applepy/applepy.py

# ApplePy - an Apple ][ emulator in Python
# James Tauber / http://jtauber.com/
# originally written 2001, updated 2011


import curses

def signed(x):
    if x > 0x7F:
        x = x - 0x100
    return x


class Memory:
    def __init__(self, size):
        self.__mem = [0x00] * size
    
    def load(self, address, data):
        for offset, datum in enumerate(data):
            self.__mem[address + offset] = datum
    
    def load_file(self, address, filename):
        with open(filename) as f:
            for offset, datum in enumerate(f.read()):
                self.__mem[address + offset] = ord(datum)
    
    def read_byte(self, address):
        assert address <= 0xFFFF
        if 0xC000 <= address <= 0xCFFF:
            if address == 0xC010:
                self.__mem[0xC000] = self.__mem[0xC000] & 0x7F # clear keyboard
        return self.__mem[address]

    def write_byte_io(self, address, value):
        self.__mem[address] = value
    
    def write_byte(self, address, value):
        if 0x400 <= address < 0x800:
            self.write_screen(address, value)
        if address < 0xC000:
            self.__mem[address] = value
    
    def read_word(self, address):
        return self.read_byte(address) + (self.read_byte(address + 1) << 8)
    
    def read_word_bug(self, address):
        if address % 0x100 == 0xFF:
            return self.read_byte(address) + (self.read_byte(address & 0xFF00) << 8)
        else:
            return self.read_word(address)
    
    def write_screen(self, address, value):
        base = address - 0x400
        hi, lo = divmod(base, 0x80)
        row_group, column  = divmod(lo, 0x28)
        row = hi + 8 * row_group
        assert row_group != 3 # @@@
        
        c = chr(0x20 + ((value + 0x20) % 0x40))
        
        if value < 0x40:
            attr = curses.A_DIM
        elif value < 0x80:
            attr = curses.A_REVERSE
        elif value < 0xA0:
            attr = curses.A_UNDERLINE
        else:
            attr = curses.A_DIM
        
        try:
            self.win.addch(row, column, c, attr)
        except curses.error:
            pass


class CPU:
    
    STACK_PAGE = 0x100
    RESET_VECTOR = 0xFFFC
    
    def __init__(self, memory):
        self.memory = memory
        
        self.accumulator = 0x00
        self.x_index = 0x00
        self.y_index = 0x00
        
        self.carry_flag = 0
        self.zero_flag = 0
        self.interrupt_disable_flag = 0
        self.decimal_mode_flag = 0
        self.break_flag = 1
        self.overflow_flag = 0
        self.sign_flag = 0
        
        self.stack_pointer = 0xFF
        
        self.setup_ops()
        self.reset()
    
