ApplePy: An Apple ][ emulator in Python

ApplePy is an Apple ][ emulator written in Python. It combines emulation of the 6502 microprocessor with emulation of the keyboard, display (including graphics mode), speaker, cassette and disk drive. This talk will provide a background to Apple ][ internals then dive into the Python code and the challenges of emulating hardware.

Transcript

1. ApplePy
   An Apple ][ emulator in Python
   James Tauber
   @jtauber
   

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3. 1977: Apple ][
   6502 at 1MHz
   Integer BASIC
   4K RAM ($1298) up to 48K ($2638)
   1979: Apple ][+
   48K RAM standard
   (additional bank-switched 16K available)
   Applesoft BASIC from Microsoft
   1983: Apple /
   /e
   64K RAM standard
   lowercase letters!
   

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4. innovations in:
   display
   sound
   disk
   why implement in hardware what you
   can do in software?
   The Original Apple ][
   

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5. May 2001: SourceForge
   implemented decent amount of 6502
   got a ROM booting but Integer BASIC didn’t work
   February 2007: Google Code
   massive refactor of 6502 code
   August 2011: Github
   patches within hours from Greg Hewgill
   completed 6502 support, low and high res graphics,
   sound, cassette and more
   ApplePy
   

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6. User
   Program
   BASIC Interpreter
   Lower-level ROM Routines
   6502 + I/O
   Logic Gates
   Transistors
   

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7. User
   Program
   BASIC Interpreter
   Lower-level ROM Routines
   6502 + I/O
   

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8. 6502 Overview
   3510 transistors
   8-bit data
   16-bit addresses
   3 general and 3 special-purpose registers
   accumulator register (A)
   2 index registers (X, Y)
   status register (P)
   stack pointer (S)
   program counter (PC)
   about 60 commands
   

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9. class CPU:
   def __init__(self):
   self.accumulator = 0x00
   self.x_index = 0x00
   ...
   self.setup_ops()
   ...
   def setup_ops(self):
   self.ops = [None] * 0x100
   ...
   self.ops[0x8A] = self.TXA
   ...
   

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10. ...
    def run(self):
    while True:
    op = self.read_byte(self.get_pc())
    self.ops[op]()
    def get_pc(self):
    pc = self.program_counter
    self.program_counter += 1
    return pc
    

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11. ...
    def TXA(self):
    self.accumulator = self.x_index
    

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12. def __init__(self):
    ...
    self.carry_flag = 0
    self.zero_flag = 0
    self.overflow_flag = 0
    self.sign_flag = 0
    ...
    

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13. def TXA(self):
    self.accumulator = self.update_nz(self.x_index)
    def update_nz(self, value):
    value = value % 0x100
    self.zero_flag = 1 if (value == 0) else 0
    self.sign_flag = 1 if (value & 0x80) else 0
    return value
    

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14. def setup_ops(self):
    ...
    self.ops[0xCA] = self.DEX
    self.ops[0xE8] = self.INX
    ...
    def DEX(self):
    self.x_index = self.update_nz(self.x_index - 1)
    def INX(self):
    self.x_index = self.update_nz(self.x_index + 1)
    

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15. LDA Addressing Modes
    LDA #$10
    LDA $10
    LDA $1010
    LDA $10,X
    LDA $1010,X
    LDA $1010,Y
    LDA ($1010,X)
    LDA ($1010),Y
    

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16. self.ops[0xA1] = lambda: self.LDA(self.indirect_x_mode())
    self.ops[0xA5] = lambda: self.LDA(self.zero_page())
    self.ops[0xA9] = lambda: self.LDA(self.immediate_mode())
    self.ops[0xAD] = lambda: self.LDA(self.absolute_mode())
    self.ops[0xB1] = lambda: self.LDA(self.indirect_y_mode())
    self.ops[0xB5] = lambda: self.LDA(self.zero_page_x_mode())
    self.ops[0xB9] = lambda: self.LDA(self.absolute_y_mode())
    self.ops[0xBD] = lambda: self.LDA(self.absolute_x_mode())
    

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17. def immediate_mode(self):
    return self.get_pc()
    def zero_page_mode(self):
    return self.read_byte(self.get_pc())
    def absolute_mode(self):
    return self.read_word(self.get_pc(2))
    def absolute_x_mode(self):
    return self.absolute_mode() + self.x_index
    def indirect_mode(self):
    return self.read_word_bug(self.absolute_mode())
    

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18. def LDA(self, operand_address):
    self.accumulator =
    self.update_nz(self.read_byte(operand_address))
    

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19. 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
    

