |
| // Set the Interrupt Disabled flag. RTI will clear it. |
| setIrqDisableFlag(); |
| |
| // Load interrupt vector address into PC |
| state.pc = Utils.address(bus.read(vectorLow), bus.read(vectorHigh)); |
| } |
| |
| /** |
| * 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 The sum of the accumulator and the 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); |
| setOverflowFlag(state.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) - (state.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) { |
| state.zeroFlag = (reg == 0); |
| state.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 & 0xff) >>> 1; |
| } |
| |
| /** |
| * 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 current Cpu State. |
| * |
| * @return the current Cpu State. |
| */ |
| public CpuState getCpuState() { |
| return state; |
| } |
| |
| /** |
| * @return the negative flag |
| */ |
| public boolean getNegativeFlag() { |
| return state.negativeFlag; |
| } |
| |
| /** |
| * @param negativeFlag the negative flag to set |
| */ |
| public void setNegativeFlag(boolean negativeFlag) { |
| state.negativeFlag = negativeFlag; |
| } |
| |
| public void setNegativeFlag() { |
| state.negativeFlag = true; |
| } |
| |
| public void clearNegativeFlag() { |
| state.negativeFlag = false; |
| } |
| |
| /** |
| * @return the carry flag |
| */ |
| public boolean getCarryFlag() { |
| return state.carryFlag; |
| } |
| |
| /** |
| * @return 1 if the carry flag is set, 0 if it is clear. |
| */ |
| public int getCarryBit() { |
| return (state.carryFlag ? 1 : 0); |
| } |
| |
| /** |
| * @param carryFlag the carry flag to set |
| */ |
| public void setCarryFlag(boolean carryFlag) { |
| state.carryFlag = carryFlag; |
| } |
| |
| /** |
| * Sets the Carry Flag |
| */ |
| public void setCarryFlag() { |
| state.carryFlag = true; |
| } |
| |
| /** |
| * Clears the Carry Flag |
| */ |
| public void clearCarryFlag() { |
| state.carryFlag = false; |
| } |
| |
| /** |
| * @return the zero flag |
| */ |
| public boolean getZeroFlag() { |
| return state.zeroFlag; |
| } |
| |
| /** |
| * @param zeroFlag the zero flag to set |
| */ |
| public void setZeroFlag(boolean zeroFlag) { |
| state.zeroFlag = zeroFlag; |
| } |
| |
| /** |
| * Sets the Zero Flag |
| */ |
| public void setZeroFlag() { |
| state.zeroFlag = true; |
| } |
| |
| /** |
| * Clears the Zero Flag |
| */ |
| public void clearZeroFlag() { |
| state.zeroFlag = false; |
| } |
| |
| /** |
| * @return the irq disable flag |
| */ |
| public boolean getIrqDisableFlag() { |
| return state.irqDisableFlag; |
| } |
| |
| public void setIrqDisableFlag() { |
| state.irqDisableFlag = true; |
| } |
| |
| public void clearIrqDisableFlag() { |
| state.irqDisableFlag = false; |
| } |
| |
| /** |
| * @return the decimal mode flag |
| */ |
| public boolean getDecimalModeFlag() { |
| return state.decimalModeFlag; |
| } |
| |
| /** |
| * Sets the Decimal Mode Flag to true. |
| */ |
| public void setDecimalModeFlag() { |
| state.decimalModeFlag = true; |
| } |
| |
| /** |
| * Clears the Decimal Mode Flag. |
| */ |
| public void clearDecimalModeFlag() { |
| state.decimalModeFlag = false; |
| } |
| |
| /** |
| * @return the break flag |
| */ |
| public boolean getBreakFlag() { |
| return state.breakFlag; |
| } |
| |
| /** |
| * Sets the Break Flag |
| */ |
| public void setBreakFlag() { |
| state.breakFlag = true; |
| } |
| |
| /** |
| * Clears the Break Flag |
| */ |
| public void clearBreakFlag() { |
| state.breakFlag = false; |
| } |
| |
| /** |
| * @return the overflow flag |
| */ |