    def setup_ops(self):
        self.ops = [None] * 0x100
        self.ops[0x00] = lambda: self.BRK()
        self.ops[0x01] = lambda: self.ORA(self.indirect_x_mode())
        self.ops[0x05] = lambda: self.ORA(self.zero_page_mode())
        self.ops[0x06] = lambda: self.ASL(self.zero_page_mode())
        self.ops[0x08] = lambda: self.PHP()
        self.ops[0x09] = lambda: self.ORA(self.immediate_mode())
        self.ops[0x0A] = lambda: self.ASL()
        self.ops[0x0D] = lambda: self.ORA(self.absolute_mode())
        self.ops[0x0E] = lambda: self.ASL(self.absolute_mode())
        self.ops[0x10] = lambda: self.BPL(self.relative_mode())
        self.ops[0x11] = lambda: self.ORA(self.indirect_y_mode())
        self.ops[0x15] = lambda: self.ORA(self.zero_page_x_mode())
        self.ops[0x16] = lambda: self.ASL(self.zero_page_x_mode())
        self.ops[0x18] = lambda: self.CLC()
        self.ops[0x19] = lambda: self.ORA(self.absolute_y_mode())
        self.ops[0x1D] = lambda: self.ORA(self.absolute_x_mode())
        self.ops[0x1E] = lambda: self.ASL(self.absolute_x_mode())
        self.ops[0x20] = lambda: self.JSR(self.absolute_mode())
        self.ops[0x21] = lambda: self.AND(self.indirect_x_mode())
        self.ops[0x24] = lambda: self.BIT(self.zero_page_mode())
        self.ops[0x25] = lambda: self.AND(self.zero_page_mode())
        self.ops[0x26] = lambda: self.ROL(self.zero_page_mode())
        self.ops[0x28] = lambda: self.PLP()
        self.ops[0x29] = lambda: self.AND(self.immediate_mode())
        self.ops[0x2A] = lambda: self.ROL()
        self.ops[0x2C] = lambda: self.BIT(self.absolute_mode())
        self.ops[0x2D] = lambda: self.AND(self.absolute_mode())
        self.ops[0x2E] = lambda: self.ROL(self.absolute_mode())
        self.ops[0x30] = lambda: self.BMI(self.relative_mode())
        self.ops[0x31] = lambda: self.AND(self.indirect_y_mode())
        self.ops[0x35] = lambda: self.AND(self.zero_page_x_mode())
        self.ops[0x36] = lambda: self.ROL(self.zero_page_x_mode())
        self.ops[0x38] = lambda: self.SEC()
        self.ops[0x39] = lambda: self.AND(self.absolute_y_mode())
        self.ops[0x3D] = lambda: self.AND(self.absolute_x_mode())
        self.ops[0x3E] = lambda: self.ROL(self.absolute_x_mode())
        self.ops[0x40] = lambda: self.RTI()
        self.ops[0x41] = lambda: self.EOR(self.indirect_x_mode())
        self.ops[0x45] = lambda: self.EOR(self.zero_page_mode())
        self.ops[0x46] = lambda: self.LSR(self.zero_page_mode())
        self.ops[0x48] = lambda: self.PHA()
        self.ops[0x49] = lambda: self.EOR(self.immediate_mode())
        self.ops[0x4A] = lambda: self.LSR()
        self.ops[0x4C] = lambda: self.JMP(self.absolute_mode())
        self.ops[0x4D] = lambda: self.EOR(self.absolute_mode())
        self.ops[0x4E] = lambda: self.LSR(self.absolute_mode())
        self.ops[0x50] = lambda: self.BVC(self.relative_mode())
        self.ops[0x51] = lambda: self.EOR(self.indirect_y_mode())
        self.ops[0x55] = lambda: self.EOR(self.zero_page_x_mode())
        self.ops[0x56] = lambda: self.LSR(self.zero_page_x_mode())
        self.ops[0x58] = lambda: self.CLI()
        self.ops[0x59] = lambda: self.EOR(self.absolute_y_mode())
        self.ops[0x5D] = lambda: self.EOR(self.absolute_x_mode())
        self.ops[0x5E] = lambda: self.LSR(self.absolute_x_mode())
        self.ops[0x60] = lambda: self.RTS()
        self.ops[0x61] = lambda: self.ADC(self.indirect_x_mode())
        self.ops[0x65] = lambda: self.ADC(self.zero_page_mode())
        self.ops[0x66] = lambda: self.ROR(self.zero_page_mode())
        self.ops[0x68] = lambda: self.PLA()
        self.ops[0x69] = lambda: self.ADC(self.immediate_mode())
        self.ops[0x6A] = lambda: self.ROR()
        self.ops[0x6C] = lambda: self.JMP(self.indirect_mode())
        self.ops[0x6D] = lambda: self.ADC(self.absolute_mode())
        self.ops[0x6E] = lambda: self.ROR(self.absolute_mode())
        self.ops[0x70] = lambda: self.BVS(self.relative_mode())
        self.ops[0x71] = lambda: self.ADC(self.indirect_y_mode())
        self.ops[0x75] = lambda: self.ADC(self.zero_page_x_mode())
        self.ops[0x76] = lambda: self.ROR(self.zero_page_x_mode())
        self.ops[0x78] = lambda: self.SEI()
        self.ops[0x79] = lambda: self.ADC(self.absolute_y_mode())
        self.ops[0x7D] = lambda: self.ADC(self.absolute_x_mode())
        self.ops[0x7E] = lambda: self.ROR(self.absolute_x_mode())
        self.ops[0x81] = lambda: self.STA(self.indirect_x_mode())
        self.ops[0x84] = lambda: self.STY(self.zero_page_mode())
        self.ops[0x85] = lambda: self.STA(self.zero_page_mode())
