Writing NES games! with assembly!!

Talk given at !!Con 2017.

I’d like to take you on a stroll down memory lane and dig into the internals of the Nintendo Entertainment System (NES) to figure out how it works. While we’re there, we’ll see how to build a game for the NES using 6502 assembly with the help of a few modern tools. We’ll gain a new respect for ’80s developers and an appreciation for the high-level languages we have today!

Transcript

1. View Slide

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6. View Slide

7. PRG ROM
   CHR ROM
   

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8. chr rom
   

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9. View Slide

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11. View Slide

12. View Slide

13. PRG ROM
    CHR ROM
    battery
    RAM
    

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14. Milestone 1
    

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15. NO WAY! OMG
    omg!! it works!!!
    is it normal that we are this excited about displaying a ball?
    

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16. NO WAY! OMG
    omg!! it works!!!
    is it normal that we are this excited about displaying a ball?
    

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17. NO WAY! OMG
    omg!! it works!!!
    is it normal that we are this excited about displaying a ball?
    

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18. NO WAY! OMG
    omg!! it works!!!
    is it normal that we are this excited about displaying a ball?
    

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19. NO WAY! OMG
    omg!! it works!!!
    is it normal that we are this excited about displaying a ball?
    

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20. View Slide

21. 00008000: 20 8e 10 40 2c 02 20 2c 02 20 10 fb 8a 95 00 9d
    00008010: 00 01 9d 00 03 9d 00 04 9d 00 05 9d 00 06 9d 00
    00008020: 07 e8 d0 e9 a9 ff 9d 00 02 e8 d0 fa 2c 02 20 10
    00008030: fb 4c b6 80 a9 01 85 02 a9 01 85 03 a9 70 8d 03
    00008040: 02 a9 70 8d 00 02 a9 00 8d 02 02 a9 01 8d 01 02
    00008050: 60 e6 00 e6 01 a5 00 8d 03 02 a5 01 8d 00 02 60
    00008060: a9 01 8d 16 40 85 05 4a 8d 16 40 ad 16 40 29 03
    00008070: c9 01 26 04 ad 17 40 29 03 c9 01 26 05 90 ec 60
    00008080: a5 05 85 07 a5 04 85 06 20 70 80 a5 05 85 09 a5
    00008090: 04 85 08 20 70 80 a2 01 b5 04 d5 08 f0 04 b5 06
    000080a0: 95 04 ca 10 f3 60 a9 3f 8d 06 20 a9 00 8d 06 20
    000080b0: a9 0f 8d 07 20 a9 3f 8d 06 20 a9 11 8d 06 20 a9
    000080c0: 30 8d 07 20 a9 0f 8d 07 20 8d 07 20 20 44 80 a9
    000080d0: 18 8d 01 20 a9 80 8d 00 20 a9 01 85 0e 20 90 80
    000080e0: a5 04 f0 03 20 00 81 20 61 80 20 25 81 4c e9 80
    000080f0: a5 04 29 0c f0 0c a5 04 29 08 f0 03 e6 03 60 c6
    00008100: 03 60 a5 04 29 03 f0 0b a5 04 29 02 f0 03 c6 02
    00008110: 60 e6 02 60 60 e6 11 a5 11 d0 fc 60 48 8a 48 98
    00008120: 48 a5 0e f0 0a a9 00 8d 03 20 a9 02 8d 14 40 a5
    00008130: 0f f0 08 2c 02 20 20 24 81 c6 0f a5 10 f0 17 a5
    00008140: 0b 8d 01 20 a5 0a 8d 00 20 2c 02 20 a5 0c 8d 05
    00008150: 20 a5 0d 8d 05 20 a9 00 85 11 68 a8 68 aa 68 40
    

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22. cc65 toolchain
    asm
    asm
    asm ca65 ld65
    asm
    asm
    obj ines
    

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23. 69
    70 main:
    71 ; Set universal background color ($3F00) to black ($0F)
    72 ; Set PPU address to $3F00
    73 lda #$3F
    74 sta PPUADDR
    75 lda #$00
    76 sta PPUADDR
    77 ; Write $0F to that PPU address
    78 lda #$0F
    79 sta PPUDATA
    80
    81 ; Set sprite palette 0 ($3F11-$3F13) to white ($30), black ($0F), black ($0F)
    82 ; Set PPU address to $3F11
    83 lda #$3F
    84 sta PPUADDR
    85 lda #$11
    86 sta PPUADDR
    87 ; Write $30, $0F, $0F to PPU
    88 lda #$30
    89 sta PPUDATA ; PPUADDR will increase by 1 each time we write to it
    90 lda #$0F
    91 sta PPUDATA
    92 sta PPUDATA
    93
    94 jsr ballSetup
    95
    96 ; Enable sprite and background rendering
    97 lda #%00011000
    98 sta PPUMASK
    99
    100 ; Enable NMI
    101 lda #%10000000
    102 sta PPUCTRL
    103
    