| public boolean getOverflowFlag() { |
| return state.overflowFlag; |
| } |
| |
| /** |
| * @param overflowFlag the overflow flag to set |
| */ |
| public void setOverflowFlag(boolean overflowFlag) { |
| state.overflowFlag = overflowFlag; |
| } |
| |
| /** |
| * Sets the Overflow Flag |
| */ |
| public void setOverflowFlag() { |
| state.overflowFlag = true; |
| } |
| |
| /** |
| * Clears the Overflow Flag |
| */ |
| public void clearOverflowFlag() { |
| state.overflowFlag = false; |
| } |
| |
| /** |
| * Set the illegal instruction trap. |
| */ |
| public void setOpTrap() { |
| state.opTrap = true; |
| } |
| |
| /** |
| * Clear the illegal instruction trap. |
| */ |
| public void clearOpTrap() { |
| state.opTrap = false; |
| } |
| |
| public int getAccumulator() { |
| return state.a; |
| } |
| |
| public void setAccumulator(int val) { |
| state.a = val; |
| } |
| |
| public int getXRegister() { |
| return state.x; |
| } |
| |
| public void setXRegister(int val) { |
| state.x = val; |
| } |
| |
| public int getYRegister() { |
| return state.y; |
| } |
| |
| public void setYRegister(int val) { |
| state.y = val; |
| } |
| |
| public int getProgramCounter() { |
| return state.pc; |
| } |
| |
| public void setProgramCounter(int addr) { |
| state.pc = addr; |
| |
| // As a side-effect of setting the program counter, |
| // we want to peek ahead at the next state. |
| try { |
| peekAhead(); |
| } catch (MemoryAccessException ex) { |
| logger.log(Level.SEVERE, "Could not peek ahead at next instruction state."); |
| } |
| } |
| |
| public int getStackPointer() { |
| return state.sp; |
| } |
| |
| public void setStackPointer(int offset) { |
| state.sp = offset; |
| } |
| |
| public int getInstruction() { |
| return state.ir; |
| } |
| |
| /** |
| * @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(); |
| } |
| |
| public String getAccumulatorStatus() { |
| return "$" + Utils.byteToHex(state.a); |
| } |
| |
| public String getXRegisterStatus() { |
| return "$" + Utils.byteToHex(state.x); |
| } |
| |
| public String getYRegisterStatus() { |
| return "$" + Utils.byteToHex(state.y); |
| } |
| |
| public String getProgramCounterStatus() { |
| return "$" + Utils.wordToHex(state.pc); |
| } |
| |
| public String getStackPointerStatus() { |
| return "$" + Utils.byteToHex(state.sp); |
| } |
| |
| public int getProcessorStatus() { |
| return state.getStatusFlag(); |
| } |
| |
| /** |
| * Simulate transition from logic-high to logic-low on the INT line. |
| */ |
| public void assertIrq() { |
| state.irqAsserted = true; |
| } |
| |
| /** |
| * Simulate transition from logic-low to logic-high of the INT line. |
| */ |
| public void clearIrq() { |
| state.irqAsserted = false; |
| } |
| |
| /** |
| * Simulate transition from logic-high to logic-low on the NMI line. |
| */ |
| public void assertNmi() { |
| state.nmiAsserted = true; |
| } |
| |
| /** |
| * Simulate transition from logic-low to logic-high of the NMI line. |
| */ |
| public void clearNmi() { |
| state.nmiAsserted = false; |
| } |
| |
| /** |
| * 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 + state.sp, data); |
| |
| if (state.sp == 0) { |
| state.sp = 0xff; |
| } else { |
| --state.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 (state.sp == 0xff) { |
| state.sp = 0x00; |
| } else { |
| ++state.sp; |
| } |
| |
| return bus.read(0x100 + state.sp); |
| } |
| |
| /** |
| * Peek at the value currently at the top of the stack |
| */ |
| int stackPeek() throws MemoryAccessException { |
| return bus.read(0x100 + state.sp + 1); |
| } |
| |
| /* |