        self.ops[0x86] = lambda: self.STX(self.zero_page_mode())
        self.ops[0x88] = lambda: self.DEY()
        self.ops[0x8A] = lambda: self.TXA()
        self.ops[0x8C] = lambda: self.STY(self.absolute_mode())
        self.ops[0x8D] = lambda: self.STA(self.absolute_mode())
        self.ops[0x8E] = lambda: self.STX(self.absolute_mode())
        self.ops[0x90] = lambda: self.BCC(self.relative_mode())
        self.ops[0x91] = lambda: self.STA(self.indirect_y_mode())
        self.ops[0x94] = lambda: self.STY(self.zero_page_x_mode())
        self.ops[0x95] = lambda: self.STA(self.zero_page_x_mode())
        self.ops[0x96] = lambda: self.STX(self.zero_page_y_mode())
        self.ops[0x98] = lambda: self.TYA()
        self.ops[0x99] = lambda: self.STA(self.absolute_y_mode())
        self.ops[0x9A] = lambda: self.TXS()
        self.ops[0x9D] = lambda: self.STA(self.absolute_x_mode())
        self.ops[0xA0] = lambda: self.LDY(self.immediate_mode())
        self.ops[0xA1] = lambda: self.LDA(self.indirect_x_mode())
        self.ops[0xA2] = lambda: self.LDX(self.immediate_mode())
        self.ops[0xA4] = lambda: self.LDY(self.zero_page_mode())
        self.ops[0xA5] = lambda: self.LDA(self.zero_page_mode())
        self.ops[0xA6] = lambda: self.LDX(self.zero_page_mode())
        self.ops[0xA8] = lambda: self.TAY()
        self.ops[0xA9] = lambda: self.LDA(self.immediate_mode())
        self.ops[0xAA] = lambda: self.TAX()
        self.ops[0xAC] = lambda: self.LDY(self.absolute_mode())
        self.ops[0xAD] = lambda: self.LDA(self.absolute_mode())
        self.ops[0xAE] = lambda: self.LDX(self.absolute_mode())
        self.ops[0xB0] = lambda: self.BCS(self.relative_mode())
        self.ops[0xB1] = lambda: self.LDA(self.indirect_y_mode())
        self.ops[0xB4] = lambda: self.LDY(self.zero_page_x_mode())
        self.ops[0xB5] = lambda: self.LDA(self.zero_page_x_mode())
        self.ops[0xB6] = lambda: self.LDX(self.zero_page_y_mode())
        self.ops[0xB8] = lambda: self.CLV()
        self.ops[0xB9] = lambda: self.LDA(self.absolute_y_mode())
        self.ops[0xBA] = lambda: self.TSX()
        self.ops[0xBC] = lambda: self.LDY(self.absolute_x_mode())
        self.ops[0xBD] = lambda: self.LDA(self.absolute_x_mode())
        self.ops[0xBE] = lambda: self.LDX(self.absolute_y_mode())
        self.ops[0xC0] = lambda: self.CPY(self.immediate_mode())
        self.ops[0xC1] = lambda: self.CMP(self.indirect_x_mode())
        self.ops[0xC4] = lambda: self.CPY(self.zero_page_mode())
        self.ops[0xC5] = lambda: self.CMP(self.zero_page_mode())
        self.ops[0xC6] = lambda: self.DEC(self.zero_page_mode())
        self.ops[0xC8] = lambda: self.INY()
        self.ops[0xC9] = lambda: self.CMP(self.immediate_mode())
        self.ops[0xCA] = lambda: self.DEX()
        self.ops[0xCC] = lambda: self.CPY(self.absolute_mode())
        self.ops[0xCD] = lambda: self.CMP(self.absolute_mode())
        self.ops[0xCE] = lambda: self.DEC(self.absolute_mode())
        self.ops[0xD0] = lambda: self.BNE(self.relative_mode())
        self.ops[0xD1] = lambda: self.CMP(self.indirect_y_mode())
        self.ops[0xD5] = lambda: self.CMP(self.zero_page_x_mode())
        self.ops[0xD6] = lambda: self.DEC(self.zero_page_x_mode())
        self.ops[0xD8] = lambda: self.CLD()
        self.ops[0xD9] = lambda: self.CMP(self.absolute_y_mode())
        self.ops[0xDD] = lambda: self.CMP(self.absolute_x_mode())
        self.ops[0xDE] = lambda: self.DEC(self.absolute_x_mode())
        self.ops[0xE0] = lambda: self.CPX(self.immediate_mode())
        self.ops[0xE1] = lambda: self.SBC(self.indirect_x_mode())
        self.ops[0xE4] = lambda: self.CPX(self.zero_page_mode())
        self.ops[0xE5] = lambda: self.SBC(self.zero_page_mode())
        self.ops[0xE6] = lambda: self.INC(self.zero_page_mode())
        self.ops[0xE8] = lambda: self.INX()
        self.ops[0xE9] = lambda: self.SBC(self.immediate_mode())
        self.ops[0xEA] = lambda: self.NOP()
        self.ops[0xEC] = lambda: self.CPX(self.absolute_mode())
        self.ops[0xED] = lambda: self.SBC(self.absolute_mode())
        self.ops[0xEE] = lambda: self.INC(self.absolute_mode())
        self.ops[0xF0] = lambda: self.BEQ(self.relative_mode())
        self.ops[0xF1] = lambda: self.SBC(self.indirect_y_mode())
        self.ops[0xF5] = lambda: self.SBC(self.zero_page_x_mode())
        self.ops[0xF6] = lambda: self.INC(self.zero_page_x_mode())
        self.ops[0xF8] = lambda: self.SED()
        self.ops[0xF9] = lambda: self.SBC(self.absolute_y_mode())
        self.ops[0xFD] = lambda: self.SBC(self.absolute_x_mode())
        self.ops[0xFE] = lambda: self.INC(self.absolute_x_mode())
        