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24. 6502 assembly
    LDA STA
    memory
    ADC SBC
    INC DEC
    arithmetics
    AND ORA
    CMP
    compare
    BCC BCS BEQ
    BMI BNE BPL
    branch
    

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25. 6502 assembly
    LDA STA
    memory
    ADC SBC
    INC DEC
    arithmetics
    AND ORA
    CMP
    compare
    BCC BCS BEQ
    BMI BNE BPL
    branch
    

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26. 6502 assembly
    LDA STA
    memory
    ADC SBC
    INC DEC
    arithmetics
    AND ORA
    CMP
    compare
    BCC BCS BEQ
    BMI BNE BPL
    branch
    

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27. 6502 assembly
    LDA STA
    memory
    ADC SBC
    INC DEC
    arithmetics
    AND ORA
    CMP
    compare
    BCC BCS BEQ
    BMI BNE BPL
    branch
    

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28. 6502 assembly
    LDA STA
    memory
    ADC SBC
    INC DEC
    arithmetics
    AND ORA
    CMP
    compare
    BCC BCS BEQ
    BMI BNE BPL
    branch
    

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29. $0600: LDA $05
    $0602: CMP #$08
    $0604: BEQ $060b
    $0606: INC $05
    $0608: JMP $0600
    $060b: RTS
    PC
    

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30. Milestone 2
    !
    

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31. Milestone 2
    !
    

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32. picture processing unit
    

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33. 256px
    240px
    

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35. 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
    10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
    20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F
    30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F
    nes palette
    

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39. y tile attrs x
    23 01 00 01
    64 x
    

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40. PPU registers
    $2000-$2007
    

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41. !
    Milestone 3
    "
    "
    "
    

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42. OMG! Wait... Is that real?
    yeah!!!
    NO WAY!!
    yeah!!!
    

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43. OMG! Wait... Is that real?
    yeah!!!
    NO WAY!!
    yeah!!!
    

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44. OMG! Wait... Is that real?
    yeah!!!
    NO WAY!!
    yeah!!!
    

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45. OMG! Wait... Is that real?
    yeah!!!
    NO WAY!!
    yeah!!!
    

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46. OMG! Wait... Is that real?
    yeah!!!
    NO WAY!!
    yeah!!!
    

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47. OMG! Wait... Is that real?
    yeah!!!
    NO WAY!!
    yeah!!!
    

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48. video frame
    vertical blank
    

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49. video frame
    vertical blank
    

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50. nmi interrupt
    

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51. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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52. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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53. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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54. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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55. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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56. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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57. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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58. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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59. loop:
    INC $05
    JMP loop
    nmi:
    ; Do stuff
    RTI
    PC
    

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60. Milestone 4
    #
    #
    "
    

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61. controllers
    $4016-$4017
    

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62. View Slide

63. 0 0 0 0 0 0 0 0
    a B se st
    

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64. $
    1 0 0 0 0 0 0 0
    a B se st
    

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65. $
    $
    1 0 0 0 0 0 0 1
    a B se st
    

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66. LDA controller1
    AND #%10000000
    BEQ notPressed

    ; A is pressed
    notPressed:
    RTS
    

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67. LDA controller1
    AND #%10000000
    BEQ notPressed

    ; A is pressed
    notPressed:
    RTS
    1 0 0 0 0 0 0 0
    a B se st
    

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68. Milestone 5
    # #
    "
    %
    &
    "
    

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69. wine + fceux
    

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70. View Slide

71. lua scripting
    

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72. emu.frameadvance()
    memory.readbyte(0x0000)
    memory.writebyte(0x0001, 0x30)
    

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73. emu.registerafter(function()
    ballX = memory.readbyte(0x0000)
    ballY = memory.readbyte(0x0001)
    gui.text(
    10,
    30,
    string.format('Ball: %02x,%02x', ballX, ballY)
    )
    end)
    

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74. View Slide

75. '
    

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76. '
    (
    lua
    

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77. !
    )
    

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78. function TestBall:test_ball_moves_down()
    memory.writebyte(ballYAddress, 0x30)
    memory.writebyte(ballYDirection, 0x01)
    emu.frameadvance()
    ballY = memory.readbyte(ballYAddress)
    luaunit.assertEquals(ballY, 0x31)
    end
    

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79. View Slide

80. View Slide

81. View Slide

82. Read the source code
    when in doubt
    

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83. View Slide

84. NExt steps
    

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85. @cjoudrey
    

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