| * Increment the PC, rolling over if necessary. |
| */ |
| void incrementPC() { |
| if (state.pc == 0xffff) { |
| state.pc = 0; |
| } else { |
| ++state.pc; |
| } |
| } |
| |
| /** |
| * Given a hi byte and a low byte, return the Absolute,X |
| * offset address. |
| */ |
| int xAddress(int lowByte, int hiByte) { |
| return (Utils.address(lowByte, hiByte) + state.x) & 0xffff; |
| } |
| |
| /** |
| * Given a hi byte and a low byte, return the Absolute,Y |
| * offset address. |
| */ |
| int yAddress(int lowByte, int hiByte) { |
| return (Utils.address(lowByte, hiByte) + state.y) & 0xffff; |
| } |
| |
| /** |
| * Given a single byte, compute the Zero Page,X offset address. |
| */ |
| int zpxAddress(int zp) { |
| return (zp + state.x) & 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 (state.pc + (byte) offset) & 0xffff; |
| } |
| |
| /** |
| * Given a single byte, compute the Zero Page,Y offset address. |
| */ |
| int zpyAddress(int zp) { |
| return (zp + state.y) & 0xff; |
| } |
| |
| /* |
| * 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 opScheduledEnd = opBeginTime + clockSteps; |
| long now = System.nanoTime(); |
| while (now < opScheduledEnd) { |
| now = System.nanoTime(); |
| } |
| } |
| |
| /** |
| * Return a formatted string representing the last instruction and |
| * operands that were executed. |
| * |
| * @return A string representing the mnemonic and operands of the instruction |
| */ |
| public static String disassembleOp(int opCode, int[] args) { |
| String mnemonic = opcodeNames[opCode]; |
| |
| if (mnemonic == null) { |
| return "???"; |
| } |
| |
| StringBuilder sb = new StringBuilder(mnemonic); |
| |
| switch (instructionModes[opCode]) { |
| case ABS: |
| sb.append(" $").append(Utils.wordToHex(Utils.address(args[0], args[1]))); |
| break; |
| case ABX: |
| sb.append(" $").append(Utils.wordToHex(Utils.address(args[0], args[1]))).append(",X"); |
| break; |
| case ABY: |
| sb.append(" $").append(Utils.wordToHex(Utils.address(args[0], args[1]))).append(",Y"); |
| break; |
| case IMM: |
| sb.append(" #$").append(Utils.byteToHex(args[0])); |
| break; |
| case IND: |
| sb.append(" ($").append(Utils.wordToHex(Utils.address(args[0], args[1]))).append(")"); |
| break; |
| case XIN: |
| sb.append(" ($").append(Utils.byteToHex(args[0])).append(",X)"); |
| break; |
| case INY: |
| sb.append(" ($").append(Utils.byteToHex(args[0])).append("),Y"); |
| break; |
| case REL: |
| case ZPG: |
| sb.append(" $").append(Utils.byteToHex(args[0])); |
| break; |
| case ZPX: |
| sb.append(" $").append(Utils.byteToHex(args[0])).append(",X"); |
| break; |
| case ZPY: |
| sb.append(" $").append(Utils.byteToHex(args[0])).append(",Y"); |
| break; |
| } |
| |
| return sb.toString(); |
| } |
| |
| /** |
| * Return a formatted string representing the next instruction and |
| * operands to be executed. |
| * |
| * @return A string representing the mnemonic and operands of the instruction |
| */ |
| public String disassembleNextOp() { |
| return Cpu.disassembleOp(state.nextIr, state.nextArgs); |
| } |
| |
| /** |
| * @param address Address to disassemble |
| * @return String containing the disassembled instruction and operands. |
| */ |
| public String disassembleOpAtAddress(int address) throws MemoryAccessException { |
| int opCode = bus.read(address); |
| int args[] = new int[2]; |
| int size = Cpu.instructionSizes[opCode]; |
| for (int i = 1; i < size; i++) { |
| int nextRead = (address + i) % bus.endAddress(); |
| args[i - 1] = bus.read(nextRead); |
| } |
| |
| return disassembleOp(opCode, args); |
| } |
| } |