    def reset(self):
        self.program_counter = self.memory.read_word(self.RESET_VECTOR)
    
    def dump(self, win, op):
        win.addstr(10, 50, "%04X got %02X" % (self.program_counter - 1, op))
        win.addstr(14, 50, "BUFFER:" +
            " ".join("%02X" % self.memory.read_byte(m) for m in range(0x200, 0x210))
        )
        win.addstr(11, 50, "A=%02X X=%02X Y=%02X S=%02X V=%02X B=%02X D=%02X I=%02X Z=%02X C=%02X PC=%04X S=%02X" % (
            self.accumulator,
            self.x_index,
            self.y_index,
            self.sign_flag,
            self.overflow_flag,
            self.break_flag,
            self.decimal_mode_flag,
            self.interrupt_disable_flag,
            self.zero_flag,
            self.carry_flag,
            self.program_counter - 1,
            self.stack_pointer))
        win.addstr(12, 50, "STACK:" +
            " ".join("%02X" % self.memory.read_byte(self.STACK_PAGE + i) for i in range(255, self.stack_pointer, -1))
        )
        
    def run(self, win):
        self.memory.win = win
        win.clear()
        curses.noecho()
        win.nodelay(True)
        while True:
            op = self.read_pc_byte()
            # self.dump(win, op)
            func = self.ops[op]
            if func is None:
                curses.endwin()
                print "UNKNOWN OP"
                print hex(self.program_counter - 1)
                print hex(op)
                break
            else:
                self.ops[op]()
            
            try:
                key = ord(win.getkey())
                if key == 0xA:
                    key = 0xD
                elif key == 0x7F:
                    key = 0x8
                # win.addstr(15, 50, hex(key))
                self.memory.write_byte_io(0xC000, 0x80 + key)
            except curses.error:
                pass
            except TypeError:
                pass
    