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20. def push_byte(self, byte):
    self.write_byte(self.STACK_PAGE + self.stack_pointer, byte)
    self.stack_pointer = (self.stack_pointer - 1) % 0x100
    def push_word(self, word):
    hi, lo = divmod(word, 0x100)
    self.push_byte(hi)
    self.push_byte(lo)
    

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21. def ADC(self, operand_address):
    a2 = self.accumulator
    a1 = signed(a2)
    m2 = self.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)]
    

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22. Memory
    0x0000 – 0xBFFF RAM
    0xC000 – 0xCFFF memory-mapped I/O
    0xD000 – 0xFFFF ROM
    

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23. Original Apple ][ ROM
    $F800-$FFFF (2k)
    Monitor: screen/keyboard I/O, disassembler, memory
    utils, multiply/divide, etc
    $F689-$F7FC (372 bytes)
    sweet-16 interpreter
    $F500-$F63C; $F666-$F668 (320 bytes)
    mini assembler
    $F425-$F4FB;$F63D-$F65D (248 bytes)
    un-used floating point routines
    $E000-$F424 (5k)
    Integer BASIC
    

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24. Display
    no separate video RAM
    scanlines are not sequential in memory
    “Venetian Blind” effect
    

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25. base = address - start_text
    hi, lo = divmod(base, 0x80)
    row_group, column = divmod(lo, 0x28)
    row = hi + 8 * row_group
    mode, ch = divmod(value, 0x40)
    There and Back Again
    BASCALC PHA
    LSR A
    AND #$03
    ORA #$04
    STA BASH
    PLA
    AND #$18
    BCC BSCLC2
    ADC #$7F
    BSCLC2 STA BASL
    ASL
    ASL
    ORA BASL
    STA BASL
    RTS
    

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26. characters = [
    [0b00000, 0b01110, 0b10001, 0b10101,
    0b10111, 0b10110, 0b10000, 0b01111],
    [0b00000, 0b00100, 0b01010, 0b10001,
    0b10001, 0b11111, 0b10001, 0b10001],
    [0b00000, 0b11110, 0b10001, 0b10001,
    0b11110, 0b10001, 0b10001, 0b11110],
    [0b00000, 0b01110, 0b10001, 0b10000,
    0b10000, 0b10000, 0b10001, 0b01110],
    [0b00000, 0b11110, 0b10001, 0b10001,
    0b10001, 0b10001, 0b10001, 0b11110],
    [0b00000, 0b11111, 0b10000, 0b10000,
    0b11110, 0b10000, 0b10000, 0b11111],
    [0b00000, 0b11111, 0b10000, 0b10000,
    0b11110, 0b10000, 0b10000, 0b10000],
    [0b00000, 0b01111, 0b10000, 0b10000,
    0b10000, 0b10011, 0b10001, 0b01111],
    ...
    

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27. I/O
    0xC000 — the keyboard
    0xC010 — keyboard strobe
    0xC030 — toggle the speaker
    0xC060 — read the cassette
    

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28. for event in pygame.event.get():
    ...
    if event.type = pygame.KEYDOWN:
    key = ord(event.unicode)
    if event.key = pygame.K_LEFT:
    key = 0x08
    if event.key == pygame.K_RIGHT:
    key = 0x15
    if key:
    if key == 0x7F:
    key = 0x08
    self.softswitches.kbd = 0x80 + (key & 0x7F)
    Keyboard
    

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29. Speaker
    reading 0xC030 toggles the speaker
    we pass around the cycle count
    everywhere
    when speaker is toggled we note the
    cycle and append to a sound wave array
    everyone once in a while, we play the
    sound wave array
    

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30. Cassette
    almost the reverse of the speaker
    given a sound wave
    when softswitch is read, convert cycle number
    to how far into the sound wave we are
    keep track of whether we’ve crossed zero since
    last time
    so far works about 10% of the time
    

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31. Hi-Res Color
    Each row of 280 pixels represented by
    40 bytes
    7 bits are the pixels
    high-bit is the palette
    odd/even bits are different colors
    palette 1: black, green, magenta, white
    palette 2: black, blue, orange, white
    

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32. Hi-Res Color
    1 0 0 0 1 0 1 0 0 1 0 1 1 1 0 0
    0 0 0 1 1 0 1 0
    try setting this magenta
    and you get this
    

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33. Disk Drive
    still not working
    darn magnets
    Woz’s brilliance is my curse :-)
    

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34. github.com/
    jtauber/
    applepy
    

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35. DEMO
    

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36. ApplePy
    An Apple ][ emulator in Python
    James Tauber
    @jtauber
    

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