    ####
    
    def get_pc(self, inc=1):
        pc = self.program_counter
        self.program_counter += inc
        return pc
    
    def read_pc_byte(self):
        return self.memory.read_byte(self.get_pc())
    
    def read_pc_word(self):
        return self.memory.read_word(self.get_pc(2))
    
    ####
    
    def status_from_byte(self, status):
        self.carry_flag = [0, 1][0 != status & 1]
        self.zero_flag = [0, 1][0 != status & 2]
        self.interrupt_disable_flag = [0, 1][0 != status & 4]
        self.decimal_mode_flag = [0, 1][0 != status & 8]
        self.break_flag = [0, 1][0 != status & 16]
        self.overflow_flag = [0, 1][0 != status & 64]
        self.sign_flag = [0, 1][0 != status & 128]
    
    def status_as_byte(self):
        return self.carry_flag | self.zero_flag << 1 | self.interrupt_disable_flag << 2 | self.decimal_mode_flag << 3 | self.break_flag << 4 | 1 << 5 | self.overflow_flag << 6 | self.sign_flag << 7 
    
    ####
    
    def push_byte(self, byte):
        self.memory.write_byte(self.STACK_PAGE + self.stack_pointer, byte)
        self.stack_pointer = (self.stack_pointer - 1) % 0x100
    
    def pull_byte(self):
        self.stack_pointer = (self.stack_pointer + 1) % 0x100
        return self.memory.read_byte(self.STACK_PAGE + self.stack_pointer)
    
    def push_word(self, word):
        hi, lo = divmod(word, 0x100)
        self.push_byte(hi)
        self.push_byte(lo)
    
    def pull_word(self):
        s = self.STACK_PAGE + self.stack_pointer + 1
        self.stack_pointer += 2
        return self.memory.read_word(s)
    
    ####
    
    def immediate_mode(self):
        return self.get_pc()
    
    def absolute_mode(self):
        return self.read_pc_word()
    
    def absolute_x_mode(self):
        return self.absolute_mode() + signed(self.x_index)
    
    def absolute_y_mode(self):
        return self.absolute_mode() + signed(self.y_index)
    
    def zero_page_mode(self):
        return self.read_pc_byte()
    
    def zero_page_x_mode(self):
        return (self.zero_page_mode() + signed(self.x_index)) % 0x100
    
    def zero_page_y_mode(self):
        return (self.zero_page_mode() + signed(self.y_index)) % 0x100
    
    def indirect_mode(self):
        return self.memory.read_word_bug(self.absolute_mode())
    
    def indirect_x_mode(self):
        return self.memory.read_word_bug((self.read_pc_byte() + signed(self.x_index)) % 0x100)
    
    def indirect_y_mode(self):
        return self.memory.read_word_bug(self.read_pc_byte()) + signed(self.y_index)
    
    def relative_mode(self):
        pc = self.get_pc()
        return pc + 1 + signed(self.memory.read_byte(pc))
    
    ####
    
    def update_nz(self, value):
        value = value % 0x100
        self.zero_flag = [0, 1][(value == 0)]
        self.sign_flag = [0, 1][((value & 0x80) != 0)]
        return value
    
    def update_nzc(self, value):
        self.carry_flag = [0, 1][(value > 0xFF)]
        return self.update_nz(value)
    
    ####
    
    # LOAD / STORE
    
    def LDA(self, operand_address):
        self.accumulator = self.update_nz(self.memory.read_byte(operand_address))
    
    def LDX(self, operand_address):
        self.x_index = self.update_nz(self.memory.read_byte(operand_address))
    
    def LDY(self, operand_address):
        self.y_index = self.update_nz(self.memory.read_byte(operand_address))
    
    def STA(self, operand_address):
        self.memory.write_byte(operand_address, self.accumulator)
    
    def STX(self, operand_address):
        self.memory.write_byte(operand_address, self.x_index)
    
    def STY(self, operand_address):
        self.memory.write_byte(operand_address, self.y_index)
    
    # TRANSFER
    
    def TAX(self):
        self.x_index = self.update_nz(self.accumulator)
    
    def TXA(self):
        self.accumulator = self.update_nz(self.x_index)
    
    def TAY(self):
        self.y_index = self.update_nz(self.accumulator)
    
    def TYA(self):
        self.accumulator = self.update_nz(self.y_index)
    
    def TSX(self):
        self.x_index = self.update_nz(self.stack_pointer)
    
    def TXS(self):
        self.stack_pointer = self.x_index
    
    # SHIFTS / ROTATES
    
    def ASL(self, operand_address=None):
        if operand_address is None:
            self.accumulator = self.update_nzc(self.accumulator << 1)
        else:
            self.memory.write_byte(operand_address, self.update_nzc(self.memory.read_byte(operand_address) << 1))
    
    def ROL(self, operand_address=None):
        if operand_address is None:
            a = self.accumulator << 1
            if self.carry_flag:
                a = a | 0x01
            self.accumulator = self.update_nzc(a)
        else:
            m = self.memory.read_byte(operand_address) << 1
            if self.carry_flag:
                m = m | 0x01
            self.memory.write_byte(operand_address, self.update_nzc(m))
    
    def ROR(self, operand_address=None):
        if operand_address is None:
            if self.carry_flag:
                self.accumulator = self.accumulator | 0x100
            self.carry_flag = self.accumulator % 2
            self.accumulator = self.update_nz(self.accumulator >> 1)
        else:
            m = self.memory.read_byte(operand_address)
            if self.carry_flag:
                m = m | 0x100
            self.carry_flag = m % 2
            self.memory.write_byte(operand_address, self.update_nz(m >> 1))
    
    def LSR(self, operand_address=None):
        if operand_address is None:
            self.carry_flag = self.accumulator % 2
            self.accumulator = self.update_nz(self.accumulator >> 1)
        else:
            self.carry_flag = self.memory.read_byte(operand_address) % 2
            self.memory.write_byte(operand_address,  self.update_nz(self.memory.read_byte(operand_address) >> 1))
    
    # JUMPS / RETURNS
    
    def JMP(self, operand_address):
        self.program_counter = operand_address
    
    def JSR(self, operand_address):
        self.push_word(self.program_counter - 1)
        self.program_counter = operand_address
    
    def RTS(self):
        self.program_counter = self.pull_word() + 1
    
    # BRANCHES
    
    def BCC(self, operand_address):
        if not self.carry_flag:
            self.program_counter = operand_address
    
    def BCS(self, operand_address):
        if self.carry_flag:
            self.program_counter = operand_address
    
    def BEQ(self, operand_address):
        if self.zero_flag:
            self.program_counter = operand_address
    
    def BNE(self, operand_address):
        if not self.zero_flag:
            self.program_counter = operand_address
    
    def BMI(self, operand_address):
        if self.sign_flag:
            self.program_counter = operand_address
    
    def BPL(self, operand_address):
        if not self.sign_flag:
            self.program_counter = operand_address
    
    def BVC(self, operand_address):
        if not self.overflow_flag:
            self.program_counter = operand_address
    
    def BVS(self, operand_address):
        if self.overflow_flag:
            self.program_counter = operand_address
    
    # SET / CLEAR FLAGS
    
    def CLC(self):
        self.carry_flag = 0
    
    def CLD(self):
        self.decimal_mode_flag = 0
    
    def CLI(self):
        self.interrupt_disable_flag = 0
    
    def CLV(self):
        self.overflow_flag = 0
    
    def SEC(self):
        self.carry_flag = 1
    
    def SED(self):
        self.decimal_mode_flag = 1
    
    def SEI(self):
        self.interrupt_disable_flag = 1
    
    # INCREMENT / DECREMENT
    
    def DEC(self, operand_address):
        self.memory.write_byte(operand_address, self.update_nz(self.memory.read_byte(operand_address) - 1))
    
    def DEX(self):
        self.x_index = self.update_nz(self.x_index - 1)
    
    def DEY(self):
        self.y_index = self.update_nz(self.y_index - 1)
    
    def INC(self, operand_address):
        self.memory.write_byte(operand_address, self.update_nz(self.memory.read_byte(operand_address) + 1))
    
    def INX(self):
        self.x_index = self.update_nz(self.x_index + 1)
    
    def INY(self):
        self.y_index = self.update_nz(self.y_index + 1)
    
    # PUSH / PULL
    
    def PHA(self):
        self.push_byte(self.accumulator)
    
    def PHP(self):
        self.push_byte(self.status_as_byte())
    
    def PLA(self):
        self.accumulator = self.update_nz(self.pull_byte())
    
    def PLP(self):
        self.status_from_byte(self.pull_byte())
    
    # LOGIC
    
    def AND(self, operand_address):
        self.accumulator = self.update_nz(self.accumulator & self.memory.read_byte(operand_address))
    
    def ORA(self, operand_address):
        self.accumulator = self.update_nz(self.accumulator | self.memory.read_byte(operand_address))
    
    def EOR(self, operand_address):
        self.accumulator = self.update_nz(self.accumulator ^ self.memory.read_byte(operand_address))
    
    # ARITHMETIC
    
    def ADC(self, operand_address):
        # @@@ doesn't handle BCD yet
        assert not self.decimal_mode_flag
        
        a2 = self.accumulator
        a1 = signed(a2)
        m2 = self.memory.read_byte(operand_address)
        m1 = signed(m2)
        
        # twos complement addition
        result1 = a1 + m1 + self.carry_flag
        
        # unsigned addition
        result2 = a2 + m2 + self.carry_flag
        
        self.accumulator = self.update_nzc(result2)
        
        # perhaps this could be calculated from result2 but result1 is more intuitive
        self.overflow_flag = [0, 1][(result1 > 127) | (result1 < -128)]
    
    def SBC(self, operand_address):
        # @@@ doesn't handle BCD yet
        assert not self.decimal_mode_flag
        
        a2 = self.accumulator
        a1 = signed(a2)
        m2 = self.memory.read_byte(operand_address)
        m1 = signed(m2)
        
        # twos complement subtraction
        result1 = a1 - m1 - [1, 0][self.carry_flag]
        
        # unsigned subtraction
        result2 = a2 - m2 - [1, 0][self.carry_flag]
        
        self.accumulator = self.update_nz(result2)
        self.carry_flag = [0, 1][(result2 >= 0)]
        
        # perhaps this could be calculated from result2 but result1 is more intuitive
        self.overflow_flag = [0, 1][(result1 > 127) | (result1 < -128)]
    
    # BIT
    
    def BIT(self, operand_address):
        value = self.memory.read_byte(operand_address)
        self.sign_flag = ((value >> 7) % 2) # bit 7
        self.overflow_flag = ((value >> 6) % 2) # bit 6
        self.zero_flag = [0, 1][((self.accumulator & value) == 0)]
    
    # COMPARISON
    
    def CMP(self, operand_address):
        result = self.accumulator - self.memory.read_byte(operand_address)
        self.carry_flag = [0, 1][(result >= 0)]
        self.update_nz(result)
    
    def CPX(self, operand_address):
        result = self.x_index - self.memory.read_byte(operand_address)
        self.carry_flag = [0, 1][(result >= 0)]
        self.update_nz(result)
    
    def CPY(self, operand_address):
        result = self.y_index - self.memory.read_byte(operand_address)
        self.carry_flag = [0, 1][(result >= 0)]
        self.update_nz(result)
    
    # SYSTEM
    
    def NOP(self):
        pass
    
    def BRK(self):
        self.push_word(self.program_counter + 1)
        self.push_byte(self.status_as_byte())
        self.program_counter = self.memory.read_word(0xFFFE)
        self.break_flag = 1
    
    def RTI(self):
        self.status_from_byte(self.pull_byte())
        self.program_counter = self.pull_word()
    
    
    # @@@ IRQ
    # @@@ NMI


if __name__ == "__main__":
    mem = Memory(0x10000)
    
    # available from http://www.easy68k.com/paulrsm/6502/index.html
    mem.load_file(0xD000, "A2ROM.BIN")
    
    cpu = CPU(mem)
    curses.wrapper(cpu.run)